Contact Information

Dr. Tony Pescatore, Ph.D.
Interim Department Chair

900 W.P. Garrigus Building Lexington, KY 40546-0215

+1 (859) 257-2686

Horse Discovery - Parasites

Horse Discovery - Parasites

External Parasites Combo


External Parasites or Ectoparasites

An ectoparasite is a parasite that lives on the skin surface of an animal--in this case, the horse. The primary ectoparasites of horses are houseflies, stable flies, mosquitoes, and, to a lesser extent, horse and deer flies. Ticks, lice and mites are also common ectoparasites in horses.

External Parasites by Type

House Fly

The House Fly (Musca domestica) is about 1/4" long, and gray with 4 black stripes on the thorax.  The house fly doesn't bite, unlike face flies, stable flies, and horn flies. They breed most commonly in manure. Mucus secretions and wounds on horses are a protein sources for the adult house fly. House flies feed around the eyes, nose, genital openings, and wounds on a horse. In high fly-populated areas, this feeding can be very annoying and potentially dangerous for the horse. 

Environmental control through good sanitation is essential to minimize house fly populations.

Face Fly

Face flies (Musca autumnalis) are annoying pests of pastured horses. These flies, closely resembling house flies, feed on secretions of the eye and nose, as well as blood and other fluids around wounds. They can use their abrasive sponging mouthparts to stimulate tear flow around the eyes. Face flies breed only in fresh cow manure so they are most problematic on horses pastured near cattle but they are strong fliers and move a mile or more from them.

Horses constantly annoyed by face flies may become nervous or skittish and seek deep shade to avoid the insects. Face flies can carry the nematode eyeworm Thelazia lacrymalis. A study in Kentucky found these nematodes in horses’ eyes and in about 1% of the face flies captured around horses.

Fly wipes and sprays may give some temporary relief. Fly masks may be needed in cases where the flies are causing severe distress.

Horn Flies

Horn flies (Heamatobia irritans), are blood-sucking pests which usually are a nuisance to cattle. Horn flies will also attack and remain on horses if they are pastured or ridden near cattle. Horn flies resemble small house flies. They feed on the back and side of cattle and horses, remaining their day and night. They leave only to lay eggs or to move to another animal. 

Horn flies breed only in freshly deposited cattle manure. The life cycle can be as short as 10 days during hot weather. This species is not known to transmit any diseases of horses.

Bot Flies

Horse bot flies are bumble bee-sized insects that dart and buzz around the head and lower legs of horses as they glue their eggs (nits) to the animal’s hairs. They cannot bite but animals may injure themselves while attempting to escape. Horses also can injure themselves as they attempt to relieve the irritation caused as newly hatched bot fly larvae (maggots) that burrow into oral tissues. 

The larval stages of the flies are spent as internal parasites in the stomach of the horses where they can cause gastrointestinal disturbances. Large numbers of bots can mechanically block the stomach outlet resulting in colic. 

Three species of horse bot flies with very similar life cycles can be found in Kentucky. Adults are active from July until killing frosts in October or November. Females lay 150 to 500 eggs during their short 7 to 10 day life span. The eggs are glued individually to hairs; placement sites vary by species. 

Eggs of the common bot fly (Gasterophilus intestinalis) are stuck onto hairs of the forelegs, chest, neck, belly, and sometimes the hind legs and flanks. The eggs incubate for 1 to 2 weeks and hatch only when licked or eaten. The moisture, warmth, and friction stimulates egg hatch. The tiny maggots burrow into the front the tongue and exit from the back of the tongue about 3 to 4 weeks later before migrating to the stomach wall. 

The female throat bot fly (G. nasalis) glues her eggs on hairs under the jaw and throat. They hatch in 4 to 6 days without stimulation. The maggots infest the gum margins and tissue around the teeth for about a month before migrating to the stomach, pylorus, and duodenum. Large numbers of maggots in the gums may cause pus pockets and irritation in the mouth. 

