 |
Figure 2 Performer Ruslana and her Wolf. Both humans and Animals are threatened by rabies. A solution for people must also involve preventing this disease in wild animals
RABIES
James A. Wilkerson, M.D.
Rabies has terrorized humanity since the dawn of civilization, and the menace continues. In industrialized nations where human rabies is rare, animal rabies abounds and humans are protected from infection only by vigorous animal vaccination programs and post exposure immunization. In developing countries tens of thousands die each year, and over ten million endure agonizing anxiety following exposure to a possibly rabid animal. 179 In the United States 15,000 to 40,000 people receive postexposure prophylaxis each year. 145
An encounter with this uniformly fatal infection, globally the most common form of viral encephalitis, leaves physicians with “a more indelible stamp of horror” than any other disease. 113 CURRENT STATUS
Globally rabies is the tenth most frequent cause of death from infectious disease. 94 The actual number of deaths is unknown because reporting in the developing countries where this infection is common is unreliable. Most of these countries do not have laboratory facilities capable of establishing a dependable diagnosis. 62 The World Health Organization (WHO) currently estimates the number of rabies deaths globally at 40,000 to 70,000 a year, an average of approximately one death every ten minutes. (http://www.who.int/mediacentre/factsheets/fs099/en/)
Some—perhaps many—human rabies infections are not diagnosed, even in nations with sophisticated medical systems. This problem was vividly dramatized in 2004 by the deaths of four United States organ transplant recipients from a donor, whose rabies infection had not been recognized. 55, 60 A recent review has suggested that rabies may be underdiagnosed in the United States. 179
In addition to being undiagnosed, Rabies is probably incorrectly diagnosed with considerable frequency. Of 33,000 human rabies deaths reported worldwide in 1997, laboratory confirmation was available for less than 0.5 percent. 62
THE RABIES VIRUS
Rabies viruses belong to the group Rhabdoviridae, genus Lyssavirus . At least seven genotypes are recognized, but genotype 1 is the only one of major significance. This genotype consists of multiple variants or lineages, each closely linked to a single mammal species such as raccoons, skunks, foxes, mongooses or various bat species. In the 1980's these variants were distinguished with monoclonal antibodies. Subsequently, analysis of nucleotide substitutions in the rabies genome by reverse transcriptase—polymerase chain reaction (RT-PCR) has allowed identification of the primary reservoirs for each variant, to map the geographic distribution of variants, and to identify virus spillover into animals and humans. 154, 62 In addition, for the past 25 years the source of many human infections has been identifiable in the absence of a recognizable exposure to rabid animals.
The rabies virus is bullet-shaped with one flat end, and contains a single strand of RNA made up of approximately 12,000 nucleotides that encodes five proteins. Three, the nucleoprotein (N), phosphoprotein (P, M 1 , or NS), and the large polymerase or transcriptase protein (L), make up the core of the virus. The other two, the matrix protein (M) and the transmembrane glycoprotein (G), form its coat. 190
The external surface is studded with perpendicular aggregates of glycoprotein, the G protein, that recognizes cell surface receptors and facilitates virus entry into cells. 4 The 505-amino-acid G protein is composed of a forty-four-amino-acid internal or “cytoplasmic” portion, a twenty-two-amino-acid hydrophobic transmembrane portion, and the large external, “antigenic” portion. 129, 191 The complete amino acid sequences of these proteins have been determined for several fixed rabies strains. 180
A lipid bilayer is closely associated with the matrix protein, and the two form a clearly defined shell for the virus. The M protein is the smallest of the rabies virus structural proteins with only 202 amino acids, 192 but makes up approximately 25 percent of the total virion protein. It is in close contact with the core, and also interacts with the internal segment (cytoplasmic tail) of the surface protein.
The core of the virus forms a tightly structured helix of thirty to thirty-five coils that extends end-to-end within the virion. The RNA genome is associated with about 1,800 closely packed molecules of the 55 kDa nucleoprotein, which together are known as ribonucleoprotein (RNP). The N protein protects the genome from digestion and keeps it in a suitable configuration for transcription. Some thirty to sixty copies of the large (~190 kDa) transcriptase-associated L protein and about 950 copies of the smaller (~38 kDa) phosphoprotein are responsible for the replication of the viral RNA. 192
RABIES IN THE U. S.
Incidence In Humans
The incidence of human rabies in the United States fell dramatically from 23.5 infections per year in the late 1940's to 1.0 infections per year during the 1980's. However, 27 infections were reported from 1990 through 1999, and 15 have been reported so far in the 2000's, most acquired directly or indirectly from bats. 41, 43, 44, 47, 55, 57, 60, 185 Human Rabies Infections in the United States, 1946 to 2004
Time Period U.S. Cases/Year Non-U.S. Infections Infections
1946–1949 94 23.5 0 1950–1959 136 13.6 0 1960–1969 38 3.8 3 1970–1979 17 1.7 6 1980–1989 3 0.3 7 1990–1999 22 2.3 5 2000–2004 15 3.8 2
Although reliable rabies vaccines and antisera first became available during this fifty-eight-year period, extensive vaccination of domestic animals, particularly household pets, and elimination of unrestrained and stray animals are considered primarily responsible for the decline in the human infection rate. 49, 50, 121 Such programs reduced the incidence of laboratory confirmed rabies in dogs from 6,949 in 1947 to 99 in 2002. 107 The annual cost for these programs is over $300 million, most of which is borne by pet owners. 155 RABIES IN WILD TERRESTRIAL ANIMALS
In the United States and Canada a vast reservoir of rabies persists in wild animals. 155 During 2003, 49 states and Puerto Rico reported 7,170 rabies infections in animals, 6,556 (91.4 percent) of which were in wild animals. However, some states accept only animals responsible for a human or domestic animal incident for rabies testing; others test all submitted specimens. Furthermore, the number of rabid wild animals that die without being detected is estimated to be more than 90 percent of the total, so the identified infections represent only a small fraction of wild animal rabies. 107
Animal Rabies in the United States, Annual Averages 1998–2002 145
Species Number of Percentage Animals
Raccoons 2,962 39.5% Skunks 2,257 30.1% Bats 1,175 15.7% Foxes 443 5.9% Cats 276 3.7% Dogs 105 1.4% Cattle 106 1.4% Horses and Mules 620 0.8% Woodchuck 50 0.7% Bobcat 30 0.4% Sheep or Goats 9 0.1% Other Wild Animals 24 0.3%
Currently several major rabies epizootics are recognized. An epizootic of rabies started in Arctic foxes in northern Canada in the late 1940's and early 1950's, and swept southward in the middle and late 1950's to involve red foxes in Alberta, Saskatchewan, Manitoba, Ontario, and Quebec. The epizootic crossed the St. Lawrence River in 1961, and involved foxes in upper New York State, although currently it appears to be limited to the adjacent Canadian provinces, which are experiencing a lower incidence of fox rabies. 10, 26, 108 The epizootic, which moved westward to involve arctic foxes in Alaska and the Northwest Territories, 152 surrounds the North Pole and may cover the largest land area of any observed outbreak. 26
An outbreak of raccoon rabies started in central Florida in the 1940s and spread at the rate of about twenty-five miles per year, reaching Georgia in the early 1960s and Alabama and South Carolina in the 1970s. In the late 1970s a second outbreak appeared on the Virginia–West Virginia border. That epizootic has now spread north to all of New England, and crossed into Canada in 1999. It has also spread south to join with the epizootic coming north from Florida in North Carolina. 42, 62, 63 The second outbreak developed when raccoons were translocated from Florida for restocking for hunters. Although the animal suppliers held legal permits and health certificates, the inclusion of some rabid animals among the more than 3,500 transshipped raccoons has been documented. 102, 189
