Observations on epidemiological and clinical aspects of gastrointestinal helminthiasis of sheep in northern Nigeria during the rainy season.

During the rainy season of 1976 severe gastrointestinal helminthiasis was observed in Yankassa sheep grazing unimproved pasture in northern Nigeria. Until mid August high mortality from acute haemonchosis was mainly in lambs. Later, lambs and ewes died from haemonchosis combined with trichostrongylosis but the course of the disease was less rapid. Animals surviving until the end of October showed combined symptoms of chronic haemonchosis and of trichostrongylosis. A rapid turnover rate of adult and developing Haemonchus contortus resulted in a replacement of the blood sucking stages by arrested early fourth stage larvae from mid August onwards. Trichostrongylus spp had a lower turnover rate and only low levels of arrested development occurred, resulting in a gradual build up of Trichostrongylus spp populations during the rainy season. About three or four generations of trichostrongylids developed throughout the rainy season. The first and second generation caused primarily acute haemonchosis, while later generations caused chronic haemonchosis combined with trichostrongylosis.