Mycobacteriosis (or fish tuberculosis), caused by aquatic Mycobacterium species, is a sub-acute to chronic disease known to affect a wide variety of wild and cultured fish of freshwater and marine species world-wide. Although Mycobacterium marinum was considered the primary causative agent of mycobacteriosis, a great number of Mycobacterium species associated with tubercle granulomas in aquarium and wild fish populations have been found. However, it has to be considered that the risk of infection in fish reared in captivity is significantly higher. The incidence of mycobacteriosis in aquarium fish has been reported to range from 10-22%. Most likely, the aquarium environment offers more favourable conditions for propagation of mycobacteria (temperature, water, oxygen etc.).
Mycobacteriosis in aquarium fish is one of the most commonly diagnosed bacterial diseases. A recent study (2006) on the presence of mycobacteria in healthy fish and aquariums has shown that the incidence of the pathogens is 'quite high'. Samples taken from home aquaria found that 18 of 42 samples taken from 19 fish contained various forms of mycobacteria. The study also showed that mycobacteria were present in the water itself, with 75.4% of samples testing positive. Mycobacteria were also found in snails and crustaceans used for fish feeding.
Further studies of home aquaria in the same year found mycobacteria in 201 of 325 (61.8%) samples. In this group of samples, mycobacteria were most frequently isolated from biofilm (77.1%), plants (68.0%) and sediment (65.9%). Mycobacteria were also isolated from 12.5% of fish food (pellets). The prevailing species were M. fortuitum (13.9%), M. marinum (8.0%) and M. gordonae (4.5%).
The genus Mycobacterium (family Mycobacteriaceae) consists of aerobic, rod-shaped, non-motile members of the Actinomycetes. Over 100 species of mycobacteria are recognized, and although they can be divided into fast- or slow-growing groups according to their replication in laboratory medium, the many species have a number of characteristics in common. Due to the presence of mycolic acids and mycolates, the cell wall is thick and waxy compared to other bacteria. As a result of these compounds and the resulting thick cell wall, mycobacteria are not typically classified by staining as Gram-positive or Gram-negative, but instead are acid-fast bacteria. In addition, mycobacteria are very hardy, surviving in harsh environments, and are naturally resistant to many antibiotics.
Many of these Mycobacterium species are considered ubiquitous in the environment, being able to survive in water or sediment. Such "environmental" mycobacteria can survive in water which contains chlorine, or varies over a wide range of pH and temperature conditions. It is speculated that the water thus serves as a natural habitat and may serve as a means of transmission. Even though environmental mycobacteria can survive outside of an animal host saprophytically, they can also be pathogenic for a wide variety of aquatic fauna and humans.
Before the end of the eighteenth century, piscine mycobacteria had been described by a group of French scientists. They reported that Mycobacterium piscium (a name which is now obsolete) was a pathogen of diseased carp, Cyprinus carpio. Since that time, much research on mycobacterial diseases in fish has been carried out. In 1926, when J. D. Aronson studied a number of saltwater fish, which had died in the Philadelphia Aquarium, USA. He found that the fish had a tuberculosis-type disease with large nodules or ulcers throughout many of their tissues, eg spleen, liver, kidney, gills, heart, and intestines. From the nodules Aronson cultured a bacterium which he named Mycobacterium marinum. Injections of bacteria from this culture killed Goldfish, Frogs, Pigeons, and Mice but had only minor effects on Guinea Pigs and Rabbits.
Disease outbreak in aquarium fish is reported to be related to management factors. However, even the healthiest aquarium can harbour the bacteria. Once present in an aquarium, a 100% infection rate of the population is conceivable. Poor aquarium management results in abnormal stress and a reduction in the normal resistance of the host. Overcrowding, accumulation of waste and organic matter in the water and increasing water temperature (above 28°C) may all be predisposing factors. Attention to water quality and good nutrition will assist the fish in fighting these chronic infections.