More than 50,000 rabies infections in raccoons have been reported in the United States since 1975. The land mass affected by this epidemic is approximately 1 million km 2 (383,000 mi 2 ) and includes the residences of 35 percent of the United States human population. The raccoon epizootic is considered particularly threatening because many raccoons live in densely populated urban and suburban areas. 62 However, the only known human rabies infection resulting from this epizootic occurred in 2003. 43 The spread of rabies from raccoons to humans appears to have been limited in part because raccoons are large animals and their bites are obvious. To some extent well-vaccinated dogs and other pets form a barrier between wild animals and humans. Perhaps of greatest significance is the nonaggressive behavior of rabid raccoons. In thirty-eight rabid raccoon incidents in Florida over a 5-year period, bites were inflicted only when humans or dogs tried to kill or capture raccoons that seemed tame. 186 Prior to the raccoon epizootic, most terrestrial rabies in the United States was in skunks. Human rabies resulting from exposure to a spotted skunk in California was reported in 1826. 61 Four epizootics are recognized, one of which is in Quebec and New York and is associated with the fox epizootic in that area. A larger epizootic started in Iowa in 1945. It has spread east to Ohio, west to Montana, north to the Canadian provinces of Manitoba (1959), Saskatchewan (1963), and Alberta (1971), and south to meet with a third epizootic that originated in Texas and has spread to surrounding South Central states, particularly Oklahoma and Arkansas. The fourth epizootic in skunks is located in northern California. 61
An increase in the number of rabid skunks in the East Coast states has recently occurred, but analysis of these infections indicate they result from raccoon rabies spilling over into skunks and are not indicative of a separate skunk epizootic. 91
Screening of rabies virus isolates from the epizootics has disclosed five distinctive patterns. Red foxes and skunks in New York and adjacent Canada present one pattern; raccoons from the Atlantic states present a second. The skunks in the South-central states present a third, and a fourth is represented only by a small outbreak in gray foxes in Arizona. However, the fifth pattern is found in skunks from the North-central states and from California, in dogs from the Mexico border states, and in a small rabies outbreak in gray foxes in Central Texas. 151
Rabies in rodents is an intriguing problem. Rodents are the animals of choice for rabies virus isolation in the laboratory, yet rabies in small free-living rodents is rare. Rodents may usually be killed rather than just infected by the bites of rabid animals, but rodents are carrion eaters and can be infected by that source. In recent years the largest number of rodent rabies infections has been in large rodents such as woodchucks that have been infected by rabid raccoons. Rabid beavers have attacked and bitten humans in North Carolina. However, no transmission of rabies to humans by rodents has been documented. 163 RABIES IN BATS
With the exception of Antarctica, rabies in bats is global. In Canada and the United States, parts of South America, Western Europe, and Australia where rabies in carnivores, particularly dogs, has been controlled, bats are the most prominent source of human rabies. 119
Rabies was diagnosed in insectivorous bats in Brazil in the 1920's and in frugivorous bats in Trinidad during the 1930's, although the principle subject of these studies was rabies in hematophagous ("vampire") bats that was being transmitted to humans. The first definitive diagnosis of rabies in nonhematophagous bats was made in a frugivorous bat that flew into a “chemist's” shop in Port of Spain, Trinidad, on September 10, 1931.
However, the incident that drew widespread attention to bat rabies occurred in Tampa, Florida, on June 23, 1953. The seven-year-old son of a ranch hand was looking for a baseball near some shrubbery when a lactating female yellow bat suddenly flew out of the bushes and bit the boy on the chest, remaining firmly attached until knocked off by the boy's mother. The ranch owner had heard of rabies in vampire bats in Mexico and insisted the bat be examined for infection. Negri bodies were found in smears of the brain, and the diagnosis was confirmed by mouse inoculation of brain tissue. The boy was given postexposure treatment and did not develop an infection. 16, 63, 180
The publicity given this event led to many more bats being submitted for rabies examination. Subsequently, rabies has been found in bats in every state except Hawaii, and in eight Canadian provinces. 16, 133 In 1989 bat rabies was reported from all forty-eight of the continental states and the District of Columbia. 50 The estimated incidence of rabies in bats in the United States is 0.5 to 1.0 percent; the incidence in bats that appear ill or injured is much higher, 7 to 50 percent. 16, 164
Rabies virus variants from bats are species specific rather than geographically specific. 151 and are distinctly different from those of terrestrial animals in the same locations, including the major terrestrial epizootics. Clearly, little exchange of infection between bats and terrestrial animals takes place, although occasional animals infected with rabies virus strains typical of bats are found. Many large areas of the United States, particularly the Pacific Northwest and New England (prior to the raccoon epizootic), report rabies in bats but in no other species. Even though cats and foxes catch and eat bats, only three of 136 cat and fox rabies isolates over a two-year period were antigenically similar to bat rabies strains. 16, 70, 151
Approximately 70 percent of human rabies infections and 75 percent of cryptic rabies deaths in the United States have been caused by the variant associated with silver-haired and the eastern pipistrelle bats, which are reclusive animals rarely found around human habitation. Infections by variants associated with bats that frequent human dwellings are much less common. Infections in other animals by this variant are also disproportionately very high. These bats are small and their bites are difficult to detect. However, in comparison with other rabies virus variants, the variant associated with these two bats replicates better in fibroblasts and epithelial cells, and replicates better at the low temperature of 34°C. These features indicate this variant is better able to replicate in the peripheral tissues involved by most bites. 114 RABIES IN DOMESTIC ANIMALS
Since rabies in dogs has been controlled, rabies infections in cats have outnumbered infections in dogs (321 to 117 in 2003.) 107 A major problem in vaccinating cats is establishing ownership. Farmers value cats for rodent control, but do not recognize them as property. Cats wander from farm to farm and contact wild animals with rabies. Capturing feral cats so they can be vaccinated is difficult. 37
Rabies is not rare in other domestic animals, including cattle, horses and mules, and sheep and goats. SOURCES OF HUMAN INFECTION
In the late 1940's and 1950's most human rabies in the United States resulted from bites by dogs or cats. Of 146 infections in the years 1946 to 1961 for which a source of exposure could be identified, dogs were responsible for 120 and cats for 9 (88.4 percent). Foxes (7), skunks (5), and bats (5) were responsible for the rest. 49 However, when rabies in domestic animals was controlled, human rabies resulting from bites by pets disappeared. Since 1966, all but two of the 15 human rabies infections resulting from exposures to rabid dogs, were acquired outside of the United States. 49, 58, 121
Prior to 1965, the CDC had recorded no human rabies occurring within the United States that had been acquired outside the country. 121 However, since then the number of infections acquired outside the United States has been significant: 3 of 15 (20 percent) between 1965 and 1970, 6 of 23 (26 percent) in the 1970's, 7 of 10 (70 percent) in the 1980's, and 7 of the 35 cases (20 percent) since 1980. Lack of knowledge about the risk of rabies in developing countries has led some travelers to disregard animal encounters and not obtain rabies immunoprophylaxis, but some of these infections have been in children who did not inform their parents of the animal contact.
