Risk factors for mortality during the first occurrence of Pacific Oyster Mortality Syndrome due to Ostreid herpesvirus – 1 in Tasmania, 2016

Abstract The microvariant genotype of Ostreid herpes virus 1 (OsHV-1 μVar) has caused repeated seasonal outbreaks of mass mortality in Pacific oyster ( Crassostrea gigas ) populations in Europe, New Zealand and in New South Wales, Australia. The first occurrence of OsHV-1 related mortality in Tasmania, Australia was in January 2016; mortality was documented and multivariable logistic regression analysis was undertaken to examine risk factors. Overall mortality was 78% but varied from 37%–92% across 6 farms and between 96% (0–20 mm length) and 33% (61–115 mm) across size classes. The odds of mortality were 3.2 to 3.8 times greater for baskets containing oysters with an average shell length ≤ 40 mm compared to those with an average > 61 mm. Generally, the greater the time on farm the lower the mortality, consistent with the increasing age and size of the oysters. The effect of stocking density on mortality changed with increasing time on farm (age). The standard density produced lower mortality than high or low stocking density particularly for spat. Mortality was nearly twice as likely when oysters were handled for routine husbandry in the 7 days prior to the outbreak compared to not handled. Ploidy and hatchery of origin did not alter the disease outcome. The variation in mortality with husbandry requires further investigation to determine if it can be exploited to help maintain productivity. Statement of relevance This descriptive epidemiological study documents the impact of the first occurrence of disease associated with Ostreid herpesvirus - 1 (OsHV-1) in Tasmania, Australia. OsHV-1 is an emerging and economically important pathogen. Describing the impact of OsHV-1 in new environments and identifying the association between farm management and mortality is important to develop effective disease control measures.