The use of 35S-labelled Toxocara canis to determine larval numbers in tissues

The potential of using 35S-labelled larvae to determine the number of second-stage Toxocara canis larvae present in the tissues of infected animals was assessed. Infective larvae were labelled by in vitro culture with medium containing 35S-methionine. The amount of radiolabel attached to larvae decayed exponentially with time and had an in vitro mean half life of 3.54 +/- 0.65 days. The 'lost' radiolabel was incorporated into proteins which formed part of the worm's excretory/secretory products. The levels of radioactivity present in different organs of BALB/c mice, infected with 35S-labelled T. canis larvae, varied over the course of infection. Initially most of the radioactivity was present in liver, but over the course of infection 35S liver levels gradually decreased and brain levels increased. By day 14 post-infection the majority of the isotope was present in the brain (p < 0.01). Assessment of antibody levels on day 14 post-infection showed that infection with 35S labelled T. canis larvae induced the production of parasite-specific IgM, IgG and IgG1 antibodies.