The effect of multiple transfers of immune serum on maturing Schistosoma bovis infections in calves.

To investigate the role of humoral factors in immunity, serum from cattle with naturally acquired immunity to Schistosoma bovis was injected intraperitoneally into calves that had been infected 4 weeks earlier with 10,000 S. bovis cercariae. Serum was injected weekly until 12 weeks post-infection to a total of 4,500 ml per calf and controls received normal serum or saline. No significant difference in worm or in faecal or tissue egg counts were seen in the three groups of recipients in spite of the observation that the serum donors had proved highly resistant to experimental challenge. In a second experiment, pre-infection or 4-, 8- or 12-week post-infection serum from donors given a single experimental infection with 10,000 S. bovis cercariae was injected intraperitoneally into groups of calves that had been infected 4 weeks earlier with 20,000 S. bovis cercariae. Injections were given weekly up to week 10 post-infection to a total of 2000-3500 ml serum per calf. In calves injected with immune serum there was a reduction in faecal and tissue egg counts and in the numbers of worms recovered as compared with the controls. In recipients of 8- and 12-week serum the reductions in faecal and tissue egg counts were higher than those in worm recovery, suggesting that 8- and 12-week post-infection sera contained factors capable of causing, in addition to worm death, suppression of worm fecundity. This provides further evidence of the importance of fecundity suppression in immunity to schistosomiasis.