Length of protection by murine vaccination with living infective third stage hookworm larvae

Objective To determine the length of protection by murine immunization with living third stage hookworm larvae (L 3) as measured by reduction in worm burden and host serologic antibody responses Methods Outbred male (Kunming strain) mice were immunized subcutaneously with 500 L 3 once every 2 weeks for a total of immunization for 3 times, and then challenged orally with 1000 L 3 for 1 to 8 weeks after the final immunization Host protective immunity was determined both by the reduction in worm burden as measured by the number of L 3 recovered from murine lungs 48 hour post challenge, as well as by measurement of circulating antibodies Histopathological responses were also examined Non immunized mice served as negative controls Results The protection by L 3 immunization declined over time One or 2 weeks after the final immunization, worm burdens were reduced 72% and 77 5% after challenge respectively In contrast, only 37% reduction in worm burden was observed when the L 3 challenge was delayed by 4 weeks and protection was almost entirely lost when there was an 8 week delay between the time of final immunization and challenge The reduced level of protection over time partially correlated with diminishing L 3 specific antibody responses Host inflammation in the lungs of immunized mice also diminished Conclusion The protection afforded by living L 3 immunization is maximal for the first two weeks after immunization, but then declines significantly over the ensuing weeks