Heat Ramping

Much has been written and spoken about the common cold in humans, despite the fact that the most reliable information must come from those researching cold viruses. The same happens with KHV in Koi, so hobbyists need to filter the facts from the variables in order to make the right choices.

The PKDA endorses heat ramping to broaden the remit of quarantine to diseases and conditions other than Koi Herpesvirus (KHV).

Case History

Consider the unfortunate Koi-keeper who nursed 17 Koi back to health after they survived a KHV outbreak. He was encouraged to heat his pond having been assured this kills the KHV virus completely. He then purchased several new Koi to make up for the fatalities and the fish thrived until the following summer. Almost overnight, he noticed his new Koi were lethargic and refusing food whereas the KHV survivors were behaving normally. The Koi-keeper was initially uncertain why his new Koi had so suddenly succumbed to the virus and it took days for him to associate the fact that his pond heater malfunctioned two weeks before his new Koi showed any signs of illness. A constant high temperature can limit outbreaks but cannot cure KHV. Carriers will still shed virus when the criteria is right, and in this case, the outbreak was triggered by the fluctuation in temperature. However, there are many who herald a high water temperature as a cure on the Internet with the best of motives, believing they are helping other Koi-keepers.

The influence of heat

Viruses can mutate and develop new requirements for survival and replication. In the case of KHV, there are now several strains of the disease proven in research to vary in many respects, particularly the type of outbreak experienced by the Koi and how long the virus lives. Every disease irrespective of whether caused by a viral, bacterial or other pathogen has its own specific incubation criteria and this has to be fully met, with temperature the primary but not the only trigger factor. There are diseases that will not develop if the pond never drops into the required range for incubation. KHV however is usually triggered in unheated garden ponds in late spring and early summer in the UK climate. Temperature inhibits as well as triggers the disease process but ponds vary in respect of ambient temperature at the outset of disease incubation and this is highly influential on whether the incubation period is completed and disease results.

The use of temperature

Heat ramping is an easy way in which hobbyists can attempt to find disease including KHV, but it should never be regarded as 100% reliable. Heat ramping has been used repeatedly in projects over the last 12 years at LFH Laboratories with carriers of various strains of KHV. In addition, Koi that have never been heat ramped are assessed in comparative studies. Situations are set up to mirror the situations most likely to trigger KHV outbreaks compared with other viral diseases, with special attention to herpes viruses as they are a common type of viral family infecting fish.

A primary outbreak of KHV can happen without chilling as a rise in temperature can initiate it. However, this type of outbreak is not comparable with disease triggered in a carrier of KHV, as there are differences between a new infection and latent disease in many respects. For example, a new outbreak in naive Koi should result in signs of disease in the whole community. In a pond in which all the Koi carry KHV only a Koi naive to the disease would show any indication of the disease during an outbreak. This is why a naive companion Koi is often suggested during quarantine and this raises ethical issues.

The carrier state

After an outbreak, the KHV virus finds a safe haven in the nervous system. The capacity of carriers to shed virus long after they contracted and survived the disease can depend on the temperature of their environment during the recovery phase and after. Disease outbreaks depend on body chemistry interactions during incubation, the natural biological cycles, and the status of the immune response, all of which vary fish to fish. The suggestion that preventing cross-infection is achievable by removing suspect Koi that are potential carriers from the pond at low temperatures is scientifically unsound. It is impossible to know when Koi are shedding virus other than in laboratory facilities and to make such a decision in a Koi pond armed only with a thermometer for guidance is foolhardy.

Incubating KHV

There are various tests for KHV not appropriate to every situation. A polymerase chain reaction test (PCR) is used to confirm KHV only during the disease process. When looking for KHV in healthy Koi suspected of being carriers it is possible to test for the presence of KHV antibodies and one such method is the ELISA test. Blood samples from every Koi are required and this can be impractical and costly. A slight drawback is that occasionally the ELISA test has been known to detect other herpes viruses creating a false positive for KHV. When tested, Koi may actually have suffered nothing more than Cyprinid Herpesvirus (CHV), commonly termed carp pox. Inconclusive results are rare but create difficult decisions whether or not to euthanase valuable Koi, particularly brood stock. There are new advances in serology, the science that deals with antibodies, and in addition, the Koi immune response creates other products that can also be utilised to test for KHV carriers. Trials of a simple screening method were completed in 2008 at LFH Laboratories and are now being assessed for commercial viability. The problem is that the cost of research for any product intended for a relatively small market often makes development unfeasible no matter how successful.