The black eggs of the nose bot fly (G. hemorrhoidalis) are laid on the hairs of the lips. They hatch in 2 to 4 days and the maggots bore into the inner lip membranes in front of the incisors. About 5 weeks later they migrate to the stomach and attach themselves to the stomach wall.

Full-grown horse bots are thick, tough skinned maggots that are blunt at the posterior end and taper in the front where two strong mouth hooks can be seen. There is a ring of prominent spines around each body segment giving the 1/2 to 2/3 inch-long whitish maggot a screw-like appearance. The mature larvae detach from the stomach wall and move through the intestine, passing out with the feces. Then they will burrow into the soil and molt to the pupal stage. Adults will emerge from the soil during the summer to complete a year-long life cycle.

Life Cycle of a Bot Fly

Control: While bot flies may or may not be noticed around horses, it is easy to look for nits (eggs) on the animal's coat. Virtually all horses in Kentucky are likely to be infested. Consequently, an insecticide, applied internally, is necessary to provide effective control. Check product labels carefully, all equine deworming drugs do not necessarily control horse bots. Before purchasing any product, read the pest list on the label and note any precautions regarding product use. Trichlorfon, an organophosphate insecticide, is available by itself (Combot) or included in some combination dewormers to provide bot control. No other organophosphate or cholinesterase inhibiting products, such as those containing dichlorvos (Vapona), coumaphos (Co-Ral), or tetrachlorvinphos (Rabon) should be applied to horses at the same time, or within several days of treatment. The product label will give specific restrictions. Ivermectrin, the active ingredient in products such as Eqvalan, Zimectrin, and Protectin 1, controls bots and other internal parasites and is not a cholinesterase inhibitor. No supplementary bot control material is needed when using products that have ivomectrin as the active ingredient. Consult your veterinarian about an appropriate parasite control program. 

Alternatives: Clipping hairs that harbor eggs is not a practical solution for these pests. Sponging areas of the fore legs where nits are attached with warm water (110o to 112oF) may stimulate some eggs to hatch and the small larvae can then be washed off. This is of limited value and would have to be repeated frequently because new eggs are attached daily while the flies are most active. 

Bots and Humans: On rare occasions, bots can invade the oral tissue of humans. The small larvae may burrow behind the lips or inner cheek and cause an uncomfortable sensation. They are unable to develop in humans so the small larvae will die within a few days. For example, rubbing or petting an area where bot eggs are attached can result in larvae on the hands and subsequent transfer to the person's mouth.

Stable Fly

Stable flies are blood suckers that resemble house flies but have piercing mouthpart that can be see jutting forward from the front of the head. 

Both males and females are blood-feeders, usually biting horses on the lower legs and flanks. Horses receiving these painful bites stamp and kick in attempts to chase these flies away. Stable flies spend an estimated 80% of their time away from animals, resting on vegetation, fences, and building surfaces. The presence of several stable flies continuously on animals is a sign of a large population and the need control measures directed at breeding sites. 

These insects visit cattle and horses only to feed. They prefer to feed low on the animal, usually on the lower parts of the legs or the flanks. Feeding by large numbers of stable flies causes severe irritation and some blood loss. They may transmit equine infectious anemia.

Stable flies are usually found around stables or paddocks where they breed in organic matter. Fermenting mixtures of bedding, hay, manure, or feed moist with water are urine are ideal places that can produce large numbers of both stable flies and house flies. Each stable fly female produces about 500 eggs and it takes from 20 to 60 days for them to complete their life cycle. 

Insecticide applications to animals may provide some temporary protection but it is difficult to keep the lower legs treated, and during daylight hours, unfed flies are continually coming to animals to feed. Reapply treatments when stable flies begin to bite again.

Residual sprays and space sprays (foggers) around barns and stables can help to reduce numbers temporarily but breeding site elimination is the most effective long term means of control.


