A recent trial1 has shown that resistance to macrocyclic lactones such as ivermectin and moxidectin-based products is reducing their efficacy against large roundworms and that pyrantel-based Strongid-P has provided the most effective treatment.

The trial, involving 26 horses on a stud in the Netherlands, demonstrated continued evidence of large roundworm eggs despite regular treatments with ivermectin and moxidectin. However, the egg counts were markedly reduced with a subsequent treatment of Strongid-P.

The investigators concluded that the most likely reason for the failure of macrocyclic lactones to treat large roundworms was that the horses in the trial had developed resistance to these wormers. They had been on a predominantly ivermectin based treatment regime for the past 10 years and it was suggested that the overuse of macrocyclic lactones may have caused this resistance to develop.

Commenting on these findings, Steven Fay MRCVS, Veterinary Adviser for Pfizer Animal Health said: “The extremely important discovery of apparent resistance to macrocyclic lactones, combined with the continued efficacy of pyrantel-based products such as Strongid-P, is likely to have a considerable impact on the current understanding of effective worming strategies. It emphasises the importance of careful selection of wormers for annual rotation to minimise the chance of resistance. For example, the majority of wormers are in the macrocyclic lactone group and it makes sense to rotate between one of these and Strongid-P on an annual basis. As a simple rule of thumb if you used a wormer whose name started with the letter “E” last year, this year you should choose Strongid-P”

Further information can be obtained by calling Pfizer Animal Health on 01737 331333 (Option 1)

1. JH Boersema, DVM Eysker, JWM Nas, Apparent resistance of Parascaris equrum to macrocyclic lactones. The Veterinary Record (2002) 150,279-281