If you cannot see images at all on my site click here for an explanation
Medical Index
First Posted: June 7, 2007
Last Updated:Oct 17, 2017

Equine Parasites

by Debora Johnson

Ascarids or Roundworms (Parascaris equorum-Large roundworms)  Bots (Genus Gasterophilus)  Chiggers, jiggers, redbugs and harvest mites  Face Flies (Musca autumnalis)  Foals: Parasite Control  Guinea Worm/Dracunculus Medinensis  Hair Worms (Trichostrongyles axei)   Lice  Lungworms/Dictyocaulus Arnfieldi)  Mange Mites   Mature Horses (Parasite Control)  Neck Threadworms (Onchocera) (Microfilariae)   Pinworms (Oxyuris equi-L4 Larvae)   Strangles: Small Strangles (Cyathostomes or Bloodworms) and Large Strongyles   Summer Sores or Habroniasis   Tapeworms   Threadworms (Strongyloides westeri)

Nothing brings more disgust to me than the thought of parasites in and on my horses feeding and living off of their host. It brings to mind a visual that actually makes me feel sick. Horses are hosts for over 150 parasites inside and outside. It is all but impossible to eradicate them, but the parasites can be managed with regular deworming, good nutrition, pasture and environmental management. All horses have internal parasites. The following statistics are staggering to me. Ninety percent of colic cases may be related to blood vessel damage caused by the migrating larvae of blood worms (Strongylus vulgaris). Fifty percent of the deaths in horses may be related to internal parasites. Internal parasites have adapted themselves to the internal environment of their host animal. Horse parasites can only exist in horses. They do not live in other animals. This is referred to as "host specific."

Although over 150 parasites live on and in horses this article will start with six of these. The first five are the most common in horses.

Delaying Dewormer Resistance: Advice Offered in Study
Drugs for the Deworming War

  • Tapeworms
  • Large and small strongyles
  • Ascarids
  • Pinworms
  • Bots


Anoplocephala perfoliata is the fancy name for tapeworms. They are also referred to as cestodes or flatworms. Tapeworms can do lots of harm if not kept in check. They can cause severe inflammation (swelling) to the walls of the intestine where they embed themselves. Colic can be the result including ileocecal intussusceptions, impaction, (blockage with intestinal content). Tapeworms interfere with gut motility. Parasitologists, individuals who study parasites, recommend at least one annual treatment with a dewormer containing praziquantel during spring or autumn for horses weaning age or older. I use Ivermectin Gold. We also use Strongid to deworm with a dose of two or three tubes. This is also suppose to control tapeworms. We have a fecal done on our horses in the spring and in the fall to keep a pulse on the parasite load.

Proglottids, which contain eggs, are segments that look linked box cars that make up a train. They mature, and break off from the end of the tapeworm. The worm does not die. Instead it stays attached to the horse's intestinal wall. When I was a child I was told if an earth worm was cut in half that both halves would live. I don't know if this actually true, however, in the case of the tapeworm these "boxcars" break off either singly or in small groups without killing the worm. Ick! It is actually true.

It is curious that tapeworms do not first develop in the horse. They begin their life cycle in mites and then are transmitted to the horse. Mites, called oribatids, live in many of our pastures. The mites eat the tapeworm eggs that are in horse manure and in our pastures. That is how the transfer takes place. Then the tapeworms grow, mature and reproduce in the horse's digestive tract. This entire cycle takes approximately four to six months. Horses between the ages of three and five and over fifteen years of age tend to have the most tapeworms. Tapeworms are not usually transmitted in drylots, barns, stalls, etc. where grass is not available because there are no mites. I would like to note here that ascarids and pinworms can be acquired from grassy or non-grassy environments. Pinworm transmission is especially rapid in stabled horses. The delicate eggs are concentrated and protected from direct sunlight.

