Fish Lice: Causes, Symptoms & Treatment

Fish lice are tiny, parasitic crustaceans that feed on the blood of fish. They can severely damage fish populations by sucking their blood and causing their skin to ulcerate. They are most common in warmer climates, where they are able to survive in freshwater habitats.

The article provides a brief on fish lice their causative agent, clinical signs, prevention, and treatment.

What are Fish Lice

Branchiuran crustaceans that parasitize both marine and freshwater fish include fish lice, which are identified as a major threat to fish health.

Argulus spp. can often be the cause of this disease, even inducing long-term sickness and death to fish species.

Fish lice are also thought to be the source of many other fish diseases.

Fish Lice (Argulus sp) are known to spread fatal diseases in heavily infected fish.

They can often lead to severe illness and death in heavily infected fish. In addition to being simply a puncturing irritant as it sucks blood, fish lice are thought to be carriers for other fish diseases.

What Causes Fish Lice | How Do Fish Get Lice


Argulus infections are usually most severe in the spring and summer. The hookworms can lodge in the scalp, claws, and gills. Inflammation at the binding site happens from the damage caused by hooks and spines on the stylet and appendages, as well as some inflammation caused by digestive juices.

In chronic infestations, the parasitic lice can be located all over the surface of the skin and fins. Argulus can also be used as an intermediate host for numerous fish diseases. It can be used to transport and efficiently transmit the carp ailment known as spring viremia.

Aeromonas salmonicida, an important bacterial pathogen, has been isolated from Argulus coregoni, and tests demonstrated that the infection rate of Aeromonas was higher when Argulus were present, but direct transmission from louse to fish has not yet been proven.

Argulus can also serve as an intermediate host for several species of nematodes (roundworms).
In addition, Argulus lice have also been known to be a carrier of other disease agents, including Rabdovirus carpio, larval nematodes, and the fungus Saprolegnia.

Life Cycle of Fish Lice (Argulus sp.)

Argulus’s life cycle includes one host, which is the fish, and spans from hatching to maturity. Crustaceans, including Argulus, have an evolutionarily complex and prolonged life cycle.

They first grow a chitinous exoskeleton and then molt, losing their entire exoskeleton in the process.

The specific length of the branchiuran’s life cycle will depend on the species that determine its existence as well as the water temperature.

The entire life cycle involves a parasitic life. The branchiarusan life cycle averages approximately 30 to 60 days.

An individual Argulus (adult) may live for several days after consumption of fish. After a male and a female couple copulate, the adult female pulls out fish hosts to lay eggs upon hard surfaces and vegetation on the ground.

After the female has laid her eggs, she will return to the fish host. The time needed for Argulus eggs to hatch will depend on the species and temperature.

Argulus japonicus eggs hatch in 10 days at 35°C but require 61 days at 15°C. At 23°C, the eggs of a closely related species, A. foliaceus, hatch in 17 days, whereas at 20°C they hatch in 30 days.

If eggs lay in the fall, they can survive overwintering until the following spring. As the metanauplius develops in certain animals, they must find a parasite within 2 to 3 days or lose their lives.

Larvae go through a series (11 to 12 molts) of development within a leafhopper before attaining sexual maturity, typically around 30 to 40 days after hatching.

Larvae can overwinter within this leafhopper’s mucus. Fish lice peak in the fall and spring. Larvae of these fish parasites attach themselves to the skin, gill chamber, and mouth.

They cause localized inflammation at the contact area due to barbs and spines on the hooks and appendages, and irritation from digestive enzymes.

In heavy infestations, there may be lice all over the fins and bodies of the fish.

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Signs and Symptoms of Fish Lice Infestation

Fish without visible infestation may exhibit miscellaneous indicators of infestation. These include
• Pinpoint or pus sores, anemia, fin and scales loss.
• Increased mucus production, lethargy, erratic swimming, reduced feeding.
• Hanging at the surface for relief from waterborne irritation and poor physical condition.
• Fish may display flashing or rubbing clothing against surfaces in an effort to ameliorate irritation.
• Local inflammation and damage at the affected location may lead to secondary infections.
• The high reproductive rate of the parasite can quickly elevate an infection. Severe infestations of hundreds or more parasites may impair the host by damaging skin and reducing the fish’s ability to regulate its internal fluid and salt levels.


Parasites can be diagnosed with the naked eye due to their large size. The victims can be seen moving over the host or swimming in the water. The parasite can also be identified on a wet mount of the affected tissue.

