Theodore R. Meyers

Viral hemorrhagic septicemia virus in North America

Abstract The first detections of viral hemorrhagic septicemia virus (VHSV) in North America were in Washington State from adult coho (Oncorhynchus kisutch) and chinook (O. tshawytscha) salmon in 1988. Subsequently, VHSV was isolated from adult coho salmon returning to hatcheries in the Pacific Northwest in 1989, 1991 and 1994. These isolates represented a strain of VHSV that was genetically different from European VHSV as determined by DNA sequence analysis and T1 ribonuclease fingerprinting. The North American strain of VHSV was also isolated from skin lesions of Pacific cod (Gadus macrocephalus) taken from Prince William Sound (PWS), Alaska in 1990, 1991 and 1993. In 1993 and 1994, the virus was isolated from Pacific herring (Clupea harengus pallasi) in Alaskan waters of PWS, Kodiak Island, Auke Bay and Port Frederick. During 1993 and 1994 the herring fishery in PWS failed from a probable complex of environmental stressors but VHSV isolates were associated with hemorrhages of the skin and fins in fish that returned to spawn. Also in 1993 and 1994, VHSV was isolated from apparently healthy stocks of herring in British Columbia, Canada and Puget Sound, Washington. Thus, the North American strain of VHSV is enzootic in the Northeastern Pacific Ocean among Pacific herring stocks with Pacific cod serving as a secondary reservoir. Although the North American strain of the virus appears to be moderately pathogenic for herring, causing occasional self-limiting epizootics, it was shown to be relatively avirulent for several species of salmonids. Pacific herring are common prey for cod and salmon and were most probably the source of the VHSV isolates from the adult salmon returning to spawn in rivers or at hatcheries in Washington State. Compelling circumstances involving the European isolates of VHSV suggest that this strain of the virus also is enzootic among marine fish in the Atlantic Oean. The highly pathogenic nature of the European strain of VHSV for salmonid fish may be the result of the exposure of rainbow trout (O. mykiss), an introduced species, in a stressful environment of intensive culture and the high rate of mutation inherent in all rhabdoviruses. Consequently, we recommend that efforts be made to eradicate the North American strain of VHSV when detected in live salmonids to reduce the possibility of its evolution into a more virulent salmonid virus.

North American Strain of Viral Hemorrhagic Septicemia Virus is Highly Pathogenic for Laboratory-Reared Pacific Herring

Abstract Specific-pathogen-free Pacific herring Clupea pallasi were reared in the laboratory from eggs and then challenged at 5, 9, and 13 months of age by waterborne exposure to low (101.5–2.5 plaque-forming units [PFU] per milliliter), medium (103.5–4.5 PFU/mL), or high (105.5–6.5 PFU/mL) levels of a North American isolate of viral hemorrhagic septicemia virus (VHSV). The fish were extremely susceptible to the virus, showing clinical disease, mortality approaching 100%, and only a limited increase in resistance with age. Mortality began 4–6 d after exposure and peaked at approximately day 7 in fish exposed to high levels of virus. Whereas the mean time to death showed a significant dose response (P < 0.001), the percent mortality and virus titers in dead fish were generally high in all groups regardless of initial challenge dose. External signs of disease were usually limited to 1–2-mm hemorrhagic areas on the lower jaw and isthmus and around the eye, but 2 of 130 infected fish exhibited extensive cutan...

Preliminary results on the seasonality and life cycle of the parasitic dinoflagellate causing bitter crab disease in Alaskan tanner crabs (Chionoecetes bairdi).

Tanner crabs (Chionoecetes bairdi) from the Sullivan Island area of southeast Alaska were sampled for 1 year to determine the prevalence and intensity of the parasitic dinoflagellate which causes bitter crab disease (BCD). The prevalence and intensity of infection were the greatest in the summer, declined in the fall and winter, and increased again in the spring. A possible relationship between softer, newer shells and higher levels of parasitism was also observed. In vivo transmission studies in the laboratory suggested there are several morphologically different forms of the vegetative cell of the BCD dinoflagellate which occur prior to sporulation of the parasite. In addition, it appears that both the two spore types produced by the parasite are infectious by injection and that there is no ploidy difference between the two spore types and the vegetative cell, suggesting that the two spore types may not represent separate sexes.

