Strongyle Infections in Horses

Strongyles are an extremely common, worldwide parasite that lives in every climate. In most cases, parasites activate the immune system to keep the infection under control, and horses show no symptoms of infection.

Horses that show symptoms of strongyles infection have a greater than usual burden of parasites and require immediate veterinary attention.

Large strongyles were once considered the main parasite of concern, however in recent years they are less and less prevalent and the diseases they cause are becoming rarer.

Small strongyles, on the other hand, are rising in prevalence. This is in part due to increased drug resistance in these parasites.

The severity of a strongyles infestation depends on which species is present, how many there are, where in the system they are, and what they are doing.

Among the large strongyles, there are three major species. Strongylus edentatus and Strongylus equinus migrate to the liver. Typically, they do little damage.

The third large strongyle is Strongylus vulgaris, also known as the bloodworm. These parasites tend to migrate to the walls of the arteries that supply blood to the small and large intestines. Their migration often causes blood clots and scar tissue, which can affect the gastrointestinal system of the horse. In severe cases, the migration of Strongylus vulgaris can impede blood flow to the intestine, causing death of intestinal tissue, called an infarction.

Symptoms of intestinal infarction include:

• Fever
• Complete or partial absence of stomach noises
• Pale mucous membranes
• Signs of severe colic

In some cases, this condition is fatal.

Among the small strongyles, there are more than 50 identified species. Small strongyles migrate into the large intestine where they make a hole in the lining of the intestinal wall and form a cyst. This encystation occurs shortly after ingestion of early larval forms by the horse.

In most cases, these larval stages develop appropriately and form adults, emerging from the intestinal cyst. In some cases, for reasons that are not fully understood but may relate to a triggering event in the climate, the larvae enter a dormant (inactive) state instead. These larvae remain encysted in the intestinal wall, and can emerge in massive numbers following seasonal changes or deworming. This condition, called larval cyathostominosis, damages the intestinal wall and leads to extensive fluid and protein loss.

Symptoms of larval cyathostominosis include:

• Sudden, excessive, watery diarrhea
• Signs of severe dehydration including tacky gums, sunken eyes, and skin that does not return to its normal position quickly when pinched
• Swelling of the chest or legs
• Severe weight loss
• Fever
• Severe colic signs

In many cases, this condition is fatal.

Strongyles are a large group of roundworms. They live part of their life cycle in the lower intestines of the horse and part of their life cycle in the environment.

The life cycle of the strongyles consists of the following stages:

• The egg which is passed from the horse to the environment in the feces
• Early larval stages in the environment which are ingested by horses grazing or drinking from infected sources
• Mature larval stages which migrate to the intestines or other systems in the body
• Adult stage which is where it attaches to the lining of the intestine and produces eggs

The length of time this cycle takes depends on the specific type of strongyles.

All horses have parasites. Typically, parasites cause no problem for the horse, either because the horse’s immune system is able to control the infestation or because the horse’s management includes an appropriate anti-parasitic routine. Only in cases where the body is overwhelmed by strongyles do symptoms occur.

Symptoms also often include colic which appears as:

• Restlessness including getting up and laying down often
• Kicking or biting at the flanks
• Groaning
• Sweating
• Curling the lip back (Flehmen response)
• Grinding teeth
• Rolling
• Labored breathing
• Pawing the ground

Horses with the symptoms of strongyles infection require immediate veterinary care. Horses with the symptoms of intestinal infarction or larval cyathostominosis require emergency veterinary attention.

Diagnostic tools include:

• Physical examination
• Bloodwork
• Rectal palpation
• Nasogastric intubation
• Fecal egg count
• History of anthelmintic use (also called anti-parasitic drugs)
• Diagnostic imaging such as abdominal ultrasound

In the case of a suspected intestinal infarction, a surgery called an exploratory laparotomy is used to remove the dying intestinal tissue and alleviate the blood flow obstruction.

In the case of larval cyathostominosis, treatment includes:

• IV fluids
• Anti-inflammatory medications
• Antibiotics to avoid secondary infections
• Nutritional support
• Blood thinners

Certain anthelmintics (anti parasitic medications), such as moxidectin, are successful at killing the small strongyles larvae in the dormant state. These products are recommended to prevent larval cyathostominosis.

Treatment for strongyles infection is anthelmintics. Over the past several decades, anthelmintics have become less effective as strongyles parasites have developed resistance to them. It has also been noted that some horses pass far greater levels of eggs into the environment than other horses. Therefore, it is recommended that fecal egg counts are used to identify “high shedding” horses and treat them with appropriate anthelmintics more frequently than other horses. “Low shedding” horses are often dewormed once or twice a year, to reduce exposure of the parasites to deworming medications and maintain the efficacy of these drugs. In cases where anthelmintic resistance is suspected, a repeated fecal egg count after deworming can help confirm the efficacy of the deworming product used.

Almost all horses are infected with strongyles parasites. Mild infections are not associated with disease or shortened lifespan.

Horses with overwhelming strongyles infections have a good prognosis with appropriate treatment, prior to the severe manifestations of disease occurring.

Horses with intestinal infarction or larval cyathostominosis have a guarded prognosis, and survivors may have a prolonged recovery period.

Thoughtful, targeted use of anthelmintics under the guidance of a veterinarian is usually sufficient for prevention of overwhelming strongyle infection. Anthelmintics that kill small strongyles during the dormant stage are recommended to prevent larval cyathostominosis.

Strongyles infection is very common, with almost all horses having some degree of infection. Overwhelming infection is less common. Intestinal infarction and larval cyathostominosis are rare.

• Anthelmintics