Bacterial Disease
Bacterial disease is the most common infectious problem for aquarium fishes. Most cases require scientific identification of the bacterial types involved and selection of a specific antibacterial agent under guidance of a veterinarian. The most common bacterial infections in aquariums are caused by organisms such as Aeromonas, Pseudomonas, Mycobacterium and Flavobacterium. They can cause diverse pathological conditions that include both acute systemic and/or chronic diseases. The effects on the fish are as varied as the signs.
External signs of bacterial infection are variable and include shallow reddened ulcers with irregular edges; loss of tail and finnage; missing or raised scales; haemorrhagic areas on the body, in the fins, and on the mouth; protruding eyes (exophthalmia); dropsy; and a protruding and inflamed vent. Dropsy is a distention of the abdomen, giving the fish a “pot belly” appearance. This is a strong indicator of disease problems which may include swelling of internal organs (liver, spleen or kidney), the build up of body fluids, parasitic problems, or other unknown causes. At this stage, the infection has usually become systemic. External lesions expose the body surface to secondary invaders as well as serve as sites for the loss of salts and body fluids. Fish stop feeding and abnormal swimming may become pronounced. When diagnosed early, treatments available from an aquarium fish specialist may help.
Bacteria can play both a beneficial and detrimental role in aquarium keeping. On the beneficial side they help break down ammonia, nitrite and organic wastes in the aquarium, which is a requirement for maintaining water quality. Conversely, bacteria can cause serious disease problems in aquarium fish. Only water quality problems exceed bacterial diseases in the area of aquarium fish mortality. Nevertheless, they are among the least known and understood elements of aquarium keeping.
The ubiquitous nature of bacteria in aquatic environments provides ample opportunity for rainbowfishes to come into contact with pathogenic and opportunistic bacteria. Such contact may lead to infection which, depending on the species and the virulence of the strains encountered, may have lifethreatening consequences. Some bacteria are even capable of causing illness in fishkeepers who may acquire infections from contaminated aquarium water.
Aquarium systems support large populations of bacteria. One study sampled water from an aquarium store and found a wide variety of potentially infectious organisms in every sample tested. Most of these are opportunistic and ubiquitous in aquatic environments. However, some bacterial growth is inhibited by the presence of other bacteria, suggesting that competition is a factor limiting the growth of some infectious bacteria.
Like other areas of aquarium keeping, bacteria have certain environmental preferences. However, most species tolerate a wide range of environmental conditions. An aquarium, with its warm water, aeration, nutrients and surfaces for colonisation, creates an environment where bacteria will flourish. Many of these bacteria live in biofilms that are located on all surfaces and in particular in the biofilter. Because the biofilter is full of different microniches, it can support the growth of a variety of microorganisms including pathogenic and opportunistic bacteria. The use of granulated activated carbon provides a readily available source of nutrient carbon for bacteria thus encouraging the growth of massive numbers of bacteria on its surface. However, bacteria are also present within the water column, and are capable of living independently away from their fish host.
Bacteria in biofilms are more resistant to a wide variety of antimicrobials including surfactants, antibiotics, phagocytic predators and drying. When conditions in the aquarium are optimum for the resident bacteria, planktonic cells are released from the biofilm and are capable of causing persistent and recurring exposure to disease and the presence of asymptomatic carriers.
In an aquarium, outbreaks of disease are usually associated with overcrowding, high or sudden change of temperature, handling, aggressive interactions, inferior water quality and poor nutritional status. All these conditions contribute to physiological changes and heighten susceptibility to infection. These management problems must be corrected for successful, long-term control of infections. Avoidance of exposure to the disease is the primary method of prevention. Keeping aquarium systems pathogen-free is an impossible task, but reducing levels of pathogens to below infective levels, should decrease the chance of fish becoming clinically infected.
Bacterial organisms may be the primary cause of disease, or they may be secondary invaders. Bacterial diseases can be transmitted by discharge from the intestinal tract and external lesions on the skin. Parasite damage and fungal infection may also allow entry and spread of the bacterial infection. In captivity, rainbowfishes will gradually develop resistance to some local strains of bacteria but may carry virulent organisms to another aquarium when transferred.
