Host and Environmental Influences on Development of Disease

While many myxozoan parasites produce asymptomatic infections in fish
hosts, several species cause diseases whose patterns of prevalence and
pathogenicity are highly dependent on host and environmental factors.
This chapter reviews how these factors influence pathogenicity and
disease prevalence. Influential host factors include age, size and nutritional
state. There is also strong evidence for host strains that vary in resistance
to infection and that there is a genetic basis for resistance. A lack of
co-evolutionary processes appears to generally underly the devastating
impacts of diseases caused by myxozoans when introduced fish are
exposed to novel parasites (e.g. PKD in rainbow trout in Europe) or when
native fish are exposed to an introduced parasite (e.g. whirling disease in
North America). Most available information on abiotic factors relates to
water temperature, which has been shown to play a crucial role in several
host parasite systems (e.g. whirling disease, PKD) and is therefore of
concern in view of global warming, fish health and food sustainability.
Eutrophication may also influence disease development. Abiotic factors
may also drive fish disease via their impact on parasite development in
invertebrate hosts.