Horse Deworming Q & A

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With all the recent press about drug-resistant parasites and how to combat them, many horsemen aren’t sure what to do. We’ve compiled a list of frequent questions in order to help you make rationale decisions.

Q.I’m wondering about fecals being positive again four weeks after ivermectin or moxidectin. Are we seeing emerging larvae growing more quickly to egg-laying stages or are these drug-resistant strains'

A. It’s parasites with a shorter life cycle. If it was just emerging larvae, we wouldn’t see shorter intervals with moxidectin because moxidectin kills encysted forms.

The recommended intervals between dewormings are based on research that looks at fecal egg counts at regular intervals after deworming. When eggs begin to reappear, you repeat the treatment.

Ivermectin kills more life stages than drugs like pyrantel, so its recommended interval is 6 to 8 weeks. Moxidectin’s interval is 12 weeks. Most other drugs are at 4 weeks.

However, what’s being seen now in some areas is egg counts returning quickly after any drug, which points to emergence of a strain with a short life cycle.

Q.Is moxidectin (Quest) as effective as the five-day fenbendazole Panacur Power Pack for encysted worm stages'

A. Yes. There’s one study that showed the five-day fenbendazole caused gut ulceration while the moxidectin didn’t. However, fenbendazole, even at double dose won’t work if the small strongyles are resistant to it.

Q.If you use the Power Pack initially on horses who appear unthrifty, is it good strategy or overkill to then use a daily wormer for a couple of months so the gut can heal without more larval damage'

A. It’s reasonable, but deworming at monthly intervals for a few months with ivermectin will accomplish the same thing by eliminating the parasites before they can mature to egg-laying stages.

Q.What dewormer method do you recommend I use'

A. We’re not keen on rotating drugs because some of the drugs commonly used in these programs have documented high rates of resistance. We worry about the use of moxidectin, as it can’t be used on foals and has a comparatively narrow safety margin. There’s also evidence that ivermectin may be losing the battle against small strongyles. Therefore, we recommend a target deworming program.

Ask your vet to run fecal egg-count checks for you, one prior to deworming and again two weeks after you deworm. Target the parasites that show up in those egg counts with the right drugs. Don’t treat horses with negative results or low levels.

Q.Make deworming easy for me. It’s too confusing.

A. Unfortunately, it’s not simple. However, if your horses are rarely exposed to new horses, they don’t leave the farm, they’re healthy and between two and 15 years old, you may be able to use ivermectin on an eight-week cycle, with an ivermectin-praziquantel combo in the spring and late fall. That said, you’ll need to pay close attention as the research on drug resistance and new strains continues.