MEDICAL ENTOMOLOGY FOR BACKPACKERS AND TRAVELLERS

Terry D. Galloway

Department of Entomology, Faculty of Agricultural and Food Sciences,

University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

All the World Over

  Canadians are world travellers, and they often visit exotic locations, sometimes seeking true wilderness adventures, or visits with indigenous people. Canadians also make extended excursions into remote areas in North America and the tropics, on holidays, working on special assignment or working with a variety of aid agencies. We too often forget in our travels, that we become exposed to local wildlife that isn't as obvious or as photogenic as zebras, hippopotami, lions, elk or deer. These include the flies, the fleas, the ticks and bugs that are potential vectors of disease-causing organisms. To be infected with these agents may mean an unscheduled visit to a local hospital, mild to severe illness, and sometimes death.

  If you plan to travel, particularly to tropical or subtropical parts of the globe, it is wise to learn as much as you can about insects and ticks that may transmit pathogens to you and your family, and about the pathogens and diseases involved. In this short summary, there is no way that I can cover all the details of every potential vector and all possible pathogens. I am not a physician, so it is not my intention to provide you with information about prophylactic drugs that may be essential for your survival in some parts of the world. I am not going to discuss the ants, bees, wasps, spiders, and scorpions which may cause life-threatening allergic reactions or envenomization by their bites or stings. What I do hope to do, is to introduce you to the groups of insects and their relatives that you may encounter, to provide you with some selected elements of their biology and vector potential, and to provide you with a list of just some of the diseases it is possible to pick up through an encounter with these insects and ticks.

Be Prepared

  There are a couple of basic principles to keep in mind when you travel to the tropics. First of all, you may only rarely encounter the kind of biting fly pressure we see nearly every summer in Manitoba. You know what I mean. There are those memorable times each summer when we have gorgeous, warm evenings when it would be just great to get out and barbecue in the back yard, or to go for a leisurely stroll around the neighbourhood......but you can't, because the second you step out the door, you are smothered by an army of hungry mosquitoes. Or when you take that extended holiday to canoe that Whiteshell route you always wanted to do, and the mosquitoes drive you into your tent at dusk, and the black flies and stable flies drive you to distraction all day. You may not ever encounter this kind of biting fly pressure in the tropics. The risk is that when you travel to these parts of the world, there may be so few mosquitoes around, that you hardly notice. Who would bother with repellent where you get only a few bites per hour. Or in some cases, there may be just a few mosquitoes biting, mainly at night while you sleep. However, where some of these biting insects are involved in the transmission of pathogenic organisms, it may take only one bite for you to acquire an infection. Whether or not you become infected may depend on the proportion of the flies biting that are themselves infected, and at what level the pathogen occurs in their bites. You can think of it as a lottery. There may only be one in a hundred mosquitoes that is infected, but what are the chances that that one mosquito successfully feeds on you?

  That brings me to my second point. If there are infected insects biting, the longer you are in the area where they occur, the greater the probability that eventually you will become infected if you don't take adequate precautions. In some cases, with yellow fever virus or malaria for example, one bite from a suitably infected mosquito is sufficient for you to become infected. In the case of other pathogenic organisms, in elephantiasis or river blindness for example, clinical disease usually only develops where people are exposed over a considerable period of time, perhaps over decades, where they are bitten repeatedly by infected flies.

  The bottom line is that you must know what the potential is for transmission locally where you visit. If you have paid for a luxury tour where you are staying in the best hotels and making only short excursions out into the countryside, you are usually less at risk than if you take off on a Morocco to South Africa safari, living out of a rickety old caravan, travelling the backwaters of the continent. Serious research into the insects you will encounter on each stage of your trip, the pathogens they transmit, and the precautions you need to take will pay dividends, and allow you to enjoy the experience you are looking for to the fullest.

Insects are People Too

  I do want you to keep in mind that there are probably more than 30 million species of insects. If you consider that each one of these different insects does things just a little differently from all other species, that means there is a lot of room for insects to make use, in one or another, of every conceivable resource. Unfortunately, the insects I am going to talk about here mostly require vertebrate blood to reproduce. In some groups, males need blood to develop sperm ( e.g stable flies), while females may need blood to develop their eggs ( e.g. mosquitoes and black flies). Some insects ( e.g. lice and fleas) are obligate parasites of birds and mammals. They are found only on the bodies of their hosts, in the case of the lice, or they may be intimately tied to the host and its nest environment, as for most fleas. In other groups of blood-sucking organisms, ( e.g ., true bugs, flies and ticks) there may be species which are obligate parasites and which are very host specific, or there may be species which will attack almost any warm-bodied animal for blood. These insects are doing only what they have to do to survive, so don't take it personally that they want your blood.

Nasty but not Necessarily Dangerous

  There are many insects which have developed the annoying habit of having adapted to living in the same places we live, or even on ourselves. These insects usually draw shrieks of anguish and shudders, but in the long run, as much as we may hate the idea, they really don't do us all that much harm. Here are a few to set you mind at ease.

