Proliferative kidney disease (PKD) is an endoparasitic disease of salmonid fish caused by Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea). This chronic, largely renal interstitial disease is caused by the extraporogonic but intracellular stages of the parasite, which cause a severe granulomatous host response.
The severity of the disease is linked to water temperature, with roughly 15 degrees °C as the cut-off: below that temperature, lesions and clinical disease are minimal.
Above that temperature, however, lesions can be severe and mortality high. Inevitably, global warming has resulted in spread of the disease.
All ages of fish are susceptible unless they have been previously exposed to the parasite: thus, immunity is possible. Similarly, some strains of fish are more resistant than others. Filter-feeding bryozoa (also known as moss animals) have been shown to be responsible for releasing T. bryosalmonae spores, even a small number of which can then infect fish and lead to PKD.
Fish with PKD are often lethargic and darker than normal. As the primary target organ, the kidney may become so massively enlarged as to be visible externally along the lateral line. Exophthalmia, ascites and anaemia are other gross features, probably the results of the disease on vascular integrity, leading to haemorrhage and reduced haematopoiesis due to the granulomatous lesions (myelophthisic anaemia?).
Both anterior and posterior portions of the kidney may be thrown into grey bulbous ridges or discrete nodules, depending on the severity of its involvement. Splenomegaly is usually present and indeed may be one of the earliest indications of infection.
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