Review of the sequential development of Loma salmonae (Microsporidia) based on experimental infections of rainbow trout (Oncorhynchus mykiss) and Chinook salmon (O. tshawytscha)

Abstract

Abstract. Loma salmonae (Putz, Hoffman et Dunbar, 1965) is a common gill parasite of salmonids, and essentially all species in the genus Oncorhynchus are susceptible. Infections occur in both fresh and salt water. Loma salmonae is directly transmissible by ingestion of spores or infected tissue. The parasite infects the wall of blood vessels of various organs, but the gill is the primary site of infection. Initial infection occurs in the intestine, and xenomas are easily detected in the gills by standard histology at 4-6 wk post-exposure. A few presporogonic stages of the parasite are found in the heart endothelium prior to xenoma formation in the gills. Ultrastructure studies of early infections demonstrated that wandering blood cells transport the meronts to the gills, and that merogony occurs in pillar cells and other cells underlying the gill endothelium. Xenomas develop in these cells, resulting in hypertrophied host cells filled with spores. Xenomas ultimately rupture, and are associated with severe inflammation in which free spores are found in macrophages. The parasites are most pathogenic during this phase of the infection, resulting in severe vasculitis and clinical disease. Both rainbow trout (Oncorhynchus mykiss) and Chinook salmon (Oncorhynchus tshawytscha) recover from infections, but free spores persist in kidney and spleen phagocytes for many months after xenomas are absent in Chinook salmon. Fish that have recovered from the infection show strong immunity against the parasite, lasting up to 1 year. Fish are susceptible to infection by other routes of exposure by spores; co-habitation, anal gavage, and intramuscular, intraperitoneal and intravascular injection. Autoinfection probably occurs following release of spores in blood vessels after xenomas rupture. The optimal temperature for L. salmonae infections is 15-17°C, with a permissive range of 11-20°C.

Guide to the identification of fish protozoan and metazoan parasites in stained tissue sections

“…L. salmonae infections in gills of coho salmon Oncorhynchus kisutch held in fresh water are also characterised by intact xenomas with minimal associated host response (Kent et al 1989). A few presporogonic stages of the parasite can be found in the heart endothelium prior to xenoma formation in the gills (Kent & Speare 2005). Loma spores stain PAS-positive and weakly to strongly Gram-positive (Kent et al 1989, Bruno et al 1995.…”

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

“…The sequential development of K. takedai infection, such as the portals of entry and migration route to the primary site of infection, has not been elucidated yet. 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.…”

Demonstrated Efficacy of a Pilot Heterologous Whole-Spore Vaccine against Microsporidial Gill Disease in Rainbow Trout

“…icrosporidial gill disease (MGDS) of salmon (Loma salmonae) is a leading cause of mortality in farmed Pacific salmon in Canada (1). During the disease, large spore-filled xenomas (cysts with contributions from host and parasitic cells) develop within the gills; when the xenomas rupture, they evoke a severe inflammatory response in the gills which can lead to death (1).…”

“…During the disease, large spore-filled xenomas (cysts with contributions from host and parasitic cells) develop within the gills; when the xenomas rupture, they evoke a severe inflammatory response in the gills which can lead to death (1). Previous work has demonstrated a strong protective immune response in those fish which recover from initial infection (1); the prophylactic efficacy of an nonadjuvanted whole-spore vaccine based on a low-virulence strain of Loma salmonae has also been demonstrated, with vaccinated fish developing up to 93% fewer gill lesions than control fish following experimental challenge with virulent L. salmonae (2).…”

“…Five weeks following exposure, fish were euthanized in a water bath containing 100 mg/liter benzocaine. The first left gill arch was removed, and the numbers of spore-filled xenomas were counted (1,5). Infection levels are expressed as xenoma counts per gill arch (XCPGA) (mean values and standard deviations [SD] are shown below) (2).…”