Resistance to benzimidazole anthelmintics in equine strongyles. 1. Frequency, geographical distribution and relationship between occurrence, animal husbandry procedures and anthelmintic usage.

A survey was conducted to determine whether benzimidazole resistant populations of equine strongyles are present in New South Wales and north central Victoria; what is their frequency and geographical distribution; which species are involved; and whether different methods of parasite control could be related to the occurrence and frequency of anthelmintic resistant populations. Resistant populations of strongyles were found over wide areas of New South Wales and in north central Victoria. There was no relationship between geographical location and the occurrence of benzimidazole resistance. The species involved were small strongyles of the sub-family Cyathostominae. There was a direct correlation between the occurrence of resistance (including the level at which it is present) and the frequency of use of benzimidazole anthelmintics. Examination of management practices showed that resistance is not an important problem on farms where different chemical classes of anthelmintics were used in a slow rotation programme; combination anthelmintic therapy (for example, benzimidazole/piperazine/organophosphates) was used and anthelmintic treatment was given at intervals of not less than 16 weeks. Tentative suggestions are made for the control of small strongyles in the light of an emerging resistance problem.