Herpesvirus anguillae (HVA) isolations from disease outbreaks in cultured European eel, Anguilla anguilla in the Netherlands since 1996

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 23°C; six outbreaks at water temperatures lower than 23°C.Decreasing the temperature to below 22°C 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.