The transmission potential of Loma salmonae (Microspora) in the rainbow trout, Oncorhynchus mykiss (Walbaum), is dependent upon the method and timing of 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), Oncorhynchus mykiss (Walbaum), were exposed to Loma salmonae by feeding on the viscera (gills, hearts and spleens) from L. salmonae -infected donor RBT (DRBT) or by cohabitation with infected DRBT. Exposure occurred 3, 7, 11 and 15 weeks after the DRBT were infected. All naive RBT were examined 7 weeks 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 branchial xenomas. Cohabiting naive RBT with DRBT, at week 7 PE and week 11 PE, resulted in the development of branchial xenomas. Xenomas failed to develop in 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 PE, developed branchial xenomas by week 7 PE. A polymerase chain reaction (PCR) was used to detect L. salmonae DNA from the water and 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 important in disease transmission and may be useful in predicting and preventing disease outbreaks in aquaculture.