Saturday, July 20, 2024

Bacterial Fish Diseases and Control Measures

Bacterial fish diseases can be very difficult to deal with, so early detection is extremely important so that they can be treated before becoming lethal to your fish. Bacterial diseases are generally categorized into two types: primary pathogens, which are not normally part of the fish’s environment, and opportunistic pathogens, which generally are.

Fish do not usually suffer bacterial diseases, unless they are farmed fish or fish restricted to a tank or bowl. Under these circumstances of potential stagnation, overcrowding, restricted space, higher concentrations of excreta and higher quantity of nutrients in the water, bacteria can gain the upper hand.

Opportunistic bacteria, though the most common problem, normally do not pose a threat to your fish, unless there is some changein the fish’s environment, such as an overabundance of rotting material or other stress or injury to your fish.

These factors allow the bacteria to invade. Most of the time, bacterial diseases in the early stages are localized. These can be indicated by fin rot, body ulcers, and red or inflamed areas on the fins and body.

If not treated, these bacterial infections may become systemic, spreading throughout the fish’s body.

This normally happens through absorption through the gills gut, or skin. Other signs of bacterial infection include swollen eyes, swollen gut, or lethargic activity. Gill disease can also be an indication of bacterial disease in fish.

These symptoms are usually found when the bacterial infection has begun to spread within your fish. Since it is impossible to accurately diagnose bacterial disease in fish merely by sight, it is important that other tests be performed on the fish, such as mucus sample testing.

The obvious signs will, however, help to narrow down problems in your fish. Pet store employees, other fish owners, and veterinarians can also provide helpful information.

Bacterial Diseases

Bacterial diseases are often internal infections and require treatment with medicated feeds containing antibiotics. Typically fish infected with a bacterial disease will have hemorrhagic spots or ulcers along the body wall and around the eyes and mouth.

They may also have an enlarged, fluid-filled abdomen, and protruding eyes. Bacterial diseases can also be external, resulting in erosion of skin and ulceration. Columnar is an example of an external bacterial infection which may be caused by rough handling.

A fundamental management objective of all fish rearing practices is to avoid and minimize stress on fish.

Stress: The aquatic environment is dynamic and constantly subject to changes in its physical, chemical and biological components.

These changes along with culture practices– stressors severely stress the physiological systems of fishes. The physiological response elicited initially is adaptive.

However,they may ultimately become maladaptive in chronic situations. Stress response Innumerable diseases are caused in fishes due to bacterial pathogens and several of them.

They occur in nursery, rearing and grow out ponds causing serious concern to fish farmers. Some of them often wipe out the entire population of fish.

Some of the important bacterial pathogens are Aeromonas hydrophila, A. salmonicida, Pseudomonas fluorescens, P.putrefaciens, Flexibacter columnaris, Edwardsiella tarda, Vibrio alginolyticus and V. Parahaemolyticus which have been identified as the most commonly encountered agents in fish diseases.

Bacterial Infections

The most common bacterial infections are caused by one of three pathogens: Vibrio, Pseudomonas or Aeromonas.

The symptoms (e.g., cloudy eyes, bloody patches, decaying or frayed fins, scratching) of these bacterial infections can be similar, and therefore it can be difficult to determine which pathogen is responsible.

Fish with an internal bacterial infection may not show any signs other than a loss of appetite and possibly a swollen abdomen.

With most bacterial infections, all fish in the pond or tank may be affected to varying degrees, so the entire pond may need to be treated.

Obviously, if we are dealing with a large pond, this could potentially be expensive. If only one fish appears to be infected and you move the infected fish to a quarantine pond as soon as possible, you may get lucky and not have to treat the display pond.

In most cases, tetracycline has been the most effective treatment for infections caused by both Vibrio and Aeromonas, though there are other antibiotics that you can use.

Remember to follow the manufacturer’s directions and remove any activated carbon from your filter before treating the aquarium. Another option is the use of medicated foods if the fish are eating well.

Aeromonas hydrophila (Bacterial Hemorrhagic Septicemia)

Gram negative motile rods: Effects many freshwater species and usually is associated with stress and overcrowding. The clinical signs and lesions are variable.

The most common finding is hemorrhage in skin, fins, oral cavity and muscles with superficial ulceration of the epidermis. Occasionally cavitary ulcers (similar to A. salmonicida) are observed. Exophthalmus and ascites are commonly observed. Spleenomegaly and swollen kidneys are common.

