Now here's another blighter of a parasite that can be easy to control, but cause a lot of damage to your koi if gone unnoticed for a period of time.
There are two types, skin flukes and gill flukes. Skin flukes are live bearers and gill flukes lay eggs. Gill flukes are distinguishable mainly by their "eyes", two or four black dots which are clearly visible on the slide. In low numbers they can be harmless and I bet at least one fish in your pond has flukes on it right now, and you wouldn't know.
So, here is a gill fluke example;
Many hobbyists struggle to control fluke outbreaks, and the main reason for this is that they tend to only treat once. They will kill the live flukes, but the eggs remain untouched by the treatment. At 20c the eggs will hatch in about 4 days, and must find a host within 8 hours or they will die. At lower temperatures they can remain as eggs for up to 6 months, so you can see why they are a constant issue. We think we have zapped them but then bang... the eggs hatch. During summer, with pond temperatures at above 16c, I would recommend redosing after 5- 7 days.
Skin flukes example;
Skin flukes are live bearers and already carrying a fully developed embryo when it is born, and will "give birth" a day later. This process carries on and a single skin fluke can produce up to 2000 offspring in one month.
Symptoms for both are the same, lethargy, lack of appetite,heavy mucus, flashing and quite common is "submarining", where the koi will lay motionless with its head pointing downwards. Secondary infections will develop, caused by the hooks that the flukes use to attach themselves to the host. Rapid gill movement is also a clue, as the flukes can damage the gill filaments. Always take a scrape from the gill area, especially, and other areas of the koi.
Treatment should be either a Praziquantel or Flubendazole based product. I have found Fluke-Solve to be very effective and easy to administer. Fluke-M is a flubendazole based treatment which is widely available. Follow the manufacturers instructions for dosage. Some hobbyists add potassium permanganate a day prior to treatment to strip excess mucus. PP, Malachite & Formalin and Chloramine T will not eradicate flukes.