The transmission potential of Loma salmonae (Microspora) in the rainbow trout, Oncorhynchus mykiss (Walbaum), is dependent upon the method and timingof exposure

The ability of a parasite to transmit from one fish to another is important in the dissemination of disease. Groups of 25 naive rainbow trout (RBT), O ncorhynchus mykiss (Walbaum), were exposed to Loma salmonae by feeding on t he viscera (gills, hearts and spleens) from L. salmonae-infected donor RBT (DRBT) or by cohabitation with infected DRBT. Exposure occurred 3, 7, 11 an d 15 weeks after the DRBT were infected. All naive RBT were examined 7 week s post-exposure (PE) to the DRBT. Naive RBT, exposed to DRBT at week 3 PE, by feeding on viscera or by cohabitation, failed to develop visible branchi al xenomas. Cohabiting naive RBT with DRBT, at week 7 PE and week 11 PE, re sulted in the development of branchial xenomas. Xenomas failed to develop i n naive RBT exposed via cohabitation to week 15 PE DRBT. Naive RBT, exposed by feeding on the viscera of DRBT at week 7 PE, week 11 PE and week 15 PE, developed branchial xenomas. The transmission potential of viscera from L. salmonae-infected DRBT at week 15 and week 20 PE was also examined. Naive RBT, fed with viscera free of visible branchial xenomas, from DRBT at week 15 PE and week 20 PEI developed branchial xenomas by week 7 PE. A polymeras e chain reaction (PCR) was used to detect L. salmonae DNA from the water an d sediments of a tank of L. salmonae-infected RBT at week 7 PE. The method and timing of exposure of naive fish to L. salmonae-infected fish are impor tant in disease transmission and may be useful in predicting and preventing disease outbreaks in aquaculture.