Lymphocystis in Saltwater Aquarium Fish

Causes, Treatment, and Prevention

Clownfish with Lymphocystis


Lymphocystis is a common virus in both saltwater and freshwater fish systems. Caused by a member of the iridoviridae family, this virus results in light-colored bumps on the surface of the fish's skin. Thankfully, these bumps do not cause serious health symptoms and tend to be self-limiting. Focusing on better health management can often clear most clinical signs, but prevention is key. Since this virus spreads through infected water, it's important to practice proper quarantine methods when introducing new fish to your aquarium. However, fish can also be asymptomatic, so it's normal for fish to experience this problem even with proper care when new tank residents are added.

What Is Lymphocystis?

Lymphocystis is caused by a Lymphocystivirus, a group of large iridoviruses, which makes the skin and fins of fish develop light-colored nodules that can be white to grey in coloration. The nodules themselves are made up of infected cells that become enlarged as the virus replicates, eventually stopping replication and causing the cell to burst. Bumps on the skin are visible because the cell is enlarged from 50,000 to 100,000 times the size of a normal, healthy cell.

Iridoviruses are found throughout all aquatic environments and can affect freshwater, brackish, and marine fishes. Other iridoviruses can also affect invertebrates and amphibians, and they are common in many parts of the world.

Symptoms of Lymphocystis in Saltwater Fish

The most common sign of lymphocystis is raised, white to grey nodules on the skin and fins. You may notice signs of this virus on the fish in your saltwater aquarium that develop in individual spots or clusters. The following signs are typically seen:


  • Raised or slightly raised nodules
  • Nodules are white to grey in coloration, can be pigmented on darker colored skin patches
  • Bumps located on the skin and fins
  • Affected areas may be individual spots or coalesce into cluster

The papilloma-like nodules of lymphocystis can vary in size and cluster patterns throughout the body of the fish. They are often concentrated on the skin and fins, but aquarium owners may notice pigmentation of the nodules on patches of their fish's skin that are darker in color.

Lymphocystis is often confused with white spot disease, given its similar appearance, but other diseases including encysted parasites, epitheliocystis, epistylis, digenean trematodes, and others can also cause these symptoms. It is important that a correct diagnosis be made by a qualified aquatic veterinarian prior to pursuing any treatment.

Causes of Lymphocystis

As with many other aquatic viruses, lymphocystis can spread horizontally throughout a system through infected water. This virus is naturally occurring in the environment, and it can even be passed through fish living in crowded conditions when being shipped to new areas. In aquariums, it most commonly develops from poor water quality or the introduction of infected fish to a tank of otherwise healthy fish. Lymphocystis is not believed to pass from parent fish to eggs. Causes include:

  • Infected water
  • Improper quarantine
  • Asymptomatic carrier

Lymphocystis has a latent stage in which a fish may be carrying the virus, but not show any clinical signs. Usually, a secondary stress trigger (such as poor husbandry or water quality issues) causes the virus to replicate and the fish to start showing clinical signs. Once a fish is carrying lymphocystis, it can potentially spread it to any other fishes in the same system.

Diagnosing Lymphocystis in Saltwater Fish

In order to determine the cause of suspicious white to grey lumps on your fish, your aquatic veterinarian will take a skin sample for microscopic evaluation. Some asymptomatic fish may have signs of the disease affecting their internal organs, which cannot be seen by aquarium owners. Your veterinarian may take biopsies or perform histopathology, electron microscopy, or molecular examinations to determine whether your fish are infected. Depending on the results, they may recommend further testing. Treatment will vary based upon the test results.

In addition to hands-on testing, environmental evaluation such as water quality testing may be included to identify any potential stressors. Lymphocystis is often made worse by stressors that may not be obvious to newer aquarium owners.


There is no treatment for lymphocystis in any species other than supportive care. It is critical to correctly diagnose the cause of any lumps on your fish and take deliberate steps to decrease any stressors in your tank. Stress impairs immune function in fish and can often lead to a lymphocystis outbreak. Asymptomatic carriers may be present in your system and never show any clinical signs, so identifying a virus-positive fish can be difficult.

Lymphocystis is often self-limiting, but supportive care is essential to the recovery of your fish. Try to be patient and provide good water quality and a proper diet as your fish recover. You can move them to a quarantine system, but the virus will already have spread around your tank once an individual fish shows clinical signs. Quarantining the most affected fish in your tank may help limit the volume of the virus to prevent severe symptoms from presenting throughout the tank's residents.

Prognosis for Fish With Lymphocystis

Lymphocystis is not typically a fatal problem. This virus typically resolves on its own within six weeks, but severely infected fish are also susceptible to other infections caused by bacteria, fungi, and parasites. Secondary infections can increase mortality rates in fish with lymphocystis, so aquarium owners should always practice supportive care during outbreaks to help their fish recover.

How to Prevent Lymphocystis

Aquarium owners cannot always prevent lymphocystis, but there are a few methods that may decrease the likelihood of this virus spreading to your tank:

Quarantine New Fish

The best method to prevent lymphocystis from entering your system is to thoroughly quarantine all new fish, including invertebrates, for 30 to 60 days. The stress of capture, transport, and introduction to a new system is often enough to trigger a lymphocystis outbreak if your fish is a carrier. This usually leads to visible symptoms that aquarium owners can recognize, which indicates that a veterinarian should diagnose the fish properly before any actions are taken.

However, given the virus's latency status, quarantine is no guarantee to prevent the spread of lymphocystis. Aquarium equipment should be disinfected if there is any suspicion that it may have come into contact with this virus. Asymptomatic carriers can often clear quarantine and go on to infect healthy fish in your tank. Thankfully, given that this disease is mostly an aesthetic concern, most fish keepers should not worry about this virus too much. Giving proper supportive care will resolve most outbreaks in a few weeks.

Keep Water Conditions Optimal

Along with safely quarantining all new fish, aquarium owners can also help reduce the likelihood of their fish contracting lymphocystis by keeping the tank's water conditions optimal. Since outbreaks of this virus can be triggered by stress, it's important to handle fish gently along with avoiding overcrowding. Any conditions that may trigger physical stress in the fish should be prevented as often as possible.

Is Lymphocystis Contagious to Humans?

Although there are other iridoviruses that can infect humans, the fish versions are not zoonotic, or cannot spread to humans. There are bacteria, such as mycobacteria, that can transfer to humans, but viruses tend to be less zoonotic.

Article Sources
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