Epizootic ulcerative syndrome

Aphanomyces invadans

This deadly water mould attacks a range of fish species that live in fresh water and estuaries. It kills most of the fish it infects. It invades their muscles and internal organs, digesting the fish alive.

About epizootic ulcerative syndrome

Epizootic ulcerative syndrome (EUS) is caused by a water mould that occurs in fresh water and estuaries in warmer waters (20°C to 30°C). This water mould attaches to damaged skin of fish, causing ulcers and killing the tissue in the infected area. As the water mould grows, it branches into the muscles and internal organs of the fish and starts digesting these tissues.

Why this is a problem for New Zealand

EUS can infect many different types of fish both in the wild and in aquaculture. It kills most of the fish it infects.

How it could get here

EUS could enter New Zealand in a live infected fish. MPI has strict measures in place to limit the chances of sick fish coming through the border.

Where would I find it

EUS is most likely to be found in home aquariums. Although it can also be spread through wild fish populations in fresh water.

How to identify infected fish

Fish infected with EUS:

  • have skin ulcers
  • float near the surface of the water
  • are hyperactive with jerky movements.

Diagnosing fish diseases requires laboratory testing. Signs of fish diseases are difficult to tell apart. Not all infected fish show signs of disease.

If you think you've found an infected fish

  • photograph it
  • note the location and any landmarks, if found in the wild
  • catch it (if you can)
  • call 0800 80 99 66 

Note: This information is a summary of this disease's potential impacts on New Zealand.

Last reviewed: