Bonamia exitiosa transmission among, and incidence in, Asian oyster Crassostrea ariakensis under warm euhaline...

Previously reported in Australia, New Zealand, and more recently in Europe, the protistan parasite Bonamia exitiosa was also reported in the mid-Atlantic region of the USA after causing serious mortalities there in the Asian oyster Crassostrea ariakensis. At the time, this oyster was being considered for introduction, and the potential consequences of introducing this species were being assessed using field and laboratory studies. B. exitiosa emerged as the most serious disease threat for this oyster species, especially under warm euhaline conditions and for oysters <50 mm in size. To better evaluate how quickly this parasite may be able to spread among C. ariakensis, we investigated B. exitiosa transmission and incidence in C. ariakensis. During a first trial, potential direct transmission of B. exitiosa was evaluated by cohabitating infected C. ariakensis with uninfected C. ariakensis under in vivo quarantine conditions. In a second experiment, B. exitiosa incidence was estimated in situ by determining its prevalence in C. ariakensis deployed in an enzootic area after 4, 7, 14, 21 and 28 d of exposure. Results suggest that under warm euhaline conditions B. exitiosa can be transmitted among C. ariakensis without requiring any other parasite source and that parasite incidence may be at least as high as 40% after only 4 d exposure to an enzootic area. These results underscored the severity of the bonamiasis disease threat to C. ariakensis and provided further evidence that efforts to build an aquaculture industry based on C. ariakensis in the eastern USA might have been thwarted by parasitic disease.