Top   Home

Small Strangles (Cyathostomes or Bloodworms) and Large Strongyles

Note: "Recent studies performed by researchers at UK have shown that ivermectin resistance is developing in the immature larvae of small strongyle parasites. The horse ingests infective larvae from the pasture that develop into adult egg-laying worms within the bowel. Small strongyles reside in the colon and cecum of the horse and undergo several developmental stages before they begin to shed eggs." Small Strongyles might be Developing Ivermectin Resistance

There are more than 40 species of cyathostomes or bloodworms (small strongyles). Horses have been seen with up to 100,000 larvae at one time. They migrate straight to the intestinal lining. When they emerge from the intestinal wall, they can cause colic. They reproduce on a continuous basis. Therefore, your horse should be dewormed every two months or daily and ongoing. The most common and most destructive of all internal parasites, in the equine, is the small strongyle (blood worm). They can cause severe intestinal damage. If they emerge from the gut wall together they do the most damage. Small strongyles can lead to anemia, diarrhea and emaciation They are found in 95% of horses and are the most dangerous to horses under two years old. They are the most common intestinal worm. Strongyles range in length from 1/2" to 2." They can be seen in horses of all ages except foals. Blood Worms

Mature strongyles are found mainly in the large intestine. The larval stages travel through the blood vessels to the heart, liver, and lungs, leaving damaging tracks or scar tissue behind. They destroy healthy tissue. The mature female lays her eggs which are passed in the manure. The eggs hatch into infective larvae that contaminate grass and hay. When eaten by your horse this cycle starts again. When horses are kept on the same pasture year after year, the number of infective larvae accumulate and multiply.

The large strongyle group contains three main species: Strongylus vulgaris, Strongylus equinus, and Strongylus edentatus. Strongylus vulgaris is by far the most deadly of the equine worms. Large strongyle block the arteries supplying blood to the gut. Migrating larval stages through the arteries of the intestine are a significant cause of colic. This migration creates a thickening in the wall of the anterior mesenteric artery where it attaches to the aorta. Blood clots in the bloodstream or ballooning of the vessel wall creates a reduction in blood flow to a large portion of the bowel. This can cause horses to colic after they eat or exercise. How bad the colic depends upon the amount of damage that has been done to the horse's circulatory system. Ruptures of an aneurysm or a complete blockage may cause death of your horse.

Foals are not likely to harbor large strongyles until they are 6 months old.


Top   Home

Ascarids or Roundworms (Parascaris equorum-Large roundworms)

Parascaris equorum, the large roundworm of horses, can be deadly to foals in their first year of life. After that, acquired immunity provides strong protection. Ascarids or roundworm infections are primarily a problem of young horses and are found in the small intestine. They can grow up to 60mm in length. If your horse has a heavy load of larvae, severe inflammation can occur. There can be major damage to the organs such as the liver and lungs. They can move up into the trachea and are coughed up and reswallowed. They can also block the bile duct. Unlike strongyles, roundworms do not attach themselves to the intestinal wall. They mature quickly and produce eggs rapidly. A horse that is heavily infected will have a pot belly. Ascarids raid allot of nutrition from your horse. These roundworms are heavy eaters. Roundworms also excrete toxins, cause digestive problems, and rob your horse of important nutrients which causes your horse not to thrive.

Ascarids can be acquired from grassy or non-grassy environments where transmission is especially rapid in stabled horses where the delicate eggs are concentrated and protected from direct sunlight. The cycle develops as the eggs pass out in the manure. The eggs do not hatch outside the host. They do embryonate. Being resistant to drying and freezing, the eggs can wait to become infective for up to five years. Sometimes the time period can be even longer. Lye is effective in destroying ascarid eggs. Also composting can cut down on the surviving ascarid egg population because heat is harmful to them. The dryness of summer is harmful to them as well.


Life Cycle and More Information About Ascarids or Roundworms in the Equine

Top   Home

Pinworms (Oxyuris equi-L4 Larvae)

Pinworm larvae live and mature in the large intestine. They feed off of the intestinal lining. You will often notice that your horse has rubbed a spot of hair off or there may be an open sore around the top of his tail. This is because the adult females move to the anal area where they lay eggs covered with a stick fluid that causes severe itching. If not treated a horse that is heavily infested with pinworms will be nervous and off his feed. There will be severe itching and rubbing of the tail and rump. As the horse tries to stop the itching by rubbing and biting he may cause skin infections and open, gaping pieces of missing skin and broken hair.