Comparative Susceptibilities of Salmonid Species in Alaska to Infectious Hematopoietic Necrosis Virus (IHNV) and North American Viral Hemorrhagic Septicemia Virus (VHSV)

Abstract Juveniles of eight Alaskan salmonid species were exposed to waterborne challenges of infectious hematopoietic necrosis virus (IHNV) and North American viral hemorrhagic septicemia virus (VHSV) to determine individual host susceptibilities for potential disease management purposes. Challenges with each of the viruses were conducted at doses of 103 and 105 plaque-forming units (pfu) per milliliter for 1 h. Duplicate tanks were exposed at each dose level; one tank was monitored for fish mortality and the other for virus replication at 5-d intervals. Arctic char Salvelinus alpinus, Arctic grayling Thymallus arcticus, and pink salmon Oncorhynchus gorbuscha were refractory to experimental infection with IHNV, whereas lake trout S. namaycush supported limited replication of the virus with associated clinical disease. Mortality of lake trout ranged 5–15% higher than mortality of fish not exposed to IHNV. Similarly exposed sockeye salmon O. nerka used as positive controls for susceptibility to IHNV sustai...

Infectious Hematopoietic Necrosis Virus: Trends in Prevalence and the Risk Management Approach in Alaskan Sockeye Salmon Culture

Abstract Infectious hematopoietic necrosis virus (IHNV) is endemic to all tested stocks of anadromous sockeye salmon Oncorhynchus nerka in Alaska and was a major cause of failure in the culture of this species until 1981. Since then, the Fisheries Rehabilitation, Enhancement, and Development Division of the Alaska Department of Fish and Game has devised and implemented a strict statewide policy requiring procedures directed towards minimizing losses of sockeye salmon alevins to IHNV. Culture practices under these guidelines have resulted in a steady yearly increase in sockeye salmon production and few fish deaths occurring from IHNV. The statewide IHNV monitoring program for cultured sockeye salmon involves the testing of all brood stocks for virus prevalence and titer. This testing has resulted in the development of a data base on IHNV occurrence in wild and hatchery stocks of sockeye salmon in Alaska during the past 14 years. This data base and data from additional research are being used to examine tre...

Detection of viruses and virus-like particles in four species of wild and farmed bivalve molluscs in Alaska, USA, from 1987 to 2009

The U.S. Alaska Department of Fish and Game has regulatory oversight of the mariculture industry that is partially administered through a statewide shellfish health policy. Possession and transport of bivalve molluscs require development of indigenous pathogen histories from diagnostic examinations of wild and farmed populations. These examinations have resulted in the detection of various infectious agents and parasites including viruses: an aquareovirus and aquabirna-like virus isolated by fish cell culture, and papilloma- or polyoma- and herpes-like virus particles within bivalve cell intranuclear inclusion bodies observed by electron microscopy. This study summarizes these results in samples examined from 1987 to 2009 and is the first description of poikilothermic viruses from Alaskan waters isolated from or observed within the tissues of 4 species of bivalve molluscs: geoduck clam Panope abrupta, native littleneck clam Protothaca staminea, purple-hinged rock scallop Crassadoma gigantea and Pacific oyster Crassostrea gigas.

