Argulus

Argulus (Branchiura) parasites are found predominantly on the skin and gills of freshwater fish. They are commonly called “Fish Lice”. Branchiura are a class or subclass of crustaceans (with segmented, chitin-encased body and articulated appendages) that are strongly dorso-ventrally compressed (flattened), oval-shaped, with strongly reduced abdomen and with few thoracic appendages. All branchiurans belong to a single family, the Argulidae. All argulids are ectoparasitic on marine or freshwater fish. Although parasitic, many species retain mobility and can swim or crawl free from their host. Attachment to the host is via maxillules developed as ventral suckers.

Argulus contains about 129 valid species and occurs in marine, estuarine, and freshwater habitats. There are only a few records of them occurring on Australian freshwater fish. However, Australian species of Argulus have been reported from aquaria. Rainbowfishes would be expected to be susceptible to infection.

Adult Argulus are visible to the naked eye. They are flattened creatures about 5-10 mm in length with a very distinctive shape and appearance. The flat, shield-like part of the body is the carapace and it is fused with the head and partially covers the thorax. Two movable compound eyes are present on the head region and are usually conspicuous. The abdomen is the tail-like part hanging off the back end. They have four pairs of swimming legs. They move about the skin of a fish very effectively and camouflage themselves well on the host. Argulus adults can survive about two weeks without a fish host. However, parasitic larva must find a host within the first 2-3 days after hatching otherwise they will perish.

Argulus may lay several hundred tiny eggs in clusters, or more typically in two or more parallel rows on a suitable surface (e.g., plant stems, stones or glass sides of aquaria), and the eggs hatch anywhere from two weeks to a month later. Argulus breed several times during the year. Development time of egg stages is heavily dependent upon the temperature of the surrounding water with development being more rapid at higher temperatures. Depending on temperature, the total life cycle may take between 6 and 20 weeks. Argulus foliaceus eggs for example hatch after just 8 days at 26°C or after several months for eggs deposited at temperatures below 10°C. This could be a strategy that evolved to enable parasites in temperate regions to survive harsh winter conditions as an egg stage and to take maximum advantage of favourable summer conditions to increase and/or maintain population sizes. Certainly overwintering egg stages have been reported for several species of this genus.

Argulus attach to fishes with the aid of two suction cup-like appendages (their first maxillae) and abundant little spines located over much of their underside. They have a long, slender pre-oral sting that they use to pierce the fishes skin and inject a toxin (or anticoagulant), then they use their mouth and mandibles to consume the blood, mucous, tissue at the puncture site. They can be damaging to fishes mainly due to fungal infections that attack at the puncture sites made by the Argulus, and they also can transmit fish viral diseases and other fish parasites.

Heavy infections may cause mass mortalities, especially in young fish. Local necrosis and ulceration may occur, serving as a site for secondary infection. Affected fish will initially attempt to dislodge the parasites by rubbing on tank sides or bottom. Eventually infected fish will stop feeding and become lethargic. Haemorrhagic spots are seen at the attachment/ feeding sites and scale loss and fraying of fins may also occur.

Much of the literature recognises that Argulosis, like many other diseases, is best defended against by good fish husbandry and stock management. Quarantining fish is vital as individual fish can then be examined for the presence of parasites and any encountered can be carefully removed using forceps. Care must be taken to examine buccal and gill cavities also because on occasion Argulus individuals have been found there.

There are numerous options available for the control, prevention and treatment of Argulus infections. Many traditional treatment methods rely on the use of chemicals such as malachite green or organophosphates, potassium permanganate (2-5 mg/L, bath) or diflubenzuron (Dimilin®) (0.01 mg/L, bath). Usually 2-3 doses at one week intervals are needed to treat the emerging larvae and juveniles. It is recommend to disinfect the aquarium and equipment to completely remove eggs. Two applications at weekly intervals of trichlorfon (0.25 mg/L at temperatures below 27°C, or with 0.50 mg/L above 27°C) added to the water is reported to be effective at removing all life stages. A bath repeated twice a week was found to be effective. Proprietary products such as Paragone® can be added to the tanks or ponds at the dose rate on the label.