Pinworm transmission is especially rapid in stabled horses where the delicate eggs are concentrated and protected from direct sunlight. Dropped eggs into the feed such as hay, grain, and grass as well as water is how the horse becomes infected. They are eaten by the horse. Adult worms actually crawl part way out of the anus to deposit their eggs on an adjacent surface. The eggs hatch outside the horse's body and become infective in a few days. The eggs can survive in the outside environment for several months. The young worms mature in the large intestine in three to four months, and the cycle begins again.

Effective Wormer Treatments

  • Fenbendazole
  • Ivermectin
  • Moxidectin
  • Oxfendazole
  • Oxibendazole
  • Pyrantel pamoate
  • Prantel tartrate- daily
  • Pyrantel tartrate - single

Pinworms can be treated successfully with the same drugs that are effective against strongyles and ascarids.


Life Cycle and More Information About Pinworms in the Equine
Equine Pinworm Oxyuris equi Not Resistant to Pyrantel Pamoate or Ivermectin by: Stacey Oke, DVM, MSc April 11 2010, Article #16157/TheHorse

Top   Home

Bots (Genus Gasterophilus)

Bots are the larvae (immature worms) of the botfly. Since these flies are common to the horse's environment, it is almost impossible for a horse not to be infected.

During the warm months of late summer and early fall, adult botflies lay their eggs on the hair of various parts of the horse, especially the chest, forelegs, throat, and nose. The eggs look like small, yellowish, hanging threads about one half the length of a small piece of white rice. Stimulated by the horse's licking, the larvae hatch and enter the horse's mouth, where they settle in the tissues of the gums, cheek and tongue. The larvae migrate to the stomach in about a month where they attach in the stomach lining. It is not unusual for several hundred bots to attach to the stomach, causing irritation, interfering with digestion, and obstruction to the opening of the small intestine.

Bot larvae are passed in the feces after about eight to ten months. They burrow into the ground and pupate. They become adult flies in about a month, ready to start the cycle again by laying their eggs on the horse.

Horses do become infected and should be treated from the time botflies or "nits" are seen on the horse until about a month after the first hard frost. Botflies are killed by freezing temperatures. Several commercial anti-bot preparations are on the market and some are relatively toxic. It is wise to consult your veterinarian as to the type of drugs and frequency of treatment against bots as a part of your overall parasite control problem. I do not like to handle highly toxic materials, even with protective gloves, so I use a wormer paste given to the horse by mouth.

Note: Bot Relatives

"Bot flies (Gasterophilus spp) are the sole equine examples of a large family of flies known as Oestridae. Oestrid flies are all parasitic, but the various species only attack very specific types of hosts. This entire family shares a common survival strategy. Rather than leaving the fate of the next generation to the vagaries of climate, Oestrids ensure the survival of their offspring by parking them inside a nice, warm mammalian host during the winter months.

The means by which bot fly larvae enter the host, and the sites they occupy during the winter vary by species. Some examples: Hypoderma spp, also known as 'warbles' or 'cattle grubs' lay their eggs on hair shafts of the cattle's rear legs. The eggs hatch, and larvae penetrate the skin and begin a long migration to the back of cattle. During this migration, one species passes through the spinal canal, and another invades the esophagus. If certain parasiticides are administered during this migration, bot larvae could be killed while in these organs, causing inflammation which could result in paralysis or bloat, respectively. If the larvae survive their migration, they form large cysts under the hide on the topline of cattle, causing severe local damage which reduces the value of the leather.

Other bot flies deposit larvae, not eggs, directly into the nasal passages of sheep and reindeer. These larvae overwinter within the sinuses of the host. Still other Oestrids lay their eggs around rodent burrows, and are commonly found under the skin of squirrels and rodents during the colder months. These latter parasites (also known as 'warbles' or 'wolves') commonly cause abscesses in kittens and sometimes adult pets during late summer and autumn.

Gasterophilus spp only occupy the alimentary tissues of equids, and probably cause less damage than most of their cousins because they don't actively penetrate any tissues."

Craig Reinemeyer, DVM, PhD Got Bots?