Fish may be infected by ingesting feed and water contaminated with faecal material, urine or exudates from diseased animals that contain mycobacteria. The sources and modes of transmission in fish may be related to the infection of invertebrates, such as freshwater snails, daphnia and shrimp. The entry of mycobacteria through skin and gill lesions caused by injury or parasitic infection should also be considered. After the organisms enter the body they may cause skin lesions or spread to other organs through the circulatory or lymphatic system.
The observation of mycobacteria in the piscine ova and tubercle granulomas in the ovary wall suggests that transovarian transmission is a definite possibility. A report from an Australian fish hatchery has provided the evidence that mycobacterium can be introduced by eggs and transmitted to the F1 generation. This observation does not confirm that ovarian transmission takes place, as the egg surface may be contaminated by peritoneal fluid containing mycobacteria. However, in 1994 a report confirmed the transmission of mycobacteria in Siamese fighting fish via transovarian passage. Acid-fast bacteria were found in the ova of diseased female Siamese fighting fish, using a fluorochrome technique.
Rainbowfishes seem to be particularly susceptible to mycobacteria infection and it is very common among captive populations. Under pathology examination, mycobacteria have been found in apparently healthy rainbowfishes. Therefore, fish should only be obtained from sources known to be free of the disease. Mycobacterium is widespread in Asian fish farms and is commonly found in imported ornamental fish from that source. I believe that it is now so widely circulated in the hobby that it is almost impossible not to be beset by it at some time.
It is generally believed that infected fishes are the main source and reservoir of mycobacteria in aquaria. Dead fish, which have died from mycobacteria infection, and live carrier fish, can spread these bacteria. Obviously, the practise of feeding sick rainbowfishes to your pet Saratoga or fluffy feline has its risks. Dead fish should be destroyed by burning. Under no circumstances should fish from an infected population be sold, moved or given away.
Clinical Signs:
Signs of mycobacteriosis in fish are variable and often resemble other diseases. Disease may be acute or chronic. The acute form of the disease occurs rarely. It is characterised by rapid morbidity and mortality with few clinical signs. The chronic form of the disease is most commonly seen and it may take two or more years for the number of organisms to grow to readily detectable numbers. Many species of fish may manifest few or no external signs of disease. Because of the slow progression of the disease, younger fish infected with mycobacteriosis show no external signs. As fish age or are stressed, the infection becomes more serious. However, in advanced stages, emaciation, cachexia (wasting, loss of weight), exophthalmia (pop-eye), ascites (dropsy), lordosis (curvature of the spine), haemorrhagic and dermal ulcerative lesions or loss of scales may be observed. Other signs of infection can be seen in the gills, which are paler than normal and show thickened areas on some filaments. Small lesions may be observed around the mouth and vent. Changes in cutaneous pigmentation include a fading of normal colour in aquarium fish or change in colouration. Affected fish generally exhibit lethargic behaviour, floating impassively on the surface of the water, with concurrent loss of appetite. The infection will smoulder in infected fish, resulting in chronic health problems and eventually, mortality in the whole population.
The incubation period varies greatly and depends on susceptibility, temperature, and severity of exposure. It is difficult to specify the length of incubation (the time from infection to the appearance of the first signs of the disease). With rainbowfishes kept in somewhat crowded aquariums and at higher temperatures, it may last only a few weeks or months.
Diagnosis:
Most cases of mycobacteriosis are not identified or more often, are simply misdiagnosed. It is recommended that fish be submitted to a laboratory for identification. Diagnosis of mycobacteriosis depends on clinical and histological signs and identification of the bacterial pathogen. Smears from scrapings of the cut surface of the spleen, kidney or skin lesions should be made and stained with Kinyoun modification of the Ziehl-Neelsen stain. Use of fluorescing dyes is also recommended. Gross and histologic lesions of necrogranulomas containing acid-fast bacilli give strong clues towards the involvement of Mycobacterium. Isolation of the pathogen can provide definitive diagnosis. Attempts can be difficult due to the fastidiousness of the pathogen. Cultures should be incubated for 2 to 30 days at 20-30°C, since M. marinum is a slow grower. Visible growth will require seven or more days of incubation. The cell and colony morphology is not distinctive. Identification may occur on the basis of biochemical characteristics, however these tests are cumbersome and time consuming. Polymerase Chain Reaction (PCR) based methods may be an alternative for diagnosis.
