Fish Diseases and Management

4.2.1.Protozoan diseases

 Velvet/rust disease: the disease is caused by Oodinium sp. The infected fish exhibit Peppery coating, yellowish, clamped fins respiratory distress (breathing hard), yellow to light brown "dust" on body. The fish may show signs of irritation, shortage of breath. This disease is highly contagious and fatal. Treatment is with copper at 0.2 mg/l (0.2 ppm) to be repeated once in a few days if necessary. Acriflavin may be used instead at 0.2% solution (1 ml/l). As acriflavin can possibly sterilize fish and copper can lead to poisoning, the water should be gradually changed after the fish recovered from the disease. Salt treatment for a prolonged duration at 2 g/l can also be effective.

Costiasis or Ichthyobodiasis: is caused by Ichthyobodo sp., a flagellated protozoan ectoparasite found commonly in the fish skin commonly called costia. Poor water quality and other stresses (especially crowding) may allow this normally mutualistic parasite to reproduce rapidly and overwhelm the host. They may be attached to host tissue or swim free. It is responsive to treatment with formaldehyde and malachite green but tougher than most protozoa. Prolonged KMnO₄ immersion and salt water bath (for freshwater fishes) could be used fro control.

White spot disease /Ich disease: The disease is caused by a ciliate Ichthyophthirius multifilis and is characterised by the presence of small white spots on the skin or gills. Infected fish may show signs of irritation, flashing, weakness, loss of appetite, and decreased activity. Gills will be pale and very swollen. Diagnosis of "Ich" is easily confirmed by microscopic examination of skin and gills. The mature parasite is large, dark in colour (due to the thick cilia covering the entire cell), and has a horseshoe-shaped nucleus. Potassium permanganate can be applied at a concentration of 2 mg/l and three treatments are given 2 to 3 days apart. A short-term bath of 250 mg/l formalin for 30 to 60 minutes followed by a water change can also be effectively used to control l the infection.

Neon Tetra Disease: The disease is caused by Plistophora, A microsporidian sporozoan parasite. It attacks the musculature of the affected fish. Infected muscle contains numerous sporoblasts containing spores. Grossly infected muscles appear white or pale. Certain bacterial skin diseases will produce similar gross lesions. Sporozoan infections are usually unresponsive to treatment and diseased fish should be removed from the tank. High mortality is usually associated with this disease.

Guppy Killer Disease: Caused by a free-living or parasitic ciliated protozoan. The infection is common in crowded conditions and in water containing excessive organic debris. These pear-shaped protozoa may be present in very large numbers when the infestation is severe. The parasite is usually unaffected by parasiticides because of its ability to burrow deeply into skin of host which ultimately protects parasites from chemotherapeutics. Best method of control is prevention through sound husbandry practices.  Epistylis: A stalked ciliate, which is commonly found in freshwater containing a high organic load and tends to colonize bottom dwelling fishes. Lesions appear pale and white in colour and resemble a fungal disease. This infection could be easily treated with formaldehyde. This disease is usually not fatal in itself but may open the fish up to secondary bacterial disease.

Trichodina: A disc-shaped ciliate protozoan found on the skin and gills of many fish. Circular rows of denticles and a ciliary girdle give this parasite a unique radial symmetry. Not harmful when present in small numbers. Treatment include formalin bath, KMnO₄ prolonged immersion.

 Hole in the head disease (Head and Lateral Line Disease): The disease is caused by Hexamita (Spironucleus), a flagellated protozoan, which may cause severe gastrointestinal disease if present in large numbers. It is a normal inhabitant of fish digestive tract. As an ectoparasite it is common to oscars and other cichlids. Physical signs of hexamitiasis include weight loss, decreased activity and refusal of food. Angel fish which are severely infected with hexamita may lie horizontally on the surface of the water with the abdomen visibly distended. The flagellates move rapidly and erratically and are easily seen in areas where the mucosa is broken. The infection is treated effectively with metronidazole (Flagyl) 25 ppm in water every other day for 3 days. Metronidazole can be administered orally at a dosage of 50 mg/kg body weight (or 10 mg/gm food) for 5 consecutive days. The medication can also be mixed with dry food using fish oil as a binding agent.

 Chilodonella: A ciliated protozoan which can cause high mortality. The parasite attacks skin and gills. Easily identified microscopically by its heart-shaped structure and slow circular motion when not crawling on the surface of the fish. Once diagnosed, this problem is easily treated with formaldehyde, malachite green or salt.