Fish lice are not insects, as their infamous name suggests, nor are they related to the lice that infest humans and other domestic animals. Argulids are branchiuran crustaceans, related more closely to shrimp and crabs, than to the head louse. Like other crustaceans, argulids must molt or shed their shell periodically to grow and mature.
Argulids are common parasites of ornamental fish, particularly koi and goldfish, and are also found on wild fish, such as largemouth bass. Approximately 100 species are recognized, but most show little host specificity. The majority occur in freshwater environments, although marine species exist.
Fish lice are primarily a problem in outdoor ponds, and are brought into aquariums on wild or pond-raised fish. Argulids are extremely irritating and infested fish rub on submerged objects or bottom substrates in an attempt to dislodge the parasite from their skin – affected fish dart to the bottom of a tank and roll onto their side, briefly revealing their lightly colored underbelly, as they attempt to rub off the parasite. This maneuver, seen with many external parasitic infestations, is referred to as "flashing."
The feeding activities of argulids are highly damaging to fish. The fish louse possesses a long stylet that it uses like a sewing machine needle to inject an enzyme into the fish that actually digests tissue. The parasite then sucks up its predigested meal through its straw-like mouthparts. Heavy infestations typically result in gradual loss of physical condition and weakening of the host to such an extent that it becomes susceptible to secondary bacterial infections, particularly at the site of attachment. Argulids are also capable of transmitting certain viruses, bacteria, and blood parasites as they feed.
The fish louse is visible to the unaided eye, and has a broad, lat,oval-shaped body, with a shell and four pairs of swimming legs. They bear some resemblance to a small pale green or brown crab. Argulids attach by a pair of hooks and two large anterior suckers, but remain free-swimming throughout their life. The parasite may periodically release itself from one fish and swim to another, but cannot live for extended periods off a fish host on which it must feed.
The adult female leaves the fish host to lay her eggs on submerged objects or vegetation. Eggs hatch in an advanced form known as a second stage copepodid and must find a host within a few days or they will die. After a series of molts the adult stage is reached and the cycle begins again. The entire life cycle takes 40 to 100 days depending on the water temperature. Outbreaks occur from spring through fall at moderate to warm water temperatures, but development is arrested below about 60 degrees F.
A diagnosis of argulid infestation is based upon identification of the parasite attached to a fish's skin. Brief saltwater dips, or other chemical baths such as formalin, dislodge the parasite from individual fish, which should then be transferred to a clean tank or pond. Infestations in ponds or large tanks are best treated chemically. Multiple treatments may be needed to eliminate all stages of the parasite. Organophosphate insecticides (trichlorfon) have been used in the past, but these products have come under scrutiny by government regulatory agencies and may no longer be available. Resistance to the organophosphates by the parasite may also develop with repeated exposure.
Chitin synthesis inhibitors (diflubenzuron and lufenuron) prevent a new shell from developing after the crustacean molts and it dies. Some of these agents are found in certain products used to treat fleas on dogs and can be prescribed by your veterinarian.
Antibiotic therapy may also be recommended to prevent bacterial infections at attachment sites.
Argulids are extremely susceptible to drying. Draining, cleaning and
allowing a pond to dry thoroughly for several days should eliminate the
parasite. If this is not practical and chemical treatments are necessary, read and follow your veterinarian's instructions carefully, and apply all medications with caution. Assure the fish is eating and provide a low stress environment by maintaining good water quality.
The keys to prevention are avoidance and quarantine. When purchasing new fish, avoid argulids by inspecting fish closely for the presence of the parasite. Also, remember that eggs are laid on vegetation and other substrates and can be introduced into a pond or aquarium on plants, rocks, or other materials. Argulids have also been known to hitch a ride on amphibians and the feathers of wading birds.
Try to limit the access of amphibians and birds to your pond. Always quarantine new fish for at least one month. This period of time will usually allow any disease signs to manifest before they have a chance to spread to your established fish population. New plants should also be quarantined, which will allow time for argulid eggs to hatch and juveniles to die before they can find a host.