EVOLUTION OF INDIVIDUAL SHEEP RESPONSES ( ANTIBODY-PRODUCTION, EXCRETION OF EGGS AND PACKED CELL-VOLUME) DURING 2 SUCCESSIVE EXPERIMENTAL INFECTIONS WITH HAEMONCHUS-CONTORTUS

Eight sheep (five ewes and three youngs of one year old) were subjecte d to two successive artificial infections with Haemonchus contortus se parated by anthelmintic treatment. Antibody response (ELISA IgG), excr etion of eggs and the level of packed cell volume were measured and co mpared during both infections. The results obtained indicated the exis tence of high individual variation in the excretion of eggs and in the intensity of antibody response. It is also possible to separate diffe rent categories of individuals : high responders (high IgG response an d low excretion of eggs), low responders (low IgG response and high ex cretion of eggs) and individuals with intermediate characteristics. In dividual membership to any one of these categories is not permanent. I n fact, some ewes (resistant to H.contortus infection during at first) have shown decreased antibody response and increased excretion of egg s during the second infection trial following lambing. This is likely related to the effect of immunodepression observed during parturition in sheep. This phenomenon should be taken into account in the current conception of plan for the selection of resistant animals against infe ctions with gastro-intestinal nematodes.