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Large Strongyles The large strongyles, known as bloodworms, consist of Strongylus vulgaris, S. equinus and S. edentatus, are long, fat worms that live in the large intestine after migrating from the liver. Of the 3 species, S. vulgaris is the most pathogenic and was in the past considered the most common and dangerous equine parasite. Eggs of the three species of large strongyle can develop into infective larvae on pasture in as little as three days if warm, moist conditions exist. Once the larvae have been swallowed, the larvae drop their protective coating, or "sheath", on their bodies and migrate to different organs for further development. The larvae of S. vulgaris move to the horse's arteries where they "go with the flow" of the blood for approximately two weeks. When they reach the mesenteric artery, the main artery that feeds the gut area, they stay there for approximately four months while they continue growing. Then they return to the large intestine through the arteries. Once in the large intestine, the larvae burrow into the intestinal cavity. After six to eight months, the worms are mature and eggs begin to pass into the manure.
S. equinus larvae move to the liver after they have shed their protective coating. They stay there for approximately six weeks, then move through various abdominal organs to the large intestine. After about nine months, adult worms lay their eggs. S. edentatus also move to the liver where they remain for approximately nine weeks, then move to the lining of the abdominal cavity where they form nodules. The larvae also form nodules in the gut wall, which they break open to serve as a doorway into the large intestine.
Of the three species, S. vulgaris does the most damage. That's because this species uses - and damages - the blood system as it moves around the horse's body. During their wanderings, the larvae rough up the walls of the arteries, leaving "tracks" in the lining. These tracks are perfect spots for blood clots. The clots break away from the walls and lodge in other blood vessels, blocking the blood flow to the intestine below the clot. The rough walls of blood vessels not only promote blood-clot development, but also weaken the walls. If the blood-vessel wall is weakened enough, it can burst, causing immediate death. If the blood clots block blood flow to the hind quarters, lameness and/or weakness can result. When they set up housekeeping in the large intestine, the worms literally bite off pieces of it. This may lead to severe colic, diarrhea, fever and anemia from the bleeding "bite wounds" in the stomach and intestine. Small Strongyles The scientific name for this group of parasites is cyathostomes, for which there are nearly 40 different. Often bright red in color, the adult worms range in size from 4 mm to 26 mm (about 1/8 inch to 1 inch). The adult female worm lays her eggs in the horse's large intestine. The eggs pass from the horse in manure, then hatch into larvae and mature through three stages, the last of which, L3, is infective to horses. L3 larvae attach themselves to grass and are eaten by the horse. After a horse swallows these larvae, they burrow into the intestinal wall, mature and emerge into the large intestine where they feed and lay eggs.
A horse severely infected with small strongyles can suffer weight loss, diarrhea and colic. However, most infestations are "subclinical", meaning that there are no outward signs of the damage that is occurring inside. New resarch shows that penetration of the horse's intestinal wall by small strongyles contributes to the disease that has traditionally been connected to larval emergence into the large intestine. In recent years, cyathostome resistance against some of the common dewormers has been observed, including ivermectin. Horses that suffer the above signs or have recurrent colic should have a fecal egg count performed prior to and after deworming to ensure that parasite resistance is not a factor. Roundworms The scientific name of roundworms is Parascaris equorum, commonly known as ascarids and roundworms. Ascarids are a rigid, heavy-bodied worm. Adults can reach up to 50 cm long (about 20 inches). Adult female ascarids lay eggs which are passed in manure to the pasture where they develop into infective larvae in 10 days to six weeks. The horse swallows the larvae, which hatch and burrow into the walls of the intestine. From there, they are carried by the blood stream into the liver and lungs, then move up the horse's windpipe. The horse coughs up the larvae and swallows them again. They mature to adults in the small intestine.
