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

Simple Summary: OsHV-1 is one of two herpesviruses known to infect invertebrates; this pathogen has emerged as the primary etiology responsible for mass mortalities in the species. The pathological characteristics, tissue and cellular tropisms of OsHV-1 in Anadara broughtonii (A. broughtonii) remain unknown. The objective of this study was to characterize the pathological changes and tissue tropism during the development of an OsHV-1 infection in A. broughtonii. The results demonstrated that hemocytes and fibroblastic-like cells were the primary cellular targets of OsHV-1. Additionally, lesions, infiltrated hemocytes, and co-localized ISH signals were identified in the muscular tissues of the foot and adductor muscle. These findings contribute to the understanding of OsHV-1 pathogenesis in Arcidae mollusks. 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.