Genetic resistance to gastro-intestinal nematode parasites in Red Maasai, Dorper and Red Maasai× Dorper ewes in the sub-humid tropics.
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Resistance to naturally acquired gastrointestinal (GI) nematode parasite infections (predominantly Haemonchus contortus) was studied in 166 Masai, 230 Dorper and 294 Masai × Dorper ewes in the subhumid coastal region of Kenya. Body weight, blood packed cell volume (PCV) and faecal egg count (FEC) were recorded at mating, 3 months after mating, 1 week before lambing and 1, 2 and 3 months after lambing for 4 separate lambings that took place between 1993 and 1996. The Masai ewes were more resistant to GI nematode infections than Dorper ewes, as shown by their lower FEC (P<0.05) and higher PCV (P<0.05) at most of the sampling times over the reproductive cycle. The breed difference for FEC was significant in the lactating ewes but not in the non-lactating ewes. At most sampling times, the crossbred ewes were as susceptible as the Dorper ewes in terms of both PCV and FEC, particularly at the 1 and 2 month postlambing samplings. Resistance was also manifested as a lower proportion of ewes having to be treated with an anthelmintic and a lower mortality rate in the Masai than the Dorper breeds. The Masai ewes were 1-2 kg lighter (P<0.05) than the Dorper ewes at all sampling times. There was a significant increase in FEC and decrease in PCV over the first 2 months of lactation in lactating ewes compared with non-lactating ewes. This periparturient increase in FEC occurred in both breeds and in the crossbreds, but was more marked in the Dorper ewes.