A koi carp with severe fluke infestation has lost body weight and has reddening to the head, shoulder, ventral fin and tail base.

Gill flukes are a common parasite found in both fresh and saltwater aquarium fish. They prefer to attach to the gills but can also be found feeding on mucus and skin cells anywhere on the body. They use hooks to attach to fish and cause significant damage to the gills and skin through attachment sites and their grazing activity. This damage can then allow bacteria to invade, resulting in a high death rate.

The flukes are so small that they are invisible to the naked eye, and require visualisation with the use of a microscope. They are flat worms that resemble tiny leech-like creatures. Pictured is a single gill filament hosting two gill flukes (star). The paleness of the gill filament indicates the fish is anaemic. The flukes cause so much irritation that the mucus layer (arrows) has greatly expanded.

Fish that are infected may hide, become lethargic, have clamped fins, a decreased appetite, “rub” their bodies on surfaces, and have difficulty breathing or lose scales. Environmental conditions such as over-stocking, aggression, poor water quality and incorrect nutrition can lower the immune system’s ability to combat the infection.

Gill flukes have a direct life cycle, meaning they are contagious and spread from one fish to another. In recirculating systems, with fish stocked at high densities, the number of parasites can increase exponentially. It is a ticking “time bomb”.

The best way to avoid disease caused by gill flukes is by implementing a quarantine protocol. If gill flukes have been diagnosed, treatments can include: freshwater dips for marine fish or saltwater dips for freshwater fish, praziquantel, formalin, trichlorfon or copper. Gill flukes lay eggs, which can be particularly resistant to treatment, so repeated treatments are required. Consult a fish veterinarian to help check whether you have flukes, and they can formulate the best method to control and eradicate flukes from your system before you have a disease outbreak.

Biography of Dr Karlee Hirakis

Dr Karlee Hirakis is a member of The Fish Vet’s team, located in Sydney and offers mobile veterinary services for aquatic patients. After graduating from The University of Sydney in 2013 she has worked in rural mixed and small animal practice on the central coast of NSW and in Sydney.

Dr Hirakis has kept numerous different types of fish throughout her life and enjoys the challenges that come with practising veterinary medicine in a unique environment. At home, she is kept busy looking after her two Doberman dogs, a cat and seven fish tanks.

Dr Hirakis is currently awaiting approval to become a certified aquatic veterinarian through the World Aquatic Veterinary Medical Association. This program includes over 150 hours of theory and practical work in aquatic animal medicine.

The Fish Vet – Sydney

Email: karlee@reddrop.net