Fasciola hepatica: An immunofluorescent study of antigenic changes in the tegument during development in the rat and the sheep

Serum from sheep was collected throughout a 30-week period of infection with Fasciola hepatica and specificity for the tissues of flukes of various ages was tested by an indirect fluorescent antibody labeling technique, using as antigen JB4 plastic-embedded sections of flukes up to 30-weeks old grown in rats. Quantitative estimates of host antibody concentration and fluke tissue antigenicity were determined by titration using serially diluted serum. Serum from early infections (pre-7 weeks) gave strong labeling over the tegument of young flukes, but the reaction became progressively weaker with older fluke tissue. This was associated with a decline in the number of T1 bodies in the tegument as revealed by electron microscopy. T1 bodies contain glycocalyx precursor substances and during development they replace the antigenically similar T0 secretory bodies characteristic of early juvenile flukes. Glycocalyx turnover may help protect the pre-bile duct flukes against immunological attack. Serum from sheep with F. hepatica infections older than 7 weeks gave moderate reaction with T2 bodies which accumulated in the tegument during the early stages of infection but only expressed their antigens on the surface about the time of entry into the host's bile ducts. The antigenicity of the gut and excretory system of flukes seemed to remain unchanged throughout adult life. Levels of host antibody specific for juvenile tegument, gut, and excretory system peaked at 3–5 weeks postinfection, and declined once the flukes entered the bile ducts. Anti-T2 antibody appeared 6 weeks postinfection and began to decline 5–6 weeks later.