Flukes belong to the family of a monogenetic named trematoda. These parasites have sets of hooks at the end portion of their body and with the help of these they affix themselves to
the selected hosts. There are two varieties of parasites found on the bodies of koi fish, skin flukes (image on the right )
and gill flukes (image on the left ). The monogenean trematodas are similar to worms and can be 2mm long. An overstocked pool with dirty water is a perfect breeding place for this kind of microscopic intruder.
Skin flukes (Gyrodactylus) measure up to 0.4mm, and gill flukes (Dactylogyrus) are up to 2mm in length. Both types of 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 a specialised structure at the posterior called the opishaptor. 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. It is hooks that are responsible for the majority of the damage to the host fish as they pierce the surface of the skin. The gill fluke has two or four noticeable dark spots on the body which are not present on the skin fluke.
Life Cycle
Monogeneans (a group of largely ectoparasitic members of the flatworm) only require one host to complete their life cycle.
Gyrodactylus are viviparous (livebearing) hermaphrodites (having both male and female reproductive organs) and when it is born it already has a fully developed embryo inside it's body, this will be born within one day of it's parents birth, within that is another embryo developing - rather like Russian dolls. This is called serial polyembryony. They are usually found on the skin and fins of the fish.
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). After release into the water they hatch in about four days at optimum temperature of 20 degrees C. The free-swimming ciliated (hair like organs) larvae must find a host within 6-8 hours of hatching to survive. At optimum temperatures (22-24 degrees C) the cycle only takes a few days to complete. However it can take as long as 5-6 months at 1-2 degrees C.
Symptoms
flashing and rubbing
lethargy
stopping feeding
sitting on the bottom with their fins clamped
milky appearance due to an increase in mucus secretion
Fish seriously infected with gill flukes (Dactylogyrus) may show increased gill movements, as the flukes damage the delicate gill tissues.
There are several parasites that cause the same symptoms so the only way to make a definite diagnosis is to examine a skin scrape under a microscope. Tissue damage caused by the hooks used to attach the flukes to their host can lead to secondary infections by bacteria and fungi.
Treatment
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!
Chemical treatments include Malachite & Formalin, Chloramine-T, Potassium Permanganate and Superverm (see note above)
Superverm must not be used with any of the goldfish family (goldfish, shubunkins, comets etc).
Chloramine-T can be used with sturgeon
For fish that can tolerate Potassium Permanganate it can be used as a bath but don't add directly to the pond because it will kill the filter bacteria.
Salt is effective against Flukes, use a salt bath for individual fish as it will kill plants and should not be used with zeolite filters as it will release ammonia, nitrite etc back into the water.
Always check the labels to make sure the treatment is suitable for your fish (see above). 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. It is recommended that extra aeration is added when treating and reduce feed levels.
With all parasites and disease, prevention is better than cure. Healthy fish will be able to fight off most diseases and parasites however, poor water quality, overcrowding and poor nutrition will cause stress and weaken the immune system.
Test the water quality before using any chemical treatment. Some symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good.