Until the 1980's, identifying the source of a number of human rabies infections in the United States was impossible. For many infected persons no animal exposure incident—even an opportunity for animal exposure—could be identified. An infectious source could not be found for 84 of 230 (35 percent) human rabies infections occurring in the United States between 1946 and 1961, 121 or for 6 of 38 (16 percent) human infections between 1960 and 1970. 48
Only since the 1980's have monoclonal antibody typing or RT-PCR nucleotide analysis allowed the source of human rabies infections to be determined when no animal exposure incident could be identified. 16, 70, 151 However, such studies have made unmistakably clear that bats are now the major source of human rabies in the United States.
Thirty-six of the forty human rabies infections acquired within the United States since 1980 are attributed to bats. Only four of the individuals had a history of a bite by a bat, and the source of infection for the others was identified by nucleotide analysis. 49, 55, 60
How the infection is transmitted remains uncertain. In 1956 and 1959, two men died of rabies after exploring Frio Cave near Uvalde, Texas. The walls and ceiling of that cave hold astonishing numbers of bats—300 to 400 per square foot. Neither had been bitten, and their infections are attributed to aerosol transmission of the rabies virus. When experimental animals of various species subsequently were placed in the cave in cages that only allowed the virus to be transmitted as an aerosol, a significant percentage developed rabies. 65, 66 Additionally, aerosol transmission of rabies to humans has occurred at least twice in laboratories. 22, 188 CDC recommends rabies vaccination for spelunkers. 52
However, nursery caves such as Frio Cave contain an astounding number of bats. Saliva and urine constantly rain down on anyone entering the cave and the blanket of guano on the floor ranges from several inches to several feet in thickness. In Frio Cave air circulation is so poor the bats warm the cave, the air is humidified by their respiration, and the concentration of ammonia from their urine is so high that the cave usually cannot be entered without respirators. 65 Similar infections in other caves have not been reported.
Unrecognized bites appear to be the source of infection for most individuals who have had no recognized bat encounters. Bat teeth are so small and so sharp that a bite may not be felt. Even the recognized bites do not appear to be particularly painful, although at least one of the individuals known to have been bitten was intoxicated with ethanol at the time. 87 For centuries South American vampire bats have been reported to bite sleeping victims without awakening them.
Limiting human rabies of bat origin is best addressed by informing the public of the risk. 133 Reducing the bat population is not an acceptable approach. Significant population reduction would be difficult, but if achieved, would be an ecological disaster because bats play such a major role in insect control.
CDC and other institutions now advocate the following measures:
· Dwellings should be “bat-proofed” by tightly covering all possible entrances, particularly roof ventilation openings, with wire screens. Protection from bats in unscreened dwellings or when sleeping outdoors can be achieved with mosquito netting.
· Contact with bats must be assiduously avoided, particularly bats that are behaving unusually. Bats are nocturnal and any activity during daylight hours should be considered abnormal. Diseased bats often are unable to fly.
· Any person who has contact with a bat, regardless of whether a bite is thought to have been inflicted, should receive postexposure prophylactic therapy unless the bat has been caught and examined for rabies.
· Any person, particularly a child, who awakens from sleep and finds a bat in the room, should receive post-exposure prophylaxis unless the bat has been caught and examined. A number of the recent bat rabies victims have been children who experienced this kind of exposure.
RABIES IN OTHER COUNTRIES Epidemiology
Rabies is found throughout the world, and although more common in tropical or temperate climates is by no means limited to those areas. Arctic foxes with rabies have been found in Alaska, Northern Canada, Greenland, Norway's Svalbard Islands, and much of Siberia. An epizootic in the Thule district of Greenland in 1958 and 1959 resulted in the death from rabies of 50 percent of the sled dogs in that area. Over 1,000 dogs in the Egedesminde district died in another epizootic in 1959 and 1960. (For reasons that are not known, transmission of rabies to humans is rare in these areas, even though exposures are common. 67 Perhaps the rabies variant is more infective for foxes and dogs.)
Rabies is not found in a few areas, all of which are landmasses or peninsulas isolated by water. Rabies does not occur in Hawaii, the only state in the United States in which rabies is not found, some of the Caribbean islands, Pacific Oceania, including Australia (although Australian bat lyssavirus is found there) and New Zealand, or Antartica. 89, 118 The two human genotype 1 rabies infections that have occurred in Australia are thought to have been acquired outside that country. 22, 54, 89
Great Britain had been free of rabies since an extensive dog confinement and vaccination program in 1903, although concern about reintroduction of rabies has been raised by the Channel tunnel and the reduction of border controls between members of the European Community. 63, 165 A 55-year-old Scotsman with a fatal infection in 2002 was the first locally acquired lyssavirus infection on that island in 100 years, but the virus was not of Genotype 1. 85
Rabies has been endemic in Japan since the tenth century. However, following World War II, members of the U.S. Army Veterinary Corps determined that no reservoir of rabies existed in the wild animal populations of Japan, Taiwan, and the Philippines — perhaps in part because wild animals were hunted for food during the war. Extensive campaigns to eliminate stray dogs (which in some areas of Japan reduced the canine population by 70 to 80 percent) and to vaccinate those remaining succeeded in eradicating the infection from Japan and Taiwan. Endogenously acquired rabies has not occurred in those islands since the late 1950's. 4, 156
The success of canine rabies eradication programs is dependent upon the society in which the programs are initiated. Such programs achieve little success in nations that are predominantly Hindu or Buddhist, because the people do not support elimination of animals that have no apparent owner. They often put out food for stray dogs. In contrast, Malaysia, a peninsula that is predominantly Muslim, has been largely free of rabies since the early 1950's. 23
Elimination of stray dogs must be combined with vaccination programs. Dogs that are eliminated because they cannot be associated with human ownership are quickly replaced. The annual turnover of the dog population in developing countries has been found to range between 30 and 40 percent. 27
Vaccination of domestic animals for rabies is limited largely to industrialized nations. In many developing countries, vaccination of animals is considered unaffordable and rabies control resources are expended on postexposure immunoprophylaxis of humans. Even though rabies immunoprophylaxis is administered to 800 to 900 persons per million inhabitants annually in such countries, the human death rate from rabies is still high, an average of nearly five deaths for each one million population annually. 28 In the United States that death rate would result in nearly 1,500 rabies deaths a year.
The magnitude of the rabies threat in developing countries is illustrated by the experience in Thailand. Rabies has been of particular interest in that country since Princess Banlusirisarn was bitten by a rabid dog within the palace grounds near Bangkok in 1911 and subsequently died. (No rabies vaccine was available in Thailand — then called Siam — at that time.) The princess's death was instrumental in establishing the Institute Pasteur of Bangkok in 1913, which was renamed the Queen Saovabha Memorial Institute (QSMI) in 1922. This Institute, a WHO Collaborating Center for Research on Rabies Pathogenesis and Prevention, has been and remains the site of many sophisticated rabies investigations.
