Herpesvirus now endemic in Australia

A review of: Whittington, Crockford, Jordan and Jones. Herpesvirus that caused epizootic mortality in 1995 and 1998 in pilchard, Sardinops sagax neopilchardus (Syeindachner), in Australia is now endemic. Journal of Fish Diseases. 2008. 31, 97-105.

In 1995, a massive epizootic occurred in pilchards, starting in Australian waters and spreading bidirectionally over 6000km, throughout a range of species, and finally reaching to New Zealand. The disease was characterized by severe branchitis and was attributed to a herpesvirus, Pilchard herpesvirus (PHV). Mortality was high with vast numbers of dead fish washing onto beaches and settling on the seafloor also. Unsurprisingly, there were measurable secondary impacts in piscivorous species including penguins, which experienced increased mortality rates and reproductive failure due to food shortage.

The cause of the outbreaks of PHV is unknown although it is hypothesized that the virus was introduced to a naïve population, or due to the reactivation of latent infection within the population. The authors set out to determine whether PHV is still present within Australian pilchard populations as previous surveys were hampered by the fish’s wide geographical distribution and effective capture avoidance behavior. Observations and modeling of the spread of PVH disease in the 1995/1998-99 epizootics suggested a single semi-continuous Australian pilchard population with direct contact between shoals of adult fish and between schools within shoals, sufficient to promote the spread of disease. However, preliminary survey results from this experiment suggest that there are 4 separate populations now present in the waters around Australia.

Samples of fish were taken randomly from each sup-population and tested for PHV. The results clearly show that three of the four subpopulations were infected. The fourth subpopulation was not sampled using the same random technique as the other three, and so true prevalence is not certain. The authors conclude that their results show PHV is endemic in S. sagax neopilchardus in Australian waters, supporting the hypothesis that the virus was introduced to a naïve population in 1995 and again in 1998 when it became established. Those fish are now all dead either due to infection or lifespan. The PHV positive fish sampled in this study were therefore recently exposed to the virus and may be immune.

The findings of this study have implications for fisheries managers. If similair other surveys are carried out in the future with comparable results, it may be beneficial to manage anthropogenic influences to limit contact between infected and non-infected populations. Current international translocations of baitfish, including pilchards, clearly place naïve populations at risk of acquiring pathogens to which there is no prior exposure or immunity. Not to mention economic losses associated with infections in aquaculture and fisheries, there are potentially larger consequences as food chains are impacted and predator prey relationships change.

This paper was a bit long-winded although the authors do highlight potential problems and unreliability of their data, especially in regard to the 4^th uninfected/ immune subpopulation. I think it would be very beneficial for more surveys like this to be carried out on a wider range of species; especially those used extensively in the production of fish meal for use in aquaculture, as many diseases can be contracted form the ingestion of previously infected food. It may help prevent some of the more important diseases, such as VNN, from having such a significant presence in aquaculture stocks.