By Jessie M Sanders, DVM, CertAqV
What is Dropsy?
“Dropsy” is not an actual disease, but a physical manifestation of kidney failure in fish, where the fish’s body balloons outward from excess water and its scales stick out like a pinecone.
Dropsy is used to describe the outer signs of a condition, not a specific condition or disease. The underlying cause of the water retention may be a bacterial infection, or it may be due to kidney failure.
Consider a fish’s environment: A freshwater fish exists in a hypotonic environment. That is, freshwater is made up of a higher concentration of water and lower concentration of solutes, like salt. This results in water similar enough to the water in the fish’s body so that the water moves freely into the fish’s skin and other tissues. The water also needs to leave its body, otherwise the fish absorbs fatal amounts of water. The kidneys are responsible for removing the excess water, pushing it out of the body and back into the environment through the gills and urinary tract. However, if the kidneys are not functioning correctly, the excess water can build up internally, leading to the bloated appearance referred to as dropsy.
Physical Signs of Dropsy
Signs of excess water retention may be from a slight rounding of the belly to a very swollen belly. If your fish's belly has changed shape it would be advisable to contact your local aquatic veterinarian for assistance.
What Causes the Kidneys to Fail in Fish?
There are many environmental influences that can affect the proper function of the kidneys. It may malfunction due to an infectious or non-infectious cause. It can also be related to stress.
Fish kidneys lie above their swim bladder, right next to their spine. Since fish lack bone marrow, in addition to removing toxins from the blood, fish kidneys are responsible for making blood cells, both red and white.
Stress can cause small or extreme changes in the production of healthy blood cells, affecting immune system function in addition to kidney function. Stress levels in fish can be a result of poor water quality, improper diet, overcrowding or mismatched species, noise pollution, parasites, and many other factors. All of these factors play into disease susceptibility and spread, and can affect the organs.
Other Causes of Kidney Failure in Fish
One of the most common kidney diseases in fish is seen in goldfish. Polycystic kidney disease (PKD) occurs when fluid-filled cysts form in the kidney, destroying normal tissue and leading to kidney failure. Buoyancy disorders may be secondary to PKD as the kidney tissue swells, compressing or displacing the swim bladder. Affected goldfish will take on the typical “dropsy” appearance. Confirmation of PKD will be made by your veterinarian using needle aspirate or ultrasound.
Although PKD is thought to be caused by a parasite, no confirmed causative agent has yet been identified. There have been some cases that have spontaneously resolved, and there are a few treatments that may bring some relief, such as hyperosmotic therapy, but in many cases, there is no cure for PKD.
Other Causes of Dropsy in Fish
Urogenital disorders in fish vary widely by species. In most captive fish species, neoplasia or tumors within the reproductive tissue is the most common urogenital disorder. Koi in particular seem to be the most predisposed to developing internal tumors. The only correction for these tumors is surgical removal.
Another possible cause of swelling in koi is egg retention late in open spawning season, resulting in eggs potentially rotting in the coelom or abdomen. Your veterinarian may inject hormones to induce spawning.
How to Treat Dropsy and Kidney Disease in Fish
Quarantine all fish that develop signs of dropsy or kidney disease as soon as possible. A simple change of water and environment may correct any underlying problems. If the condition reappears when the fish is re-introduced to the main aquarium, this indicates there is a hidden stressor in the environment that needs to be resolved.
Many different disease processes may present very similarly to kidney failure. If you suspect your fish is ill, check your water chemistry right away using a liquid-based test kit. If all your readings are within range, contact your local aquatic veterinarian to help diagnose your fish.