Characterizing physiological and genetic differences in the early immune response to Haemonchus contortus in resistant and susceptible sheep

This dissertation compares immune responses of resistant and susceptible sheep to infection with Haemonchus contortus during the peri-parturient period and larval stage of infection. Identification of immunological events resulting in parasite resistance in St. Croix hair sheep may provide better targets for differential gene expression analysis and eventual discovery of selectable markers for parasite resistance. Antibody levels of hair ewes and composite Dorset x Finnsheep-Rambouillet wool ewes were measured during breeding and again after parturition. Results demonstrated that hair ewes had higher levels immunoglobulin-A after infection and maintained a higher level of circulating antigen-specific antibody when compared to wool ewes. To characterize immune responses to the larval stage of infection, hair and wool lambs were sacrificed at 0, 3, 5, and 7 d after infection with H. contortus. Neutrophil migration to abomasal mucosa and lymph node development were higher in hair sheep than in wool sheep. Gene expression analysis indicated no difference in the abomasal lymph node as both breeds expressed a general T-helper cell type 2 (TH2) response. However, profound differences in TH2 responses were observed in the abomasal mucosa, where hair sheep expressed more IL-4, -13 and -33 than wool sheep. These data thus document the presence of immunological differences between the breeds. Immune responses to larval parasite infection in wool sheep are generally suppressed and may increase the magnitude and duration of infection whereas immune responses to larval infection in hair sheep was more robust and more strongly polarized towards a TH2