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Fish Lice

Fish lice (Argulus japonicus) are the commonest of the larger visible parasites found on Koi. It is a saucer shaped parasite about 1 cm in diameter when adult. The young are miniature versions of the adults and all stages of fish lice will feed on Koi. They are usually spread by purchasing already infected Koi or aquatic plants. Fish lice lay sticky eggs on objects in the pond. The life cycle in comparison with the anchor worm is fairly simple as many of the metamorphic stages take place inside the egg itself. Fish lice moult numerous times and mature individuals cover a range of sizes. The complete life cycle can take up to 100 days but this will be influenced by the water temperature and other environmental factors. At a temperature of 16 degrees Celsius, it takes approximately six weeks to hatch. It is unlikely that the number of fish lice will diminish in the pond without the aid of chemicals to eradicate them, although in lower temperatures the rate of reproduction will decrease.


The fish lice look fairly aggressive and possess some good attacking mechanisms. The body is oval shaped and flat. The males and females look very similar. The eyes are located on either side of the head. It has four pairs of legs but these are used for swimming only, because the parasite clings to the body with of the fish with numerous barbs, hooks, spines and suckers. These can cause some damage to the body of the koi, but the real damage is caused by the mouth that resembles a hypodermic needle. This mouth apparatus contains mandibles that resembles jaws and are situated on the ventral surface. The stylet or needle is located between the mandibles. It is used to stab the victim and then to inject the fish with an anticoagulant to ensure a continuous blood flow for the parasite. When a fish louse stabs a koi, it also injects a substance that is an attractant to other lice, which feed at the same site. Over a period of time, this results in open lesions and ulcers. This feeding behaviour of fish lice creates the possibility of anaemia and secondary infections by bacteria, viruses, or fungus. It further spreads disease from fish to fish as it moves on to new hosts.



Adults are easy to spot on the body of a koi when they move, although their dark grey colouration can be a good camouflage on fish with sumi markings, like Showa or Shiro Utsuri. If they don’t move, they can be mistaken for black spots. The juveniles are very hard to see with the naked eye, but in an examination bowl they can easily be seen swimming in the water around the fish. They return at intervals for a snack and then detach themselves once more. The fish louse does create disinterest in food, even if relatively few lice are present. If the infestation is heavy, the koi may be irritated, scratching on objects or on the bottom of the pond. Red marks are also caused by the action of the parasite. Sometimes behavioural changes can be observed before the parasite is actually seen.





Carefully check all new fish for parasites and remove them with tweezers. If fish lice are suspected, it is worth-while to do the first treatment during quarantine with organophosphate to get rid of the parasites before the females have time to lay eggs.


If you spot a Koi in your pond with fish lice, the chances are that that a number of your Koi will also be infected. Check all the Koi for adult lice and remove any that you find with tweezers. The best treatment remains organophosphate but frequent treatments will be necessary to eradicate the lice as the eggs will hatch over an extended period.


Last Updated on Saturday, 10 January 2009 22:54