1) Cockroaches   

  There are more than 4,000 species of cockroaches, most of which are tropical and subtropical. They vary in size from the tiny Nocticola (3mm long) to the giant among cockroaches, Macropanesthia rhinoceros , which is 65mm long, and weighs up to 20 grams. There are numerous cosmopolitan species that have accompanied humans around the world and live together in our homes and workplaces, including in Manitoba. Even on the University of Manitoba campus, if you walk quietly along the tunnel connecting the Agriculture buildings, you may be fortunate enough to see one of these interesting little beasts. There has been all out war declared on our faculty's cockroach population in the last nine or ten years, but there are still a few around. There are native wood roaches in Canada, usually found in rotting logs, under bark or in forest leaf litter. These wood roaches, Parcoblatta sp., were unknown in Manitoba until the summer of 1997, when specimens turned up in light traps along the Winnipeg River and in Nopiming Provincial Park. They are probably quite widely distributed in eastern Manitoba, but it's just that few people take the time to look for them.

  All six legs of cockroaches are similar and they can run surprisingly fast - just try to catch one. The body is oval-shaped and flattened, and the head is hidden from above by the front of the thorax. Cockroaches are generally creatures of the night and they will avoid the light if they can. Hence, their antennae are extremely long and filamentous, and serve as one of the foremost sensory organs. When we had American cockroaches infesting our building, they liked to take shelter under the toilet seats during the lights were turned on in the morning. Imagine the reaction when contemplative visitors would be seated there, and the antennae of a hidden cockroach would brush their exposed buttocks! Most, but not all, species are winged, with four wings lying flattened over the back at rest. The forewings are parallel-sided and somewhat leathery, while the hind wings are larger, membranous and folded beneath the forewings. There are usually two distinct cerci at the tip of the abdomen.

  Cockroaches will eat almost anything organic using their chewing mouthparts. Unfortunately, they are messy eaters, and contaminate food items with saliva and faeces, imparting a characteristic and decidedly unpleasant odour. It is for this reason that cockroaches have such an unsavoury reputation. One of the disadvantages of working in entomology is that most people learn to recognize the smell associated with cockroaches and it can be rather off-putting when trying to enjoy a meal in an infested restaurant! Cockroaches will also consume paper, labels from jars, and books. However, more than 99% of the known species are not found in association with humans, and are confined to very specific habitats. Females of many species enclose their eggs in capsules called oothecae, and tuck these oothecae in cracks and crevices or in corrugations in cardboard. There are a few species that retain the ootheca until the eggs hatch and then appear to give birth to live young. Juveniles look very much like small adults, but do not have wings.

  In warm climates, many species of cockroaches move freely in and out of buildings, flying about at night, attracted to lights. Don't be surprised if you see them in you hotel room or cabana on the beach. They can be everywhere. The list of potentially pathogenic organisms that have been isolated from cockroaches is truly impressive. Most of these organisms cause gastrointestinal disorders, and include such things as Salmonella , Shigella , and Clostridium . The precise role of cockroaches in the dissemination of these pathogens is unclear, but who wants cockroach vomit or faeces in the food anyway? Of greater concern is the evidence that under certain conditions, cockroaches may be responsible for the spread of infectious hepatitis. If you can, avoid crowded housing and places where there are clearly large numbers of cockroaches about. It has now also been shown that prolonged exposure in cockroach-infested living quarters may be responsible for development of severe respiratory allergies. This comes as no surprise to entomologists who have studied cockroaches for many years, over which time they will have spent long hours looking after their cultures of roaches.

2) Bedbugs

  These wafer-thin little devils are a scourge wherever they occur. There are nearly 100 described species, all of which are wingless, and most of which you are unlikely to ever encounter. They are parasites of bats. However, the human pest, the bed bug ( Cimex lectularius ), is the most dreaded. They require blood for development and reproduction, and take this at night from their unsuspecting, sleeping hosts. They especially like to feed around the neck and scalp area, but they will feed elsewhere on the body as well. During the day, they hide in cracks, crevices, bed springs and mattress seams. Sometimes the only sign that they are present will be tiny specks of blood on the pillows, or greasy black faecal deposits in areas where the bugs hide, their harbourages. Infestations usually become established when someone brings a used bed or bedding into a home from an infested area. It is sometimes recommended that the bedposts can be set into cans of water or oil, and the bed bugs can't swim across to get to the bed. However, I was once told that under these circumstances, a fellow discovered that the bed bugs crawled up the walls, across the ceiling, and then fell down onto their sleeping victim below! An innocent intruder, often mistaken for the bed bug, is the swallow bug, Oeciacus vicarius . This bug is intimately associated with the Cliff Swallow in North America, although there are records of occurrence in the nest of other species of swallow. You can distinguish these bugs, which do not normally bite humans, from bed bugs because swallow bugs are hairy.

  Although bedbugs are known to acquire a variety of pathogenic organisms, there is no conclusive evidence that they are important vectors of any of them. If you have to sleep in a room where there are bedbugs, you can take comfort in knowing that you probably won't be infected with anything unpleasant, and that you will have contributed to the continued survival of their population. And it seems that the probability of this happening is increasing in many developed countries around the world. You will find media reports claiming that the prevalence of bedbugs is on the rise. Whether this is the result of greater restrictions on the use of effective residual insecticides for domestic use, the development of insecticide resistance in bedbug populations, or increased awareness and media hype is difficult to tell.

