Pathobiology of Water Molds in Fish: An Insight into Saprolegniasis

Saprolegnia is an aquatic pathogen with a fishy appetite—it develops on farmed and wild fish populations as a notoriously destructive ‘water mold’. The etiologic agent of Saprolegniasis, Saprolegnia is an opportunistic oomycete that is of significant interest in the aquaculture industry due to its financial impact and widespread effect. Previously, infection models studying the effects of Saprolegnia utilized methods that were injurious to fish and did not mimic a natural outbreak, thus making it difficult to use to evaluate new treatments for the disease. By developing a novel zebrafish (D. rerio) egg infection model, a new insight into the pathogenesis and progression of the disease in vivo is possible. Once completed, the model could serve as a platform to quickly identify and test alternative treatments to formalin. This study compared existing culture techniques of Saprolegnia and developed a novel staining method to view the pathogen during host invasion. Zebrafish eggs were infected with zoospore suspensions and monitored carefully. It was found that in all treatments, the zoospore concentrations were too low to infect healthy zebrafish eggs reproducibly. However, almost all zebrafish that were exposed to Saprolegnia colonized hemp seeds developed an infection within 24 hours. Different concentrations of Fluorescent Brightener 28 were assessed to develop an efficient staining protocol to visualize the disease progression. This staining technique could potentially be utilized to help quantify and track the infection in fish eggs. Future studies should consider the use of zebrafish as model organisms in Saprolegnia research