Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design


Division of Animal and Nutritional Sciences

Committee Chair

Scott Bowdridge

Committee Co-Chair

Janet Tou

Committee Member

Janet Tou

Committee Member

Jianbo Yao

Committee Member

Rosana Schafer

Committee Member

Anne Zajac


Haemonchus contortus is a hematophagous, gastrointestinal parasite affecting small ruminants. Parasitism by H. contortus is the greatest concern of sheep producers in the United States and accounts for billions of dollars in economic loss worldwide. However, different breeds of sheep display differences n natural resistance to H. contortus. St. Croix hair sheep have a remarkable ability to clear H. contortus without the aid of anthelmintics and develop a robust T helper type 2 (Th2) immune response, preventing establishment of infective stage 3 larvae (L3), when compared to susceptible Suffolk sheep. However, the mechanism of development of immune response and differences between resistant and susceptible breeds is not fully understood. In these studies, response of ovine monocytes, from resistant and susceptible breeds of sheep, to whole L3 were evaluated in vitro. Monocytes were isolated from peripheral blood mononuclear cells and cultured with L3 over 6 to 15 hours to measure gene expression and effector function, by measuring larval ATP and motility. Separation of peripheral blood mononuclear cells (PBMC) into monocytes and lymphocytes demonstrated a clear effector function of monocytes against L3. Monocyte and macrophage activation state have been associated with helminth infection and play an important role in induction of Th2 type responses for overall host protection. Inhibition of inducible nitric oxide synthase in Suffolk monocytes rescued susceptibility, whereas inhibition of alternative pathways reduced effector function in both breeds, and thus, monocytes required arginase-1 for full effector function. Though St. Croix derived monocytes up-regulated genes associated with classical (M1) activation, early activation of alternative markers (M2) were present compared to Suffolk derived monocytes. Interleukin-13 (IL-13) is a cytokine critical to Th2 type immune response and promoting alternative activation of monocytes in context of helminth infection. Inhibition of IL-13 in monocyte culture dampened effector function in St. Croix derived cells. However, larval morbidity was not IL-13 dependent. Independent of cellular mechanisms, IL-13 was observed to cause larval paralysis, reducing larval speed and distance, identifying a previously unknown role of IL-13. Overall, St. Croix displayed increased paralytic activity of from monocyte culture supernatant, in conjunction with heightened monocyte-mediate larval morbidity, and therefore may be reflective resistance to H. contortus infection and development of full host protective immunity.