Koi herpesvirus: water temperature decides the outcome, not the purchase date
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
A koi bought in cold water can look perfectly healthy for weeks, and then the pond empties out at the first real warm spell. A study published in 2003 by Gilad, Yun, Adkison and colleagues in the Journal of General Virology explains why, by varying nothing but water temperature after an experimental infection with koi herpesvirus (, doi 10.1099/vir.0.19323-0). Our other guidance is gathered in the fish travel and safety section.
Contents · 6 sections
Three temperatures, three radically different outcomes
Koi infected under identical conditions were then held at distinct temperatures. At 23 °C, cumulative mortality reached 95.2%. At 18 °C, it came to 85.0%. At 13 °C, no mortality was observed at all. The virus, the inoculum and the fish were comparable: only the water changed. One point that is often distorted deserves clarifying here: at 28 °C the authors describe a disease that progresses faster with a mortality they call lower — but the range they report, 89.4 to 95.2%, overlaps the figure recorded at 23 °C. What clearly changes between these two warm levels is the speed of the episode; nothing licenses presenting 28 °C as a protective temperature.
Warming alone is enough to trigger the disease
The most telling result for a pond keeper is the transfer experiment. Fish that had been exposed to the virus and then held at 13 °C, and therefore looked unaffected, were moved to 23 °C. Mortality started quickly. The cold water had not cleared the virus, it had merely prevented the disease from expressing itself. A fish bought and introduced at the end of winter can therefore go through quarantine without showing a single sign, then set off an outbreak once the pond climbs past a certain threshold in spring.
Reactivation is possible months later
That same delay has been found on an even longer timescale. A study published in 2005 by St-Hilaire, Beevers, Way and colleagues in Diseases of Aquatic Organisms observed reactivation of the virus in three out of five experimental tanks, several months after the initial exposure, and this reactivation also appeared to depend on temperature. A pond that has come through an episode without visible losses is therefore not necessarily a cleared pond.
What these studies do not show
They cover koi and common carp, and say nothing about goldfish. They do not show that a healthy pond becomes contaminated by warming up: temperature is a revealer, not a cause. They test no treatment, and the viral growth optimum reported between 15 and 25 °C was measured in a fin cell line, not in live fish — forums that turn this into a "thermal protocol" are extrapolating from cell-culture laboratory data. Deciding on a health management plan is a matter for a vet competent in aquaculture.
What this changes when introducing a fish
The practical consequence is clear: a quarantine carried out in cold water proves nothing, because it takes place in the range where the disease cannot express itself. Watching a new fish within a temperature range where a problem would actually be visible, and avoiding introductions just before a thermal rise, is worth more than an isolation period set by the calendar. See our quarantine protocol before introduction and our comparison of goldfish and koi in a garden pond.
Frequently asked questions
Is a koi that survived an episode cured for good?
The 2005 data show that reactivation is possible several months after exposure. A survivor should therefore be treated as a fish to keep an eye on, not as a neutral animal for the rest of the pond.
Should a pond be heated to "cure" the virus?
No, and the study never suggests it: it tests no treatment, and the temperatures described as unfavourable to the virus were measured in cell culture. Find all our guides on the fish hub.
An accessory from our shop for this use: Stick-on digital thermometer for aquarium and terrarium glass (€18.90) · the whole shop
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.