Viral, bacterial and fungal diseases of Siluroidei, cultured for human consurnption

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INTRODUCTION stressors (Snieszko, 1974;MacMillan and Tucker, 1985;Hanson and Grizzle, 1985;Plumb and Olah, Fish cultured for consurnption often suffer diseases, 1984;Fedoruk, 1981). Examples of such stressors bath from infectious and non-infectious ohgins might be (1) husbandry factors like crowding (Ventura (Durborow et al., 1991). Most attention is paid to and Grizzle, 1987), capture and hauling, (2) waterinfectious pathology, but it is questionable if this related problems like toxicants (Faisal et al., 1988; is justified because many diseases have a patho- Tucker et al., 1984), temperature and oxygen extremes physiological background related to environmental (Plumb and Olah, 1984;Walters and Plumb, 1980), and rapid environmental changes (Ciembor et ul., 1995), (3) social interactions (Wise et al., 1993), and (4) transport (Blazer, 1992). Stress results in a higher susceptibility for diseases, especially infectious ones (Ellis, 1981). The prevalence of such diseases depends on the interaction between fish, pathogen and environment (Snieszko, 1974). In infectious fish diseases carrier state animals can play an important role. Carrier state fish are asymptomatic animals which harbour the pathogens without any clinical symptoms. They start spreading the pathogen mostly under unpredictable circumstances, which can result in a sudden onset of a disease in an obviously healthy population. Carrier state fish are difficult to detect because usually they are present in a low number in a population, thereby hiding the pathogen and hampering its isolation. Also in diseases of Siluriformes carrier state fish can play a significant role (Klesius, 1992b;Mqolomba and Plumb, 1992).
There are fish diseases of which the etiology is not exactly known (Boon et al., 1987 b). Micro-organisms might be involved in these respective disease processes. However, nearly always, opportunistic micro-organisms are found in these (chronically) diseased fish. In some cases, it is not (yet) possible to experimentally induce the pathology of such diseases, and consequently the relationship between disease and micro-organism(s) have still to be proven.
Only one bacterial pathogen has a specific pathogenicity for one of the Siluriformes: Edwardsiella ictuluri (Blazer et al., 1985). However, Morand (unpublished results) isolated E. tarda from European catfish suffering a septicaemia with haemorrhagic barbels and fins.  (Farkas and Olah, 1984). The latter bacteria does not seem of significant economical impact for the catfish culture.
As usual in fish culture, fungal infections in catfish species are nearly al1 opportunistic infections after primary lesions.

Treatment of infectious diseases
Although the best treatment of diseases is an effective prevention, treatment by drugs is regularly needed. Such treatment has to be supported by environmental optimization. If the use of drugs is not effective, like in viral diseases, other methods have been used e,g. isolation andlor eradication of infected populations. Large enough fish can be slaughtered for consumption if no hazards for consumers exist. For a proper treatment of bacterial fish diseases drugs must be available. Unfortunately, knowledge about correct use of antimicrobial drugs in fish culture is scarce. Research to support the registration of promising therapeutic agents is urgently needed (Meyer, 1991). Nearly no drug dose levels used in fish culture practice are based on scientific research. However, in the USA and The Netherlands pharmacokinetic studies of different dmgs have been done for channel-(Ictalurus punctatus) and African catfish (Clarias gariepinus) respectively (Stehly and Plakas, 1993;Van der Heijden et al., 1994). Beside pharmacokinetic characteristics, results of drug sensitivity tests of the respective bacterial strains are needed for a proper treatment of bacterial infections. Large differences exist between countries concerning the legal possibilities of drug use. For instance in Canada, only one antibiotic (Oxytetracycline) is registered and approved for use in fish medicine, while in the Federal Republic of Germany only Trimethroprim/Sulphadiazine is fully licensed (Bernoth, 1992). ln contrast, in The Netherlands no drug is approved for use in fish medicine. Therefore, in the latter country drug use in fish disease trcatment is always off-label, with ail the risks for the veterinarian who prescribed the drug. Because of unscientific and illegal use of chemotherapeutants drug-resistant bacteria have been developcd in many parts of the world. In this context Depaola (1995) reported tetracycline resistance by bacteria in response to oxytetracycline-contaminated catfish feed. To achieve really effective treatments more attention has to be paid to (1) proper diagnosis, (2) "preventive treatment", (3) treatment period, and (4) dosage rates for effective disease control (Wellborn and Schwedler, 1980).

