Encephalomyelitis among turbot associated with a picornavirus-like agent

Abstract

An outbreak of encephalomyelitis among larval turbot ScophthaLmus maximus caused very heavy mortalities. Bacteriological findings were very unspecific, and parasites were not observed. By electron microscopy large numbers of virus particles were found in the brain and medulla of the diseased fish. Based on the size of the particles and their position in the cytoplasm of the infected cells it is suggested that the particles belong to the picomaviridae. The neurotropic character of the disease further suggests that the virus might be an enterovirus. Parallel with the development of hatching and larval rearing techniques of new species in aquaculture, new disease problems of parasitological, bacterial, and viral origin occur. In turbot production, vibriosis appears to be a common problem, and we have hitherto isolated Vibrio anguillarum serogroups 01, 02a, and 02b as well as members of the Vibrio splendidus/pelagius group. A more complex ulcerative disease has been described in Spain involving 3 facultative pathogens, Vibrio carchariae, Myxobacteria, and a protozoan resembling Cryptocaryon spp. (Devesa et al. 1989). Viral agents demonstrated in turbot include 'Herpesvirus scophthalmi' (Buchanan & Madeley 1978), infectious pancreatic necrosis virus (Castric et al. 1987) and a virus which was suggested to be a member of the Reovirus group (Lupiani et al. 1989). This report deals with an investigation of a disease associated with heavy mortalities among young turbot Scophthalmus maximus in a Norwegian fish farm. Materials and methods. The disease problem appeared in an extensive procruction unit near Bergen, Norway. Turbot larvae were reared in 18 floating bags (50 m3) in a maine lagoon (140 000 m3). The bags were stocked with 80 000 to 150 000 eggs per batch at the Author for correspondence O Inter-ResearchIPrinted in Germany beginning of June 1989. The temperature was 12 to 13 C at the time of stocking, but increased gradually to 17 or 18°C over the next 2 to 3 wk, with surface water temperatures reaching 20 C around noon. In late June the weather changed, and heavy rainfall, accompanied by a drop in surface temperatures to between 14 and 15C, occurred. The larvae were fed zooplankton collected from the lagoon. They seemed to develop normally until the beginning of July, when heavy mortality occurred. In the first batch of larvae, the mortality appeared during weaning to moist pellets (weight 50 to 100 mg). Later, the clinical signs associated with the mortality in the first batch were observed among other batches of larvae in the lagoon. The last batches, stocked in August 1989, died within 3 wk after hatching. Fish transferred to weaning tanks that exhibited mortalities were treated with antibiotics (Nifurazolidone). This did not reduce mortality. The initial clinical signs were reduced feeding activity followed by a darkening in pigmentation. Diseased fish became lethargic, often lying abdomen-up on the bottom. Atypical swimming, such as rotating, spinning, and horizontal looping, was observed when the fish were disturbed. The clinical signs indicated disturbance of the central nervous system and were followed by 100 % mortality. Samples were taken from internal organs (pronephros, liver, spleen) and plated on tryptic soy agar containing 2 % NaCl or marine agar (Difco) with 5 % citrated calf blood, and TCBS agar (Difco). Incubation was at 20°C for 48 h. Resulting isolates were characterized according to West & Colwell (1984) and Larsen (1985). Gills, skin, and intestine were examined for the presence of parasites by light microscopy. 5 k g 5. & ,< a k g b