by Ashley Daily
Photos courtesy of Equine Medical Associates, Inc.
Parasites are organisms that attach themselves to a host to suck blood or obtain nutrients. Most internal parasites are species specific, meaning equine parasites survive only in horses, cattle parasites survive only in cattle, and so on. External parasites, however, are not picky and will feast on any host they come into contact with.
The most common equine internal parasites are strongyles, ascarids (roundworms), tapeworms, pinworms, botfly larvae, and threadworms.
Parasites can rob the horse of nutrients, inhibit the growth of a young horse, and compromise a horse’s athletic abilities.
Parasites most likely to infect Oklahoma horses are roundworms and strongyles, according to J. Mike Johnston, D.V.M., of Equine Medical Associates, Inc., in Edmond.
Roundworms
Roundworms are especially large, with females as long as 15 inches. Adult horses generally develop an immunity to roundworms, but foals and yearlings become infected by consuming grass or licking stall surfaces that have been contaminated by eggs.
Roundworms typically lay eggs in manure. When larvae hatch, they stay in the environment 10 days before becoming capable of infecting a new host. Once larvae are ingested, they burrow into the small intestine, migrating through veins until they reach the lungs. Inside the air spaces of the lungs, larvae are coughed up and swallowed, where they return to the small intestine to become egg-laying adults, starting the cycle again.
“When we see a foal with a snotty nose and an occasional cough, we consider the intestinal parasite roundworm (ascarid) larvae migrating through the foal’s lungs,” says Johnston. “Foals often will eat the adult’s manure, thus ingesting large quantities of roundworm eggs and larvae. We can reduce the incidence of roundworms in foals by deworming the mare 30 days prior to foaling and using an effective dewormer in the foal monthly that first year.”
It is important to consult your veterinarian about choosing a safe, effective deworming agent and establishing a schedule for pregnant mares and foals.
Strongyles
Strongyles are classified as large or small. Fortunately, the use of effective dewormers has dramatically reduced the number of infestations of large strongyles. Small strongyles are now the most common. Small strongyles have a similar appearance to longer ones but are only about one-fourth inch to one inch long. Large strongyles can grow to two inches. Strongyle eggs are passed in manure. Larvae then hatch and live on pasture, waiting to be ingested by a host, where they will take up residence in the horse’s intestine.
Larvae can be spread throughout the pasture by heavy rains. Larvae are resistant to harsh environments, even surviving freezing temperatures because of a protective sheath. Some larvae can survive three months or even a year. However, a hot, dry environment will often kill them.
Once larvae are ingested, they migrate into various parts of the body, often settling in the arteries, hence the strongyles’ other name, bloodworm. One type of strongyle that is responsible for causing the most damage to the blood supply leading to the intestines is S. vulgaris.
Signs that a horse might be infected with large strongyles are diarrhea, weakness, anemia, and weight loss.
Small strongyles have a similar life cycle to large strongyles, but they do not migrate beyond the intestinal walls. They live in the colon wall.
Horses infected with small strongyles might not show any outward signs, but horses with severe infestations might develop diarrhea and be slow to shed.
Tapeworms and Botfly Larvae
Horses become infected with tapeworms by ingesting pasture mites that carry larvae. Tapeworms can be as long as three inches.
Botfly larvae hatch from tiny yellow eggs attached to your horse’s legs. When horses lick their legs, the warm saliva activates the hatching of the eggs. Larvae burrow into the horse’s gums and tongue, where they incubate for three weeks.
Deworming
Although no horse is completely free of internal parasites, owners can minimize the effects the organisms have. The key is knowing how and when to administer a deworming agent.
In the 1960s, a common practice was to deworm horses every two months or so, rotating the type of dewormer that was used, a practice still used by some horse owners today. However, in addition to wasting money on expensive and sometimes unnecessary treatments, that practice gave rise to “superworms” that developed a resistance to some commonly used deworming medications. Today, several species of worms have developed resistances.
The best way to know if your horse needs to be dewormed is by having your veterinarian do a fecal egg count (FEC). The FEC measures the amount of strongyle and roundworm eggs contained in each gram of your horse’s manure. The American Association of Equine Practitioners (AAEP) recommends that an FEC be performed at least twice a year on every horse in your herd.
Some owners use a daily dewormer, which is mixed into a horse’s grain ration.
“The use of a daily dewormer can be an effective means of controlling internal parasites. We must first clear the adult parasites in the horse’s digestive tract with an appropriate dewormer,” notes Johnston. “The daily dewormer works by controlling the infective larvae the foal picks up. In the adult horse, we need to look at the economics of daily dewormer versus the use of fecal samples and the regular use of broad-spectrum dewormers as recommended by your veterinarian.”
Keep in mind that a daily dewormer must be used every day because skipping a dose can allow worms to develop.