Aquatic mycobacteria can be detected in tissue sections using Ziehl-Neelsen staining, and characterisation is usually based on growth rate, pigmentation, optimal growth temperature and biochemistry. Definitive identification of the type strains, M. marinum, M. fortuitum and M. chelonae, is however, not possible using these conventional methods.
Post-Mortem
Mycobacteriosis lesions occur along the gut or in the skin and gills. Gross or microscopic greyish-white miliary granulomas may be found scattered, grouped or coalescing within any number of organs. Lesions can be hard or soft, range from 80-500 µm in size, and may have a caseous necrotic centre. The spleen, kidney and liver are the most commonly affected. Peritonitis and oedema may be apparent. In severe cases, visceral organs will be swollen and fused by whitish membranes around the mesenteries large caseous necrotic areas.
Treatment:
Mycobacterial infections of fish should be considered non-treatable. Unlike most other bacterial diseases, there is no cure for mycobacteriosis and it will progress despite your best efforts. Efforts to eliminate infection in affected populations with prolonged use of antibiotics have not been successful as mycobacterium species are mostly resistant to conventional antibiotics. There are some reports that a combination of antimicrobials, including streptomycin, ethambutol, cycloserine, cotrimoxazole, rifampicin and tetracyclines, has been used and thought to be effective. Among these drugs, cotrimoxazole or rifampicin with ethambutol have been used most frequently. Isoniazid and rifampicin have been recommended for treating mycobacteriosis in marine fish.
The customary treatment for infected fish is euthanasia and the disinfection of the aquarium before restocking. If several fish become infected in the same aquarium, it is usually assumed that the survivors are carriers and that they be treated accordingly. Following depopulation, the entire system, especially the filter bed, must be thoroughly disinfected with a mycobactericidal product (see below). In addition, all equipment that has been in contact with the infected fish should be disinfected. Gloves should be worn when handling infected fish or cleaning contaminated tanks or other equipment. Hands should be washed thoroughly afterwards with 70% isopropyl alcohol and a bactericidal soap.
Break down the original infected aquarium and any other tank use as a treatment or quarantine tank and disinfect them with a strong chlorine solution. Use Calcium hypochlorite 65% to disinfect any tanks, which are in the vicinity of others housing live fish. Granular chlorine does not volatilise as readily as liquid chlorine (Sodium hypochlorite). In a poorly ventilated fishroom, fumes from liquid chlorine can cause fish kills in adjacent tanks. Concentrations of 200 mg/L for 30 to 60 minutes or 100 mg/L for 24 hours should be effective for disinfections of tanks, substrate, and submersed equipment (keep filters running during treatment).
Always use chlorine with caution as repeated use and extended exposure of the silicon sealant to strong chlorine solutions will destroy or render the adhesive bond ineffective on glass aquariums with disastrous results. Chlorine will dissolve synthetic material like sponge filters, but most plastics are unaffected. Calcium hypochlorite is an oxidising agent and should not be exposed to intense heat, acids, or organic compounds because it is a fire hazard, particularly if wet. In some cases, explosion may occur. Always wear eye protection and rubber gloves when handling large quantities of chlorine. Chlorine can be neutralised by adding Sodium thiosulfate to the solution (7.5 grams of Sodium thiosulfate will neutralise the chlorine present in 5 litres of a solution of 200 mg/L).
Veterinarians at the National Aquarium in Baltimore, USA recommend using chlorine to clean the tank and substrate, etc., and then spray isopropyl alcohol onto the glass, and allow it to dry. They recommend the alcohol as they found that chlorine does not kill all mycobacteria. They use chlorine to remove/oxidise organic material to assure the alcohol contacts all mycobacteria in/on the tank. Remove all residues of disinfectant from the aquarium before reuse. (Denise Petty DVM, pers. comm. 1998).
