Date Awarded

2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Andrew R Wargo

Committee Member

Kimberly S Reece

Committee Member

Troy D Tuckey

Committee Member

David T Gauthier

Abstract

Emerging infectious diseases in aquatic systems, both in aquaculture and in the wild, are a global concern. Many have proposed an uptick in marine diseases as a result of environmental changes including a warming climate, habitat modifications, trade and transfer of wildlife and aquaculture products, pollution, overharvesting of resources, and other anthropogenic impacts. These perturbations can disturb the delicate host-pathogen relationships and result in new diseases or exacerbate the existing diseases in a population. Diseases can lead to several direct and indirect effects in the ecosystem such as population declines and extinctions, and thereby a change in population dynamics, as well as loss of numerous ecosystem services. Marine diseases can also tremendously distress the local communities that rely on them by affecting their livelihoods, sustenance, and culture. Understanding the etiology, epidemiology, and the impacts of a disease is the key to its management. However, this can be particularly challenging in cases of understudied and emerging diseases, as there are several unknowns and time is of the essence for disease mitigation and future prevention. Here, we investigated an emerging infectious disease, called the ‘red sore disease (RSD)’, in American eels in the Chesapeake Bay region with the aim of enhancing the sustainability of American eel fishery and aquaculture. The American eel is the only species of freshwater eel found in the North America and is an ecologically and economically important finfish. RSD is characterized by skin lesions on eels and is reported to cause high mortality by the eel distributors in the Chesapeake Bay. Our findings provide new information on the biotic and abiotic drivers of RSD in the American eels in the Chesapeake Bay region. First, we explored the microbial and environmental correlates of RSD in American eel aquaculture in the Chesapeake Bay region by sampling diseased animals at two different aquaculture operations. We developed a scale to score the severity of RSD in eels. By using standard bacteriological and molecular methods, we identified the disease to be associated with bacteria in the genera Aeromonas and Vibrio. While previous studies have implicated Aeromonas, this is the first report identifying Vibrio as a potential causative agent for RSD in American eels. We also found that increasing temperature was positively associated with the presence of Vibrio spp. in the aquaculture operations. Next, we conducted in vivo laboratory challenges using American eels as experimental animals, to confirm if Vibrio vulnificus, the most commonly isolated Vibrio species from diseased eels, is a causative agent of RSD in American eels by applying Koch’s postulates. We also assessed the role of temperature on clinical severity of RSD. We observed that V. vulnificus led to similar lesions, when injected into fish, as previously documented for RSD in aquaculture. In addition, we found the mortality to be greater and more acute at higher temperatures in this experiment. Lastly, we investigated the etiology and epidemiology of RSD in wild-caught American eels from the York River, Virginia by collaborating with a local commercial eel potter and quantifying the prevalence of RSD bi-weekly. Using previously established methods, we again found aeromonads and vibrios to be the dominant bacteria isolated from diseased eels. Septicemia with Vibrio spp. was also observed and prevalence in the wild was associated with higher temperatures and the longer holding time (number of days eels were held in underwater floating cages). These results expand our overall understanding of RSD in American eels in the Chesapeake Bay region. Our findings suggest that a warming climate may exacerbate RSD in the wild. High temperature and handling stress may also exacerbate the disease in aquaculture. We have identified some best practices for the growing aquaculture industry. Recommendations for reducing disease-associated mortality in aquaculture include improvement of water quality by addition of adequate mechanical and biological filters, maintenance of optimum water temperature, and reducing handling trauma and injury by reducing holding time. Given the prevalence of RSD observed in the wild, there are also important implications for considering disease-associated mortality in the assessment of the wild eel stock along the Mid-Atlantic coast. Further research on RSD in American eels is warranted, particularly as V. vulnificus has zoonotic potential and environmental modeling predicts its abundance to increase in the Chesapeake Bay. Safety of eel handlers in aquaculture operations also needs to be ensured. Critical directions for future research have been detailed and include biochemical profiling of the etiological agents as well as additional investigation of environmental stress as a driver of RSD.

DOI

https://dx.doi.org/10.25773/v5-vhth-4a33

Rights

© The Author

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