October - Worm of the Month

Classes, or families, of equine dewormers have an effect on different types of parasites. Oxibendazole is a member of the benzimidazole family, which includes dewormers typically used to treat various worm species including nematodes (roundworms - 'nema' being a Greek word meaning 'thread') and trematodes (flukes - 'trema' meaning 'hole').

According to Wikipedia, benzimidazole is a 'heterocyclic aromatic organic compound consisting of the fusion of benzene and imidazole' and is a commercially available compound that has fungicidal properties that 'acts by binding to the fungal microtubules and stopping hyphal growth.'

The Merck Veterinary Manual Web site (MVM.com) reports that benzimidazoles can be given to ruminants (sheep and cattle), swine, dogs, cats, and birds as well as equines, and that in horses they are most effective at removing mature strongyles, while less effective at removing strongyle larvae.

Benzimidazoles are absorbed from the gastrointestinal (GI) tract; MVM.com also says that variables such as 'species, dosage, formulation, solubility, and operation of the esophageal groove reflex' can affect the rate and extent of benzimidazole uptake, which depends on prolonged contact time. MVM.com suggests that reducing feed intake or repeating dosages at specified intervals can increase benzimidazole effectiveness, by reducing the transit time of digested matter and increasing available medication in the system.

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While benzimidazoles have a wide
safety margin, their use has been decreasing
due to widespread parasite resistance.
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Specifically, oxibendazole is used to target large roundworms, large and small strongyles, pinworms, and threadworms. It is considered ineffective against bots, but carbon disulfide can be used concurrently for bot control when administered by a veterinarian. It's important to have an accurate body weight for your horse, since body weight defines dosage.

MVM.com also reports that while there is a wide safety margin in using benzimidazoles, their use is rapidly decreasing due to 'the widespread development of resistance and the availability of more efficient and easier to administer compounds.'

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If you want to read up on specific parasites or dewormers we've covered, check our Archive for past Worm of the Month articles.

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