Genetic analyses of polymorphisms on ovine chromosomes 5 and 20 and their effect on resistance to internal parasites

A trial was conducted to evaluate the association between polymorphisms in, or close to, candidate genes and faecal egg counts (FEC) in sheep. Corriedale and Polwarth weaned ewe lambs from commercial flocks were submitted to three successive experimental challenges in naturally contaminated pastures in Southern Brazil. During the trial, Haemonchus contortus was the predominant parasite species. Individuals were phenotyped according to their post-challenge FEC averages. Polymorphisms at interleukin genes (IL-3, IL-4 and IL-5), located at sheep chromosome 5 (OAR5) and the microsatellite markers BM1815, CSRD226, OarHH56, and OMHCI, located at sheep chromosome 20 (OAR20), were tested. A forced PCR-RFLP method to genotype ovine IL-5 was also developed and described. Associations between phenotypes and genotypes were tested by allele substitution analyses. There was no significant association between logFEC and the polymorphisms examined here, except for IL-4 in the Corriedale population. Results showed that FEC averages were significantly reduced when IL-4*B allele, the most frequent allele, was replaced by either IL-4*A or IL-4*C alleles (P < 0.05).