Retrospective Analysis of Antigen Prevalences of Renibacterium salmoninarum (Rs) Detected by Enzyme-Linked Immunosorbent Assay in Alaskan Pacific Salmon and Trout from 1988 to 2000 and Management of Rs in Hatchery Chinook and Coho Salmon

Abstract Five species of anadromous Pacific salmon Oncorhynchus spp. and eight species of nonanadromous fish, including trouts Oncorhynchus and Salvelinus spp., Arctic char S. alpinus, and Arctic grayling Thymallus arcticus (collectively called “trout”), from Alaskan waters were examined by enzyme-linked immunosorbent assay for annual and total prevalences of Renibacterium salmoninarum (Rs) from 1988 to 2000. The total prevalence of Rs antigen was lower among the Pacific salmon and highest among the nonanadromous salmonid species grouped as “trout” that were residents in many of the watersheds. These trout species were likely reservoirs of Rs infection for many of the wild and hatchery stocks of Pacific salmon. Chinook salmon O. tshawytscha and coho salmon O. kisutch comprised 80% of the salmon database, and 45% of these two species were hatchery broodstocks at four facilities where eggs were routinely culled from Rs-positive parent fish. The cyclical occurrences of Rs antigen in the coho and Chinook salm...

Healthy Juvenile Sockeye Salmon Reared in Virus-Free Hatchery Water Return as Adults Infected with Infectious Hematopoietic Necrosis Virus (IHNV): A Case Report and Review of Controversial Issues in the Epizootiology of IHNV

Abstract Eggs of sockeye salmon Oncorhynchus nerka were spawned from a wild broodstock each fall from 1985 to 1989. The eggs and resulting juvenile fish were incubated and reared as separate families in virus-free freshwater. Each spring of the following year smolts were released into seawater at the hatchery, when they were about 2–6 g in body weight. At spawning, the yearly prevalence (100 × number of virus-positive fish/number examined) of infectious hematopoietic necrosis virus (IHNV) in the wild broodstock ranged from 0% to 100% whereas no IHN disease or virus was ever detected in the progeny at the hatchery. A small number of adult fish from these juvenile releases first returned to the hatchery in 1990. No virus was detected in the 69 fish examined after maturation in virus-free freshwater. In 1991, a greater number of adult fish returned to the hatchery and IHNV was detected at very high titers in fish that matured in both freshwater and seawater. Virus prevalences were as high as 99% in female fi...

Prevalence of Viral Erythrocytic Necrosis in Pacific Herring and Epizootics in Skagit Bay, Puget Sound, Washington

Epizootics of viral erythrocytic necrosis (VEN) occurred among juvenile Pacific herring Clupea pallasii in Skagit Bay, Puget Sound, Washington, during 2005-2007 and were characterized by high prevalences and intensities of cytoplasmic inclusion bodies within circulating erythrocytes. The prevalence of VEN peaked at 67% during the first epizootic in October 2005 and waned to 0% by August 2006. A second VEN epizootic occurred throughout the summer of 2007; this was characterized by disease initiation and perpetuation in the age-1, 2006 year-class, followed by involvement of the age-0, 2007 year-class shortly after the latters metamorphosis to the juvenile stage. The disease was detected in other populations of juvenile Pacific herring throughout Puget Sound and Prince William Sound, Alaska, where the prevalences and intensities typically did not correspond to those observed in Skagit Bay. The persistence and recurrence of VEN epizootics indicate that the disease is probably common among juvenile Pacific herring throughout the eastern North Pacific Ocean, and although population-level impacts probably occur they are typically covert and not easily detected.

Summer Mortalities and Incidental Parasitisms of Cultured Pacific Oysters in Alaska

Abstract During the summer of 1987, surface seawater temperatures in Alaska were unseasonably warm, periodically approaching 20°C with salinities of 29‰ in late July and early August. During this period at least one Alaskan oyster grow-out station sustained excessive mortalitygreater than 50%-within a group of 150,000 18-month-old Pacific oysters Crassostrea gigas. These oysters continued to die despite later reported declines in both seawater temperature and salinity. Histological examination of moribund oysters indicated mature or nearly mature gametes in both sexes and infiltration of tissues by opportunistic secondary invaders composed primarily of various bacterial types and the flagellate protozoan Hexamita sp. An incidental, but potentially pathogenic, rickettsia-like organism that formed cytoplasmic inclusions within vesicular connective tissues was also observed in some of the oysters. Circumstances associated with this oyster mortality were similar to those accompanying summer mortalities of raf...