AffectedAnimals:
Foals, younger horses, and on occasion debilitated older horses are most at risk for developing clinical signs of roundworm infection.

Overview:
Equine roundworms, or Parascaris equorum, are parasites that usually affect younger horses. The disease is especially common in horses raised on pastures that are used for new foals on a yearly basis. These pastures usually have a high number of Parascaris equorum eggs, which horses will eat while they graze. After the horse ingests these eggs, and the eggs hatch, roundworms will infect various organs, causing illness to occur.

Common symptoms of a roundworm infection include respiratory signs such as coughing and pneumonia, colic, pain in the abdomen, depression, and failure to thrive. A high concentration of roundworms may cause life-threatening intestinal rupture or obstruction.

Treatment and prevention of Parascaris equorum require a regular deworming schedule for all horses that are out to pasture, and good pasture management.

Clinical Signs:
Clinical signs of Parascaris equorum include respiratory signs such as coughing and nasal discharge, depression, anorexia, lethargy, a poor coat, reduced weight gain, failure to thrive, diarrhea, a pot belly, and varying degrees of colic.

Symptoms:
See clinical signs.

Description:
Parascaris equorum usually infects young horses or foals living on pastures with other infected horses. Parascaris equorum eggs are shed by infected horses, and may survive in the soil for many years, thus contaminating entire pastures for long periods of time.

Once ingested, roundworm eggs will hatch into larvae once within a horse's intestines. These larvae migrate out of the intestines and into organs such as the lungs and liver. Some larvae move from the lungs up the trachea, or windpipe, and into the oral cavity. Here they are swallowed again, reaching adulthood in the small intestines.

The migrating larvae damage the lungs, liver, and small intestines. The lungs and liver may suffer significant damage resulting in respiratory signs such as coughing, nasal discharge, and, in severely affected horses, pneumonia. Other symptoms include depression, loss of appetite, and debilitation.

Adult worms are large, white, and as long as 20 to 30 centimeters. Large numbers of adult worms may clog the digestive tract and block the passage of digesta, decrease normal gut movements, and may cause the intestines to rupture their contents into the abdomen. Consequently, roundworm infection in horses is a serious condition that may cause sudden death in the case of intestinal rupture or obstruction.

Because their immune systems are not developed enough to kill the worms, foals and young horses are most susceptible to infection with Parascaris equorum. As horses mature, their immune systems develop a resistance to the parasites. However, adult horses usually retain enough worms to continually shed eggs into pastures, thus infecting new foals each year. After about three months of age, foals also begin to pass eggs into the environment, further contaminating the soil.

Diagnosis:
The veterinarian usually will perform a fecal floatation examination for roundworm eggs to diagnose the disease. Because a negative test does not rule out roundworms, the diagnosis must take into account the horse's clinical signs, its deworming schedule, and farm history.

Prognosis:
With proper deworming, the prognosis for horses that are not severely affected is good. In foals infected by a large number of worms, deworming must be performed with extra care; killing the worms too quickly may cause either shock or obstruction to the bowels.

Transmission or Cause:
Horses become infected with Parascaris equorum by eating roundworm eggs found within the soil of contaminated pastures. These eggs may survive for many years, and in varying climates and weather conditions.

Treatment:

Eliminating Parascaris equorum from a farm requires a strategic deworming program involving all horses out to pasture. Deworming is started when foals are seven to eight weeks old, and treatment is performed either daily or at two-month intervals, until the foal is about one year old.

Deworming treatments include pyrantel tartrate, which can be used for daily treatment, and medications that can be used at two-month intervals, such as ivermectin, pyrantel pamoate, and oxibendazole. However, many kinds of parasites are developing a resistance to certain medications, a veterinarian should examine all horses and foals on a farm in order to determine the types of parasites infecting the animals as well as an appropriate deworming schedule.

Prevention:
Preventive measures for Parascaris equorum require decreasing the number of parasite eggs shed by horses into the environment by implementing a regular deworming schedule and good pasture management.

Pasture management is best accomplished by raising foals on different pastures from year to year, and separating weanlings from yearlings in order to prevent younger horses from infecting older ones. Additionally, manure should be allowed to compost before it is used on pastures with horses, since fresh manure contaminates the soil with eggs. Horses should be put on fine gravel lots rather than dirt or barren lots. Avoiding overgrazing is also a good preventative measure -- either pastures should be rotated, or horses should be provided two to three acres each.