Counting the dead to determine the source and transmission of the marine herpesvirus OsHV-1 in Crassostrea gigas

Abstract

Marine herpesviruses are responsible for epizootics in economically, ecologically and culturally significant taxa. The recent emergence of microvariants of Ostreid herpesvirus 1 (OsHV-1) in Pacific oysters Crassostrea gigas has resulted in socioeconomic losses in Europe, New Zealand and Australia however, there is no information on their origin or mode of transmission. These factors need to be understood because they influence the way the disease may be prevented and controlled. Mortality data obtained from experimental populations of C. gigas during natural epizootics of OsHV-1 disease in Australia were analysed qualitatively. In addition we compared actual mortality data with those from a Reed–Frost model of direct transmission and analysed incubation periods using Sartwell’s method to test for the type of epizootic, point source or propagating. We concluded that outbreaks were initiated from an unknown environmental source which is unlikely to be farmed oysters in the same estuary. While direct oyster-to-oyster transmission may occur in larger oysters if they are in close proximity (< 40 cm), it did not explain the observed epizootics, point source exposure and indirect transmission being more common and important. A conceptual model is proposed for OsHV-1 index case source and transmission, leading to endemicity with recurrent seasonal outbreaks. The findings suggest that prevention and control of OsHV-1 in C. gigas will require multiple interventions. OsHV-1 in C. gigas, which is a sedentary animal once beyond the larval stage, is an informative model when considering marine host-herpesvirus relationships.

The complex interactions of Ostreid herpesvirus 1, Vibrio bacteria, environment and host factors in mass mortality outbreaks of Crassostrea gigas

“…Therefore, direct transmission could occur by close contact (Whittington et al . ). In fact, the higher mortality of oysters in baskets compared to oyster on ropes may be due to the direct transmission between oysters in the closer proximity within baskets (Pernet et al .…”

“…The data obtained from experimental populations of C. gigas during natural epizootics of OsHV‐1 disease in Australia showed that direct transmission could have occurred at a local scale (<40 cm separation distance)(Whittington et al . ).…”

“…On the other hand, indirect transmission (vector particles are required) could occur over both short and long distances (Whittington et al . ). However, indirect transmission is more common and important at larger scales (>40 cm between cultivation units, metres between groups, kilometres between sites) (Whittington et al .…”

“…However, indirect transmission is more common and important at larger scales (>40 cm between cultivation units, metres between groups, kilometres between sites) (Whittington et al . ). Recent observations in Australia suggest that OsHV‐1 may be transmitted by plankton in the water (Paul‐Pont et al .…”

Disease threats to farmed green-lipped mussels Perna canaliculus in New Zealand: review of challenges in risk assessment and pathway analysis

“…by contact), dispersal in the water column (e.g. through tidal currents) is likely to play a significant role in the local spread of disease (Whittington et al 2018), especially in areas with a high density of farms in close proximity (Murray 2013, Pernet et al 2016). Furthermore, since most shellfish aquaculture operations in New Zealand are open water systems, it is reasonable to assume that an organism newly established in wild populations will eventually spread to adjacent or related farmed stocks and vice versa .…”