Effect of the number of infected fish and acute exposure period on the horizontal transmission of Loma salmonae (Microsporidia) in rainbow trout, Oncorhynchus mykiss

Abstract

Loma salmonae is an intracellular obligate parasite of farmed chinook salmon ( Oncorhynchus tshawtscha) and is the causative agent for microsporidial gill disease (MGD). The infection occurs mainly in secondary gill lamellae, with the formation of large white spore-laden xenomas within the endothelial and pillar cells. The overall objective of the study was to investigate the minimum exposure time required for disease transmission between naive rainbow trout ( Oncorhynchus mykiss) (RBT) and L. salmonae-infected fish using the cohabitation challenge model. In trial I, low (1), medium (5) or high (10) numbers of infected RBT were added to tanks containing 45 naive fish for a cohabitation period of 21 days. Trial II limited the cohabitation period between five L. salmonae-infected fish in a tank of 45 naive RBT to 1, 12, 24 or 96 h. For both trials, each treatment level was completed in duplicate and an additional tank of 45 naive RBT served as the control and did not receive infected fish. Beginning on day 21 post exposure (PE) and continuing biweekly until day 83 PE and 105 PE for each trial, respectively, fish were evaluated for branchial xenomas to determine xenoma onset time and intensity. The effect of adding one infected fish produced similar median xenoma onset time and mean xenoma intensity compared with adding 5 or 10 infected fish (trial I). Significant differences were observed amongst the survivor curves for the fish exposed to infected RBT for 1, 12, 24 and 96 h, with the median onset time equal to 63 days for the lowest exposure period. This study demonstrated that L. salmonae transmission was possible with as little as 1-h exposure time between 5 cohabiting infected and 45 naive RBT. Adding five L. salmonae-infected fish (at week 7 PE) to a group of naive RBT for a period of 1 h resulted in over 94.5% disease prevalence in cohabiting fish.