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Diseases of Aquatic Organisms

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DAO 68:141-147 (2006)  -  doi:10.3354/dao068141

Assessing the accuracy of a polymerase chain reaction test for Ichthyophonus hoferi in Yukon River Chinook salmon Oncorhynchus tshawytscha

Christopher M. Whipps1,*, Tamara Burton2, Virginia G. Watral1, Sophie St-Hilaire3, Michael L. Kent1

1Center for Fish Disease Research, Department of Microbiology, 220 Nash, Oregon State University, Corvallis, Oregon 97331-3804, USA
2Alaska Department of Fish and Game, Division of Commercial Fisheries, 333 Raspberry Road, Anchorage, Alaska 99518, USA
3Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209, USA

ABSTRACT: Ichthyophonus hoferi Plehn & Mulsow, 1911, is a cosmopolitan, protistan pathogen of marine fishes. It is prevalent in mature returning Chinook salmon Oncorhynchus tshawytscha in the Yukon River watershed, and may be associated with prespawning mortality. We developed and evaluated a polymerase chain reaction (PCR) test for I. hoferi using primers specific to the parasite’s small subunit rDNA. The test has a minimum detection limit of approximately 10–5 parasite spores per reaction and does not cross-react with the closely related salmon parasites Dermocystidium salmonis or Sphaerothecum destruens. Sensitivity and specificity of the PCR test used on somatic muscle and heart tissue for detecting infected fish were determined using 334 Chinook salmon collected from the Yukon River at 2 locations (Tanana and Emmonak) in 2003 and 2004. The true infection status of the fish was determined by testing somatic muscle, heart and kidney tissue using histological evaluation, culture, and PCR. The severity of infection was grouped into 2 categories, light and heavy infection. The probability of detecting a heavily infected fish (sensitivity of the test) was generally much higher than the probability of detecting light infection, suggesting that more than one tissue and/or method should be used to accurately detect light or early infection by I. hoferi. The probability of correctly identifying a negative fish (specificity of the test) was always greater than 94% regardless of the tissue used, infection severity, sampling site or year of collection.

KEY WORDS:Ichthyophonus hoferi · Mesomycetozoea · Chinook salmon · Alaska · Polymerase chain reaction · Small subunit ribosomal DNA

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