Epidemiological studies of parasitism in sheep and reproduction in horses : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies (Epidemiology) at Massey University
Anthelmintic resistance is a well-recognised problem for control of nemotodes in sheep in most countries of the world. The climatic conditions in New Zealand are particularly favourable to the survival and development of gastrointestinal nemo=atodes. As a consequence, gastrointestinal parasitism is a major impediment to profitable sheep raising in New Zealand.
A random postal survey of 300 sheep farmers in the southern North Island region of Manawatu was conducted with the purpose of examining current farming and drenching practices and investigating possible risk factors in the development of anthelmintic resistance. The results of this study, reported in Chapter 2, revealed a high degree of awareness and concern about the problem of resistance, but also a lack of understanding on how grazing management strategies should be combined in order to achieve integrated control over gastrointestinal nematodes while minimising the use of anthelmintic drugs. Only 31% of respondents had performed at least one drench test on their property. Amond testing farms, prevalence of resistance approached 70% and involved benzimidazole products in all but one case.
Subsequently, a trial was undertaken to investigate the economic consequences of anthelmintic resistance in growing lambs on commercial farms (Chapters 3 and 4). Five farms with a history of resistance to benzimidazole drenches were selected. The effects of three treatment strategis on nematode egg counts, bodyweight gains and susceptibility to diarrhoea were compared between groups of ewe lambs. Suppressively treated lambs performed significantly better than effectively treated lambs, which in turn performed better than ineffectively treated lambs. However, a partial budgeting analysis carried out by means of a stochastic simulation model (Chapter 4) indicated that effective treatment yielded the highest net returns. The model also showed that the range of possible outcomes oscillated substantially around the mean, reflecting the degree of uncertainty about the outcome on any single farm due to variation between farms.
Chapter 5 describes a study which was carried out with the objective of evaluating two management strategies for breeding mares after foaling. Mares were examined on day 7-9 postpartum by palpation and ultrasound. Pregnancy rates, pregnancy loss rates and time from foaling to conception in two groups were compared. The statistical power of the study as determined by power analysis, was insufficient for the observed differences to reach statistical significance.