Several mosquito species feed on horses. While their bites are annoying and can cause skin reactions in sensitive animals, some species can transmit equine infectious anemia and several equine encephalitis viruses., such as West Nile Virus and Eastern, Western and Venezuelan Encephalomyelitis. 

Floodwater mosquitoes breed in temporary pools that develop in low lying areas of fields. They can fly several miles from their breeding sites to feed on animals, usually at night. They can be a painful annoyance but usually are not a threat to animal health. 

Some species, such as the house mosquito Culex pipiens, use stagnant water in buckets, water troughs, gutters, and artificial containers for larval development. They generally feed on birds and occasionally mammals so they are important vectors of encephalitis. House mosquitoes feed at night and will enter barns and stables to find a blood meal. Fly wipes and repellents can provide only limited protection against mosquitoes.

Horse Fly and Deer Fly

Horse flies and deer flies are bloodsucking insects that can be serious pests of horses and riders. Horse flies range in size from 3/4 to 1-1/4 inches long and usually have clear or solidly colored wings and brightly colored eyes. Deer flies, which commonly bite humans, are smaller with dark bands across the wings and colored eyes similar to those of horse flies. Attack by a few of these persistent flies can make outdoor work and recreation miserable. The numbers of flies and the intensity of their attack vary from year to year. 

Numerous painful bites from large populations of these flies can interfere with grazing because animals under attack will bunch together. Animals may even injure themselves as they run to escape these flies. Blood loss can be significant. Twenty to 30 flies feeding for 6 hours would take 20 teaspoons. This would amount to one quart of blood in 10 days. They also may transmit diseases, such as equine infectious anemia. 

Female horse flies and deer flies are active during the day. These flies apparently are attracted to such things as movement, shiny surfaces, carbon dioxide, and warmth. Once on a host, they use their knife-like mouthparts to slice the skin and feed on the blood pool that is created. Bites can be very painful and there may be an allergic reaction to the salivary secretions released by the insects as they feed. The irritation and swelling from bites usually disappears in a day or so. However, secondary infections may occur when bites are scratched. General first aid-type skin creams may help to relieve the pain from bites. In rare instances, there may be allergic reactions involving hives and wheezing. Male flies feed on nectar and are of no consequence as animal pests. 

Horse flies and deer flies are intermittent feeders. Their painful bites generally elicit a response from the victim so the fly is forced to move to another host. Consequently, they may be mechanical vectors of some animal and human diseases.

LIFE CYCLE: The larvae of horse fly and deer fly species develop in the mud along pond edges or stream banks, wetlands, or seepage areas. Some are aquatic and a few develop in relatively dry soil. Females lay batches of 25 to 1,000 eggs on vegetation that stand over water or wet sites. The larvae that hatch from these eggs fall to the ground and feed upon decaying organic matter or small organisms in the soil or water. The larvae, stage usually lasts from one to three years, depending on the species. Mature larvae crawl to drier areas to pupate and ultimately emerge as adults.

Life Cycle of Deer Flies

PROTECTING ANIMALS: Permethrin-based sprays are labeled for application to livestock and horses. These insecticides are very irritating to the flies and cause them to leave almost immediately after landing. Often, the flies are not in contact with the insecticide long enough to be killed so they continue to be an annoyance. These flies will swarm persistently around animals and feed where the spray coverage was not complete (underbelly or legs) or where it has worn off. Repeated applications may be needed. Check the label about minimum retreatment intervals. Pyrethrin sprays also are effective but do not last as long as permethrin. Horse flies and deer flies like sunny areas and usually will not enter barns or deep shade. If animals have access to protection during the day, they can escape the constant attack of these annoying pests. They can graze at night when the flies are not active. 

