Protective immunity by vaccination with plasmid DNA encoding a viral glycoprotein (G) has long been assumed to result from the induction of a specific immune response. We report here that the initial protection may be due to the induction of alpha/beta interferon, with long-term protection due to a specific response to the encoded viral G. DNA vaccines encoding the Gs of three serologically unrelated fish rhabdoviruses were used to vaccinate rainbow trout against a lethal challenge with infectious hematopoietic necrosis virus (IHNV). All three vaccines, each encoding the G gene of either IHNV (IHNV-G), snakehead rhabdovirus (SHRV) (SHRV-G), or spring viremia of carp virus (SVCV) (SVCV-G), elicited protective immunity against IHNV. Vaccinated fish were challenged at 30 or 70 days postvaccination with lethal doses of IHNV. At 30 days postvaccination, only 5% of fish that had received any of the G vaccines died, whereas more than 50% of the control fish succumbed to virus challenge. When fish were vaccinated and challenged at 70 days postvaccination, only 12% of the IHNV-G-vaccinated fish died compared to 68% for the SHRV-G-and 76% for the SVCV-G-vaccinated fish. Assays for trout Mx protein, an indicator of alpha/beta interferon induction, showed that only fish vaccinated with a G-containing plasmid produced high levels of Mx protein in the kidneys and liver. Interestingly, at day 7 after virus challenge, all of the fish vaccinated with the IHNV-G plasmid were negative for Mx, but the SHRV-G-and SVCV-G-vaccinated fish still showed detectable levels of Mx. These results suggest that DNA vaccines in fish induce an early, nonspecific antiviral protection mediated by an alpha/beta interferon and, later, a specific immune response.Antiviral DNA vaccines carrying a gene for a major antigenic viral protein have received considerable attention as a new approach to vaccine development, especially when traditional vaccines have failed. They offer the advantage of mimicking a viral infection, resulting in host production of a single viral protein that is correctly folded and modified, and eliciting both cellular and humoral immune responses (9, 48). DNA vaccines have been developed for a wide variety of viruses, including influenza virus (14, 46), human immunodeficiency virus (7,15,42), rabies virus (38), hepatitis B virus (10), rubella virus (41), and foot-and-mouth disease virus (19). Genetic vaccines have also been developed for several other pathogens, including Mycoplasma pulmonis (29), Mycobacterium tuberculosis (34), Plasmodium yoelii (17), and Schistosoma japonicum (49).For fish viruses, DNA vaccines have been developed for infectious hematopoietic necrosis virus (IHNV) (2, 33) and viral hemorrhagic septicemia virus (6), both rhabdoviruses belonging to the Novirhabdovirus genus. Laboratory trials with fish indicate that these vaccines are considerably more effective in protecting fish from lethal challenge with homologous virus than either the traditional killed vaccine or the subunit vaccine we had developed previous...

Biotype 2 (BT2) variants of the bacterium Yersinia ruckeri are an increasing disease problem in U.S. and European aquaculture and have been characterized as serovar 1 isolates that lack both peritrichous flagella and secreted phospholipase activity. The emergence of this biotype has been associated with an increased frequency of enteric redmouth disease (ERM) outbreaks in previously vaccinated salmonid fish. In this study, four independent specific natural mutations that cause the loss of both motility and secreted lipase activity were identified in BT2 strains from the United States, United Kingdom, and mainland Europe. Each of these was a unique mutation in either fliR, flhA, or flhB, all of which are genes predicted to encode essential components of the flagellar secretion apparatus. Our results demonstrate the existence of independent mutations leading to the BT2 phenotype; thus, this phenotype has emerged separately at least four times. In addition, BT2 strains from the United Kingdom were shown to have the same mutant allele found in U.S. BT2 strains, suggesting a common origin of this BT2 lineage. This differentiation of distinct BT2 lineages is of critical importance for the development and validation of alternative vaccines or other treatment strategies intended for the control of BT2 strains.

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Pectoral fin tissue of white sturgeon was investigated as a potential non-lethal sample source for the detection of white sturgeon iridovirus (WSIV) infection. Histopathology and polymerase chain reaction (PCR) results using fin tissue were compared with the standard lethal histopathology sampling method that utilizes head tissue. Tissues for each of the three sampling methods were collected weekly for 8 weeks from individual sturgeon undergoing an experimental cohabitation challenge with fish infected with the Abernathy isolate of WSIV. Non-lethal fin histopathological evaluation did not reveal infection during the first 3 weeks of sampling, while non-lethal PCR and the lethal method were variable. However, all three sampling methods were equally capable of identifying infection from 4 to 8 weeks post-exposure. Of the survivors tested, all were negative by PCR and the lethal method, and only one fish was identified as being positive by non-lethal fin histopathology. In another experiment, all three sampling methods were applied to asymptomatic WSIV carriers in a case study conducted at the Kootenai Tribal Sturgeon Conservation Hatchery. Results showed that both lethal and non-lethal fin histopathology were equally effective in detecting infection, but PCR was unable to identify this strain of WSIV. Depending on the virus isolate, these results suggest that non-lethal sampling of fin tissue (histopathology or PCR) is comparable with the lethal sampling method at identifying WSIV infection once infection is established, and under certain circumstances may provide an alternative to lethal sampling.

It is thought that white sturgeon iridovirus (WSIV) is transmitted vertically from adult white sturgeon Acipenser transmontanus to progeny, and that wild adults are carriers of this virus. Based on this assumption, egg disinfection trials were initiated using wild Kootenai River white sturgeon. Over 2 consecutive years, post-fertilized eggs were disinfected with iodine at concentrations ranging from 0 to 400 ppm. Eggs were incubated and progeny were reared on either de-chlorinated municipal or Kootenai River water. Juvenile sturgeon (mean weight 3.0 g) from these treatment groups were then subjected to a density stress (15 or 20 g l -1