The semi-translucent lumps resembling blobs of white to pink candle wax that occasionally spontaneously appear on koi are the common symptoms of what is generally known as carp pox. Carp pox is the visible symptom of the herpes virus Cyprinid Herpesvirus 1, or CyHV-1. It is closely related to CyHV-2 which attacks haematopoietic (blood cell producing) organs such as the liver, spleen, and kidneys of goldfish. Itís also related to CyHV-3 which is more often called KHV. Only CyHV-3 is a reportable disease.
It should be emphasised that a fish with carp pox is in no danger of contracting, or being a carrier of koi herpesvirus, KHV, unless it has also been exposed to KHV. This is because although the three CyHV viruses are related, they are totally separate viral species and none of them can spontaneously mutate into either of the other two.
All three viruses are incurable and symptoms will frequently appear after periods of stress. Carp pox symptoms often occur after winter in unheated ponds and disappear as the fish grows to maturity.
Carp pox is an aesthetic disfigurement rather than a life threatening condition except in the most extreme cases and no attempt should be made to remove or burst the lesions. Any open carp pox lesion will shed the virus into the pond and, as with all viruses, itís contagious. Also, as with all herpes viruses, carp pox is incurable although, as time passes the severity of the lesions is likely to be reduced.
If bacterial diseases can be cured why is it that these viral diseases cannot?
To understand why bacterial diseases can be cured but viral diseases such as carp pox canít it may be helpful to first consider how human beings function. We are born, we have to take on nutrients (eat) and then reproduce before we die. We call this process life and we can be killed by chemicals that are toxic to us.
Bacteria are simpler life-forms but they behave similarly. They are ďbornĒ by a process called binary fission in which a parent bacterium divides into two daughter bacteria. Both daughter bacteria take on nutrients in order to grow and then each of these two daughter bacteria go on to divide into two new daughter bacteria. This process is common to all bacteria and so they can be described as being alive. Not all bacteria are harmful, whether to koi or to us, some are even beneficial but where a bacterium causes an illness or an infection, we can use a variety of methods to interrupt their life processes or terminate them prematurely. In this way it is possible to end or to cure a bacterial infection.
Viruses are completely different. They do not follow processes that could be described as living, therefore they cannot be killed. The carp pox virus, in common with other viruses, is far smaller than a bacterium and consists of a piece of genetic material such as RNA or DNA.
A virus cannot reproduce by itself, but can insert itself into the cells of a host. As these infected cells reproduce making copies of themselves, they also make copies of the virus within them. These new viruses then break out of the infected cell and go on to infect other cells.
Itís possible to destroy a virus before it infects its hostís cells but once it has infected them it becomes impossible to destroy it without destroying the host cell that it is hiding in. In other words, a treatment that would destroy the carp pox virus would also kill the fish it has infected.
The best way to remove a viral infection is for the immune system to try to seek and destroy virions (virus particles) as fast as the hijacked cell is producing them. The immune system can't remove a virus that's already inside a cell without attacking the fish's own cells.
The koi immune system can recognise that an infected cell is behaving oddly and destroy them, one at a time, without damaging uninfected cells. This is why some treatments claim to cure carp pox but these are usually immunostimulants that merely boost the immune system to help it combat infected cells.
But that strategy can't remove the original virus and any replica viruses that have been reproduced inside an infected cell and have made it as far as infecting another cell. The infection will continue, albeit at a lower level until conditions change and the virus becomes dormant. Stress due to cold is one sure way to "wake up" a dormant carp pox virus. At summer temperatures, the immune system becomes more efficient and learns to recognise and destroy viruses that aren't inside cells yet. The warmer temperatures also send the virus actually inside cells into a dormant phase but it will wake up and begin again when cold stress returns which is why the same fish get carp pox year after year.
It seems to be generally thought that carp pox is something that only occurs when the water temperature falls. Although carp pox often appears after winter, it isnít always the case. The CyHV-1 virus is commonly carried by carp and is usually suppressed by the immune system but expresses when the immune system is compromised. Carp have two immune systems; the specific or acquired immune system, and the non-specific system.
The specific immune system is chiefly comprised of leucocytes (white blood cells) which recognise any invading pathogens that they have encountered before and deal with them very efficiently by engulfing and consuming them. It is at its peak efficiency at around 25įC with a full complement of white blood cells. It becomes increasingly less effective as the temperature falls because there are fewer and fewer white blood cells until, at about 12.8įC there are no white blood cells at all.
The non-specific immune system is less efficient and is a general defence system that's designed to recognise and attack anything that it doesnít recognise as "self". It never stops working and will always provide a measure of protection even at low temperatures.
It's the falling off with temperature of the efficiency of the combined immune system that frequently means that the lesions which are the obvious symptoms of carp pox are generally associated with low temperatures. However carp pox symptoms can be expressed at any temperature. The ever-present virions that hide inside normal cells normally reproduce only relatively slowly. While this is the case, as they break free of their host cells in search of new cells to infect, they are easily engulfed and destroyed by white blood cells. However, if the immune system becomes compromised by stress or cold, the reduced number of white blood cells can't keep up with the rate at which the virus is being replicated in its host cells and the waxy looking lesions appear.
As time passes and with each successive infection, the immune system eventually learns to react so quickly to replica viruses being released that the symptoms they cause, (the waxy spots), are barely noticeable which is why it is often said that carp pox goes away as a fish matures. That isn't true, once a fish (or us) has contracted a herpes virus, it doesn't go away. The right conditions will trigger it into activity again, but over time, the immune system will have got better at dealing with it.
An immune system that is overloaded with trying to keep an outbreak of carp pox in check may not have the spare capacity to also deal with another pathogen that it could normally have resisted. This is why it is often recommended to use Chloramine T during a carp pox outbreak. It doesn't have any direct effect on the carp pox virus but generally lowers the environmental count of opportunistic bacteria or other pathogens and reduces the risk that a fish will also succumb to any other infection.
What to do with a koi thatís showing carp pox? If you like this fish then there is no reason not to keep it because it will usually improve and reduce the severity of subsequent outbreaks as it grows. If a low water temperature is the stressor that has allowed this outbreak to occur, try to keep the pond a little warmer next year. The virus will be less active and, at the same time, the specific immune system will be better able to cope and learn for the future.
Another question thatís often asked is; ĒWhat water temperature will prevent a carp pox outbreak?Ē This cannot be answered with an exact number because it depends on what else in the pond might be acting as a drain on the fishís immune system. The immune system isnít only compromised by low temperatures but by non-ideal water parameters or any other immune suppressors such as stress. However, it should be remembered that the main part of the specific immune system depends on the action of leucocytes (white blood cells) and these are completely absent below 12.8įC. This means that the koi immune system(s) will be unable to effectively combat the virus below that temperature but will become more able as temperatures rise above 12.8įC provided that no other non-ideal conditions are present to lessen its efficiency.