Rethinking Worm Control in Australian Horses

While the traditional deworming routine of treating every horse on a fixed schedule and rotating dewormers from one class to the next has long been practiced by Australian horse owners, the emergence of resistance to all available dewormer classes has created the need for a change to this traditional practice. One of the objectives of the Australian Equine Parasitology Advisory Panel (AEPAP) was to formulate contemporary guidelines for parasite control horses in Australia. A review of these guidelines was published in the Australian Veterinary Journal in 2025.

Why the Old Approach has Changed

The latest guidelines explain that frequent, indiscriminate use of dewormers has driven widespread resistance amongst the primary equine parasite groups, namely cyathostomins and Parascaris. Resistance is now reported in Australia across all the major drug classes used in horses, which means that routine “rotation” of products is no longer a reliable strategy. Instead, the focus has shifted to preserving the effectiveness of the products that remain effective.

Which Parasites Matter Most

For adult horses, small strongyles (cyathostomins), are the primary parasite of concern. In foals and young horses under 12 months of age, Parascaris is the main target as it can cause serious disease in this age group and the eggs persist for a long time in the environment. Tapeworms, bots, pinworms, and other parasites are typically addressed as part of a broader parasite control program rather than as separate stand-alone targets.

Faecal Egg Counts are Central

The key diagnostic tool is the faecal egg count (FEC). The test helps identify which horses are shedding the most strongyle eggs and therefore contribute most heavily to pasture contamination. In practice, the guidelines classify horses as low shedders below 200 eggs per gram, moderate shedders at 200 to 500 eggs per gram, and high shedders above 500 eggs per gram. These categories reflect egg shedding, not total worm burden or disease severity. The value of the test lies in guiding decisions, not replacing clinical judgement.

Testing for Resistance

To test whether a product is actually effective on your property, a faecal egg count reduction test, or FECRT, can be used to assess treatment efficacy against cyathostomins and Parascaris. Where possible, it should be done regularly so you know which products remain useful and which have lost effectiveness. Shorter-than-expected egg reappearance periods (how quickly eggs reappear after treatment) can be an early sign that a product is no longer performing as it once did.

Treatment Should Match Age and Risk

The recommended approach changes as horses age. Foals and weanlings need more structured treatment because Parascaris is the major concern in that age group. Young horses from one to three years of age often shed more strongyle eggs and usually still need more frequent treatment than mature horses. As horses mature and their innate resistance to parasites increases, many can move to targeted treatment based on repeated faecal egg counts. Adult horses can often be managed with one to two treatments per year if they are low shedders, while moderate and high shedders may need more frequent treatment. Older horses may once again become high shedders, particularly if they develop conditions such as Cushing’s disease, so they should not be assumed to remain low-risk simply because of age.

Rotation is Out, Strategy is In

It is now clear that drug rotation alone is not a sustainable solution. Using different classes of dewormer in rotation, without testing whether they are effective, is a misguided approach. What matters is choosing a product or combination that is proven to work for that horse on that property. Combination products should not be assumed to be effective just because they contain multiple active ingredients. If resistance is already present, even combinations can fail.

Management Beyond Medication

New horses should be quarantined away from the resident herd and treated as if they may harbour resistant parasites. The guidelines recommend treatment that covers the main parasite groups and, ideally, faecal egg counts are performed before and after treatment to confirm effectiveness. Preventing the introduction of resistant parasites is far easier than trying to eliminate them after they become established.

Manure removal is one of the most effective tools available, especially when it is done frequently enough to prevent larvae developing and spreading onto pasture. Pasture spelling, sensible stocking density, and mixed grazing with cattle or other non-equid species can also help reduce contamination pressure.

One registered non-chemical product, BioWorma, is identified as having scientific support, although the authors note that more field work is still needed before it can be unconditionally recommended. By contrast, the evidence for the effectiveness of herbal products and diatomaceous earth is poor. For clients using those approaches, close monitoring is even more vital.

Practical Message for Horse Owners

For horse owners, the practical takeaway is straightforward. Use faecal egg counts to guide treatment, confirm that your dewormers are still effective, avoid routine product rotation, and combine medication with sound hygiene and pasture management. The goal is a program that is effective now and remains effective in the future.

Horses less than 12 months should be treated with a product targeted against Parascaris. Horses 1-2 years of age should be treated with a product targeted against strongyles. Horses older than 3 years of age should have frequent faecal egg counts. Low shedders can be managed with one or two treatments at strategic times during the year, whereas moderate and high shedders will require more frequent treatment, guided by appropriate testing.

Reference: Beasley, A., Abbas, G., Hughes, K., El-Hage, C., Jacobson, C., Bauquier, J., Wilkes, E., Carrigan, P., Cudmore, L., Hurley, J., Beveridge, I., Nielsen, M. & Jabbar, A. “Australian guidelines for equine internal parasite management.” Australian Veterinary Journal 2025;103:151-158. DOI:10.1111/avj.13424

If you have any questions about parasite control in your horses, please feel free to reach out to our team at admin@beaequine.com.au.

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