What is Ichthyobodo necator?
Ichthyobodo necator (often called Costia) is a tiny, single-celled parasite that lives on the skin and gills of freshwater fish. You can’t see it without a microscope. It has two little thread-like “tails” called flagella, which help it swim.
It can survive in both warm and cold water and multiplies very quickly, especially when fish are stressed.
When Ichthyobodo infects fish, it causes a disease called ichthyobodosis.
What does it do to fish?
This parasite damages both the skin and the gills. Fish infected with Ichthyobodo may:
- Lose weight and condition
- Become very tired and slow
- Produce lots of mucus
- Struggle to breathe
- Show patchy, greyish skin
- Die suddenly in severe cases
Inside the gills, the parasite causes cell damage and swelling. This reduces the fish’s ability to breathe properly. In bad outbreaks, the gills can fill with fluid or become misshapen, making the fish unable to control water movement through its body.
Young fish and overwintering fish are the most at risk.
Why is it so dangerous?
- Fish can die from even light infections
- The parasite multiplies very quickly
- Outbreaks happen in both cold winters and warming spring temperatures
- Stressed or overcrowded fish are extremely vulnerable
- There is no practical treatment for lakes and ponds
Ichthyobodo is a major cause of winter and early spring fish losses.
How do we reduce the risk?
There is no effective fishery-wide chemical treatment.
Good fishery management is the key:
✔️ Reduce stress
Avoid overcrowding, poor water quality, and sudden handling.
✔️ Stock fish carefully
Fish are very vulnerable during spring and early summer when water warms up.
✔️ Keep stock densities sensible
Too many fish → rapid spread.
✔️ Monitor water quality
Regular checks for oxygen, ammonia, and temperature help spot early problems.