Effectiveness of pyrantel treatment in donkeys: evidence for resistance

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Faecal egg count reduction tests on two herd subpopulations, Herd A (n=299, sample size n=32) with suspected Pyrantel resistance and Herd B (n=229, sample size n=22) with an unknown resistance status.

  1. To identify helminth parasite infections in 3 donkey populations that have different treatment practices and resistance status.
  2. To compare anthelmintic uses in each population and their impact on infection.

Three different reduction rate thresholds were used to calculate resistance due to industry wide failure to recognise one recommended rate of reduction. Of Herd A sample animals, 81.3% failed to reduce by at least 95%, 62.5% failed at 90% and 56.3% failed at an 80% reduction. These results were statistically significant to a confidence interval of 95%. 72.7% of sample animals in Herd B failed to reduce by 95%, and 63.6% and 40.9% failed at 90% and 80% respectively. However, Herd B results could not satisfy a 95% confidence interval, meaning resistance was suspected but not confirmed. Parasite loading distribution was similar between the two groups, 38% and 32% of donkeys were responsible for 80% and 78% of the parasite load of Herds A and B respectively. Microscopic examination of the faecal samples was performed and examination of the structure of the eggs revealed morphological differences but no speciation could be performed at this stage. Limitations to the study included natural variations in egg shedding; as a control sample from Herd A showed a mean reduction rate of 400epg in the absence of any anthelmintic treatment therefore it cannot be assumed entirely that Pyrantel was solely responsible for the reduction in FEC in either Herd A or B. Direct comparisons between Herd A and B results are limited due to relatively higher day 0 FEC for Herd B, seasonal differences (of up to 16.6°C) between the two sampling periods, variations in sample collection methods.


Pyrantel resistance was confirmed in Herd A even at the most conservative reduction rate of 80% and suspected in Herd B. Pyrantel administered at the current dose rate to these groups does not appear to adequately control faecal egg counts. Action needed worldwide to develop recognised protocol for detecting resistance. Further investigation required into management strategies and alternative methods of parasite control.