Prevention of infectious diseases
Prevention of outbreaks of infectious diseases have to be supported by avoiding unnecessary stress. When this is not possible, stress will induce infectious disease outbreaks if the pathogen is present in the fish population. Mostly these diseases are treated with antimicrobial drugs. Because treatment of infections with drugs may lead to (1) immunosuppression (Van der Heijden et al., 1992) and ( 2 ) development of bacterial resistance, other techniques to attack fish diseases have 10 be applied. The latter can be supported by using high feed quality. Quantity and quality of food and feed are determining factors for fish health (Fracalossi and Lovell, 1994) as measured by factors indicating specific (Henken et al., 1987) and a-specific resistance (Schippers et al., 1994). For channel catfish it has been proven that fish health is influenced by the diet. Vitamin C and dietary lipid can manipulate disease resistance (Duncan and Lovell, 1994;Sheldon and Blazer, 1991). Immunomodulators, such as glucan, can be helpful too in potentiating the immune system (Chen and Ainsworth, 1992).
Fish can be immunized using dead or alive microorganisms, or different kinds of treated antigens from the micro-organisms. For administration of vaccines different methods are used (1) injection, (2) orally by feed, (3) immersion and (4) spray (Newman, 1993). The method used depends on fish age and fish value, as well as on the fish farmers technical knowledge. However, it must be emphasized that health management problems should not be masked by using indiscriminately vaccination strategies. Such problems must be solved first, whereafter vaccination might not be needed any more.
Moreover it is stressed that the key for disease control in tropical countries, where drug and vaccine use is generally limited and impractical, lies in the prevention of stress conditions through adequate farm management practices (Fedoruk,198 1).

Viral diseases
Three pathogenic viruses for Siluriformes species have been isolated so far: one herpes virus and two irido viruses.
The herpes virus is pathogenic for I. punctutus (Wolf, 1988). The two irido viruses arc related to the amphibian virus frog virus-3 and the Epizootic Haematopoietic Necrosis Virus (EHNV) of redfin perch and rainbow trout, and are pathogenic for S. glanis and I. melas, respectively (Hetrick and Hedrick, 1993).

Channel catjïsh virus disease
Virus -Channel catfish virus disease (CCVD) is caused by an acute infection of young-of-the year I. punctatus by the Ictalurid Herpesvirus 1 (CCV), one out of a group of at least 17 distinct viral agents isolated from or observed in fish (Hetrick and Hedrick, 1993). The enveloped virus particles are between 175-200 nm, ether sensitive, loose infectivity in glycerol, do not haemagglutinate and require the envelope for infectivity (Wolf, 1988). The virus survived for only two days at 25OC in pond water while at 40°C the virus can survive one month under the same conditions (Plumb, 1988). Ce11 cultures mostly used for CCV are the ictalurid lines BB and CCO. Al1 cultures show syncytium development, followed by pyknosis and lysis (Wolf, 1988). The optimal temperature is between 25-30°C.
Clinical symptoms -The onset of the disease is sudden and characterized by haemorrhages. Often the disease results in a high mortality (Hetrick and Hedrick, 1993). Clinical signs are not specific. Beside decreased appetite, mortality, changed behaviour (head-high andlor hanging posture and convulsive swimming in spirals) and haemorrhages, a swollen abdomen and exophthalmia were obsemed in clinical CCV infections (Wolf, 1988).
Pathology -At post mortem examination an intraperitoneal yellowish/reddish fluid is found. The visceral mass seems to be anaemic and the intestinal tract is only filled with yellowish mucoid material (Wolf, 1988). Histologically, CCVD is characterized by severe haemorrhagics with generally edema and marked necrotic changes in kidneys, liver, gastro-intestinal tract, spleen, musculature, neural and pancreatic tissue (Plumb et al., 1974). Occasionally, intracytoplasmatic eosinophilic inclusions will be obsemed in the liver cells (Major et al., 1975).
Diagnosis -CCVD diagnosis can be confirmed by viral isolation and identification andfor serologically. Isolation is easy from victims or fingerlings taken during a CCVD outbreak or at the most a few days afterwards. A positive culture is characterized by syncytia. Identification is obtaincd by a serum neutralization test or a fluoresccnt antibody technique.
Carrier state animals can be a problem because of latent infections in survivors. l~eucocytes may be a reservoir in those ti$h (Chinchar et al., 1993).
Treutment -Since CCVD is a viral discase no trcatment is availablc. Only in case of secondary infections by opportunistic bactcria, these can bc treated by antimicrobial drugs.
Most extrcme treatinent is total destruction of the inrccted and diseascd population. Thcreafter, prccautions have to be taken with respect to obtain a CCV free population, for instancc selection of CCV free channel catfish farms for the providing of CCV free fry.
Prevention -The most effective prcvention is avoiding the infection by buying CCV free fry or fingerlings frorn channel catfish farms with CCV-free broodstock. Therefore, certification of such farms can be useful in the prevention of CCVD. Furthermorc, use of more resistant hybrids is possible in en~ootic areas (Plumb and Chappel, 1978).
Vaccination seeins to become of great importance (Plumb, 1988) if the problerns with carrier state fish and related certification difticultics (Leong and Frycr, 1993) have been solved.

