Internal parasites are small organisms that live a portion of their life cycle in a host animal. They live in internal organs, body cavities, and tissues while gaining their nutritive source by feeding on the host animal. The horse is affected by many different species of parasites. The nature and extent of damage varies with the parasite.

Parasite infestation causes loss of nutrients or blood from the host, resulting in serious medical problems. Horses heavily burdened with parasites will have a loss of condition due to a depletion of nutrients and blood, decreased growth, and reduced reproductive and athletic performance.

Numerous internal parasites infect horses, but there are only a few that commonly cause significant health problems. To establish an effective parasite control program, it is important to first understand the life cycle of parasites. Successful prevention and control programs are effective because they interrupt the life cycles of parasites. In some areas of the country, the primary class of internal parasites that cause health problems for horses are nematodes, such as large and small strongyles, ascarids, and tapeworms. Other internal parasites perhaps of lesser significance, such as pinworms and botfly larvae, are often considered when designing a parasite control program.

Strongyloides (Threadworm)

Strongyloides westeri is an intestinal parasite that can infect foals as early as four days after birth. A foal becomes infected by ingestion of larvae in the dam’s milk or by penetration of the foal’s skin by infected larvae in the bedding. However, the larvae are not present in colostrum. The larvae migrate through the lungs and the small intestine. The life cycle can be completed in less than two weeks. This creates the potential for severe infestations in a relatively short time. Foals will quickly develop immunity to these parasites and lose the intestinal infection of adult parasites by 60 to 90 days of age.

The primary medical problem of a strongyloides infection is diarrhea that may not respond to treatment. Some foals will become dehydrated and develop other problems related to chronic diarrhea. Treatment of mares with an anthelmintic effective against strongyloides within 24 hours of birth significantly reduces transmission of this parasite to foals.

Craig Wood, University of Kentucky