Histologically, multifocal areas of necrosis in the spleen, liver, kidney and heart with numerous rod shaped bacteria are observed. Diagnosis is rendered by culturing the organism from affected animals:

Remember this is a common water saprophyte with a great variation in virulence in serotypes. Disease is transmitted via contaminated water or diseased fish.

Bacterial infections, caused by motile members of the genus Aeromonas, are among the most common and troublesome diseases of fish raised in ponds and recirculating systems.

The wide- spread distribution of these bacteria in the aquatic environment and the stress induced by intensive culture practices predisposes fish to infections.

Motile aeromonad infections have been recognized for many years and have been referred to by various names, including motile aeromonad septicemia (MAS), motile aeromonad infection (MAI), hemorrhagic septicemia, red pest, and red sore.

In this publication, they are referred to simply as aeromonas infections. Aeromonas bacteria causing these infections are called aeromonads. Whether acting alone or in mixed infections with other organisms, the motile aeromonads are responsible for significant financial losses annually.

All species of fish, scaled and unscaled, are susceptible to infection. Under certain conditions mortalities can approach 100 percent. Aeromonas infections also occur in other vertebrates, including frogs, turtles, alligators and, sometimes, humans.


Bacterial Fish Diseases and Control Measures

Aeromonas hydrophila, A. sobria, A. caviae, and possibly other aeromonads, are capable of producing disease in fish.

While all members of this group are small, motile, gram-negative, rod-shaped bacteria and all share certain biochemical characteristics, their scientific names are constantly under revision and subject to change in the future.

Numerous strains of these bacteria exist, and they vary greatly in their ability to cause disease. In general, strains isolated from the environment are less pathogenic than those isolated from diseased fish.

The marked genetic diversity among different aero- monad strains has made it difficult to develop effective vaccines.

Read Also : Types of Fish Diseases, Determining if Fish are Sick, What to do if Fish are Sick, and The Significance of Fish Disease to Aquaculture

Clinical Signs or Symptoms

Signs of disease associated with aeromonas infection are non-specific and may be easily confused with other diseases.

Infections vary greatly in appearance and may be seen in the skin only, as an internal systemic disease (septicemia), or as a combination of both.

Outbreaks may be chronic (long-term) and affect only small numbers of fish or may produce acute (intense and short-term) infections accompanied by rapidly increasing, high mortality rates.

In unscaled fish (e.g., catfish), there is often fraying and reddening of fins, accompanied by irregular, variably sized areas of depigmentation (paleness) that can develop anywhere on the body surface. The skin overlying these sites is eventually lost, exposing the muscle below.

These open sores or ulcers may remain superficial or they can be extensive and invade deeply into muscle, revealing underlying bone in some cases.

These ulcers often have ragged white margins bordered by a narrow zone of hemorrhage. In scaled fish (e.g., largemouth bass), skin lesions begin as small hemorrhages within scale pockets (Fig. 4) that can rapidly expand to larger areas. Affected scales are eventually lost and ulcers form.

Aeromonas infection may also include any or all of the following external signs: exophthalmia (popeye), abdominal distention (swelling of the abdomen), and pale gills. Scaled fish often accumulate edema (fluid) in their scale pockets.

This condition, called lepidorthosis, creates a roughened or bristled appearance. Skin lesions caused by aeromonads often have fungus or columnaris bacteria present.

Fish affected only with skin lesions may continue to feed and survive for extended periods, despite the presence of severe ulceration (Fig. 3). Daily mortalities associated with this chronic form of disease may be low, but can rise to high levels over time.

The internal or septicaemia form of disease typically follows a more acute course with a sudden onset of relatively high mortalities.

Affected fish usually do not eat and commonly will be seen swimming lazily near the water’s surface or in shallow areas of a pond. If disturbed, the fish move into deeper water, but typically return to the surface within a short period of time.

The internal organs may be enlarged, reddened or pale, or have a mottled pattern of deep red hemorrhage interspersed with pale areas of tissue destruction or necrosis.

Organs with significant tissue necrosis become weak and are easily damaged when handled. The intestinal tract typically will be devoid of food, reddened, and filled with cloudy yellow or bloody fluid and mucus.

The abdomen may be filled with clear, cloudy or bloody fluid. The gall bladder will be filled with large amounts of green bile. Highly virulent (deadly) strains may cause sudden mortalities with few external or internal signs of disease.

