Ultrastructural Study of the Late Stages of Loma Salmonae Development in the Gills of Experimentally Infected Rainbow Trout

Abstract

The main objective of this investigation was to examine the ultrastructural features of gills from rainbow trout experimentally infected with Loma salmonae to determine the morphological events that occur during the late stages of development of this parasite. Peripheral distribution of the mature parasites inside round xenomas was observed at weeks 5 and 6 postexposure (PE), but eventually the parasite occupied the entire xenoma. Degenerative changes were observed only in immature parasites at week 7 PE, and eventually an inflammatory reaction with a cellular infiltration was directed against mature spores. Round, flattened, and irregular shaped xenomas were observed at week 8 PE. The round xenomas showed a severe inflammatory response with disintegration of the xenoma membrane. This event was accompanied by eversion of polar tubes within the attacked xenoma and by the simultaneous presence of 2 tubular appendages, the type I and II tubules. Flattened xenomas were observed below the endothelium of gill lamella arteries. The irregular xenomas were located in the connective tissue of the gill filament and showed multiple projections occupied by spores. Both flattened and irregular xenomas showed no evidence of inflammatory reaction. An earlier proposed hypothesis is expanded to explain how L. salmonae is implanted beneath lamellar endothelium and within filament connective tissue.

Microsporidian xenomas in fish seen in wider perspective

“…The journey of the already dividing merozoites of L. salmonae ends (perhaps attracted by high O 2 levels) in the gill vascular spaces between the pillar cells (Rodríguez-Tovar et al 2003b). Then, either the pillar cell phagocytizes the parasite from the leucocyte and converts into a xenoma, or the extensions of the pillar cells retract to make space for the leucocyte, which turns into a xenoma itself.…”

“…Other leucocytes stay confined between the endothelium and basement membrane after having exited from the blood vessel. How much of tissue specificity and parasite tropism is involved in the case of L. salmonae and especially in other xenoma-forming microsporidia has still to be investigated (Rodríguez-Tovar 2003b).…”

“…This localisation is associated with host mitochondria because of the need of active parasite cell division (Rodríguez-Tovar et al 2003b) but by weeks 5 and 6 mature spores have already occupied that area. Although on week 5 and 6 p.e.…”

Microgemma vivaresi (Microsporidia: Tetramicridae): host reaction to xenomas induced in sea scorpions, Taurulus bubalis (Osteichthyes: Cottidae)

“…They did not find secondary xenomas developing near intact xenomas. Rodríguez-Tovar et al (2003) observed polar tubes extruded from spores of L. salmonae in rounded xenomas invaded by phagocytes. They proposed that this might be a mechanism for autoinfection, if nearby leucocytes could sustain infections if inoculated with sporoplasms via polar tubes.…”

“…(Rodríguez-Tovar et al 2002). In the later study (Rodríguez-Tovar et al 2003) three types of xenoma were identified: rounded xenomas derived from pillar cells, flattened xenomas lying under the arterial endothelium, and irregular xenomas in connective tissue of the gill filament. Host reaction against the rounded xenomas was first observed at six weeks p.i.…”