Bacterial diseases in fish

Identify and cure bacterial diseases in fish

Bacterial illnesses occur frequently in fish kept in both aquariums and in ponds. Sometimes they can be difficult to identify and treat.

Most of these bacterial infections are already present in the pond or aquarium lying dormant waiting for their chance to infect the fish. Although the fish can be cured with some attention, it is next to impossible to completely eradicate the bacteria from the pond or aquarium. And if ever the fish become weakened or their immune system is compromised, bacteria will reinfect the fish.

Common aquarium bacteria

The three most common types of bacteria are Aeromonas species, Pseudomonas species and Vibrio species. With vibrio species of bacteria occurring mostly in saltwater fish.

The main causes of stress to fish is poor water quality such as ammonia from fish waste, low, high or rapid changes in temperature, chlorine in the water and fish bullying.

Any damage to the skin or even just to the protect mucous may allow bacteria to enter the exposed or damaged skin. This damage may be caused by injury from scratching against an object or another fish bite or even a large parasite on the skin.

Poor feeding can also lead to a bacterial infection. Lack of protein can lead to a low antibody level in the fish leaving the fish less able to fight off infection. Also fish need vitamin C and vitamin A to maintain a good immune system.

Signs of bacterial infection in fish

Peracute infections are when there are sudden, unexpected deaths with no obvious external signs. This is when an bacterial infection overwhelms the fish and it dies before it can mount any effective immune response. This is sometimes mistaken for water quality or poisoning problems. It can only be diagnosed properly on post mortem examination.

Acute infections are when the fish show normal signs of septicemia with blood streaking and red blotches on the skin and fins. The infected fish will also lose its appetite, become sluggish and have clamped fins. Infected fish become reclusive.

Younger and weaker fish become infected first and most of these will die if the infection is serious. It can be confused with parasitic infections because of the blood streaking and red blotches.

Chronic infections usually show up as ulcers on the body. This is the most obvious sign of long term infection. The ulcer can either be small and well defined or a large erosion of the body. Complications usually set in at the site of the ulcer such as with fungus. The ulcer also allows water to flow into the body tissue and body salts to leach out.

Sometimes this influx of fluid can lead to a buildup of fluid inside the body. The affected fish will develop a swollen abdomen and the eyes may protrude. Also the scales may stick out giving a pine cone appearance. The protruding scales may also be limited to one area of the fish’s body where infection infection has become established in the scale pockets.

Bacterial gill disease is also a long term bacterial infection. This usually occurs after ammonia or chlorine has damaged the fish’s gills and infection slowly sets in. Symptoms include laboured breathing, fast breathing, and gasping near the water surface. The gills appear swollen with discolouration.

Treatments for bacterial infections in fish

Antibiotics are the only effective remedy for bacterial infections the fish cannot fight off by itself. Antibacterial medicines from your local pet shop are usually not effective at all.

If the type of bacteria is not known then a wide spectrum antibiotic is usually called for.

How to treat fish with bacterial infections

Seriously ill fish or non feeding fish may need injecting with an antibiotic. Fish that are still feeding can be fed with food that is impregnated with an antibiotic. To make sure the fish eats the medicated food, withhold feeding for 24 hours.

If the antibiotic is in tablet form then crush the tablet into a powder then mix with a small amount of vegetable oil to form a thick paste. Mix this paste with some powedered fish food. Allow the pellet that you have made to dry before feeding the fish.

Treating fish ulcers

Anesthetise the fish first. Then gently remove any dead tissue using a cotton wool pad. Gently apply a povidone-iodine treatment such as Wokadine. Remember this must be diluted as per instructions. After this plaster a thin layer of mouth ulcer treatment onto the area as protection against any influx of water.

Do not re-treat ulcers. Make sure you do a proper job the first time. Re-treating will disturb the healing wound and healing will have to start from scratch again.

Do not delay in treating. Late treatments are usually unsuccessful. Your chances of successful treatment are much increased if you treat the fish before the disease spreads within the fish.

The most important part of treatment is not the fish, but rather treat the aquarium conditions that caused the disease in the first place. Causes may be from overcrowding, overfeeding with decomposing uneaten food, or poor biological filtration.

Mycobacterial infection (Fish tuberculosis)

These bacteria are free living in the aquarium. They can often be found in the gravel or in the mulm that collects on the aquarium floor. Infection is usually by mouth. Fish often sift through gravel or mulm or eat dead fish infected with mycobacteria. The second way for a fish to get infected is through a cut or abrasion.

After the initial infection, it takes 6 weeks before symptoms start showing themselves.

The fish’s immune system is only partially effective against mycobacteria. The fish’s antibodies just try to overwhelm the bacteria. This has the effect of slowing down the progress of the infection. Normally the gut, liver and kidneys get infected. Infection is urinated out by the fish with the potential to infect other fish. Damage to the kidneys leads to dropsy.

The infection can travel throughout the body. Infection to the liver causing digestive problems, into the muscles near the skin causing ulcers, brain infections that cause abnormal behaviour, spinal infections that erode the spine causing spinal deformities.

Symptoms of mycobacteria in fish

All fish species as well as amphibians are susceptible to mycobacteria. Carp related fish such as goldfish and all the anabantids are especially susceptible.

Symptoms vary depending on which part of the body is infected. Some fish just simple waste away with caved in stomach areas. Other fish develop dropsy and pop eye. And some fish may develop sudden spinal deformities. Finally most most develop ulcers along the body.

Treatment of mycobacteria in fish

Antibiotics are essential for successful treatment. The aquarium has to be cleaned and sterilised and the equipment and items in the aquarium should be thrown away or thoroughly cleaned and sterilised. Gravel, stones, ornaments, and plastic plants should be thrown away.

Columnaris disease (Flexibacteria columnaris)

The bacteria is long and slender and can move around. This bacteria forms a large group. High temperatures can cause an outbreak of this disease. Flexibacteria can grow on uneaten food. There are two stages to the disease. A low activity stage where the fish keeps the illness in check and a very active stage where symptoms start to appear. At this stage the disease can spread very quickly killing the fish before you can effectively treat it.

The first sign of infection is usually a large white spot somewhere on the body that develops a red or pink outline. White erosions then starts to appear on the body, gills and fins of the fish. The white areas usually have the red or pink outline.

Symptoms of flexibacteria in fish

The most obvious symptom is large whitish patches on the body, gill covers or fins of the fish. These white patches are usually have a white or pink outline.

Secondary symptoms include: Discoloration on the body or fins or a general loss of body colour, rapid or laboured breathing with gasping at the surface, fins may be held close to the body and fins may become ragged with bloody streaks.

When the bacteria infects the mouth it is called mouth rot or mouth fungus (it is not a fungal infection, but fungus may set in as well). Strands of flesh, bacteria or mucous may move in and out of the fish’s mouth as it breathes.

Treatment of flexibacteria in fish

Use antibiotics to treat this bacteria. If the fish is not eating then you have to inject the antibiotic in the anesthetised fish. For feeding fish crush the antibiotic tablet into a powder and mix with crushed fish food. Add some vegetable oil to the mix to create a thick paste. Allow the paste to dry before feeding. If the fish refuse it then don’t feed them for 24 hours. When hungry they should eat their medicine.


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