Abstract
The effects of temperature and infection by Ichthyophonus were examined in juvenile Pacific herring (Clupea pallasii) maintained under simulated overwinter fasting conditions. In addition to defining parameters for a herring bioenergetics model (discussed in Vollenweider et al. this issue), these experiments provided new insights into factors influencing the infectivity and virulence of the parasite Ichthyophonus. In groups of fish with established disease, temperature variation had little effect on disease outcome. Ichthyophonus mortality outpaced that resulting from starvation alone. In newly infected fish, temperature variation significantly changed the mortality patterns related to disease. Both elevated and lowered temperatures suppressed disease-related mortality relative to ambient treatments. When parasite exposure dose decreased, an inverse relationship between infection prevalence and temperature was detected. These findings suggest interplay between temperature optima for parasite growth and host immune function and have implications for our understanding of how Ichthyophonus infections are established in wild fish populations.
Highlights
Ichthyophonus hoferi, a highly pathogenic parasite of marine and anadromous fishes, is currently ubiquitous in Pacific herring (Clupea pallasii) populations throughout the NE Pacific [1,2,3]
To ensure that animals were parasitefree and immunologically naıve at initiation of experiments, a cohort of specific-pathogen-free (SPF) Pacific herring was reared for this study at the USGS Marrowstone Marine Field Station (MMFS) (Nordland, WA, USA)
Disease processes do vary with temperature in juvenile Pacific herring infected with Ichthyophonus
Summary
Ichthyophonus hoferi, a highly pathogenic parasite of marine and anadromous fishes, is currently ubiquitous in Pacific herring (Clupea pallasii) populations throughout the NE Pacific [1,2,3]. Phenotypic [4] and genotypic [5, 6] differences among isolates of I. hoferi suggest that there are multiple sympatric species in the region Due to this taxonomic uncertainty, here we refer to the parasite by its generic name. In addition to causing direct mortality from disease, Ichthyophonus infection may predispose juvenile herring to indirect mortality through predation [10], presumably as a result of decreased swimming performance [11, 12]. This type of indirect mortality is difficult to demonstrate but should be expected considering the extensive tissue damage that can result from ichthyophoniasis. The physiological cost of ichthyophoniasis and resulting reduction in fitness could compound the effects of other environmental and biological stressors experienced by juvenile Pacific herring
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