Exploring the Genetic Control of Resistance to Gastrointestinal Helminth Infections in Sheep and Mice

This paper reviews research being undertaken by ILRI and collaborators to identify Quantitative Trait Loci (QTL) for resistance to gastrointestinal (GI) nematodes in the resistant East African Red Maasai sheep and in inbred strains of mice. Six F1 rams, crosses of Red Maasai (resistant) and Dorper (susceptible) sheep, were bred and then backcrossed to pure Red Maasai and Dorper ewes to produce about 1200 double backcross progeny in six families over a 3-year period (1998-2000). These progeny were monitored for faecal egg count (FEC) and blood packed red cell volume through two cycles of natural challenge on pasture (predominantly with Haemonchus contortus), followed by one artificial challenge with H. contortus under controlled indoor conditions. Following the artificial challenge, the sheep were slaughtered and worm counts made at autopsy. Genome-wide QTL mapping is underway with the aim to identify the major chromosomal regions that determine the resistance of the Red Maasai parents. In parallel to the sheep work, we have also been studying the genetic control of helminth resistance in mice. An initial mapping of genes controlling resistance to Heligmosomoides polygyrus (a natural trichostrongyloid GI nematode of mice) in an F2 crossbred population of resistant SWR and susceptible CBA inbred lines of mice has located 14 QTL on 12 different chromosomes affecting both the parasitological (FEC at weeks 2, 4 and 6 of the challenge period and worm count at necropsy at 6 weeks) and immunological traits that were measured. Five of these QTL were associated with two or more traits.