Life Cycle and More Information on Bots in the Equine

Top   Home

Threadworms (Strongyloides westeri)

Threadworms affect mainly foals. They are mostly transferred to the foal by the mare's milk. If the mare is given an effective wormer treatment the day of foaling, most of the threadworm transmission does not occur. However, if the larvae enter through the skin the next stop is the lungs. There they cause bleeding and respiratory problems. After they mature the worms go up the windpipe, are coughed up and swallowed where they end up in the small intestine. If left untreated in foals, threadworms can cause diarrhea, weakness, weight loss and the foal may fail to thrive and grow at a normal rate.

Top   Home

Hair Worms (Trichostrongyles Axei)

Hair worms are mainly seen in foals. The adult worm occurs in the stomach and small intestine, irritating and eroding the finger like projections or villi of the gut. Hair worms can cause severe damage and may cause bleeding into the intestines. Dark, foul smelling diarrhea can occur because the villi are unable to digest food properly.

Top   Home

Lungworms Dictyocaulus Arnfieldi)

Foals are quite susceptible to lungworms. In fact, if they have a large load of lungworm larvae foals can die from lungworm infection. The larvae go through the walls of the intestine into the circulatory system and travel to the lungs. If there are large numbers of lungworm larvae present in the lungs, the lining of the small air sacs in the lungs can become irritated causing a severe cough. Older horses are less susceptible because they develop an immunity.

Top   Home

Summer Sores/Habroniasis

Photo by Andrzej Polozowski
Summer Sores, Jack sores, Bursatti, Habroniasis and Draschia megastoma in Horses
The adult habronema lives in the stomach of the horse.

Photo by Cookie Originals
Habronema infection (known as habronemiasis) commonly manifests as "summer sores," ulcerations of moist areas of horse's skin.

Summer Sores or Habroniasis - Presents much like Neck Threadwords AKA onchocera or Microfilariae below-Like with Onchocera, ask your vet if your horse should have the rhino/flu shot. It can cause flare ups of uveitis in some horses if the parasite is in the eye.

Veterinary Corner 08/01: Summer Sores (Habronemiasis)

Top   Home

Neck Threadworms/(Onchocera)or (Microfilariae)

Ask your vet: Through my research for HorseHints, I found that horses with uveitis or neck threadworms (Onchocera, Microfilariae), should probably not have the rhino/flu shot. The shot has been shown to cause flare-ups of uveitis in some of these horses. Horses who have onchocera that has traveled to the eyes are definitely at risk for flare-ups. "The major viral infections linked to equine uveitis are respiratory equine herpesvirus and influenza virus. There are other possibilities but it has been difficult to link them directly...The most common parasite connected with uveitis is onchocerca." Equine Recurrent Uveitis: Information for the Horse Owner For that reason we do not introduce even the killed virus vaccine of the Rhino/Flu shot to my husband's horse, Rusty. We have weighed the risks and decided since we do not show or compete with our horses, and since we are in a stable boarding environment with only 3 horses, no other horses in or out, it has not been necessary. Rusty is also a gelding. If there is an outbreak in our area then, of course, we may revisit this decision. In that case we would premedicate with a shot of banamine or give bute for two days prior to the injection.

What is a Neck Threadworm, and can it Hurt My Horse?
How to Fight the Big Fight against Neck Threadworms
Treatment of equine onchocerciasis with ivermectin paste/PubMed