3) Head Lice and Crab Lice

  There are well over 500 species of sucking lice world-wide, all of which feed exclusively on blood. Most species are very host specific, though as a group, they are found on a wide variety of mammals, including seals, though, oddly, none are parasitic on bats. For those of you that have been infested with lice, you know that they may be extremely irritating, especially when they are present in large numbers. Female head lice ( Pediculus humanus capitis ) cement their eggs (also called nits) to hairs on the head. There is another subspecies, known as the body louse ( Pediculus humanus humanus ) which lays its eggs among the clothing as well. The head louse is a curse for school teachers and health nurses, as epidemics commonly will sweep through the children under their supervision. At one time, head lice were easily controlled using insecticidal shampoos. However, since the early 1990's, there have been increasing reports of product failures in many parts of the world, and it appears that there are widespread populations of head lice which are now resistant to many registered control products.

  A person usually becomes infested with head lice as the result of close contact with an infested person. Lice walk very inefficiently of bald surfaces, so prolonged periods of head-to-head contact favour transmission. It is possible to become infested by sharing combs or brushes with infested people, or by wearing an infested person's hat. However, head lice do not live long when they are not on their host and they are very good at holding onto the hair of their host, so your chances of picking up lice randomly in the environment are not great.

  Crab lice ( Pthirus pubis ) are fantastic looking animals with broad shoulders and enormous claws. They are smaller than most people imagine, if you have never seen one. They are normally found clutching human pubic hair, but in the case of heavy infestations, they may also be found in the hair of the armpits, eye brows, beard or moustache, anywhere that coarse hair grows. As with head lice, they lay their eggs on the hair follicles and feed only on blood. In this case too, infestation most often occurs as a result of prolonged, intimate contact, hence their common name in French, "papillons d'amour". In rare instances, crab lice may be transmitted on contaminated towels, bed clothing, and perhaps even on toilet seats.

  Fortunately, neither head lice nor crab lice are known to transmit serious, disease-causing pathogens, and a person can always shave themselves for complete louse control. Body lice, or cooties, on the other hand, are cause for greater concern. Body lice transmit a rickettsial pathogen, called Rickettsia prowazekii , the causative agent of epidemic typhus in humans. Although there has not been an outbreak in North America for more than 100 years, there are known enzootic foci in flying squirrels in the eastern United States. Transmission in humans as a result of a body louse infestation is more likely in parts of Africa or China, where epidemics have occurred within the last 15 years. Epidemic typhus is extraordinarily infectious, and a person becomes infected when the contaminated faeces of infected lice are scratched through the skin, rubbed into the mucous membranes or inhaled.

4) The Human Bot Fly

  There are many species of flies that may deliberately or accidentally invade the flesh of a living person. If this happens, it is a condition known as myiasis. However, I only want to describe for you the one species that I see with surprising frequency, the human bot fly or tórsalo ( Dermatobia hominis ) of Central and South America. This is a truly amazing fly that you might encounter along wooded areas in river valleys and lowland areas from Mexico to Argentina. The adult flies are quite large, about the size of a small bumble bee, and they have no mouthparts. As adults, they never feed. The females mate and then abduct other flies that they use to transport their eggs to a warm-blooded host. They may tackle a mosquito or stable fly, for example, and cement their eggs onto the abdomen of the other fly. When this fly lands on a host to take its own blood-meal, the attached eggs of the bot fly hatch, and the tiny maggots burrow under the skin, often through the opening left from the bite of their taxi fly. This fly is not very host specific, and other animals, including monkeys, cattle, dogs, and sheep, as well as humans may be infested. The bot fly maggot begins to feed and increases enormously in size, twisting and gyrating inside the pocket that forms around them beneath the skin. They must create a hole in the skin through which to breathe, so there is always an opening associated with the swelling and pain created by their presence. Here they remain for the duration of their development, which can last for one to three months. I most often see people who have visited the neotropics and have returned home, completely oblivious to the developing surprise they will find later, one that they failed to report to Agriculture and Agri-Food Canada when asked if they are bringing any live animals into the country. If no action is taken, the maggot will complete its development and eventually squeeze out through the hole in the skin and fall to the ground, where it will eventually pupate and emerge as an adult fly. It would be only the most curious and tolerant traveller that would wait for this to happen, though I have tried to convince a few to do so. Typically, infested people see their physician and have the poor, unsuspecting maggot removed before it completes its development. It is interesting that only very rarely is there any secondary infection associated with a bot fly infestation.

5) Those Pesky, Pesky Flies

  I am going to include a whole range of different, closely related flies in this section. Most of them breed in decaying organic matter, sometimes including animal faeces and manure. For that reason alone, you may not desire close contact with these flies.

  House flies ( Musca domestica - cosmopolitan), face flies ( Musca autumnalis - North America and Europe), bazaar flies ( Musca sorbens - Africa, Asia and many Pacific Islands), and bush flies ( Musca vetustissima - Australia) are all non-biting flies that can drive people to distraction by their persistent habits of crawling all over our homes, our food, and our bodies. They all must feed on a liquid diet, but they can obtain the nutrients from solid food by repeatedly vomiting on it and softening it with their sponging mouthparts, until it is sufficiently dissolved to be able to slurp it up. They frequently regurgitate their gut contents and defaecate where they walk, and consequently, as with cockroaches, may spread various potentially pathogenic organisms that are responsible for gastrointestinal distress. On the other hand, they may crawl about your face, exploring every orifice, enjoying the patches of moisture that they find.

