Lymphocystis in Fish: What It Is and Can Goldfish Get It?

Lymphocystis is a virus that makes a fish's skin cells balloon into white or cream bumps, often clustered like a tiny cauliflower on the fins. It looks alarming, but it is usually harmless and tends to clear on its own within a few weeks once stress and water quality are sorted out.

Goldfish are a different story. The University of Florida's extension veterinarian, writing for UF/IFAS, lists goldfish, koi and the rest of the carp family among the fish that do not get lymphocystis. So if your goldfish has white growths, it's more likely something else, and the list of look-alikes below covers the usual suspects. One of them, Ich, can kill a tankful of fish quickly and needs a fish vet right away.

What lymphocystis is

Lymphocystis disease is caused by an iridovirus called lymphocystivirus (lymphocystis disease virus, or LCDV). It infects fibroblasts, the connective tissue cells found throughout a fish's body, most often in the skin, fins and gills.

Infected cells turn into "virus factories." A normal fibroblast is about 0.01 mm across. An infected one swells to anywhere from 0.1 mm to more than 2 mm, which is 50,000 to 100,000 times its normal volume, according to UF/IFAS. That is why single cells become visible to the naked eye. When a cell has finished making virus, it bursts and releases virus particles into the water.

Lymphocystis has a couple of much nastier relatives in the same virus family, the ranaviruses and megalocytiviruses, which can cause severe, body-wide disease. Lymphocystis is the mild one.

What it looks like

• Small to medium wart-like bumps on the fins, skin or gills, cream to gray, or tinted by the fish's own color where they sit over pigment.
• Clusters that look like a raspberry or a cauliflower. Merck describes the lesions as benign and cauliflower-like, usually on the fins.
• Bumps that appear over days and last for weeks.
• Pop-eye now and then, when a growth forms in or behind the eye and pushes it forward.
• A fish that acts normally. Most affected fish eat and swim like their tankmates. A heavy load on the gills or body can change swimming or breathing, and growths around the mouth can get in the way of eating.

Infections aren't always visible. Some fish carry smaller infected cells, or growths on internal organs, that only show up under a microscope.

Which fish get it (and which don't)

Lymphocystis has been reported in more than 125 freshwater and saltwater fish species from 34 families. It affects the more evolutionarily advanced bony fish: cichlids, gouramis, killifishes, sunfishes, gobies, butterflyfishes, damselfish and clownfish, dottybacks, snook, drum and sea basses. Merck adds painted glassfish and several marine aquarium fish, flounder are among the documented hosts of the named virus species, and in the wild it is often seen in walleye.

In a mixed tank, usually only one or two species show signs, even when others are exposed.

The fish that don't get it, per UF/IFAS:

• Cyprinids (the carp family): goldfish, koi, barbs, rasboras, danios, and rainbow and redtail black sharks
• Catfishes
• Salmonids (trout and salmon)

The government of Quebec's wildlife disease page says the same about carp, catfish and salmonids.

Bumps on a goldfish? What it could be instead

Since goldfish and koi aren't on the lymphocystis list, white or pink bumps on them need a different explanation. Even on a species that does get lymphocystis, early nodules are easy to confuse with other problems, so UF advises having the diagnosis made by a fish health professional. These are the main look-alikes:

The one to never guess about is Ich. A keeper who decides "it's just lymphocystis, it'll pass" when the fish actually has Ich can lose the whole tank. Merck notes Ich needs immediate, thorough treatment, and a vet confirms it by looking at small samples of skin mucus, fin or gill under the microscope.

How it spreads and what sets it off

Lymphocystis spreads through fish-to-fish contact and contact with infected tissue. Virus also enters the water when infected cells burst. Skin damage from spawning, fighting, parasites or rough handling gives the virus a way in. UF/IFAS links outbreaks to crowding, shipping and the stress of a new tank, poor water quality and other stressors, and water from a source that holds infected fish.

It doesn't appear to pass from parents to offspring through eggs or sperm. The tricky part is that fish can be infected without visible bumps, so once one fish shows signs, others in the group are probably carrying it too.

How long it lasts

Lymphocystis is self-limiting. In most warmwater fish the bumps clear after a few weeks. In cool or coldwater species it can take up to about 6 weeks.

Fish don't mount much of an immune response until the infected cells burst. Bumps can come back later, but repeat bouts are usually milder.

Deaths are uncommon. The real risk comes when a fish is covered in growths or the gills are heavily involved: breathing can suffer, and bacteria, parasites or fungi can move in as secondary infections. That's when a fish can get seriously sick.

