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Argulus (Fish Louse, Fish Lice)
by Natalie Steckler and Roy P. E. Yanong
Argulus species (Family: Argulidae), more commonly known as fish lice, are members of a large group of branchiuran parasites that infest and cause disease in fish. The argulids are crustaceans and are related to crabs, lobsters, and shrimp.
There are approximately 100 different species of Argulus distributed worldwide that, depending upon species, can infest freshwater and saltwater fishes. The three most-
Argulus is dorsoventrally flattened (i.e., flattened from the topside to the underside), has an oval to rounded body covered by a wide carapace (outer protective covering), and two compound eyes (Figure 1). A thin, needle-
Adult Argulus sp. Note eye spots and round suckers on “head” end, and spermatheca on “back” posterior end.
Argulus Life Cycle: (Figure 2)
Argulus has a direct life cycle, meaning it only requires one host (the fish) to completely develop from an egg to a mature, reproducing adult. All crustaceans, including Argulus, develop and grow through a complex series of molts (i.e., by shedding their outer surface, or “exoskeleton,” which is made of a compound called chitin) and multiple life stages. Unlike copepods—another group of crustacean fish parasites, such as Lernaea (the Anchorworm) branchiurans continue to molt periodically even after reaching maturity. Some chemical control methods (see Diagnosis and Management) kill the parasite by inhibiting the molting process.
While the branchiuran life cycle averages 30 to 60 days, the actual duration depends on the parasite species and the water temperature. All life stages of both sexes are parasitic—unlike the non-
The time required for Argulus eggs to hatch will vary, depending upon 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 laid in the fall, eggs are capable of over wintering (surviving) until the following spring. In many species, the first stage larvae (known as the “metanauplius”) must find a parasitic host within 2–3 days of hatching or they will die. Once attached to the fish host, juveniles undergo a series of molts (11 molts or 12 “stages” in A. foliaceus) until they reach sexual maturity, roughly 30–40 days after hatching. Juveniles can overwinter within the fish’s mucus.
Disease in Fish Caused by Argulus:
Argulus infestations tend to peak in the summer and fall. The lice can be found attached to the skin, gill chamber, and mouth. Localized inflammation occurs at the contact site because of mechanical damage from hooks and spines on the stylet and appendages, and irritation from digestive enzymes. In heavy infestations, the fish lice may be seen all over the skin and fins of the fish and in the water column (Figures 3 and 4). Fish without visible lice may show non-
Individual adult and late stage juvenile Argulus are easily seen with the unaided eye
Heavily infested Koi. Note readily visible oval parasites in throat (ventral) area of head, as well as others scattered throughout the body.
Although fish may tolerate low and even moderate levels of Argulus with very few signs of disease, localized inflammation and damage at the affected site may lead to secondary infections. The parasite’s high reproductive rate can quickly escalate an infection. Severe infestations of hundreds of parasites may debilitate their host by damaging skin and reducing the fish’s ability to balance their internal fluids and salt levels. Secondary pathogens, such as the bacteria Aeromonas and the water mold Saprolegnia, are often seen concurrently with Argulus infestations.
Argulus is also capable of acting as a mechanical vector or intermediate host for several fish diseases. The parasite can carry and transmit spring viremia of carp, a reportable viral disease of Koi, common carp, and goldfish, among other hosts. Aeromonas salmonicida, an important bacterial pathogen, has been isolated from Argulus coregoni, and experiments demonstrated higher rates of Aeromonas infection when Argulus are present, but direct transmission from louse to fish has not yet been proven. Argulus can also serve as the intermediate host for several species of nematodes (roundworms).
Diagnosis and Management:
Because of their size, older stages of Argulus can be diagnosed with the naked eye. The parasites are visible moving on 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 may rapidly leave the fish once it is disturbed or removed from the water. Filtering water from the system through a fine mesh net may also help capture free-
Drug choice and length of treatment for Argulus infections should take into consideration the life cycle of the parasite, which varies from 30 to 60 days depending on temperature and species. Treatment should target all life stages, including eggs, juveniles, and adults, both on the fish and in the environment. Adult parasites can be manually removed from the affected fish, but this is impractical in many situations and is an incomplete solution because eggs, unattached juveniles, and adults will still be present in the environment. Fish can be moved to a clean tank and treated with the appropriate drugs, while eggs in the original system are eliminated either by cleaning and disinfecting the tank or allowing it to dry completely. However, drying may be difficult in humid areas, and at cooler temperatures eggs can survive much longer time periods. Optimal water quality should be maintained for the duration of any treatments.
Several medications have historically been used for bath treatment of Argulus, but potential resistance to treatment, current availability, legality of use (especially in food fish species), dosage rates and associated costs, and fish species’ sensitivities may reduce options. It is best to work with a fish health specialist. There are currently no FDA-
Emamectin benzoate (SLICETM, Merck Animal Health) is an in-
Effective management of an Argulus outbreak must include proper identification of the parasite, treatment of all life stages on the fish and in the environment, and proper drug use. Drugs available for treatment of Argulus infestations are limited, so inclusion of a fish health professional in the diagnosis and management will help minimize the biological and economic impact of an Argulus outbreak.
Because of potential challenges with controlling this parasite, especially in food and pond fish, bio security measures should be instituted and followed to minimize introduction or transmission to other ponds, systems, or facilities.
Incoming fish, particularly wild-
Argulus infestations are not uncommon in wild or pond-
Within this category of “Fish Health” here on koisale.com we wish to thank Dr. Roy Yanong from the University of Florida IFAS Extension for his permission to utilize much of the information contained here-
I don’t have a problem with your using the information, and I think overall, you are on the right track and we do need to educate folks much better. My main concerns are:
Knowing when to seek help or When and How to Look for Fish Veterinarian
This is the website for the American Association of Fish Veterinarians (which several friends and I formed a few years ago, and now has over 130 fish vet members around the U.S.). We are still inputting data for the find-