Catjish Iridovirus diseasr
Krus -Catfish iridovirus has been isolatcd for the first time frorn young yellow bullhcad (I. melas) undergoing an acute haemorrhagic syndrome. Virions of 150-160 nm have only be found in cytoplasm and has a shape identical to EHNV. They are susceptible to chloroform and 5-iodo-2-desoxyuridirie (Pozet et al., 1992), the latter indicating that the virus contain lipids in the envelope and a DNA genome.
There is a similarity with other iridoviruscs isolated from sheatfish and rcdfin perch. This suggests that the host range is greater than the species froin which the virus is isolated (Hedrick et al., 1992).
Clinical symptoms -The disease is characterized by a high mortality (up to 100%) and haemorrhages in the pectoral and pelvic girdles and in the fins (Pozet et al., 1992).
Pathology -Post mortem examination reveals haemorrhages in the viscera and in the abdominal cavity. Histopathologically, degenerativc changes like acute necrosis of the haematopoictic tissues (kidney and spleen) and edema of the Iiver can be observed ( P o~e t et al., 1992).
Diagnosi.~ -The virus can be detected by culture on EPC monolaycrs. lmnzunitj, treatrnent and prevention -Concerning immunity, trcatment and prevention no reports havc been published so far. Gcneral rules with respect to viral fish discascs might be takcn into account.
Cliilicxd symptom.s -Sheatfish virus disease is a systemic one with a high mortality (up to 100 %) and evokes spiralic swimming and pctechial haemorrhages in the skin and, occajionally, in the eycs and barbels .
Pathology -Post mortem examination reveals necrosis of thc haematopoietic tissues (kidney and spleen) without a marked host inflammatory response. This is also the case for EHNV in redfin perch (Langdon and Humphrey, 1987). Di~cgnosis -The virus can be isolated from sheatfish using monolayers of scveral fish cell lincs (BF-2, RTG-2 and FHM) at temperatures bctwcen 20-30°C. Cytopathogcnic effect is charactcrized by ceIl rounding, prcsence of cytoplasmatic inclusions and eventual lysis.

