Serum interleukin-4 (IL-4) production is associated with lower fecal egg count in parasite-resistant sheep

• Haemonchus contortus challenged St. Croix lambs generated a fecal egg count of <50.
• St. Croix sheep have greater circulating red blood cells.
• Increased serum IL-4 in St. Croix sheep is associated with decreased fecal egg count.

The objective of this experiment was to determine serum interleukin-4 (IL-4) concentration in response to Haemonchus contortus infection in parasite-resistant and -susceptible lambs. St. Croix (STC) (resistant) and Suffolk/Hampshire crossbred (SX) (susceptible) lambs were either not infected (n = 5/breed), given a primary inoculation (n = 5/breed) or challenged infected with H. contortus (n = 5/breed). Each inoculum given consisted of 10,000 L3 larvae. Blood was collected daily for 14 days and then weekly to day 49. Feces were collected on day 0 and weekly until day 49. Challenged STC lambs generated significantly lower fecal egg count (FEC) (1520 eggs/g SX vs. <50 eggs/gram STC; P < 0.001) and had higher PCV (34% vs. 29%; P < 0.001). Serum IL-4 concentrations of primary-infected STC and challenge-infected SX lambs were greater during early infection (days 0–7) than mid (days 14–28) and late (days 35–49) infection, but was significantly reduced (P < 0.001) by day 49. Challenge-infected STC lambs incrementally increased serum IL-4 from early to late infection. Change in serum IL-4 concentration during early, mid and late infection indicated IL-4 concentration in challenge-infected STC lambs increased during mid and late infection. These data demonstrate that parasite-resistant St. Croix sheep generate a potent Th2-response, as measured by elevated serum IL-4 concentration, which is associated with a marked FEC reduction.