In the 1990's the Institute's postexposure rabies clinic treated about 18,000 patients with new animal bites each year — an average of almost fifty new patients a day! Furthermore, these patients were only 28 percent of the estimated 64,000 Thais who receive postexposure therapy annually, many of whom are residents of rural or remote portions of the country and are treated by local physicians. 183 However, the number of human deaths from rabies in Thailand has declined from about 400 a year in the 1970's to 70 in 1999 even though dog rabies has not been controlled. 62
Other developing nations have similar rabies problems. WHO agencies have estimated that eighty-seven countries and territories with a total population of about 2.4 billion people are afflicted with endemic canine rabies. 28 SOURCES OF HUMAN INFECTION
Although domestic animals are rarely the sources of human rabies in the United States and other developed countries, in developing countries the vast majority of human rabies — 99 percent by some estimates — is the result of exposure to rabid dogs. 29, 63, 175, 180 In Thailand, although rabies has been found in an array of exotic tropical animals that includes tigers and leopards, between 1979 and 1985 90.6 percent of human infections resulted from dog bites and an additional 6 percent followed unknown events. The remaining 3.6 percent followed cat attacks. 183
Other animals, particularly bats, do transmit rabies. Hematophagous or “vampire” bats are a major source of animal and human rabies in South and Central America, the only areas where such bats are found. Their range extends between northern Mexico and northern Argentina — basically between the tropics of Cancer and Capricorn — and fossils indicate vampire bats have inhabited those areas for 2.5 million years. 84 These animals consume 20 to 25 ml of blood at a feeding, and although cattle are their preferred food source, a study in Colima, Mexico, found human blood in the stomachs of 15.7 percent of 70 vampire bats. 12
Human rabies of vampire bat origin was first recognized in 1929 in Trinidad when Negri bodies were found in the brains of seventeen individuals, mostly school-age children, who died with acute ascending paralysis. Subsequently, small epidemics have been recognized almost every year in that country. (Interestingly, almost all of the rabies transmitted by vampire bats in Latin America is paralytic in type rather than furious.) 177 Human rabies resulting from vampire bat bites has been reported almost every year from Mexico, but was first reported from South America in 1953 when nine of forty-three diamond miners who slept outdoors died of a mysterious illness. Autopsies of five of the miners disclosed rabies. In an outbreak in two rural communities in the Amazon Jungle of Peru during the first four months of 1990, twenty-nine of 636 residents (4.6 percent) died with a rapidly progressive illness characterized by hydrophobia, fever, and headache. Rabies virus was isolated from the brain of the only individual upon whom necropsy was possible. Ninety-six percent of the victims had a history of bat bites, although bats also had bitten twenty-two percent of unaffected community members. 5
Human infection is not the only major problem resulting from rabies transmitted by vampire bats in Central and South America. Currently, migrating epizootics of vampire-bat-transmitted bovine paralytic rabies annually kill thousands of animals; the estimated cost in 1980 was $500 million. 5, 62 Efforts to control these epizootics have included vaccination of cattle, and attempts to limit the vampire bat population by administering anticoagulants, usually warfarin.
Interestingly, meat is often consumed from cattle slaughtered at the first, virtually pathognomic signs of disease, and paralysis of the hindquarters. Even normal appearing animals may have infected brains. Four of 1,000 (0.4 percent) apparently healthy cattle selected at random at the Mexico City slaughterhouse of Ferrería were found to be infected by rabies when investigated with fluorescent antibody staining and animal inoculation of brain tissue. However, no cases of human rabies from this source have been reported. 12
Mongooses are the major source of rabies in South Africa and in some Caribbean Islands. 13, 79 The yellow mongoose is the main reservoir of rabies in South Africa. 180 The small Indian mongoose, imported many years ago in an effort to control rodents, 95 is an important reservoir and vector of rabies in Cuba, the Dominican Republic, Grenada, Haiti, and Puerto Rico. 180 In Grenada, from 1968 to 1984, mongooses accounted for 787 (73 percent) of 1,078 cases of animal rabies on the island. Of 208 human exposures requiring antirabies therapy, mongooses were responsible for 119 (57 percent). The possibility of eliminating the animals by hunting or trapping appears remote in view of the island's topography and the animal's skill at adapting to its surroundings. However, recently reported studies have found that mongooses will take oral baits, so oral vaccination appears feasible if an appropriate vaccine can be identified. 111 Interestingly, 20 to 40 percent of the mongooses have naturally acquired antirabies antibodies, possibly from having survived infection. 73
Investigators in Nepal found fifty-one travelers who required immunoprophylaxis following rabies exposure during a two-year period. Thirty-six of these encounters were with dogs, but ten were with monkeys at Swayambunath, the “Monkey Temple,” a Hindu shrine popular with tourists. The bites were inflicted when monkeys leapt for food carried by visitors. 148
Human-to-human transmission of rabies is rare. Eight documented cases were in individuals who received corneal transplants (two from the same person) from individuals whose neuroparalytic disorder was not recognized as rabies . 53, 83, 98 In 1996 Fekadu and his colleagues reported two apparent instances of human-to-human rabies transmission in Ethiopia. A 41-year-old woman, who died of rabies 33 days after her five-year-old son died of the same infection, had been bitten on a finger by her son. A five-year-old boy, who developed rabies 33 days after his mother died of that infection, had been repeatedly kissed on his mouth by his mother, apparently passing infected saliva to him. 81
In July 2004 CDC reported four cases of human rabies transmitted by organ transplants from a single donor. The male donor was hospitalised in Texas with “severe mental status changes,” a low-grade fever, and neurologic imaging findings indicative of a subarachnoid hemorrhage. That lesion expanded rapidly in the 48 hours after admission and lead to cerebral herniation and death. An autopsy was not performed. Only after the organ recipients' deaths from rabies was the donor's history of having been bitten by a bat discovered.
The lungs were transplanted to a male who died of intraoperative complications. The liver and one kidney were transplanted to males and one kidney was transplanted to a female, all of whom died of rabies 27, 37, and 39 days later. The fourth victim had a liver transplant from another donor, but a segment of iliac artery from the first (rabid) donor was inserted during the procedure. This recipient died of rabies approximately a month after the transplant. 55, 60
One case of human rabies appears attributable to transplacental infection. However, a number of mothers dying of rabies encephalitis have given birth to healthy babies, presumably because the virus travels through nerves — viremia has never been documented — and cannot reach the placenta or fetus. 83, 98
FEATURES OF HUMAN RABIES
Mortality
Rabies in humans, once it has become clinically apparent, is uniformly fatal. No other infection is so lethal or progresses so rapidly. In the 1970's intensive support allowed three humans with clinical rabies to survive. 56, 93, 128 Three rabies survivors have been reported subsequently. The first five infections were vaccination failures, and at least three of the five survivors had severe residual neurologic deficits, severe enough to be fatal 34 months later in one person. 1, 112 The rabies virus was not cultured from any of these patients and at least one may have had a reaction to neural derived vaccine rather than an actual infection. 101
In October 2004 a 15-year-old girl in Wisconsin became the first human to survive clinical rabies without vaccination. This young lady, who developed symptoms one month after she failed to report a recognized bite by a bat, was admitted to the Medical College of Wisconsin, Milwaukee five days later. On the second hospital day CDC confirmed the presence of rabies virus-specific antibody in serum and cerebrospinal fluid. Because she had evidence of an adequate immune response, because brain pathology in humans succumbing to rabies largely reflects secondary complications rather than a clear primary process, and because clinical reports have included the hypothesis that death results from “neurotransmitter imbalance” and autonomic failure, her physicians — with her parents' approval — elected to treat her with antiexcitatory and antiviral drugs. Rabies vaccine and rabies immune globulin were not administered.
