The prevalence and impact of proliferative kidney disease (PKD) and myxosporidiosis has been investigated in wild fish stocks in the UK, over 1,500 fish representing 17 species being examined. PKD was recorded in brown trout, grayling and pike, the causative agent, the PKX cell, being identified with the aid of light and electron microscopy. A further 27 myxosporean species were also noted, with Myxobolus cotti (syn. M. jiroveci), in the brain of bullheads Cottus gobio being recorded for the first time. Studies on the structure and development of Myxidium lieberkuehni in pike revealed several previously undescribed features. Comparative morphological studies were undertaken to assess affinities of PKX with known myxosporean species. Results indicated similarities with early presporogonic stages of several myxosporean species, especially those belonging to the genus Sphaerospora. The apparent rarity of spore formation associated with PKX infections in the hosts examined focussed attention on species of Sphaerospora as possible sources of infection to salmonids. Studies concentrated on the possible involvement of the 3-spined stickleback, Gasterosteus aculeatus and its renal parasite, Sphaerospora elegans, in PKD transmission. A re-description of this parasite (recently elevated to "type species" for the genus), was prepared. Laboratory experiments using rainbow trout PKX cells successfully transmitted the infection to rainbow trout, brown trout, brook trout and grayling; however sticklebacks challenged with PKX cells did not appear to become infected. Rainbow trout challenged with S. elegans spores and presporogonic stages showed no evidence of sphaerosporosis or PKD. Experiments designed to investigate the possible role of tubificid worms in PKD transmission provided inconclusive results. Field studies provided data on the pathogenesis of PKD in grayling and showed this species to be highly susceptible to the disease.

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