Studies on the pathogenicity and immunology of Trichostrongylus colubriformis (Giles, 1892) Looss, 1905 infection in the domestic rabbit, Oryctolagus cuniculus

Abstract

The host- parasite relationship of a hybrid strain of domestic rabbit, Or ctola,gus cuniculus, and the ruminant nematode Trichostronrylus colubriformis was studied. The main techniques used to follow the infections were faecal egg counts, live weight gains, worm counts at autopsy, haematology and serum biochemistry. The newly -weaned rabbit was found to be a moderately good host for the nematode. Most animals developed patent infections and worm egg production was of varying size and duration. The sex of the host appeared to have little influence on the numbers of worms establishing or on the subsequent course of the infections. Adaptation of the parasite to the rabbit by successive passage through 16 senerations indicated that total egg production by the worms increased over the first three generations, dropped in the fourth, rose up to the seventh and then stabilised. Further evidence for such adaptation was provided by an experiment in which it was shown that the total egg production by a fifth generation rabbit -adapted strain of the worm was significantly greater than that of a sheep strain.Doses as high as 15,000 infective larvae of either strain of the worm did not appear to be very pathogenic in the rabbit and acute disease was produced on only three occasions. The most consistent clinical sign was anorexia. Most animals showed only transient clinical signs of disease or none at all and a true chronic trichostrongyliasis as described in sheep and goats was never seen.An age -related resistance was observed in rabbits aged 13 weeks and older and this was manifested mainly by a reduction in the numbers of worms reaching the adult stage.Most newly- weaned rabbits eventually developed acquired resistance to a primary infection and expelled their worm burdens apparently in a self -cure reaction. _bout this time a transient eosinophilia often accompanied by a basophilia occurred in the peripheral blood. After this the rabbits were resistant to a challenge infection which they ejected within 12 days. A similar blood response was seen during the first 11 days of this infection. However a minority cf rabbits failed to self -cure after their primary infections and these rabbits were not then resistant to a challenge infection. No blood response was observed in these or in worm -free controls. during a primary infection.Experience of primary infections, chemically abreviated after 12 days by thiabendazole did not appear to give the rabbits the ability to resist a challenge infection but the results were inconclusive because of interference by age resistance.No variation from the normal was observed in the serum proteins of rabbits infected with the nematode.The current theories relating to the function of the eosinophil and basophil leucocytes of mammals are discussed and possible roles for these cells in the mechanism of the self -cure reaction to T.colubriformis in the rabbit are suggested. It is pointed out that the rabbit -T.colubriformis host- parasite relationship affords a useful system for studying the phenomenon of self-cure.It is also noted that the pathogenicity and immunology of T.colubriformis infections in the rabbit are different to those in the guinea pig.