Two different aspects of the immunology of rabbits infected with Fasciola hepatica were investigated, the first being the development of host resistance to repeated infections with this parasite and the second was the in-vitro response of lymphocytes of infected animals to the presence of an extract prepared from this fluke.
Three other parameters proved to be of value in monitoring the development of the disease. The serum glutamic dehydrogenase levels and the numbers of peripheral eosinophils were both elevated while the parasites were causing parenchymal damage to the livers but fell rapidly following successful chemotherapy. The establishment of infection also resulted in the production of antibodies which could be detected in gel- precipitin tests within three to four weeks after infection.
Diamphenethide (Coriban, Burroughs, Wellcome and Company) was shown to have no apparent effect on the immature stages of F.hepatica four to five weeks after infection at an oral dose rate of 240 -500mg per Kg body weight. Later studies revealed that rafoxanide (Flukanide, Merck, Sharp and Dohme) was more effective against the immature stages of the flukes than either nitroxynil (Thodax, May and Baker) or diamphenethide.
No evidence of acquired resistance was observed in rabbits given a single challenge infection nine weeks after an initial infection with either 100 or 500 meta - cercariae, this initial infection having been removed by the administration of two doses of rafoxanide given four and five weeks after infection.
owever, in a later study in which the challenge infection was given after two similar previous infections had been curtailed with rafoxanide, there was some reduc- tion in the number of flukes recovered, together with changes in the gross pathology of the livers. It was not possible to decide whether this resistance was immunologically mediated or was purely or partly the result of the fibrosis and other changes in the tissues of the livers.
Both the morphological changes and the uptake of tritiated thymidine by peripheral lymphocytes of rabbits infected with F.hepatica showed evidence of an in -vitro response to the presence of an antigen prepared as a 1:5 v/v extract in Minimal Eagles Medium from adult F.hepatica somata. This antigen is also slightly mitogenic for lymphocytes from uninfected rabbits but there was a significant increase in the response within two weeks of infection.
The parameters of this technique were studied and it was found that maximum stimulation could be achieved by incubating the lymphocytes for four days and allow- ing labelling to occur during the final 24 hours. The optimal number of lymphocytes in whole blood cultures of 1 ml total volume was 0.5 million and maximum labelling occurred in the presence of 0.0625 ml of the fluke antigen per culture.
Similar levels of response were shown by lympho- cytes obtained from rabbits given either 100 or 500 metacercariae and there was no immediate change in the response after effective chemotherapy. There was some evidence of a slow diminution in the response in pro- longed infections.
Challenge infections did not cause a further increase in the level of lymphocyte stimulation. It was also shown that there was both an absolute and a relative increase in the numbers of large lympho- cytes in the peripheral blood of rabbits infected with F.hepatica.
There was increased uptake of tritiated thymidine by unstimulated cultures from animals infected with F.hepatica compared with similar cultures from uninfected rabbits. This was probably related to the increased number of large lymphocytes into the peripheral circula- tion of the infected animals.