Fish can act as asymptomatic carriers of disease. In other words, they may be immune to a specific pathogen but still be able to shed the organism into the water or transfer it to other fish by contact. Sick and dead fish are often major reservoirs of disease-causing organisms. For this reason, sick, moribund (dying), and dead fish should be removed as soon as possible from the aquarium to prevent continued shedding of the bacteria into the water. Equipment, including nets, siphon hoses and buckets, can also transfer disease-causing organisms to another aquarium.
There are few published scientific reports describing bacterial diseases of rainbowfishes. Aeromonas hydrophila is usually associated with haemorrhagic septicaemia and has been isolated from several native fish species.
Mycobacterium and Pseudomonas species of water borne bacteria are cosmopolitan and ubiquitous, and are often isolated from sick rainbowfishes. However, numerous bacterial organisms have been reported from other aquarium fishes. Aeromonas is probably the most commonly encountered bacterial pathogen of freshwater aquarium fishes. Several species of these bacteria are ubiquitous organisms and opportunistic pathogens that take advantage of stressed and immunocompromised fishes. Environmental stresses such as crowding, temperature extremes, poor nutrition, and transport all may predispose fishes to infection.
The most common bacterial infections in aquariums are caused by organisms such as Aeromonas, Pseudomonas, Mycobacterium and Flavobacterium. They can cause diverse pathological conditions that include both acute systemic and/or chronic diseases. External signs of bacterial infection are variable and include shallow reddened ulcers with irregular edges; loss of tail and finnage; missing or raised scales; haemorrhagic areas on the body, in the fins, and on the mouth; protruding eyes (exophthalmia); distended abdomen (dropsy); and a protruding and inflamed vent.
Dropsy is a distention of the abdomen, giving the fish a “pot belly” appearance. This is a strong indicator of disease problems which may include swelling of internal organs (liver, spleen or kidney), the build up of body fluids, parasitic problems, or other unknown causes. At this stage, the infection has usually become systemic. Systemic cases frequently result in a hemorrhagic septicaemia leading to inflammation and necrosis of gastrointestinal tract, kidney, muscle, and spleen. External lesions expose the body surface to secondary invaders as well as serve as sites for the loss of salts and body fluids. Behavioural signs include a reduction in feeding and listlessness. Fish stop feeding and abnormal swimming may become pronounced. The effects on the fish are as varied as the signs.
Treatment Fish in the early stages of infection may respond favourably to antibiotic treatment and environmental modification. Any underlying environmental problems that might result in stress on the fish should be corrected immediately. Infected fish should be immediately removed from the main aquarium and isolated in a quarantine or treatment tank. It is preferred that infected fish are removed from the system for treatment; however, it is acknowledged that this is not always possible. If treatment is undertaken in the main aquarium, a large water change (50-75%) together with thorough gravel cleaning in order to remove excess waste (faeces, uneaten food, detritus, etc.) from the substrate should be undertaken. This includes the removal of algae from tank walls and particulate matter from the filter (change filter wool or wash sponge etc.).
Most aquarium medications are affected by variations in water chemistry such as pH, carbonates, chlorides and dissolved or particulate matter. In water that is high in dissolved organic wastes the chemicals will oxidise the wastes rather than attacking the disease causing agents. This has the effect of lowering the effectiveness of the chemical dosage. Failing to ameliorate water conditions while treating sick fish with medication will usually either prevent the medication from being effective or will cause the disease to recur after the treatment is completed. Most chemicals used to treat fish diseases can be toxic to nitrifying bacteria at therapeutic levels for fish. Antibiotics are generally toxic and have a severe detrimental effect on nitrifying bacteria.
Therefore, it is advisable to take biofilters off-line during treatment and to do a large (75%) waterchange prior to reestablishing the filtration system.
Bacterial diseases are commonly treated with antimicrobial agents (antibiotics) and can be successfully treated if diagnosed early. An experienced fish health professional should carry out treatment for all but the most common bacterial disease problems that your fish experience. Most cases will require scientific identification of the bacterial types involved and selection of a specific antibacterial agent. However, even veterinarians with laboratory diagnostic experience cannot make an accurate diagnosis of some problems without microscopic examination of the fish or cultivation for bacteria. In extreme bacterial disease outbreaks, it may be necessary to destroy infected stock and eliminate the responsible pathogens by sterilisation of ponds, tanks and equipment.
