Partial Immunity in Sheep Induced by Haemonchus sp. Isolates from Pronghorn Antelope: Effect of Age of Sheep Host and Chemical Abbreviation of Infections

Abstract

Domestic lambs were less susceptible to isolates of Haemonchus sp. from pronghorn antelope than to an homologous strain of the parasite. These isolates were evaluated as immunizing agents. In controlled experiments with worm-free lambs, antelope strains stimulated an immune response comparable to that of the homologous strain. Increasing the size of the immunizing inoculum did not increase the degree of the response. Because of the practical disadvantage of using a live vaccine, of the immunizing infections was investigated. An appreciable protection was demonstrated following of immunizing infections with phenothiazine at 34 to 37 days but not at 21 days. In experiments involving immunizing infections of 34 days duration, 5to 6-month-old lambs developed a greater degree of protection than did 2to 3-month-old lambs. These results suggest that heterologous strains and should be investigated further as aids in controlling haemonchosis of ruminants. For several years, efforts have been under way in this laboratory to measure the adaptability to domestic sheep of strains of Haemonchus sp. originating in wild ruminants. This line of work was pursued in the hope that some of the wild ruminant strains might be less pathogenic to domestic sheep but at the same time would impart to the host animal some degree of protection against subsequent infections with homologous strains. In 1958, Allen et al. reported in abstract that under controlled laboratory conditions and using pure infections, domestic lambs were significantly less susceptible to Haemonchus sp. from bighorn (Ovis canadensis mexicana), and Barbary sheep (Ammotragus lervia), than to an homologous strain. The results of this work were reported in detail in 1964 (Samson et al.). In 1961, Allen and Samson reported in abstract that vaccination with a strain of Haemonchus sp. from pronghorn antelope protected lambs under laboratory conditions against challenge with an homologous strain. We were unable to demonstrate such protection under field conditions (unpublished data), possibly because of Received for publication 13 November 1969. This work was carried out in cooperation with the New Mexico State University, Agricultural Experiment Station, and was a contribution to the Western Regional Project W-35-Nematode Parasites of Ruminants. * Journal article 351, Agricultural Experiment Station, New Mexico State University, Las Cruces, New Mexico 88001. the contribution of the patent immunizing infections to the overall worm populations on pasture. Subsequently, our efforts were directed toward improvements in these immunization procedures. This report gives data concerning the comparative susceptibility of lambs to antelope and sheep strains of Haemonchus sp., the use and timing of abbreviations of immunizing infections, and the relation of age of lambs to their capability of developing resistance to the parasite. The use of of immunizing infections in the veterinary field was suggested by Soulsby (1961) and is based on the fact, as demonstrated by Soulsby and others, that molting larvae produce functional antigens. The reasoning is that after the larvae have gone through the molting process, producing antigens which have been utilized in antibody production, the removal of the immunizing infections by means should not alter the resistant state of the animal. Some experiences with this procedure have been reported by Christie et al. (1964a, b) and Christie and Brambell (1966). The term chemical abbreviation was used in the former publication and the authors attributed it to Campbell (1963). Manton et al. (1962) demonstrated the importance of age of lambs in immunity to Haemonchus contortus, older lambs being more responsive than young. That age is important has been confirmed by Urquhart et al. (1966a, b).