Insect-borne Equine Diseases

Practicing good fly control can minimize the threat of the many common insect borne diseases that torment horses around the world. By Melanie Wallace for EQUUS magazine.
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Practicing good fly control can minimize the threat of the many common insect borne diseases that torment horses around the world. By Melanie Wallace for EQUUS magazine.
Protecting the face and ears from flies eliminates a major source of irritation for horses. | © Mandy Lorraine

Protecting the face and ears from flies eliminates a major source of irritation for horses. | © Mandy Lorraine

Some flies transmit disease mechanically, directly transferring infectious organisms from the blood, mucus or lesions of an infected horse to the next horse upon which they feed. Other flies serve as intermediate hosts in the life cycle of transmitted parasites, passing immature forms on to their next-stage host--the horse--during feeding. Still others use the horse as an incubator of sorts, for their own young. And then there are the hordes of flying insects who are just plain irritating, firing up skin allergies with the venom of their bites. The following list of maladies, though hardly all-inclusive, represents the most common fly-borne diseases that torment the world's horses.

Mechanical dermatitis. Stable-fly and horn-fly dermatitis results from the destruction wrought by the bite itself, which can cause wheals, inflammation and, if scratched repeatedly, abrasions, scabbing and scarring. The stable fly's bite often leaves a nodule that bleeds and scabs, and some horses develop painful crusts on attacks. Abdominal ulcerations and hair loss result from the horn fly's brutal feeding technique, in which the insect first rasps the skin surface to encourage blood flow, then makes its painful bite.

Allergic dermatitis. The tiniest villains--black flies and Culicoides--are the usual perpetrators of irritating and unsightly skin allergies. Antigens in their saliva incite many horses' immune systems to hyperreaction. Anesthetic at first, black-fly bites become painful and itchy as small, fluid-filled blisters or nodules form. Black flies usually attack in swarms, and particularly sensitive animals may become listless, weak and unsteady from the toxicity of the bites, even to the point of death. Swarms of midges, punkies, no-see-ums and other gnats are responsible for sweet itch (also called summer eczema and Queensland itch), perhaps the best-known and most pervasive form of allergic dermatitis. Skin eruptions occur on an affected animal's forehead, withers, back, rump, tail base and even the stomach area, and the allergy is complicated by the fact that it recurs annually for the rest of the horse's life, unless he is moved to an area free from such pests.

Surface parasites. Whether they are introduced directly into an open wound or make their way to the exterior following complicated internal odysseys, fly-borne parasites commonly contribute to chronic surface lesions. Blowflies (Calliphoridae) and screwworm flies (Callitrogae) lay eggs directly on wounds, and the hatching maggots invade the injured tissue and feed on it. If invaded wounds are left untreated, the severe lesions and subsequent infections can be deadly. After warble flies lay their eggs on an animal's skin, the hatched larvae bore directly into the skin and then migrate to tissues near the backbone and into the esophagus. The larvae remain dormant for months until the onset of warm weather signals them to migrate to the skin of the horse's back. The tissues around the larvae swell, creating "warbles" or lumps complete with minute holes through which the larvae breathe. Eventually the larvae escape through these openings, drop to the ground and burrow into the soil, but occasionally warbles mineralize instead, creating permanent nodules known as occult sarcoid tumors. Rarely, warble-fly larvae also migrate to the brain, where they cause acute neurological disease and death.

Parasites that use flies as intermediary transportation to their horse hosts include the skin-aggravating Habronema and Onchocerca and the eye-invading Thelazia species. Habronema hitchhike on biting stable flies and houseflies to gain entry through wounds or while flies are imbibing fluids from the horse's lips and nostrils. The larvae that mature in the horse's stomach after being swallowed aren't particularly troublesome, but those infecting wounds inhibit healing and encourage excessive tissue growth in the open wound. This reaction may in part be allergic, for the lesions tend to recur on those infected parts of the body every summer, hence their common name of "summer sore."

Midges pass along the thread-like filarial worms called Onchocercae. Though most horses tolerate these skin invaders without incident, some sensitive hosts develop skin eruptions on the head, neck, belly and chest, which, in turn, invite further fly attacks. Though the lesions heal rapidly, they frequently recur; dry, scaly skin shedding dandruff-like particles, particularly along the mane and withers, belly and chest, is the most common sign of Onchocercae infection and is easily confused with Culicoides itch. Eye worm probably afflicts a third of the world's horses, yet it does surprisingly little damage. House-and face flies act as vectors for this parasite, which normally remains in the inner corner of the eye but can sometimes be seen crawling over the eyeball itself. The parasite causes tearing, and the discharge may attract even more flies and subsequent eye infections.

