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Argulus (Fish Lice)

Despite their name fish lice are not true lice, but rather a type of crustacean. They are more likely to be a problem in the summer months when the water is warm, as their life cycle is affected by temperature. As more and more keepers have heated ponds, they may be encountered anytime.

Fish lice are one of the biggest parasites (5-10 mm) and visible with the naked eye. Argulus adhere to the flesh by means of sucker-type discs. Argulus can attack a Koi anywhere, but particularly around the fins and the mouth. Once attached, the parasite pierces the flesh using stinger mouthparts and feed on the fish’s blood and body fluids. When this occurs a toxin is released as the skin of the Koi is punctured, and the toxin can be powerful enough to kill small fish, although it tends to have a lesser effect on larger Koi. The area around this puncture site is liable to secondary infection and may be attacked by fungus or bacteria. The toxin released by the fish lice may cause some Koi to behave erratically as intense irritation may result from its effects.

A close up shot showing its eyes at the top of the picture. The two prominent circular structures near the top are the suckers used to hold onto the fish while feeding. Just below and in-between the suckers is the proboscis-like mouth that it uses for feeding.

Visible Lice should first be killed by dabbing them with Potassium Permanganat. Then, carefully remove from the lice using tweezers. It is paramount that you kill the parasite before attempting to remove it. It will make it easier to remove and less stressful to the fish. It is a good idea to then dab topical ointment on the lesions to prevent secondary infections. It is recommended that you feed your fish medicated food to prevent a bacterial infection from occurring.

Agulus fish lice


Classic behavior of a fish with fish lice include:

  • Swimming erratically (including rubbing and scrapping against things in the pond)
  • Lethargic
  • Stop Feeding
  • Small Koi may be seen to be producing higher than normal levels of mucus.

Medicated Food Might Control Fish Lice

By: Robert Wells, University of Florida

The University of Florida's Tropical Aquaculture Laboratory in Ruskin, Fla., tested a medicated feed on goldfish and Koi and found that the product effectively controlled lice on the fish, the university reported.

The medicated feed could provide a legal approach to treating the infestations, according to the study& principal investigator, Roy Yanong, VMD, associate professor and extension veterinarian at the university. The relatively few products used to control fish lice are either no longer being manufactured or are illegal, he said.

Yanong said additional research is needed to determine the long-term effectiveness of the feed as well as its effectiveness in treating anchor worms and related parasites.

Funded by the U.S. Department of Agriculture’s Cooperative State Research, Education and Extension Service, the study, which was published in the Journal of Aquatic Animal Health, is being used to support the release of a medicated feed for Koi and goldfish lice treatment, the university reported.

“The nice thing about having a feed, an oral medication, is that it’s much more targeted and so less drugs is needed,” Yanong said, comparing the feed to other methods that treat the water instead. “Fish that are infected will be specifically the fish are going to be given the drug.”

Resources: Water Garden News, Magazine


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