In situ hybridisation identifies the gill as a portal of entry for PKX (Phylum Myxozoa), the causative agent of proliferative kidney disease in salmonids

PKX (Phylum Myxozoa) is an important pathogen affecting salmonid culture in Western Europe and North America. All of the available oligonucleotide pro bes developed for the PCR amplification of PKX DNA were examined for their ability to detect PKX in fixed tissue sections using in situ hybridisation. Out of the 12 probes examined only four stained PKX in tissue sections. Th e specificity of these probes to PKX was examined by testing them individua lly against a range of myxosporean infections. One of the probes (1032) cro ss-reacted with Sphaerospora truttae infecting brown trout kidney and stain ed this parasite in tissue sections, while probe 6R stained stickleback DNA . The results from these studies allowed for an optimised, relatively rapid , in situ hybridisation protocol to be developed for PKX detection. Using t his protocol, a preliminary study was conducted on the life history of the parasite in the rainbow trout Oncorhynchus mykiss. This demonstrated the pr esence of PKX in the gill arch 3 days after initial exposure in an enzootic river. It is suggested that a portal of entry for PKX is the gill. From he re, it migrates to the kidney where the disease progresses as previously de scribed.