Viral Hemorrhagic Septicemia – an important potential threat to the Salmonoid farming industry in Norway

Abstract

Viral Hemorrhagic Septicemia is caused by an aquatic Rhabdoviridae called Viral Hemorrhagic Septicemia Virus. It has several genotypes and sub-genotypes. It is a serious transmissible disease with a wide range of hosts, both in fresh water and marine habitats. VHSV is a huge threat to the salmonoid farming industry. The disease is distributed across the world, and in Norway several genotypes have been identified both in wild living and farmed salmonoid species. Several factors affect pathogenicity, such as host, life stage, immune status and water temperature. Virulence varies among the strains. Transmission is horizontal through direct or indirect contact with contaminated fish. Vertical transmission is not proven, but eggs can be contaminated. External mechanical vectors such as wild life has also been identified. Incubation and the course of the disease depends on the water temperature. Generally, higher mortality overall are seen in endemics in lower water temperature. Clinical signs include sudden death, lethargy, erratic swimming behaviour, exophthalmus, ascites and hemorrhages around the orbits or base of fins. Diagnosis of the disease is mainly done from tissue samples, detecting the viral RNA through RTPCR. Other methods can also be used, such as viral cultures, antibody detection, serology or immunohistochemistry. The diseases is listed as notifiable by the OIE, and several countries has several different eradication or control strategies implemented. In Norway, a continuous surveillance and control program is part of the eradication, as well as stamping-out strategies in case of outbreaks. Norway has not had any outbreaks since the 1970’s except in 2007. Several risk factors has been identified, among these are species proximity, species affinity to VHSV strains, proximity to other establishments or areas of infection, feeding with untreated fresh or frozen infected fish and water temperature and stress. Three main points are outlined as key for prevention, these are restriction of movement of fish on the farm, strict biosecurity and diagnosis and report of any abnormal signs in the fish. Other preventative measures might be vaccines in the future, but these are still being researched.