The epidemiology of gastrointestinal nematodes of dairy cattle in central Kenya

The epidemiology of H. placei and of other gastrointestinal nematodes in ye arling dairy cattle was examined on two farms in Kiambu District, central K enya during each of 13 one-month periods from April 1993 to April 1994. On each farm, 32 newly weaned dairy calves were given a single dose of albenda zole and then placed on experimental pastures. Twelve of the animals were d esignated for bi-monthly slaughter (n = 2) and analysis of worm population characteristics and 20 were designated for blood and faecal collection and for weighing. Two parasite-free tracer calves were grazed alongside the wea ner calves each month throughout the study period and were also slaughtered for analysis of worm populations. Faecal egg counts, haematological and se rum pepsinogen determinations, herbage larval counts, and animal live weigh t changes were recorded monthly. The study revealed that Haemonchus placei, Trichostrongylus axei, Cooperia spp. and Oesophagostomum radiatum were res ponsible for parasitic gastroenteritis and that H. placei was the predomina nt nematode present in the young cattle on both farms. Faecal egg counts fr om resident cattle and necropsy worm counts revealed that pasture larval le vels were directly related to the amount of rainfall. The total worm burden s in the animals were highest during the rainy season (March-June and Octob er-December) and lowest during the dry seasons (July-September and January- February). The very low recovery of immature larvae of H. placei from the t racer calves indicated that arrested development is not a feature of the li fe cycle of this parasite in central Kenya. The maintenance of the parasite population depended on continuous cycling of infection between the host an d the pasture. The agroclimatic conditions of the study area were such that , in general, favourable weather conditions for the development and surviva l of the free-living stages of gastrointestinal nematodes existed all year round.