Fish-species-specific: Diseases of channel catfish
Edward.riella ic.taluri diseuse Bacteria -E ictaluri is a gram-ncgative rod that survives in catfish and pond bottom mud (Newton et al., 1989). Infection with the micro-organism can cause an acute or chronic discase. The bacteria measuring 0.75 x 2.5 /Lm at 26°C or 5 to 7 /Lm at 37'C (Plumb and Vinitnantharat, 1989) is weakly motile with peritrichous flagella and cytochrome oxidase negativc. Optimum growth temperature is bctwcen 25 and 30°C. On Hrain-Heart Infusion (HHI) agar, colonies will bc 2 mm in diameter and smooth, circular and slightly convex (Hawkc, 1979). There seems to be a high homogencity between the different strains according to the study of Newton and Triche (1993) who characterized the lipopolysaccharides of the outer Aquat. Liviri: Hcv~ur.. Vol. 9. Hors sbrie -1996 membrane of 40 E. ictuluri strains isolated in the USA.
Clinical symptoms -E. ictuluri disease of channel catfish manifests itself in two clinical entities e.g.
(1) an acute gastro-intestinal septicemia followed by mortality, and (2) a chronic disease characterized by a "hole-in-the-head" lesion which may procced to septicemia too (Shotts and Blazer, 1986). The mortality is temperature depcndent; 25°C seems to be the optimal temperature for mortality of channel catfish after exposure to E. ictaluri (Baxa et al., 1992). Diseased fish are listless, hanging at the surface tail down, horizontally spiralling, rapid spinning or display other erratic swimming behaviour (Blazer et al., 1985;Hawke, 1979). On the skin, white spots and circular slightly-raised epithelial lesions can be observed over the entire body. Petcchial haemorrhages are found around the mouth, at the fin base and on the ventral and lateral body sides, as well a swollen abdomen, exophthalmia, and pale swollen gills. In-chronic diseased fish an open lesion is found caudomedially on the frontal bones of the skull (Blazer et ul., 1985;Hawke, 1979).
Furthermore, E. ictaluri produces chondroitinase which may play a role in the pathophysiology of the chronic disease inducing cartilage degradation (Waltman et al., 1986).
Diagnosis -Beside routine culture procedures on BHI agar, an ELISA for detecting serum antibodies was developed by Klesius and CO-workers (1991) which is a reliable tool for diagnosing and monitoring of E. ictaluri infections.
Immunity -Despite the general belief among animal production scientists that there is no relationship between specific plasma antibody levels and protection against the respective specific infections it can be calculated from the data given by Klesius (1992 a ) that in channel catfish a significant relationship between antibody level against E, ictaluri and protection against the related disease exists ( r = 0.70, y < 0.01). This was also observed by Vinitnantharat and Plumb (1993) and it agrees with Klesius's statement that acquired immunity is important in resistance against E. ictaluri infection of channel catfish, thereby making this species an excellent candidate for vaccine development. But, although vaccines against E. ictaluri have been developed, none of them are used on a large scale because of the difficulty in finding the adequate opportune time in the life cycle of the channel catfish for immunization. Chen and Ainsworth (1992) found that administration of glucan greatly reduced I. punctatus mortality from experimental infection with E. ictaluri. These rcsults indicate that glucan potentially could be utilized prophylactically as an immunomodulator in channel catfish.
Although the problem of carrier state fish needs more research (Thune et al., 1993), there are indications that carrier state fish can play a role in the epizootiology of the disease (Mqolomba and Plumb, 1992;Thune and Johnson, 1992). This carrier state can be induced by use of an antimicrobial drug to prevent enteric septicemia of channel catfish (Klesius, 1992 b).
Treatrnent -Different antimicrobial drugs which are generally used in animal husbandry can be uscd for treatment of Edwardsiellosis of channel catfish. Mostly, medication is given by medicated feed. Sarafloxacin proved to be effective against Ewardsiellosis. Plumb and Vinitnantharat (1990) observed a significant reduction of channel catfish mortality by E. ictaluri of +75% using 10-14 mg of this druglkg fish body weighîfday for 5 days. Also a potentiated sulfonamide (Romet-30 super (TM)) seemed to be effective against E. ictaluri. In four field trials Plumb et al. (1987) observed a mortality reduction of f 60% using 50 mg Rometkg fish body weighîfday during five days. Beside an increased survival rate, an improved weight gain was observed also after treatment of Edwardsiellosis with Sarafinand Romet-medicated feeds Johnson et al., 1993).
Special attention has to be given ta sensitivity tests of the E. ictaluri strains isolated from disease outbreaks, since shifts in sensitivity for drugs occur. This provides the opportunity to change the dmg of first choice, if necessary.
Prevention -Most important preventive measure is to avoid contamination of a fish farm by strict control of fish to be stocked for clinical and subclinical outbreaks and for the presence of carrier state animals. Therefore, a screening program has to be developed using techniques which make it possible to screen large numbers of fish in a short period.
Optimal fish condition is important too. Durve and Lovell (1982) and Li and Lovell (1985) observed a relationship between dietary vitamin C content and resistance against E. ictaluri infections. In future immunomodulators which potentiate the immune defence mechanism can be used prophylactically on a large scale to increase fish resistance.

