Pacific oyster mortality syndrome: a marine herpesvirus active in Australia

Genotypes of Ostreid herpesvirus 1 (OsHV-1) known as microvariants cause the disease Pacific oyster mortality syndrome (POMS). Since its appearance in NSW in 2010, OsHV-1 microvariant has prevented the farming of Pacific oysters (Crassostrea gigas) in the affected estuaries near Sydney, following the initial massive outbreaks^1,2. The arrival of the disease in southeast Tasmania in January 2016 has put the entire $53M industry in Australia in jeopardy³. The virus is a member of the Family Malacoherpesviridae⁴, which includes several invertebrate herpesviruses. The OsHV-1 genome consists of 207 439 base pairs, with organisation similar to that of mammalian herpesviruses. However, OsHV-1 contains two invertible unique regions (U_L, 167.8 kbp; U_S, 3.4 kbp) each flanked by inverted repeats (TR_L/IR_L, 7.6 kbp; TR_S/IR_S, 9.8 kbp), with an additional unique sequence (X, 1.5 kbp) between IR_L and IR_S⁴_. Unlike many herpesviruses which are host specific, OsHV-1 strains have been transmitted between marine bivalve species⁵ and the virus is transmitted indirectly. The virus may have relatively prolonged survival in the environment, has extremely high infection and case fatality rates, and latency is unproven. Along with pilchard herpesvirus^6–8 and abalone ganglioneuritis virus^9,10, it is part of a dawning reality that marine herpesviruses are among the most virulent of pathogens. Finding solutions for industry requires more than laboratory-based research.

In 2008, the microvariant genotype OsHV-1 µVar¹¹ emerged as a dominant isolate against a background of prior endemic OsHV-1 strains in France, and has devastated the C. gigas industry there. Similar microvariant genotypes of OsHV-1 have since appeared throughout Europe, in New Zealand, and in Australia¹, with similar devastating impact (Figure 1). In 2010–11 when an emergency response was required in Australia, almost no information existed on the epidemiology of OsHV-1 infection, and the only management responses internationally that could be drawn from were to develop resistant oysters through selective breeding approaches. In Australia, a breeding program commenced immediately but it was complemented by research to identify mechanisms of viral transmission and environmental triggers, in order to develop husbandry recommendations to mitigate losses.

Figure 1. Since its appearance in NSW in 2010, OsHV-1 microvariant has prevented the farming of Pacific oysters (Crassostrea gigas) in the affected estuaries near Sydney. Dead oysters in a tray after an OSHV-1 mortality event.

Oysters are often grown in the intertidal zone. Replicated experiments were conducted in the Georges River estuary near Sydney in summer 2011–12 and 2012–13, to determine whether immersion time in seawater influenced viral exposure and mortality in different age classes of oyster (Figure 2). The findings were remarkably consistent. Reduction in immersion time by about 2 hours per tide cycle led to 50% reduction in mortality in valuable adult oysters^12,13. Furthermore, assumptions about uniform seawater exposure to virus were shattered by observations of highly clustered infection and mortality patterns at scales of 1 km to a few cm. This was consistent with distribution patterns of estuarine plankton¹⁴. Accurate counts of thousands of live and dead research oysters placed in multiple locations in successive summers led to a hypothesis: that OsHV-1 may be carried in plankton¹⁵.

Figure 2. Oyster farmer Len Drake with a tray of experimental oysters to be placed in the Georges River to study the transmission of the disease.

Most of the Pacific oysters grown in Australia are produced in Tasmania in large commercial hatcheries, certified free from diseases, and then shipped to farmers in New South Wales, South Australia and Tasmania to be grown and marketed. For this reason the occurrence of OsHV-1 in Tasmania was predicted to be a massive risk to the entire industry. Based on the idea that OsHV-1 was probably carried on organic or inorganic particles with the plankton, experiments were conducted to prevent mortality by filtering seawater or aging it by sedimentation to remove the causative agent. Filtration would not have been an obvious solution for a viral aetiology were it not for the epidemiological observations, and evidence that testing a pellet derived from low speed centrifugation of seawater improved detection of the virus¹⁶ (Figure 3). Controlled experiments on these water treatments were successful¹⁷ and the results were taken up by the hatcheries. In January 2016, when POMS emerged in Tasmania, one hatchery had appropriate water treatment in place and did not experience mortality in its stock while a second had not completed its water treatment program and was severely affected by the disease.

Figure 3. Olivia Evans collecting water samples to test for OsHV-1.

The state jurisdictions implemented quarantine controls over affected estuaries in both NSW and TAS and this has been very effective in slowing the spread of the disease. This has been accompanied by research to confirm appropriate disinfection protocols. Surprisingly, OsHV-1 remained viable for at least a week within dried oyster tissues, such as those that might adhere to plastic farm trays and baskets in which oysters are grown. Disinfection of such material with high concentrations of chlorine was ineffective. However, quarternary ammonium compounds were effective¹⁸. Biosecurity controls apply only to oysters and oyster farming equipment, leaving many potential pathways for spread of the virus. Pleasure boats, commercial shipping, and oceanic currents may all play a role in international and regional spread of aquatic pathogens.

Pathogen distribution and disease expression may be favoured by environmental conditions. POMS occurs in Sydney estuaries when water temperature is ~22−26°C^1,2; this is ~5^oC higher than in France^19,20. An experimental infection model has been developed and used in Australia to confirm these field observations on water temperature; it revealed an important interaction between water temperature and infectious dose^21,22. As part of FRDC funded research we have established water temperature monitoring on oyster leases in all major C. gigas farming areas in Australia to provide insight into a very dynamic thermal environment. The aim is to develop a risk management and early warning system.

Over time a combination of biosecurity, husbandry and genetic approaches will need to be applied to continue farming Pacific oysters in Australia.