  Stable flies ( Stomoxys calcitrans ) are very similar to house flies in appearance. However, the principle difference here is that the stable flies, both males and females, feed exclusively on blood. They have shiny, black, dagger-like mouthparts with which to penetrate the skin, and in so doing produce a sharp, stabbing pain. They are very fast and responsive to your defenses, so you may not even see them at first, especially because they prefer to attack your ankles. If you have ever shared a canoe trip with a few stable flies, you know that they can bite through socks, denim, almost anything, it seems, and they will follow you to the ends of the earth. Despite the stress they may bring you, they are not known to transmit any pathogenic organisms to humans.

Nasty and Maybe Even Downright Deadly

  Now we move on to the rogues' gallery of blood-feeding arthropods. These are the animals that may transmit pathogens to humans in their quest for blood. These are the ones that you do need to be concerned about, and to consider as possible threats while travelling abroad.

1) Kissing Bugs or Conenoses

  Most of the more than 100 species of blood-feeding kissing bugs (Reduviidae, Triatominae) are found in South America, though there are a few found in India and Southeast Asia. They are known as kissing bugs because of their habits of creeping out at night to feed from the lips of their sleeping victims, using their long, needle-like mouthparts. Their bites are almost painless, so the sleeping victim seldom stirs as a bug feeds, even as they take in many millilitres of blood. Before humans took up residence in thatched huts, kissing bugs likely were content to feed on the variety of tree-top and hole-nesting mammals. However, with the advent of the thatch hut constructed by humans, and perhaps even with the domestication of animals, many species have adapted to the readily available blood source by sharing accommodations with us. Very often, in gathering the thatch for their homes, people actually acquired these bugs in the process, and incorporated them right along with construction. Even the immature stages require a blood meal, and it is, in fact, the engorgement with blood that triggers moulting to the next developmental stage. After they mate, females will feed and then begin laying their eggs in sheltered areas, often by cementing them singly or in small clusters to the substrate.

  Nearly all species of kissing bugs seem to be possible vectors for an important pathogen, the protozoan ( Trypanosoma cruzi ) which causes Chagas' disease, and one which infects humans and more than 100 different species of wild mammals. However, there are only about a dozen species of kissing bugs, all found in Mexico, Central or South America, that are considered important vectors for human infection. It is interesting that most of these species defaecate at the same time as they are feeding, and this is a critically important characteristic. Although the bite of the kissing bug is relatively painless, there is often a delayed sensitivity reaction that creates considerable itching some hours after the bug has fed and fled the scene. When a person who has been bitten wakes up in the morning, they respond to the intense itching, and begin to scratch. In so doing, they scratch some of the bug's faeces into the bite wound, or through the skin. Of course the infective stage of the pathogen is found in the bug faeces, and so a person becomes infected. The pathogen undergoes development in the infected person, and eventually stages infective to a feeding kissing bug appear in the blood, to complete the cycle.

  You are most likely to encounter kissing bugs where housing is rustic. The bugs do best where they have lots of places to hide during the day, so houses constructed of thatch and loosely fastened materials are usually home to the greatest numbers of bugs. The best control for these bugs is improved housing construction and, in some cases, the application of residual insecticides on the walls and ceilings of a home.

2) Mosquitoes

  You might think that all Manitobans need to know about mosquitoes is that there can be a lot of them! In fact, apart from their abundance in the province, there are at least 45 different species recorded. That's nothing to the more than 3,000 species described worldwide, among some of the most important vectors of human pathogens. In Manitoba, most of our species belong to the genus Aedes or Ochlerotatus , which includes our most important pest species. Aedes and Ochlerotatus larvae develop in snow-melt pools, spring runoff, summer flood-water, or tree holes. All of these species overwinter as eggs, and hatch under suitable environmental conditions of water levels and temperature. The larvae are aquatic, and have a long breathing tube (called a siphon) on the end of the abdomen, through which they take in air directly from the surface. The pupae are comma-shaped, very active, and they breathe air at the surface, but through a pair of respiratory trumpets on the top of the thorax. Only adult females may feed on blood, using their thin, needle-like mouthparts which they insert directly into capillaries. However, as you travel toward the tundra regions of Manitoba, there may be a higher incidence of autogeny ( i.e. females can lay eggs without a blood-meal). One hypothesis for this phenomenon is that in the north, potential hosts are less predictable, and weather conditions more often restrict flight periods for blood-seeking females. Therefore, females that can reproduce without the requirement for blood, at least for the first batch of eggs, will be more successful.

