Circulating excretory-secretory antigen levels and specific antibody responses in mice infected with Toxocara canis.

Circulatory excretory-secretory antigen levels and IgM and IgG responses to larval antigens were monitored in the serum of 20 BALB/c mice that had been given approximately 500 infective eggs of Toxocara canis by stomach tube. Other groups of mice received different doses of infective eggs, ranging from 5 to 1,250 eggs. Excretory-secretory antigens were collected from culture fluid in which mechanically hatched larvae of T. canis were maintained. An indirect enzyme-linked immunosorbent assay was used to monitor specific antibody responses. Circulating antigen levels were monitored using a direct ELISA which incorporated an IgG fraction of a rabbit antiserum to the excretory-secretory antigens as a capture antibody and a biotin-conjugated form of the same rabbit IgG as the second antibody. The antigen-specific IgM response was evident the first week of infection and peaked 3 to 6 weeks post-infection. The antigen-specific IgG response first appeared the second week of infection and peaked at 6 to 8 weeks post-infection. Both isotype levels stayed near their peak values for the remainder of the study. In the untreated sera, circulating antigen was initially evident and highest the first week of infection; the antigen concentrations dropped by the third month of infection to low, but significant, levels that persisted for the duration of the study. The administration of greater than 25 eggs produced antigenemias. There appeared to be a positive linear trend between the number of eggs given and the amount of antigen in the circulation.