Graduate Thesis Or Dissertation
 

An epidemiologic study of the seasonal incidence of nematode parasites in sheep on irrigated pastures in Central Oregon

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  • The epidemiology of nematode infections in ewes and lambs on irrigated pastures was investigated during one reproductive cycle in a commercial operation in Central Oregon. Primary objectives were to determine the sources of parasitism, the effect on lamb weight gains, and the effectiveness of control procedures in minimizing the detrimental effects of parasitism. Methodology included fecal nematode egg counts, animal necropsies and the use of tracer lambs. The relationships between blood trace mineral levels, parasitism and suboptimal weight gain in lambs and calves were also examined. High pasture burdens of infective larvae were found in spring and early summer from overwintering larvae and in late summer from autoinfestation by ewes and lambs. Lamb weight gain from March until mid-July was 0.48 lb/head/day. From mid-July until September when the study terminated, lamb weight gain declined (-0.03 to 0.28 lb/head/day). Parasite burdens were low to moderate early in the grazing period, but increased markedly during the mid-July to September period, despite monthly deworming of the flock. It appeared that parasite burdens, high summer temperatures and the lack of shade, and possibly low cobalt and selenium levels were responsible for the late summer decline in weight gain. Evaluation of control measures indicated that intensive deworming prior to parturition eliminated arrested or inhibited larvae in the ewes. The periparturient relaxation of resistance, however, permitted development of infective larvae acquired from the pre- and post-lambing pastures. Maturation of these larvae caused a marked rise in post-parturient egg per gram counts of the ewes with subsequent relatively high rates of pasture contamination. Monthly deworming of the flock was moderately effective in reducing parasite burdens and pasture contamination. Alternate grazing of pastures by cattle and sheep, and ad lib. feeding of phenothiazine in salt were not effective as control measures.
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