Experimentally induced Haemonchus contortus infections in the rabbit.

Haemonchus contortus originally derived from an Ontario field strain was used to infect laboratory rabbits. Both enshealthed and artificially exsheathed larvae were given at doses of up to 10 million larvae per rabbit. Only artifically exsheathed larvae were infective and a dose of at least 50,000 was required to produce a significant level of infection. H. contortus completed its development in the rabbit, but at a slower rate than in the sheep. Although a few fecund females were found 35 days after infection no eggs were observed on faecal examination. A high proportion of early fourth stage larvae were recovered from numerous rabbits including some which were examined well beyond a postinfection period when all worms should have been in the adult stage. This phenomenon occurred when freshly harvested and exsheathed larvae were used. Post-exsheathed cold conditioning of larvae at 5degreesC further increased the proportion of early fourth stage larvae (maximum proportions were found after 21 and again after 79 days of cold conditioning and thereafter declined). Whether such larvae were inhibited cannot be stated definitely. However, several of these larvae contained crystalline rods within their soma, and the structure of the crystals was similar to those previously described from inhibited H. contortus larvae recovered from sheep. It was concluded that the laboratory rabbit could be a useful host for further studies on some aspects of the biology and pathogenicity of H. contortus.