How can you reduce internal parasite infection and manage anthelmintic resistance on your farm?

Do you know which of your horses has parasites? Chances are they all do, to one extent or another. What’s more important is knowing which horses have the highest fecal egg counts and which anthelmintic (deworming) treatments are effective on your farm. These are the first steps in developing a strategic horse parasite control plan for your farm.

When adopting such a management strategy, the primary goal is to prevent horses from amassing extremely high worm burdens, which, although uncommon, can cause signs of colic, diarrhea, and weight loss. Two secondary goals help achieve this:

1. Reducing the worm burden on pastures (which reduces horses’ rate of infection and reinfection); and
2. Maintaining the long-term ­efficacy of available dewormers.

​Equine parasitologist Martin Nielsen, DVM, PhD, Dipl. ACVM, an associate professor and the Schlaikjer professor of equine infectious disease at the University of Kentucky’s Gluck Equine Research Center, in Lexington, says that in managing your horses’ own parasite population, you are managing that of the entire farm.

“The worm population includes the worms in each of the horses and also in manure and on the pasture,” he says. “We need to understand that we are treating the parasites, not the horses. Each horse carries a biological sample of the parasite population present on the farm, and we need a coordinated effort for the entire population before we can claim that we have a parasite control program.”

Contrary to the approach taken in the early days of modern dewormers, worm control isn’t about eliminating parasites completely, Nielsen says. That’s proven to be ­impossible.

“Horse worms such as cyathostomins (small strongyles, which primarily affect the equine large intestine) are ubiquitous and affect all grazing horses,” he says. “But they only cause disease extremely rarely and only when infections reach extremely high levels.”

Frequent anthelmintic treatments are not needed to keep adult horses healthy. “What is needed,” says Nielsen, “are properly timed treatments with effective anthelmintics administered at the appropriate time of the year, which correspond to the parasite life cycles and the levels of parasite egg shedding in individual horses.”

But Fecals Don’t Show Everything

Fecal egg counts do not distinguish large strongyle (the most dangerous but least common parasite in horses) eggs from small strongyle eggs. Equine parasitologists recognize this as one of the limitations of FECs. Other constraints the American Association of Equine Practitioners (AAEP) lists include:

• They do not accurately reflect the horse’s total adult strongyle or ascarid burden; a higher egg count just means more eggs, not necessarily more worms;
• They do not detect immature or larval stages of parasites, including migrating large strongyles and ascarids, and/or encysted cyathostomins (small strongyles embedded as cysts in the large intestine wall);
• Standard fecal techniques often underestimate tapeworm infections, so a modified FEC, as well as serum and saliva tests, exist for detecting these parasites; and
• They usually miss pinworm eggs, which adhere in packets around the anus rather than shedding in feces.

Veterinarians can use several techniques to overcome these limitations—for example, they can use PCR and ELISA testing to detect bloodworms (large strongyles) and an ultrasound technique to quantify ascarid burdens, among others.

Despite its limitations, the FEC is the only effective means of ­determining ­anthelmintic efficacy, estimating the extent of a horse’s strongyle infection, and monitoring ascarids in foals and yearlings.

Identifying Effective Drugs

Even more important than checking shedding status with FECs is using them to check that your dewormers are working, Nielsen says. “After you have dewormed the horses you identified as having high FECs, go back and retest them” 14 days later, he says. “What you really want to know is how many eggs they are shedding after treatment.”

Anthelmintic resistance occurs when worms that survive a treatment breed and pass on to their offspring the ability to survive that drug class. Over time the proportion of the worm population that’s able to survive the deworming treatment increases—that is, the worms have become resistant to the dewormer.

High numbers of strongyle and/or ascarid eggs post-treatment indicate the dewormer was not entirely effective, which can be caused by a number of factors. Aside from anthelmintic resistance, the horse owner might not have administered an adequate dose of product (e.g., they underestimated the horse’s weight and gave a lower dose), or they administered it correctly but the horse spat it out.

Understanding Resistance

Nielsen reiterates that each horse in a herd is a biological sample of the same parasite population.

“One horse in a herd won’t have resistant worms without others also having (them),” he says. “If only one horse in a follow-up sample comes back with a high count, it’s safe to assume that horse was not treated for some reason, rather than the problem being dewormer resistance.”

If all or most of the horses still have worms after treatment, it warrants further investigation but doesn’t necessarily mean they have resistant worms. Treat the horses with anthelmintics again, using the same active ingredients as the first time, and redo the FEC two weeks later.

“Resistance doesn’t show up in one test and then disappear in the next,” Nielsen says. “If the second treatment is effective, the problem the first time was not caused by anthelmintic resistance. If the second treatment is still not effective, it is highly possible that you have a problem with resistance on your property.” Parasitologist Caroline Jacobson, BVMS, PhD, senior lecturer of biochemistry and nutrition in Murdoch University’s School of Veterinary and Life Sciences, in Western Australia, also emphasizes the importance of using FECs to check treatment efficacy.

