Parvicapsula pseudobranchicola (Myxosporea) in farmed Atlantic salmon Salmo salar: tissue distribution, diagnosis and phylogeny

Abstract

Parvicapsula pseudobranchicola infections in farmed Atlantic salmon in Norway are associated with low-grade to significant mortalities. The parasite is found as mature spores in pseudobranchs, but has also been detected in the gills, liver and kidney. Diagnosis has relied on the detection of Parvicapsula spores, with the pseudobranch being the preferred organ. A better understanding of the epizootiology of this myxosporean is a prerequisite for appropriate management and control. Hence, early detection of infections and life cycle studies are needed. We sequenced the small subunit (ssu) rDNA (18S) from P. pseudobranchicola and developed a sensitive diagnostic PCR protocol. This allowed us to (1) identify appropriate tissues for diagnostic assays, (2) examine the intraspecific variation in ssu rDNA in the parasite's Norwegian range, (3) examine annelid potential primary hosts and (4) obtain additional ssu rDNA sequences of marine Parvicapsula species to perform a phylogenetic study. Primers were constructed targeting the ssu rDNA from P. minibicornis. With these we obtained a partial ssu sequence of the P. pseudobranchicola type isolate. A new set of primers (PCF3/PCR3) was constructed for diagnostic purposes. These were tested against DNA from the host and several myxozoan species infecting Norwegian salmon. The primers give a positive product of 203 bp and pick out P. pseudobranchicola in salmonids. They also amplify the congeners P. unicornis and P. asymmetrica infecting unrelated fish. The PCR protocol developed showed a greater sensitivity than light microscopy. The pseudobranchs were always positive and are the recommended organ for PCR diagnostics. There was no sequence variation between geographic isolates from farmed salmon. Preliminary examinations of marine polychaetes and oligochaetes collected from farm sites with parvicapsulose problems were negative. A comparison of the sequence of the ssu rDNA from P. pseudobranchicola with that of other myxozoans shows that it groups closely together with P. unicornis and P. asymmetrica. The closest relative to this group is P. minibicornis.

Disease interaction and pathogens exchange between wild and farmed fish populations with special reference to Norway

“…Parvicapsula pseudobranciola infects the pseudobranch of salmon and has been reported to cause mortality in farmed fish (Karlsbakk et al, 2002;Nylund et al, 2005). Several species of Myxosporidian parasites are reported from gadoids, Sphaeromyxa.…”

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

“…They are dark in colour, have eye bleedings, and some show cataracts and exophthalmia. The pseudobranchs of all affected individuals show changes, being swollen, granulomatous, haemorrhagic or partly eroded (Nylund et al, 2005). Heavily infected salmon become runts, probably due to vision impairment or blindness.…”

Parvicapsula bicornis n. sp. and P. limandae n. sp. (Myxozoa, Parvicapsulidae) in Pleuronectidae (Teleostei, Heterosomata) from Denmark

“…shows a high sequence similarity with a P. asymmetrica Shulman, 1953 sequence from the type host Cyclopterus lumpus (cf. Nylund et al 2005) (see Fig. 5 which also shows the phytogenetic position of P. bicornis).…”

“…The PCR primers used were Ecf and Myxgen4r (Kent et al 2000, Nylund et al 2005) with additional primers constructed based on small subunit rDNA (SSU rDNA) sequences from Parvicapsula spp. and other members of the marine group of Myxosporea: forward MarF1 (RRG CGT GCC TTG AAT AAA GC) and MyxF2 (CGC GCA AAT TAC CCA ATC CAG AC), reverse LyR2 (CCT TGC GAT TGT ACT CTC CC) and MarR2 (STA GCG ACG GGC GGT GTG).…”

“…Since members of Parvicapsula are often pathogenic to marine fish, it is important to increase knowledge of the biology, including the life cycles, of members of this genus. Some species are pathogenic in mariculture of salmonid fish (Hoffman 1984, Kent et al 1997, Karlsbakk et al 2002, Sterud et al 2003, Yasutake & Elliott 2003, Nylund et al 2005.…”