Altered tissue distribution of Loma salmonae: effects of natural and acquired resistance

Abstract

This study compared and contrasted the fate of the microsporidian Loma salmonae, a branchial pathogen of salmonids of the genus Oncorhynchus, upon exposure of (1) naive susceptible rainbow trout (RT) O. mykiss, (2) naive RT passively immunized with sera from RT previously exposed to L. salmonae, (3) previously exposed and resistant RT and (4) two species believed to be innately resistant to the parasite, Atlantic salmon (AS) Salmo salar and brook trout (BT) Salvelinus fontinalis. The fish were infected per os with viable L. salmonae spores. The infection was followed in the fish by detection of parasite DNA by polymerase chain reaction (PCR) at several times post‐infection. Spore germination and intestinal invasion by the parasite occurred in all groups of fish. In the susceptible RT, parasite DNA was detected in the heart by day 3 post‐exposure (PE), followed by the gill at 2 weeks PE, whereas visible xenoparasitic complexes (xenomas) were detected by week 4 PE. In the passively immunized RT, the parasite’s fate was similar to that of controls, however, its arrival in the heart was delayed by 1 week. A delay was also detected in RT which had been previously exposed to L. salmonae and then recovered from disease. In these resistant fish, the parasite was able to reach the heart by week 3 PE, however, it failed to reach the gill and form xenomas. In AS and BT, the parasite reached the heart and gills quickly, where it remained for 2 weeks before being cleared; xenomas never formed. We speculate that failure to complete the life cycle in AS, BT and resistant fish might be because of interference by the immune system in the development of the parasite, resulting in the absence of disease in these fish.

Comparison of Susceptibility to Kabatana takedai (Microspora) among Salmonid Fishes

“…As for L. salmonae, it has been demonstrated that the parasite penetrates through the gut epithelium and migrates to the heart, where early merogonic stages occur, followed by xenoma formation in the gills (Kent and Speare, 2005). Sanchez et al (2001) suggested that Atlantic salmon and brook trout Salvelinus fontinalis, the resistant species to L. salmonae, did not block the parasite invasion at the entrance in the gut, and that the parasite failed to develop into xenoma inside the fish body. K. takedai is not likely to follow the same migration pattern to L. salmonae, because cyst formation occurred simultaneously at the heart and trunk muscle.…”

Induction time for resistance to microsporidial gill disease caused by Loma salmonae following vaccination of rainbow trout (Oncorhynchus mykiss) with a spore-based vaccine

“…The use of an inactivated spore-based vaccine was identified as a necessary first step in the quest for commercial vaccines against microsporidian infections [27]. However, rainbow trout passively immunised with sera from L. salmonae-resistant fish showed a delay in the early stages of the life cycle with the arrival of the parasite in the heart, suggesting that humoral factors played a limited role in the initial immune response [28]. Unfortunately, the precise mechanisms of resistance in fish to microsporidia are unknown.…”

“…Unfortunately, the precise mechanisms of resistance in fish to microsporidia are unknown. However, it has been suggested that a cell-mediated immune response is mounted based on the intracellular nature of L. salmonae [1,13], minimal humoral response [28] and the induction of strong protection in recovered fish [12,13,28]. Similarly, rainbow trout inoculated with either a pathogenic or attenuated vaccine strain of Cryptobia salmositica, a haemoflagellate protozoan, showed strong cell-mediated immune responses during infection as well as during protection against cryptobiosis [18]; cell-mediated immunity increased three weeks following vaccination against C. salmositica and good correlation was observed between cell-mediated immunity and the control of parasitaemia [18].…”

Fish microsporidia: Immune response, immunomodulation and vaccination

“…Passive immunization with serum from resistant fish seems to have a limited or null role in protection against the initial stages of the parasite in the gut. This was observed in naive rainbow trout inoculated with serum from L. salmonae infected fish [72]. Although the presence of transferred Ab delayed parasite infection for just one week, it was attributed to low levels of Ab conferred during the passive Ab administration.…”