Department of Pathology and Clinical Studies, Faculty of Veterinary Science, University of Khartoum, Department of Animal Production, College of Agriculture, London School of Hygiene and Tropical Medicine, Winches Farm Field Station, P.O. Box 32, Khartoum North, Sudan
Calves were immunized with Schistosoma bovis by a single experimental exposure to 10,000 normal cercariae. Some of these calves were perfused 14 weeks later, and a part of their worm loads was surgically transplanted into groups of normal recipient calves: “WPR” group calves received 500 pairs of worms; “MR” group calves received between 650 and 1,000 male worms alone. All three groups were subsequently challenged 10 weeks after surgery with 20,000 cercariae, as were a previously unexposed group of controls (“CC”). Mean post-challenge fecal egg counts in the animals immunized with cercariae (“PC” group) rose to a maximum of only 60 eggs per gram (e.p.g.), compared to 376 e.p.g. in the CC, and maximum fecal egg counts in the WPR and MR groups were also somewhat lower than in the CC, at 152 and 250 e.p.g., respectively. In spite of the much lower fecal egg counts in the PC than in the CC group, calculated adult “challenge” worm recoveries were only reduced by 11%, but PC group tissue egg densities derived from the challenge were 78–100% lower than in the CC. The WPR and MR groups had 43% and 37%, respectively, fewer worms than the CC, and mean tissue egg densities were lower by 39–63% and 63–76%, respectively, though in most cases there were no statistically significant differences from the CC. Thus, judging by the results of the fecal and tissue egg counts, calves exposed to a whole primary experimental infection can strongly resist superinfection, and it appears that some resistance can also be stimulated in the absence of the migratory stages by transplantation of adult worm pairs, and in the absence of both migratory stages and eggs by the transplantation of adult male worms.