Parasites - Fish Lice

Argulus foliaceus. Photo by Donald Hobern

Fish lice, also known as carp lice or koi lice in ponds, are parasitic crustaceans that are thought to be quite prehistoric based on their morphology. However, due to their small size and obvious lack of bones or other sturdy structures, they possess no fossil record so scientists can only speculate at their exact evolutionary record. They are able to live in both freshwater and marine environments, making them a potential problem for almost any type of fish! 

 With over 170 recognized species, fish lice are found all over the world and are all well-suited to many small ecological niches as well as entire, broad ecosystems.

Most members of this family are between 3 and 35 millimeters in size, or ranging from a length equivalent to only the width of two pennies up to over an inch, depending upon the species, location, and life stage.

Most Argulidae members have a broad, ovular body shape covered by a carapace, somewhat similar in appearance to the common ticks that are found in forests and grasslands.

However, fish lice have quite visible black eyes, antennae that have fused together to form a barbed proboscis that also house easily observable suckers for latching onto fish, and typically four pairs of spiny, leg-like appendages stemming from their thoracic region (the area that includes the thorax and abdomen). With their sucking mouthparts, they feed on the mucus, blood, or both of the fish that they attach to.

The naked eye can typically determine when fish lice are present, as even the smaller species are just large enough to see clearly, though if you aren’t sure a Microscope can of course be used to aid in identification.

What are the symptoms of Fish Lice parasite in koi?

The most obvious sign of fish lice infestations are the presence of fish lice on your fish, which look like small green or brown dots. However, when these are not visible, there are a variety of other signs and symptoms to watch out for.

1) Lethargy

Fish lethargy is caused by anaemia if fish lice have been attached long enough.

2) Haemorrhages

Haemorrhages, or blood pooling at the surface of the skin due to blood vessels bursting, can be found around sites where fish lice are or have been attached.

3) Paleness

Paleness, or a dulling in colour, is due to blood and nutrient loss from fish lice.

4) Flashing and Flicking

Flashing and Flicking behaviours are most often exhibited when fish are uncomfortable and are trying to alleviate their skin discomfort and pain.

5) Scale loss

Scale and skin loss is common when multiple fish lice have attached themselves.

6) Fin Erosion
If plagued by fish lice, you may notice your fish’s fins becoming tattered and decreasing in size as blood and nutrients are lost through parasitism.

7) Excessive Mucous

This response typically occurs toward the beginning of fish lice parasitism, as the fish’s body is trying to defend itself. However, many fish lice will simply just feed on this mucous.

8) Rubbing

Affected fish are likely to be seen rubbing against rocks, plants, and pond edges to try and rid the fish lice from their skin.

9) Secondary Infections

The longer that your fish have been infested, the more likely it is that they’ll develop secondary infections such as koi herpesvirus, larval nematodes, ich, and fungal infections. If you notice these symptoms, chances are that fish lice have been around for some time.


Treatment Recommended 

• Potassium Permanganate (PP) 1.5 grams per 220 gallons or 1.5ppm. Keeping the water purple for a minimum of 2 hours should effectively kill Lice. The general rule is to keep it purple for 4 hours, additional 1/4 or 1/2 doses may be required in ponds with heavy bio load. When water starts to go brown the PP is spent and not doing anything else. To neutralise PP either Hydrogen Peroxide (HP) or Sodium Thiosulphate (ST) can be used. Always add lots of extra air to a pond when treating with PP as it is an oxidising agent. Where possible avoid feeding for 24 hours prior to using PP. It is usually a good idea to do a deep clean of the filers and system to avoid having more organic matter in the pond for the PP to consume. 

Formaldehyde and Formalin (F) Not to be used below 13̊C MG 2% 10ml per 176 UK gallons F 30% at 10ml per 140/150 UK gallons 

• Salt Remember NOT to use salt if Formalin or PP is in the pond already. Use an accurate salt meter to ensure the percentage or ppt is safe. 0.3% starting dose to slowly increase to 0.6% over 24 hours. Any further salt increases should be done slowly. *Remove any Zeolite from the filters as the salt will release ammonia from it. Keeping on top of pond and filter maintenance will help reduce the occurrences. 
Any treatments added are done so at your own risk. It is your responsibility to know your pond volume and to calculate dosages correctly. 

Golden Rules Of Treating Koi

Scrape the Koi first before treatment

Identify which Parasite is causing the problem before treatment

Make sure the pond water is salt-free because it can harm Koi when used with most chemicals

> Calculate the Volume of Water accurately

Turn off the UV Unit when treating your Pond

Test your Water Parameters before Treatment

> Follow the label’s instructions and do not overdose. Chemicals can cause serious damage to your Koi if amounts are miscalculated