Parasites in Fish
Large Parasites
Only a few parasites visible to the unaided eye ordinarily attack fishes. These include fish lice (Argulus), leeches, anchor worms (Lernaea), and grub worms (Clinostomum). Most of these parasites will be found on pond fish or aquarium fish recently brought in from ponds. They can also be introduced on plants, pond water or poorly cleaned live food. In nature these parasites are rarely serious causes of illness but in confined quarters they can overpower the fish.
The fish louse is about 1/8 to 1/4 inch long and is an oval, clear, blister-like animal when seen on the side of a fish. The fish’s skin under the parasite will frequently show a blood spot. The lice may occasionally be seen swimming freely in the water. Leeches found in the aquarium may attack fish. They measure an inch or more when mature, but are very flexible and can stretch remarkably.
Anchor worms enter the fish’s body as a microscopic form, and only weeks or months later when they protrude from the skin is the hobbyist aware of their presence. They form a blister on the surface of the skin with the worm protruding from it.
Grub worms are the immature stage of a larger fluke and cannot multiply in the aquarium. They are approximately 1/8 to 1/6 of an inch long. They may protrude from the skin in a blister with a white center.
The treatment of infestations with all of these is similar and simply involves mechanical removal of the parasite. In the case of the fish louse and leeches, the organism can be simply picked off with a tweezers. The point of attachment on the fish can be treated with a drop of mercurochrome to help prevent secondary infection with bacteria or fungus. The anchor worm is fastened very strongly and may be difficult to remove without injuring the fish. The fish can be held in a moist net and a drop of concentrated salt solution should be placed directly on the parasite. Then tweezers can be used and will require considerable tension to remove the worm. A large fish may be injured in this process and smaller fish may be killed. Grub worms may be removed similarly by opening the skin cyst containing the parasite and removing it with a needle or tweezers. Mercurochrome may also be used on the wounds resulting from the anchor worm and grub worm.
Small Parasites
Two flukes are common parasites of aquarium fishes. These are Dactylogyrus (gill flukes) and Gyrodactylus (skin flukes). Both organisms are directly transmitted from one fish to the other and the rate of transmission is partially dependent on crowding. The affected fish will have a shimmying behavior and will be seen scratching themselves against gravel, plants, etc. A strong magnifying glass will show the slender skin flukes on the surface of the fish. Gill flukes usually cannot be seen; at times they may be visible at the edge of the gill cover. The skin and gills will develop increased amounts of mucus with a slimy appearance and may show small blood spots. A formalin dip is effective in treatment of individual fish before they are introduced into the aquarium.
A contaminated tank however cannot be effectively treated with formalin. Certain precautions must be taken. The hobbyist must make sure the sponge has no soap in it. Various brands may have other toxic elements in them and the sponge should preferably be tested on a less expensive fish in an isolated tank. The pH in the tank should be checked with an accurate pH kit and should be maintained at a slightly alkaline level during the treatment (pH 7.2 to 7.4). The treated fish should be observed frequently and about half the water changed if the fish shows signs of distress. Use one standard size copper sponge for a ten-gallon tank. The copper may be removed after the fish’s symptoms disappear. Copper pennies are also effective. Use twenty pennies per gallon of water. Remove them in twenty-four hours. This treatment is also effective against a number of other parasitic organisms and will be mentioned later.
Humans also are subject to superficial skin infections with flukes. Swimmers in the Great Lakes area are familiar with the “Swimmer’s Itch” which is due to a short lived infestation of the skin with larval forms of the fluke which does not normally parasitize human beings. The infection is self limited but annoying. Fish flukes in aquariums do not attack humans.
