Localization and Tissue Tropism of Ostreid Herpesvirus 1 in Blood Clam Anadara broughtonii

OsHV-1 caused detrimental infections in a variety of bivalve species of major importance to aquaculture worldwide. Since 2012, there has been a notable increase in the frequency of mass mortality events of the blood clam associated with OsHV-1 infection. The pathological characteristics, tissue and cellular tropisms of OsHV-1 in A. broughtonii remain unknown. In this study, we sought to investigate the distribution of OsHV-1 in five different organs (mantle, hepatopancreas, gill, foot, and adductor muscle) of A. broughtonii by quantitative PCR, histopathology and in situ hybridization (ISH), to obtain insight into the progression of the viral infection. Our results indicated a continuous increase in viral loads with the progression of OsHV-1 infection, reaching a peak at 48 h or 72 h post-infection according to different tissues. Tissue damage and necrosis, as well as colocalized OsHV-1 ISH signals, were observed primarily in the connective tissues of various organs and gills. Additionally, minor tissue damage accompanied by relatively weak ISH signals was detected in the foot and adductor muscle, which were filled with muscle tissue. The predominant cell types labeled by ISH signals were infiltrated hemocytes, fibroblastic-like cells, and flat cells in the gill filaments. These results collectively illustrated the progressive alterations in pathological confusion and OsHV-1 distribution in A. broughtonii, which represent most of the possible responses of cells and tissues to the virus.