ASPECTS OF THE HUMORAL AND CELL-MEDIATED IMMUNE-RESPONSES OF SHEEP TO THE KED MELOPHAGUS-OVINUS (DIPTERA, HIPPOBOSCIDAE)

Infestations by the sheep ked, Melophagus ovinus, stimulated the production of antibody toward sheep-ked proteins. Both primary and secondary antibody responses closely followed the rise and fall of 2 consecutive ked infestations of either innately resistant or susceptible sheep. Maximum titers coincided with peak ked populations and were attained 5.5 and 4 months after primary and secondary infestations, respectively. Acquired resistance was demonstrated for innately resistant and susceptible sheep, since both groups had significantly lower secondary infestations even though their antibody responses were similar to those observed for primary infestations. Lambs passively sensitized with immunoglobulin (IgG) developed resistance at the same rate as control lambs, and their peak ked populations and ked mortality were similar to those observed in controls. The proliferative response of peripheral blood lymphocytes to phytohemagglutinin (PHA) was significantly depresses at 5, 7, 19, and 21 wk postinfestation; however, a significant response to ked antigen was observed at 5 and 21 wk postinfestation. It is postulated that acquired resistance to keds is a chronic inflammatory response combined with specific immune effector elements.