Internal parasites. In addition to the aforementioned worms that live in various organs and body cavities, fly-transmitted internal parasites include protozoa that live in the horse's bloodstream as well as one familiar digestive-system resident. They are no less harmful for being less visible. Botflies, the most ubiquitous of the insects that use the horse to continue their own species, spend the majority of their months-long life cycle in the stomach. Botflies lay their yellow eggs on the hair of horse's forelegs, shoulders and chins, where they're accessible to his mouth. When the horse bites or licks these areas, the larvae are encouraged to enter his mouth, hatch and burrow into the tissues before returning to the esophagus, where they will be swallowed. Once in the stomach, second-and third-stage larvae attach to the stomach walls and spend approximately nine months there before being passed in feces. The mature flies that metamorphose from the manure begin the cycle again. Although bot infections are generally considered relatively benign, a heavy infestation may trigger bouts of colic and cause stomach ulcers or perforations and consequent discomfort or death. Trypanosoma, the most destructive protozoal parasite transmitted by biting flies in Asia, North Africa and Central and South America, causes severe neurological disease in man and beast. In Africa, tsetse flies serve as an intermediate host for the development of the parasite and act as its primary carrier. Called sleeping sickness in people because the parasite invades the central nervous system and induces torpidity, the cattle-and-horse manifestation goes by a variety of names--nagana, surra or mal de Cadera--depending upon the region and the form it takes. Affected horses run fevers, lose weight and develop swellings on their legs, belly and chest. Whether rypanosoma infections take days or months to develop, they are generally fatal to both animals and people.

Fungal diseases. Only a minute number of the world's flourishing fungi population infect animals, but, as you might guess by now, flies can be involved in these transmissions as well, particularly biting stable flies and houseflies attracted to wounds. Implanted into damages tissue, fungi can grow beneath the skin, causing swelling and chronic local infection. Equine granular dermatitis, clinically called phycomycosis or mycotic dermatitis and popularly known as rain scald, Florida horse leech and swamp cancer, produces the ulcerating skin lesions that give the impression that the skin has been scalded. Mycetoma, another fungal infection, gives rise to revolting tumor-like swellings which drain as they expand, resulting in the humane destruction of those animals not promptly and effectively treated. Extended periods of wet, warm weather encourage fungal disease and the flies who serve as willing carriers.

Bacterial and viral diseases. Flies are the handmaidens of contagion, spreading disease from infected matter to living creatures and from the ill to the formerly well. The infectious bacteria or viruses are sometimes transmitted through blood contact and sometimes through contact with mucous secretions ad pus. Without biting flies, the togavirus responsible for pinkeye, which is transmitted into the bloodstream rather than into the eyeball itself, wouldn't get anywhere. And the passage of the virus responsible for the most common skin tumors of adult horses--the wartlike or fibrous equine sarcoids--would be considerably slowed.

In general, blood-borne diseases pose the greatest dangers to horses, and often biting flies are crucial to their spread. Anthrax is among the most dreaded bacterial diseases transmitted by flies. Often confused with acute poisoning or snakebite, it is characterized by fever and sudden death, and it is frequently associated with dry, dusty conditions. Though biting flies can transmit the disease through blood contact, the infectious organisms, when exposed to air, form spores and may be dispersed by wind, water, scavengers and non-biting flies.

Deadly viral infections spread by biting flies are far more common in the United States. Equine infectious anemia (EIA), also called swamp fever because of its prevalence in wet, warm areas where biting flies and mosquitoes flourish, is thought to be transmitted primarily by large blood feeders, such as horseflies and deerflies, but can also be passed from horse to horse by smaller biting insects and by houseflies attracted to sores or wounds. Symptoms of fever, weakness and swelling limbs or belly may vary in intensity and there may be remissions, but there is no specific treatment, and affected horses must either by permanently segregated or slaughtered. Although mosquitoes get the primary blame for the spread of Eastern and Western equine encephalomyelitis (EEE and WEE), a viral brain infection transmissible and generally fatal to both horses and human beings, the small, swarming species of biting flies may also play a role.

Those same tiny no-see-ums are believed to be among the vectors in the spread of African horse sickness (AHS), one of the equine world's most dreaded viral diseases.

As the name indicates, the deadly cardiopulmonary infection is endemic to the African continent, but international movement of infected equidae can--and recently has--spread AHS to other parts of the world, where, undoubtedly, some species of Culicoides or other biting fly will be on hand to transmit the disease to local horses. Flies offer an additional route for contagion to the many bacterial and viral diseases that pass themselves along from horse to horse quite effectively without the help of flying intermediaries. The highly contagious microorganisms responsible for equine influenza, rhinopneumonitis and strangles are transmitted when horses inhale theme, usually from nasal and bowel discharges, but virus particles can also be carried by face flies and houseflies as they go from one set of nostrils to another. Vesicular stomatitis, a viral disease that causes painful sores in horses' mouths, is another condition whose transmission is augmented by stable flies and houseflies drawn to sores or wounds. All in all, a thriving fly population ups the opportunity for the spread of disease, whether the insects are critical or merely incidental to the contagion process.

Excerpted from an article that first appeared in the June 1991 issue of EQUUS magazine.