Lymphocystis in Freshwater Fish

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Lymphocystis is a common iridovirus found in both freshwater and marine fish. Mostly causing a change in appearance by the presence of pink or white bumps anywhere on the fish's body. Unfortunately, there is no treatment, but the disease is self limited and does not cause any serious clinical signs.

What Is Lymphocystis?

Lymphocystis is a viral disease in fish. A member of the iridovirus family, these viruses are classified by having double-stranded DNA structures. They 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.

Signs of Lymphocystis in Freshwater Fish

  • Raised bumps on skin
  • Pink to white in color
  • Anywhere on body, including fins
  • Can appear as hazy film when infection first sets in

The main clinical sign of Lymphocystis in freshwater fish is raised skin nodules. They may present with a few scattered nodules or a cluster of dozens. They can appear anywhere on a fish's body, including the fins and oral cavity. Early infections may present as a thin film on the fish's body. Differentials for these symptoms include external parasites, bacterial or fungal infections. For koi, carp pox (Cyprinid herpesvirus-1) and hikui should also be differentials. This self-limiting disease will not cause any severe health issues and only alter the fish's external appearance.

Causes of Lymphocystis

The virus spreads between direct contact and within the aqueous environment. Once in shed from an infected fish, lymphocystis can survive in the surrounding water for up to 1 week. 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 completed.

How is Lymphocystis diagnosed?

To diagnose lymphocystis, your veterinarian will take a skin scraping and analyze it under the microscope. This is done to rule out common parasitic infections, such as white spot or Ich. Epitheliocystis can present similarly to lymphocystis, but will also be present on the gills, unlike lymphocystis. Samples can also be taken for histology analysis by a veterinary pathologist. This will involve your veterinarian cutting a small sample of skin and lesion off your fish and packing it in formalin. These samples may take a few days or weeks to process.

Treatment of Lymphocystis

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

How to Prevent Lymphocystis

It is very hard to catch lymphocystis outbreaks even with proper quarantine practices. 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 is not going to affect the fish's overall heath, other than appearance, keeping stress low for aquarium fish is the best goal to limit the spread of lymphocystis.

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.