Common parasite control strategies ‘outdated’

Parascaris equorum
Parascaris equorum

Parasite control guidelines have been published by the American Association of Equine Practitioners (AAEP), with a warning that common strategies for parasite control in adult horses are based largely on knowledge and concepts dating back more than 40 years.

“Much has changed over this time, necessitating a re-examination of recommendations for parasite control,” according to the guidelines, formulated by the AAEP’s Parasite Control Subcommittee, set up under the association’s Infectious Disease Committee.

In response, the AAEP formed a task force to produce recommendations for helping veterinarians develop improved strategies and programs for parasite control in horses.

Two strongyle-type eggs, at top, with a D-shaped tapeworm egg (Anoplocephala perfoliata).
Two strongyle-type eggs, at top, with a D-shaped tapeworm egg (Anoplocephala perfoliata). © Martin Krarup Nielsen

The true goal of parasite control in horses was to limit parasite infections so animals remained healthy and clinical illness did not develop, the committee said.

“The goal is not to eradicate all parasites from a particular individual,” the committee said. “Not only is eradication impossible to achieve, the inevitable result is accelerated development of parasite drug resistance.”

The committee noted that important changes in the parasitic fauna of horses had occurred. Strongylus vulgaris and other large strongyles were now rare, while cyathostomins (small strongyles) were now the major parasite of concern in adult horses, and Parascaris equorum remained the most important parasite infecting foals and weanlings.

“Anthelmintic resistance is highly prevalent in cyathostomins and Parascaris equorum, and this must be factored into treatment decisions.

“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 committee said.

“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 committee described traditional parasite control programs involving rotational treatment with deworming agents at regular intervals as outdated.

It said decades of frequent anthelmintic use had promoted development of high levels of anthelmintic drug resistance in cyathostomin and P. equorum populations, which emphasized that the traditional approaches for parasite control were not sustainable.

Frequent anthelmintic treatments were not needed to keep adult horses healthy, it said.

“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.”

The committee said worm control programs were best viewed as a yearly cycle starting at the time of year when worm transmission to horses changes from negligible to probable.

“Ultimately, each farm (with veterinary guidance) should develop its own program tailored to the specific needs of the farm and each animal. There is no such thing as a ‘one size fits all’ program.

” … all adult horses should benefit from a basic foundation of one or two treatments per year. Low egg-shedding horses with naturally strong immunity to cyathostomins will need no other treatments because the two treatments have covered the needs of the other parasites and these horses are protected naturally from cyathostomins by their immune state.

“In traditional deworming programs, repeated treatment of low shedder horses every 2-3 months accomplishes little to improve their health, but it does promote drug resistance.

“Moderate and high egg shedders will need a third or fourth treatment for cyathostomins.

“Any additional treatments would be given on an “as needed” basis depending on whether a specific parasitic infection or disease is diagnosed.

The committee urged caution over what it described as alternative remedies.

“An increasing number of so-called organic or herbal dewormers are appearing in tack shops, but the efficacy of these products 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 guidelines can be read at

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