  There are several other genera of mosquitoes in Manitoba. Culex and Culiseta deposit their eggs in rafts which float on the surface of semi-permanent pools, and the larvae hatch directly into the water. Culex tarsalis is rarely as abundant as the Aedes and Ochlerotatus mosquitoes in Manitoba and has little nuisance status. However, it is considered to be the primary vector of the Western Equine Encephalitis and West Nile Viruses to horses and humans and is the target of surveillance using light/carbon dioxide traps and flocks of sentinel chickens. Anopheles spp. are found in weedy, permanent water, and include the species of mosquitoes known to transmit the malaria parasites to vertebrates. Fortunately, we don't have human malaria in Manitoba, though there are species found in birds. Mansonia perturbans is an unusual species found in permanent marshes and slow-moving streams. It is a savage mosquito, and is most active just after dark during mid summer. The larvae are bizarre because they have abandoned the mode of surface respiration, and instead, have a special attachment on the abdomen that allows them to tap into the stems of aquatic plants and take oxygen directly from the plant tissues. Wyeomyia smithii is our only entirely autogenous species of mosquito. These delicate adults lay their eggs inside the leaves of purple pitcher plants, which grow only in acid sphagnum bogs. Also peculiar for Manitoba mosquitoes, W. smithii overwinters as a larva, frozen into a core of ice inside the pitcher plant leaf beneath the snow.

  In the tropics, mosquitoes may breed in a great variety of standing water habitats. Water may be in tree holes, axils of plants, rock pools, coconut husks, crab holes, or in containers around human habitation. They may even be breeding in the water jugs that are in the same room where you sleep.

  In the tropics, you must concern yourself with the possibility that you may be infected by any of a number of important pathogens, even though the numbers of mosquitoes you see about are not anywhere near the numbers that you encounter at home in Manitoba. These pathogens may be protozoan, such as malaria, or viruses, such as Yellow Fever Virus, Dengue Haemorrhagic Fever Virus, or Murray Valley Encephalitis Virus, to name a few, or nematode worms, such as the parasite that may cause the spectacular symptoms associated with elephantiasis. The number of specific pathogens is far too large to try to cover here. However, regardless of the type of pathogen involved, the infective agents are transmitted with the bite of the female mosquito. In most cases there is some degree of development of the pathogen within the mosquito, and sometimes there may even by replication of the pathogen, so that the probability of transmission may increase over time. When a person becomes infected, the outcome may vary, depending on the nature of the specific pathogen. When some pathogens ( e.g . West Nile virus) are innoculated into a person being bitten by an infected mosquito, the person may not necessarily become ill, but even if they do, there is rarely enough virus circulating in their blood for another mosquito to become infected when it feeds. With other pathogens ( e.g. Yellow Fever Virus or malaria), a person may become infected following the bite of the mosquito, and after some time, they may become ill, and the pathogen will replicate in their body, ultimately becoming freely available in the blood to be picked up when other mosquitoes feed.

  As if all this isn't complicated enough, you must understand that not all species of mosquitoes are able to transmit all pathogens, and among those that can transmit a particular pathogen, not all species of mosquitoes are equally efficient at doing so. Even in an area where there is a species of mosquito that is very good at acquiring and transmitting a pathogen, not every female mosquito will be infected, and the proportion of mosquitoes in a population that is infected will vary over time. The bottom line is that unless you have an awful lot of sophisticated and up-to-date information about the epidemiology of a pathogen in an area you are visiting, you will not be able to figure out the probability that the mosquito feeding on your arm is going to infect you. Phewf! I told you it was like a lottery.

  When travelling to a particular area, you should check with public health authorities before you go, to learn what you can about any pathogens that are prevalent in that area during the time you expect to be there. You must conduct your research immediately before you leave for such areas, as recent outbreaks of malaria in the Dominican Republic demonstrate. In this case, many resort communities were considered to me safe, where it was assumed that no malaria was being transmitted to humans. However, in November of 2004, human cases of malaria in recent visitors to these areas began to emerge. As a result, travel advisories were issued, and prophylactic treatments were recommended for travellers.

  Make absolutely certain that you are vaccinated where it is appropriate, and that you are taking the effective prophylactic drugs. Remember that the malaria parasites, for example, have developed resistance to some drugs. You should not rely on past experience in obtaining prophylactic drugs, but rather you should seek expert advice about the best precautions to take, before you travel, and follow the instructions you are given to the letter unless there is some medical or practical reason for you to do otherwise. If you are in an area where it is known that mosquitoes are carrying human pathogens, minimize you exposure. Wear protective clothing during periods of peak mosquito attack, or schedule your own outdoor activity to minimize your likelihood of exposure, if you can. Apply repellents if it is practical to do so. Make certain that you use bed nets at night where they are recommended. Bed nets are often treated with residual insecticides, such as permethrin, to increase their efficiency. Now, that's a lot to remember, but by taking appropriate precautions, you may reduce the risk that you will become infected.

3) Black Flies

  Canada is notorious for its black flies. Poems have been written about them; they have been immortalized in song. Anyone who has spent time outdoors in the north and boreal regions of Canada has learned to hate these fierce little devils. My grandfather always maintained that a black fly would cut a piece of flesh from your body and then fly away with it to eat its meal while sitting on a fence post! This is not quite true, even though it might feel like it sometimes. Rather, the female black fly uses its scissor-like mouthparts to create an open wound in the skin, and then it laps up the blood as it pools in the wound. This is in contrast to the mosquitoes, which have needle-like mouthparts that are inserted directly into the blood vessels.

