Paul Hershberger

Climate Change Influences on Marine Infectious Diseases: Implications for Management and Society

Infectious diseases are common in marine environments, but the effects of a changing climate on marine pathogens are not well understood. Here we review current knowledge about how the climate drives host-pathogen interactions and infectious disease outbreaks. Climate-related impacts on marine diseases are being documented in corals, shellfish, finfish, and humans; these impacts are less clearly linked for other organisms. Oceans and people are inextricably linked, and marine diseases can both directly and indirectly affect human health, livelihoods, and well-being. We recommend an adaptive management approach to better increase the resilience of ocean systems vulnerable to marine diseases in a changing climate. Land-based management methods of quarantining, culling, and vaccinating are not successful in the ocean; therefore, forecasting conditions that lead to outbreaks and designing tools/approaches to influence these conditions may be the best way to manage marine disease.

Ichthyophoniasis: An Emerging Disease of Chinook Salmon in the Yukon River

Abstract Before 1985, Ichthyophonus was unreported among Pacific salmon Oncorhynchus spp. from the Yukon River; now it infects more than 40% of returning adult Chinook salmon O. tshawytscha. Overall infection prevalence reached about 45% in the Yukon River and about 30% in the Tanana River between 1999 and 2003. Mean infection prevalence was greater in females than males in the main-stem Yukon River during each of the 5 years of the study, but the infection prevalence in males increased each year until the difference was no longer significant. Clinical signs of ichthyophoniasis (presence of visible punctate white lesions in internal organs) were least at the mouth of the Yukon River (∼10%) but increased to 29% when fish reached the middle Yukon River and was 22% at the upper Tanana River. However, clinical signs increased each year from 7% in 1999 to 27% in 2003 at the mouth of the river. As fish approached the upper reaches of the Yukon River (Canada) and the spawning areas of the Chena and Salcha rivers...

Amplification and transport of an endemic fish disease by an introduced species

The introduction of American shad from the Atlantic to the Pacific coast of North America in the late 1800’s and the subsequent population expansion in the 1980’s resulted in the amplification of Ichthyophonus sp., a Mesomycetozoean parasite of wild marine fishes. Sequence analysis of the ribosomal DNA gene complex (small subunit and internal transcribed spacer regions) and Ichthyophonus epidemiological characteristics indicate a low probability that Ichthyophonus was co-introduced with American shad from the Atlantic; rather, Ichthyophonus was likely endemic to marine areas of the Pacific region and amplified by the expanding population of a highly susceptible host species. The migratory life history of shad resulted in the transport of amplified Ichthyophonus from its endemic region in the NE Pacific to the Columbia River watershed. An Ichthyophonus epizootic occurred among American shad in the Columbia River during 2007, when infection prevalence was 72%, and 57% of the infections were scored as moderate or heavy intensities. The epizootic occurred near the record peak of shad biomass in the Columbia River, and corresponded to an influx of 1,595 mt of infected shad tissues into the Columbia River. A high potential for parasite spillback and the establishment of a freshwater Ichthyophonus life cycle in the Columbia River results from currently elevated infection pressures, broad host range, plasticity in Ichthyophonus life history stages, and precedents for establishment of the parasite in other freshwater systems. The results raise questions regarding the risk for sympatric salmonids and the role of Ichthyophonus as a population-limiting factor affecting American shad in the Columbia River.

Effects of temperature on disease progression and swimming stamina in Ichthyophonus-infected rainbow trout, Oncorhynchus mykiss (Walbaum)

Rainbow trout, Oncorhynchus mykiss, were infected with Ichthyophonus sp. and held at 10 degrees C, 15 degrees C and 20 degrees C for 28 days to monitor mortality and disease progression. Infected fish demonstrated more rapid onset of disease, higher parasite load, more severe host tissue reaction and reduced mean-day-to-death at higher temperature. In a second experiment, Ichthyophonus-infected fish were reared at 15 degrees C for 16 weeks then subjected to forced swimming at 10 degrees C, 15 degrees C and 20 degrees C. Stamina improved significantly with increased temperature in uninfected fish; however, this was not observed for infected fish. The difference in performance between infected and uninfected fish became significant at 15 degrees C (P = 0.02) and highly significant at 20 degrees C (P = 0.005). These results have implications for changes in the ecology of fish diseases in the face of global warming and demonstrate the effects of higher temperature on the progression and severity of ichthyophoniasis as well as on swimming stamina, a critical fitness trait of salmonids. This study helps explain field observations showing the recent emergence of clinical ichthyophoniasis in Yukon River Chinook salmon later in their spawning migration when water temperatures were high, as well as the apparent failure of a substantial percentage of infected fish to successfully reach their natal spawning areas.

