Biting flies are feared around the world for their painful bites, inflicted on both animals and humans. Like houseflies, which have no piercing mouthparts, biting flies can spread disease-causing pathogens that hitchhike on the flies’ legs, abdomen, or mouthparts, transmitting disease from ill to susceptible animals. This effect is reinforced by their feeding habit of severing capillaries near the skin surface to drink the blood that pools there.
Given these characteristics, it is surprising that biting flies are not more important pathogen spreaders than they actually are. Many of the dozens of diseases they are suspected of spreading are based on having identified those pathogens somewhere on a fly in the past. But documented transmission by biting flies between animals has been rare, for several reasons.
Who Are the Biting Flies?
“Biting flies” are not a coherent taxonomic group. They are members of the Order Diptera, or true flies, but this includes an estimated one million or so species, including mosquitoes. This post will consider some of the better-known biting flies, including horse flies and deer flies (Family Tabanidae) and stable flies and tsetse flies (Family Muscidae).
These biting flies all have one pair of wings (as do all Diptera, from Greek meaning “two wings”), and the worm-like larvae that hatch from the adults’ eggs look nothing like the adult fly. This contrasts with ticks, for example, whose younger stages look like miniature versions of the adults. The development from larva to adult fly, called metamorphosis, occurs in a hard casing called a pupa.
Some horse fly and deer fly larvae hibernate in temperate climates, waiting up to 3 years before finally forming a pupa. The adult emerges from the pupa after 1-3 weeks in most cases, after which life for the fly is fleeting, typically lasting only 2-3 weeks.
Maturation of eggs in the adult female fly usually requires a blood meal (unlike the New World screwworm fly we saw in a previous post). After mating and a blood meal, the female oviposts (lays) one or a few batches of several hundred eggs in a suitable area. Many horse and deer flies, for example, attach their eggs to leaves overhanging water. Others place them on fresh manure.
Each species of biting fly has its preferred host species to feed upon, anatomical site to bite, time of day to feed, preferred temperature and humidity to be active. Stable flies (genus Stomoxys) are attracted to livestock both outdoors and inside, often hanging around horse stables, feeding on the legs, sides, back, and abdomen of their victims. They will often painfully probe people on their lower legs for a good place to grab a meal.
Horse flies, despite their fierce reputation, rarely bite people, preferring large animals such as cattle and horses. Deer flies, on the other hand, are less picky, feeding on people, livestock of all sizes, and even birds.
Biting flies use numerous cues to locate their hosts. Horse and deer flies, for example, recognize a suitable meal by size, shape, color, contrast, movement, emission of carbon dioxide (and many other volatile compounds), and the polarization properties of light reflected from the animal, to name a few. Females are often attracted to animals with dark-hair or large-spots, and avoid striped coat patterns, as on zebras. Even covering a zebra in fly-attracting odors is not enough to bring the flies in for a meal.
Some deer flies wait in large numbers on forest edges to attack any unfortunate host passing by. They often favor dark-colored animals, or dark areas on black and white animals such as Holstein cattle.
Many biting flies are also strong fliers with high dispersal potential. Horse and deer flies can travel 1 to 2 km each day in search of a meal. While most stable flies remain their whole lives within a 5 km-radius, they can travel much greater distances when their blood meal hosts are scarce.
The Damage Done
Some of the most significant damage done by biting flies is the result of their harassment of livestock. Cattle in areas with heavy biting fly infestations tend to take bigger bites of grass by cutting off the grass blades lower down than they otherwise would. Burying their heads deep into the pasture to feed in this manner disturbs flies swarming around the animal’s face.
However, this increased feed intake per bite is more than offset by the tendency of fly-harassed cattle to concentrate in areas where the flies are least bothersome. These places have little grazing, or the grass there is quickly eaten down, so the cattle end up consuming less. A 20% reduction in weight gain and over 50% decrease in milk yield has been documented in beef and dairy cows, respectively, due to high stable fly numbers.
Another important impact of biting flies is their role as mechanical vectors (as opposed to biological vectors – see below), carrying pathogens from affected animals to susceptible ones. Many horse and deer flies can consume four times their weight in blood at a single meal. They initiate their feeding with a scissor-like cut to the skin, severing capillaries near the skin surface. Blood pools at the bite site and is pulled into the fly’s alimentary canal while a different tube infuses saliva into the pool that acts as an anticoagulant and dilates nearby blood vessels. The feast attracts other flies that may be carrying pathogens too.
The pain inflicted by the fly’s bite is well suited to disease spread. The swooshing tail or rub on a tree trunk to dislodge a biting fly usually sends the fly to try its luck on a different, nearby animal if it has not yet begun to feed in earnest. Any pathogens it may have picked up on one can be transferred in this way to many other animals.
Once the fly has settled in to feed on a blood pool, however, it is much more difficult to dislodge it, even with direct hits from a fast-moving tail-head or swift kick from a hoof.
Studies have shown that only about 1 in 10 horse flies is able to take a full meal on the first bovine they attempt to feed on. 90% are interrupted and move to one or more other hosts, taking pathogens with them.