Losses from aeromonas infections seldom exceed 50 per- cent; however, mortality is strongly influenced by the general health status of the fish population, stress level, and virulence of the particular bacterial strain infecting the fish. Mortalities occasionally approach 100 percent in fry and small fingerlings.


Chemical treatments with potassium permanganate (KMnO4), at a rate of 2 to 4 parts per million (milligrams/liter), are sometimes useful in the treatment of infections limited to the skin.

Potassium permanganate treatments are of greatest value when fish are feeding poorly or not at all, and medicated feed is not an option. Potassium permanganate is presently on deferred status by the Food and Drug Administration; it may be used but is not officially approved.

Systemic infections can only be successfully treated by the use of medicated feeds containing antibiotics.

For medicated feeds to be effective, it is essential that an early diagnosis is made and that the fish be fed as soon as possible, before the disease causes them to stop eating.

Treatment with medicated feeds will not be effective if a large portion of the fish population has already stopped feeding.

Usually there is not enough time to wait for the results of antibiotic sensitivity tests. Sensitivity testing, however, may indicate that a different, more effective medicated feed should be used.

Oxytetracycline (Terramycin¤) is approved for control of motile aeromonad infections in catfish. Sulfadimethoxine plus ormetoprim (Romet¤) is approved for controlling Edwardsiella ictaluri (ESC) infections in catfish.

Fin rot and tail rot

Fin rot and tail rot in hatcheries, nurseries and grow out ponds have been reported in young and adult fishes. The disease is contagious and is capable of causing immense damage. Fin and tail rot in young fish are due to a mixed infection of A. hydrophila and Pseudomonas fluorescens.

Short motile Gram-negative rods with polar flagella. Lesions similar to Aeromonas hydrophila with a hemorrhagic septicemia resulting in hemorrhage of the fins and tail and ulceration of the skin. Most times it is an indication of other problems, such as injury or poor environmental conditions.

Fish rot is indicated by the erosion or shredding of the fish’s fin, normally beginning at the outer end. It can be bacterial or fungal, and can be caused by a variety of common reasons, such as poor water conditions, diet, or injury.

Bacterial Fish Diseases and Control Measures

Once diagnosed, fin rot is generally easily and quickly treated, as the fish’s fins can be regenerated, unless the condition progresses past the base of the fin.

This is extremely dangerous to the fish as, with bacterial infections, it can be absorbed into the body of the fish and subsequently kill the fish.

Fungal fin rot usually rots more evenly, and produces a white edge; while bacterial fin rot produces more of a jagged edge. In many cases, both can be present at the same time, creating the need to treat both kinds. In more advanced cases of each, there may be redness or inflammation.

The type of fin rot in your fish will determine the method of treatment you use. Fungal fin rot is easiest to treat, generally by swabbing the infected area with an antiseptic, such as malachite green, (also called aniline green and a number of other names).

Bacterial fin rot in fish is normally treated with a form of antibiotic in a quarantine tank. This can be stressful to your fish, however. I more preferred method is the use of food containing an antibiotic. In advanced cases, the rotted area may be removed, but the fish will have to be sedated.

Fin rot is easy to diagnose and treat, and most times your fish will survive, if treated, properly. It is important to remember, though, that since fin rot is normally the result of some other stress to your fish or its environment; you must also address these issues in order to prevent it from reoccurring.


Control measures adopted are bath treatment in 1:2000 copper sulphates for 2 minutes or swabbing of concentrated copper sulphate solution in the affected fishes.

Gill and Fin Rot


Myxobacteria of the genus Flexibacter (Cytophaga), including F. columnaris, the commonest etiological agent of fin rot and skin lesions (“saddle back”) of freshwater fish, are readily recognized in direct microscopic examination of a squash of an affected tissue by their long thin (filamentous) structure.

Myxobacteria are isolated on Cytophaga agar medium, re-cultured on Nutrient, 5% sheep blood,


Gill lesions in the acute clinical condition (usually caused by F.columnaris) are necrotic and often rapidly expanding, and death is more rapid. On the skin, acute lesions are often confined to the head and back (“saddle back”)).

Such lesions are white or yellow with reddish zone of hyperaemia around the periphery and comprise of bacterial cells and necrotic tissue covering haemorrhagic ulcers.

Histology reveals epidermal spongiosis and a subsequent necrosis which extends into the dermis.

