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In 1995 and again in 1998/9, millions of pilchards (Sardinops sagax neopilchardus) were found dead or dying off the coast of Australia. Some pilchards in New Zealand were also affected during the 1995 epizootic. The epizootics moved progressively in a "bushfire-like" manner against the prevailing currents at a rapid speed. An uncharacterised herpesvirus, pilchard herpesvirus (PHV), was associated with both mortality events and, based on its occurrence in affected fish and the disease pathogenesis, generally accepted as the causative agent, although experimental evidence of its role was not established. Until recently, rapid and sensitive methods to detect PHV were not available and so research into the characteristics of this virus were limited. New methods developed and optimised for the detection of PHV include Polymerase Chain Reaction (PCR), in situ hybridization (ISH) and real-time PCR. These methods were then applied to a number of pilchard samples that were obtained at various times during the two outbreaks.

Some sequence data for PHV was obtained. These data are the first step in obtaining information at the molecular level as to how this virus fits within the Herpesviridae. Since acquiring this initial sequence data, techniques have been applied to try to further characterise PHV. At the time of starting this project, opportunities to increase the known sequence were limited due to the inability to grow the virus in culture, the limited virus stocks and the best virus material only being part purified and of unknown viability. The additional sequence data obtained is still preliminary at this stage and requires further analysis.

Three molecular methods have recently become available for detecting PHV (ISH, PCR and Real-time PCR) and were used to detect PHV in samples from fish collected pre-, during and post-outbreak. Samples collected before 1995 were not available to test. The real-time PCR

detected virus in fish collected 4 days before the epizootic front, during the front, and 8 days after the front had passed. The samples that were available for this study were archival and limited the information that could be obtained on the length of the virus incubation period and persistence of the virus after the epizootic front had passed. However, the information obtained was consistent with modelling by Murray et al. (2003) that predicted an incubation period of up to 12 days, depending on the wave speed of the epidemic. The detection of PHV in samples after mortalities had ceased, that appeared to be healthy by histology and were not positive by ISH or conventional PCR, indicates that virus persisted in the population of survivors at low levels.

In 1999 an experiment was carried out to transmit PHV to apparently healthy pilchards in an attempt to (1) show that the virus could be transmitted to uninfected pilchards, (2) demonstrate that the infected pilchards develop gill lesions characteristic of the disease and (3) show that the virus can be re-isolated from experimentally infected fish. At the time of the experiment the only available methods for detection of PHV were histology and electron microscopy. As attempts to grow the virus in vitro were unsuccessful the lack of virus detected in experimentally exposed pilchards suggested that virus transmission was unsuccessful. Because the technology is now available to apply molecular techniques to detect PHV, the transmission trial samples have been re-analysed.

ISBN

1 877098 96 5

Publication Date

10-2006

Series Number

15

Publisher

Department of Fisheries, Western Australia

City

Perth

Keywords

Pilchard herpesvirus; PCR; In situ hybridization; Transmission trial

Disciplines

Aquaculture and Fisheries | Environmental Monitoring | Immunology of Infectious Disease | Management Information Systems | Management Sciences and Quantitative Methods | Marine Biology | Natural Resources Management and Policy | Sustainability | Virology

Comments

FRDC Project No. 2002/044

Fisheries Research Contract Report No. 15 - Aquatic Animal Health Subprogram: pilchard herpesvirus infection in wild pilchards (FRDC Project No. 2002/044) Final report

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