Characterizing Peripheral Cellular and Humoral Immune Responses to Haemonchus contortus in Sheep

Gastrointestinal nematode parasitism is the most widely reported health concern among sheep producers worldwide. The most concerning of which is Haemonchus contortus; a blood-sucking nematode found in the abomasum of sheep. Worldwide anthelminthic resistance has prompted exploration of alternative methods of parasite control. Many studies have shown St. Croix hair sheep posses an immunologically mediated resistance to H. contortus infection, however no studies have utilized a true parasite naive animal for immune characterization. In this study, development of peripheral immune response was characterized in parasite resistant St. Croix lambs and parasite susceptible Suffolk crossbred Lambs. Lambs were separated into three treatment groups: naive, primary and challenge infection and experimentally infected with H. contortus larvae. St. Croix lambs had drastically reduced fecal egg count (FEC) during primary infection and were able to reduce FEC to 0. No FEC was observed in challenged St. Croix lambs. Suffolk crossbred lambs had significantly higher FEC in primary and challenge infections, which persisted to the end of the experimental period. St. Croix lambs maintained significantly higher packed cell volumes (PCV) and challenged St. Croix lambs generated significantly higher numbers of circulating eosinophils and monocytes. Additionally, St. Croix lambs had significantly higher levels of circulating IgA and H. contortus antigen specific IgG while Suffolk crossbred lambs had declining levels of antibody. These data indicate St. Croix lambs generate and maintain a more robust immune response to H. contortus infection.