Replication of a herpes-like virus in larvae of the flat oyster Tiostrea chilensis at ambient temperatures

Abstract

Veligers removed from brooding Tiostrea chilensis (Philippi, 1845) experienced ~95% mortalities over 3 to 4 d at 16 to 18°C that appeared to be associated with a herpes-like virus. Ultrastructural observations of post-removal veligers showed the presence of early viral replication or putative latent stages at 4 h, all stages of replication at low levels at 24 h, which increase to high levels at 48 h, followed by mortalities at 72 h onwards. Initially, infected interstitial or epithelial cells had an enlarged nucleus with a wavy outline in which heterochromatin was marginated. With continued increase in size, nuclei became smooth in outline with reduction or loss of heterochromatin. Capsids with lucent cores (LCC) and empty capsids appeared in the nucleus, often in association with tubular structures ~65 nm in diameter that were composed of subunits in a helical configuration that contained a tubular core ~35 nm in diameter. Empty capsids and LCC sometimes occurred in paracrystalline arrays. Partial nucleolar disaggregation and encapsidation of dense fibrillar material preceded envelopment entering and de-envelopment leaving the perinuclear cisterna, tegumentation in cytoplasmic vesicles, and egress. Groups of dense cytoplasmic filaments 30 to 35 nm in diameter occurred in some infected cells. Apparently normal cells with a few intranuclear empty capsids and/or LCC at 4 h post-removal may represent latent infections. Replication was not observed in larvae held at 24 to 27°C, but a few cells had enlarged hypochromatic karyolytic nuclei, and 1 to 2 capsids were observed in them, at 48 h. Viral replication was similar to that of ranid herpesvirus 1 (Lucke tumour) infections. This is the fifth ostreid species from which herpesviruses have been reported.