Fish lice (Argulus)
Key facts: Fish lice (Argulus) are branchiuran crustacean parasites, visible to the naked eye, that attach to the skin, fins and gills of koi and feed on blood and bodily fluids.[1][2]
Fish lice are members of the genus Argulus (family Argulidae), a group of branchiuran crustaceans related to crabs, lobsters and shrimp, with roughly 100 species worldwide and about 138–140 accepted species described in the genus.[1][4] Three species—Argulus foliaceus, A. japonicus and A. coregoni—are the most studied freshwater forms and are commonly found infesting koi, goldfish and other cyprinids.[1]Infestations are not uncommon in wild or pond-raised freshwater fish and tend to peak in summer and fall.[1] Because fish lice can be identified with the naked eye and are a common parasite of koi, they are frequently listed alongside anchor worm as one of the parasites koi keepers are expected to recognize visually.[5][6]Biology and appearance
Argulus is dorsoventrally flattened, with an oval to rounded body covered by a wide carapace and two compound eyes.[1] A needle-like stylet in front of the mouth tube pierces the host to allow feeding on bodily fluids, and adults have two prominent suckers—developed from hook-like structures in juveniles—used to attach to the fish, along with spines and hooks on the appendages that aid movement.[1] Older juveniles and adult A. foliaceus are approximately 3–7 mm long by 2–4 mm wide, with females larger than males and distinguishable by paired seminal receptacles at the rear of the body.[1] A separate pictorial guide describes juveniles as approximately 1–3 mm and adults as 5–20 mm, with a flattened, saucer-like shape, two suckers and two eyespots, and notes the parasite can glide over the fish while feeding.[2] According to a secondary source, Argulus species sit tightly against the host body to minimize the risk of detachment, feed on mucus and skin cells as juveniles, and shift to blood feeding with age as they move from the fins to the body.[4]Life cycle
Argulus has a direct life cycle, completing development from egg to reproducing adult on a single fish host, and unlike some other crustacean parasites it continues to molt even after reaching maturity; some chemical treatments work by disrupting this molting process.[1] The full life cycle averages 30 to 60 days depending on species and water temperature.[1] All life stages of both sexes are parasitic, and adults can survive for several days off the fish host.[1] After mating, females detach to lay eggs on hard surfaces and vegetation in the environment before returning to a fish host.[1]Egg hatching time depends on temperature: A. japonicus eggs hatch in 10 days at 35 °C (95 °F) but require 61 days at 15 °C (59 °F), while eggs of the closely related A. foliaceus hatch in 17 days at 23 °C (73 °F) and 30 days at 20 °C (68 °F).[1] Eggs laid in autumn can overwinter and survive until the following spring.[1] The first-stage larva, called the metanauplius, must find a host within 2–3 days of hatching or it will die; once attached, juveniles pass through 11 molts (12 stages in A. foliaceus) before reaching sexual maturity roughly 30–40 days after hatching, and juveniles can overwinter in the fish's mucus.[1]Signs in koi and disease effects
Fish lice attach to the skin, gill chamber and mouth, producing localized inflammation from mechanical damage by hooks, spines and digestive enzymes.[1] In heavy infestations lice may be visible across the skin and fins and even swimming in the water column, while fish without visible lice can still show non-specific signs such as pinpoint hemorrhages, anemia, fin and scale loss, increased mucus production, lethargy, erratic swimming, reduced feeding, surface hanging and poor body condition.[1] Affected fish commonly "flash" or rub against surfaces to relieve irritation.[1] A commercial koi health resource similarly lists visible parasites, erratic swimming and scratching as signs of fish lice, and notes that disk-shaped parasites visible under magnification distinguish argulosis from the worm-like anchor worm.[6]Although fish may tolerate low or moderate numbers of lice with few signs, the damage can lead to secondary infections, and a parasite's high reproductive rate can quickly escalate an outbreak; severe infestations of hundreds of parasites may impair the fish's ability to balance internal fluids and salts.[1] Secondary pathogens such as Aeromonas bacteria (see Aeromonas and ulcers) and the water mold Saprolegnia (see Saprolegnia (fungus)) are often found alongside Argulus infestations.[1] A species profile on koi and goldfish aquaculture also lists fish lice among the parasites, along with anchor worm, monogenean flukes and protozoans such as Ich, Trichodina and Chilodonella, that affect fish more severely when water quality is poor.[5]Role as a disease vector
