Response of resistant and susceptible Brazilian Somalis crossbreed sheep naturally infected by Haemonchus contortus

This study was carried out to evaluate the performance of Brazilian Somalis sheep to natural infections by gastrointestinal nematodes. During 98 days, 75 weaned sheep, initially 3-4 months old, were kept on the same pasture and evaluated. Fecal and blood samples were collected for parasitological and hematological exams. After this period, the eight most resistant and the eight most susceptible animals were selected based on their individual averages of nematode fecal egg counts and were slaughtered for worm burden determination and nematodes identification. Abomasum and abomasum lymph nodes were also recovered for gene expression analysis. The animals selected as resistant had lower fecal egg counts during experimental period and smaller worm burdens than the susceptible ones (P<0.05). The genus Haemonchus, followed by Trischostrongylus and Oesophagostomum, were identified in composite cultures. Haemonchus contortus was the specie identified in the abomasum. Packed cell volume and total plasma protein means were higher in the resistant group (27.2 % and 6.1 g/dL) than in the susceptible one (22.5 % and 5.3 g/dL), respectively. Regarding cytokine gene expression, IL-4 (P<0.05) was up-regulated in the abomasum of resistant animals and TNF-alpha (P<0.03) and IFN-gamma (P<0.03) in susceptible ones. In abomasum lymph nodes, IL-4 (P<0.04) and IL-13 (P<0.05) were up-regulated in the resistant animals and IFN-gamma in the susceptible one (P<0.01). This work provides further evidence that, within a given animal breed, individuals have different responses when infected by gastrointestinal nematodes. Resistant animals who responded more quickly and efficiently to these infections activated a TH2-type response.