Mycobacteria are resistant to many commonly used bactericidal agents at standard dosage rates, including chlorine bleach and quaternary ammonium compounds. As much as 10,000 ppm chlorine has been reported necessary to kill mycobacteria. Mycobacteria are sensitive to 60-85% alcohol. In addition, there are special products marketed specifically to decontaminate mycobacteria-infected surfaces. However, if these are used, efforts must be made to remove all residues of disinfectant.
Personal Experience:
Case # 1: In early 1997, I transferred some 4-year-old Goyder River rainbowfishes from their present aquarium to a larger 600-litre aquarium. I had raised 30 individuals and decided to split them in half. The 600-litre aquarium contained a mixture of fully-grown rainbowfishes. Most of them were more than 4 year old with some specimens as old as 9 years. Over the ensuing weeks the Goyder's, one by one, stopped feeding and started to 'hang' just below the surface of the water. Apart from laboured breathing, and looking as though they just had a very large meal, no other symptoms were apparent - death followed soon after. When only 3 individuals were left from the original 15 that had been transferred, I decided that I needed some confirmation of their disorder. I took the remaining 3 fish to veterinarian, Dr. Stephen Pyecroft BVSc (Hons) of Aquatic Diagnostic Services International Pty Ltd for examination.
Stephen's diagnose showed that "Disseminated caseating pyogranulomatous inflammation possibly due to Mycobacterium infection" and "Hepatic Lipidosis" (fatty liver disease). He commented that "The severity and chronicity of the pyogranulomatous inflammation suggests this is the primary disease process. Special stains have shown the presence of acid-fast bacilli consistent with Mycobacterium spp. These organisms were found in the macrophages in the liver and kidney. The hepatic lipidosis is quite severe and could well be associated with hepatoencephalopathy although histological evidence of this was not detected in the brain sections examined. The lipidosis was found in all those examined."
"The fish I concentrated on for the histopathological examination definitely showed the greatest degree of pathology and because of the diagnose of mycobacteriosis we must suspect that the total clinical picture observed is due to this problem. There is no ignoring the fact that these fish on the whole were over weight and that the hepatic lipidosis present would have eventually caused their demise had the TB not caused their final problems."
"The picture is still not that clear and I personally believe that the nutritional imbalance leading to the lipidosis is the major management factor that will need to be addressed. However the fact that a mycobacterial infection is present must, in these fish, be accepted as the primary cause of disease."
What that means in layman terms is that the fish were overweight and infected with mycobacteria. My conclusion from all this was that the 600-litre aquarium was the culprit and knew somewhere down the track that I would have to destroy all the fish and sterilise the tank with chlorine. This belief was confirmed as I continued to have disease outbreaks in this aquarium with some fish displaying similar symptoms while others also developed external lesions. This aquarium was treated with a strong chlorine treatment and all fish and plants destroyed. It is interesting to note that the remaining 15 fish (now 6 years old) in the original aquarium are still doing well, albeit on a somewhat reduced and modified diet, and are showing no signs of disease whatever.
Case # 2: About 8 months after the above episode I presented Stephen Pyecroft with six young (1-year-old) specimens of Melanotaenia oktediensis. All six specimens had what I refer to as "Blackhead Disease" in varying degrees. This disease exhibits itself as a black darkening of one side of the head only. Two of specimens also had small skin eruptions on one side of the body and one also showed the darkening skin colouration along one side of the posterior portion of its body. The most severely affected fish would swim with their head up and tail down and showed an increased respiratory rate. This disease (blackhead) seems to be common among rainbowfishes as I have seen it often and many other hobbyists have spoken to me about this problem. It also seems to be particularly prevalent among Goyder River rainbowfish.
Stephen found that most of the fish had enlarged kidneys, which had a granulated pale colour and protruded beyond their normal position. Granulomas were also present in the spleens and around abdominal organs. Acid fast (Ziehl-Nielsen) stains were preformed on impression smears from most of the affected organs and the presence of acid fast bacteria was confirmed.
The Diagnose: Disseminated granulomous inflammation - nephritis, hepatitis, and peritonitis.
