Infectious Hematopoietic Necrosis (IHN)

Where you live shapes how likely you are to encounter Infectious Hematopoietic Necrosis. It’s north america (pacific drainages, endemic), japan, korea, continental europe (introduced) — but globalised trade in ornamental fish has made geography a weaker predictor every year.

What follows is the practical version: what to look for, what to do, what not to do, and where this disease sits in the broader landscape of freshwater aquarium health.

Before going deeper, here’s the disease in one block:

• Pathogen. Infectious haematopoietic necrosis virus (IHNV) (Virus)
• Typical hosts. Salmonids: rainbow trout, sockeye salmon, chinook salmon, Atlantic salmon
• Reported distribution. North America (Pacific drainages, endemic), Japan, Korea, continental Europe (introduced)
• Temperature window. Disease at 46–59 °F (8–15 °C); rare above 59 °F (15 °C)
• WOAH-listed (notifiable). Yes — internationally notifiable
• Reference image datasets. USFWS NWFHS tested via cell culture (EPC/CHSE-214)

Background on the agent

Infectious haematopoietic necrosis is caused by Infectious haematopoietic necrosis virus (IHNV), a virus in the Rhabdoviridae.

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 Salmonids: rainbow trout, sockeye salmon, chinook salmon, Atlantic salmon. 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.

How to spot it before it spreads

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

• Fry: darkened skin, bulging eyes (exophthalmia), distended abdomen, faecal cast trailing from vent, spiral swimming. One or both eyes protrude visibly from the socket.
• Hemorrhages at fin bases. Pinpoint red dots or larger red patches — especially under the fins and around the gills.

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.

Two signs in the same fish is suggestive. Three is a working diagnosis. Don’t wait for the full set before you act.

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 Disease at 46–59 °F (8–15 °C); rare above 59 °F (15 °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.

Your move, step by step

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; iodophor egg disinfection prevents transmission; some DNA vaccines available

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.

What doesn’t work (and why)

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.

After treatment: what comes next

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.

Prevention going forward

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 46–59 °F (8–15 °C); rare above 59 °F (15 °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.

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.

A note on look-alikes

More than one disease shares the early signs of Infectious 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

• Viral Hemorrhagic Septicemia — for keepers narrowing down between this and a look-alike condition.
• Streptococcosis — for keepers narrowing down between this and a look-alike condition.

Source

Primary reference: WOAH Aquatic Manual Ch. 2.3.4; USFWS NWFHS Lab Manual Ch. 1.

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

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