Ascarids are especially dangerous to foals aged one year or younger. Severe infection in horses this young can build up quickly and lead to liver and lung damage, poor overall growth and even death. As the larvae are moving through the blood system, foals can experience coughing, fever, pneumonia and lead to other respiratory infections. Ascarids in the adult stage live in the small intestine where they can cause colic, blockage, ruptured gut and death. Severely infected foals may experience alternating bouts of foul-smelling diarrhea and constipation. Infected foals can lose weight, but my have a pot-bellied appearance. They often have low energy. Foals need to be on an adequate deworming program from one month of age in order to reduce the risk of ascarid impaction, which usually requires surgical intervention to correct. Tapeworms The equine tapeworm (Anoplocephala perfoliata) was once though to be of no consequence to its host. However, in the past 15-20 years, these parasites have been found to be associated with colic and their control is essential to horse health. Although there are 3 species of equine tapeworms (Anoplocephala perfoliata, Anoplocephala magna and Anoplocephaloides mamillana), A perfoliata is the most prevalent and pathogenic. It measures 1-2 inches long and about ½ inch wide. It attaches to the intestine at the ileocecal junction. This junction is at the end of the small intestine (ileum) as it attaches to the cecum and there is an important valve between the two parts. Inflammation causes by the parasite leads to dysfunction of these portions of the intestinal tract.Tapeworm infestation does not cause the typical “wormy” appearance we associate with other intestinal parasites. Tapeworms are extremely hard to diagnose as the parasite or its eggs are rarely passed in the feces. Therefore, fecal exams are usually negative for this parasite. Recent development of blood tests that test for the presence of antibodies made by the horse against the tapeworm have allowed an assessment of the presence of infection. In addition, the level of antibody detected correlates with the intensity of infestation.The life cycle of the tapeworm also differs from other parasites in that it involves an intermediate host, the forage mite. This mite ingests tapeworm eggs that are passed in the horse’s manure. The eggs mature inside the mite to the infective form (cysticercoids). Because the mites are frequently present in grass, hay and straw, the mites accidentally become ingested by the horse, release its contents and infect the horse. The cysticercoids developed into adult tapeworms and attach to the intestinal wall. Eggs are passed when the tapeworm body segments, filled with eggs, detach and are passed in the feces. Young and old hoses appear to be most affected by this parasite.The following abnormalities have been noted with tapeworm infections:· Thickening of the ileum· Ulceration of the mucosa (inner lining of the intestine)· Thickening or dysfunction of the ileocecal valveDue to the above changes, the diseases associated with tapeworms include:· Small intestinal rupture· Intussusceptions· Small intestinal twists· Ileal impactions· Spasmodic colic: the most common disease caused by tapeworms It is likely that a large number of mild colic episodes (“gas colics”) may be due to unknown tapeworm infections. Bot Flies Bot flies are known scientifically as Gastrophilus intestinalis and G. nasalis. Adult flies are brown, hairy and bee-like, with one pair of wings, and measure about 3/4". The larvae (bot) is also 3/4" long with a narrow, hooked end and a broad, rounded body. After a 3-week developmental period in the mouth, bot fly larvae migrate and attach themselves to the mucus lining of the horse's stomach and remain there during the winter. After about 10 months, they detach from the lining and are passed out of the body through the feces. The larvae burrow into the ground and mature. Depending on the conditions, adults emerge in 3 to 10 weeks. Adult females deposit eggs on the horse's legs, shoulders, chin, throat and the lips. Depending on geographic location, the life cycle of bot flies is not fixed to only certain times of the year and bot larvae can be active in horses anywhere from August to May.
Egg laying begins in early summer. Eggs of different species differ in color and placement. G. intestinalis lays up to 1,000 pale yellow eggs on the horse's forelegs and shoulders. Moisture and friction from the horse licking itself cause the eggs to hatch in about seven days. G. nasalis lays about 500 yellow eggs around the chin and throat of the horse. These eggs are not dependent on the horse licking them to hatch. After hatching, G. intestinalis larvae are licked into the mouth. G. nasalis burrows under the skin to the mouth, there wandering through the mouth for about a month before migrating to the stomach for overwintering.
Horses that show no outward signs of illness can be severely infested, giving no clue of the potential damage occurring inside. However, some horses do show signs of infestation, including an inflamed mouth area and stomach irritation. Infestation with bot larvae may cause holes in the stomach lining. If the infestation is severe, the opening from the stomach to the intestines may be blocked, which can cause irritation, ulcers and even colic. The burrowing larvae can cause small tears in the skin, which can become infected. "Dive bombing" adult flies cause nervousness in horses. Pinworms The scientific name of the pinworm is Oxyuris equi. The female pinworm gives the species its common name, due to her characteristically long, pinlike tail and large body. Females can be as long as 150 mm (about 6 inches) in length. The male pinworm is much shorter, reaching a maximum length of 12 mm (about 1/2 inch). Pinworm eggs are whitish yellow in color and are usually found in masses near the horse's anus. Larvae mature in the large intestine, feeding off the intestinal lining for four to five months. Adult female worms move from the large intestine to the horses's anal area. The eggs she lays are covered with a sticky fluid that causes severe itching.
