Diagnostics and prevention of infectious and fungal diseases of commercial fish

Fish diseases in pond farms are caused by biotic factors (viruses, bacteria, algae, fungi, helminths, crustaceans) and abiotic environmental disturbances — from the impact of toxic substances to failures in the hydrochemical regime. An outbreak of infection or infestation leads to mass fish mortality and a decrease in the water body's productivity. To preserve the stock, the fish farmer must timely identify clinical signs of diseases and adhere to a complex of sanitary and preventive measures.

Infectious and fungal diseases: rubella and saprolegniosis

Rubella (aeromoniasis) is the most common and dangerous infectious disease of carp and common carp. In fish farming practice, three independent forms of pathology are distinguished according to the type of pathogen: bacterial aeromoniasis, bacterial pseudomoniasis, and spring viral disease. The disease manifests itself mainly in spring and the first half of summer. In the acute course, the fish exhibit dropsy, exophthalmia (pop-eye), scale protrusion, and hemorrhages on the body surface, while in the subacute course, these symptoms are supplemented by the formation of whitish ulcers with a reddish rim. In the second half of summer, rubella proceeds in a chronic form and is accompanied by ulcers all over the body and on the fins.

Improving the health of a fish farm affected by rubella requires radical measures. It is practically impossible to restore a water body in which sick fish remain. The only reliable method is to conduct a summer fallowing period.

1. Drain the pond and completely harvest the fish contained within it.
2. Send the fish for disposal or sell them through the retail network.
3. Subject the drained pond bed to thorough disinfection during the summer fallowing period.

Fungal diseases also cause serious damage to the stock. Saprolegniosis develops when lower fungi enter traumatic injuries on the body of fish kept in unfavorable conditions. Fungal hyphae penetrate muscle tissues and destroy them, covering the body surface with a coating similar to dirty cotton wool. To prevent saprolegniosis, fish injury is avoided, and malachite green, brilliant green, or common salt are applied.

A variety of saprolegniosis is Staff's disease, which affects carp fingerlings in the winter period. Fungi initially develop in the nasal cavities of the fish, then cover the entire skin of the head and can penetrate the brain tissue. In hatchery facilities, the main problem becomes saprolegniosis of eggs: the pathogen affects dead eggs and quickly spreads to live ones. Fungal growth in incubators is prevented by treating the water with ultraviolet lamps.

• Duration of branchiomycosis course — 2–8 weeks
• Risk group for Staff's disease — carp fingerlings in winter
• Risk group for GNE — two-year-old carp

Gill pathologies, SBDI and parasitic infestations

Gill disease of unknown etiology (GNE, or gill necrosis) is one of the most dangerous pathologies, causing fish death. The disease develops in early and mid-summer, mainly affecting two-year-old carp. The pathogen of gill necrosis has not yet been found, and the symptoms are similar to branchiomycosis. A frequent cause of an outbreak is a violation of the hydrochemical regime, primarily an increased content of ammonia nitrogen. For the prevention of GNE, favorable growing conditions are created in water bodies, quicklime, bleach, or calcium hypochlorite are used, and dead fish waste is removed in a timely manner.

Branchiomycosis is caused by a microscopic fungus that destroys gill tissue. Diseased fish refuse feed, gather at the banks, and take a vertical position. The disease manifests itself in the summer, lasts for 2–8 weeks, and leads to mass fish mortality.

Swim bladder inflammation of carp (SBDI) affects carp, common carp, and their hybrids of various ages. The cause of the disease has not been definitively established, but the view of its viral nature is the most justified. The infection is transmitted through contact between sick and healthy fish, as well as through water and soil. The disease is accompanied by damage to the walls of the swim bladder and often spreads to other organs; diseased fingerlings die during the wintering period. There are no specific drugs against SBDI, but with strict quarantine, fish eventually develop immunity, and the disease subsides.

Parasitic organisms are constantly present in natural water bodies. At low densities, they do not harm the fish, but if conditions are violated, their numbers increase, causing disease. Among cyprinids, the following infestations are the most common:

• Ichthyophthiriasis;
• Trichodiniasis;
• Costiasis;
• Apisomosis;
• Philometroides;
• Bothriocephalosis;
• Khawiosis.

All the listed parasitic diseases are successfully treated provided that a timely diagnosis is made.

Invasive pathologies of fish pose a serious threat to fish farms, leading to mass mortality of juveniles and a decrease in fish productivity. The causative agents of helminthiases and parasitic infestations develop with the participation of intermediate hosts — trash fish, small crustaceans, mollusks, and birds. This requires the implementation of a clear prevention system and regular diagnostics.

Triaenophorosis is caused by a cestode and mainly affects trout, perch, and cyprinid species. The pike acts as the primary carrier of the disease, so water bodies must be isolated from wild fish. During heavy infestation, fish lose weight significantly, the abdomen becomes swollen, and the mucous membranes turn pale. As a result of the invasion, there is mass mortality of fry and fingerlings.

Philometroides of carp is provoked by roundworms (nematodes). Female parasites reach 90–160 mm in length and inhabit the scale pockets of the head and pectoral fins. Male nematodes are much smaller and are localized in the area of the swim bladder. Externally, the females appear as sluggish pink-red rings. The disease is transmitted from infected farms during fish transport, and highly effective drugs are used for treatment.

Ichthyophthiriasis is considered a highly dangerous disease causing mass mortality of young stock in all fish species. Its causative agent is a ciliated infusoria, or "fish louse," which develops under the fish skin. The mature infusoria leaves the host, attaches to underwater objects, and forms a cyst. After division, daughter cells swim freely in the water for 2–3 days and attach themselves to the fish, severely weakening it.

Helminthic infections of the intestine and eyes are also widespread in ponds and natural water bodies. Bothriocephalosis is caused by tapeworms that infect the intestines of carp, bream, blue bream, grass carp, silver carp, roach, ide, barbel, and catfish. Diplostomiasis is developed by fluke larvae with the participation of the pond snail and fish-eating birds. With diplostomiasis, fish exhibit clouding of the lens, formation of a cataract, blindness, pop-eye, inflammation of the eye membranes, and impaired blood circulation.

• Length of philometroides nematode females — 90–160 mm
• Viability of infusoria daughter cells in water — 2–3 days
• Categories of nutritional diseases — 2 groups

Parasitic infusoria develop on the surface of the gills and body of all cultivated fish species, causing trichodiniasis, apisomosis, costiasis, scyphidiosis, and trichophryosis. The diagnosis is established based on the results of scraping microscopy. Drugs for treatment and prophylactic processing during transportation or transplanting are selected taking into account the condition of the fish and the environment:

• Malachite green;
• Brilliant green;
• Violet "K";
• Bright green;
• Potassium permanganate;
• Common salt;
• Formalin solution;
• Prevention of trash fish entering the water body.

Nutritional diseases of fish and their prevention

Nutritional diseases cause significant damage to fish farming, worsening growth rates and causing the death of fish of all ages. Pathologies develop due to errors in feeding or low quality of the feed base. Preventive measures and special observations allow for the timely prevention of the development of these diseases.

All disease conditions in this group are divided into two categories:

• Diseases associated with the use of compound feeds that are not balanced in terms of fat, protein, carbohydrate, mineral, and vitamin composition;
• Diseases that arose after the consumption of low-quality feed.