Pathobiology of Loma salmonae: Progression of infection and modulating effects of intrinsic and extrinsic factors

Loma salmonae is a microsporidian parasite of salmonid fishes, which forms xenoparasitic complexes (xenomas) in the gills of fish. Rupture of xenomas to release the infective spores results in branchial inflammation and, in some cases death. In this study, the polymerase chain reaction (PCR) and in situ hybridization (ISH) were used to monitor the progression of L. salmonae infection. Extreme temperatures have a strong suppressive effect in the progression of infection. Low temperature (5°C), delays the parasite arrival in the heart, and although arrival in the gills is not delayed, by week 4 PE it was no longer detected and xenomas failed to form. High temperatures (21°C) drastically upset the progression of infection with L. salmonae. Parasite DNA can be detected in the heart, gills, and spleen by 3 and 7 days PE, but further development is blocked and the parasite is no longer detected after this time. Thus temperature can be used effectively for the control of infections with L. salmonae . Further studies addressed the effects that acquired immunity has in the progression of infection with L. salmonae. In fish that are passively immunized, the arrival of the parasite in the heart is delayed for 7 days, but the disease progresses and xenomas form in the gills. In contrast, fish that have acquired immunity by previous exposure to the parasite block the passage of L. salmonae from the heart to the gills (and xenoma formation), after an extended delay to reach the heart. As comparison, L. salmonae is unable to complete its life cycle in Atlantic salmon and brook trout, although the parasite is able to infect the fish, it is cleared from the host system after 2 weeks PE. To probe the efficacy that vaccination against L. salmonae may have on salmonids, naive rainbow trout were exposed to L. salmonae SV, a variant of L. salmonae isolated in the laboratory with low-virulence for salmonids of the genus Oncorhynchus (rainbow trout, Chinook salmon, coho salmon), and a strong preference for brook trout and Arctic char. When the exposed rainbow trout juveniles were subsequently challenged with typical spores from L. salmonae 16 weeks after first exposure to L. salmonae SV, they were protected, presenting 14 times fewer xenomas per filament than the naive controls. The marked degree of reduction in numbers of xenomas that formed after challenge suggests that use of the attenuated strain should be further considered as a means to protect fish in regions where the parasite is endemic. (Abstract shortened by UMI.)...