Captured fish should be examined quickly because Argulus parasites can leave the host quickly, especially after they are disturbed or taken out of the water.

The water filtration process with a fine net or wedge-shaped strainer may help capture free-swimming Argulus adult or juvenile stages for identification.

Adult and juvenile stages of this species (much like adults but lacking suckers) are easy to identify and can be identified by a veterinarian, but identification should be confirmed by a specialist.

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How to Get Rid of Fish Lice | How to Kill Fish Lice

Treatment of Argulus infestations in fish encompasses the proper identification of the source of the contamination, the elimination of parasites in food and water, and therapy with drugs.

Drugs believed capable of curing Argulus infections are few, so consultation with a fish health physician during the initial diagnosis stages is essential in controlling the outbreak’s biological and financial impact.

Fish can be moved to a clean tank and treated with the treatment appropriate for the type of fish, while eggs in the original system can be cured either by removing the tank and treating it appropriately to completely dry or cleaning the tank to disinfect it.

Lengthy exposure to a trichlorfon pesticide, such as Dylox 80 (Bayer), which acts on the central nervous system, has been an effective treatment when dosed at 0.25-0.50 mg L active ingredient, once a week for four treatments.

Potassium permanganate ( 1.3 mg/L applied twice over 3 days) has been found to be effective in killing the juvenile and adult stages of freshwater species.

The goldfish were fed the medicated diet emamectin (50 gm/kg body weight per day) for 7 consecutive days. Emamectin is a member of the avermectin family and is effective to treat fish lice infestation.

How To Remove Fish Lice

In the past, it has been challenging to eradicate Argulus sp infestations; therefore, chemical treatments have been used (e.g., salt, formaldehyde, potassium permanganate, formalin, trichlorfon, emamectin benzoate, powdered quicklime).

Ivermectin has been used as an anti-parasite medication via oral intake, but this medicine is approved for use in food animals and is only available through another path veterinarian’s prescription.

How to Prevent Fish Lice

The best way to avoid an Argulus infestation is through good biosecurity, such as screening and quarantine of arriving fish and constant monitoring of all fish.

Incoming fish, including wild-caught or pond-raised stock, should be quarantined, monitored, and sampled to minimize the risk of introduction.

Argulus outbreaks, once recognized, should be handled quickly. Source water should be evaluated, and examined to ensure that it does not contain pathways for the introduction of Argulus eggs.

Ideally, water should be filtered or obtained through a fly fishing source that’s Argulus-free.

Read Related Article: Flukes In Fish (Causes, Symptoms & Treatment)

Is Fish Lice is Harmful to Humans

Fish lice are small parasitic insects that attach themselves to the body of fish but do not affect humans as well. Generally, fish lice are harmless to humans, but they can cause itching and discomfort if they are picked up by mistake.

Fish lice infestations generally disappear on their own once the fish is killed or the parasite is eliminated from its environment.


What is Sea Lice on Fish?

There are a number of different types of sea lice on fish, but the most common is called Scalene Flatfish Louse. It can be found on a variety of fish species, including cod, haddock, halibut and salmon.

The flatworm-like creature attaches itself to the fish’s skin with its sharp claws and feeds off of blood and parasites. There are also parasitic copepods that can attach themselves to the fish’s skin and feed off of fluids in its body.

Can You Eat Fish With Gill Lice?

Many people are unaware that they can eat fish with gill lice. This is because the lice do not attach themselves to the skin; instead, they live in the fish’s scales.

While this information may not be reassuring to those who are afraid of eating raw or undercooked seafood, it is important to remember that gill lice are not infectious and will not cause any health problems if ingested.

Can Fish Lice Live Out of Water?

Fish lice are parasites that can’t live out of water. They are found on a variety of fish, including catfish, bass, and trout. Fish lice attach themselves to the fish’s skin with their legs and suck blood.

The parasites can be difficult to spot and often go unnoticed until they cause an infection. Fish lice can be spread through close contact with infected fish or through water that has been contaminated with the parasites. If you find evidence of fish lice on your fish, it is important to take steps to remove them and prevent the infection from spreading.

Final Word

In conclusion, it is evident that fish lice are a serious problem for both commercial and recreational fishers. They can cause extensive damage to fish populations, and they are a nuisance to anglers.

There are several ways to control fish lice, but the most effective methods are chemical treatments. These treatments can be expensive and hazardous, so it is important to use them only when necessary. There are also some natural methods of controlling fish lice, such as using salt or vinegar.