Koi Herpes Virus (KHV) & Spring Viremia of Carp (SVC)
Overview
Koi are generally hardy fish. They're descended from the common carp and are tough, essentially omnivorous fish with the ability to withstand a range of living conditions.
Koi health and disease is essentially a balancing act (an “equilibrium”) between stocking density, water/environmental conditions, parasites, and the fish itself.
It was once said that “if you take care of the environment, the fish will take care of themselves”. This was true until some of these viruses started showing up with increasing regularity.
Introduction to the viruses
There are two known viruses of importance to koi. There are other viruses, but these matter because they can quickly kill fish and are highly contagious:
- SVC / Spring Viremia of Carp (Rhabdovirus carpio)
- KHV / Koi Herpes Virus
These viruses are similar and dissimilar. Some of their differences and similarities are important.
| Spring Viremia of Carp (SVC) | Koi Herpes Virus (KHV) |
|---|---|
| Recently discovered? No. Described in the literature more than forty years ago. | Recently discovered? No. Reported in Japan years before its first outbreak/discovery elsewhere; documentation exists back to the 1980s. |
| Kills fish? Can kill groups of fish; reported mortalities may be 20–30% with supportive care and optimized environment; secondary infections can be a major factor. | Kills fish? Often 70–90% of exposed fish that have not been previously exposed. |
| Seasonal Yes — spring. | Seasonal Yes — spring and fall. |
| Endemic to USA? Debated; lack of testing may have hidden earlier cases. Some believe it contributes to spring die‑offs. | Endemic to USA? Doubtful; spreads in “groups,” and virulence may limit long-term persistence. Carrier state is complex. |
| Lesions Pale/white lesions may occur due to bacterial co‑infection; skin can become pink/red as infection progresses. | Lesions Pale/white gill lesions; excess slime (especially head/nape); blotchy body color; internal organs may be damaged. |
| Diagnostic challenges Reportable status creates reluctance; RNA virus requires extra PCR step; false negatives can occur depending on timing/conditions. | Diagnostic challenges Hidden/occult carrier states may be hard to prove; false negatives possible outside ideal replication temperatures. |
| Carrier states Survivors may exist; carrier state not fully confirmed. | Carrier states Survivors are said to be clear and not re‑infectable; detection in asymptomatic fish can be difficult. |
| Kind of virus RNA virus, rhabdo (bullet) shaped. | Kind of virus DNA virus (herpes virus). |
| Testing Culture and reverse PCR testing exist. | Testing Culture, swabs/nucleic acid tests, PCR testing. |
| Reportability Reportable by law. | Reportability Not legally reportable (as of the source material) and largely unregulated. |
| Prevention Prevent exposure. | Prevention Prevent exposure. |
| Control / treatment Support fish in ideal environment and control secondary infections (food, injections). Survival can be high with aggressive support. | Control / treatment Losses may be reduced if fish are rapidly warmed above ~80°F; cooling can slow outbreaks outside the ideal temperature range. |
Spring Viremia
The key issues around SVC include its reportable status and the practical impact of testing. Testing is possible but is often avoided due to cost and consequences. Well cared-for fish can sometimes survive, and optimally housed fish may not even show signs.
Some providers focus on supportive antimicrobial care “past” the ravages of the virus rather than seeking definitive diagnosis.
KHV
KHV tends to affect a narrow temperature range and can “hide” outside those temperatures in asymptomatic fish. Diagnosis can be missed if fish aren’t actively viremic.
If you’re considering buying fish, there’s no security unless the fish have been through a full seasonal cycle (cold → warm) that can reveal occult infection.
KHV strategy notes
The original article includes a real-world strategy discussion for winter outbreaks: stop heating, avoid supercooling from waterfalls, maintain mid-water circulation for aeration/degassing, remove dead fish promptly, and then control warming in spring.
Errors or omissions are possible; some points are based on hearsay or opinion from the source material. Newer information may exist.