Stephen comments were "As we have discussed before, the dark areas on the skin are most likely due to a malfunctioning in either the pigment cells or the nerves that control the pigment cells in that area of the skin. The findings of a generalised infection with Mycobacterium sp. would be suggestive that the localisation of the dark pigmentation is due to the formation of local abscesses, which are then causing the expression of the major clinical sign. Most of these cases of "blackhead syndrome" in rainbowfish that I have investigated have had a primary infection with Mycobacterium sp. There may be other primary causes of this distinct clinical sign but in these fish it was piscine TB."
Zoonosis:
Mycobacteriosis is different from most other fish diseases that you are likely to experience in your aquarium. This is because mycobacteria are capable of causing a wide range of dissimilar symptoms in infected fish and its ability to cause disease in humans. Human infections caused by pathogens transmitted from fish or the aquatic environment are quite common. Mycobacteriosis is particularly significant among infections transmissible from fish to human beings. The significance of fish mycobacteriosis as zoonosis is evident from case reports published in scientific papers. Ninety-nine publications dealing with the infection of 652 cases of human beings with M. marinum appeared between 1966 and 1996. Of those infections 76% were associated with the aquarium environment.
Mycobacterium marinum has a well-documented zoonotic history. In 1951 it was found that this bacterium was able to infect people who frequented swimming pools. For this reason, the skin infection was termed swimming pool granuloma. Since then, several authors have noted the association of the skin infection with aquariums and tropical fish and today is generally referred to as "fish keeper's disease".
One of the first infections of an aquarium keeper due to Mycobacterium marinum was reported in 1958, and two more cases turned up in California in 1962. These were a 37-year-old woman and her 18-year-old son, who ran a pet shop. Both of them cut their fingers while cleaning a particular freshwater fish tank. Some weeks later they developed a number of nodules near the injury - small, slightly tender swellings which periodically enlarged and discharged pus. Mycobacterium marinum was isolated both from the nodules and from the fish tank.
In 1970, a similar case was reported in Sweden, and identified Daphnia as a possible source of infection. Mycobacterium marinum was not only found in the patient's skin lesions, but also in snails and fish in the patient's aquarium, and in mud in a pond from which Daphnia had been collected and fed to the fish. Also in 1970 three more cases of Mycobacterium marinum infection were reported in California, all of aquarists who cut their hands just before or during work on a tropical aquarium.
The first cases in the Southern Hemisphere were reported from Auckland, New Zealand, in 1971. One was a tropical fish keeper at the Auckland Zoo, another a pet store owner, and a third a part time assistant in a pet store. Mycobacterium marinum was isolated from all three cases. Tanks at the zoo and the pet store, and the elephant pond at the zoo, from which the keeper collected Daphnia, were checked for the presence of Mycobacterium marinum. It was found in five tanks at the zoo, and in dead fish but not at the other sites. In Australia, several cases of "fish keeper's disease" have been reported. In Queensland alone, 29 culture-proven cases of mycobacterium infections were recorded by the Tuberculosis Reference Laboratory between 1971-1990; two of which resulted in amputation.
Although several hundred cases have been reported in the scientific literature, transmission from fish to human is rare. In most cases the infected individual has been in contact with high numbers of mycobacteria, with a break in the skin at the site of original infection. The most commonly infected sites involved are the fingers and hands. Allergic dermatopathies have also been reported on the skin of aquarists handling water in which affected fish have been reared. Anyone who suspects they may have been exposed to mycobacteriosis from handling infected fish should contact their physician and inform them of the nature of the exposure. Diagnosis and treatment may be difficult, especially in view of emerging antibiotic resistance in fish pathogens.
However, "fish keeper's disease" is not a focal infection of the skin. A case of mycobacteria infection contacted from mouth syphoning water from a fish tank has been reported. It concerned an individual who experienced a throat infection that wouldn't get better, and was eventually diagnosed as fishtank granuloma (Practical Fish Keeping, Jan. 1998). So next time you do a water change and take a big suck on the end of the syphon hose - just think of this article.
© Copyright Adrian R. Tappin
Created June, 1999
Updated May, 2008
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