Coma was induced with ketamine and midazolam, she was intubated and maintained on a ventilator, and received intravenous ribavirin and amantadine. (Ketamine is a dissociative anesthetic, but is also a N -methyl-D-aspartate (NMDA) antagonist, and the NMDA receptor has been speculated to be one of the rabies virus receptors.) Later she also was given benzodiazapenes and supplemental barbiturates. She recovered slowly, was removed from isolation on the thirty-first day, and was discharged from the hospital on the seventy-sixth day. Attempts to isolate the rabies virus, detect viral antigens, or identify rabies RNA in two skin biopsies and nine salivary specimens were uniformly unsuccessful. Five months after her initial hospitalization she was alert and communicative, but had choreoathetosis, dysarthria, and an unsteady gait. 57,187
No other person who has not been vaccinated has survived clinically evident rabies. (Subclinical human infections probably occur.) The clinical phase of rabies encephalitis rarely lasts more than a few days to a few weeks, and infected persons are severely incapacitated. 83, 2 The catastrophe of rabies is compounded by the young age of many of its victims: 40 to 50 percent are fifteen-years-old or younger. 83
INCUBATION PERIOD
For many years some human rabies infections have been thought to follow prolonged incubation periods. In 1987 a 13-year-old boy who had immigrated from the Philippine Islands six years earlier died with rabies determined by RT-PCR to be of Philippine dog origin. He had not been out of the United States since he arrived. 45 The second documented Australian rabies patient, a ten year old girl of Vietnamese origin, had experienced no identifiable animal contact since she had left North Vietnam six years and four months earlier, and the virus responsible for her death was of an immunotype found in Southeast Asia. 22, 89
Joshi and Regmi have reported an individual who had an apparent incubation period of 1100 days (three years). 105 The first documented patient with rabies reported in Australia, a ten year old boy who died in 1987, probably resulted from a monkey bite inflicted in northern India sixteen months earlier. 54 An eighteen year old Mexican man who died in Oregon in 1989 was infected with rabies virus of a strain to which he could not have been exposed for at least ten months, although no history of any type of exposure could be obtained. 48 Even longer incubation periods of ten and nineteen and one-half years have been reported, but these occurred in areas where rabies is endemic and a second exposure in the intervening period could not be ruled out. 153
Confirmation of such prolonged incubation periods was achieved in three immigrants into the United States from the Philippines, Laos, and Mexico. Nucleotide analysis disclosed rabies viral amino acid compositions essentially identical to the patterns from rabies viruses isolated from dogs in their native countries, and unlike rabies viruses found in the United States. These individuals had been in the United States for six years, four years, and eleven months before the onset of clinical disease. 153, 155
Such prolonged incubation periods may explain the inability to recall any animal exposure by some patients. However, the possibility that in the past rabies infections resulted from an unrecognized source, such as an undetected bite by a bat, cannot be dismissed.
Almost 99 percent of human rabies infections are clinically manifest in less than one year, typically two to twelve weeks. 14, 22, 83 The median incubation period in the United States for persons diagnosed between 1960 and 1990 was twenty-four days for those fifteen-years-old and younger, but forty-three and one-half days in people older than fifteen. Fixed (laboratory) strains of virus tend to produce a shorter incubation period than wild or “street” strains. In 1960 in Brazil, sixty people were injected with vaccine that had been inadequately inactivated. Sixteen developed rabies and the incubation periods ranged from four to sixteen days. 83
The size of the viral inoculum clearly influences the incubation period. Experimental animals injected with large numbers of viruses develop clinical infections significantly faster than those receiving small inocula. (Interestingly, small inocula resulted in greater central nervous system histologic damage and more widespread infection outside the central nervous system, particularly in salivary glands. 78 ) PATHOGENESIS OF CENTRAL NERVOUS SYSTEM INFECTION
Immediately after a bite (or investigational injection) rabies virus can be identified at the site with fluorescent antibodies, and remains near the wound or injection site for hours to weeks, depending on the animal species. Viral antigen can be demonstrated in muscle, and viral particles budding into the sarcoplasmic reticulum and from the sarcolemma have been demonstrated by electron microscopy. 61 The virus appears to enter both motor and sensory nerves, probably through motor endplates and neuromuscular spindles. 11, 143
Passage of the virus through peripheral nerves was demonstrated in 1887 when rats 71 and rabbits 72 were protected from rabies following injections of the virus in their hind legs by sectioning the sciatic nerve. After entry into peripheral nerves, the virus travels at a rate of about 5 to 10 mm per hour to neuronal cell bodies such as dorsal root ganglia. 14, 61 Replication can begin at this site, and prolonged esconcement at this site has been suggested as one explanation for prolonged incubation periods. 143 Upon reaching the CNS, the virus is widely disseminated with extreme rapidity, almost simultaneously with entry, but the manner in which the virus disseminates throughout the CNS is not known. Viremia has not been documented. Plasma membrane budding from infected to uninfected neurons, or dissemination through intercellular spaces or cerebrospinal fluid have been suggested. Clusters of viral particles at neuromuscular junctions, the reduction of viral infectivity by nicotinic acetylcholine receptor competitors, and other data suggest that the virus recognizes cholinergic binding sites and perhaps enters peripheral and central nerve fibers through those sites. The large numbers of muscle cholinergic binding sites in foxes, which are exquisitely sensitive to rabies, and the small number of such sites in opossums, which are highly resistant to rabies, support this hypothesis, and possibly explains the mechanism of sensitivity or resistance. 14 The glycoprotein that coats the viral particle is a major determinant of neuroinvasiveness, and alteration of this protein can markedly alter the kinetics of CNS viral spread. 11
Viruses can be isolated from cerebrospinal fluid—a significant antibody concentration in this fluid is considered diagnostic of CNS rabies infection—and spread by this route could be quite fast. 147 Additionally, rabies virions have been identified in intercellular spaces in the CNS by electron microscopy. Rabies antigen can be found in essentially all parts of the CNS, and although limited mostly to neurons, can also be found in oligodendrocytes. 99
The rabies virus can infect a wide variety of cells in culture; no explanation for its localization to neurons in vivo has been found. 106
After wide CNS involvement, the virus passes centrifugally through neural axoplasm to a wide variety of tissues, including salivary glands, corneas, and skin of the head and neck, sites at which identification of the virus may aid the diagnosis of clinical illnesses. The route of spread to the periphery was demonstrated over ninety years ago when Bartarelli sectioned nerves to salivary glands and found that the glands subsequently did not contain rabies virus. 21 Even within the salivary gland the virus appears to spread by neural networks and not between adjacent epithelial cells. 61
An element of immunopathology is produced by disseminated rabies infection. Among persons exposed to rabies, those immunized with early vaccines who subsequently developed infections did so more rapidly than unvaccinated individuals, a phenomenon termed “early death.” Experimental confirmation of this feature has been achieved by injecting mice with a lethal quantity of rabies virus and immunosuppressing a portion of them. The immunosuppressed animals survived 20 to 25 percent longer than unsuppressed animals, but their survivals were shortened to those of the control animals when they were injected with antirabies antibody. Additionally, cytolytic T-cells appear to be a significant component of the protective response to rabies virus. Avirulent strains of rabies virus induce rabies specific cytolytic T-cells, but virulent strains do not. 116