PROTECTING YOURSELF: Deer flies are usually active for specific periods of time during the summer. When outside, repellents such as Deet and Off (N-diethyl-meta-toluamide) can provide several hours of protection. Follow label instructions because some people can develop allergies with repeated use, look for age restrictions.   Pyrethroid-based repellents are for application to clothing only but typically provide a longer period of protection. Repellents can prevent flies from landing or cause them to leave before feeding but the factors that attract them (movement, carbon dioxide, etc.) are still present. These flies will continue to swarm around even after a treatment is applied. Light colored clothing and protective mesh outdoor wear may be of some value in reducing annoyance from biting flies. In extreme cases, hats with mesh face and neck veils and neckerchiefs may add some protection. 

CONTROL: It is difficult to impossible to locate and/or eliminate breeding site of horse flies and deer flies. They breed in environmentally sensitive wetlands so effects of drainage or insecticide application on non-target organisms or water supplies is a concern. Also, these insects are strong fliers that can move in from some distance away. Breeding sites may be very extensive or some distance away from where problems are occurring. Fortunately, horse flies and deer flies are sporadic problems for specific times of the year. Some adaptation in behavior or use of repellents can allow enjoyment of the outdoors.


The American dog tick (Dermacentor variabilis) and the lone star tick (Amblyomma americana) are potential pests of horses. Ticks live in overgrown areas containing tall grasses and brambles and small mammals that are hosts. They can infest horses pastured or ridden in those types of areas. Ticks do not survive well in dry sunny areas such as those in well-maintained pastures. 

Insecticidal sprays and wipes can provide some repellent effect and temporary protection against these blood-feeding arthropods.

Horse Lice

Two biting lice (Bovicola equi and Trichodetes pilosus) and a sucking louse (Haematopinus asini) can infest horses. Their entire life cycles are spent on the host animals. Infestations are greatest during the colder months of the year. Signs include generally poor condition including scruffy skin, unkempt coat, and abrasions or scabs from excessive rubbing and scratching.

The sucking horse is about 1/8 inch long when full grown. It has a very narrow, cone-shaped head, and flat, dirty gray body. When the insect is not feeding its tube-like sucking mouthpart is drawn back into the head. The bites of these lice are every irritating, which causes horses to rub off patches of hair. Heavily infested horses can suffer enough blood loss to become anemic. The life cycle takes 4 to 5 weeks. They can survive off the host for only 2 to 3 days.

Horse Louse Life Cycle

The horse biting lice have distinctive broad, rounded heads and flat bodies with slender legs. They use chewing mouthparts to feed on dry skin, skin secretions and hairs. The life cycle from egg to adult takes about a month. Biting lice are most often found on the head, mane, shoulder, and at the base of the tail.

In chronically infested herds, a few lice survive on hot weather on susceptible animals. Their numbers then increase rapidly in cooler weather and lice are spread throughout the herd by direct contact as animals bunch. Lice also can be transferred on shared items such as blankets. Lice can be introduced into “clean” herds by new animals that are brought in without first going through a course of treatment.

A variety of products can be used for louse control. The key is to treat animals twice at about 2 week intervals because the egg stage is not killed by treatments. The interval between applications allows eggs to hatch so the nymphs are exposed to the insecticide. Saddle blankets, brushes, and halters from infested animals can be immersed in hot water for several minutes to kill lice and reduce the chance for transfer to uninfested animals.

Internal Parasite Combo

Internal Parasites or Endoparasites

Internal parasites are organisms that live a portion of their life cycle in a host animal. They live in internal organs, body cavities, and tissues while gaining their nutritive source by feeding on the host animal. The horse is affected by many different species of parasites. The nature and extent of damage vary with the parasite. 

Heavy parasite infestation causes loss of nutrients or blood from the host, resulting in serious medical problems. Horses heavily burdened with parasites will have a loss of condition due to a depletion of nutrients and blood, decreased growth, and reduced reproductive and athletic performance. 

Numerous internal parasites affect horses, but there are only a few that commonly cause significant health problems. To establish an effective parasite control program, it is important to first understand the life cycle of parasites. Successful prevention and control programs are effective because they interrupt the life cycle of parasites. 

