Ick (also known as ich) is short for ichthyophthiriasis and is caused by a parasite called Ichthyophthirius multifiliis. This organism is a protozoan, which is a general term for single-celled animals. Although it is only made up of a single cell, it has many features that make it extremely good at what is does. In fact, ick is so successful it has become the most common disease of captive goldfish, and is responsible for more goldfish deaths than any other disease.

Trophozoite

This is the phase of the ick life cycle when the parasite is living in the skin of the goldfish. It is a spherical shape with a cup shaped mouth at one end. The mouth contains very fine hair-like structures called cilia, that beat rhythmically and continuously inwards. These cilia scrape tissue from the body of the goldfish and pass it into the mouth.

Goldfish produce a natural reaction to the trophozoite - they surround it with a hard casing that forms a cyst. It is these cysts that are the most noticeable symptom of ick - the appearance of being sprinkled with salt.

Depending on the temperature of the water, the trophozoite grows to between 0.1mm and 1mm wide, and after about a week it falls off the goldfish. It swims in downward spirals to the bottom of the aquarium, or sticks to a plant or ornament.

Tomont

Once the released trophozoite has settled, it absorbs its own mouth and secretes a gel around itself, becoming known as a tomont (or trophont). It then starts to replicate inside the gel casing. The cells produced by the first few divisions form an internal wall on the gel casing, but the rest of them develop into parasites.

The number of cells produced in the tomont can reach 1000, and will usually be at least 600. This phase of the ick life cycle will be complete in anything between 8 hours and several weeks, depending on water temperature.

When complete, the tomont bursts and releases the individual cells called theronts. These look for goldfish to burrow into.

Theront

Theronts (also known as tomites) are microscopically small and oval in shape. They are good swimmers and move about the aquarium until they find a goldfish to latch on to and burrow into.

They swim with cilia and are thought to burrow into goldfish skin using these cilia, and possibly a combination of mechanical and chemical attacks on the integrity of the goldfish skin using a pointed structure and a capsule of chemicals found at the front of the parasite. It is after they burrow into the skin that they grow their mouths.

Theronts can survive for a maximum of 4 days without finding a goldfish to feed on, and in most cases they will die after 2 days.

How long does it all take?

It all depends on water temperature. The warmer the water, the quicker the ick life cycle is complete. At a temperature of about 20 Celsius (70 Fahrenheit) the cycle would take up to 4 weeks. Raising the water temperature to 26 Celsius (80 Fahrenheit) would see the process complete in about a week.

As the optimum water temperature for goldfish is at the lower end of the scale, the ick life cycle will take much longer in a goldfish aquarium or pond.

Tags: ich, ichthyophthiriasis, Ichthyophthirius multifiliis, ick

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