  Black flies breed exclusively in running water, and the larvae are highly adapted to survive in this challenging habitat. They attach themselves to the substrate using a circlet of hooks which latches into a patch of silk they lay down specifically for this purpose. The larvae are essentially legless (though they do have one anterior proleg), but have enlarged feeding fans with which they filter passing organic matter from the water column. They pupate directly on the substrate, often inside slipper-shaped cocoons, and emerge into a gas bubble which rises to the surface and releases the adult. Some species are partially or entirely autogenous, but it is the blood-feeders that cause significant annoyance and economic losses to humans, livestock and wildlife.

  Wherever black flies that feed on humans occur, they can be extremely annoying. They may be numerous enough to drive people to distraction. More serious is when people develop sensitivity to the saliva in their bites. When this happens, people may develop enormous welts and bumps, and may suffer from black fly fever or even anaphylactic shock. Black flies are not generally known for their ability to transmit pathogenic organisms to humans, but there is one notable example among the filarial nematodes - onchocerciasis or river blindness. The adult worms are found coiled up in nodules under the skin or deep in the tissues. The males are relatively small, only 3-5cm long, but a female may attain a total length of 80cm. Just think about that for a moment. These adult females release the infective stage (microfilariae) into the bloodstream, and these make their way to the capillaries under the skin. The adult female worms may live for more than a decade, releasing the microfilariae the whole time. In fact, it is the large numbers of microfilariae over many years which move through the tissues and through the eyes that eventually may cause blindness and structural changes in the skin. Humans become infected when bitten by a variety of species of black flies which are found in Africa and in central and South America. However, it takes many years of heavy exposure before the symptoms of blindness to appear in infected people.

  Black flies bite mainly during the day, and personal protection is by far the most effective means of avoiding adverse reactions sustained from their bites and possible infection with filarial worms. In some parts of the world, there have been massive efforts to reduce or even eliminate species of black flies that transmit onchocerciasis.

4) Tsetse Flies

  Tsetse flies are a fascinating group of just over 20 species found throughout southwestern, central and east Africa. Males and females feed only on blood, and their bites can be extremely painful and annoying. They are persistent biters, and never fail to ruin a picnic when they are around. The really peculiar element of their life history is that female flies have a special adaptation in their reproductive tract. There is a structure referred to as a "uterus", in which the female nurtures each of her developing maggots, one at a time. The egg hatches in the uterus, the maggot begins feeding from a gland that produces its food, and eventually matures there. The female gives birth to a fully mature maggot, one which pupates immediately and eventually emerges as an adult fly.

  Unfortunately, these interesting flies also have the ability to transmit a protozoan parasite that causes trypanosomiasis, or African sleeping sickness in humans. A fly acquires the parasite from the blood of an infected person, the parasite develops and multiplies inside the fly, and when the parasites migrate into the salivary glands of the fly, they may be transmitted to another host when the tsetse fly feeds.

  Although there have been enormous efforts directed to the control and elimination of tsetse flies in Africa, they are still present in many areas where you will be at risk of infection. Treatment for trypanosomiasis is based on a number of drugs that may cause any number of unpleasant side affects. However, the best advice to you is to avoid being bitten.

5) Sand Flies

  I am using the term "sand flies" here in a strict sense. A sand fly to a New Zealander or Australian is a black fly to us. The sand flies I am referring to here belong to a small group of about 600 or so species of what are also more precisely called phlebotomines, mostly found at tropical and semitropical latitudes around the world. These blood-feeding flies breed in the soil, rock pools, animal burrows, manure, and many different areas where there are accumulations of damp organic matter. The adults are generally small, less than 5 mm in length, but the females can often be recognized by their peculiar habit of running over the skin, stopping briefly, and then running again. There are many harmless relatives, called moth flies, that are a common nuisance in homes in Manitoba, where they breed in the traps of kitchen and bathroom fixtures, feeding as larvae on the built-up gunk you find there.

  Among the most common pathogens picked up by travellers to Central and South America, or in tropical and subtropical Africa, Asia and Europe, is cutaneous leishmaniasis. This condition is caused by a protozoan parasite that normally infects a variety of native mammals. When a person is infected, they may exhibit no signs of infection for several months, making a correct diagnosis, long after you have returned home, less certain. Then at the site of the bite, they may develop a mild to severe ulcer. Secondary ulcers may occasionally develop, in some cases causing hideous disfigurement. Visceral leishmaniasis, or kala azar, may begin as a lesion or ulcer at the site of the bite, but it ultimately progresses into lesions on the major organs. If untreated, visceral leishmaniasis usually results in death. Sand flies may also transmit a variety of exotic viral pathogens.

  Protective clothing and repellents are the most effective means of avoiding sand fly bites. Bed nets are commonly used where sand flies are a problem.

8) No-see-ums

  The common name of this group of flies is as good description as you need to remember. Species that bite humans are usually very small, perhaps only 2-3 mm in length. Many of the important species have spotted wings, but you will have to look very closely to be able to see this feature. Most of the troublesome species breed in damp soil and organic matter or in a wide range of aquatic and semi-aquatic habitats. Only the females take blood, but they do so with impressive style for such a tiny fly.