Susceptibility of Three Stocks of Pacific Herring to Viral Hemorrhagic Septicemia

Laboratory challenges using specific-pathogen-free Pacific herring Clupea pallasii from three distinct populations indicated that stock origin had no effect on susceptibility to viral hemorrhagic septicemia (VHS). All of the populations were highly susceptible to the disease upon initial exposure, with significantly greater cumulative mortalities occurring in the exposed treatment groups (56.3-64.3%) than in the unexposed control groups (0.8-9.0%). Interstock differences in cumulative mortality were not significant. The virus loads in the tissues of fish experiencing mortality were 10-10,000 times higher during the acute phase of the epizootics (day 13 postexposure) than during the recovery phase (days 30-42). Survivors of the epizootics were refractory to subsequent VHS, with reexposure of VHS survivors resulting in significantly less cumulative mortality (1.2-4.0%) than among positive controls (38.1-64.4%); interstock differences in susceptibility did not occur after reexposure. These results indicate that data from experiments designed to understand the ecology of VHS virus in a given stock of Pacific herring are broadly applicable to stocks throughout the northeastern Pacific.

Chronic and persistent viral hemorrhagic septicemia virus infections in Pacific herring.

Chronic viral hemorrhagic septicemia virus (VHSV) infections were established in a laboratory stock of Pacific herring Clupea pallasii held in a large-volume tank supplied with pathogen-free seawater at temperatures ranging from 6.8 to 11.6 degrees C. The infections were characterized by viral persistence for extended periods and near-background levels of host mortality. Infectious virus was recovered from mortalities occurring up to 167 d post-exposure and was detected in normal-appearing herring for as long as 224 d following initial challenge. Geometric mean viral titers were generally as high as or higher in brain tissues than in pools of kidney and spleen tissues, with overall prevalence of infection being higher in the brain. Upon re-exposure to VHSV in a standard laboratory challenge, negligible mortality occurred among groups of herring that were either chronically infected or fully recovered, indicating that survival from chronic manifestations conferred protection against future disease. However, some survivors of chronic VHS infections were capable of replicating virus upon re-exposure. Demonstration of a chronic manifestation of VHSV infection among Pacific herring maintained at ambient seawater temperatures provides insights into the mechanisms by which the virus is maintained among populations of endemic hosts.

Up in arms: Immune and nervous system response to sea star wasting disease

Echinoderms, positioned taxonomically at the base of deuterostomes, provide an important system for the study of the evolution of the immune system. However, there is little known about the cellular components and genes associated with echinoderm immunity. The 2013–2014 sea star wasting disease outbreak is an emergent, rapidly spreading disease, which has led to large population declines of asteroids in the North American Pacific. While evidence suggests that the signs of this disease, twisting arms and lesions, may be attributed to a viral infection, the host response to infection is still poorly understood. In order to examine transcriptional responses of the sea star Pycnopodia helianthoides to sea star wasting disease, we injected a viral sized fraction (0.2 μm) homogenate prepared from symptomatic P. helianthoides into apparently healthy stars. Nine days following injection, when all stars were displaying signs of the disease, specimens were sacrificed and coelomocytes were extracted for RNA-seq analyses. A number of immune genes, including those involved in Toll signaling pathways, complement cascade, melanization response, and arachidonic acid metabolism, were differentially expressed. Furthermore, genes involved in nervous system processes and tissue remodeling were also differentially expressed, pointing to transcriptional changes underlying the signs of sea star wasting disease. The genomic resources presented here not only increase understanding of host response to sea star wasting disease, but also provide greater insight into the mechanisms underlying immune function in echinoderms.

