Carp pox has the doubtful honor of being the oldest documented fish disease. It is activated when the water temperature falls below 57° F (14° C). Because of its disfiguring effect, carp pox may reduce the value of a particular Koi. Even though it is not very infectious, fish with carp pox should not be bought, but it is not essential to remove fish carrying it from your collections.
This disease is not caused by a poxvirus but by Cyprinid Herpesvirus (CHV-1). It produces epidermal hyperplasia, presenting as superficial milky white to pink colored plaques on the skin and fins. The lesions are soft and waxy, not wary and rough. They extend 1-2 mm above the surface of the skin and should not be scraped. The carp pox lesions usually occur on the upper lower lip, the leading edge of the pectoral fins, anywhere on the ventral and anal fins, and sometimes on the back area, under the dorsal fins. Adjacent plaques may merge into larger raised area. The lesions are benign and eventually slough. This Koi disease is not fatal but can produce scarring. In severe cases large areas of the body and fins can be covered with plaques. The fish will swim and eat normally in most instances.
Experimental exposure of carp to CHV-1 revealed that these growths appeared five to six months after infection. After a period of time, the growths will shed and disappear, with no apparent trace that they ever existed. A large percentage of infected fish do not eliminate the virus from their body; instead, it "hides" from the immune system in certain cells of the body, including the cranial nerves, spinal nerves, and some skin tissues.
Such infected fish are highly likely to remain infected for the rest of their lives, with recurrences of the infection and hence reappearances of the papilloma's usually occurring around seven to eight months after their disappearance.
CHV-1 recurrence occurs most often in late fall, with the number of growths and the number of fish showing signs increasing over the winter and into the spring. If you are wondering why this happens, think of cold sores (a human herpes virus). Cold sores on a human usually show up when you are unwell or sick. When you are sick your body is less able to fend off infections because your immune system is suppressed. The same happens with Koi in cold weather. As temperatures drop, their immune systems do not work so well and the virus is able to multiply. With the arrival of spring the carp pox growths shrink and fall off, because as the weather improves and temperatures increase, the fish are better able to mount an effective immune response, thereby stopping viral production and shedding the wax-like papilloma's.
Generally, for most healthy adult Koi, CHV-1 represents a nuisance condition. It temporarily disfigures the individual but is of no consequence. In young carp it can be fatal, and it has been linked to skull abnormalities. In other cases, CHV-1 has been suggested as a trigger for the formation of malignant tumors in carp, especially squamous cell carcinomas. These are invasive skin tumors that are often found around the mouth and may require surgery or possibly chemotherapy to control.
High stocking densities will help the virus to spread, as will anything that damages the protective integrity of the skin and mucous covering, such as ectoparasites (especially Argulus spp.) and trauma or abrasions.
Signs to Look for:
This virus is usually seen only in the varieties of cultured carp (Cyprinus carpio), such as mirror carp, leather carp and Koi. It can infect other cyprinids such as Crucian carp and grass carp but is rarely fatal, even in the young of these species. This virus triggers way-like growths on the skin and fins. In those cases where the virus triggers tumors to form, then these may appear as rough, wart-like masses often around the mouth but potentially anywhere on the body surface. Heavy losses may be encountered in young fry.
CHV-1 could be mistaken for other causes of firm swellings such as fibromas, granuloma, (areas of thickened inflammation), and epitheliocystis. This last disease is infectious and is caused by the bacteria-like organism chlamydia. It triggers whitish growths to occur not unlike Lymphocystis, an iridovirus.
Strict quarantine of all newly bought stock will stop the introduction of this virus into established collections. If you don't want it in your collection, don't buy from ponds or vats where there are fish showing typical pox growths. In particular, this applies to those retailers that keep their larger Koi for sale in pond along with some resident fish; if these permanent fish are infected, they will continually infect any new, non-infected Koi that are brought in by the retailer.
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