I have had two horses with Onchocera. It is often misdiagnosed because many vets have not seen it. My husband and I purchase 2 year olds and then proceed to do our own training and handling, etc. A vet check will not pick this up because it does not usually show signs until later (around 2 years of age). You will see several types of symptoms. The horse will get summer sores or bumps along their top and bottom lines. These are caused by the dead worms. Sometimes the bumps will break and infect. The horse will often rub his bottom line along the ground rocking back and forth because of irritation. You might see large losses of hair on the horse's body. Also, the mane and tail hairs break off and can get quite nasty looking. Along the back neck line where the mane attaches, you can feel indentations in a tidy row down the horse's neck line. The neck threadworms migrate to this area and die. This is what you are feeling. Sometimes the Onchocera gets off its usual path to the top and bottom line of the horse, and travel to the eyes instead. We have had both happen. The horse tends to rub his head on everything because of the irritation. "The most common parasite connected with uveitis is onchocerca. Culicoides, a biting midge of the Ceiatopogonidae family, is believed to be the primary transmitter. The adult lives in the connective tissue of the horse's neck and the microfilariae travel throughout the body. The most common signs of it are sores breaking out on the midline of the horse's stomach, base of the mane and withers and uveitis in the horse's eye(s) (French, 1988). Uveitis occurs when there are large quantities of dead microfilariae in the eye. Normally the eye can handle the live ones but the dead give off large amounts of antigens and these cause inflammation in the eye (Schwink, op cit p. 560). Ironically for horse owners, onchocerca can sometimes first be identified by the onset of symptoms following worming with ivermectin. This is one drug that will effectively kill off the young microfilariae, but at the same time by doing its job it can initiate uveitis if a large quantity of the microfilariae are in the eye at the time of worming. After uveitis has started, some owners find that administering bute or banamine several days before and after worming can control the inflammation so that the uveitis does not flare up every time the horse is wormed. This also might indicate that in dealing with abandoned or abused horses who may not have been wormed on a regular basis, consulting a veterinarian regarding the possibility of onchocerca microfilariae in the eye before worming may prevent uveitis in addition to its other problems. A conjunctival biopsy can be used to identify onchocerca microfilariae in the eye, but it does involve using auriculopalpebral nerve block and topical anesthesia. Once the inflammation has quieted, treatment can commence. Diethylcarbamazine and ivermectin are two drugs that are used (Cook, 1983)." Equine Recurrent Uveitis

You will see a constant water stream out of one eye or both eyes. There may be a white or yellow mucous in the eye on a regular basis. Many times vets will think this is a blocked tear duct. Again, there may be hair loss around the face and eye area. Ivermectin is the conventional treatment, but it did not really work for our horses. My vet of 24 years, was an incredible vet. Nancy Sitarz had a diagnostic sense unlike any that I have ever seen. Unfortunately, she is not in private practice any more. We surely miss her. She had a way of thinking outside the box. We had Wedgewood Pharmacy compound a pill for our horses that has worked really well after the Ivermectin did not. It is called Diethylcarbamazine Citrate. The capsules are each 400 mg. It is basically heart worm medicine compounded into capsule form. We open two capsules, once a week, and place the powder in a cup of grain and the horse eats it. No problem! Spring time is when the Onchocera becomes most active. This has worked for both horses. You can discuss this with your vet. If not managed, complications can be blindness if in the eyes, and if left untreated, adult worms can travel into the ligaments and tendons of the horse. The horse may eventually fall and stumble. They can cause swelling and bumps because of the hardening of dead worms.

Unlike many other worms the neck threadworm has an indirect life cycle. The threadworm relies on another organism to get it to the horse. The biting midge transmits the threadworm from horse to horse. It can, however also be transferred in utero, as well.

According to Dr. Robert Ogelsby, Onchocerca is a "parasitic filarial worm (nematode)." The adults live in the connective tissues of horses. There are several different types of onchocerca - Onchocerca Reticula prefers the suspensory tendon and flexor ligament where it can cause swelling and lameness. Onchocera Cerviculus is found in the ligamentum nuchae . Their larvae, or microfilariae, are ingested by gnats and midges. Horse reactions to the larvae can vary ranging from mild irritation, skin scurf (weeping patches with thick scabs embedded in the hair primarily on the lower legs and muzzle; easily confused with sunburn), lesions, swelling, and even unsoundness. Sometimes the microfilaria migrate into the eye of the horse. Some believe this is implicated in some forms of uveitis. While sweet itch is a distinct and separate condition, the midline dermatitus caused by neck thread worms is often misdiagnosed as sweet itch.