Sequence analysis of the internal transcribed spacer (ITS) region reveals a novel clade of Ichthyophonus sp. from rainbow trout

The mesomycetozoean parasite Ichthyophonus hoferi is most commonly associated with marine fish hosts but also occurs in some components of the freshwater rainbow trout Oncorhynchus mykiss aquaculture industry in Idaho, USA. It is not certain how the parasite was introduced into rainbow trout culture, but it might have been associated with the historical practice of feeding raw, ground common carp Cyprinus carpio that were caught by commercial fisherman. Here, we report a major genetic division between west coast freshwater and marine isolates of Ichthyophonus hoferi. Sequence differences were not detected in 2 regions of the highly conserved small subunit (18S) rDNA gene; however, nucleotide variation was seen in internal transcribed spacer loci (ITS1 and ITS2), both within and among the isolates. Intra-isolate variation ranged from 2.4 to 7.6 nucleotides over a region consisting of approximately 740 bp. Majority consensus sequences from marine/anadromous hosts differed in only 0 to 3 nucleotides (99.6 to 100% nucleotide identity), while those derived from freshwater rainbow trout had no nucleotide substitutions relative to each other. However, the consensus sequences between isolates from freshwater rainbow trout and those from marine/anadromous hosts differed in 13 to 16 nucleotides (97.8 to 98.2% nucleotide identity).

Viral tropism and pathology associated with viral hemorrhagic septicemia in larval and juvenile Pacific herring.

Viral hemorrhagic septicemia virus (VHSV) genotype IVa causes mass mortality in wild Pacific herring, a species of economic value, in the Northeast Pacific Ocean. Young of the year herring are particularly susceptible and can be carriers of the virus. To understand its pathogenesis, tissue and cellular tropisms of VHSV in larval and juvenile Pacific herring were investigated with immunohistochemistry, transmission electron microscopy, and viral tissue titer. In larval herring, early viral tropism for epithelial tissues (6d post-exposure) was indicated by foci of epidermal thickening that contained heavy concentrations of virus. This was followed by a cellular tropism for fibroblasts within the fin bases and the dermis, but expanded to cells of the kidney, liver, pancreas, gastrointestinal tract and meninges in the brain. Among wild juvenile herring that underwent a VHS epizootic in the laboratory, the disease was characterized by acute and chronic phases of death. Fish that died during the acute phase had systemic infections in tissues including the submucosa of the gastrointestinal tract, spleen, kidney, liver, and meninges. The disease then transitioned into a chronic phase that was characterized by the appearance of neurological signs including erratic and corkscrew swimming and darkening of the dorsal skin. During the chronic phase viral persistence occurred in nervous tissues including meninges and brain parenchymal cells and in one case in peripheral nerves, while virus was mostly cleared from the other tissues. The results demonstrate the varying VHSV tropisms dependent on the timing of infection and the importance of neural tissues for the persistence and perpetuation of chronic infections in Pacific herring.

Passive Immunization of Pacific Herring against Viral Hemorrhagic Septicemia

The plasma of Pacific herring Clupea pallasii that survived laboratory-induced viral hemorrhagic septicemia (VHS) epizootics contained humoral substances that, when injected into naive animals, conferred passive immunity against the disease. Among groups exposed to viral hemorrhagic septicemia virus (VHSV), injection of donor plasma from VHS survivors resulted in significantly greater survival (50%) and significantly lower tissue titers (1.5 x 10(5) plaque-forming units [PFU]/g) than the injection of plasma from VHSV-naive donors (6% survival; 3.7 x 10(6) PFU/g). Additionally, the magnitude of the protective immune response increased during the postexposure period; plasma that was collected from survivors at 123 d postexposure (931 degree-days) provided greater protection than plasma collected from survivors at 60 d postexposure (409 degree-days). These results provide proof of concept that the VHSV exposure history of Pacific herring populations can be determined post hoc; furthermore, the results can be used as the foundation for developing additional high-throughput diagnostic techniques that may be effective at quantifying herd immunity and forecasting the potential for future VHS epizootics in populations of wild Pacific herring.