Ichthyophoniasis: An Emerging Disease of Chinook Salmon in the Yukon River

Abstract

Before 1985, Ichthyophonus was unreported among Pacific salmon Oncorhynchus spp. from the Yukon River; now it infects more than 40% of returning adult Chinook salmon O. tshawytscha. Overall infection prevalence reached about 45% in the Yukon River and about 30% in the Tanana River between 1999 and 2003. Mean infection prevalence was greater in females than males in the main-stem Yukon River during each of the 5 years of the study, but the infection prevalence in males increased each year until the difference was no longer significant. Clinical signs of ichthyophoniasis (presence of visible punctate white lesions in internal organs) were least at the mouth of the Yukon River (ϳ10%) but increased to 29% when fish reached the middle Yukon River and was 22% at the upper Tanana River. However, clinical signs increased each year from 7% in 1999 to 27% in 2003 at the mouth of the river. As fish approached the upper reaches of the Yukon River (Canada) and the spawning areas of the Chena and Salcha rivers (Alaska), infection prevalence dropped significantly to less than 15% in females on the Yukon River and less than 10% for both sexes in the Chena and Salcha rivers, presumably because of mortality among infected prespawn fish. Age was not a factor in infection prevalence, nor was the position of fish within the run. The source of infection was not determined, but Ichthyophonus was not found in 400 Pacific herring Clupea pallasi from the Bering Sea or in 120 outmigrating juvenile Chinook salmon from two drainages in Alaska and Canada. Freshwater burbot Lota lota from the middle Yukon River were subclinically infected with Ichthyophonus, but the origin and relationship of this agent to the Chinook salmon isolate is unknown.

Isolation and Characterization of Some Pathogenic Agents Causing Wobbling Syndrome in Cultured Oreochromis niloticus

“…Prevalence of infection was 12.5%, mostly infected tissues of the gas bladder with I.hoferi macroscopically had white nodules mainly near blood vessels with various degrees of congestion different from neighboring transparent portions of the same specimen and this may be attributed to the relatively more affection of highly vascularised organs which identify Icthyophonus systemic nature (Spanggaard, et al 1994 andEl-Khattib andElyas, 2003). Growth of nonseptated microhyphae and tubular club shaped macrohyphae resembled that of Kocan et al (2004). The recorded histopathological changes in gas bladder of naturally infected O.niloticus were sloughing in the epithelial layer, subepithelial edema with mononuclear cells infiltration in the submucosa and congestion in the blood vessels of the submucosa with eosinophilic granular cells infiltrations.…”

“…This result is in agreement with Mai et al (2005) who recorded that the effectiveness of Mirazid in treatment of fish parasites. In case of I.hoferi infection, (McVicar, 1999 andKocan, et al 2004) suggested that no results of chemotherapy but efforts should be focused on disease prevention. According to the present study, invitro sensitivity test of isolated bacterial strains to different chemotherapeutic agents revealed that all isolates were sensitive to oxytetracycline while vary in susceptibility to the other chemotherapeutic agents (Table 3).…”

“…Icthyophonus hoferi is worldwide distributing organism transmitted by feeding spores from dead infected fish. It is a cosmopolitan parasite that infects many organs while the primary target tissue for the parasite is the heart muscle and disseminated to other visceral organs and the somatic muscle tissue (McVicar, 1999 andKocan, et al 2004).…”

Impact of Ichthyophonus infection on spawning success of Yukon River Chinook salmon Oncorhynchus tshawytscha

“…After that, no incidence of Ichthyophonus infection was reported until the late 1990s, when the infection became widely noticeable throughout the river drainage. The source of Ichthyo phnus infection for Yukon River Chinook salmon is un known, but it is believed that infection occurs in the ocean prior to freshwater migration through ingestion of infected prey (Criscione et al 2002, Jones & Dawe 2002, Kocan et al 2004a. Ichthyo phonus primarily infects cardiac muscles but spreads throughout the body of a host, which reduces swimming performance and stamina of the host, and leads to its mortality (Tierney & Farrell 2004, Kocan et al 2006.…”

“…Ichthyo phonus primarily infects cardiac muscles but spreads throughout the body of a host, which reduces swimming performance and stamina of the host, and leads to its mortality (Tierney & Farrell 2004, Kocan et al 2006. During the period 1999 to 2003, Ichthyo phonus infection prevalence in the Yukon River was about 25% at the river mouth, peaked at 35% in mid-river, and was about 10% at the spawning grounds (Kocan et al 2004a). This decline from mid-river to spawning grounds was attributed to mortality of the infected fish, in which about 60% of the infected fish die en route to the spawning grounds (Kocan et al 2004a).…”