Internal parasites are divided into 2 groups: Nematodes or round worms (ascarids, large and small strongyles, pinworms) and Cestodes or tapeworms.

Internal Parasites by Type

Large Strongyles

Adult large strongyles are found firmly attached to the walls or free in the contents of the large intestines. There the females deposit large numbers of eggs that are voided to the outside in the feces. From these eggs, larvae hatch and develop into infective stages which climb blades of grass. Horses swallow these larvae while grazing, and the ingested larvae penetrate into the intestinal walls. These larvae are very resistant to harsh environmental conditions and can survive in a freezing environment up to 31 weeks at winter temperatures, compared to up to seven weeks at summer temperatures. 

The large strongyle larvae then migrate to various organs and arteries, inflicting damage in the organs and arterial walls. Blood clots form where the artery wall is damaged and sometimes these blood clots break away and plug up an artery leading to the intestines and hind legs. This causes colic symptoms and lameness, and sometimes death due to the blocking of an artery. Large strongyles mature about 6 to 11 months after larvae are ingested.

Small Strongyles

Small strongyle larvae limit their migrations (the larvae burrow or encyst) into the intestinal wall, not in blood vessels or vital organs. Therefore, they do not inflict the severe damage caused by larval large strongyles. Slowly the larvae mature and are released to the intestinal lumen. This causes a bit of inflammation on the intestinal wall, which is quickly resolved with no further consequences. Horses with heavy infestations will show clinical signs of diarrhea and negative growth performance. These clinical signs usually occur when large numbers of the encysted cyathostomes emerge from the gut wall, resulting in inflammation. The severity of clinical signs is related to the degree of damage to the intestines, which varies with the level of infection. Sometimes all the small strongyles migrate from the intestinal wall to the intestinal lumen at once, causing an extensive damage to the intestinal wall, which leads to severe bloody diarrhea and even death. This is called Cyathostomiasis. At present, it is unclear why the cyathostomes emerge all at once. It is postulated that the occurrence of larval cyathostomiasis may be associated with seasonal factors (winter/spring in northern areas, spring/summer in southern areas), antiparasitic treatment within two weeks, young horses (less than six years). 

Small strongyles develop to maturity about 6 to 10 weeks after horses consume the larval stages.

Strongyles affect horses of all ages, but the young are especially susceptible. Some signs of strongyle infection are fever, loss of appetite, loss of weight, depression, progressive weakness, anemia, recurrent colic, diarrhea, and death.


Ascarids is a very large roundworm (females may be up to 15 inches long), yellowish white parasite that may pass out in the feces of foals and young horses. Typically, adult horses develop an immunity to this parasite; therefore, roundworms primarily infect young horses less than 2 years of age. 

The female worm deposits large numbers of eggs in the intestine, and these worm eggs are voided to the outside in the feces. In about two weeks, these eggs become infective and the horse picks them up and swallows them while grazing. The eggs hatch larvae that burrow into the small intestines, migrating through veins to the liver, heart, and eventually the lungs. After migrating in the air spaces of the lungs, the parasite larvae are coughed up and swallowed. The roundworm larvae are returned to the small intestine, where they mature to egg-producing adults, completing the life cycle. The life cycle takes about 3 months. 

Physical damage, such as inflammation and scarring of liver and lung tissue, occur in the horse during migration. Adult roundworms can cause physical damage due to intestinal blockage or intestinal rupture. The damage ranges from mild digestive upset and lower feed absorption to severe colic. 

Roundworms are more injurious to young horses than to older ones. They damage the liver and lungs, and cause digestive upsets. Sometimes, they cause rupture of the small intestine and consequent death of the foal .Clinical signs of ascarid infection may include potbelly, rough hair coat, and slow growth. Some young horses develop nasal discharge accompanied by a cough as a result of larvae migration.

