Flukes (Gyrodactylus and Dactylogyrus)

Photos © Paul K. Vaughn

by Karen Paul

Skin flukes (Gyrodactylus) and gill flukes (Dactylogyrus) are monogenean trematodes of the family Dactylogyridae. Gyrodactylus measures up to 0.4mm and Dactylogyrus up to 2mm in length. Flukes can be found in small numbers on healthy fish, living on mucus and skin debris, without apparently causing any harm.

The adult fluke attaches itself to its host by the opishaptor, a specialised structure on the rear end. The Gyrodactylus opishaptor has eight pairs of marginal hooks and one pair of median hooks. Dactylogyrus has seven pairs of marginal hooks and one pair of median hooks. These hooks are responsible for the majority of the damage to the host as they hook into the surface of the skin. The skin fluke does not have the two or four dark spots on the body that are present on the gill fluke.

Opishaptor (left) and the dark spots on the body of Dactylogyrus (right)

Life Cycle

Monogeneans only require one host to complete their life cycle.

Gyrodactylus is usually found on the skin and fins and is a viviparous (livebearing) hermaphrodite (has both male and female reproductive organs). When it is born Gyrodactylus is already carrying a fully developed embryo, which will be born within one day of its parent's birth, and within this second embryo a third embryo is developing. This is known as serial polyembryony. After giving birth to the embryo in its uterus Gyrodactylus can mate and produce additional offspring at a rate of one every 5-10 days. A single Gyrodactylus may contain up to 3 daughter generations and in 30 days can produce a population of more than 2000.

Dactylogyrus is usually found on the gills and is oviparous (egg-laying). The eggs are release into the water and hatch in about 4 days at 20°C. The free-swimming ciliated larvae must find a host within 6-8 hours of hatching to survive. The length of the life cycle is dependent on water temperature. At 22-24°C the cycle only takes a few days to complete but can take as long as 5-6 months at 1-2°C.

Symptoms

Flukes may cause the fish to flash or rub against the sides and bottom of the pond to relieve the irritation. The fish may also be lethargic, stop feeding and sit on the bottom with their fins clamped. The skin may have a milky appearance due to an increase in mucus secretion. As there are a number of parasites that cause the same symptoms the only way to make a definite diagnosis is to examine a skin scrape under the microscope. The tissue damage caused by the hooks used to attach the flukes to their host can lead to secondary infections by bacteria and fungi.

Fish seriously infected with gill flukes (Dactylogyrus) may show increased gill movements, as the flukes damage the delicate gill tissues.

Treatment

Chemical treatments include Malachite & Formalin, Chloramine-T, Potassium Permanganate and Superverm. Any treatment method must take into account the species of 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. Superverm must not be used with any of the goldfish family (goldfish, shubunkins, comets etc).

Always check the labels to make sure the treatment is suitable for your fish. If in doubt consult a vet. Do not overdose. Chemical treatments are toxic not only to the parasite but also to the fish, especially those already weakened by disease. Always add extra aeration when treating and reduce feed levels.

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

Salt is effective against Flukes but will kill plants and should not be used with zeolite filters as it will release ammonia, nitrite etc back into the water. Use a salt bath for individual fish.

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

Before using any chemical treatments it is advisable to test the water quality. Some symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good.

Back to top