Pathologic alterations in the CNS infected by rabies are surprisingly mild, unless supportive care has kept the patient alive for several weeks, which allows much more extensive, necrotic lesions to develop. 133, 143 Leptomeningeal congestion is the only grossly visible change typically found. Mild edema may be present if the patient has been hypoxic. Pressure grooves are rare. The meninges may be cloudy if severely inflamed. 124
Typical histologic features are perivascular cuffing by mononuclear inflammatory cells, microscopic collections of reactive glial cells known as Babes nodules following his description in 1892, 6 and areas of neuronal degeneration and neuronophagia. Some leptomeningeal inflammation is usually present. Very recently spongioform degeneration similar to that found in prion diseases has been described in animals, particularly in skunks. 61
Van Gehuchten and Nelis described a striking proliferation of the capsular cells surrounding ganglionic neurons that pushed these cells apart and separated them by a dense cellular layer. The neurons also contained degenerative changes. Subsequent studies have found the Van Gehuchten-Nelis changes to be present in almost everyone dying with rabies, and to be a much more consistent and reliable diagnostic feature than viral inclusions. 166, 167 Negri bodies are the best known histologic feature of rabies and were the first viral inclusions to be found. Negri described these cytoplasmic inclusions in 1903 and considered them parasites that caused the disease. 117 (Entirely independently, Bosc described identical inclusions in two separate papers published the same year, but he is rarely recognized—the fickleness of fame. 30, 31 These bodies are eosinophilic cytoplasmic inclusions that contain a small, basophilic inner body or innerkörperchen . Inclusions are found almost entirely within neurons, are most common in Ammon's horn and Purkinje cells of the cerebellum, but may be seen in any part of the central nervous system, particularly in humans. The bodies may be seen in other tissues, such as salivary gland, skin, cornea, and pancreas, as well, but are not seen in the ependyma and choroid plexus.) The appearance of the inclusion bodies varies in different animal species, and uninfected animals such as cats commonly contain cytoplasmic inclusions that easily could be confused with rabies bodies. 159
By electron microscopy three types of bodies have been identified, one composed of a granular matrix of viral protein and typical virus particles, a second composed of matrix and tubular structures, and a third composed of matrix alone. Invaginations of cytoplasm into the inclusion give rise to the innerkörperchen , indicating that Negri bodies and lyssa bodies are both diagnostic of rabies. 124 CLINICAL FEATURES
“Although the clinical features of classical rabies are said to be too well known to require description, few clinicians practicing outside the tropical endemic zone have ever seen the disease, and the few cases presenting in Europe and North America are often misdiagnosed.” 177
Pain, paresthesias, or symptoms such as burning, itching, or numbness at the site of the bite or in that limb is the most common early symptom. Paresthesias may result from proliferation of rabies virus in the spinal cord at the level at which the nerves from the bite site enter. 83 In Thailand, this initial symptom often takes the form of severe itching that can lead to frenzied scratching and extensive excoriations. 177 This symptom is so well known among the Thai people, and animal bites are so common in that country, that any cause of itching or dysesthesia, even contact dermatitis, can lead to anxiety months to years after an animal exposure. 183
Systemic symptoms usually develop later—local symptoms may not appear at all—and are largely nonspecific. One-third or less of all patients initially have symptoms that suggest the etiology of their infection to physicians who do not commonly encounter the disease. Complaints may be constitutional: malaise, chills, fever, or fatigue. Symptoms that suggest an upper respiratory infection are common and include sore throat, cough, and dyspnea. Gastrointestinal symptoms include anorexia, dysphagia, nausea and vomiting, abdominal pain, and diarrhea. Headache, vertigo, irritability, or anxiety and apprehension suggest CNS involvement. However, even advanced rabies often has nonspecific features. 49, 121 Hypoventilation and hypoxias are common during the prodrome and early acute neurologic phase, and the cause is not understood. Cardiac involvement is common, and is manifested by tachycardia out of proportion to the fever; hypotension, congestive failure, or cardiac arrest may ensue. 83
Two clinical forms of human rabies are recognized. The furious, encephalitic, or agitated form that is associated with periodic episodes of hyperactivity, restlessness, or agitation is considered most typical. This form of rabies is characterized by periodic opisthotonic spasms or convulsions, particularly in response to tactile, auditory, visual, or olfactory stimuli (aerophobia and hydrophobia.) Episodes of disorientation, sometimes with hallucinations or violent behavior, often alternate with periods of lucidity, which can be particularly horrifying because the patient recognizes the nature of his illness. The terror associated with hydrophobia has been labeled “powerful but indescribable.” Episodes of priapism, increased libido, insomnia, nightmares, and depression may suggest a psychiatric disorder. Patients maintained with supportive therapy progressively deteriorate, become comatose, lose peripheral nerve function, lose brain stem function, and die. 83, 177
Hydrophobia has been described in only 32 percent of recent United States patients, 83 although one recent United States rabies victim, an eleven-year-old boy, was so afraid of water he would not even take a bath. 46 Experienced observers in Thailand have described hydrophobia as a violent, jerky contraction of the diaphragm and accessory muscles of inspiration that is triggered by attempts to swallow liquids and by other stimuli. Usually it is not associated with neck or throat pain or with laryngopharyngeal spasms. It has been likened to respiratory myoclonus (Leeuwenhoek's disease). When patients lapse into coma, hydrophobia is typically converted into cluster breathing with long apneic periods. 177
The variability of the clinical manifestations of rabies may result from heterogeneity in wild or “street” rabies viral populations (as contrasted with “fixed” viral strains maintained in laboratories), even in the viruses infecting a single animal. Rabies viruses isolated from a boy who died after being bitten by a fox and propagated by intracerebral inoculation of white mice produced three distinctly different forms of disease in the mice. 90
The second clinical form of rabies, the paralytic, dumb, or Guillain-Barré-like form is characterized by progressive paralysis without an initial furious phase. Even though the paralytic form of the disease does not appear to be as familiar to some health care providers, 20 to 30 percent of human rabies victims present in this manner. 2, 183 (Other animals also have furious or dumb rabies presentations. 119 ) Paralytic rabies is more common after rabid vampire bat bites, in persons injected with fixed virus strains, and in persons who have received postexposure vaccination. 83 Distinction from Guillain-Barré syndrome may be difficult, although individuals with that disorder usually do not have urinary incontinence, which is common in rabies infected persons. 100
Individual case reports make clear that every patient is different. The most common misdiagnoses are psychiatric or laryngopharyngeal disorders. 100 Physicians at QSMI, who have a vast clinical experience, consider inspiratory spasms to be the most reliable clinical sign of rabies, particularly in comatose patients, regardless of whether the disease was initially furious or paralytic. Such respirations also have been described as rapid, irregular, or jerky—apneustic.
