Lymphocystis in Fish

Lymphocystis in fish is a viral disease that causes light-colored bumps to appear on the surface of fish skin. Although there is no cure, the bumps don't typically 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. Here's what aquarium hobbyists should know about lymphocystis in fish.

What Is Lymphocystis in Fish?

Lymphocystis in fish is a common disease that causes pink or white bumps to appear anywhere on a fish's body. It's caused by Lymphocystivirus, a member of the iridoviridae family, and it's common in both saltwater and freshwater fish systems. These viruses are found throughout the animal kingdom, including amphibians, invertebrates, and both freshwater and marine fish. They are related to megalocytivirus, which are also members of the iridoviruses.

The pink or white nodules that occur in lymphocystis 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.

Signs of Lymphocystis in Fish

The main sign of lymphocystis in fish is the appearance of raised skin nodules, which may vary from a few scattered nodules to a cluster of dozens. These nodules, also called warts or tumors, are often pink, white, or gray and can appear anywhere on a fish's body, including the fins and oral cavity. 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. Early infections may present as a thin film on the fish's body.

Lymphocystis a self-limiting disease that rarely causes serious health issues, but it can significantly alter a fish's external appearance. Contact a fish veterinarian if you notice bumps, nodules, or a film on your fish. A correct diagnosis must be made by a qualified aquatic veterinarian before pursuing any treatment.

Lymphocystis is often confused with white spot disease, given its similar appearance. Other potential causes of these symptoms include external parasites and bacterial or fungal infections. Epitheliocystis can present similarly to lymphocystis, but will also be present on the gills, unlike lymphocystis. For koi, carp pox (Cyprinid herpesvirus-1) and hikui should also be ruled out by a vet.

What Causes Lymphocystis in Fish?

Lymphocystis in fish is caused by Lymphocystivirus, a virus that spreads between direct contact and within the aqueous environment. Once the virus is shed from an infected fish, it can survive in the surrounding water for up to a week. 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. Stress impairs immune function in fish and can often lead to a lymphocystis outbreak.

Some fish may be latent carriers, in which they carry the virus, but do not show clinical signs. Due to the long incubation period of weeks to months, the virus may not show up until well after most quarantine protocols have been completed.

How Do Vets Diagnose Lymphocystis in Fish?

To diagnose lymphocystis, your veterinarian will take a skin sample and analyze it under a microscope. They may also take biopsies or perform histopathology, electron microscopy, or molecular examinations to determine whether your fish are infected. Depending on the results, the vet may recommend further testing.

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

Asymptomatic carriers may be present in your system and never show any clinical signs, so identifying a virus-positive fish can be difficult.

Treatment of Lymphocystis in Fish

There is no treatment for lymphocystis other than supportive care. Often, clinical signs of lymphocystis are exacerbated by other stressors in the tank, such as poor water quality, poor diet, or inappropriate temperatures. Alleviating some of these issues may help fish recover their previous appearance. Since lymphocystis does not typically affect anything other than external appearance, it is of minimal concern to most fish owners.

You can move fish 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. The 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 in Fish

Due to its lengthy incubation time frame and presence of non-symptomatic carriers, even the most prominent fish keepers may have lymphocystis in their tanks and not even realize it. Since it's unlikely to affect the fish's overall health, the best way to limit the spread of lymphocystis is to keep stress low for aquarium fish.

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.

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.

Optimal Water Conditions

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?

Thankfully, lymphocystis is not a zoonotic disease and therefore cannot transfer to humans. There are other viruses within the iridoviridae family which can affect frogs, snakes, and insects.