Viral Haemorrhaginc Septicaemia (VHS) Disease Information

Introduction

VHS is a serious disease of farmed trout causing up to 80% mortality, our stocks are naive to the disease meaning potentially high mortalities should the virus escape into our trout populations.

Petechial haemorrhages on muscle tissue of Rainbow trout with VHS. © Crown copyright 2005

Etiological Agent

A Rhabdovirus of the genus Novirhabdovirus

Geographical Distribution:

VHS was first reported from a rainbow trout farm in Denmark (egtved) and has since caused significant losses in rainbow trout farms in most countries in continental Europe. It has also been reported in Japan, U.S.S.R and the USA. Great Britain and Eire have each had a single outbreak of VHS, in farmed Turbot.

The VHS virus has been isolated from the marine environment, in the Baltic and North seas, the Atlantic Ocean and off the Pacific coast of North America.

Susceptible Species:

VHS is principally a disease of farmed rainbow trout, but most salmonid fish are considered susceptible, as are whitefish (Coregonus spp), grayling and pike. The disease has also been reported from farmed turbot, and wild Pacific herring.
The virus has been isolated from numerous marine fish species, in the Pacific Ocean: Pacific herring (Clupea pallasi), Pacific salmon (Oncorhynchus spp.), Pacific cod (Gadus macrocephalus) Pacific sandlance (Ammodytes hexapterus) and pilchard (Sardinops sagax); and in the Atlantic Ocean: Atlantic Cod (Gadus morhua), haddock (Gadus aeglefinus), poor cod (Trisopterus minutus), rockling (Rhinonemus cimbrius), sprat (Clupea sprattus), herring (Clupea harengus), whiting (Merlangius merlangus), blue whiting (Micromesistius poutassou), lesser argentine (Argentina sphyraena), Norway pout (Trisopterus esmarki). Diagnostic tests have suggested the presence of the virus in other marine species, from which the virus has yet to be isolated.

Epizootiology

Outbreaks of VHS in rainbow trout typically occur between temperatures of 7 and 140c, and often cause acute mortalities. At lower water temperatures an extended disease course with a lower but more protracted mortality may be observed. At temperatures between 15 and 180C the disease takes a short course with low mortality. The disease is transmitted horizontally through contact with infected fish or water. Large numbers of virus are shed in the fishes faeces, urine and sexual fluids. There is no vertical transmission of the virus.

The potential result of a VHS outbreak. © Crown copyright 2005

Disease Signs

Outbreaks are typified by a sudden increase in mortality rates. Symptoms include darkening of the body, swollen eyes (exopthalmia), distended abdomen and haemorrhaging at the vent and bases of the fins. Fish may become moribund and congregate around the pond outlets and sides. They may also show erratic swimming behaviour such as darting, spiralling and swimming on their sides. Internal signs include haemorrhaging on the surfaces of the visceral fat and internal organs and throughout the musculature, with accumulation of fluid in the abdominal cavity.

Diseased Trout with VHS. © Crown copyright 2005

Diagnosis

Diagnosis is by virus isolation in tissue culture, followed by confirmatory immunological testing, including enzyme linked immunosorbent assay (ELISA), and neutralisation.

Treatment & Control

There is no treatment for VHS. As a List II notifiable disease there is a legal obligation to report any suspected outbreaks of VHS to the Fish Health Inspectorate.

If the disease were found on a farm in Britain, movement restrictions would be applied to all farms on the same river catchment, and attempts would be made to eradicate the disease. All contact sites to the positive farm would be rapidly investigated to look for evidence of the source and any potential spread of the virus.

The approved status of any infected area would be lost until the disease had been eradicated, and a testing programme, to confirm the eradication, completed.