In Western culture, the fly was suspected of evildoing long before science proved its guilt. It was "otherwise called a divell or familiar," according to Scot's 1584 Discoverie of Witchcraft, for it was believed that devils frequently assumed its form to transport themselves and, more insidiously, find their way into private places. The fly's merciless mission to pester, harass and madden its victims did little to help its image.
Nor did its penchant for life's primal elements--blood, sweat, tears and offal--and its attraction to wounds, mucus and manure go unnoticed or unjudged. Indeed, "flie-blowen flesche," swollen, oozing and maggot infested, represented the epitome of impurity.
Today's horseman is no less intolerant, holding fast to the belief that the only good fly is a dead fly. These culprits transmit contagious diseases, parasitic infections, and fungal spores. Many insect-borne ailments are debilitating; some are life threatening, even lethal. Sheer numbers guarantee that the flying menaces will get their digs in against your horses. Though nature has given even the most phlegmatic horse, one who's as insensitive as stone to your leg aids, the sensitivity to feel a single fly light on his side, all the efforts of his twitching fly-shaker muscles, swishing tail, tossing head and stamping feet will not win a lasting peace in his battle with bugs.
Horses may injure themselves in a stampede to escape a particularly maddening insect attack; they may be bitten or kicked as they crowd together for protection. They've been known to stand in water until their joints swell to prevent flies from biting their legs, and quite commonly they injure their eyes as they look for relief by rubbing their heads against their knees and stall doors, trees and posts. Self-mutilation occurs when horses bite themselves to ward off flies or rub their tails, manes and patches of skin until raw to relieve the itch of fly-induced dermatitis. Horses will even straddle stumps or lie down on the ground and drag themselves along on their bellies to assuage the pain caused by the fly-transmitted filarial worm.
The unmitigated discomfort of fly attacks can cause classic signs of stress -- anxiety, restlessness, depression and listlessness. If continually swarmed, even the most stoic horse may stop feeding, lose weight and suffer a lowered resistance to disease.
For such tiny creatures, flying insects wield and inordinate degree of power over considerably grander animals, leading one to wonder if those medieval notions about devils and such aren't so far off the mark after all.
It's A Fly's World
From the tropics to temperate climes, from desert lands to tundra, no place in the equine world is fly free. Some varieties, such as the 20-odd species of tsetse flies (Glossina) in sub-Saharan Africa, are habitat specific and found nowhere else in the world. But Trypanosoma, a protozoal disease agent transmitted by the tsetse fly, does its neurological damage in other regions as well, thanks to biting horseflies (Tabanidae) and stable flies (Stomoxys). Black flies (Simuliidae), also called buffalo gnats, are nearly universal tormentors, as are the 800-plus species of Culicoides, popularly called midges, punkies, and no-see-ums. Horn flies (Haematobia irritans) infest the cooler regions of both northern and southern hemispheres, while warble flies (Hypodermatidae) and botflies (Gasterophilus intestinalis and G. hemorrhoidalis) flourish in just about all climates. Face flies (Musca autumnalis) and houseflies (M. domestica), along with their continent-specific relative, the Australian bush fly, are annoyances most everywhere horses are kept.
Though it's a fly's world, chances are that your horse won't be bothered by all types of flies during even the worst of fly season, for most species require specific elements in their environments to survive and reproduce. Though some flies use the horse as their breeding ground, laying their eggs on wounds, legs, necks or chests, most do not. Face and horn flies prefer to deposit their offspring in cow dung; horseflies lay their eggs on vegetation, over water or in wet soil; black flies and buffalo gnats use fairly fast-running water as their nurseries. Some biting midges lay their eggs in wet, fecal-rich soil, others in swamps. In general, wetness and rot are the common ingredients in these little devils' ideal habitats.
If your environment is inhospitable to specific flies, your herd won't suffer from their presence. But you can count on at least a handful of species to threaten your horses' health, no matter where you life and no matter how short your fly season happens to be. Once the flies' prime time is over, however, the danger hasn't necessarily passed, for fly-induced ailments often persist well beyond the insects' short life spans.
Excerpted from an article that first appeared in the June 1991 issue of EQUUS magazine.