Koi herpesvirus (KHV) is a virus that infects common carp (Cyprinus carpio) and its ornamental varieties, such as koi and ghost carp.
Since being first reported from Israel and the USA in 1998, KHV has spread rapidly through the international fish trade and is now one of the most significant viral diseases of carp worldwide.
KHV was first confirmed in carp fisheries in England in 2002, following mortality investigations conducted by National Fisheries Services (NFS) Brampton. Despite controls, the virus has spread rapidly and is now widespread in England. To date, NFS Brampton have investigated over 120 cases of KHV, providing support and advice to help fisheries recover and avoid future outbreaks. This is a vital aspect of KHV disease as fishery management is both a trigger for the disease as well as the main route for avoiding future losses.
Fish with KHV typically show signs of lethargy, respiratory distress and erratic behaviour. Many die. Infected individuals may linger at the water surface or congregate in the margins. Clinical signs of KHV can vary but include severe gill pathology such as patches of necrotic (dead) tissue and erosion. These are frequently accompanied by fungal, bacterial and parasitic infections. Other signs of KHV include enophthalmia (sunken eyes) and abnormal changes to the internal organs. In many cases, KHV occurs alongside other parasites and diseases, making it difficult to identify one clear cause.
Two main factors commonly trigger KHV disease: temperature and stress. Viruses often have temperature ranges in which they become more active and pathogenic. Outbreaks of KHV generally occur in late spring and summer when water temperatures rise above 16°C. Outside of these temperatures, the virus may be less active or lie dormant without causing any signs of disease.
Increased stress is also a key factor that triggers KHV disease. This is why fishery management is so important for disease outbreaks in carp fisheries. Intensive fishery management practices, leading to poor environmental conditions, overcrowding and excessive angling pressure, can increase stress within the fish population. This reduces the immune response of carp, making them more susceptible to infections. KHV outbreaks most commonly occur in intensively managed fisheries with very high stock densities.
Fish stocking poses the greatest risk of introducing the disease to your fishery. The difficulty is detecting the virus in healthy fish, as carp may carry the virus without showing signs of disease. Current surveillance efforts for KHV are based only on KHV disease, not the virus itself, so there is a risk that KHV can spread undetected. It is also possible to transfer the virus on wet fishing tackle, nets and boots, or by the transfer of infected water. Good fishery management and biosecurity are therefore critical in minimising the risk of transferring KHV into your fishery.
KHV disease can only be confirmed through specific laboratory tests. It is not possible to confirm KHV from the bank side or by external examinations alone. Lethargy and gill necrosis can be caused by many different factors including poor water quality, toxins, algal blooms, parasites, bacteria and fungi. It is not uncommon to find a range of different infections and problems during KHV outbreaks. Confirming the cause of any disease outbreak requires detailed laboratory examinations in combination with assessments of environmental conditions, presence of other pathogens and understanding of fishery management practices. Such investigations are conducted by NFS Brampton and you should contact your local Environment Agency fishery officer for further guidance and information.
Report any fish mortalities or disease problems to the Environment incident hotline 0800 80 70 60, or to your local Environment Agency officer, immediately.
If necessary, a fish disease investigation may be conducted at the Environment Agency’s fish disease laboratory. This includes comprehensive testing for parasites, bacteria, fungi and viruses including KHV, as well as assessments of fishery management practices and environmental conditions.
KHV is a notifiable disease and it is a legal requirement to notify the government's Fish Health Inspectorate (FHI) upon suspicion of any notifiable disease. We have a legal duty to tell the FHI immediately if we suspect KHV during our mortality investigations and we work closely with the FHI to protect our fisheries from important diseases.
Since the detection of KHV in carp fisheries in 2002, control measures have been in place to limit the spread of the virus in England. These were initially placed by the Environment Agency under Section 30 of the Salmon and Freshwater Fisheries Act, 1975. This involved movement restrictions on all waters where the virus was confirmed.
In 2007, KHV became a notifiable disease under the Aquatic Animal Health (England and Wales) Regulations. This made it a legal obligation to report suspicion of the disease in any farmed or wild fish to the FHI. We support fisheries with KHV by providing fishery management advice and guidance, to understand the reasons for the outbreak and what can be done to avoid problems arising again in the future.
As most KHV outbreaks are triggered by either environmental conditions or intensive fishery management, there are many things that can be done to limit losses and prevent a re-occurrence of disease.
If you experience fish disease problems, or you would like more information about these conditions, please contact: National Fisheries Services, Environment Agency, Bromholme Lane, Brampton, Huntingdon, PE28 4NE.
Call: 07825 111723
Further information can be found on the EA's KHV factsheets.
Last date edited: 5 January 2021