What is it?
Koi herpesvirus disease (KHVD) is a very infectious herpesvirus infection capable of inducing a contagious and acute viraemia
in common carp (Cyprinus carpio) and varieties such as koi carp and ghost carp.
Since 1998, there have been cases in the UK, Europe, US, Indonesia, Israel
and Japan.
Where and when might it occur?
Outbreaks of disease generally occur at water temperatures between 16˚C to 25˚C.
The virus only appears to infect Cyprinus carpio which includes koi, ghost koi and common carp. Goldfish and other cyprinids are not known to be affected by or carry the virus.
Water is the major abiotic vector. However, animate vectors (eg other fish species, parasitic invertebrates and piscivorous birds and mammals) and fomites may also be involved in transmission.
Diagnosis
During a KHVD outbreak there will be a noticeable increase in mortality in the population. All age groups of fish appear to be susceptible to KHVD, although, under experimental infection, younger fish up to one year old are more susceptible to the disease.
On closer examination of individual fish, typical clinical signs include pale discolouration or reddening of the skin, which may also have a rough texture, focal or total loss of epidermis, over- or under-production of mucus on the skin and gills and pale discolouration of the gills.
Other gross signs include enophthalmia (sunken eyes) and haemorrhages on the skin and base of the fins, and fin erosion.
Fish may also become lethargic, separate from the shoal and gather at the water inlet or sides of a pond and gasp at the surface of the water. Some fish may experience loss of equilibrium and disorientation but they may also show signs of hyperactivity.
Morbidity of affected populations can be 100 percent, and mortality 70 to 80 percent, but the latter can be as high as 90 or 100 percent.
Secondary and concomitant bacterial and/or parasitic infections are commonly seen in diseased carp and may affect the mortality rate and display of signs.
Control and treatment
There is currently no treatment available for this disease.
Methods to control and prevent KHVD should mainly rely on avoiding exposure to the virus coupled with good hygiene and biosecurity practices.
This is feasible on small farms supplied by spring or borehole water and a secure system to prevent fish entering the farm via the discharge water.
Source: Defra and OIE