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On Worms and Wormers

Since we were all advised to worm every horse at six weekly intervals by an animal pharmaceutical industry keen to cash-in on that recommendation, we now have widespread resistance to the common equine wormers that are available on the market.

The tricky thing about worming is that there is so much contradicting advice. There are those who would have us feed our horses diatomaceous earth or pumpkin seeds. There is no evidence that any natural therapies work at all, but the laboratory research that proves that is paid for by Virbac (as are some of the studies in the sources below).

Further compounding the problem is that a faecal egg count will only show the presence of adult worms who are reproducing at the time of the test, and not larvae-stage worms, adults that haven't produced eggs at the exact time of the test or parasites not in the gut like neck thread worm.

* If your horse is only itchy on the neck, chest or underside of the belly and not at the base of the tail. Check out these articles.

Horses can have high faecal egg counts and look perfectly healthy.

So what do you do?

The only wormers in Australia that are effective at treating encysted-stage strongyles are moxidectin (Equest) and fenbendazole (Panacur), so they should be part of your worming strategy.

Dr Ann Nyland has written extensively on wormers in Australia.

“Only moxidectin (as a single dose) and fenbendazole (for 5 consecutive days) are effective against some stages of encysted cyathostomes. Moxidectin might not kill all EL3s in one dose. The registered claim of Equest shows it is currently the only wormer effective as a single dose against encysted stages of small strongyles, developing stages, late encysted stages, and aids in the control of early encysted stages including inhibited larvae (it is approximately 90% effective). Fenbendazole (though note that some resistance has been demonstrated) has been shown to be effective against inhibited EL3 if given for 5 consecutive days (this would be off-label in Australia), at a dose of 10mg/kgBW (which is the recommended dose for Panacur here in Australia, but is actually double the dose recommended in other countries)! No wormer other than moxidectin at a single dose, or fenbendazole for 5 consecutive days at the dose described above, has any effect whatsoever on these encysted small strongyles.”

Recent studies show that there doesn't appear to be much difference to long-term health whether you worm your horse twice annually, every six weeks, or only when your horse's faecal test reaches a certain level. However, the standard recommended by the Department of Agirculture (for livestock generally, not equines specifically) is worming in the A months - April and August - which is when the conditions are the best for intestinal parasites to thrive. This would be a good time to worm with moxidectin or 5 days of fenbendazole, irrespective of your other strategies. It's also a good idea to worm any horses coming on to the property with either moxidectin or fenbendazole before they arrive.

Ivermectin is next door to useless at the doses on the tube, which is why it is so cheap, but it is effective against onchercerca cervicalis, and that is nasty. Anecdotally, many horse owners say that their vet has never mentioned onchercerca, and just gave repeated doses of cortisone for itch. If that has been your advice, maybe research into other treatment options. The sources listed below might be a good starting point. I give my own horses a double-dose of ivermectin two weeks apart if they are itchy at the base of the neck. Tolerances are quite high, and so it is a worthwhile precautionary measure.

Aside from wormers:

Adequate zinc in the diet helps with all immune responses, and so a mineral supplement, such as any of the Sound Advice Trace Mixes that provide optimal amounts of zinc will help a horse manage their parasite burden.

"Zinc is important at multiple steps in the production of immune system cells and their function including direct cellular killing of organisms as well as antibody production."

Paddock rotation, along with collecting manures, and/or harrowing are the only 'natural' therapies that would be an effective contributor to your parasite management.


Published Online:28 Jan 2019

Anthelmintic efficacy against equine strongyles in the United States.

Belaw, J.L.; Garber, L.P.; Kopral, C.A.; Phillippi-Taylor, A.M.; Traub-Dargatz, J.L. VetNielsen, M.K.; Branan, M.A.; Wiedenheft, A.M.; Digianantonio, R.; Scare, J.A.;

Veterinary Parasitology 2018.

Monitoring equine ascarid and cyathostomin parasites: Evaluating health parameters under different treatment regimens Martin K. Nielsen, Erica K. Gee, Alyse Hansen, Tania Waghorn, Julie Bell, Dave M. Leathwick Equine Veterinary Journal (2020)

Anthelmintic resistance in sheep - Laboratory methods that assist in detecting and diagnosing anthelmintic resistance in sheep. Lyndal-Murphy M, 1987

The Worm has Turned

Kaye Meynell and Dr Ann Nyland

What You Don't Know About Worms Will Surprise You

Ebook Dr A Nyland

The Disturbing Truth About Neck Threadworms and Your Itchy Horse

June 12, 2013 by Jane @ THB

How to Fight the Big Fight against Neck Threadworms

October 2, 2013 by Jane @ THB

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