Immunohistochemical and PCR studies of wild fish for Tetracapsula bryosalmonae (PKX), the causative organism of proliferative kidney disease

Abstract

Tetracapsula bryosalmonae, previously referred to as PKX, causes proliferative kidney disease (PKD) in salmonids and is an economically important myxozoan pathogen in salmonid culture. A variety of molecular and immunological tools have been developed to detect the parasite. To determine the specificity of four monoclonal antibodies (MAbs) raised against T. bryosalmonae, archive material of fish infected with various myxosporean species was obtained and immunostained. Wild fish were also collected from enzootic waters and examined for T. bryosalmonae infection using immunohistochemistry and the polymerase chain reaction (PCR). Three of the MAb probes appear to be specific for T. bryosalmonae while only two of the five sets of primers tested appeared to specifically amplify T. bryosalmonae DNA. The results of the immunostaining and the PCR demonstrate that T. bryosalmonae occurs in the tubules of grayling Thymallus thymallus L., brown trout, Salmo trutta L. and Atlantic salmon, Salmo salar L. outside of the PKD season (June‐September) in the UK. This confirms the results of previous studies that these species are the preferred fish hosts for the parasite in the UK.

Transmission Biology of the Myxozoa

“…Severe outbreaks of the disease were also noted in grayling (Thymallus thymallus) (Hoffmann & Dangschat, 1981). Northern Pike (Esox lucius) is the only nonsalmonid fish species, in which extrasporogonic stages similar to those of T. bryosalmonae were found (Seagrave et al, 1981, Morris et al, 2000a. It was observed that fish become resistant against reinfection with T. bryosalmonae after surviving the disease (Ferguson, 1981, Foott & Hedrick, 1987.…”

Evaluation of malacosporean life cycles through transmission studies

“…The sporogonic stage is found in the kidney tubule lumen 2 to 3 wk after the extrasporogonic stages are first observed. Sporogonic stages can persist in the kidney long after the recovery of the fish from clinical disease (Kent & Hedrick 1986, Morris et al 2000. Development of sporogonic stages in at least some fish hosts proceeds to the production of malacosporean-like spores containing 2 capsulogenic cells and 1 sporoplasm surrounded by 2 unstrengthened (soft) valve cells (Kent & Hedrick 1986, Hedrick et al 2004).…”

“…Prior to the discovery of phylactolaemate bryozoans as hosts of the 'PKX' parasite, researchers screened many aquatic invertebrates and other fish species from sites enzootic for PKD in an attempt to locate the source of the infection. For instance, Morris et al (1999) surveyed 35 000 oligochaete worms. In another study, 60 to 80 species of aquatic invertebrates were investigated, including Gammarus sp., caddis flies, leeches and snails , M. Longshaw & S. Feist unpubl.…”

“…So far there is no evidence that fish, other than salmonids and pike, act as hosts of Tetracapsuloides bryosalmonae (Hedrick et al 1993, Adams & Morris 1999. A number of other potential fish hosts have been investigated, but none have been identified (Adams & Morris 1999).…”

“…A number of other potential fish hosts have been investigated, but none have been identified (Adams & Morris 1999). Furthermore, the broad diversity of habitats associated with PKD outbreaks and T. bryosalmonae infections in bryozoans provide evidence that no single fish can be an obligate host.…”