Disease of Freshwater Fishes

DISEASE OF FRESHWATER FISHES There are a large number of diseases of fishes, many of which, happily, will never be seen by the aquarist. For many fish diseases neither cause nor cure is known, and the aquarist is warned that it is easy to spend more money buying cures for a fish than the fish is worth and that after some ‘cures’ the fish is worse off than before the treatment. The classification of disease in fishes falls into two groups: congenital and acquired. CONGENITAL DISEASES These are diseases with which a fish is born, some of them being genetic in origin — missing gill plates and fins, for example — and are capable of being transmitted to its offspring; others are due to environmental influences during the incubation period of the egg. ACQUIRED DISEASES Trauma Fishes can be seriously injured by other fishes or by rough handling when netted. Excessive heat or cold can also damage or kill a fish, and water conditions are very important; excessive acidity or alkalinity or rapid changes in water conditions can make fishes ill or kill them. Infections These are very important in the fish tank as the disease can spread and every fish in the tank become infected. Many infections with viruses or bacteria are not recognized but two protozoal infections — White Spot and Velvet disease — are very characteristic and very common. WHITE SPOT DISEASE is an infection with Ichthyophthirius multifilius and the disease is always transmitted from an infected fish or an infected piece of equipment. Diagnosis is difficult in the early stages when the infected fish may have only one white spot, but such a fish can transmit the disease to every other fish in the tank. The white spot is a cystic stage of the parasite and on maturity it leaves the host and falls to the bottom of the tank. Multiplication takes place within this cyst and after a few days many daughter cells are present. The cyst then bursts, freeing the daughter cells which are free living and each seeks a new host and then burrows under its skin, feeding off the skin and mucus. When mature the parasite forms a cyst and the cycle is repeated. Once the White Spot parasite has left a fish it must find a new host within seven days or it will die. The infected fish appears to have an itch and scratches against rocks. Then the white pin-point spots gradually develop; these are very few at first and are often seen best in the fins. If untreated they become very numerous and can kill the fish. Raise the temperature to 82°F (27°C); this speeds up the development of the cysts into the free-living form which can be easily killed as it is very susceptible to methylene blue. Add a sufficient quantity of one per cent solution of methylene blue to the water to make the tank a deep blue colour (about 1 ml/gallon repeated as colour fades until all spots are gone). Methylene blue kills many species of plants so put the fishes in an unplanted tank for this treatment. The planted tank with all its fishes removed should be left empty for two weeks, by which time any parasites will have died from the lack of a host. VELVET is another infectious disease caused by a protozoan parasite. Oodinium limneticum attaches itself to a fish and sends rhizoids between the skin cells of its host through which it extracts tissue juices. The parasite grows for about seven days and can be seen as a pin- point golden spot. In mild infections, with just one or two parasites, it is very easily overlooked. The parasite matures, withdraws its rhizoids, drops off and forms a cyst within which the parasite divides producing 256 daughter cells. These then mature and escape from the cyst and are free swimming by means of two flagellae. Each ‘swarmer’ then swims about until it finds a fish— it can encyst again if it does not find one, probably remaining viable for some time. This parasite contains chlorophyll and can probably live a short time in well-lit tanks without a host. Very often fishes do not appear ill until the infection is very heavy. The parasites are seen on dark- coloured fishes as yellowish in colour, ‘dusted with sulphur’, but in brightly coloured fishes they appear white. This parasite can kill; particularly heavy losses are caused in young fry. A one per cent solution of methylene blue (about 2 ml/gallon repeated as colour fades) kills the ‘swarmer’ stage of the parasite. At higher temperatures the life cycle is speeded up so raise the temperature to 80°F (27°C). Good proprietary cures are also available.

Disease of Freshwater Fishes

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