Aetiology
Viral haemorrhagic septicaemia (VHS) is a viral disease of mainly rainbow trout. The virus belongs to the genus Novirhabdovirus in the family Rhabdoviridae. The virus is related to infectious haematopoietic necrosis (IHN) virus and is genetically more related to Lyssa virus genus (rabies virus) than with vesiculovirus genus (VSV). VHSV is serologically relatively homogeneous and can be identified using mono- and polyclonal antibodies. There are a number of different sero-and genogroups that can be distinguished by the use of monoclonal antibodies in neutralization test or ELISA or by RFLP analysis or sequencing.
A major eradication program moved for VHS came into force on 1 April 2009 and since there has been no VHS outbreaks and Denmark is currently regarded as free of VHS. Based on EU legislation, the Northern and Southern Jutland is Category I (disease free). In between these two zones are a number of farms registered as Category I segments. The rest of Denmark is registered as Category II (monitoring program in order to obtain Category I status). Sea farms have Category III status (known to be infected but are not covered by an application to obtain disease-free status).
Symptoms and pathology
The incubation period is under farming conditions usually 1-3 weeks and depends on fish age, virus concentration and especially water temperature - incubation periods of up to three months in winter has been observed. The clinical symptoms evolve from acute / sub acute in 2-3 weeks. Initially sudden nervousness and restlessness among the fish without detectable specific symptoms is observed. Later darkening, protruding eyes, pale gills and lethargy. Haemorrhages can often be seen around the eyes and in skin and muscle. The abdomen can be distended by fluid. Towards the end of an outbreak haemorrhages are usually less pronounced, while abnormal swimming behaviour with rotation around the longitudinal axis takes over. Characteristic findings in the acute / subacute phase are dark colored, lethargic fish that are just below the water surface along sides of the ponds. Sick fish do not try to flee when touched.
When dissecting the fish haemorrhages are typical findings. These are usually in muscles, skin, gills, liver, spleen, kidneys, heart, brain, swim bladder and in the perivisceral adipose tissue. The spleen is often mildly swollen while the middle and rear of the intestines in contrast to findings in bacterial septicaemia usually is devoid of feed, pale and atonic. By cutting of back musculature characteristic petechial bleedings can be observed.
For more information on the disease and diagnosis please consult the OIE Manual of Diagnostic Tests for Aquatic Animals, chapter 2.3.9. and the EU Diagnostic manual for VHS and IHN.