Red Head Diseasr
Hucteriu -The causative bacteria are gramnegative, oxidase-positive, curved and motilc rods and identified as Vibrio sp. Hlood agar cultured colonies are round convex, surrounded by a zone of total haemolysis. The Vibrio sp. related to discases in sheatfish can also grow in freshuatcr outside the host (Farkas and Malik, 1986).
Clinical symptoms - Farkas and Malik (1986) isolated a Vihrio sp. from S. glatzis fry during an epizootic. The disease was characterized by red spots in the head region. Therefore thcy named this disease "Red Head" disease.
Pathology -Histological cxaniination revcals an infiltration of brain tissue by erythrocytcs. The brain is edemic and exudate with bactcria is formcd. In acute cases the meninges and epidermis ruptures with fatal results (Farkas and Malik, 1986). This pathology differs from other infections with Khrio sp. in other fish species which more likely a septiccmia inducc (Thunc et al., 1993).
Diagnosis -The diagnosis has to be made by clinical inspection focused on the head region and isolation and identification of the bacterial agent.
Treatmerzt -Vibrio sp. involved in the discase can be treated by antimicrobial drugs. Different antibiotics are suitable (Farkas and Malik, 1986).

Preventiorz -Measurcs to prevent Red Head
Disease are related to buying Vibrio free fry and avoiding contact with infected populations. Specific measures remain to study.

Motile Aeromonad Septicaemia
Bacteria -Motile aeromonads arc free living motile mesophilic bacteria, which are considcred primary and secondary pathogens of aquatic organisms (Leblanc et ul., 1981). Diseases caused by motile aeromonads have been described in many fish species (Newman, 1982). Motile aeromonads can be divided into three different species, e.g. A. hydrophilu, A. sobriu and A. cuviue (Popoff, 1984). The motile aeromonads are pathogenic for mammals (including man) and birds too (Davis II et al., 1978;Shane and Gifford, 1985;Wohlgemut et al., 1972). A. hydrophila finds access to the interna1 organs through the digestive tract or through the injured skin under crowded conditions and high temperatures (Ventura and Grizzle, 1987).
Motile aeromonads are gram-negative, non-acidfast, rod-shaped, and motile by generally monotrichous polar flagella. The cells are usually not capsulated. They are aerobic and facultative anaerobic (Post, 1987) and can be isolated from al1 types of freshwater and brackish water habitats (Thune et al., 1993). The pathological effects of A. hydrophila are mainly related to extra cellular products (Thune et al., 1993) of the S-Iayer of thc bacteria (Ford and Thune,199 1) which are responsible for the cytolytic and fibrolytic capacitics of A. Iijdrophila, destroying host defence systems and contributing to the invasive power of the bacteria (Bach et u1.,1978). Supriyadi and MacLean (1986) reported that Aerornoriu.v sp., espccially A. hydrophila, cause sccondary infections of Clarius sp. kept under intensive conditions. Thcre are indications that C. batruchus is more susceptible to infection\ with A. hq'drophila than common carp (Cyprinus carpio) and giant gouraini (Osphronem~is goururni).
Cliriic~al Syniptoms -Clinical signs depend on the form of the disease but are a-specific in al1 cases. The acutc form is characterized by exophthalmia and a distended abdomen. The chronic form shows more deep dermal ulcers with haemorrhages and inflammation (Post, 1987;Thune et al., 1993).
Patliology -At post mortern the acute form is charactcrized by edema, haemorrhages and diffuse necrosis (Thunc et ul., 1993). The namc Motile Aeromonad Septicemia (MAS) is originated from this septicemic picture. The chronic form is charactcrized by local necrosis in the epidermis and musculature and by petechia on the serosa and in the muscles (Thune et al., 1993). Fish with MAS have a severe branchitis as indicated by lcucocytic infiltration and dilation of the gill's central venous sinus. In addition the nuclei in the branchial epithclium are enlarged (Grizzlc and Kiryu, 1993).
Diagnosis -For a proper diagnosis different tests are available. The bacteria can easily be cultured and identified but more sophisticated diagnostic techniques are also possible. From these variouï tests the slide agglutination procedure proved effective under field conditions. However, the fluorescent antibody technique is more sensitive than other procedures and can be performed in less time (Eurell et al., 1978).
Immunity -Both cellular and humeral immune responses after primary infection havc been describcd for rainbow trout and carp (Post, 1966 andBaba et al., 1988), suggesting the same for the Siluriformes. Despite the availability of vaccines against MAS vaccine use is not common bccause of the small geographical area of efficacy due to occurrence of specific strains in small areas (Schachte in Newman, 1993).
Treatment -In clinical cases antibiotic treatments are indicated. Because of changing sensitivity patterns of the aeromonads this should be done according to results of sensitivity tests of the isolated aeromonad strains.
Some antimicrobial drugs used in human medicine have been used for treatment of bacterial septicemia in C. batrachus. Saha et al. (1990) fed catfish supplementary feed mixed with Trimethoprim and Sulphamethoxazole (Bactrim) for I O days. They found the drug effective, both as treatment and as prevention of the disease.
Prevention -The best prevention is to avoid . stressful periods. Stress factors advance clinical bacterial infection as proven by Faisal et ul. (1988). They found in C. guriepinus stressed by a molluscicide (Bayluscide) a depletion of haematopoietic tissue, an increased susceptibility to pathogens, and an appearance of granulae with bactcria.

