Cyathostomes in horses in Canada resistant to pyrantel salts and effectively removed by moxidectin.

This research focuses on the resistance of certain parasites, namely cyathostomes, found in horses in Canada to pyrantel salts, a common anti-parasitic medicine, and how these parasites were effectively treated by using moxidectin instead.

Experiment and Findings

• The research was conducted on a farm in 1997, where fecal samples were taken from horses, specifically yearlings and mares, to estimate the number of strongyle eggs per gram of feces. The findings showed that 11 out of 15 yearlings, which were fed grain with pyrantel tartrate for 66 days, still had strongyle eggs in their feces.
• This showed that the horse parasites were resistant to pyrantel tartrate. The yearlings were then divided into three groups to test the efficacy of continuous pyrantel tartrate feeding, a single pyrantel pamoate treatment or a single moxidectin treatment when it comes to reducing the number of strongyle eggs in the horses’ feces.
• When two of three yearlings were given pyrantel tartrate or pamoate, there was no reduction in the eggs per gram of feces. However, when the yearlings were treated with moxidectin, the number of eggs per gram of feces was reduced to zero.

Hypothesis and Further Testing

• The researchers presumed that the 66 days of pyrantel tartrate was not enough time for the development of resistant cyathostomes and that the resistance might have been due to the extensive use of pyrantel pamoate on the farm over many years for controlling tapeworm infection.
• To test this hypothesis, 12 mares with strongyle eggs in their feces were randomly divided into two groups for different treatments: pyrantel pamoate at label dose or moxidectin. Again, moxidectin proved to be effective by reducing eggs per gram of feces to zero in the mares, while pyrantel pamoate’s efficacy was less than 80%.
• The outcome of the experiments depicted cross-resistance to the different pyrantel salts used, with only cyathostomes identified in the culture. 

Implications of the Study

• This research provided critical insight into the growing issue of resistance among parasites to common medication. It demonstrated the existence of resistance to pyrantel salts in cyathostomes, which are a type of parasitic worm in horses.
• The success of moxidectin against these parasites suggested it as an effective alternative for treating parasitic infections in horses when pyrantel salts fail to deliver results.