Comparison of Immune Responses During Gastrointestinal Helminth Self-Cure Expulsion Between Resistant Gulf Coast Native and Susceptible Suffolk Sheep

Abstract

The immune response to the self-cure phenomenon seen during gastrointestinal nematode (GIN) parasitism of small ruminants was compared between sheep breeds that are resistant or susceptible to Haemonchus contortus infection. Fifty-four Gulf Coast Native (resistant) and Suffolk (susceptible) lambs were allowed to acquire a natural GIN infection on pasture and were then randomly allocated into 4 groups. After being moved to parasite free housing for 2 months, lambs were given a challenge infection of 20,000 H. contortus L3. Fecal egg counts (FEC) were monitored throughout the study and animals were necropsied at 0, 1, 3, and 7 days post infection (DPI). FEC decreased beginning at 3 DPI in both breeds, with Native lambs having a higher percent reduction in FEC at 3 and 7 DPI compared to Suffolk lambs. Both Native and Suffolk lambs were able to expel their existing adult population. However, while Native lambs also successfully cleared the larval challenge, Suffolk lambs did not. The numbers of eosinophils within the abomasal mucosa reflect the magnitude and timing of fecal egg count reductions in both breeds. Additionally, the elevated levels of eosinophils within the abomasal mucosa of Native lambs at 3 DPI are likely to be involved with the clearance of the larval burden via eosinophil mediated larval killing. Suffolk lambs displayed a delayed cellular response that resulted in larvae to entering the mucosa before sufficient eosinophilic response could be established. Elevated mast cells within the abomasal mucosa coincide with the clearance of adult GIN and along with elevated levels of IL-13 seen in both breeds suggest their involvement. The results confirm that self-cure is an immune mediated response that can occur in both resistant and susceptible breeds of sheep while differences in the magnitude and time course of immune responses may prevent susceptible sheep from fully clearing the infection.