A large number of blood-borne bacteria, viruses, protozoa, and other parasites is suspected of being spread by biting flies (see Table 1 below for a list). The word “suspected” is important because the great majority of pathogens appearing in Table 1 are listed because they have been found at one time or another stuck to the legs, mouthparts, abdomen, or inside of biting flies. The latter is significant in stable flies because these often regurgitate part of their last meal onto a host before starting a new meal.
But whether some of these hitchhikers are actually transmitted to the animals the flies feed upon, and in high enough doses to cause disease, is in most cases far from certain.
For some pathogens, transmission from one host to another via a fly bite has been documented in the laboratory. While this reinforces suspicions, it still does not mean that such transmission occurs regularly, if at all, in nature. Yet even laboratory evidence is not easy to come by. Neither horse nor deer flies, for example, will mate in captivity, precluding large-scale experiments on them.
A Perfect Storm
What is most surprising is that biting flies do not play a more prominent role in disease transmission. It appears that many factors must be just right, at just the right time, and occurring in just the right order for biting flies to transmit most pathogens mechanically from an infected animal or person to a susceptible one – and this rarely happens.
For example, infected animals should have high levels of the pathogen in their bloodstream at the time of the fly bite. Large numbers of both infected and susceptible animals must be in the herd, and must be physically near to one another. Also important are the density of biting flies in the area, size of the mouthparts of the biting flies, amount of blood on the mouthparts of flies whose feeding is interrupted, and many other factors that are unique to each pathogen, fly species, habitat, etc.
Another barrier to disease transmission is the amount of time between feedings by biting flies. Individual horse and deer flies average a blood meal every 5-7 days, much too long for most pathogens to survive exposed to the environment, clutching to the hairs on a fly’s leg. Even for stable flies, which feed once, sometimes even twice, a day during warm weather, many hours pass between meals, greatly reducing pathogen survival rates.
As a result of these numerous barriers, biting flies probably do not initiate outbreaks of disease, but they can help spread or prolong outbreaks once they occur in a population by other means.
Biting Flies as Biological Vectors
Mechanical vectors, as we have seen, essentially pick up pathogens from one source and carry them unchanged to another source – a kind of parcel delivery service. But some biting flies also act as biological vectors, in which either multiplication or development of different pathogen life stages occurs in the fly before transmission to a susceptible animal.
One of the more significant to humans is the nematode (roundworm) Loa loa, which causes “eye worms” and other problems across West and Central Africa. Deer flies (genus Chrysops) take up microfilariae (the pre-larval stage of these worms) from the blood of an infected person. Non-infectious at this stage, the microfilariae migrate to the fly’s fatty tissues and develop through several larval stages, after which they move to the host fly’s mouthparts. These larvae are released onto a person when the fly takes its next blood meal, then burrow into the skin.
Also of great importance to people and animals are the trypanosome protozoa that cause sleeping sickness in humans and similar diseases in animals. These parasites undergo numerous biological changes inside their tsetse fly (genus Glossina) vectors before the latter’s bite infects a new host.
Interestingly, tsetse flies (and the other biting flies covered in this post) can also act as mechanical vectors for trypanosomes. This is most evident in South America, where Trypanosoma vivax was introduced by at least 1920 without its normal biological vector, the tsetse fly. The disease nevertheless spread and is maintained to a large extent, if not exclusively, through mechanical transmission by biting flies. In Africa, biting flies can mechanically vector some trypanosomes in afflicted areas, but they appear unable to sustain these pathogens in the absence of tsetse flies.
In another twist, the non-biting human botfly (Dermatobia hominis) of South and Central America oviposits its eggs on various biting insects such as mosquitoes, ticks, as well horse and deer flies. As the parasitized fly feeds on a person, livestock or other mammal, the eggs hatch and the larvae drop onto their victim’s skin, burrowing into the bite wound caused by the fly. The larvae develop under the skin over the coming 8 weeks, then crawl out and drop off to pupate and emerge as adults.
The biting flies discussed in this post probably contribute to the spread of dozens of pathogens, but in most cases only rarely – when a series of ideal conditions are met.
Stable flies appear to be of little to no significance for spreading disease to people. In animals, horse and deer flies generally play more important roles in pathogen transmission than do stable flies. For example, with the protozoan Besnoitia besnoiti, 3 horseflies have been shown to transmit this cattle parasite to a susceptible animal, whereas some 50,000 stable flies were required for transmission.
Despite the damage they cause, biting flies are here to stay. Controlling their populations, let alone eradicating them, is difficult, requiring lots of ingenuity and, when that fails, lots of insecticide (to which flies can become resistant).
Nor are the consequences of fly eradication known. As a reminder of how nature works when we try to manipulate insect populations, the draining of salt marshes for mosquito control in many parts of the world is believed to have created good habitat for horse fly and deer fly larvae, leading to greatly increased local populations.
In any event, biting flies (with the possible exception of tsetse flies that serve as biological vectors of sleeping sickness in people and livestock) are generally much less important as disease vectors than other arthropods such as mosquitoes, ticks, biting midges, sand flies, and others. We should get used to living with them.
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