Cytophaga (Flexibacter) columnaris disease
Bucteria -Cytophaga columnaris is a gramnegative, rod-or filament shaped ("pine tree needles") bacteria, characterized by gliding motility. It is proteolytic, ferments sugars and is strict aerobe (Post, 1987). The bacteria causes explosive diseases of freshwater fish kept above 18°C. Often the infection becomes clinically when the fish is simultaneously infected with other micro-organisms (Marks et al., 1980).

Clinicul symptoms -Infections with Cytophuga
sp. are mostly secondary infections. Fish suffering environmental stress and /or injuries, are highly susceptible. Especially young stages (eggs, larvae and fry) are attacked, invaded and destroyed by the bactena.
The most dominant clinical aspect of an infection with Cytophaga columnaris is local thickening of the mucus on the skin, followed within a short time by a Cotton wool-like layer, covering the primary skin lesions. A-specific symptoms like exophthalmia, haemorrhages and hydrops of the abdomen can often be observed. Clinical signs depend on strain pathogenicity.
Puthology -No specific post mortem characteristics are known. Sometimes pale interna1 organs are observed, possible related to toxins produced by the bacteria.
Diagnosis -Diagnosis is made by combining the clinical aspects, especially the cotton-like layer, and analyzing results of the smears of the suspect skin spots.
Immunity -Immunity is not a significant factor in the prevention and treatment of the disease. Mostly, affected fish, especially young ones, will die and recovering of the disease is time consuming. Therefore, fattening of a recovered (and diminished) population often prove unattractive.
Treatment -Different antimicrobial drugs can be used for treatment. Sensitivity tests have to be done to select a suitable drug. However, without changing the environment, antimicrobial treatment is a waste of money.
Prevention -The best prevention is proper husbandry. In cases where stress cannot be avoided and the risk of an infection is high the use of antimicrobial drugs as a preventive measure is advised.

