The Pathology of Systemic Iridoviral Disease in Fish S. Gibson-Kueh, P. Netto * , G. H. Ngoh-Lim, S. F. Chang, L. L. Ho, Q. W. Qin † , F. H. C. Chua ‡ , M. L. Ng § and H. W. Ferguson { AgriFood and Veterinary Authority of Singapore, *Electron Microscopy Unit, Faculty of Medicine, † Tropical Marine Science Institute, ‡ Aqualife Aquatic Health Services, § Microbiology Department, Faculty of Medicine, National University of Singapore, Singapore, and { Institute of Aquaculture, University of Stirling, Stirling, Scotland, UK Summary Iridoviruses have been associated with severe disease and economic loss in farmed food fish and ornamental fish, with mortality often reported to reach 50% or more. In the present study, three tropical marine food fish species and four tropical freshwater ornamental fish species with systemic iridovirus infections were examined histopathologically and ultrastructurally. Light microscopy consistently revealed pale to intensely basophilic hypertrophied virus-infected cells in spleen, kidney and intestine from all seven species. Ultrastructural examination showed changes in the vascular endothelium overlying hypertrophied virus-infected cells suggestive of pressure necrosis. Viral isolation was improved by the use of fibroblastic cell lines. This, together with the sub-endothelial location of infected cells in all infected species examined, suggests that systemic iridoviruses are mesotheliotropic. q 2003 Elsevier Ltd. All rights reserved. Keywords: fish; iridovirus; lymphocystis disease; systemic iridoviral disease; viral infection. Introduction Iridoviruses have been implicated as the cause of severe disease, mortality and economic loss in farmed food fish and ornamental fish, as well as in wild fish (Langdon et al., 1986; Langdon and Humphrey, 1987; Plumb et al., 1996). Hetrick and Hedrick (1993) recognized distinct groups of histopathological lesions in iridoviral infec- tions of fish. Such infections may be localized or systemic. Localized lesions include those of lymphocystis disease of the skin and fins (Smail and Munro, 1989; Wolf 1988a), iridoviral infection of the skin and gills in the white sturgeon (Acipenser transmon- tanus Richardson)(Hedrick et al., 1990), and viral erythrocytic necrosis (VEN) in Atlantic cod (Gadus morhua Linnaeus) (Reno and Nicholson, 1981; Wolf 1988b; Smail and Munro, 1989). Systemic iridoviral infections have been described in a variety of freshwater food fish species (Ogawa et al., 1990; Pozet et al., 1992; Whittington et al., 1994; Reddacliff and Whittington, 1996; McGrogan et al., 1998; He et al., 2000), tropical freshwater ornamental fish (Armstrong and Fergu- son, 1989; Anderson et al., 1993; Rodgers et al., 1997) and marine food fish species (Inouye et al., 1992; Bloch and Larsen, 1993; Chua et al., 1994; Matsuoka et al., 1996; Chou et al., 1998; Nakajima et al., 1998; Jung and Oh, 2000). The first iridoviral disease to be described in fish was that caused by lymphocystis disease virus (LDV), which is capable of infecting a great variety of freshwater and marine species. Histopathologically, J. Comp. Path. 2003, Vol. 129, 111–119 doi: 10.1016/S0021-9975(03)00010-0, available online at http://www.sciencedirect.com on 0021–9975/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. Correspondence to: S. Gibson-Kueh, Aquatic Animal Health Branch, Animal and Plant Health Laboratory Division, AgriFood and Veterinary Authority of Singapore, 60 Sengkang East Way, Singapore 548596.