Epizootic Hematopoietic Necrosis (EHN)

Open the tank lid in the morning, glance at the redfin perch (perca fluviatilis), and you’ll know something is off.

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

The fast version, for keepers who want context before reading further:

• Pathogen. Epizootic haematopoietic necrosis virus (EHNV) (Virus)
• Typical hosts. Redfin perch (Perca fluviatilis), rainbow trout (Oncorhynchus mykiss); other percid and salmonid species susceptible
• Reported distribution. Australia (endemic); not reported elsewhere
• Temperature window. 52–68 °F (11–20 °C); severe disease above 54 °F (12 °C)
• WOAH-listed (notifiable). Yes — internationally notifiable

Recognizing it in the tank

Run through this list with the lights up and the fish settled — a startled fish reveals less:

• Sudden mortality. Fish that were normal yesterday are dead this morning, often with no other visible signs.
• Focal liver necrosis, splenomegaly, hemorrhages in kidney. Patches of grayish, dead tissue with a clear boundary against healthy skin.
• Outwardly often few signs. A change worth noting and timing — write down when it first appeared.

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.

You don’t need every sign on the list to make the call. A clear match on two or three combined with a recent stress event — new fish, temperature swing, ammonia spike — is usually enough to start treatment.

Treating the infection

Action items, in priority order:

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 available; control via biosecurity and stamping-out

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.

For the keeper reading this with a problem in front of them right now: don’t medicate in the next 15 minutes. Spend that 15 minutes on observation and water testing. The diagnosis you’ll arrive at from the cooler analysis will be better than the one you’ll commit to under stress. Two hours of right treatment beats two days of wrong.

Background on the agent

Epizootic haematopoietic necrosis is caused by Epizootic haematopoietic necrosis virus (EHNV), a virus in the Iridoviridae.

Knowing what’s actually doing the damage is what lets you pick the right medication on the first try, rather than burning through a string of failed treatments while the fish gets worse.

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 Redfin perch (Perca fluviatilis), rainbow trout (Oncorhynchus mykiss). 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.

Water and environment factors

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 52–68 °F (11–20 °C); severe disease above 54 °F (12 °C). 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.

Routes of transmission

Routes of transmission are the leverage point for prevention:

Direct contact and water; horizontal transmission via cohabitation; virus stable in water and frozen tissue.

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

Bad advice you’ll see online

A short list of the moves that make this worse — not theoretical, these are the mistakes that come up over and over in forum threads:

Don’t dump random medications hoping something works. Once a disease is on the no-treatment list, the move is biosecurity, not chemistry. Aggressive multi-medication regimens stress the fish further.

Don’t treat with antibiotics. They don’t touch viruses. Antibiotic dosing on a viral case just disrupts your biofilter and adds stress, both of which work against recovery.

Don’t add another fish during an active outbreak. Even if the new fish looks healthy, you’ve now committed the new arrival to the same exposure with no chance to quarantine. Wait until the affected fish has been symptom-free for at least 4 weeks.

Don’t increase temperature blindly. It’s a common forum suggestion that helps for some parasites and hurts for several bacterial and viral conditions. Check the pathogen first.

Timeline and expectations

Recovery is the wrong framing for an untreatable disease. What you’re managing is containment. What that looks like in practice:

• Days 1–7: Acute mortality phase. Remove dead fish promptly to limit pathogen spread through cannibalism or scavenging.
• Weeks 2–4: Survivors stabilize. Don’t assume they’re cured — they may be carriers.
• Month 2 onward: No new fish for at least 90 days, ideally longer. Re-stocking too soon resets the clock.

In a commercial setting, the move here is depopulate-disinfect-restock with confirmed-clean fish from a different source. In a home tank, hobbyists sometimes successfully keep a stable group of survivors long-term, but you’ve committed that system to never receiving outside fish again.

Keeping it out of your tank

The boring stuff that works, in order of impact:

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 52–68 °F (11–20 °C); severe disease above 54 °F (12 °C). 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.

A note on look-alikes

More than one disease shares the early signs of Epizootic Hematopoietic Necrosis. 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

• Branchiomycosis — for keepers narrowing down between this and a look-alike condition.
• Largemouth Bass Virus — for keepers narrowing down between this and a look-alike condition.

Source

Primary reference: WOAH Aquatic Manual Ch. 2.3.1.

Read the full source: https://www.woah.org/en/disease/epizootic-haematopoietic-necrosis/

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