Chronic myxobacterial infections cause extensive hyperplasia in the gills, with resulting fusion of the lamellae and sometimes also with proliferation of mucus glands and chloride cells. Proliferation in the skin occurs at the tips of fins and at skin folds.

Bacterial Fish Diseases and Control Measures

Other pathological manifestations of myxobacterial infection reported elsewhere, which might also be relevant to African fish are as follows Fin rot condition has been reported only from fish outside Africa (Richards & Roberts, 1978).

It induces a severe epidermal and dermal edema, a fibrinous exudates overlaid with cytophaga bacteria, and cellular exudates, with subsequent sloughing and progressive erosion of the fins. Myxobacteria may be displaced by secondary opportunistic saprophytic bacteria.

Bacterial Fish Diseases and Control Measures

Cotton wool disease, affecting tropical aquarium fish, is suggested to result from irritation of gill lamellae, producing catarrhal exudates over the gills which serve as an attractive substance for cytophaga proliferation.


Myxobacteria are ubiquitous opportunists of the aquatic habitat. Many, particularly those causing gill and skin rot conditions, will colonise only damaged or ulcerating integument, necrotic tissue or irritated mucus excreting epithelium (cotton wool disease).

Acute gill and skin infections are most often associated with handling, or mechanical damage of skin and gills in hauling and sorting.

Adverse ambient conditions-low temperatures, excessive organic load, ammonia or nitrites, are important contributory factors in such situations.

The same environmental parameters, as well as other adverse growing conditions (overcrowding and inadequate feeding or nutrition), predispose fish to chronic myxobacterial diseases. Dispersed toxic substances may play an important role in the etiology of chronic myxobacterial infections (tail rot etc.).

Read Also : Ecological Effects of Parasites on Fish


The symptoms of dropsy in fish are a distended or swollen body with protruding scales. Dropsy is also known as ascites. The affected fish may appear lethargic and may not eat.

Dropsy isn’t a disease itself, but actually a symptom of another underlying disease in the fish. The reason the fish’s body swells is due to a buildup of fluid in its body cavity and internal organs.

Bacterial Fish Diseases and Control Measures

This may occur if the fish’s kidneys are affected, for example. There can be many different causes of fish dropsy, and it is usually difficult to know exactly what the cause is.

Notice the protruding scales on this goldfish with dropsy.

However, one common cause of dropsy in fish is a bacterial infection byAeromonas. Aeromonasis normally present in all aquarium water; however, it sometimes causes illness in fish, especially fish that are stressed due to poor water quality from overcrowding or infrequent partial water changes.

Also, some species of Aeromonas are more pathogenic than others. Infection by Aeromonas will sometimes cause red streaks or sores on the fish’s body (but not always).

Dropsy is not a specific disease, but rather a symptom of a deteriorated health condition. The epidermis and body cavities get filled with fluid and scales protrude out from their pockets leading to severe anemic condition.

A mixed infection of A. hydrophila and myxozoan parasite or malnutrition is frequently the cause of infectious dropsy in fish.

With dropsy, the fish will have visible swelling and projected scales. This is the result of a fish not being able to regulate the amount of fluid in a part of its body. The affected area is typically the abdomen; specifically, it is most often the visceral cavity that houses a number of organs, such as the stomach, intestines, gall bladder and kidneys.

The failure to regulate fluids is a symptom; therefore, there is usually some other disease involved that starts the process (caused by poor water quality, stress, internal bacterial infections, parasites, viruses and tumors).

Although dropsy is fairly easy to diagnose, the cause is much harder to determine; however, the primary cause is usually attributed to a bacterial infection. The causative agent can be introduced to the pond through food, poor water quality or through the introduction of other fish to an established pond.

Although dropsy is not highly contagious, the affected fish should be removed and placed in a quarantine pond. Dropsy can be spread from the affected fish, which can possibly produce stress among the other fish and make them more vulnerable to dropsy or other conditions.


Although there are no present medications that can effectively cure fish stricken with dropsy, your first line of defense is to administer a wide- spectrum antibiotic in the condition’s early stages.

The affected fishes could be effectively treated by Terramycin in the feed. Application of neem leaf and leaf extract in the pond water, lime in soil, change of pond water with fresh water can control this disease within 2/3 days.

Unfortunately, the prognosis of fish affected with dropsy is not very good. By the time the fish has swollen up and the scales project outward, the internal damage may be too expensive to repair and for the fish to recover.

