Koi herpes virus infection
The koi herpes virus infection is an epidemic, acute to subacute viral infectious disease of carp and koi carp caused by the koi herpes virus (KHV) . It has been a notifiable animal disease in Germany since the end of 2005 .
The incubation period is between a week and several months, depending on various factors such as stress and the condition of the fish. If the disease breaks out, the mortality rate is usually between 80% and 100% over a period of 24 hours to 14 days.
Pathogen
Koi herpesvirus (KHV) was described as a herpesvirus in 2000 by Ron Hedrick of the University of California, Davis . It is not yet known whether this virus is identical to the herpes virus cyprini, which was described in Japan in 1990. The Koi Herpes Virus is classified as a DNA virus of the virus group Herpesviridae (Herpesviruses). The virus first appeared and was described in 1998 in Israel.
The pathogen occurs in koi carp and farm carp. Other non-related cyprinids such as grass carp ( Ctenopharyngodon idella ) or golden crucian carp ( Carassius auratus ) do not always seem to remain unaffected by the virus . This information is secured insofar as KHV has already been detected in goldfish and, provided they do not fall ill, they act as carriers. What is certain is that the virus only affects carp fish.
transmission
Like most viruses, the Koi herpes virus is transmitted through direct contact. This can be the fish themselves through skin contact and gill excretions, water or other liquids that have come into contact with infected fish. Fiddling in the water with your hands, working with objects such as nets or nets, or coming into contact with footwear are sufficient to transmit the virus. Waterfowl can also introduce the virus into water, although the virus does not survive outside of its habitat for more than two hours. Without a carp as a host, the virus remains active in the water for a maximum of 14 days. In most cases, the virus is introduced into the stock by new fish.
Course of disease
The course of the disease is rather unspecific and many of the symptoms that occur are hidden by other diseases or appear as secondary infections, which then do not indicate a KHV infection. These include, above all, poor water values (ammonia, lack of oxygen, overfeeding or poorly functioning filter systems), but also bacterial diseases that can be dragged into the pond. However, the highest loss rates are observed in the temperature range between 18 ° C and 25 ° C. The outbreak of the disease is already possible at 8 ° C. According to the latest research, the virus must have adapted to European conditions. At temperatures above 30 ° C and below 4 ° C ( in vitro conditions) the Koi herpes virus does not appear to be infectious . The course of the disease is also decisively influenced by other environmental and housing conditions (water quality, stocking density, stress).
The clinical picture shows the following symptoms:
- apathy
- Separation from the swarm
- increased slime formation
- Shredded mucus shedding
- Gasping at the surface
- Necrosis on the gills (white or red spots)
- Necrosis and bleeding in the skin
- Unwillingness to eat or refusal to eat
- sunken eyes
- Standing upside down
- Color loss
- rough skin (sandpaper effect)
- Bleeding in and on organs
- Necrosis in organs
- swollen spleen
- altered kidney
Pathologically, the Koi herpesvirus infection is characterized by an interstitial inflammation of the kidneys , inclusion bodies in the gills or intestines, formation of lobes on the gills, giant cells and an altered blood count.
diagnosis
The pathogen can be detected by polymerase chain reaction (PCR) or the LAMP test ( loop-mediated isothermal amplification ) (see Koi herpesvirus detection ).
quarantine
In quarantine disease suspicious and freshly imported animals must. This also applies to animals with poor fitness. In the case of valuable animals, the ponds must be secured against birds flying with nets. New additions are to be placed separately for at least 14 days at at least 20 ° C, ideally at different temperature ranges and a duration of around four weeks. If there are no symptoms of the disease after two weeks, at least one Koi, better two, from the existing pond should be moved into quarantine to buy a new one. Bringing new additions to the old stock must be avoided at all costs.
Embarrassing hygiene in relation to equipment and people can also counteract contamination of the coi population. It is urgently recommended that koi from areas known from KHV such as China, Israel, Japan or Thailand are cleaned for the above mentioned purposes before being brought into koi ponds. Time to be kept in quarantine. Buyers should only purchase animals from dealers who have their Koi tested using a PCR analysis, who can provide evidence of negative test results from an institute recognized by veterinary medicine and who have an appropriate quarantine facility. Merchants who do not provide this proof should refrain from buying. Mixing animals from different areas of origin should also be avoided. The acquisition of so-called immunized (not to be confused with vaccinated!) Animals must be viewed critically, as these animals can also carry the pathogen latently and thus re-infect healthy stocks. With the introduction of a real vaccine (see below), such fish have not been on the market since around 2006. COD can be very contagious. A contaminated splash of water can be enough to endanger the entire population.
treatment
There is currently no treatment with the prospect of a cure. Infected animals can be treated (immunized) with a temperature increase of over 30 ° C over a period of over 30 days so that they can live with the virus. However, if these animals survive, they become carriers. These carriers can then bring contagious viruses into circulation at any time, which can infect and kill non-immunized animals. If the disease has broken out, the entire population may have to be culled and the pond drained. An intensive disinfection treatment should then follow.
According to R. Hoffmann (Institute for Zoology, Fishery Biology & Fish Diseases at the LMU Munich), immunizing measures against herpes viruses in animal husbandry are generally of particular problem, since animals infected with herpes virus are life-long virus carriers. Vaccination of healthy animals could therefore lead to vaccination protection, but this did not guarantee that the vaccinated animal would not become an undetected virus carrier when it came into contact with the herpes virus. Herpesvirus vaccines are therefore not suitable for eliminating the disease from animal populations. It is precisely the carriers (i.e. the healthy virus carriers) who, according to his observation, repeatedly excrete viruses in the course of their lives and could infect animals that have not yet been infected.
This view is currently contradicted by the contrary experiences from Israel, which survived a severe epidemic with extreme losses and successfully rebuilt a carp production protected by vaccines.
The vaccinated animal does not become ill, the previously uninfected koi becomes seriously ill and in most cases dies. Despite claims to the contrary, it has not yet been established that fish vaccinated with the above vaccine have ever transmitted field virus.
