PKX, the Causative Agent of Proliferative Kidney Disease (PKD) in Pacific Salmonid Fishes and Its Affinities with the Myxozoa1

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Abstract
Proliferative kidney disease (PKD), caused by an unclassified protozoan (PKX), is reported from Pacific salmon, Oncorhynchus tshawytscha (Walbaum) and O. kisutch (Walbaum), and steelhead trout, Salmon gairdneri Richardson, held at the Mad River Hatchery in California, USA. The cumulative mortality attributed to the disease was 95, 13, and 18%, respectively. The mortalities were greatest at mean water temperatures of 12.degree.-14.degree. C during July 1983. The ultrastructure of the PKX organism and its associated pathology during clinical disease in all 3 spp. were consistent with those of the parasite in rainbow trout (S. gairdneri) as described in European outbreaks. Significant mortalities did not occur after Aug. at which time the parasite could no longer be detected in the salmon species. The steelhead continued to exhibit parasites in the kidney interstitium and epithelium and lumens of the tubules. Myxosporidan trophozoites and developing spores were also observed in the lumens of the kidney tubules of these fish. Although a mixed infection with another parasite may have occurred, evidence suggests that the myxosporidans are later stages of PKX. They were only observed in fish exposed to water with the infective stage and were particularly prominent in recovering fish. The PKX organism is similar to UBO, a unclassified protozoan of carp suspected to be an early stage of Sphaerospora renicola Dykova and Lom. Both parasites infect the blood and kidney, divide by endogeny, and are released by disintegration of the primary mother cell. The intraluminal myxosporean forms show similarities to Sphaerospora spp. in that they are monosporous and sporoblasts are formed within pseudoplasmodia. It is possible that PKX migrates to the lumen of the kidney tubule and subsequently sporulates. If the myxosporean forms are later stages of PKX, then it would belong to the phlyum Myxozoa.