Good resistance status on horse farms using selective deworming

Dewormer resistance in horses is a growing global problem resulting from decades of regular deworming. As no new dewormers are expected to come on the market in the near future, it is of utmost importance that the efficacy of our currently available dewormers is preserved as far as possible. Researchers have now shown that Sweden, which makes extensive use of so-called selective deworming, has managed to maintain a good effect of the deworming agent ivermectin, which is unusual in an international perspective.

In order to reduce its use, all equine dewormers are available on prescription in Sweden. So-called selective deworming is advocated, whereby adult horses are only dewormed if they excrete more than a specific amount of eggs of the equine small bloodworm or if they are infected with more pathogenic parasites, such as the large bloodworm or tapeworm.

Sixteen Swedish farms using selective deworming were included in the study. All farms showed over 99% reduction in bloodworm eggs 14 days after treatment with ivermectin and therefore no signs of resistant parasites. The study also looked at how long it took for the horses to start excreting eggs again after treatment. On the majority of farms, it took at least eight weeks before eggs started to appear in the droppings again. However, the study showed that horses in stud farms, riding schools and farms that did not use separate summer and winter pastures started to excrete bloodworm eggs earlier, compared to other farms.

In conclusion, the study suggests that a selective deworming strategy can reduce the risk of developing drug resistance.

Link to the publication