What helps (and what doesn't)

There is no treatment that speeds up recovery, and no vaccine. The disease runs its course. What you can do is give the fish the best conditions to clear it and keep the virus from spreading.

1. Get the diagnosis confirmed. A vet can take a fresh sample of a bump (a wet-mount biopsy) and look for the giant, balloon-like cells. This is how lymphocystis is told apart from Ich and the other look-alikes.
2. Test the water. Merck calls a complete analysis of water from the affected tank "critical" when fish are sick. Ammonia and nitrite problems are common, especially in tanks less than about 6 weeks old.
3. Remove the stressors. UF says most warmwater cases resolve in a few weeks as long as husbandry is good: good water quality, good nutrition, sensible stocking, compatible tankmates, and no other stressors such as parasites.
4. Isolate visibly affected fish if you can. Moving fish with obvious growths cuts down the number of fish actively shedding virus. It won't catch every carrier.
5. Watch eating and breathing. Growths around the mouth or on the gills deserve a vet's look.

What doesn't help: medicating the tank "for lympho." No medication speeds recovery, and treating blind can mean missing what is really wrong. If a secondary bacterial, fungal or parasitic infection develops, your veterinarian diagnoses it and chooses the treatment. Don't squeeze, pop or cut growths off yourself. A vet may remove a growth to examine it.

Keeping it out of your tank

• Quarantine new fish. UF/IFAS recommends at least 1-2 weeks, with 30-60 days preferred, watching the new fish every day. Merck gives 30 days as the minimum. Shipping and acclimation stress are exactly what trigger outbreaks, so quarantine catches them before they reach the main tank.
• Use clean water. A water source that has held infected fish raises your risk.
• Keep separate nets and buckets for the quarantine tank.
• Disinfect equipment after handling infected fish. UF/IFAS lists the disinfectants, strengths and contact times shown to inactivate iridoviruses, including household bleach. Wear gloves and eye protection, ventilate the room, and rinse and dry everything well so no residue goes back into a tank.
• Handle fish gently and avoid crowding. Both cut the skin damage and stress that let the virus in.

When to call a fish vet

Call now:

• White spots spreading fast, or fish gasping or dying (possible Ich)
• A fish breathing hard, or growths covering much of the gills
• Several fish sick or dying at once

See a fish vet soon:

• New bumps you can't explain, on any fish, so the cause is confirmed before you treat anything
• Bumps you can't explain on a goldfish or koi
• Growths around the mouth, or a fish that has stopped eating
• New patches or sores appearing on top of the growths (possible secondary infection)

Mention at a checkup:

• Repeat bouts in a fish that has had confirmed lymphocystis before
• A fancy goldfish's wen that is growing or changing
• Questions about setting up quarantine for new arrivals

Many general practice vets don't treat fish, so it helps to find one ahead of time. The American Association of Fish Veterinarians has a Find a Fish Vet map for North America. If no one is listed near you, AAFV suggests asking your regular vet to consult one, contacting AAFV, or asking a local aquarium society for a referral. Fish are among the easier pets to care for, but they still need a vet who knows fish.

Quick answers

Can people catch lymphocystis? It is not known to be transmissible to humans, according to Quebec's wildlife disease page. Wash your hands after tank work anyway.

Is it contagious to my other fish? Yes, to susceptible species, through contact and virus shed into the water. Other fish in the group may already carry it without visible bumps.

Will it come back? It can. Bumps sometimes return, but a repeat bout is usually milder than the first.

Can goldfish get lymphocystis? Not according to UF/IFAS. Rule out Ich first, then work through the look-alike table above. When in doubt, a fish vet can check a sample under the microscope.

Originally written by Jaclyn. Updated by the I Love Veterinary editorial team.

Sources

• UF/IFAS Extension: Lymphocystis Disease in Fish (FA181), Roy P. E. Yanong, VMD
• Merck Veterinary Manual: Viral Diseases of Fish
• Merck Veterinary Manual: Parasitic Diseases of Fish
• Merck Veterinary Manual: Mycotic Diseases of Fish
• Merck Veterinary Manual: Neoplasia of Fish
• Merck Veterinary Manual: Management of Aquarium Fish
• ICTV: Genus Lymphocystivirus
• Gouvernement du Quebec: Lymphocystosis
• Veterinary Record Case Reports: Surgical management and histopathology of wen overgrowth and neoplasia in four oranda goldfish (Schreeg et al., 2021)
• FishBase: Carassius auratus morphology summary
• American Association of Fish Veterinarians: Find a Fish Vet

This article is for general information and is not a substitute for advice from your veterinarian.