SUSCEPTIBILITY OF SUFFOLK AND GULF-COAST NATIVE SUCKLING LAMBS TO NATURALLY ACQUIRED STRONGYLATE NEMATODE INFECTION

Three trials compared responses to naturally-acquired strongylate nema tode infection between suckling Suffolk and Gulf Coast Native (Native) lambs which grazed together. In Trial 1 (1992), infection in 14 lambs of each breed was monitored from birth to 12 weeks of age using fecal egg count (FEC) and blood packed cell volume (PCV). In Trial 2 (1993) , two age-matched lambs of each breed were sacrificed at seven and ten weeks of age to estimate nematode burdens. In Trial 3 (1994), infecti on in 18 lambs of each breed was monitored (FEC, PCV, white blood cell count, differential leukocyte count, and anti-Haemonchus contortus im munoglobulin level) from birth to 8 weeks of age, at which time six ag e-matched lambs of each breed were sacrificed to estimate nematode bur dens. The remaining 24 lambs were monitored until 12 weeks of age. In both Trials 1 and 3, infection in Native lambs peaked and then decline d between 6-10 weeks of age. Infection in Suffolk lambs continued to i ncrease as evidenced by increasing FEC with concomitant reduction in P CV, higher morbidity and mortality (Trial 1), and number of anthelmint ic treatments required (Trial 3). In Trials 2 and 3, the principal nem atode found at necropsy was H. contortus, and infection level was cons istently lower (> 64%) in Native compared with Suffolk lambs. In Trial 3, there was no difference between breeds for WBC, any leukocyte type , or anti-H. contortus immunoglobulin level. These results demonstrate d that suckling Native lambs developed resistance to H. contortus infe ction during their first exposure to infection at an age when they are considered immune incompetent and colostrally transferred anti-ti. co ntortus immunoglobulin did not appear to be involved in the resistance .