What causes brooklynellosis?

Brooklynellosis is caused by a ciliated protozoan known as Brooklynella hostilis Lom and Nigrelli, 1970. B. hostilis is found pretty much all over the world, but is more common in warmer waters (Lom, 1995). It infects most marine teleosts (bony fishes) but has not been detected in wild fish (Lom, 1995). Brooklynella has no free-living stages (Lom, 1995).

The parasite attacks the gills of the infected fish and with heavy infections, the surface tissue of the gills is destroyed and the parasites feed on tissue debris and ingest blood cells (Lom, 1995).

The signs of brooklynellosis

As the parasite causes respiratory difficulties (Lom, 1995), the infected fish show rapid or laboured breathing and may swim close to the water surface in an attempt to get sufficient oxygen. The gills may haemorrhage and small red spots (petechiae) may be present (Lom, 1995). A mild inflammatory reaction may be evoked by a light infection, but a heavy infections will usually cause the death of the fish. Noga (2000) reports that skin lesions are also often associated with brooklynellosis.

Prevention of brooklynellosis

Lom (1995) suggests that ciliated protozoan parasites, such as Brooklynella, only cause problems when the fish are stressed. Maintaining good water quality and low stress levels may be sufficient to prevent Brooklynella in aquaria. As the introduction of new fish is generally stressful to the fish, quarantining a fish for a number of weeks before it is moved to a display tank will allow both observation for signs of disease or parasites and enable the fish to regain any condition lost in the capture and transport process.

Treatment of brooklynellosis

Both Lom (1995) and Noga (2000) recommend formalin baths of between 0.125 and 167 mL/L for 30 to 60 minutes. The warmer your water, the less formalin that should be used as both formalin and increasing temperature reduces the oxygen saturation of the water.

The bath water should be matched closely with the tank water and probably the best way is to take water from the tank and place it in a bucket, add the formalin. mix well and treat the fish. Replace the removed water with fresh seawater (artificial or natural depending on what is normally used). Add an airstone to the dip bucket to ensure good aeration (and then toss the airstone away). After the dip is completed, dilute the dip water with 4 times the volume of tap water and discard.

To each gallon (or 4 litres) of bath water, around 0.6 mL of formalin should be added to get the desired dosage.

Have a second bucket of tank water ready to transfer the fish to when the treatment is complete or the fish shows signs of stress. The second bucket allows the fish to be "rinsed" before it is placed back into the tank.

Don't use formalin if the water temperature is greater than 27°C (80°F).

If the fish appear to have trouble breathing - more so than before the bath started - remove them.


Lom J. 1995. Trichodinidae and other ciliates (Phylum Ciliophora) In: P T K Woo (ed.) Fish Diseases and Disorders. Volume 1: Protozoan and Metazoan Infections. CAB International, Wallingford, Oxon. pp 229-262.

Noga E.J. 2000. Fish Disease: Diagnosis and Treatment. Iowa State University Press, . 367pp.

Last updated: November 25, 2003