Koi Herpesvirus (KHV) Disease

Where you live shapes how likely you are to encounter Koi Herpesvirus. Reported distribution: worldwide. Trade in live fish does the rest.

You’ll see it called several things — KHV is the same thing. The naming inconsistency is part of why misdiagnosis is common.

Quick facts — the structured picture, before we get into the practical detail:

• Pathogen. Cyprinid herpesvirus 3 (CyHV-3) (Virus)
• Typical hosts. Common carp and koi (Cyprinus carpio, all varieties and subspecies, including hybrids); goldfish and grass carp may carry but not show disease
• Reported distribution. Worldwide; first identified in late 1990s in Israel/USA/Germany; widespread in US, Europe, Asia
• Temperature window. Disease at 64–82 °F (18–28 °C); below 61 °F (16 °C) and above 86 °F (30 °C) virus is largely inactive
• WOAH-listed (notifiable). Yes — internationally notifiable
• Reference image datasets. Roboflow Fishlens v1, CuraPeces (Hugging Face)

Background on the agent

Koi herpesvirus disease is caused by Cyprinid herpesvirus 3 (CyHV-3), a virus in the Alloherpesviridae.

Understanding the agent matters here because the wrong treatment class — antifungals for a bacterium, salt for a parasite that doesn’t tolerate it — wastes time you don’t have.

Important distinction: this is viral. That means antibiotics do nothing — they target bacterial cell machinery viruses don’t have. What you can do is support the fish through it: clean water, low stress, gradual return to feeding. The fish’s own immune system is the only thing that will clear a viral infection. Antibiotics dumped ‘just in case’ just damage the biofilter.

This is on the World Organisation for Animal Health (WOAH) notifiable list. If you suspect it in a commercial or institutional setting, there are reporting obligations that vary by country. Home hobbyists usually never encounter it — but if you import unusual species, it’s on the radar.

Susceptible species: primarily Common carp and koi (Cyprinus carpio, all varieties and subspecies, including hybrids). Different species within the same family show different vulnerability — even closely related fish can have wildly different clinical outcomes from the same pathogen. If you keep mixed species, observe each one independently; the apparently-healthy one may simply be a quieter carrier.

What to look for on your fish

Here’s what you’ll actually see on the fish, ranked roughly by how early in the infection it shows up:

• Severe gill necrosis (white/brown patches, swelling), sunken eyes, sandpaper-like skin, mucus overproduction then mucus loss. Patches of grayish, dead tissue with a clear boundary against healthy skin.
• Mass mortality (80–100%) within 24–48 h of clinical signs. Fish that were normal yesterday are dead this morning, often with no other visible signs.

Observation discipline:

Pick one fish at a time and watch only that fish for 60 seconds. The temptation is to scan the whole tank looking for problems — that’s how you miss the subtle ones. One fish, full attention, then move to the next. The whole-tank scan is for triage; the per-fish look is for diagnosis.

Trust the pattern, not any single sign. White spots alone could be a dozen things; white spots plus flashing plus loss of appetite narrows the field fast.

Differential diagnosis — narrowing it down

Differentials worth considering before you medicate:

• Bacterial septicemia. Sudden mortality with internal hemorrhage looks viral but is more often bacterial. The difference matters: antibiotics work on one, not the other.

If two stay in the running after symptom comparison, the deciding factor is usually recent tank history: which condition matches the last 2 weeks of events better?

How it gets into a tank

Where it comes from and how it gets between fish:

Direct contact, water, contaminated equipment; recovered fish are lifelong latent carriers and can re-shed virus when stressed; plankton (rotifers) may carry KHV DNA.

The implication for keepers with multiple tanks is direct: dedicated nets, dedicated siphons, dedicated buckets per system. Color-code them.

The environmental side of the equation

A tank-side checklist for the 2 weeks before symptoms appeared:

• New fish in the last 4 weeks. The single most common trigger, by a margin. If you skipped quarantine, this is your most likely vector.
• Recent temperature swing. A 3 °C / 5 °F change in either direction stresses the immune system enough to enable opportunistic pathogens.
• Ammonia or nitrite reading above zero. Even brief spikes damage gill tissue and open the door for secondary infection.
• Filter maintenance event. A full filter clean (vs. a rinse) can crash the bio-load briefly. Watch the next 48 hours closely.
• Aggressive tankmate. Bite wounds and abrasions are infection sites. The ‘pretty cichlid’ becomes the disease vector when a flank wound stops healing.
• Overstocking creeping up. Each new fish raises ambient pathogen load and competition for oxygen. Disease load rises non-linearly with stocking density.

For this specific disease, temperature matters: outbreaks concentrate at Disease at 64–82 °F (18–28 °C); below 61 °F (16 °C) and above 86 °F (30 °C) virus is largely inactive. If your system runs through that band seasonally, raise vigilance during those weeks. Pond keepers in temperate climates should bookmark this — spring and autumn are when problems land.

Keeping it out of your tank

Long-term prevention comes down to a small set of repeatable habits:

Quarantine new arrivals for at least 4 weeks. This is the single biggest lever you have. Most introductions of disease into established tanks come from un-quarantined new fish or live plants/decor with adherent water. A separate 10-gallon tank with a sponge filter is sufficient for most species and costs less than one decent fish.

Stable water parameters. Test weekly even when the tank looks fine. Ammonia and nitrite at zero, nitrate under control (under 20 ppm for sensitive species, under 40 ppm for hardier ones), pH consistent across tests. Most outbreaks follow a measurable water-quality slip the keeper didn’t catch — not because the parameters are bad in absolute terms but because they shifted enough to stress the fish.

