ACQUIRED-RESISTANCE AGAINST SCHISTOSOMA-BOVIS AFTER SINGLE OR REPEATED LOW-LEVEL PRIMARY INFECTIONS IN GOATS

Experimental Schistosoma bovis infections were studied in groups of si x goats for 32 weeks. Two types of primary infections, a single exposu re to 400 cercariae per goat or repeated (trickle) exposure to 200 cer cariae per goat weekly for 10 weeks, were followed by a challenge infe ction with 2000 cercariae per goat in week 16 of the experiment. Perio ds of high faecal egg excretion were associated with reduced weight ga in, anaemia, hypoalbuminaemia and blood eosinophilia; these changes gr adually diminished with decreasing faecal egg excretion but never disa ppeared completely. The pathogenic effects of the trickle exposure exc eeded those of the single primary exposure. After the challenge, the g oats excreted few or no additional eggs and no additional clinicopatho logical changes were observed in the group previously exposed to a tri ckle infection. However, the primary single infection did not provide full protection against the effects of the challenge infection.