In 1995 and again in 1998, millions of pilchards (Sardinops sagax neopilchardus)
were found dead or dying off the coast of Australia and also in New Zealand. The
epizootics moved progressively, at a rapid speed against the prevailing currents. A
previously unrecognised herpesvirus, Pilchard herpesvirus (PHV), was identified as
the causative agent.
Until recently, rapid and sensitive methods to detect PHV were not available and
based on a previously identified and conserved 373 bp region of the genome,
polymerase chain reaction (PCR), in situ hybridisation and real-time PCR methods
were developed for the specific detection of PHV in formaldehyde-fixed and frozen
tissues of pilchards. Real-time PCR was shown to have greater sensitivity than a
conventional PCR and in situ hybridisation for the detection of PHV infection.
The PCR assay and sequence analysis of the amplification products was used to
compare the 373 bp region of the genome from strains obtained during the 1995
and 1998 epidemics. Significant differences between the strains were not detected.
Additional sequence data was obtained adjacent to the 373 bp of known PHV
sequence, which did not match any sequence in any of the genetic databases, and
this will be invaluable for further study of the pilchard herpesvirus and the
development of improved detection methods.
The molecular-based methods of virus detection developed were applied to a reexamination
of virus in paraffin-embedded tissues taken from fish during an attempt
to transmit the virus to wild caught pilchards in 1999. The results obtained
confirmed previously equivocal results that transmission of PHV to wild caught
pilchards was achieved, although this experiment failed to demonstrate that
transmission of the virus resulted in severe lesions typical of those seen in the
epizootics.
Using formaldehyde-fixed samples from fish collected during the 1998 PHV
epizootic, virus was detected in fish collected 4 days prior to the occurrence of the
epizootic even though the fish then appeared clinically normal, during the epizootic,
and 8 days after mortalities had ceased.
An investigation of wild pilchards collected from 4 Australian pilchard subpopulations
using real-time PCR demonstrated that PHV was present in the gills of
13.75% of 800 fish sampled, indicating that the virus is now endemic in the
Australian pilchard population. Variation in the prevalence of PHV infection in the 4
subpopulations was detected, higher in western and southern populations than in
populations from the east coast. The endemic nature of PHV infection in the
pilchard population explains why there have been no further epizootics with mass
mortalities since 1998.
| Identifer | oai:union.ndltd.org:ADTP/221847 |
| Date | January 2007 |
| Creators | mcrockford@agric.wa.gov.au, Melanie Crockford |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Melanie Crockford |
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