Paranucleospora theridion (Microsporidia) infection dynamics in farmed Atlantic salmon Salmo salar put to sea in spring and autumn

Abstract

The microsporidian Paranucleospora theridion (syn. Desmozoon lepeophtheirii) is a parasite of Atlantic salmon Salmo salar and also a hyperparasite of the salmon louse Lepeophtheirus salmonis. The parasite develops 2 types of spores in salmon, cytoplasmic spores in phagocytes and intranuclear spores in epidermal cells. The former type of development is assumed to be propagative (autoinfection), while the epidermal spores transfer the parasite to lice. Development in lice is extensive, with the formation of xenoma-like hypertrophic cells filled with microsporidian spores. We show that salmon are infected in the absence of lice, likely through waterborne spores that initiate infections in the gills. During summer and autumn the parasite propagates in the kidney, as evidenced by peaking normalised expression of P. theridion rRNA. Lice become infected during autumn, and develop extensive infections during winter. Lice mortality in winter and spring is likely responsible for a reservoir of spores in the water. Salmon transferred to sea in November (low temperature) did not show involvement of the kidney in parasite propagation and lice on such fish did not become infected. Apparently, low temperatures inhibit normal P. theridion development in salmon.

Investigation of co‐infections with pathogens associated with gill disease in Atlantic salmon during an amoebic gill disease outbreak

“…The pattern of infection in relation to D . lepeophtherii corresponds with the infection pattern observed by Sveen, Øverland, Karlsbakk, and Nylund (), wherein, the gill load of D. lepeophtherii was highest in autumn followed by a decline over the winter months. In this study, the decline in gill load continued through the following summer until the end of the sampling.…”

Applicability of a food chain analysis on aquaculture of Atlantic salmon to identify and monitor vulnerabilities and drivers of change for the identification of emerging risks

“…Sea lice and wild fish may act as carriers and reservoirs of viruses and other pathogens/parasites (Ferguson et al, 2010;Gonzalez et al, 2011;Jorgensen et al, 2011;Rimstad, 2011;Graham et al, 2012b;Sveen et al, 2012;Aamelfot et al, 2014;Dixon et al, 2016)…”

“…In salmonids the parasite develops in cells of the reticuloendothelial system (Nylund et al, 2011). The relation between infected lice possibilities of transferring the parasite to salmon has been investigated (Sveen et al, 2012) and it was shown that salmon are infected in the absence of lice, likely through waterborne spores that initiate infections in the gills. During summer and autumn the parasite propagates in the kidney.…”

“…Cleaner fish are also mentioned as potential reservoir for a range of biological hazards, including parasites, bacteria and viruses such as Paramoeba perurans, (Treasurer, 2012;Oldham et al, 2016). Interestingly, also parasitic organisms such as sea lice could form a reservoir for "hyper parasites" that infect salmon, such as Nucleospora theridion (Sveen et al, 2012). Another emerging pathogen is the free-living, facultative ectoparasites (amoeba) Neoparamoeba perurans which can cause Amoebic Gill Disease (AGD) .…”

Biology of a new xenoma-forming gonadotropic microsporidium in the invasive blotchfin dragonet Callionymus filamentosus

“…Fish microsporidia are known to be temperature sensitive (Antonio & Hedrick 1995, Sveen et al 2012, and low temperatures retard and even arrest development in some species (Olson 1981, Zenke et al 2005. In the microsporidium Loma salmonae, temperatures below 9°C interrupt the sporogonial development and production of xenomas (Beaman et al 1999).…”