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Breakthrough in Fish Disease Mystery

Media Brief
For Immediate Use
Wednesday 1 December 1999

Scientists at Imperial College, London and the University of Reading have identified the origin of a mysterious disease responsible for a high death rate of fish in freshwater farms, according to work published this week in the journal Parasitology (See Notes to Editors 1).

Fish farming has become a cheap and efficient means of producing large quantities of certain types of fish. However the concentration of large numbers of fish in fish farms brings problems, not least the enhancement of the spread of disease.

In recent years fish farming has been used extensively for raising rainbow trout and salmon. The disease known as Proliferative Kidney Disease (PKD) was hardly known at the beginning of the century but has developed into a serious problem in farming these fish in Europe and North America. The infection appears regularly in late spring or early summer, when a parasite infects the kidneys and spleen causing the fish immune cells to multiply out of control, destroying these organs and resulting in anaemia, bloating, discolouration and death.

Mortality can be extremely high and sometimes entire fish stocks can be wiped out. This can result in significant economic loss from the 15,000 tonnes of trout produced annually by the fish farming industry in the UK worth £25 million.

It has been known since the 1950s that a microscopic parasite is the cause of the disease. In the 1980s it was shown that the organism belongs to a group known as Myxozoa but, because it does not develop to its final form in fish, it has not been possible to identify it further. Thus, it has been referred to as PKX, "X" signifying the "unknown" cause of proliferation in kidney. It has been widely believed that PKX completes its normal development in another host and research groups in the UK, continental Europe and North America have searched for this host without success until now.

Professor Elizabeth Canning and Dr Cort Anderson at Imperial College and their colleague Dr Beth Okamura at Reading, supported by the Natural Environmental Research Council, have now made the surprising discovery that an obscure group of freshwater organisms, known as bryozoans or "moss animals" harbour the infective stages of PKX(2).

They first used the new tools of molecular biology, by which they showed identical genetic material (DNA) of PKX in the bryozoans and rainbow trout, thus identifying bryozoans as the mystery hosts. Professor Canning said "Since then, we have observed the actual parasite in bryozoans, studied its development and named it Tetracapsula bryosalmonae, the name reflecting the names of the bryozoan and salmon hosts". Also, in collaboration with scientists Steve Feist and Matt Longshaw at the MAFF* Centre for the Environment, Fisheries and Aquaculture Science, Weymouth, final proof of the discovery was obtained when the infection was transmitted experimentally from bryozoans to rainbow trout which subsequently developed the disease.

The discovery has far-reaching implications for fish farming. For the first time the natural source of infection has been identified. This raises the possibility that infection can be controlled. Although elimination of bryozoans in rivers is not likely to be possible or environmentally desirable a reduction in bryozoan abundance could be achieved. Regular removal of bryozoans growing near the water input of fish farms might reduce the incidence or severity of the disease. However, a vaccine based on infective stages released from bryozoans perhaps offers the best long term hope of control by preventing infection from becoming established.

*Ministry of Agriculture, Fisheries & Food


For further information contact:

Professor Elizabeth Canning, Imperial College of Science, Technology and Medicine, London. Tel: 01344 294244 Fax: 01344 294339 Email: (Out of hours number 01344 884757)

Notes to editors

1. The research is reported in the article "Molecular data implicate bryozoans as hosts for PKX (Phylum Myxozoa) and identify a clade of bryozoan parasites within the Myxozoa" by C.L. Anderson, E.U. Canning and B.Okamura, published in the peer-reviewed journal, Parasitology Vol. 119 Issue 6, 1 December 1999.


Proliferative Kidney Disease (PKD), a condition associated with high mortality in salmonid fish, represents an abnormal immune response to the presence of an enigmatic myxozoan, which has been designated simply as PKX organism because its generic and specific status are obscure. Phylogenetic analyses of partial sequences of the 18s rDNA of PKX and of myxozoan parasites infecting the bryozoans Cristatella mucedo, Pectinatella magnifica and Plumatella rugosa, including the previously named Tetracapsula bryozoides from C. mucedo, showed that these taxa represent a distinct clade that diverged early in the evolution of the Myxozoa before the radiation of other known myxozoan genera. A common feature of the myxozoans in this clade may be the electron-dense sporoplasmosomes with a lucent bar-like structure, which occur in T.bryozoides and PKX but not in the myxozoans belonging to the established orders Bivalvulida and Multivalvulida.

Variation of 0.5-1.1% was found among the PKX 18s rDNA sequences obtained from fish from North America and Europe. The 18s rDNA sequence for T.bryozoides showed that it is a distinct taxon, not closely related to PKX but some sequences from myxozoans infecting 2 of the bryozoan species were so similar to those of PKX as to be indistinguishable. Other sequences from the new myxozoans in bryozoans at first appeared distinct from PKX in a maximum likelihood tree but, when analysed further, were also found to be phlogenetically indistinguishable from PKX. We propose that at least some variants of these new myxozoans from bryozoans are able to infect and multiply in salmonid fish, in which they stimulate the immune reaction and cause PKD but are unable to form mature spores to complete their development.

AUTHORS C.L. Anderson1, E.U. Canning1 and B.Okamura2

1Department of Biology, Imperial College of Science, Technology and Medicine, London SW7 2AZ, UK.
2School of Animal and Microbial Sciences, The University of Reading, Whiteknights, PO Box 228, Reading RG6 6AZ, UK.

2. Bryozoa occur as inconspicuous, branching, plant-like growths on submerged branches and other surfaces in rivers and lakes, sometimes in great profusion. Their rapid build-up in pipes and channels can cause great problems in the water industry.

3. The Natural Environment Research Council (NERC) is the UK´s leading body for research, survey, monitoring and training in the environmental sciences. It sponsors research and postgraduate training in universities, and has a network of Centres and Surveys across the UK. For more information, visit their web site at

4. Imperial College of Science, Technology and Medicine is the largest applied science and technology university institution in the UK, with the largest annual turnover (£310 million in 1997-98) and the largest research income (£210 million in 1997-98). It is consistently rated in the top three UK university institutions for research quality, with an aggregate score of 6.09 out of 7 in the 1996 Research Assessment Exercise. Web site at