Localization of the Initial Developmental Stages of Loma salmonae in Rainbow Trout (Oncorhynchus mykiss)

Abstract

Abstract. The intracellular microsporidian parasite Loma salmonae affects salmonids of the genus Oncorhynchus and is a significant cause of economic losses in pen-reared Chinook salmon (O. tshawytscha) in British Columbia. Loma salmonae infection is easily recognized by the xenomas that form in the gills, but early stages of infection are difficult to detect in histologic sections. In situ hybridization (ISH), using an L. salmonae-specific digoxigenin-labeled single-stranded DNA probe, was used to detect the parasite during the early stages of infection. Loma salmonae was detected in the gut mucosal epithelium as early as 24 hours postexposure (PE), and it localized in the lamina propria of the intestine within 24 hours of infection. After the parasite was detected in the lamina propria, dividing merogonic stages in infected cells in the heart were detected by ISH as early as 2 days PE, providing the first evidence of parasitaemia and hematogenous distribution of this parasite in infected blood cells. The parasites inside the infected cells appeared to be undergoing merogony as they passed through the heart, indicating that proliferation may start at the site of infection, before the parasite arrives to the gills for their final developmental phase. This is the first time that L. salmonae passage through the intestinal wall and migration to the heart has been visualized; however, the identity of the cells harboring the parasite has yet to be determined.

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

“…However, placement of infectious spores directly on gills of Chinook did not result in infection Shaw et al (1998) found spores and possible sporoplasms in the intestinal epithelium shortly after exposure by standard histology. Sánchez et al (2001a) further confirmed this observation and found the parasite in epithelial cells and the lamina propria of the small intestine as early as 12 h post-exposure (p.e.) by using a Loma-specific in situ hybridisation (ISH) probe (Fig.…”

“…The gut is the initial site of infection , Sánchez et al 2001a. Fish can be experimentally infected per os by intubation of spores ), infected tissue (Kent et al 1995, Speare et al 1998a or by co-habitation .…”

“…1). The parasite is likely transported from the gut in leucocytes (Rodríguez-Tovar et al 2002), and early merogonic stages have been observed by ISH (Sánchez et al 2001a) or PCR (Sánchez et al 2000) in the heart endocardium a few days after infection. Loma salmonae is first detected in the gills about 2-3 wk p.e.…”

“…Loma salmonae is first detected in the gills about 2-3 wk p.e. (Sánchez et al 1999, 2001a, Rodríguez-Tovar et al 2002. In the gills, initially meronts are found within cells underlying the endothelium (Fig.…”

Microsporidian xenomas in fish seen in wider perspective

“…However, merogonial proliferation may presumably start in the cells that were initially infected. Sánchez et al (2001), using in situ hybridisation technique, have found that Loma salmonae migrates from mucosal epithelium to the lamina propria of the intestine before reaching the final destination in the gills. The dividing merogony stages were then detected within infected blood cells in the heart as early as day 2 post exposure (p.e.…”