Many veterinary schools and agricultural diagnostic laboratories are familiar with fish disease. The most common tissues collected for bacterial culture and sensitivity in ornamental fish are blood, kidney, liver, and spleen. Diagnosis can be made through gill biopsy, skin cytology, faecal examination, or necropsy. Determining the cause of death in a fish is important for maintaining the health of other fish in the same environment. Due to rapid autolysis, fish necropsies should be performed promptly as soon as they are found.
For the detection and identification of bacterial pathogens in populations of fish showing disease signs, ideal samples are multiple (five or more) moribund fish or those showing clinical signs typical of the disease outbreak. For the detection of subclinical infections in populations of asymptomatic fish, larger sample numbers may be necessary. Fish that are found dead at the time of sampling are usually not acceptable for bacteriological examination, unless they are known to be very fresh. Bacterial invasion of fish skin and other organs occurs rapidly after death, particularly in warm water. The samples should be kept cool or at the fish’s water temperature during transport and delivered rapidly to the laboratory. Delays and lack of temperature control may confound results. It is best to contact the laboratory. Some organisms (e.g., Flavobacterium, Vibrio) require special media and temperatures, and Mycobacterium may take several months to culture.
If the fish have a bacterial disease and the causative agent has been identified, a sensitivity test should be performed to ensure that the correct medication is used. The incidence of resistant bacteria is high and a sensitivity test will show the resistance of the disease-causing bacteria to various antibiotics. Bacteria have the ability to develop and transfer drug resistance. The occurrence of multiple antibiotic resistance by bacterial pathogens is becoming more evident in commercially farmed aquarium fishes, especially in regions where antibiotics are widely and indiscriminately used. When bacteria become resistant to a specific antibiotic, even high concentrations of that drug will not be effective.
Certain antibiotics also have been shown to suppress the immune system, potentially making aquarium fish more susceptible to viral or parasitic infections.
Antibiotics are usually taken internally to control bacterial infections. Therefore, medicated feed or injection, are preferred for treating systemic (internal) bacterial infections. Dose rates are based on fish weight and are expressed as weight of chemical per weight of fish per day for a specified number of days. Improper doses may result in an ineffective treatment or mortalities. However, the effectiveness of oral antibiotic therapy has been inconsistent and, as a consequence, mortalities continue to occur.
Fish often stop eating as a bacterial disease progresses, so early diagnosis and treatment are essential to ensure that infected fish consume the medicated feed. This may be compounded by unpalatable feed caused by the presence of the drug itself, or the fish may be too sick to consume the normal amounts, which makes the problem worse. Withholding food for a few days prior to feeding medicated food may aid in acceptance. It has been suggested to enhance the food with cod-liver oil.
If commercial medicated feed is not readily available, it is possible to mix your own feed in small quantities. A qualified fish health professional should be contacted for help in calculating the appropriate quantity of medication needed before using this treatment. The powdered antibiotic is combined with a binder and then added to the feed. Antibiotics can be introduced into artificial feed by making a solution of 5% gelation, cooling the solution to room temperature and adding the antibiotic. The gelatineantibiotic solution can then be mixed with the diet, which is then stored in a refrigerator at 4-6° Celsius. Vegetable or fish oil can work well as binders also.
The feed and antibiotic must be mixed thoroughly to assure even distribution of the drug to all the feed. If you use the antibiotic to coat dry feed then the coated feed should be spread out to air dry. After several hours of drying, the feed can be re-bagged and stored under proper conditions. This can be a time-consuming process. In addition, a significant quantity of the antibiotic may leach out of the homemade medicated feed before being consumed by the sick fish.
Baths and dips are not as effective as medicated feed or injection, particularly for systemic bacterial infections, because of generally poor internal absorption of the antibiotic being used. This method of application however, can be used for surface infections such as fin rot, bacterial gill disease, superficial fungal infections and ectoparasitic infestations. Another disadvantage with bath type treatments is that a lot more antibiotic is required when compared with oral (feed) treatments or injections. Consequently, fish treatment by bath usually requires a separate container or tank. Additional aeration may also be required for bath treatments. Accurate calculation of the volume of water in the tank is also required.