Prevalence and control of strongyle nematode infections of horses in Sweden

Strongyle nematodes constitute the most important group of helminths in horses. In this PhD project, based on five separate studies, the overall aim was to investigate the present parasite status of horses in Sweden and to determine the efficacies of the commonly used anthelmintics. It is hoped that the results will serve as a foundation for the development of relevant control strategies in horse establishments. In a national survey strongyle nematodes were found to be ubiquitous in Sweden, and it was established that young horses on studs had the highest faecal output of strongyle eggs. Furthermore, horses in the south of the country shed more eggs than those in the central and northern parts. Eggs from the pathogenic species Strongylus vulgaris were identified in 14% of 110 farms investigated. In a subsequent study, the prevalence and species composition of small strongyles were investigated in 27 horses on a stud in southern Sweden. Following anthelmintic treatment, worms expelled in faeces were collected and identified to the species level. In addition, the expulsion method was evaluated. Fifteen species of the tribe Cyathostominea were documented for the first time in Sweden. The mean number of species per horse was nine. The presence of anthelmintic resistance in populations of small strongyles was investigated in two studies using different detection methods, namely the larval development assay (LDA) and the traditional faecal egg count reduction test (FECRT). In the first of these studies, it was concluded that LDA was not a reliable tool for routine monitoring of horse establishments. This was due to difficulties in data interpretation and inexplicable variation in the development of parasite larvae within and between assay plates. The latter study revealed that benzimidazole resistance was widespread despite a low use of these drugs for the past 20 years. Moreover, the first case of pyrantel resistance in Sweden was documented. Finally, a questionnaire study on parasite control practices on Swedish horse establishments was performed. Most respondents were concerned about anthelmintic resistance, although they considered parasites to be a minor problem in their horse establishments. The survey highlighted the need for changes in routines in order to slow the spread of anthelmintic resistance.