Internal Parasites & Symptoms

Small Redworms (Cyathostomes)

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The small strongyle (cyathostomins) is considered to be the number one nematode and most common intestinal parasite of adult horses. They become encysted within the intestinal wall by the immune cells, they can reside up to two years before taking their place as adult worms in the intestine. Sudden emergence of the larvae is known as cyathostominosis, and can follow deworming that removes the adults, or in response to favorable conditions for development in the pasture. Significant inflammation and injury to the intestinal wall can cause profuse diarrhea and can be fatal. The severity of clinical signs is related to the deree of damage to the intestines, which varies with the level of infection. Unfortunately, there is no current test available to assess the larval burden in the horse.

 

Symptoms can include:

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• Weight Loss

• Diarrhea

• Colic

 

Horses can be infected with small strongyles and not show overt signs of disease. Small strongyles have been implicated to cause subclinical effects.

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Symptoms can include:

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• Decreased Feed Efficiency

• Decreased Rate of Gain

• Decreased Performance

 

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Large Redworms (Strongyles)

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There are several different varieties of strongyles worms included in the term large redworms. They are considered to be one of the most dangerous worms to infect horses; they eat through the lining of the gut wall and travel through the blood vessels of the gut causing significant bleeding and damage. After being ingested, the eggs of the worms reach the horses bloodstream. While they are in the blood vessel, they tend to block blood flow, which damages the tissue that is supplied by the blood vessel.

 

Symptoms can include:

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• Colic

• Rapid Weight Loss

• Diarrhea

 

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Pinworms (Oxyuris Equi)

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Pinworms are one of the causes of tail rubbing in horses. The female crawls out of the anus and lays her eggs on the skin in this region. As a consequence of rubbing, horses can spread pinworm eggs throughout the horse’s environment. Transmission can occur in stalls and from contact with grooming materials, tail wraps, fence posts, etc. Furthermore, pinworm eggs are hardy and can persist on the perianal region and in the environment for relatively long periods of time. Use of dewormers should always be followed by a thorough cleansing of the area under and around the tail and anus to prevent reinfection.

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Symptoms can include:

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• Tail Rubbing

• Intense Itching

• Loss of Hair

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Lung Worms (Dictyocaulus Arnfieldi)

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Lung worm is an infection of the lower respiratory tract in horses, usually resulting in severe bronchitis is often accompanied by chronic pneumonia, pulmonary edema, and secondary bacterial infection, caused by the parasitic roundworm Dictyocaulus arnfieldi. The infection can cause severe coughing in horses and can be difficult to distinguish from other respiratory diseases. Donkeys, which usually show few signs of the infection, are the prime source of pasture contamination for horses.

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Signs of lung worm infection range from moderate coughing with slightly increased respiratory rates to difficult or labored breathing in older horses. Infections with few or no visible signs can occur in foals and donkeys. Heavy infections can also block small airways, leading to fatality, particularly in foals.

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Symptoms can include:

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• Cronic Coughing

• Elevated Respiratory Rate

• Bilateral Nasal Discharge

• Exercise Intolerance

• Rapid Weight Loss

 

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Bots (Gastrophilus)

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Stomach bots are not worms, but rather the larvae of the botfly. Female botflies lay their eggs by attaching them to the hairs of the horse. Different species lay their eggs on different parts of the horse’s body (legs, jaw, lips, etc.). The eggs are tiny, yellow/orange in color and attach to the hairs of the legs or throat. The eggs on the legs are stimulated to hatch by the lip action and warm saliva as the horse licks its leg. The eggs around the nostrils and lips spontaneously hatch in one to one and a half weeks. Larvae attach and burrow into the tongue and gums of the mouth and incubate there for three weeks. Bot larvae can cause small ulcers on these areas of the mouth. After incubation, they are swallowed and attach to the lining of the non-glandular or upper part of the stomach or the duodenal ampulla. Bots spend approximately nine months attached to the stomach lining before passing in the manure. This typically occurs in late winter to early spring. 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, subsequently developing into adult flies.

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The adult flies are active from late spring to the killing frost in the late fall. Botfly larvae likely cause minimal damage to the stomach, but may cause problems such as obstruction or damage to lining of stomach if present in large numbers.

 

Oral Stage Infection Symptoms can include:

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• Excessive Salivation

• Head Shaking

• Tongue Irritation

• Chewing Issues

• Ulcerations In and Around the Mouth

• Inflammation in the Throat

• Ulceration in the Stomach

 

Stomach Stage Infection Symptoms can include:

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• Colic Symptoms

• Gastric Ulcer Symptoms

• Tongue Irritation

• Chewing Issues

• Ulcerations In and Around the Mouth

• Inflammation in the Throat

• Ulceration in the Stomach

 

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Roundworms (Ascarids)

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The horse roundworm is a very large (females may be up to 15 inches long), yellowish white nematode (worm) that may pass in the feces. The life cycle of the roundworm starts when the horse consumes grass, feed or from surfaces such as feeders, stall walls or water contaminated with the eggs. Adult females pass eggs into the horse’s feces, where they spend about 10 days in the environment before they are capable of infecting a new host. Development of the egg to second stage larvae requires the correct temperature (77 to 95 degrees), but once developed to that stage, these hardy larvae can persist for 5 to 10 years.

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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 three 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, ranging from mild digestive upset and lower feed absorption, to severe colic, due to intestinal blockage or intestinal rupture. Intestinal blockage is more likely following the use of an effective anthelmintic or dewormer, as the now dead parasites essentially bind up in the intestine.

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Roundworms are generally found in horses less than 3 years old and most commonly found in weanlings and yearlings. They are especially dangerous for horses with a weak immune system.

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Symptoms can include:

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• Potbelly

• Dull/Rough Hair Coat

• Weight Loss

• Depression/Lethargy

• Nasal Discharge with or without a Cough

• Stunted Growth

• Diarrhea

• Liver Damage

• Intestinal Blockage & Rupture

 

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Tapeworms (Anoplocephala Perfoliata)

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The adult equine tapeworm is a flatworm, approximately 3 inches in length. The life cycle of this parasite requires an intermediate host, which is a tiny pasture mite that feeds on horse feces. Gravid segments (or those carrying eggs) of the tapeworm break off and pass through the horse.

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A horse can then become infected with tapeworms by consuming grass or hay contaminated with mites containing tapeworm eggs in their infective stage. There are typically millions of these pasture (orbatid) mites on a pasture. Adult tapeworms live or attach to a horse’s intestines at the ileocecal junction, or where the small intestine terminates at the cecum. Heavy infections of tapeworms can cause inflammation, ulceration and thickening of this area of the intestines. The injury to the intestines can be severe enough to cause obstruction or other serious intestinal abnormalities, which may result in acute intestinal pain and may require surgery. Pasture-housed horses are at a greater risk of tapeworm infestation. There does not appear to be any age-related immunity to tapeworms, as they are found in all ages of horses.

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Symptoms can include:

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• Diminished Performance

• Pain and Discomfort

• Increase Flatulence

• Colic Symptoms

• Depression/Lethargy

• Weight Loss

• Nutritional Deficiency

• Dull/Rough Hair Coat

• Lack of Shedding in Warm Weather