The importance of environmental stress on the outcome of the encounter between a pathogen and a susceptible host cannot be overemphasized. Fish kept under intensive conditions are constantly exposed to a wide range of stressors and the fish will attempt to adjust physiologically. However, any stressor hat exceeds the fish’s ability to adapt may be lethal or will facilitate the infection by opportunistic pathogens or parasites which may be present in the water.
In large bodies of water such as earth ponds,raceway or in the wild , environmental conditions are normally very stable. If these conditions change, the fish can move away to areas where the stressors are reduced. But , in small bodies of water, such as in the case of an hatcheries or production ponds or high stocking density tanks, environmental conditions may change rapidly, with no avenue of escape for the fish. In addition to this, many man-made activities such as handling ,water changing or draining and feeding are brought to bear on the fish. This stressful situation may be sufficient to trigger off a disease or parasitic infection in the population and which may result in mortality.
High stocking rates of fish in a small tanks results in increased levels of metabolic wastes from the uneaten food, excretal and others and this leads to decreased levels of dissolved oxygen.
Moreover, highly stocked ponds or tanks also greatly facilitates successful transmission of a pathogen from one fish to another by increasing the possibilities of an infected fish coming into contact with a susceptible one.
The most important diseases, infections and parasites of African catfish under culture conditions are bacterial diseases and fungal infections (particularly during the early life history) and various internal and external parasites. Viral diseases have not yet become a problem in catfish culture but as the industry grows it is highly likely that viral infections will manifest themselves and this is evident from developed countries experience in culture of related fishes such as channel catfish.
Bacterial diseases :
They are found everwheere and are said to feed on dead organic substances. Factor which can promotes bacterial diseases includes overstocking, presence of dead organic matter in the fish pond,low dissolved oxygen, and wound on the body of fish
Symptoms of bacterial diseases includes darkening of fish body, slowness, sluggish movement in the water bodies,fish clustering at water inlets or close water inlets, red/brown spots on the body of the fish,visible red streak at the base of the fins and swelling of the abdomen with protrusions.
Bacterial diseases can be prevented or control by proper feeding, hygienic water environment, removal on injured/dead fishes, proper records, avoids excessive algae blooms.
One of the most characteristic signs of a bacterial infection is an exponential increase in the mortality rate. As soon as the farmer notices an increase in the mortality rate, he should seek help. A sudden mass mortality on the other hand is normally caused by environmental factors such as a sudden drop in temperature, clogged water inflows or toxic substances in the water. The pre-disposing factors which results in these bacteria becoming problematic is stress, caused by poor water quality, poor nutrition and handling The most troublesome bacteria in African catfish are facultative and opportunistic Aeromonas hydrophila and Pseudomonas spp. Myxobacteria, causing fin rot in fish has also been reported in African catfish.
TREATMENT OF BACTERIAL DISEASES
African Catfish are important fish breeds and have high commercial values, to achieve success in catfish farming business the understanding of the potential danger occasioned by diseases and parasites are worthy considerations. The article is meant to serve as a guides to catfish diseases and control. It is a known fact that very little is known about diseases and parasites of African catfish. This is attributed to the fact that the animal is very hardy, showing a greater resistance to diseases and parasites than other fish such as tilapia (perhaps this is one of the reasons why the African catfish is such a successful species throughout Africa and the middle East and in areas to which it has been introduced in the whole world), or it might be a consequence of the low level intensity of African catfish farming.
It must be stated that catfish like any other fish species can be kept healthy without the need for medication if all condition necessary for growth are kept at optimal level, in other words if conditions such as bad handling, overcrowding, poor water quality, unsuitable food, unhygienic environments, and other adverse situation are put under control.
Most infection are opportunistic in nature, they have access to the fish when they are stressed and the body immunity have been impaired. So a balance in the fish body, environment result in good health and anything otherwise leads to diseases conditions.
Diseases are categorized into pathogenic groups bacteria, fungi, parasites, virus and other non pathogenic diseases which maybe nutritionally induced or environmentally caused.
This may be the result of injuries sustained in the handling process, which are then secondarily infected by fungi. Fungi are difficult to treats and as such hygienic hatchery practices, reduction of environment stress on the fish must be avoided and finally activities that can result in injury to the fish must be minimized .
Ubiquitous opportunistic fungi of the genus Saprolegnia are commonly found in focal infections on the skin of large catfish maintained in aquaria. In many cases Saprolegnia appears shortly after collection and transport of catfish. Eggs and larvae of catfish are highly susceptible to fungal infections, which are also notoriously difficult to treat effectively. Saprolegnia is the most common fungus affecting catfish, particularly developing eggs, larvae and juveniles.
Fungus can be treated with zinc free malachite green or formalin, Methylene Blue or a cocktail of malachite green and formalin. Treatment: Malachite green oxalate: Eggs: 500 ppm but no longer than 10 seconds or 0.1 ppm for 1 hour. Larvae and juveniles: In tanks 0.15 ppm for 1 hour or 0.2 ppm in small ponds for 1 hour. Formalin: Eggs, larvae and juveniles 10-15 ppm for 1 hour Methylene Blue: Use at concentration of 3 – 5 ppm for eggs, larvae and juveniles. Formalin / Malachite Green cocktail: In tanks 0.5 ppm Malachite Green and 100 ppm Formalin for 10 minutes. Sodium chloride: Can also be used at 2% but is not very effective. (Some of this products have been banned for usage on fish in some countries especially US)
VIRAL INFECTIONS IN CATFISH FARMS
The first issue about viral infection is that they do not have a cure , but prevention and control is the ultimate objectives of a successful farmer. Very little is known about viral diseases in African catfish. Two syndromes of unknown etiology was recorded namely : 1. Ruptured Intestine Syndrome 2. Broken Head Disease The Ruptured Intestine Syndrome which occurs mainly during the fingerling stage (3 – 5 g) concerns a rupture in the caudal part of the intestine. Broken Head Disease concerns the destruction of the arborescent organs (airbreathing organs), which leads to inflammation of the skull resulting in a lateral skull break. A similar syndrome has also been reported for Clarias batrachus and Clarias macrocephalus in Asia . The lack of information on viral and bacterial diseases can probably be correlated to the age and extent of African catfish husbandry on the continent and elsewhere. At best, excluding the early research which was undertaken on African catfish culture in the 1960’s in Egypt, the species has been farmed on a commercial and small scale basis for not longer than 15 to 20 years. In comparison to the trout farming industry which was started in Denmark in 1890 the African catfish industry is still in its infancy. We predict that more diseases will become manifest as the industry ages and becomes more intensive. It has been found that Channel Catfish Virus is capable of infecting cells of C. batrachus and there is no reason why this virus could not also infect C. gariepinus. Channel catfish have been introduced into west Africa and this disease constitutes a potential risk to African catfish species, if introduced with its American host.
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