Lymphocystis disease virus persists in the epidermal tissues of olive flounder, Paralichthys olivaceus (Temminch & Schlegel), at low temperatures M Hossain 1 , S-R Kim 1 , S-I Kitamura 2 , D-W Kim 1 , S-J Jung 1 , T Nishizawa 3 , M Yoshimizu 3 and M-J Oh 1 1 Division of Food Science & Aqualife Medicine, Chonnam National University, Yeosu, Korea 2 Center for Marine Environment Studies, Ehime University, Matsuyama, Japan 3 Faculty of Fisheries Science, Hokkaido University, Hakodate, Japan Abstract Olive flounder artificially infected with lympho- cystis disease virus (LCDV) were reared at 10, 20 and 30 °C for 60 days, to compare LCD-incidence. In the fish reared at 20 °C, lymphocystis cells ap- peared on the skin and fins at 35 days post-chal- lenge, and the cumulative LCD-incidence was 80% at 60 days. High levels of LCDV, with a mean polymerase chain reaction (PCR) titre of 10 6 PCR- U mg )1 tissue, were detected in the fins and skin of LCD-affected fish at 20 °C, but were not detected in the spleen, kidney, brain and intestinal tissues of these fish. No LCD clinical signs were observed in the fish reared at 10 °C and 30 °C; however, a low level of LCDV (10 3 PCR-U mg )1 tissue) was de- tected in the fins and skin of these fish. By increasing the rearing temperature from 10 to 20 °C, lymphocystis clusters appeared on the skin and fins of the fish with no previous LCD clinical signs within 33 days after the temperature change. It was shown that permissive cells for LCDV infection exist in the epidermis of olive flounder. At low temperatures, small amounts of LCDV were able to persist over a period extended for a further 45 days in the fish epidermis, even though the fish showed no LCD clinical signs. The optimum growth temperature of LCDV is near 20 °C. Keywords: epidermis, lymphocystis disease virus, Paralichthys olivaceus, persistent infection, temper- ature. Introduction Lymphocystis disease (LCD) is a viral disease occurring in marine, brackish and fresh water teleost fish worldwide. LCD is characterized by the development of whitish clusters of abnormally enlarged cells on the skin and fins of affected fish (Nigrelli & Smith 1939). LCD is not fatal, but its unsightly appearance may cause serious economic losses. The causative agent of LCD (LCDV) is a large icosahedral DNA virus in the genus Lymph- ocystivirus of the family Iridoviridae (Brown 1986). The hypertrophied cells, generally named lympho- cystis cells, have a thick hyaline capsule, an enlarged nucleus, and prominent basophilic cyto- plasmic DNA inclusions (Samalecos 1986). Since an electron microscopic study on LCDV by Walker (1962), many attempts have been made to elucidate the ultrastructure of lymphocystis cells and LCDV (Zwillenberg & Wolf 1968). During the past few decades, researchers have investigated the development of lymphocystis cells containing inclusions and capsules, both in vivo (Walker & Weissenberg 1965) and in vitro (Walker & Hill 1980), and have succeeded in their molecular characterization and genome analysis (Schnitzler & Darai 1993; Tidona & Darai 1997, 1999; Zhang, Xiao, Xie, Li & Gui 2004). Culture isolation and detection techniques for LCDV have also been Journal of Fish Diseases 2009, 32, 699–703 doi:10.1111/j.1365-2761.2009.01048.x Correspondence M-J Oh, Division of Food Science & Aqualife Medicine, Chonnam National University, Yeosu, Korea (e-mail: ohmj@chonnam.ac.kr) 699 Ó 2009 Blackwell Publishing Ltd