Aetiology
Koi herpes virus (KHV) disease is a viral disease caused by the koi herpes virus in the family Herpesviridiae. The disease was first observed in Israeli koi carp production and has since spread to many countries, also several EU countries.
Symptoms and pathology
Among naturally infected fish, the disease has only been observed in Cyprinus carpio. That is common carp, C. carpio carpio, koi carp, C. carpio koi and ghost sharp, C. carpio goi. The disease is temperature dependent and occurs at temperatures between 16-25 ºC.
The disease process can be fast. During a disease outbreak there will be increased mortality. All age groups appear to be susceptible, but especially young fish up to one year will suffer. The fish become lethargic, separates itself from the shoal and stands at the water intake or the sides of the pond and gasp in the surface. Some fish may show signs of loss of balance and disorientation. There may also be signs of hyperactivity. Typical signs include discoloration or reddening and/or roughening of the skin. There may be focal or total loss of epidermis over- or under-production of mucus on skin and gills. Sunken eyes and bleeding in the skin and at fin base may also be seen. White necrosis in gills often followed by secondary bacterial infection is one of the main characters. Histopathology is nonspecific and variable, but inflammation and necrosis of the gills are reliable features.
Hypertrophy of the branchial epithelium and fusion of secondary lamellae and adhesion of gill filaments are also typically signs. Inflammation and necrosis can also be observed in other organs, especially in the kidney, but also in spleen, pancreas, liver, brain, gut and oral epithelium.
Diagnosis
KHV grow only poorly in fish cell cultures. It is therefore necessary to perform PCR on gills and kidney tissue to detect the agent.
The Diagnostic Manual for KHV Disease is not yet available. Please find recommendations for diagnostic procedures in the report “Report of the workshop “KHV PCR diagnosis and surveillance” 12-13 November 2009, Central Veterinary Institute, Lelystad, The Netherlands” made by a KHV expert working group under the EPIZONE network.
For more information on the disease and diagnosis please consult the OIE Manual of Diagnostic Tests for Aquatic Animals, chapter 2.3.6.