Parasite control in horses: Don’t rely on blind faith

This image of a horse's caecum - the first portion of the large bowel - shows the damage that can be caused by encysted strongyles, also known as cyathostomins. Encysted strongyles are very resilient. ©
This image of a horse’s caecum – the first portion of the large bowel – shows the damage that can be caused by encysted strongyles, also known as cyathostomins. Encysted strongyles are very resilient. © Martin Krarup Nielsen

How do you know if your parasite control program is working?

Fresh parasite control guidelines released by the American Association of Equine Practitioners stress the importance of monitoring the effectiveness of any equine worm control program.

A team of experts recently finished updating the guidelines, first published in 2013.

The guidelines lay out a roadmap for effective parasite control while minimizing the risk of resistance developing.

University of Kentucky equine parasitologist Dr Martin Nielsen, who heads the team which updated the guidelines based on the latest evidence, stresses the need for routinely checking the effectiveness of treatment programs.

“There is no way one can just assume that something works, and then just treat blindly over and over again,” he told Horsetalk.

The guidelines, originally published in 2013, consist of a combination of strategic treatments for all horses, which vary depending upon age, and targeted treatments for adult horses identified as high shedders of strongyle eggs.

“People often get very confused about these things,” Nielsen says.

“They may understand that calendar-based treatments are no longer recommended, and can be led to falsely believe that everything now has to be based on egg counts.

“I see this over and over again, and this is definitely not the case. We are actually still recommending some treatments that are calendar-based.”

The guidelines describe how the threat from large strongyles has subsided in managed horse populations, with the control of cyathostomins (small strongyles) now central to worm management programs.

“The cyathostomins may be a major target in mature horses,” Nielsen says, “but we should never forget the large strongyles.

“Evidence demonstrates very clearly that they can reemerge if not considered in parasite control programs. The AAEP guidelines reflect this.”

All grazing horses are infected with cyathostomins.

“But they are relatively mild pathogens and only produce disease when infections reach extremely high levels,” the guidelines point out.

“Thus disease from strongyle parasites is much less of a concern in adult horses today than it was decades ago when Strongylus vulgaris [large strongyle/bloodworm] was highly prevalent.

Frequent worming treatments are therefore not needed to keep adult horses healthy.

“What is needed are properly timed treatments with effective anthelmintics administered at the appropriate time of the year, which correspond to the epidemiological cycles of transmission and the relative parasite burdens in individual horses,” according to the guidelines.

The true goal of parasite control in horses is to limit parasite infections so animals remain healthy and clinical illness does not develop.

“The goal,” according to the paper, “is not to eradicate all parasites from a particular individual.

“Not only is eradication impossible to achieve, the inevitable result is accelerated development of parasite drug resistance.”

Dr Martin Nielsen led the panel which has updated the AAEP's parasite control guidelines for horses.
Dr Martin Nielsen led the panel which has updated the AAEP’s parasite control guidelines for horses.

Rotation between dewormers is no longer recommended. Strongyle resistance is well documented against pyrantel, fenbendazole, and oxibendazole, and these drugs can therefore only be used if a fecal egg count reduction test (FECRT) has shown good effectiveness.

However, resistance among species of Parascaris – the most important parasite infecting foals and weanlings – is still uncommon for these drugs, making them often solid choices when targeting this parasite.

Such resistance must be factored into treatment decisions, according to the team of experts.

“Adult horses vary greatly in their innate susceptibility to infection with cyathostomins and their level of strongyle egg shedding and thus, require individualized attention to their parasite control needs,” the panel wrote.

“Horses less than about three years of age require special attention as they are more susceptible to parasite infection, and are more at risk for developing disease.”

The panel says traditional parasite control programs involving rotational treatment with deworming drugs at regular intervals are commonly recommended by vets.

“However, this approach is based on concepts and strategies developed more than 50 years ago when Strongylus vulgaris was the most important parasitic pathogen of horses.

“The rationale for this parasite control scheme was rather simple: to kill S. vulgaris worms before they could mature and lay eggs that would contaminate the environment.

“This approach was very successful in controlling S. vulgaris infections, and disease from S. vulgaris is now very rare in managed horse populations.

“It is noteworthy that cyathostomins (small strongyles), were not considered important pathogens at that time, as their pathogenic potential was over-shadowed by S. vulgaris.

“However, that situation has changed and currently, cyathostomins (small strongyles), are recognized as a primary equine parasite pathogen.

“Similarly, Parascaris spp. is recognized as a major parasitic pathogen in foals and weanlings, and Anoplocephala perfoliata (the tapeworm) has been recognized as a cause of ileal colic in the horse.”

The biology, life-cycles and host-parasite dynamics of these parasites are very different from S. vulgaris, the panel points out. “Thus strategies designed for controlling S. vulgaris will not be appropriate or very effective for controlling these parasites.”

They say decades of frequent anthelmintic use have selected for high levels of drug resistance in cyathostomin and Parascaris populations. This emphasizes that traditional approaches for control are not sustainable and that new strategies are needed.

The panel says the goal of any parasite control program is to minimize the risk of parasitic disease, control parasite egg shedding, maintain the effectiveness of the drugs used, and avoid further development of resistance as much as possible.

The experts traverse the parasites that infect horses and the drugs available for their control.

They say deworming programs for adult horses should evaluate the effectiveness of the drugs used on each farm at least every three years.

A basic foundation of treatments should be considered for all horses, comprising one or two yearly treatments to target large strongyles, tapeworms, bots, and spirurid nematodes, responsible for causing summer sores. In most cases, one or two yearly treatments will achieve this goal.

All further treatments should target horses with a high strongyle contamination potential.

Treatments should target seasons of peak transmission, usually spring and fall.

A different approach is recommended for foals, weanlings and yearlings. During the first year of life, foals should receive at least four anthelmintic treatments. First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure its effectiveness against ascarids.

A second deworming is recommended just before weaning (approximately 4-6 months of age). An extra treatment can be justified before weaning if the time period between the two treatments exceeds three months.

At weaning, a fecal egg count is recommended to determine whether worm burdens are primarily strongyles or ascarids, to facilitate the right choice of drug class.

Third and fourth treatments should be considered at about 9 and 12 months, primarily be targeting strongyles. In areas where tapeworms are present, a tapeworm-directed treatment should be included in the 9-month treatment, or before the end of the first calendar year.

Recently weaned foals should be turned out onto the “cleanest” pastures with the lowest parasite contamination levels.

Yearlings and two-year-olds should continue to be treated as “high” shedders, and receive three to four yearly treatments, depending on the duration of the grazing season.

The panel warned against under-dosing horses and foals, saying that weight tapes or scales should be used.

They say strongyle-directed treatments can be decreased when hot summers or freezing winters knock back larval survival and/or transmission.

The panel also stated its view on so-called organic or herbal dewormers, saying their effectiveness has never been demonstrated in formal, controlled evaluations.

“These products exist primarily because they exploit differences in the labeling requirements for drugs versus non-drug items.

“Before a drug can earn label claims for activity against parasites, this fact must be proven unequivocally to the Food and Drug Administration (FDA) by extensive efficacy and safety testing.

“Once a dewormer is approved by the FDA, the claims that can be made in advertising that drug are regulated by the FDA.

“In contrast, products that are not considered drugs do not require FDA approval for marketing, so advertisers of non-FDA approved products can say just about anything they want, and their products do not have to be effective.”

The updated AAEP Parasite Control Guidelines can be read here.

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