Since 1996, Herpesvirus anguillae (HVA) infections were diagnosed 19 times at 14 Dutch eel farms with European eels (Anguilla anguilla). The eels showed various clinical symptoms, but predominantly apathy, haemorrhages in their skin and fins, ulcerative skin and fin lesions, haemorrhages in the gills, congested gill epithelium, a pale liver, and a mortality up to 10%. Eel gills were in more cases HVA positive than the corresponding organs. Therefore, gills should be sampled for HVA-isolations in diseased eels. Often a bad water quality induced the virus propagation. Eleven outbreaks occurred at water temperatures higher than 23degreesC; six outbreaks at water temperatures lower than 23degreesC. Decreasing the temperature to below 22degreesC worked well in 4 cases. The agent was isolated on Eel Kidney (EK-1) cells, and characterized as a herpesvirus with electron microscopy, typed by digestion of virus DNA with restriction enzyme HindIII, and its DNA fragments were analysed by gel electrophoresis. All Dutch HVA isolates showed a similar DNA pattern. Comparison of the DNA of the Dutch viruses and Japanese isolates revealed a small difference after digestion with HindIII, however.

Eight outbreaks of “red spot disease” were diagnosed between March and June 1997 in diseased European elvers Anguilla anguilla (L.), maintained at 23–25°C in The Netherlands. The elvers originated from Southwestern France and Northern Portugal. In all cases, Pseudomonas anguilliseptica was isolated in pure culture. The mortalities varied from 3% to 20%, within 2–3 weeks of the initial outbreak. The isolates were phenotypically similar to French isolates from sea bream Sparus aurata (L.), sea bass Dicentrarchus labrax (L.) and turbot Scophthalmus maximus (L.). They were sensitive to neomycine, trimethoprim, trimethoprim+sulphadiazine, and sulphonamids, and some were sensitive to oxytetracycline, flumequine, and oxolinic acid. Mortality ceased when the water temperature was raised to 26–27°C. One isolate was tested for pathogenicity by experimental infection in post elvers and larger European eels, and showed low pathogenicity.

In 1998 a cytopathogenic agent was isolated from diseased European eel (Anguilla anguilla) in an eel-kidney cell line (EK-1 cells) and was characterized as a herpesvirus by electron microscopy. This virus multiplied well in EK-1 cells but poorly in cells of different fish species: RTG-2 cells of trout and FHM cells of Fathead minnow. The virus appeared to be related antigenically to the Herpesvirus anguillae isolated from European and Japanese eel (A. japonica) in Japan by Sane et al. (M. Sano, H. Fukuda and T. Sano (1990). Isolation and characterization of a new herpesvirus in eel. In: Pathology in Marine Science 3rd: 1988. T.O. Perkins and T.C Cheng eds., Ac. Press, New York), The importance of this herpesvirus as a cause of disease of eel in aquaculture is emphasized by four more isolations since this first isolation. and by eight more isolations at retrospective examination of tissue homogenates of diseased eel, which had been stored in the freezer since 1996.