“By the time we see treatment failure in adult worms evident as poor egg count reduction, resistance to that treatment in the worm population is already advanced,” she says.

The earliest sign of emerging resistance in strongyles to the macrocyclic lactones (dewormers that target parasite nervous systems) is reduction in the time it takes for FECs to rise after treatment—the egg reappearance period. Macrocyclic lactones, such as ivermectin and moxidectin, revolutionized equine parasite control when they were released in the 1980s and ’90s, respectively, and continue to be important worm control drugs. Maintain- ing their efficacy is a key component of a parasite management program.

“If FECs start to rise sooner than expected, this is a warning sign that resistance is emerging, and steps should be put in place to (keep resistance from) worsening to a point where treatments become ineffective,” says Jacobson.

“This is already happening all over the world,” Nielsen adds. “We commonly see egg reappearance periods around four weeks now, where it used to be nine to 13 weeks for ivermectin and 16 to 22 weeks for moxidectin.”

Maintaining Refugia

The old recommendation to deworm all horses every six weeks hastened the development of anthelmintic resistance. “Every time you use a dewormer you’re pushing the worm population toward resistance,” Jacobson says. “It’s important to use dewormers carefully and not just de- worm on a calendar schedule. Sustainable control relies on understanding the important parasites in your region, targeting treatments to horses that need treating, and using treatments that are currently effective against the target worms.”

She underscores the importance of maintaining refugia, the population of worms that doesn’t get exposed to de- wormers because they are on pasture as eggs or larvae or in untreated horses.

“Susceptible worms in refugia can effectively ‘dilute’ drug-resistant worms in the population,” she says. “Interbreed- ing of the susceptible worms with any resistant worms slows the accumula- tion (of) resistance to treatments in the worm population. Restricting deworming treatments to horses with high FEC and leaving horses with low FEC untreated is an important tool to manage refugia for sustainable parasite control programs.”

Worm control must strike a balance between reducing the risk of disease in horses and managing the emergence of resistance in worms.

“Because we value them as individuals, the loss of a single horse is too great a loss,” says Jacobson. “That means we err on the side of caution and develop control programs that aim to prevent disease, but do so with a long-term view of maintaining the effectiveness of the treatments we have.”

Nonchemical Parasite Management

Managing the worm burden on pastures remains an important aspect of parasite control. You can accomplish this by removing manure regularly from en- closures and treating horses with high egg counts appropriately. Where horses are kept in small areas, pick up and compost manure (or remove it from the horse’s environment altogether).

“Nonmedical means for worm control are important,” Nielsen says. “Cross- grazing with ruminant animals is potentially very effective. Chain harrowing or mowing can be useful, but timing is crucial and is dependent on where you are and the climate you have. Mowing in warm, wet conditions just spreads the worms around, meaning all the pasture is then contaminated. But in very hot weather, spreading manure around can be an effective control strategy, because the larvae will die off much quicker.”

Recently, a commercial biological product has become available in Australia that might serve as a nonchemical parasite control tool. It’s composed of fungal (Duddingtonia flagrans) spores that feed on parasite larvae in manure after being fed to livestock (TheHorse. com/165635).

“We don’t know how the product is going to work out yet and how it could be implemented into a strategy as an adjunct to anthelmintic therapy,” says Nielsen, “but the principle is well-researched.”

The Bottom Line

Managing parasites and preventing worm-related disease in horses is a complex topic, with many things to consider. The AAEP recommends all adult horses receive “one or two yearly treatments to target large strongyles, tapeworms, bots, and spirurid nematodes responsible for causing summer sores (Habronema spp and Draschia spp).”

Beyond this basic treatment, owners and veterinarians should use egg counts to guide their parasite control decisions, keeping in mind the following considerations:

• How are the horses housed? (Alone? In herds? At pasture? In stables?)
• What are they used for? (Do they travel? Are they stressed?)
• What ages are the horses? (Foals? Youngsters? Old horses?)
• What is the local climate? (Hot? Cold? Wet? Dry?)
• These factors can affect egg count levels, worm burdens, when parasites will be most active, and how and when they should be targeted. (See TheHorse. com/157317 and TheHorse.com/148466 for specific recommendations.)

Deworming horses is not a simple matter of dropping into the shop and buying a product, going home, and administering it. Work with your veterinarian to develop an appropriate strategy, beginning with fecal egg counts. Then, determine what anthelmintics are still effective on your farm. “That comes before anything else,” says Nielsen. “And there is no way of telling without (FEC) testing.”