Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand?

This research article discusses the rising issue of resistance among equine helminth parasites to traditionally administered anthelmintic drugs in New Zealand. Proposed alternatives include reliance on targeted and selective approaches based on parasite population distributions specific to each animal, non-chemical means, and combinations of anthelmintics.

Anthelmintic Resistance in Equine Helminth Parasites

• This research revolves around the growing problem of anthelmintic resistance in equine helminth populations. This is a major concern as many commonly used products could become inadequate due to this resistance.
• The research notes that significant reliance has been placed on the macrocyclic lactone (ML) group of anthelmintics globally. Although this form of medication has been effective, resistance to these products, the benzimidazoles (BZ), and the tetrahydropyrimidines, like pyrantel, has been reported.
• In New Zealand in particular, resistance has already been reported against the ML and BZ groups, but so far, none has been recorded against pyrantel.

Alternatives to Interval-based Anthelmintic Regimens

• One proposed solution to the growing problem of resistance is to rely on characteristics of individual parasite populations in horses. Instead of administering anthelmintics at regular intervals, the medication would be given when the horse exceeds a predetermined level of shedding parasite eggs.
• Such an approach could potentially control the majority of parasite fecundity while still allowing a population of susceptible parasites to remain, thus preserving some effectiveness of the drug.
• This approach, however, would require adequate monitoring and the means of dealing with parasites and lifecycle stages that aren’t readily visible with routine diagnostic methods.

Non-chemical Means of Parasite Control and Combinations of Anthelmintics

• The research also suggests more non-chemical means of parasite control, such as the regular removal of faeces from pasture, thereby reducing the risk of new infections.
• The under-dosing of horses should be avoided and high efficacy anthelmintics should be preferred.
• The paper also endorses the use of combinations of anthelmintics, especially ones targeting the same spectrum of parasite species, in order to achieve greater levels of efficacy and potentially slow down the development of resistance.