THE EPIDEMIOLOGY OF NEMATODE INFECTIONS IN SHEEP IN A COOL TROPICAL ENVIRONMENT

The epidemiology of nematode infections in Menz sheep was studied in t he highlands of Ethiopia at the International Livestock Research Insti tute (ILRI), Debre Berhan Research Station, using a series of tracer l ambs grazing contaminated pasture for either 4, 16, 32 or 48 weeks fro m July 1992 to June 1994. The basic nematode seasonal infectivity patt ern was expressed in terms of relative numbers of third-stage larvae ( L-3) available on pasture for different months. Data from faecal nemat ode egg counts, pasture larval recoveries and worm counts from the tra cer lambs were used to investigate the infectivity pattern, Four nemat ode species of economic importance: Longistrongylus (Pseudomarshallagi a) elongata, Trichostrongylus 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 peak s 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 rai ny season (i.e. March-May) were not conducive to the development and s urvival of nematode eggs and the free-living stages, hence little or n o transmission occurred. Rainfall and humidity seemed to be the most i mportant factors for the development of eggs and free-living stages. T he 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 ar e discussed in relation to control strategies. (C) 1997 Elsevier Scien ce B.V.