Parasite diversity and anthelmintic resistance in two herds of horses

Diversity of parasite populations was compared between two herds of horses, one a regularly treated herd the other a feral herd which has bad no anthe lmintic treatment for at least 25 years. Eggs obtained from fecal samples o f both herds were tested for anthelmintic resistance by use of an in-vitro larval hatch/development assay (LDA), DrenchRite(TM). A fecal egg reduction test was also performed with the domesticated herd using fenbendazole, pyr antel pamoate and ivermectin. Cyathostomes were the predominant group of worms present in both herds. Tri chostrongylus axei was seen in both herds, but Strongylus equinus, Strongyl us vulgaris, Gyalocephalus capitatus, Poteriostomum spp, and Strongyloides westeri were only found in the feral horses. Larvae of Strongylus edentatus were found in a single domesticated horse. Fecal egg reduction tests with the domesticated herd showed a 32% egg count reduction for fenbendazole, a 93% reduction with pyrantel, and a 99% reduction with ivermectin. From the LDA, anthelmintic resistance was evaluated by determining the resistance ra tio of the domesticated herd compared with the feral herd. For benzimidazol es in the domesticated herd, 45% of the cyathostome population was 9.4 time s more tolerant than the feral herd's parasite population. The parasite pop ulation in the domesticated herd was 1.5 times more tolerant to Levamisole, and 1.7 times more tolerant to the benzimidazole-levamisole combination th an the parasite population within the feral herd. 9% of the parasite popula tion in the domesticated herd was 90 times more tolerant to avermectins tha n the feral herd's parasite population, even though a subpopulation of worm s in the feral herd were tolerant to low concentrations of avermectins desp ite never being previously exposed to this class of anthelmintic. (C) 1999 Elsevier Science B.V. All rights reserved.