In addition, most of the QSMI patients have myoedema, particularly in the region of the chest, deltoid muscles, and thighs .180, 183 However, this phenomenon, a brief, unpropegated, localized muscle contraction that appears in response to percussion with a tendon hammer, has not been confirmed as an important sign of paralytic rabies. 100
Because the signs and symptoms are so nonspecific, and often are rather mild at onset, many patients with rabies are not hospitalized the first time they are seen by a physician. (One patient who died in the United States between 1960 and 1980 was never hospitalized.) When admitted, most have a fever, which may be mild, but commonly is above 103 ° F (39.4 ° C). Of the thirty-eight individuals who died of rabies in the United States between 1960 and 1980, twenty-four (63 percent) had difficulty swallowing, but only half of those had definite hydrophobia or aerophobia. Twenty-seven (72 percent) manifested excitement or agitation, twenty-four (63 percent) had paralysis or weakness, and twelve (32 percent) had hypersalivation. Dysesthesias at the exposure site were described by nineteen (79 percent) of the twenty-four individuals who had an identifiable bite or similar exposure. 2
Twenty-six of the twenty-eight patients diagnosed before death, but only four of eight patients diagnosed after death, had a history of an animal exposure. All twelve patients with hydrophobia were diagnosed before death. 2
The duration of illness for patients not given supportive care averages 7.3 days, and ranges from 2 to 23 days. For patients that are given supportive care, the average duration of illness is 25.3 days, and the range is 7 to 133 days. 2, 159 “The following [therapeutic] measures have...been tried in clinical rabies, but without any evidence of effectiveness: administration of vidarabine; multisite intradermal vaccination with cell-culture vaccine; administration of a -interferon and rabies immunoglobulin by intravenous as well as intrathecal routes; and administration of anti-thymocyte globulin, high doses of steroids, inosine pranobex, ribavirin and the antibody-binding fragment of immunoglobulin G.” 180 In a recent review of the management of rabies in humans a combination of some specific therapies was suggested. The authors point out that essentially no individuals with clinical rabies survive, and the best therapy often is palliative. 101 SUBCLINICAL RABIES
Although clinical rabies in humans is a uniformly lethal infection with only six recognized exceptions, subclinical infections probably occur. Low titers of rabies virus neutralizing antibodies have been found in Canadian Inuit hunters and their wives, and in unimmunized students and faculty members of a veterinary medical school. 100 In Nigeria, 28.6 percent of 158 healthy humans, who had no history of exposure to rabies or of antirabies prophylaxis, were found to have serum-neutralizing antibodies against rabies. (Antibodies against Mokola virus [Genotype 3] and Lagos bat virus [Genotype 2] also were found in 7.5 percent and 2.5 percent of these individuals.) The investigators suggested that these individuals had been infected, but the infection had been halted before the virus had entered nerves. An attenuated strain of rabies virus also has been suggested as the cause of such antibodies, but an as yet unidentified virus of the Lyssa group, or even some other cross-reacting infectious agent, could be responsible. 120 Among forty-eight family members of Peruvians who died following rabid vampire bat bites in 1990, seven had antirabies antibody levels that ranged from 0.14 to 0.66 IU, and could not be related to exposure to bats or other animals, or to other epidemiologic events. 5
Some animals, including dogs, have significant amounts of nonspecific virus-neutralizing antibodies in their sera. Up to 20 percent of raccoons in Virginia and Florida 146, 189 and 20 to 40 percent of mongooses on Grenada have antirabies antibodies, which may be evidence of nonfatal infections. 73 Up to 80 percent of bats in crowded nurseries may have rabies antibodies. 16 (The only finding considered diagnostic of prior rabies infection is antibodies in cerebrospinal fluid. 76, 77 Even clinical rabies is not always fatal in animals. Pasteur observed that some dogs recovered from early symptoms of rabies and subsequently could not be infected by rabies virus injections. 123 Recoveries from infections that produced paralysis have been described in two dogs; 80 recovery from clinical rabies has also been described in mice, donkeys, bats, 12 and pigs. 15 UNDIAGNOSED RABIES
Clearly many rabies infections are not diagnosed in developing countries. The possibility that in industrialized countries a number of human rabies infections are not diagnosed and the true incidence of this infection is higher than reported is suggested by several considerations. The clinical manifestations usually are nonspecific, and many infections are diagnosed late in the course of their illness as the result of testing for any identifiable cause of encephalitis, or subsequently by autopsy examination of the central nervous system. Eight of the thirty-eight (twenty-one percent) human rabies infections in the United States in the 1960's and 1970's were diagnosed after death. Subsequently the percentage of infections diagnosed postmortem has been higher. 48, 179
In recent years in the United States most of the individuals with rabies have no recognized exposure to potentially infective animals, yet an account of animal exposure often is the only stimulus for laboratory identification of rabies as the cause of the encephalitis. 48 All but four of the individuals with rabies of bat origin had no history of bite, and approximately half had no history of any bat exposure.
The significance of undiagnosed human rabies is uncertain. When the diagnosis is made, members of the patient's family and all health care personnel who had a significant exposure to the patient are given postexposure immunoprophylaxis. However, human-to-human transmission of rabies has been reported only following tissue or organ transplantation, except for two individuals in Ethiopia. 81 LABORATORY DIAGNOSIS OF RABIES
Conventional laboratory procedures are not helpful in establishing a diagnosis of rabies. Cerebrospinal fluid protein and leukocyte counts may be modestly elevated, but these changes are nonspecific.
Currently available techniques for a definitive laboratory diagnosis of rabies usually are nondiagnostic in the early days of infection, and become useful only a week or more after the onset of illness. The diagnosis of rabies should be confirmed as quickly as possible so the number of persons exposed to the infection can be limited, and therapy for those exposed can be initiated promptly. In industrialized nations, the number of persons exposed to a hospitalized rabies patient can number in the hundreds. 131, 163
If rabies is suspected, a complete set of samples should be collected for testing by all currently available diagnostic procedures. Consultation is available from CDC twenty-four hours a day, seven days a week, and should be obtained. ( 877-554-4625; http://www.cdc.gov/ncidod/dvrd/rabies/ ) However, samples taken antemortem can not definitively rule out rabies. If infection is seriously suspected, repeated sampling is needed. 163 Samples can be transported overnight to CDC or to state laboratories.
The rabies virus is systemically disseminated shortly after central nervous system infection, and often can be detected with labeled antibodies or by RNA extraction. Currently detection of rabies RNA in saliva by RT-PCR appears to be the most reliable procedure. Saliva should be collected with a sterile eyedropper or pipette and placed in a small sterile container that can be sealed securely. No preservative or other material should be added.
A 5 or 6 mm punch biopsy of hair bearing skin with at least ten hair follicles from the nape of the neck is commonly used; it should be placed on sterile gauze, moistened with sterile water or saline, and placed in a sealed container with no other fixative or preservative.
At least 0.5 ml of serum (not whole blood) is needed to test for antibodies by immunofluorescence and virus neutralization. If no vaccine or rabies immune serum has been administered, the presence of antibodies is diagnostic and testing for CSF antibodies is not needed. At least a similar volume of CSF is needed for antibody studies.
Rabies virus can sometimes be found in imprints from the cornea. An opthalmologist should prepare the smears after consultation with the rabies testing laboratory to avoid damaging the cornea. RT-PCR and immunostaining are used to identify the viral antigens.
Brain biopsies should not be routinely performed because human rabies is so rare in industrialized countries, and no effective treatment is available. If a biopsy is performed to rule out another condition, it can also be examined for rabies. 163
Rabies virus often can be isolated from body fluids, particularly saliva and cerebrospinal fluid. The murine neuroblastoma cells used for isolation of rabies are more susceptible to that virus than any other cell line tested, and culture on such cells can usually provide a diagnosis within twenty-four hours. Mouse inoculation may take fifteen to thirty days, although the time can be shortened by sacrificing mice starting five days after inoculation and examining the brains with fluorescent antibodies. 159, 180 In industrialized nations, facilities for such studies are limited almost entirely to state and national laboratories. In developing countries they often are unavailable.
