Mortality events in farmed oysters throughout the coastal landscape of North Carolina (NC) have stymied growth in the oyster aquaculture industry. Histological analyses of oysters harvested during these mortality events indicate that disease is playing a role in the loss of valuable product. To better understand etiology and onset of disease, longitudinal studies have been conducted to characterize both oyster condition and surrounding environmental conditions during periods of stress. However, previous efforts have not always succeeded in capturing critical moments in the progression of disease development. To combat these shortcomings and increase resolution, a daily sampling campaign was undertaken at a lease that has consistently experienced severe mortality over the past decade.
For 63 consecutive days, a subset of triploid oysters (Crassostrea virginica), water samples, and environmental parameters were collected from a single site. Histology sections were taken from each oyster censored and water samples were subjected to analysis via ddPCR (droplet digital PCR) to quantify pathogen presence within the water column. Using novel molecular assays designed to quantify multiple Vibrio spp. by targeting the GH18 domain of the chiA gene, our analyses revealed a strong microbial signal in the water column prior to the first observations of mortality. The peaks in concentration of two distinct Vibrio clades occurred 6 days before the first observed death, revealing a small window of time between initial challenge and eventual onset of mortality. On any given day, no more than 5 oysters were found dead, suggesting that the general perception of mortality is often colored by a lack of resolution with weekly assessments often appearing more jarring. In contrast, daily observations demonstrate a slower progression of disease. Given that the increase in concentration of subpopulations of Vibrio bacteria in the water column occur prior to the first instance of mortality, this signal could offer predictive capacity and indicate when mitigation or preventative measures would be most effective.