Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

First Advisor

James E. Miller


Haemonchus contortus is a serious constraint for sheep production in tropical and subtropical regions of the world. The use of genetically resistant animals is a promising alternative for controlling nematode infections without relying on the use of anthelmintics. Extensive epidemiological studies demonstrated that Gulf Coast Native sheep are naturally resistant to H. contortus infection, but the mechanism underlying this resistance is not well known. The main purpose of this research was to define the components of the immune response that may be involved in resistance of GCN to H. contortus infection. Three studies were conducted. In the first study, a group of neonate lambs was treated with dexamethasone for ten weeks while grazing on pasture with their dams. All lambs were monitored weekly for blood packed cell volume (PCV), white blood cell differential, and fecal egg count (FEC) for the duration of the study. The second study was similar to the first one but post-weaned lambs were used. They were kept in dirt floored pens and experimentally infected. Treated lambs showed higher FEC and nematode burden and lower PCV and antibody titer to H. contortus whole nematode antigen, compared to controls. These studies indicate that there is a component of the immune response that may play a role in the natural resistance of GCN sheep to H. contortus infection. To further characterize components of the immune response, a third study was conducted in which a group of GCN lambs was depleted of their CD4+ T lymphocytes and challenge with H. contortus infective larvae. The lambs in the treatment group received serial injections of mouse monoclonal antibody to sheep CD4+ T lymphocytes. Lambs in the treatment group showed higher FEC and nematode burden than the controls. These results indicated that CD4+ T lymphocytes were important in immunity of GCN sheep to H. contortus infection. This research has contributed to the better understanding of the mechanism(s) underlying natural resistance of GCN sheep to Haemonchus contortus infection.