RABIES IN ATTACKING ANIMALS
Prior to 1903, the diagnosis of rabies in attacking animals was based entirely on the clinical features of the disease, or on the presence of unusual material in the animal's stomach that evidenced bizarre behavior. (The dog that attacked Joseph Meister, the first recipient of Pasteur's antirabies vaccine, was diagnosed as rabid because it had hay, straw, and wood in its stomach. 7 ) In that year Negri and Bosch described the typical neuronal cytoplasmic inclusions, and for many years the laboratory diagnosis of rabies in animals depended upon the detection of such bodies. However, only 60 to 80 percent of infected animals have identifiable inclusions. 162 Typical inclusion bodies are scarce in Arctic foxes, for instance. 67
Immunofluorescent detection of rabies antigen in smears of cerebral tissues, which was introduced in 1958 and became widely used in the early 1960's, 88 is far more reliable in experienced hands. Comprehensive analyses of the performance on survey examinations established that in major United States public health laboratories the sensitivity of fluorescent antibody examination is nearly 100 percent. Fluorescent antibodies are used to identify rabies antigen in tissue culture (see below) because the rabies virus produces few cytopathogenic changes.
The major shortcoming of the procedure is its reduced sensitivity for rabies antigen in decomposed brain tissue. Some investigators have concluded that failure to identify rabies antigen with fluorescent antibodies in partially decomposed brain tissue can not justify withholding postexposure therapy for exposed individuals. 162
A procedure to confirm negative diagnoses is essential. Rabies diagnosis by intracerebral mouse inoculation was introduced in 1935, and demonstrated that only 85 to 95 percent of rabies infections could be identified by examination for inclusion bodies alone. 157 Eventually most laboratories, even those in developing countries, adopted adult or suckling mouse inoculation. Tissue-culture inoculation began with inoculation of chick embryo cells in 1942. 159 Now tissue-culture isolation on mouse neuroblastoma cells, which is far faster—only twenty-four hours are needed to confirm a positive diagnosis in contrast to fifteen to thirty days for mouse inoculation—and is also the most sensitive technique for confirming negative immunofluorescence examination results, at least in laboratories with suitable facilities and appropriate personnel. 32, 141, 159 The rabies virus does not produce cytopathic changes in tissue culture and fluorescent labeled antibodies have to be used to diagnose the infection.
POSTEXPOSURE RABIES THERAPY
If the effectiveness of the therapy for the fifteen-year-old girl in Wisconsin is confirmed, 57,187 it will revolutionize the treatment of rabies in humans who have developed a clinical infection. However, postexposure treatment for humans will still consist of reducing the viral inoculum by cleansing the wound as thoroughly as possible, administering antiserum to help control viral reduplication and spread (passive immunization), and administering rabies vaccine to establish immunity to the virus before signs of infection appear (active immunization). “Rabies vaccination is a race between the active immunity induced by vaccination and the natural course of infection.” 170 Identifying Exposure
Exposure to rabies is divided into bite and nonbite categories. A bite is considered a significant exposure if it penetrates the skin. Scratches that break the skin are also considered significant exposures because the claws could be contaminated by saliva. Unprovoked attacks are more likely to have been inflicted by a rabid animal than provoked attacks, but determining whether a dog or cat bite has been provoked is frequently difficult. Bites that occur while feeding or handling apparently healthy animals are generally regarded as unprovoked, but some animals, particularly dogs, may bite anyone walking by or riding a bicycle.
Nonbite exposure consists of contamination of cutaneous wounds—including scratches, abrasions, and weeping skin rashes—with saliva, cerebrospinal fluid, or brain tissue from a rabid animal. If the material is dry, it is considered noninfectious. 33 Contamination with urine from a rabid animal or person is not considered an exposure even though rabies viruses have been isolated from kidneys and urine. A laboratory technician cut by broken specimen container was given postexposure therapy. 62
Importantly, the rabies virus does pass through intact mucous membranes, and any mucous membrane contact—particularly membranes of the oral cavity or conjunctiva—with saliva or other infectious material from a possibly rabid animal is considered an exposure.
Exposure of medical personnel or family members caring for patients with rabies is a significant problem. High-risk contact is defined as a percutaneous (through needle sticks or open wounds) or mucous membrane contact with saliva, cerebrospinal fluid, or brain tissue—which are essentially the same as bite and nonbite exposures to rabid animals. Individuals who have had a high-risk contact should receive postexposure immunoprophylaxis. However, routine infection isolation procedures, including respiratory precautions, minimize the risk for medical personnel caring for patients with rabies. 33
Individuals who have not had a high-risk contact do not need postexposure immunoprophylaxis, although such treatment is sometimes administered to allay anxiety.
In areas where canine rabies is not enzootic, which includes all of the United States except for the area along the border with Mexico (particularly southern Texas) a healthy domestic dog or cat that bites a person should be confined and observed for ten days, particularly if the animal has been previously vaccinated. A veterinarian should evaluate any illness during confinement. If rabies is suspected, the animal should be humanely killed and its head should be shipped to a qualified laboratory. 52 The head must be refrigerated during shipped. Examinations for rabies can not be reliably performed on decomposed brain tissues. 163
Such confinement and observation was judged safe for exposures to ferrets in 1998. Scientific evidence that the same quarantine period would be adequate for wolf hybrids does not exist because studies of pathogenesis and virus shedding have not been performed. Hybrids that bite humans should be euthanized immediately and their heads should be shipped to reliable laboratories. 119
The significance of the laboratory's qualifications was emphasized by the death from rabies of a United States citizen in 1981 after he was bitten by a dog in Mexico. The dog's head was shipped to a Mexican laboratory, where it was examined with Seller's stain instead of a fluorescent antibody technique. Because no evidence of rabies was found with this less sensitive technique, he was not given postexposure therapy. 122 (In a Thai investigation, thirteen of 404 rabid animals diagnosed with fluorescent antibodies did not have Negri bodies identifiable with Seller's stain. 149
Any stray or unwanted animal should be killed immediately and its head submitted for rabies examination. Euthanasia does not have to be delayed for further development of the infection in an attacking animal for a reliable diagnosis to be made. 163 No one in the United States has died of rabies when the attacking dog or cat has been healthy after ten days of observation. 153 However, dogs injected with an Ethiopian strain of rabies virus excreted virus in the saliva up to thirteen days before signs of disease were observed, 79 and dogs that have recovered from experimental rabies excrete virus in saliva for as long as six months after recovery. 82
If an attacking dog or cat is rabid or is suspected to be rabid, postexposure therapy should be initiated at once. However, a reliable diagnosis of the presence or absence of rabies in animals can usually be completed in less than one day. 163 If the dog or cat escapes and is not suspected to be rabid, the Immunization Practices Advisory Committee (ACIP) of the U.S. Department of Health and Human Services recommends that local public health officials be consulted. 52 In a study of postexposure therapy practices in the Emergency Departments of a number of university hospitals from all parts of the United States, the most frequent inappropriate administration of therapy was for animals that could be observed for ten days. The most common failure to administer postexposure therapy was for animals that could not be observed. The investigators emphasized that physicians often failed to use the well-informed c |
