Costia Ichthyobodo fish parasite

Costiasis  or Costia  is a freshwater ciliated (having hairlike organs) protozoa (unicellular) parasitic species called Ichthyobodo necatrix. Formally known as species Costia necatrix.

These are stalked protozoa which attach to the skin or gills of fish to fed and can swim and attach themselves to other fish and then reproduces to infect other fish.
They are quite a common parasite in Koi or outdoor Goldfish.

Costia are sometimes found in small numbers on the skin and gills of fish without appearing to cause any harm. They live on cellular debris and the natural defence system of the fish semm to keep the parasite to an acceptable level.
However when large numbers establish themselves it becomes a serious threat and seemingly the natural parasitic behaviour takes over.

This is especially true when the fish's natural defences are taken up coping with other factors such as overcrowding, low oxygen, poor water quality or other parasitic infestation.
Costia can kill a host rapidly as it attacks the gills, depriving the fish of oxygen and allowing secondary infection of fungus and bacteria in the wounds. 
As it is a cold-water parasite, in the spring it is particularly dangerous as it attacks when the immune system is weaker after the winter.

Life Cycle

Reproduction is by cell division and this is temperature dependent. As it is a cold-water parasite it reproduces rapidly at temperatures between 10°C - 25 degrees C but does not appear to survive above 30 degrees C.
The life cycle is completed in 10 - 12 hours at 25°C. At temperatures below 8 degrees C Costia encloses  as if in a cyst, reverting when conditions become more favourable. They must then find a host within a short time to survive.


Signs of infection

excess mucous production
respiratory distress, show increased gill movements
general debilitation/lethargic
fish resting on the substrate
clamped fins
flashing and rubbing
skin has a milky apperance due to increased mucus production
may be white 'spider-web' lesions

As with Chilodonella these symptoms are not exclusive to Costia and may be caused by other parasites or environmental conditions.
The examination of skin and gill scrapes under a microscope is needed to make a positive diagnosis.


Treatment

Please note Before using any treatment please take into account the fish being treated. Not all treatments are safe with all fish.
 Do not use Formalin, Potassium Permanganate or any treatments that state not to be used with Golden Orfe (Leciscus sp.) or Rudd (Scardinius reythrophathalmus) if you have sturgeon in your pond!
Chloramine-T can be used with sturgeon.

Chemical treatments include Malachite & Formalin, Chloramine-T and Potassium Permanganate (see warning above)

If in doubt about wether or not a treatment is suitable, consult a vet. Do not overdose. These are chemical treatments, and as such, are toxic not only to the parasite but also to the fish. It is sensible practise to add extra aeration and reduce feed levels when treating with chemicals.

Be cautious before using any chemical treatments. Symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good. It is advisable to test the water quality before using any treatment.

For fish that can tolerate it Potassium Permanganate can be used as a bath but don't add it to the pond because it will kill the filter bacteria.

Salt can be useful against Costia but some strains have become salt resistant. Salt will also kill plants and should not be used with zeolite filters as it will release ammonia, nitrite etc back into the water. A salt bath can be effective for individual fish.

Practise caution before using any chemical treatments. Symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good. It is advisable to test the water quality before using any treatment.


Prevention

Prevention is better than cure as with all parasites and disease. Healthy fish will be able to fight off most diseases and parasites. Poor water quality, overcrowding and poor nutrition will cause stress and weaken the immune system.