Elucidation of Schistosoma japonicum population dynamics in pigs using PCR-based identification of individuals representing distinct cohorts

The study reported here investigated the interactions of successive infecti ons and acquired resistance of pigs to challenge infections of Schistosoma japonicum. Two morphologically indistinguishable geographical isolates from China (from Anhui and Zhejiang provinces) were used for the infections. Th e worms of the two isolates were distinguishable by PCR-linked restriction fragment length polymorphism analysis of the nicotinamide adenine dinucleot ide phosphate dehydrogenase I gene of the mitochondrial genome. Thirty-two pigs divided into seven groups were used in the experiment. Two groups rece ived a single infection by either the Anhui or the Zhejiang isolate. In Cha llenge Groups 1, 4, 6, 8 and 12, a primary infection of the Zhejiang isolat e was followed by a challenge infection with the Anhui isolate at week 1, 4 , 6, 8 or 12 after the primary infection. In this way it was possible to de termine whether worms recovered by perfusion originated from the primary or the challenge infection. Only the challenge infection at week 1 resulted i n a higher worm burden when compared with a single primary infection with t he Zhejiang isolate. The results showed that challenge worms were able to e stablish, and that the proportion of worms originating from challenge infec tion increased at the later challenge infections, however without an increa se in the total number of worms. In addition, mixed pairs of the two isolat es were found in all challenge-infected groups. The results indicate that p igs are able to mount a partial resistance against re-infection with S. jap onicum by 4 weeks after a primary infection, but that worms of the challeng e infections eventually replace the primary infection. The finding of mixed pairs of the two isolates indicates that worms of S. japonicum are either polygamous or able to wait in solitude for up to 12 weeks for a partner. (C ) 1999 Australian Society for Parasitology Inc. Published by Elsevier Scien ce Ltd. All rights reserved.