VIRAL LOAD OF LYMPHOCYSTIVIRUS IN TARGET AND NON-TARGET TISSUES OF NATUALLY INFECTED GILTHEAD SEABREAM (SPARUS AURATA)

Lymphocystis disease virus (LCDV), (Lymphocystivirus, Iridoviridae) is the causative agent of lymphocystis, a chronic self-limiting disease described worldwide in over 125 species of fish. In the Mediterranean area, gilthead seabream (Sparus aurata) is the most affected species. Lymphocystivirus infection causes single or clustered tumour-like nodules that are localized to the skin and fins. Although lymphocystis is frequently benign, it may be responsible for significant economic losses related to poor growth rate, non-marketability of injured fish and secondary bacterial infections. Recent surveys have shown a systemic spread of the LCDV and a persistent infection in fish after recovery. The limited availability of effective techniques for the diagnosis and study of Lymphocystivirus, due to their poor and difficult cultivability in vitro, has long reduced the possibility to study this infection. The recent development of a real-time PCR suitable for the detection and quantitation of the LCDV DNA genome in tissues of clearly or hidden infected subjects, represents a valuable new tool for pathogenesis study of lymphocystis. The presence of viral DNA was investigated in target (pectoral and caudal fins) and non-target (spleen, brain, eye) tissues of 12 naturally infected gilthead seabream. The infected fish were ranked on the basis of presence of typical lesions (4 subjects), scarring due to the regressive phase of the disease (4 subjects) and absence of any clinical alterations (4 subjects). The viral DNA was detected in all tested fish and tissues. Different viral loads were detected in target and non-target tissues showing statistically higher amount of viral DNA in pectoral and caudal fins (106-107 viral DNA copies/l) than that detected in internal organs (103-105 viral DNA copies/l). The distribution of the viral genome showed a similar pattern regardless of the clinical stage of the fish. In the caudal-pectoral fins and spleen viral loads were detected in decreasing, but not statistically significant, values in diseased, in regression and recovered subjects, respectively. The viral DNA was always detected in recovered fish with variable values in different organs (102-107 viral DNA copies/l) showing a persistent infection after symptom remission. The present study confirmed systemic and persistent infection adding a quantitative analysis.