|Koi Herpes Virus As Explained to Aquaculturists
I would like to discuss is the "emergence" of Koi Herpes Virus in the ornamental Koi hobby. Well, it's not really a "recent" thing is it? It was first described as early as 1996 in Japan. Researchers identified it as a Corona virus and it was a serious but narrowly experienced event, killing a lot of a few fish groups, and then seemingly disappearing. It showed up again in a group of Japanese fish being held in England. The fish were being moved en masse to Israel because it was unknown whether some as yet undetected intoxication or water quality issue was causing their rather rapid demise. The fish went to Israel and contaminated (and killed) a formidable amount of fish on a Koi farming kibbutz there. Israeli researchers were forthright and published mightily on the virus, and the preponderance of information we have on Koi Herpes Virus comes from the original work done in Israel - and so the virus is erroneously referred to as an Israeli phenomenon.
What Is It?
Koi Herpes Virus, hereinafter referred to as KHV is a herpes virus that features VERY high morbidity and nearly 100% mortality. The virus is spread to vulnerable fish under stress, usually cohabited in confined facilities. Crowding, and co morbidity with Costia or other pathogens seems to increase it's virulence. The KHV virus has certain temperature-related "windows" of opportunity or action that have been instrumental in its occurence and control.
I have been professionally involved in countless cases in North America and most of them feature the same sorts of hallmarks:
1. Introduction and mixing of new fish without quarantine
2 Fish have come from cold water, usually under seventy degrees
3. Fish have been transported and have Costia co morbidity
4. Most fish are of Asian origin however; I have seen few cases from Japan. The incidence in Taiwan, Malaysia and China is tenfold higher.
Many cases were confounded as far as their recognition because the fish came from spring fed, cool ponds and were only warm while they were in quarantine or during transport. When fish "break" with clinical KHV infections, but are placed in cool water again, their symptoms can subside and then the rate of their decline and death can become deceptively slow. Recognizing the hallmarks of KHV infection can help identify the disease regardless of the rate of their demise.
Clinical Signs of KHV
1. Fish skin begins to peel, especially at the nape of the head.
2. Fish become reddened and appear abraded overall
3. Gills are damaged by the removal of the epithelial cells secondary infections rapidly set in and gills summarily necrose.
4. Lack of oxygen causes the fish to become fearless and unaware.
5. Internal organs may liquefy and an unusually copious amount of fibrin can be found in these peritoneums
6. Kidney is damaged, but fish often die before that effect is grossly observable.
Proper diagnosis of KHV is via any of three diagnostic tests. First, the PCR test (polymerase chain reaction) test that uses an enzyme linked immuno-sorbent assay and detects crucial segments of specifically identifiable DNA from the virus. Another test can actually detect circulating antibodies via agglutination and is wonderful for finding out whether a fish has recently been infected or not. The third test is called In Situ Hybridization and it uses enzyme markers to find and illuminate viral DNA or particles in fixed tissues. This last test doesn't just prove the presence of the virus as the PCR test does. The in Situ hybridization test proves actual infection because it demonstrates the virus INSIDE the cell.
The importance of the tests:
PCR testing reliably shows the presence or absence of the virus on tested samples. It does not prove infection and absence of virus particles doesn't conclusively prove lack of infection. Presence of the virus at detectable levels varies as the infection progresses.
Agglutination testing holds promise on VERY Expensive Koi as a prepurchase examination; to wit, before the purchase of a valuable fish, the buyer should require an agglutination (antibody) test be run to ensure they are buying non-exposed / non-infected specimens. If no antibodies are found in a mature fish in good clinical condition, it is assumed (with all that implies) that the fish is not infected and has not recently been infected, as antibodies can circulate for up to a year after infection.
Finally, in Situ hybridization is a way of looking at various tissues to see where, and if the virus "hides" in neural, meningeal, brain or other tissues latently. It also proves for research purposes that a fish that you're bleeding for agglutination testing and antibody studies were in fact infected when that has been experimentally attempted. It's the only and best "proof" that the fish actually contracted the disease and can therefore contribute meaningfully to latency and sustainable antibody studies.
Vaccine research is underway at several universities, heavily funded by scant hobbyist donations and industry participation. Very, very little support for KHV research has come from the actual breeders and vendors of Koi. The majority has come from hobbyists, and companies that supply dry goods such as food to the industry. The amounts raised are disappointing, and the funds are being distributed to many different organizations for various studies. The result will be that the research needed to characterize the virus and develop immunization protocols, technologies and resources will be splintered among many different research facilities, which will end up with many entities having all the pieces but no one entity being able to bring them to bear on the virus and its eradication. So until then, control of the virus and its effects, to save fish lives and restore collections of fish have been developed.
Because of the above industry-funding allocation snafus, I wanted to contribute some thoughts and experiences on "control" of KHV. The virus is easily quelled by heating infected fish to 83-86o F. I have found that heating infected fish at a rate of ONE degree Fahrenheit PER HOUR is fast enough and slow enough to permit control of the viral symptoms without killing the fish via too-rapid warming. As I often say, aeration while heating is supposed to be absurdly aggressive short of creating a Jacuzzi effect that tosses the fish out... within A short time of achieving temperatures in the low eighties Fahrenheit, fish begin to exhibit more normal body comportation and success rates have been nothing short of amazing. Survivors by heat treatment have not been infective for years after their recovery, however, careful examination of tissue samples and various agglutination tests are only underway, and not yet available.