Seasonal changes in transmission of and recovery from Hyalophysa lynni in commercial shrimp

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Megan Tomamichel and James E. (Jeb) Byers, Odum School of Ecology, University of Georgia, Athens, GA, Richard J. Hall, Center for the Ecology of Infectious Diseases, University of Georgia, Athens, GA, Tina L. Walters and Marc E. Frischer, University of Georgia, Skidaway Institute of Oceanography, Savannah, GA

Background/Question/Methods
Emerging infectious diseases in marine systems can cause mass mortality events in fished species, threatening food security and livelihoods of coastal communities. Because both hosts and parasites are sensitive to abiotic factors including water temperature, ocean warming could exacerbate disease emergence. The ciliate Hyalophysa lynni infects commercially harvested penaeid shrimp in the southeastern United States. Infection with the ciliate induces gill melanization impairing respiration and possibly resulting in reduced shrimp harvests. H. lynni infection prevalence peaks in warm water months, up to 95% in some years, and declines in cooler months. It is unknown if these changes are due to seasonal heterogeneities in parasite transmission and/or host recovery. We experimentally measured parasite transmission and shrimp recovery between June 2020 and February 2021. We first harvested wild caught shrimp to detect baseline infection prevalence and then treated the remaining shrimp with anti-parasitic medications to remove H. lynni infections. We placed individually caged medicated shrimp into the Skidaway River, Georgia for 10 days. As a control, we held medicated shrimp in artificial seawater for the same length of time. After the exposure period all shrimp were assayed for the presence of H. lynni to quantify parasite transmission rates.
Results/Conclusions
Seasonal changes were detected in both the transmission of and recovery from H. lynni. Baseline prevalence of H. lynni was positively correlated with water temperatures: winter had the lowest ambient prevalence (25%) while the summer had the highest ambient prevalence (80% and 83%). In all seasons except for winter, the chemical procedure was effective at eliminating H. lynni infection. During field trials on medicated shrimp, H. lynni was transmitted to 83% and 100% of shrimp during the two summer experiments compared to 7% and 18% prevalence of shrimp contemporaneously held in artificial seawater. During fall and winter, only 27% and 20% of shrimp deployed were infected with H. lynni, while shrimp remaining in artificial seawater had 6% and 33% prevalence, respectively. These experiments demonstrate that cooler seasons strongly decrease the transmission of H. lynni. Winter temperatures also decrease the efficacy of the medication treatment. It remains unclear if seasonal differences are due to changes in the parasite’s life history, changes in the immune system of the shrimp host, or both.