Most cases of dropsy are fatal. Treatment for aquarium fish: If you have a fish with a swollen, distended body typical of dropsy, it is best to isolate the fish and treat it in its own treatment tank if possible.

You can treat the fish by adding a small amount of epsom salts to the tank. Epsom salts consist of magnesium sulfate as opposed to sodium chloride found in regular aquarium salt (and table salt).

Adding epsom salts to your tank will help to draw some of the excess water out of the fish’s body cavity and tissues. Don’t add more than 2.5 teaspoons of epsom salts per 10 gallons of water.

You should feed the affected fish antibacterial fish food for 7-10 days if the fish is still eating. You can also add some Maracyn2 to the aquarium that the fish with dropsy is in.

Maracyn2 treats gram negative bacterial infections, such as Aeromonas, and is absorbed through the fish’s skin from the water. However, unless you start the treatment early the fish may still not survive.

It is also possible that the dropsy may be caused by something other than a bacterial infection, such as a virus, or some other cause. If this is the case then the antibiotics will not be effective.

You can often prevent bacterial and other diseases in your fish by doing regular partial water changes (20-25 %) in your tank each week. In fact, if you have the time, doing partial water changes twice a week is even better. And of course, don’t overcrowd your fish or over feed them.

Control of Infections by Facultative Pathogenic Bacteria in Fish

Antibiotic therapy by use of medicated feeds appears to be a feasible undertaking, as long as the infected fish are still willing to eat.

Drug sensitivity tests of the pathogen targeted for treatment must be repeatedly performed to ensure efficacy of antibiotics added to feeds.

Application of medicated feeds, as a non-specific prophylactic measure, is economically wasteful, harmful to the environment (damaging nitrification processes and primary production) and promotes drug resistance amongst pathogens in the habitat.

In the case of streptococcal infection, use of medicated feeds in polyculture is not cost-effective as carp, which always comprise the greater portion of the pond biomass, are intractable to infection.

Vaccination has been repeatedly considered as a potential solution and some research is in progress. There are, however, serious doubts as to how effective this vaccination may be and whether it will be economically worthwhile.

The feasibility of vaccination use will largely depend on the degree and durability of protection afforded by immersion vaccination or via feeds.

Specific immune response has been induced in tilapia to various antigens and to some bacterial infections.

Elevation of antibody titers to a level of 100% protection to challenges during two to five weeks was obtained in Nile tilapia (O. niloticus) following injection of formalin-killed A.hydrophila and Freunds complement adjuvant vaccines (Ruangapan et al., 1986).

Attempts to vaccinate reared tilapia (O. niloticus) fingerlings with lyophilized Aeromonas hydrophila in the Ivory Coast were, however, unsatisfactory (J-P. Coquelet, unpublished report).

Serum agglutinating antibody titer of tilapia hybrids rose following injection with a V.parahemoliticus bacterin both with and without Freunds complement adjuvant, while titers for naturally occurring antibodies in the overwintering fish were usually low.

Immunization trials by immersion showed significant efficacy in protecting tilapia from a challenge for about 60 days.

Neither significant elevation in agglutinin titers nor protection against challenge was obtained in Nile tilapia immunized with formalin-killed Edwardsiellatarda, using the hyperosmotic infiltration method, it was found that vaccination using toxoid, rather than bacterin, elicited a greater antibody response and they recommended the use of detoxified endotoxin in streptococcal infections of yellow tail.

There is a serious risk of introducing bacterial infections into Africa (notably Streptococcus, Pasteurella and mycobacterium; see 3.2), which are prevalent in countries outside Africa specializing in tilapia culture, with genetically improved culture seed – breeders, fry and apparently also eggs.

Only an adequately enforced ban on such imports will secure African habitats from these infections.

In summary, bacteria are everywhere. But if bacteria are all bad, why aren’t all fish sick? Some bacteria are good, some are bad and some are only bad under certain, specific conditions or circumstances. Fish have defenses against bacteria, but they don’t always function properly.

This module introduce you to the bacteria world and how they are identified , what causes them to be pathogenic, how they are transmitted and how the diseases associated with then develop.

You discover how fish respond to bacterial infection and how he fishes systems and human control methods work. The problem of bacterial pathogens on finfish culture, their etiology and specific control methods.

Read Also : The Role of Irrigation in Addressing Water Scarcity For Agriculture


Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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