Resistance to deworming agents in roundworms in Swedish stud farms

Fluke is a parasite that primarily affects foals and young horses, as they lack the resistance to the parasite that the adult individual has developed. In most cases, roundworm causes no or mild symptoms, such as weight loss, coughing and nasal discharge, but a large parasite burden can cause severe constipation in the foal's small intestine, with a questionable prognosis for its survival. The recommendation for treatment of foals in Sweden is to deworm all foals against roundworm at 8-10 and 16-18 weeks of age.

Historically, several groups of deworming agents have been possible treatment options, but since the early 2000s, widespread resistance has developed to agents within the macrocyclic lactones drug group.

It is now recommended to use other drug groups for treatment of roundworm, such as pyrantel and fenbendazole, but resistance has also been documented in recent years. This global and gradually increasing problem with the development of multi-resistance in the horse roundworm to our currently available deworming agents poses a real threat to foal health.

In order to investigate the current resistance situation in Sweden to the currently most commonly used dewormer for the treatment of roundworm, fenbendazole, the effect of this drug was investigated on 11 Swedish farms with a total of 92 foals under one year of age. Resistance to fenbendazole was detected on four farms, with an efficacy of between 45-96%.

There was a statistically significant increased risk of resistant roundworms on farms with more than 40 foals per year. In addition, large farms were more likely to deworm their foals more frequently than the nationally recommended two treatments. Remarkably, only one of the farms studied dewormed according to the national recommendations, and six of the farms dewormed their foals three or more times during the foals' first year of life.

In conclusion, the study shows the presence of established resistance of the equine roundworm to fenbendazole. The results of this study are very worrying, given the serious damage that the parasite can cause. Although several farms in the study expressed concern about deworming drug resistance, only four of the farms regularly checked the drug efficacy when deworming their foals.

Link to the article

Reference

Martin, F, Halvarsson P, Hedberg Alm, Y, Tydén E. Occurrence of fenbendazole resistance in Parascaris spp. on breeding farms in Sweden. Vet Parasitol. 2024; 110272.