  Your most frequent encounters with no-see-ums (also sometimes called sand flies, incidentally) will usually result in no more than severe annoyance. People often fail to notice that they are being bitten, either because these flies are most active at dusk so they don't show up easily, or because they are so small that most people can't imagine them to be a problem, even if they do see them. Their saliva is very potent, however, and many people have very strong reactions to the bites, a reaction that may not develop into the characteristic, super itchy, running sores until a day or two after being bitten. Most of the viral pathogens transmitted by no-see-ums affect domestic animals and wildlife. Exceptions which may cause disease in humans are Oropouche virus (South America and the Caribbean), Crimean-haemorrhagic fever virus (Africa and Asia) and Dugbe virus (Africa).

  Personal protection is generally the only practical means of avoiding attack from no-see-ums. Protective clothing and repellents are effective, but it might also be wise to avoid activity outside during periods of peak flight of these little devils. Unfortunately, some species will also enter houses quite readily. If this turns out to be the case, bear in mind their small size when you decide to purchase screen for the windows.

7) Horse Flies and Deer Flies - Tabanidae

  The 50 species of tabanids in Manitoba have been the subject of research for many years. The Manitoba Horse Fly Trap, was first developed here at the University of Manitoba by A.J. Thorsteinson and his students and colleagues in the 1960's, and is now the standard means of collecting and studying this group of flies all over the world. The tabanids can be very broadly divided into two groups, the horse flies and the deer flies. The horse flies include Hybomitra and Tabanus spp., which are large and robust, attack mainly large animals, and which inflict major pain with their bite. They can be serious pests of pastured cattle, horses, and wildlife. I have seen moose come charging out of the forest onto the road and run for their lives, trying to escape these persistent hunters. If you have ever parked a vehicle at the roadside where these flies are abundant, you know that it is wise to keep the windows rolled up! If you don't, you will find the inside of the vehicle alive with hundreds or even thousands of horse flies dancing about the windows. Deer flies ( Chrysops spp.), on the other hand, are smaller, with darkly patterned wings, and which sit at rest in a delta design. These flies also have a painful bite, but in particular, they fly around the head and shoulders, causing considerable annoyance just by their presence. You know they are going to nail you...it's just a matter of when. We do have a few other unspecified genera of tabanids in the province, Atylotus spp., for example. However, these species are rarely encountered, are of no economic consequence, and therefore have attracted little attention. Worldwide there are well over 4,000 different species, most of which are most active on bright, warm sunny days.

  The larvae of tabanids are aquatic, or at least are found in wet soils. Many are predacious, and have savage mouthparts. If you try to pick up larvae of the larger species, they have the ability to lacerate the skin and draw blood. They feed on other invertebrates in the habitat, including their conspecifics. There are published records of larger larvae attacking and feeding upon small vertebrates, toads, for example.

  Horse flies and deer flies most often transmit pathogens mechanically. That is, they acquire a pathogen while feeding on an infected host. Because they cause so much pain when they bite, they often elicit a strong defensive reaction from that animal, and the feeding fly is disturbed. They are very persistent, and will continue to attempt to feed until they are full. If they resume feeding on a different but nearby host, the pathogens adhering to the mouthparts may be inoculated into the next host. In these cases, there is no development or replication of the pathogens on the mouthparts of the fly at all. A variety of viruses, bacteria and protozoa may be transmitted in just this way. One exception is African eyeworm ( Loa loa ) that causes loiasis. This nematode must undergo development inside the female fly before it can be transmitted to a new host. The adult worms, which may be up to 7 cm in length, are found under the skin, particularly in the upper body regions. They get their common name, eyeworm, by their disconcerting habit of moving about under the conjunctiva of the eye.

8) Fleas

  Adult fleas are highly sophisticated ectoparasites of mammals and birds. They are wingless and laterally flattened so that they can move easily among the hairs and feathers of their hosts. Their bodies are covered with rows of backward-directed hairs some of which on the head, thorax or abdomen may be heavily sclerotized and modified to form conspicuous combs, called ctenidia. The mouthparts are adapted for piercing the skin and sucking blood. The heads of males have a dorsal groove into which the ventral margin of the female abdomen fits during mating. The antennae of the males are prehensile, and during mating are extended dorsally to clasp either side of the female's abdomen. The hind legs of most species are adapted for making incredible leaps, enhanced by a nearly perfect elastic protein called elastin. There are several species which are largely confined to their host's nests and in which the ability to jump has been lost. The larvae are generally conservative in structure, white, legless, and covered by numerous long hairs called setae. The larvae are most often found in the nest of the host and feed on organic debris and faecal pellets from the adults, though there is morphological evidence that larvae of some species must be predators. There are several rather peculiar larvae that are nearly always found on the body of their host, for example on arctic hares on the Barren Grounds of Canada's arctic islands.