Argulus is capable of acting as a mechanical vector or intermediate host for other fish diseases.[1] It can carry and transmit spring viremia of carp (SVC), a reportable viral disease of koi, common carp and goldfish (see Spring viremia of carp (SVC)).[1][7] Experimental work has demonstrated that the SVC virus can be isolated from Argulus foliaceus removed from infected carp, and that the parasite can transfer the virus between diseased and healthy fish under experimental conditions, alongside the leech Piscicola geometra.[3] Aeromonas salmonicida has also been isolated from Argulus coregoni, and experiments found higher rates of Aeromonas infection when Argulus is present, although direct louse-to-fish transmission has not been proven.[1] More broadly, Argulus is considered able to transmit bacteria and viruses between fish in addition to causing direct damage itself, and the parasite can also serve as an intermediate host for several species of nematodes.[8][1]Diagnosis
Because of their size, older stages of Argulus can be diagnosed with the naked eye, visible moving on the host or swimming in the water, and can also be identified on a wet mount of affected tissue; captured fish should be examined quickly since the parasite may leave the fish once disturbed or removed from water.[1] Filtering system water through a fine mesh net can help capture free-swimming adults or juveniles for identification, though confirmation by a fish health professional is recommended, a step consistent with general guidance on diagnosis using scrapes, microscopes and fish vets.[1] A disk shape under magnification helps distinguish Argulus from the worm-like anchor worm (Lernaea), described further at Anchor worm (Lernaea).[6][2]Treatment and management
Treatment should account for the parasite's 30–60 day life cycle and must target eggs, juveniles and adults both on the fish and in the environment, since manual removal of adults is often impractical and incomplete.[1] Fish can be moved to a clean system while the original tank is disinfected or allowed to dry completely, though drying can be difficult in humid climates and eggs can survive longer at cooler temperatures; water quality should be maintained throughout treatment.[1]There are no FDA-approved drugs for Argulus, and options are limited by resistance, availability, legality and species sensitivity, so working with a fish health specialist is advised.[1] Historically, the organophosphate trichlorfon has been effective at 0.25–0.50 mg/L active ingredient weekly for four treatments, though the main approved product is no longer manufactured.[1] Diflubenzuron and lufenuron interfere with molting and have also been used, and potassium permanganate (10 mg/L for 30 minutes, or 1.3 mg/L applied twice over three days) has been shown effective against juvenile and adult stages in freshwater, though it is not recommended in marine systems.[1] Increased salt concentration and formalin do not appear effective against Argulus in freshwater.[1] The in-feed product emamectin benzoate has experimentally controlled Argulus in goldfish and koi but is only available in the United States through an FDA Investigational New Animal Drug study.[1]Prevention and biosecurity
Because controlling established Argulus outbreaks is challenging, biosecurity measures are recommended to minimize introduction or spread between ponds and systems.[1] Incoming fish, particularly wild-caught or pond-raised stock, should be quarantined, observed and sampled to reduce the risk of introducing the parasite, consistent with general quarantine practice for new koi.[1] Source water should ideally be filtered or drawn from a fish-free and Argulus-free source, since outbreaks should be managed quickly once recognized.[1] Potentially infested fish should not be mixed with clean fish without proper quarantine, a precaution also relevant when assessing general signs of illness in newly acquired koi.[1]Common questions
Can you see fish lice on koi with the naked eye?
Are fish lice the same as anchor worm?
Can fish lice spread other diseases to koi?
Does salt treatment get rid of fish lice?
According to University of Florida IFAS Extension, increased salt concentration does not appear to be effective for controlling Argulus in freshwater, nor does formalin.[1]
How long does an Argulus infestation take to clear up?
Because the life cycle ranges from about 30 to 60 days depending on temperature and species, and eggs can survive in the environment for weeks, treatment protocols typically require repeated applications over several weeks to target all life stages.[1]
Related koi topics
References
- ^ UF/IFAS EDIS FA184: Argulus (Fish Louse) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA184. Retrieved November 2026.
- ^ UF/IFAS EDIS FA115: Common Freshwater Fish Parasites Pictorial Guide: Crustaceans. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA115. Retrieved November 2026.
- ^ Manual of Diagnostic Tests for Aquatic Animals, ch. 2.3.9 Spring viraemia of carp (WOAH). World Organisation for Animal Health. woah.org/fileadmin/Home/eng/Health_standards/aahm/current/2.3.09_SV…. Retrieved November 2026.
- ^ Argulus (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Argulus. Retrieved November 2026.
- ^ SRAC Publication No. 7201: Species Profile: Koi and Goldfish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/7201%20Species%20Profile-%20Koi…. Retrieved November 2026.
- ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
- ^ UF/IFAS EDIS VM106: Spring Viremia of Carp Virus (SVCv). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM106. Retrieved November 2026.
- ^ UF/IFAS EDIS FA100: Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 2: Pathogens. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA100. Retrieved November 2026.