Transmission pattern of the parasitic dinoflagellate Hematodinium perezi in polyculture ponds of coastal China

Abstract Since 2012, Hematodinium perezi epizootics have occurred frequently in cultured swimming crabs Portunus trituberculatus in polyculture ponds along the coast of Shandong Province, China. These outbreaks have resulted in significant economic losses to local farmers. The cryptic transmission pattern of H. perezi has hampered the prevention and control of the pathogen in affected farms. The mudflat crab Helice tientsinensis is one of the dominant wild crabs in the region with considerable prevalence of the parasitic infection covering the full culture period of P. trituberculatus. Mudflat crabs can migrate in and out of polyculture ponds during water exchange and may act as an important reservoir host and source of H. perezi for newly seeded P. trituberculatus. During 2019, we carried out continuous field investigations and sentinel experiments in the polyculture pond system, as well as a series of laboratory infection trials, to study whether waterborne transmission of H. perezi can occur in polyculture ponds, and to determine whether H. perezi can be transmitted from H. tientsinensis to cultured P. trituberculatus. The sentinel experiment verified waterborne transmission of H. perezi to cultured P. trituberculatus in the polyculture pond. The cohabitation trial further indicated that H. perezi was transmittable from H. tientsinensis to P. trituberculatus through waterborne transmission, whereas P. trituberculatus was not able to acquire H. perezi infection by feeding on tissues of infected H. tientsinensis. The results highlighted the transmission pattern of the parasitic dinoflagellate H. perezi in the polyculture pond systems widely used along the coast line of China. In addition, the mudflat crab H. tientsinensis was confirmed as a reservoir host of H. perezi in the epidemic area, and confirmed as the likely source of the parasite during the intensive culture of P. trituberculatus.