March 2019 - Bad News Blood Worms
Written by by Tom Lenz, DVM, MS, DACT
Wednesday, 27 February 2019 05:13
PDF Print E-mail


Understanding the parasite lifecycle is important to management.

by Tom Lenz, DVM, MS, DACT

Veterinary Parasitologists are reporting an increase in resistance by equine internal parasites to routinely used anthelmentics (dewormers). Most horse owners and some veterinarians are still following a 40-year-old plan for internal parasite control that recommended deworming every horse in the herd or on the premises every two to three months.

Today, large strongyles have all but been eliminated from well-managed horse herds. Small strongyles (bloodworms) have become the most significant internal parasite in adult horses and they have developed resistance to many commonly used dewormers.

The bad news is that no new dewormers are coming to the market in the foreseeable future. The good news is we can minimize parasite resistance by understanding the parasite’s life cycle and implementing a strategic deworming program.

Life Cycle

Parasites enter the horse’s body when the horse eats grass that harbors third-stage bloodworm larvae. The larvae pass through the horse’s intestinal tract and take up residence in the horse’s large colon where they burrow into the lining and form cysts.

During the next few weeks, the parasite develops into fourth-stage larvae that produce inflammation as they emerge from the cyst. The adult small strongyles lay eggs that are distributed with the horse’s feces where they hatch into infective larvae.

In well-nourished, unstressed horses, the intestinal inflammation caused by strongyle larvae causes decreased feed efficiency and performance. In poorly-nourished horses, the result is weight loss, poor growth, anemia, diarrhea and/or colic.

Like all living organisms, internal parasites periodically mutate. When a horse is administered an anthelmentic, the parasites that have mutated and developed resistance to that class of dewormer survive and produce offspring.

If we continue to use the same class of anthelmentics, we will kill susceptible parasites and select for the resistant ones.Our deworming program becomes ineffective; we’re wasting money and not protecting horses.

Ask your veterinarian to run fecal egg counts on individual horses or at least 10 percent of the herd. Next, treat the animals by weight with an accurate dose of dewormer labeled for strongyles. Then do post-treatment fecal egg counts 14 days later.

FECRs near 90 percent indicate that the anthelmentic is effective and you can continue to use it. FECRs of less than 80 percent indicate your dewormer is not effective. Once you have determined the anthelmentics that are not effective for your horses, don’t use them.

Phase 2

Horses with low fecal egg counts (less than 200 eggs) require deworming twice a year, in the spring and fall, with the fall dewormer containing a drug that kills tapeworms and bots. Horses with a moderate fecal egg count (200-500 epg) should be dewormed three times a year: spring, fall and midsummer. Horses with high fecal egg counts (more than 500 epg) should be dewormed spring, midsummer, late summer and fall to minimize strongyle contamination.

Article provided courtesy of AAEP Alliance Partner, AQHA. About the author: Thomas R. Lenz, DVM, M.S., Diplomate of the American College of Theriogenologists, is a trustee of the American Horse Council, past chairman of AQHA’s research committee and past president of the American Association of Equine Practitioners.