Strongyloides Westeri

Strongyloides westeri is an intestinal parasite that can infect foals as early as four days after birth. A foal becomes infected by ingestion of larvae in the dam’s milk or by penetration of the foal’s skin by infected larvae in the bedding. However, the larvae are not present in colostrum. Foals will quickly develop immunity to these parasites and lose the intestinal infection of adult parasites by 60 to 90 days of age.

The primary medical problem of a strongyloides infection is diarrhea that may not respond to treatment. The diarrhea that occurs in foals at about 1 – 2 weeks of age (when the mare presents the foal heat) has been related to strongyloides infestation. Some foals will become dehydrated and develop other problems related to chronic diarrhea.


Stomach bots are not worms but, rather, the maggot of the bot fly. The adult bot fly resemble the honeybee in general appearance. Female botflies lay their eggs by attaching them to the hairs of the horse. Different species of the bot fly will lay their eggs on different parts of the horse’s body, but the eggs are always attached to horse’s hair, between the jaw bones, on the short hairs of the lip and on the forelimb and shoulder. 

The eggs, containing first stage maggots (or larvae), hatch either spontaneously or after being licked by the horse. These larvae then migrate, or enter the mouth, attach themselves to the lips, tongue, gums, and other mouth tissues, and burrow into these tissues, principally the tongue. After about three weeks, they emerge from these tissues, are swallowed or migrate down the throat, and attach themselves to the lining of the stomach. Bots spend approximately nine months attached to the stomach lining before passing out with the manure. Damage in the stomach includes obstruction of the flow of food from the stomach to the intestine, and irritation of the stomach lining at the site of attachment of the bots. Bots may cause rupture of the stomach and death of the horse. They affect horses of all ages. These larvae pupate into adult flies. 

The life cycle depends on the parasite larvae overwintering in the stomach, then passing out in the manure in spring and subsequently developing into adult flies. The adult flies are active from late spring to the killing frost in the late fall. Botflies have no trace of mouth parts and never feed. They live their adult life cycle on stored energy. They have yellow stripes and resemble a sweat bee.


Pinworms are usually quite harmless. The most common sign of a horse with a heavy infestation of pinworms is itching of the anus caused by adhesive eggs deposited on the skin. The eggs of pinworms are picked up by horses from contaminated feed, water, bedding, stable walls, fences, or other fixtures. The eggs are swallowed and the worms mature in the colon and rectum. These worms are irritating, causing the horse to rub its tail, with resulting loss of hair and sometimes injury to the tail and rump. The eggs from worms are smeared around the anus and some ruptured worms are voided in the feces. Pinworms do not create a serious health problem because their life cycle is not very harmful to the horse.

Cestodes or Tapeworms

Tapeworms are flat segmented woms that parasitizes the intestines. The body is composed of a ribbon of individual segments or proglottids. Each proglottid contains various body systems including a complete reproductive tract. The most posterior proglottids become gravid or literally filled with eggs and then separate, passing out in feces of horses.

Tapeworms have no mouth parts and cannot actively ingest food. Nutrients are absorbed through the walls of the proglottids.

The life cycle of tapeworms includes the definitive host or horse in which they mature and an intermediate host (a type of mite) in which immature stages are found. Horses, as they graze or eat other feed, accidentally ingest mites infected with immature stages. Inside the horse they develop into adult tapeworms in about two months. Tapeworm eggs within gravid proglottids, or free, pass in horse feces and are eaten by free-living mites. Infected mites are then eaten by horses and the tapeworm cycle continues. 

Tapeworm infection is generally not detrimental to the health of the horse. However, their preferred area of attachment is the ileo-cecal valve. Large numbers of worms may cause reduction of this opening, and that can cause serious problems. These effects seem to be more prevalent in weanlings and yearlings.

Contact Information

Dr. Tony Pescatore, Ph.D.
Interim Department Chair

900 W.P. Garrigus Building Lexington, KY 40546-0215

+1 (859) 257-2686