The biting activities of tabanids cause major problems for horses, other domestic animals and people. Tabanids slash and suck and can cause relatively large bite wounds which may continue to bleed after the flies have left. Also, prevention of biting by tabanids is difficult because they find their hosts primarily by sight, and so insect repellents are more or less ineffective. Tabanids also act as vectors for several helminth and protozoan parasites of domestic animals, wildlife and people.
Horse flies, deer flies and clegs are related, therefore, to mosquitoes and to the other flies of importance in veterinary parasitology.
Host range and geographic distribution
Although there are differences in life cycles in different genera and species, the basic pattern is similar. Adult female tabanids need several blood meals prior to mating and laying eggs. Mating occurs when females enter a swarm of males, and it starts in the air and finishes on the ground. Eggs are laid on plants or rocks or debris overhanging water. The newly hatched larvae move to a suitable substrate, often mud at the edge of a pond. Larvae of Tabanus prefer soil adjacent to water, those of Chrysops prefer very wet environments and those of Haematopota prefer soil. There are up to 11 larval instars, some of which may take several months before moulting to the next, and then pupation. The larvae eat small crustaceans and each other. Under ideal conditions the life cycle can be completed in four to five months.
Pathology and clinical signs
Tabanids blood feed by slashing and sucking and can create relatively large wounds. The preferred sites of biting depend on the genus and species of fly. Bites by tabanids can be very painful, and will continue to bleed after the fly has left. Biting by tabanids is a major source of annoyance, and horses, cattle, sheep and other livestock can be affected, together with wildlife and people.
Tabanids can serve as vectors for Elaeophora schneideri (arterial worm) of sheep in the western US, as well as Loa loa (Calabar swellings) of people in West Africa, Dirofilaria roemeri of macropod marsupials, Leucocytozoon species of birds, Haematoproteus metchnikovi of turtles, and the harmless Trypanosoma theileri of ruminants, which occurs in domestic and free-ranging hosts in Canada. Tabanids can also serve as mechanical vectors for Anaplasma marginale, Francisella tularensis and Bacillus anthracis, as well as for Trypanosoma evansi and T. vivax of livestock in the tropics.
Treatment and control
Effective control of tabanids is extremely difficult, particularly because they often locate animals by sight, and this makes prevention of biting using chemically-based means essentially ineffective. Pyrethrins, permethrins, combinations as well as products containing butoxypolypropylene glycol plus resmethrin have label claims for repelling, controlling or killing tabanids as well as other fly species in horses.