Koi herpesvirus disease is a serious infection of koi and common carp caused by cyprinid herpesvirus 3 (CyHV-3). Breathing trouble, gill damage, lethargy, and deaths can raise concern, but these signs also occur with other problems. A veterinarian and diagnostic laboratory are needed to investigate a suspected outbreak.
What the Signs Can Tell You
Look for a change across the group: unusual breathing, fish staying near the surface, skin or mucus changes, and unexplained losses. Do not lift gill covers or repeatedly catch distressed fish to compare them with an online photograph. Record when the signs began, water temperature, test results, recent arrivals, and any treatment history.
What to Do While Seeking Help
- Check circulation, aeration, ammonia, nitrite, pH, and recent refill or contamination events.
- Contact a fish veterinarian promptly if several fish decline or deaths occur.
- Pause fish transfers, sales, shows, and sharing wet equipment while the cause is investigated.
- Ask the professional what samples are needed before shipping fish or disposing of material.
Testing, Carriers, and Temperature
Survivors can remain carriers. A normal appearance or a negative result on one sample cannot rule out every carrier situation. Test choice, sample quality, timing, and exposure history affect interpretation. Heating fish is not a reliable way to certify them free of KHV; do not use an internet heat-treatment recipe as a release test.
Reduce Introduction Risk
Buy with traceable records, maintain a separate quarantine protocol, and keep equipment separate between groups. Quarantine reduces risk but cannot guarantee freedom from every pathogen. Ask about past exposure and testing rather than accepting a disease-free label without details. Goldfish not showing typical KHV disease should not be treated as proof that a mixed system is safe.
Further reading: UF/IFAS: Koi Herpesvirus Disease.