White-spot disease in fish

Ichthyophthirius multifiliis ('Ich') & Cryptocaryon irritans

Two species of parasites cause white-spot disease in fish:

  • Ichthyophthirius multifiliis (also known as ‘Ich’) in freshwater fish
  • Cryptocaryon irritans in marine fish.

Both species are ciliated, which means they are covered in tiny hair-like projections that they use for locomotion.

Caudal fin of barramundi infected with Cryptocaryon irritans ('white-spot') Caudal fin of barramundi, Lates calcarifer, showing the characteristic 'white-spot' appearance due to infection with Cryptocaryon irritans

They follow similar routes of infection and have very similar lifecycles, as follows:

  • Infective stage (theronts): free swimming, actively seek out host fish and burrow into skin.
  • Feeding stage (trophonts): feed and grow under upper layer of fish skin.
  • Reproductive stage (tomonts): leave fish and fall to bottom, undergo multiple internal divisions, finally release numerous free swimming infective stages.

The parasites feed on fish host tissue, debris and fluids causing proliferation and swelling of host skin cells (a condition called epithelial hyperplasia). 

These feeding stages form the characteristic visible white spots on the skin and gills.  Severe infestations can develop rapidly in closed aquarium systems.  Where no treatment is provided, the number of parasites on each fish will increase quickly and will generally result in the death of susceptible fish.

Unlike some parasites, white-spot disease can infect and kill many different species of fish. This is known as low host-specificity.

A range of treatments are available through pet stores, but prevention of infection through quarantine of fish and other aquarium material is the best line of defence.

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