Don’t overstock. Crowding raises ambient pathogen load and lowers individual immunity. The most disease-free tanks I’ve seen are the ones that look slightly empty. A rule of thumb worth more than the standard ‘inch per gallon’ is to stock for half the bioload your filter is rated for, leaving headroom for water-quality buffering.

Equipment hygiene. Dedicated nets, siphons, and buckets per tank. If you must share, disinfect with a bleach dip (1:19 with water, 60 seconds, thorough rinse, then a 24-hour air dry). Color-coded gear per tank is a 5-dollar fix for what otherwise becomes a recurring problem.

Watch the fish daily. Two minutes per tank, every morning, with the lights on full. You catch the first hour of trouble that way — and the first hour matters. Build it into a routine you can’t skip; with the coffee, with the dog walk, whatever sticks.

Source matters. Buy from sellers who actively quarantine their incoming stock. The premium price reflects fewer disease introductions downstream. A fish that’s 30% more expensive but doesn’t bring in pathogens has saved you more than the markup.

Don’t ignore live plants and decor as vectors. Snails, tubifex worms, and any wet surface from another system carries water and the microbes in it. Rinse new plants thoroughly; consider a hydrogen peroxide dip (3% solution for 30 seconds, then rinse) for plants from unknown sources.

Mind the temperature range. Outbreaks are concentrated at Disease at 64–82 °F (18–28 °C); below 61 °F (16 °C) and above 86 °F (30 °C) virus is largely inactive. If your system runs there seasonally, raise vigilance during those weeks. For pond keepers, this often means heightened spring and autumn watching when water temperatures pass through the danger band twice a year.

First-response steps

What I do when I see this in a tank I’m advising on:

Isolate the affected fish. A bare-bottom hospital tank — heater, sponge filter pre-cycled, no substrate, no decor — lets you medicate at proper dosing without nuking the display’s biofilter or invertebrates. The hospital tank needs to match the display in temperature and parameters; sudden changes are an additional stressor the sick fish can’t afford.

Confirm the diagnosis before you medicate. Photo the symptoms (see the photo tip section), compare against reference images, and run the case through the Symptom Checker. Picking the wrong treatment class is worse than waiting an extra 12 hours to confirm. Don’t medicate blind.

Specific treatment. No treatment; raising water temperature to 86 °F (30 °C) may suppress mortality but does not clear infection; vaccines available in some regions

Test the water on the source tank. Even after moving the fish out, your display almost certainly has an underlying parameter issue that enabled the disease in the first place. Test ammonia, nitrite, nitrate, pH, and (where applicable) salinity, KH, and temperature. Fix what you find before reintroducing anything.

Do a 25% water change on the display. Not 50% — that’s a parameter shock to the remaining fish. 25% with temperature-matched, dechlorinated water, then test again 12 hours later.

Quarantine new fish for a minimum of 4 weeks. During an active outbreak, don’t add anything to the system. Adding a new fish into a sick tank is committing it to the same exposure with zero protection.

Track everything. A simple notebook entry per day — date, observations, treatment dose, water-test numbers — pays off if you need to consult a vet or if the same issue recurs in 6 months. Memory is unreliable here; written records aren’t.

If you found this article in panic mode — meaning you’re staring at a sick fish right now — do these three things first, in this order:

1. Take a photo. Multiple angles. Both the lesion and the whole fish. 2. Test the water. Ammonia and nitrite at zero is non-negotiable. If either is above zero, that’s an active emergency on its own. 3. Set up the hospital tank. Even if you’re not 100% sure yet, having it ready buys you time when the diagnosis firms up.

When this is past DIY

When to escalate beyond DIY:

• The fish has stopped eating for more than 48 hours despite clean water.
• Lesions are deeper than the scale layer, or you can see exposed muscle.
• A second fish has shown the same symptoms after treatment started.
• The condition is on the WOAH notifiable list — there may be a reporting obligation depending on your country.
• You’re working with a high-value specimen (breeding pair, show koi) where a definitive diagnosis is worth the consultation cost.

Most countries have aquatic-specialist vets reachable by email. Send them the photo set, the water-test numbers, and a 30-second video of the fish swimming.

Pond-scale considerations

Koi-specific notes:

Koi behave differently from aquarium fish during disease. They’re stoic — sick koi often eat normally until the day before mortality. Don’t wait for appetite loss as your early warning.

Daily visual checks should focus on three things:

1. Posture in the water column. Healthy koi cruise the lower third. A koi hovering near the surface or hanging in one spot is signaling. 2. Flashing behavior. Even one rub against a pond wall is worth noting. Three in a day means parasites until proven otherwise. 3. Skin reflectivity. Hold a flashlight at a low angle across the back. Healthy koi reflect cleanly; affected fish look matte.

A note on look-alikes

More than one disease shares the early signs of Koi Herpesvirus. Before you commit to a treatment course, run your symptoms through the Diagnose by symptom tool to make sure you’ve not missed a closer match.

Treatment success rate on this disease comes down to how fast you act. If you’ve read this far, you’ve already done the part most keepers skip.

You may also want to read

• Infectious Pancreatic Necrosis — for keepers narrowing down between this and a look-alike condition.
• Epizootic Hematopoietic Necrosis — for keepers narrowing down between this and a look-alike condition.

Source

Primary reference: WOAH Aquatic Manual Ch. 2.3.6; IFAS VM113 (Yanong & Hartman, 2024).

Read the full source: https://www.woah.org/fileadmin/Home/eng/Health_standards/aahm/current/2.3.06_KHV.pdf

Editorial review by the Fishy Farmacy team. Last reviewed: May 2026.