Rotational and calendar-based Worming strategies are outdated and no longer recommended. Instead, owners are now encouraged to work with their veterinarians to implement diagnostic driven worming plans. The overall aim of a diagnostic driven program is to reduce the risk of parasite related disease in our horses.
In order to achieve this, owners need to:
Unfortunately, due to the indiscriminate use of wormers in the past three decades, equine worms are now widely resistant to commonly used wormers. Resistance refers to the parasite’s ability to survive a worming treatment. The overuse of wormers means that we have been very successful at killing the susceptible worms, but what’s left behind are the resistant worms. These resistant worms then reproduce which leads to more resistant worms developing in your horse.
No horse property is free of drug-resistant parasites and no worming product is free of resistant issues. The only way to know if the wormer is effective in your horse, is through resistance testing.
Resistance testing refers to the percent reduction in parasite eggs between a sample collected just before worming and another sample collected 14 days post-worming. It’s really simple to do and your vet will be able to guide you with the appropriate reduction percentages for each worming product.
The majority of worming treatments are administered unnecessarily. We won’t ever beat worms and we shouldn’t be trying to. Horses have evolved alongside their worms for centuries and there’s even some evidence to suggest they might be beneficial. Studies in other species have found that parasites are important in the development of immunity and prevention of auto-immune and allergic diseases in the host 1,2. A very recent study also found that reducing the frequency of worming is not associated with any negative health risks to the horse3.
Most adult horses, on well managed properties are considered low risk for parasite-related disease and should not require more than one worming treatment per year. Strongyles (cyathostomins) are arguably the most important cause of parasitic disease and they should be the focus of any worming strategy in mature horses.
Adult horses (more than six years old) develop robust (but variable) immunity to cyathostomins and we frequently forget they can tolerate relatively large burdens of cyathostomins without developing clinical disease 4. Ivermectin and moxidectin are generally the most effective against strongyle parasites. Other wormers (such as pyrantel) may also be effective, but should only be used if it’s shown to be effective on your property.
How to worm adult horses:
Minimising pasture contamination and implementing these 10 simple tips will help you reduce the need for frequent worming treatments on your property.
The rise in resistant parasites is an alarming trend and there is no one-size-fits-all approach to Worming your horse. Owners should consult with their veterinarian to develop a diagnostic driven Worming plan. The ultimate aim of these plans will be to develop a strategy which prevents the development of parasite-related disease whilst minimising the use of wormers.