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The parasitic dinoflagellates of the genus Hematodinium have been considered one of the most important emerging pathogens for a broad range of marine crustaceans around the world. In China, frequent outbreaks of Hematodinium infections have caused serious economic losses for local farmers since 2004. Wild crabs were recently indicated to play a vital role in the transmission and spreading of the Hematodinium disease in polyculture pond systems. Based on PCR amplification and histopathological examination, we demonstrated that H. perezi can naturally infect a wild crab species, Hemigrapsus takanoi, which were collected from the waterways located on the coast of Rizhao or Weifang, Shandong Peninsula, China. According to the sequence similarity analysis and phylogenetic analysis, the Hematodinium isolates were identified as H. perezi and belonged to genotype II. The prevalence of H. perezi ranged from 3.3% to 5.7% in H. takanoi originating from Rizhao (n = 165 wild crabs) and from 0.9% to 20.0% in that originating from Weifang (n = 1386 wild crabs), respectively. To our knowledge, H. takanoi is, for the first time, reported as a new host for Hematodinium. Given the wide distribution of H. takanoi on the coasts along the Shandong Peninsula and the relative high prevalence of infection we monitored in our study, we speculate that H. takanoi contributes to the introducing and spreading parasitic Hematodinium between ponds via waterways in a poly-culturing system. Findings in this study broaden the host range of this parasite and expand the scope of our surveillance for Hematodinium disease in China., (© 2022 John Wiley & Sons Ltd.)

Abstract: Since 2004, the parasitic dinoflagellates Hematodinium emerged as causative agent of the “milky blood disease” in cultured Portunus trituberculatus, Scylla serrata and Exopalaemon carinicauda along the coastal areas of southern China; while, no Hematodinium infections had been reported in northern China. We sampled P. trituberculatus in polyculture ponds and adjacent coastal waters from two sites in Shandong Peninsula, the major culture region in northern China which contributes to one third of the national gross outcome of the crab species. Hematodinium infections were identified in 10% of P. trituberculatus randomly sampled from polyculture ponds, while no infections were observed in wild crabs collected from adjacent coastal waters. Filamentous trophonts and amoeboid trophonts were observed in diseased crabs. The Hematodinium sp. shares similar morphologic features and causes typical pathological changes in hepatopancreas, heart, and muscles of infected hosts as other Hematodinium spp. The laboratory trials indicated that this parasite was infectious and caused pathological alterations to crab hosts as those diseased crabs from culture ponds. Molecular analysis indicated that the isolates were closely related to those reported in southern China. Our findings indicate an alarming broad distribution of the infectious pathogen along the coastal areas of China. The polyculture pond system widely used in major culture regions in China may prompt transmission or spread of Hematodinium among susceptible hosts, which urges pressing investigation of its major routes of transmission in such culture system along with better understanding of its life cycle. [Copyright &y& Elsevier]