Most of the effects of pinworm infection are on the horse's appearance and comfort level. Heavily infected horses may be nervous and may decrease eating. The severe itching makes the horse rub his tail and rump so relentlessly that the tail hairs break off. This condition is called "rat tail". Also, rubbing, biting and scratching can open up the skin in the tail and anal area to infections. Threadworms The scientific name for intestinal threadworms is Strongyloides westeri. Threadworms are long and hair like. Adults grow to be 8 to 9 mm in length (about 2/5 inch). Larvae mature to adult worms in the small intestine. Infection can occur by eating infective larvae or by penetration through the skin. If the larvae enter the horse's system through its skin, they move to the lungs, then up the wind pipe where they are coughed up and swallowed. They mature in the small intestine, where adult females lay the eggs that are passed out in the manure. The infective larvae of the threadworm can either be swallowed by the horse or the larvae can go through the horse's skin. Young foals can also be exposed to the worm by nursing from their infected mothers.
If the larvae enter through the skin the next stop is the lungs, where they can cause bleeding and respiratory problems. Inflammation and rashes can develop where the larvae penetrated the skin.
The worst damage occurs in untreated foals. Infected through their mothers' milk, they can suffer diarrhea, weakness, weight loss and failure to thrive and grow at a normal rate. Veterinarians generally recommend deworming your foal early in life and as often as every month until weaning. Diarrhea in 7-10 day old foals may be caused by intestinal threadworms. Deworming may be beneficial in these cases. Be very careful to read your dewormer's label - some products cannot be used on foals this young. Hairworms The scientific name for hair worms is Trichostrongylus axei. The adult worm is tiny (0.5 cm long) and hair-like. The adult worm occurs in the stomach and in the small intestine, irritating and eroding the finger-like projections, or villi, of the gut, damaging the capillaries and lymph vessels within the villi. The horse eats grass infected with eggs of the stomach hair worm. The eggs hatch and the larvae move into the stomach where they mature and lay more eggs which are passed in the manure.
When a horse is infected with hair worms, he is usually also infected with other worms. Damage caused by the hair worm can be worsened by other worms in the system. Signs of infection may be dark, foul-smelling diarrhea, because the damaged villi are unable to digest and absorb properly in the intestine. If there is severe damage to the villi, the underlying small vessels could be damaged as well, causing bleeding into the intestines. Bleeding may lead to anemia and loss of condition. Foals are very open to infection by hair worms, so broodmares should be dewormed and moved to clean pastures. Lungworms The scientific name of lungworms is Dictyocaulus arnfieldi. The horse lungworm is long and slender, measuring about 2 inches (25 to 70 mm). The larvae go through the walls of the intestine and into the circulatory system. They are carried in the circulatory system into the lungs, where they mature. The eggs pass through the horse's system through the manure. Female lungworms lay eggs containing larvae. The horse eats the eggs of the lungworm off damp grasses. Donkeys are common carriers of lungworms and are usually responsible for the transmission of lungworms to horses.
If there are large numbers of the larvae present, the lining of the small air sacs in the lungs, called the bronchioles, may become irritated causing the horse to have a severe cough, difficulty breathing and loss of appetite. These are signs of bronchitis. Infection is usually light in older horses because they develop resistance to the parasite and usually have no signs. If foals are infected, they could die from a lungworm infection because they have less immunity. Stomach Worms The scientific names of stomach worms are Draschia megastoma and Habronema muscae. Adult worms are white in color and about 10 to 25 mm long (about 3/8 inch to 1 inch). The larvae of these parasites are ingested by the common housefly or stable fly maggots, which develop in manure. The worms develop inside the flies' maggots. The flies mature and then deposit the larvae on the lips, nostrils, wounds and other naturally moist areas of the horse. The larvae may stay in the area of the wound or may be swallowed by the horse.
A small wound infected with Habronema and Draschia larvae can grow much larger, and the larvae prevent healing. These chronic, non-healing wounds are commonly called "summer sores". Larvae deposited in the eyes can cause conjunctivitis (swelling of the mucus lining of the eye lids). Larvae that are eaten can cause gastritis and the formation of tumor-like growths, which may rupture.
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