Saprolegnia sp.
Fungus -The family of Suprolegniaceae ubiquitous in the world, primarily in freshwater. The fungi havc long, branched non-septate hyphae and reproduce primarily asexually forming zoospora. The optimum growth tempcrature is betwcen 15 and 30°C (Post, 1987).
Clinicul symptoms -Fungus diseased fish show fluffy cotton-like, white grey to grey brown structures on the skin, fins, gills and eyes. Affected eggs show the same (Post, 1987).
In Clariidae fungal infections often occur. During a survey in a Nigerian freshwatcr fish pond 24 fungal species belonging to 6 fungal genera were isolated of which the major part were Suprolegnia sp. C. gariepinus had the highest amount of isolates. There were similarities between the isolates from the pond fish and thosc isolated from the hatchery from where the fish were obtained (Ogbonna and Alabi, 1991). Fungi are mostly sccondary invaders. Firstly, somc tissues with a decreased resistance against microorganisms were infestated. For instance, after Gas Bubble Discase with degcnerationlnecrosis of the peripheral fin tissue caused by air-embolism of small blood vessels andlor capillaries, clinicül Saprolegnia infections appear very soon (Boon et ul., 1987a).
Puthology -Xu Dhai and Rogers (1991) observed that epidermic cells in the lesions related to fungal infections were necrotic. After penetration of the dcrmis fibroblasts and collagen lamellae were damaged.
In channel catfish, Bly et al. (1992) observed a complete lack of bacteria and leucocytic infiltration around the site of fungal penetration during the winter period ("winterkill syndrome"). Therefore, they suggested that winterkill syndrome should be regarded as an immunodeficiency disease of fungal origin.
Diagnosis -Clinical diagnosis is confirmed by examination of direct smears of affected spots. At low magnification characteristic fungal hyphae can be observed.
Treutment -Saprolegniosis can be treated by using chernicals by bath, like malachite green (5 mg/L for one hour), or by dipping, like sodium chloride (5% for one or two minutes). However, in many countries it is forbidden to use malachite green for treatment of food fish diseases due to teratogenic properties of the drug (Post, 1987). Recently a negative effect of malachite green on the haematocrit of rainbow trout (0. mykiss) related to haemolysis within 12 hours post treatment was described (Tanck et ul., 1995). A same effect in other fish species is most likely.
Prevention -Saprolegniosis is seldom a primary disease. Therefore, the best prevention is good husbandry practice avoiding mechanical damage and al1 other kinds of stress.

Diseases of Siluroidei
Diagnosis -The disease is diagnosed by the presence of the ulcers combined with histopathological characteristics.
Treatment -Because bacterial involvement is most likely, a treatment with a broad spectrum antimicrobial drug can be helpful to minimize the damage to catfish populations suffering from this disease.
Prevention -No specific preventive measures are known. A proper husbandry system is the only one.

CONCLUSION
From the present review it is clear that in the culture of catfish problems with infectious diseases exist. However, compared to the culture of other fish species, for instance salmonids with a number of important viral and bacterial diseases, the number of pathogens related to catfish diseases with a significant economic impact is low. At the present state only one viral (by CCV) and one bacterial disease (by E. ictuluri) can be a threat for catfish industry. However, both diseases are diagnosed in North America. In Europe, Asia and Africa no comparable catfish-specific infectious diseases are diagnosed suggesting that they do not exist in those parts of the world. However, the latter is doubtful and may be related to the difference in scientific input in catfish disease research between especially the USA and the rest of the world as a reflection of the economic impact of the respective national catfish productions. That may also be the main reason for which the etiological agents of different Clarias sp. diseases are not described yet. A comparison can be made with the research in viral fish diseases, e.g. from the eighties onwards scientists paid more attention to those which resulted in a dramatically increased number of identified viruses and related diseases (Hetrick and Hedrick, 1993).
Moreover, it is known that in industrial fish production, like the channel catfish production in the USA, epidemics occur more easily cornpared to small scale catfish production like in other countries. Furthemore, it is suggested that for example C. gariepinus is very infectious-disease resistant. This implicates that if this fish species becomes diseased it is nearly always related to bad culture conditions. It is likely that this is the same with C. batrachus.
This explains the involvement of mainly secondary infections in the diseases of those species.
With the development of catfish culture industry, it can be expected that the prevalence of viral, bactenal and fungal diseases will increase, followed by an increase in drug (ab)use and need for vaccination. If so, research in catfish diseases has to make a shift in near future into improving catfish health by optimization of husbandry systems and increasing the disease resistance of catfish, The latter also implies development of a better genetic capability of the fish to cope with pathogens.