Treatment: Some worming protocols state that regular worming with ivermectin will kill the larvae of neck thread worms and that there is no way to kill the adults. If you check out the COTH thread, you will read about many people finding success with a double (by weight) worming with Equimax, repeated in two weeks. The Federal Food and Drug Administration has approved the use of ivermectin and praziquantel oral paste for the treatment of dermatitis caused by neck threadworm microfilariae. However, the COTH thread's suggested dosage was established by antedotal accounts of success treatments. Baba Yaga's Mirror Blogspot

Other research info can be found by following the following links:

Onchocerciasis/Merck Vet Manual
Veterinary Notes for Horse Owners By Captain M. Horace Hayes, Roy Knightbridge
Onchocera in Horses From Western Canada and Northwestern US
Hypersensitivity of Horses to Culicoides Bites in British Columbia
Efficacy of Ivermectin Against Onchocerca Cervicalis Microfilarial Dermatitis in Horses.
Equine Board Review
Can Neck Threatworms Hurt My Horse?

Top   Home

Guinea Worm or Dracunculus Medinensis

Dracunculus Medinensis or Guinea Worm

Dracunculus Medinensis or Guinea Worm

Top   Home

Chiggers, Jiggers, Redbugs and Harvest Mites

Chiggers , also called jiggers, redbugs and harvest mites, are among the tiniest of arachnids. Only the microscopic 1/125-inch larvae feed on blood; chigger nymphs and adults do NOT bite.

1/125-inch larvae

Chigger Bite on Human Skin-Nasty, itchy, and long lasting! Our skin doctor gave us an RX for Vanos Cream--it helps allot!

Top   Home

Face Flies (Musca Autumnalis)

Face flies are a real bother and feed on secretions around the eyes, nostrils and mouth. They are after the water. Good pasture and barn management help to prevent lots of flies. Also, fly masks can be worn by your horse. Repellent is often used which contains pyrethrum. I use white vinegar and it seems to work well and is really inexpensive. We also have fans at the barn and the horses can come and go as they wish. You can stable your horse during peak fly activity.

Top   Home


There are two species of lice that can infest horses. They cause skin irritation, self mutilation and loss of condition by your horse. Lice peak in late winter and spring, and once established, recur annually. Horse lice do not affect humans or other types of animals. They are host specific. Lice spread from infested horses, tack, rubbing on trees, fences, barn, etc. Lack of grooming of long winter coats set up conditions for lice. Lice live only a few days on the horse. Lice are blood suckers and mainly found under the forelock, mane, tail and fetlocks. Bad infestation can go full body. They are common on the backline, neck, head and flanks. Look for grayish lice eggs under the forelock. Do it quickly as lice do not like light and move quickly when exposed to it. To treat lice the horse is thoroughly washed with an insecticidal produce that your vet will give you. Organophosphate insecticides are often used. Treatment is usually repeated in 7 to 10 days in order to break the life cycle. It takes at least 14 days for the contaminated equipment to become lice free as well. If your horses are kept in a stable, they must be taken out of the stable so that they do not get reinfected. The lice must be deprived of their blood sucking ways! Get the horse out!

Mange Mites

Mange mites cause severe itching and are spread by contact. Call your vet for a diagnosis and treatment schedule.

Top   Home

Parasite Control In Foals

Up to one year of age foals should be treated with drugs effective against ascarids at 2 month intervals. This program should begin at 2 months of age, depending upon the time of year. Treatments for cyathostomes should be included. Treatment for S. vulgaris should begin after a foal is 6 months of age. This program needs to be customized to your particular situation: climate, season of year, when the foal was born. Discuss this with your vet.

Top   Home

Mature Horses (Parasite Control)

I worm my horses every two months. (Note: We have cut back on the worming schedule as some parasites are becoming resistant to the wormers. We are following the recommendation of our vet and also several seminars that my husband and I have attended are recommending the same. You can read about this recent information on HorseHints) The horse with onchocerca I give Ivermectin on the off months. My home page has a worming schedule that works really well. All horses should be wormed at the same time, otherwise, worming does little good. One untreated horse can contaminate the entire environment for all the other horses. A daily worming schedule can also be done. Daily wormer can be put in each horse's food. However, there many vets do not recommend this way of worming because the parasites often build up an immunity to the treatment and recent research also shows that many parasites are building up a resistance to Ivermectin.

See also Dr. McKenzie's article on Internal Parasites
Keeping Worms At Bay

Top   Home

Internal Parasite Slideshow

This article is for informational purposes only. I am not a vet. Always check with your vet.

Medical Index