The epidemiology of nematode infections in sheep in a cool tropical environment

The epidemiology of nematode infections in Menz sheep was studied in the highlands of Ethiopia at the International Livestock Research Institute (ILRI), Debre Berhan Research Station, using a series of tracer lambs grazing contaminated pasture for either 4, 16, 32 or 48 weeks from July 1992 to June 1994. The basic nematode seasonal infectivity pattern was expressed in terms of relative numbers of third-stage larvae (L3) available on pasture for different months. Data from faecal nematode egg counts, pasture larval recoveries and worm counts from the tracer lambs were used to investigate the infectivity pattern. Four nematode species of economic importance: Longistrongylus (Pseudomarshallagia) elongata, Trichostronglylus colubriformis, Haemonchus contortus, and Dictyocaulus filaria, were recovered from sheep. The largest numbers were recovered during the wet season (i.e. July to November) with peaks in late August and early September. During this 2 year study period, the seasonal pattern of sheep gastrointestinal nematodes was clearly defined. An important finding was that conditions during the short rainy season (i.e. March-May) were not conducive to the development and survival of nematode eggs and the free-living stages, hence little or no transmission occurred. Rainfall and humidity seemed to be the most important factors for the development of eggs and free-living stages. The period of acquisition of third-stage larvae from pasture was found to be relatively short, suggesting that a strategic control programme is feasible with minimal anthelmintic use. Results from the studies are discussed in relation to control strategies.