Key words: PKD, temperature, bryozoans, salmonids 

TEMPERATURE-DRIVEN PARASITE PROLIFERATION IN INVERTEBRATE HOSTS PORTENDS SALMONID DECLINES

Beth Okamura (1)* and Sylvie Tops (2)

1  School of Biological Sciences, Lyle Building, University of Reading, Reading, RG6 6BX, UK  b.okamura@reading.ac.uk

2  Schering Plough Animal Health, Breakspear Road South, Harefield, Uxbridge, Middlesex, UB9 6LS, UK  sylvie.tops@spcorp.com

Proliferative kidney disease (PKD) is caused by the myxozoan parasite, Tetracapsuloides bryosalmonae,  which develops in freshwater bryozoan hosts prior to infecting salmonid fish. The disease is provoked by seasonal increases in temperature and is manifested as a massive cell-mediated immune response. PKD can cause great economic loss to trout farms and salmon hatcheries and is increasing in prevalence and severity in wild fish populations, possibly as a result of increasing temperatures. The relationship between PKD and temperature may be explained by several mechanisms including enhanced bryozoan growth leading to an increase in host resources, induction of parasite proliferation or earlier activation of the fish immune system. We investigated whether temperature influences the development of parasites in bryozoans and show that increasing temperatures drive the proliferation of infective stages by provoking, accelerating and prolonging the production of spores from cryptic stages that reside in the host body wall as covert infections. Since the temperature-driven induction of overt infections can have little to no effect on bryozoan hosts, our results provide a wake-up call and predict that PKD outbreaks will increase in magnitude and severity in wild and farmed salmonids as a result of climate-driven proliferation of infective stages in invertebrate hosts.