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Fish Lice

By | Last Updated: 31st August 2018

What is fish lice

Fish lice (Argulus spp) are parasitic crustaceans that infest by attaching themselves to a fish with their spines and hooks, causing disease in the fish [1, 2].

There are around 100 species of fish lice, some of which can infest both saltwater and freshwater fishes [2]. The infections are most likely to occur in summer and fall [2].

What do fish lice look like

Adult lice can easily be seen with the naked eye, either swimming in the water or moving on the affected fish [2]. They have a flattened, oval or rounded body with an outer protective covering, but the juveniles are typically smaller than the adults and lack the suckers [2]

Fish Lice

What happens when the parasites affect a fish

The infection can spread rapidly because of the high reproductive rate of the parasite, resulting in a severe infestation that lessens the ability of the fish to maintain the balance between its body fluids and salt levels [2]. It can also act as a vector and bring in disease-causing microorganisms like Aeromonas, Saprolegnia, Rhabdovirus carpio, and larval nematodes [1].

Which Fish are more likely to get it

Infestations with these parasitic crustaceans are usually seen in freshwater pond and aquarium fishes, including:

  • Goldfish
  • Sunfish
  • Common Roach
  • Pike
  • Koi
  • Salmon
  • European Horse Mackerel
  • Blue Bream
  • Betta
  • Trout
  • Zander
  • Silver Bream
  • Carps
  • Catfish
  • Ide
  • Tench
  • Minnows
  • Common Rudd
  • Pumpkinseed
  • European Perch

The three most commonly studied Argulus species affecting freshwater fish include A. foliaceus, A. coregoni, and A. japonicus [1, 2].

Signs and symptoms of fish lice

The lice usually attach to the mouth, gill chamber, and skin of the affected fish causing irritation and inflammation at the site of contact, due to the damage from the spines and hooks [2]. In a heavily infested fish, you may be able to see the parasites scattered throughout its skin and fins [2]. However, sometimes the lice remain invisible, still causing certain symptoms.

Check your fish to see if it has:

  • Tiny red, brown, or purple spots on the body (pinpoint hemorrhage) caused by bleeding underneath the skin due to the parasites attaching to the body [1, 2]
  • Irregular scale pattern and/or torn fins [1]
  • Cloudy, patchy areas on the skin, as studies have shown fishes affected by the parasite to produce more mucus [1]

Behavior changes in your fish to watch out for:

  • Swimming erratically, with sudden direction and speed changes [1]
  • Not feeding as often as it used to [2]
  • Showing signs of lethargy, like avoiding swimming and hanging at the water surface [2]
  • Trying to remove the lice and lessen irritation by rubbing against surfaces [2]

Fish Lice Salmon

Causes: How may your fish get the louse

In most cases, the infection transmits from:

  • Introduction of an infected fish into the aquarium [3]
  • Addition of wild plants to the fish tank without cleaning and disinfecting them [3]
  • Addition of water contaminated with adult Argulus parasites and their eggs [2]

How to treat the fish lice disease

The parasite often leaves the infected fish the moment it realizes you are trying to catch it [1]. So, as soon as you spot a louse, quickly capture the infected fish with a net to keep it from leaving its host and attaching to another fish [1]. Contact your fish health professional for proper treatment measures, including necessary medications, to prevent a rapid increase of infestation [1].

Treatment measures you can take at home

Isolating the infected fish, and removing the lice and their eggs from the tank

Quarantine the infested fish in a clean tank and then treat it with the prescribed drugs [2]. Maintaining optimum water quality (by managing the temperature, ammonia, nitrate, nitrite, and pH levels) through the entire course of treatment is essential for effectively eliminating all the life stages of lice [2].

Also, transfer the unaffected fish (that show no symptoms and no lice are visible on them) to another clean tank so the old tank can be treated. Throw out the water from the contaminated tank, dry it completely, followed by cleaning and sterilizing the whole system to remove the unattached adult and juvenile lice along with their eggs [2].

Medication for fish lice

Although no FDA-approved drugs are currently available, some medications may be prescribed by the vet while the duration of the treatment varies depending on the life cycle of the Argulus parasite [1, 2].

Pesticides like diflubenzuron, lufenuron, and trichlorfon are commonly prescribed for killing the adults and larval stages of the lice [1, 2]. Other treatments, including the use of chemicals such as potassium permanganate, salt, formaldehyde, and formalin, may be necessary [1]. Emamectin benzoate, an FDA INAD (Investigational New Animal Drug), is also known for controlling Argulus infestations in koi and goldfish [2].

Can you prevent the fish lice disease

Since it can be a challenge to control this parasitic infestation, proper aquarium care measures should be taken, in addition to making sure that whatever you are putting inside your aquarium (any new fish, or decorations and accessories) is properly disinfected [2].

Perform partial water changes as frequently as necessary, and provide fresh, clean, chlorine-free water [4]. Check water parameters regularly and maintain optimum water temperature, hardness, pH, and nitrite levels in the tank [5]. Disinfect any equipment that is used for handling the fish, such as nets, in a solution of bleach and tap water [1].

Isolating the affected fishes as soon as possible from the healthy fishes may also help to prevent the infection from breaking out.

Can it be fatal for your fish

Low to moderate levels of infestations can be cured if the infection is kept in check with prompt treatment [2]. If left uncontrolled, it can weaken the host and result in its death [1].

Is it contagious to humans

According to some experienced fish keepers, it is not contagious to humans. However, it is still recommended to cautiously handle the sick fish by wearing gloves and sterilizing any equipment used in the aquarium.


    1. Fish Lice (Argulus japonicus) in Goldfish (Carassius auratus) – Ncbi.nlm.nih.gov
    2. Argulus (Fish Louse) Infections in Fish – Edis.ifas.ufl.edu
    3. Goldfish Lice Treatment – About-goldfish.com
    4. Aquarium Maintenance Tips and Fish Care Guidelines – Algone.com
    5. How To Keep Your Fish Free From Disease – Theaquariumguide.com

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