Epizootiology of Parvicapsulaminibicornis in Fraser River sockeye salmon, Oncorhynchusnerka (Walbaum)

Abstract

Late-spawning Fraser River sockeye salmon, Oncorhynchus nerka, stocks have suffered signi®cant prespawn mortality associated with an unusually early freshwater migration pattern and the myxosporean parasite Parvicapsula minibicornis. Surveys of migrating adult salmon from several spawning populations were conducted in 1999 and 2000 to determine the extent of infection with P. minibicornis, when and where the parasite ®rst becomes detectable during migration, and whether early migrating stocks might be used as sentinels to assess risk of infection in late-spawning stocks. Posterior kidney, preserved in 95% ethanol, was examined for P. minibicornis in stained histological sections and using a polymerase chain reaction (PCR) test. The prevalence of this parasite in all Fraser River sockeye salmon stocks examined was high (range 47±100% infected). In contrast, P. minibicornis was not detected in the ®sh tested from the two sockeye salmon stocks outside the Fraser River drainage in either 1999 or 2000. The parasite was also not detected histologically or by PCR in the kidney tissue of the ®sh from the Fraser River that were sampled in salt water or early during their freshwater migration up the river. These ®ndings and the progression in the prevalence and intensity of infection as the ®sh from three stocks (early Stuart, Weaver Creek and Cultus Lake) were monitored over time, suggest salmon acquired the parasite either in the lower Strait of Georgia or in the lower Fraser River before the con¯uence of the Harrison River. In both 1999 and 2000 the parasite was present in all Fraser River sockeye salmon stocks sampled, which suggests that early Stuart salmon may be valuable as a sentinel stock for the presence of the parasite in later-spawning stocks.

Myxobolus arcticus and Parvicapsula minibicornis infections in sockeye salmon Oncorhynchus nerka following downstream migration in British Columbia

“…Epidemiological evidence indicates transmission of P. minibicornis to salmon as they migrate through the lower Fraser River. In adult salmon, prevalence and severity progressively increase with distance migrated in the river (St-Hilaire et al 2002, Jones et al 2003. In the present study and with the exception of the aforementioned 2010 catches, there was a trend of increasing prevalence in juvenile salmon with distance from the Fraser River plume, and this was statistically significant for the Lower Johnstone Strait.…”

“…Our detection of P. minibicornis confirms an earlier study in which the parasite was reported in juvenile Fraser River sockeye salmon in the Strait of Georgia. St-Hilaire et al (2002) used PCR to demonstrate the infection in approximately 60% of 67 juveniles collected from an unreported location in the lower Strait of Georgia. P. minibicornis was reported in juvenile Chinook salmon O. tshawytscha in the Klamath River, California, USA (Foott et al 2004), where infective actinospores are shed by the polychaete host, Manayunkia sp.…”

Site preference of myxozoans in the kidneys of Hungarian fishes

“…Trophozoites of this parasite (PKX cells) multiplying with internal cleavage are located free among interstitial cells (Clifton-Hadley et al. 1984, Kent et al 1994, St-Hilaire et al 2002, but the possibility that PKX cells are located in small capillaries in the interstitium cannot be excluded. This latter possibility is supported by observations that similar PKX cells occur in other organs as well, and could serve also as an explanation for the route of further development, viz.…”

“…Among them, P. minibicornis Kent, Whitaker & Dawe, 1997, which is the best studied species (Kent et al 1997, St-Hilaire et al 2002, seems to follow the Myxidium type development involving initial glomerular stages.…”

Estimating the future decline of wild coho salmon populations resulting from early spawner die-offs in urbanizing watersheds of the Pacific Northwest, USA

“…Scholz, personal observation). Therefore, the urban coho mortality phenomenon is dissimilar from other documented circumstances of adult salmon dying prematurely at the spawner life stage because of disease, predation, altered flows, inaccessible spawning habitat, and other nonchemical stressors (St-Hilaire et al 2002;Kocan et al 2004;Keefer et al 2010).…”