  Fleas are notorious as vectors of the bacilli that cause Black Death, bubonic plague, transmitted primarily from rats and other rodents to humans. Although plague is now found all over the world where suitable rodent hosts and fleas are found, you don't have to travel far from home to become infected. Plague has never been found in Manitoba, but it is known in southwestern Saskatchewan, Alberta and British Columbia. At one time, it was isolated in Border County, North Dakota, a little too close for comfort! Most species of fleas are found in association with various mammals, but there are many species which specifically attack birds. Some fleas are host specific, and may be found on only one host species. At the other extreme, there are species which seem to be able to attack a wide variety of hosts. Most are free living as adults, but females of the tropical chigoe fleas are an interesting exception. Female chigoes are initially only about 1 mm in length when they attach to their host, in humans often between the toes or under the toe nails. They are slowly engulfed by host tissue, where they increase greatly in size until they attain the size of a pea under the skin. At this stage, only the anus and reproductive opening protrude through an opening in the skin surface. The irritation caused by these infestations is something you can easily do without!

  There are a few interesting examples where reproduction in the flea is regulated by reproduction in their host by the presence of circulating reproductive hormones in the blood. In an interesting example of convergence, larvae of the Australian flea, Uropsylla tasmanica , are found living as parasites under the skin of dasyurid marsupials, and look very much like the larvae of warble grubs found on cattle.

9) Ticks

  After mosquitoes, ticks are probably the most important group of arthropods as vectors of pathogens. They aren't insects, since the adults have eight legs instead of six, but that makes them no less important. For some strange reason, most people absolutely detest ticks. I'm not sure why that is. Perhaps it's because they hide in the long grass and get onto your body without your knowing. Then they stealthily crawl over your skin until they find a suitable place to feed, where they embed their mouthparts, and slowly begin to extract your blood, all without being detected, until they are engorged and the size of a grape! Yup, that's probably why.

  There are two types of ticks: soft ticks and hard ticks, the latter being the far more important in public health. Most hard ticks hatch from eggs as tiny six-legged larvae. These larvae attach to a passing host, take a blood meal, and then moult to an eight-legged nymph. The nymph takes a blood-meal and then moults to the adult stage. The adults feed and mate, and the females fall from their host and lay their eggs, as many as 3000 or more, somewhere on the ground. Depending on the species of tick, they may spend their entire active life cycle on one host, as is the case for the winter moose tick that is such a problem in some parts of Manitoba, or, more commonly, they may drop off each host to moult to the next stage. Our wood tick does this, and must wait in the grass for a new host before they can get a blood-meal and resume their development. The life cycle of our wood ticks generally takes two years to complete, the first winter spent as a larva, and the second winter as an adult.

  Ticks may transmit a wide variety of different pathogens, including viruses ( e.g ., tick-borne encephalitis, Powassan encephalitis), bacteria ( e.g ., Lyme borreliosis, ehrlichiosis), rickettsiae ( e.g. , Rocky Mountain spotted fever), and protozoans ( e.g. , babesiosis). To add to the grief of pathogens transmitted by ticks, many people suffer severe localized sensitivity to tick bites, or they may acquire secondary bacterial infections that may require treatment. Some species of ticks found in western North America (including British Columbia) and in Australia are also known to cause tick paralysis. This toxic response to the saliva of a feeding tick can result in paralysis and even death, if the tick is not removed in time. It is a flaccid, ascending paralysis, which usually begins as weakness in the legs, progressing to the trunk, ultimately causing paralysis of respiratory muscles.

  Personal protection is by far the most effective means of avoiding ticks bites and tick-borne infections. If you are in an area where ticks are present, tuck you pant legs into your socks to keep ticks, initially at least, on the outside of your clothing. Ticks are more easily seen on light-coloured clothing, too. If you are going to spend a lot of time in tick-infested habitat, you might even consider cutting your hair short, so the little devils will be easier to find. Treatment of socks and pant legs with repellent will also provide some protection. Before you settle in after a long day of tramping in the field, check yourself carefully for any ticks that are wandering around on you, or that may have begun to feed. If you find an attached tick, grasp the tick gently with tweezers, or with your fingers, and pull them out using gradual, steady pressure. Do not squeeze them, smash them, twist them, burn them, or smother them in petroleum jelly or mineral oil. You don't want to irritate them and have them regurgitate saliva into the wound. Remember that in most cases, even if you have been attacked by an infected tick, the sooner you remove it, the less likely you are to be infected.

Happy Trails

What I really hope to have accomplished in this brief sojourn into the world of arthropod vectors, is to raise you level of awareness and to peak you interest. Carry with you the warning that a little knowledge can be a dangerous thing. With that in mind, I make the following suggestions for you to have a safe and enjoyable trip:

1) Read, read, read - When you prepare for the trip of a lifetime, you probably go to great lengths to study tourist brochures, maps, guides to the sites, currency, local culture and language. How much time do you spend reading about the insects you may encounter or the pathogens they can transmit? Go to your local library, surf the internet, talk to friends with experience in the places you are to visit, consult with your physician and with public health officials. Do whatever it takes to know what to expect when you arrive.

2) Be prepared - Consult with your physician and with provincial or federal tropical disease specialists. Make sure that you have been vaccinated against pathogens that you may encounter where you are going, and that while you are there, you take every precaution to reduce the risk of exposure.

3) Relax - Once you are aware of the risks of vector-borne diseases and you are armed with the best information and medical precautions you can, you can rest assured that you are in the best position possible to enjoy your travel experience