There are several diseases which can affect rainbowfishes. Opportunistic bacteria and parasites can cause dermal and systemic infections. Intestinal nematodes can cause chronic wasting (anorexia) and considerably damage to the intestinal lining. Additionally, water moulds and fungi may also present disease problems in poorly managed aquariums. However, one of the most common and problematic diseases of captive rainbowfishes is mycobacteriosis.
The increasing popularity of rainbowfishes has resulted in a significant increase in the number of commercial operators breeding, rearing and distributing rainbowfishes. This increases the potential for dissemination and exacerbation of infectious diseases, such as mycobacteriosis. Rainbowfishes are a highly susceptible species based on numerous reports and anecdotal observations. Perhaps rainbowfishes differ from other aquarium fishes in their immunological response to mycobacterial organisms.
Two terms are used to describe the disease, either "piscine tuberculosis" or "mycobacteriosis". Mycobacteriosis is usually a sub-acute to chronic disease of fish where the etiologic agent is an acid-fast bacillus in the genus Mycobacterium. Chronic proliferative mycobacteriosis is characterised by the formation of granulomas, while subacute and acute forms of the disease are associated with necrosis and acid-fast bacilli scattered diffusely among the kidney, liver, spleen, and often all visceral organs. Under pathology examination, mycobacteria have been found in apparently healthy rainbowfishes. Often no external signs are present until advanced stages of the disease occur, at which time non-specific signs present including emaciation, hemorrhagic and dermal lesions, lethargy, and death. Many Mycobacterium species are ubiquitous in the aquarium hobby and trade, making control by avoidance of these pathogens very difficult. Furthermore, there is currently no effective treatment for mycobacteriosis in rainbowfishes.
Source of Infection
It is generally believed that infected fishes are the main source and reservoir of mycobacteria in aquaria. Mycobacteriosis can also be acquired through the ingestion of mycobacteria present in the aquarium environment, which usually have their origin in detritus derived from dermal lesions, faecal material or exudates etc., shed by diseased animals that contain mycobacteria. The sources and modes of transmission in rainbowfishes 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.
It is suspected that ovarian transmission from parent to offspring may occur. A report from an Australian fish hatchery in 1977 provided evidence of mycobacteriosis transmission from eggs to the F1 generation. This observation does not confirm that ovarian transmission takes place, as the egg surface may have been contaminated by peritoneal fluid containing mycobacteria. However, research in 1994 confirmed the transmission of mycobacteria in Siamese fighting fish (Betta splendens), via transovarian passage. Acid-fast bacteria were found in the ova of diseased female Siamese fighting fish, using the fluorochrome technique. Transovarian transmission has also been reported in Xiphophorus maculatus and Danio rerio. The observation of mycobacteria in the fish's eggs and tubercle granulomas in the ovary wall suggests that transovarian transmission is a definite possibility.
Mycobacteriosis disease outbreak in aquarium fish is often reported to be related to management factors. However, even the healthiest aquarium can harbour the bacteria. A variety of bacterial pathogens are always present in an aquarium, even if the system is maintained in optimal condition. Most of them are ubiquitous in aquatic environments and the non-expression of their virulence could be ascribed to a good management of the system and to a good physiological status of the fish. Moreover, the presence of bacteria described as producers of inhibitory compounds, suggests that the indigenous microbiota can control pathogenic organisms in aquarium systems.
Although there is no firm evidence to confirm that environmental stress can cause mycobacteriosis infection, it has been suggested that an unnatural environment, such as an aquarium, may actually promote the disease. Fish should be maintained under optimal conditions. Inappropriate aquarium conditions can result 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. Beyond overcrowding and confinement, aquarium fish are also subjected to other stressors such as handling, fluctuating temperatures, poor water quality, and social stresses. High numbers of mycobacteria have been correlated with warmer temperatures, low dissolved oxygen and pH. Such factors exacerbate the susceptibility of fish to disease and thus further increase morbidity and mortality in the population. Attention to water quality and good nutrition will assist the fish in fighting these chronic infections. Poor nutritional health can greatly enhance the progression and severity, and reactivation of disease. Once present in an aquarium, infection rates can vary from 10 to 100%. The severity of the disease is influenced by a number of interrelated factors, including bacterial virulence, the kind and degree of stress exerted on the population of fish, the physiologic condition of the host, and the degree of resistance inherent within specific populations of fishes.
Prevention
Rainbowfishes should be obtained from specific pathogen-free sources and quarantined when received. In addition, knowledge of the origin and aquarium practices of your source can help you prevent future potential problems. Breeders should maintain separate young brood fish populations and avoid using older brood fish. Breeders who breed from wild stock don't generally have a problem.
A better understanding of the pathogenesis of mycobacteriosis, including factors affecting host susceptibility, may enable aquarists to manage this pathogen and prevent potential disease outbreaks through effective aquarium practices. The implementation of preventive measures in controlling chronic mycobacteriosis is particularly relevant, due to difficulties in treatment, and different fish species probably have different levels of sensitivity. Clearly, prevention and appropriate routine disinfection should be viewed as the primary means to control mycobacteria in aquarium systems. The chronic nature of mycobacteriosis means that it is often too late for any remedial action to be taken once the first cases have been observed and diagnosed. The same protocol can be used in quarantine systems, at least on a periodic basis, to prevent potential concentration of mycobacteria. Although fish should be quarantined for at least 4-8 weeks before being placed in their main aquarium, most fish become clinically affected after a longer period of time. Direct lethal sampling of a quarantined population, with histopathology and culture, may be necessary to detect subclinical infections.
Ideally all equipment such as nets, hoses, buckets, etc. that comes into contact with stock (diseased or healthy) should be immersed into a strong biocide regularly (such as hypochlorite or iodophores), ideally after each use, to achieve sterilisation. Quaternary ammonium compounds can also be used. All this products however, must be rinsed adequately prior to reuse because all these compounds are toxic to fish.
Clinical Signs
Early signs of mycobacteriosis may be subtle or unapparent, and visual clinical signs often do not develop until the disease has become widely systemic. Clinical signs of mycobacteriosis are not specific to the disease and often resemble other diseases. They can vary in occurrence and severity and infected fish may manifest few or no external signs of disease. Clinical signs can vary between fish species and the species of mycobacteria can also influence the clinical symptoms observed. Mycobacteriosis is generally a chronic, slowly progressive disease. 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 months to years for the number of organisms to grow to readily detectable numbers. There is ample evidence that these organisms are capable of adapting to prolonged periods of dormancy in tissues, and that this dormancy is responsible for the latency of disease.
Chronic mycobacteriosis infections manifest themselves primarily as swollen white patches or lumps on the body that turn into red or pale lesions. Fish with only skin infections may have several types of concealed lesions. Both the dermis and epidermis are eroded and the underlying musculature becomes severely necrotic. At this stage, the infection has usually become systemic and the infection on the surface of the skin may occur throughout the peritoneum and musculature. Internally the liver, kidney, and spleen may be impaired.
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. Nevertheless, mycobacterial lesions have been observed in rainbowfishes as young as three months old. It is difficult to specify the length of incubation (the time from infection to the appearance of the first signs of the disease). The incubation period varies greatly and depends on susceptibility, temperature, and severity of exposure. With rainbowfishes kept in substandard conditions and at higher temperatures, it may last only a few weeks or months.
If clinical signs develop, emaciation, cachexia (wasting, loss of weight), exophthalmia (pop-eye), ascites (dropsy), skeletal deformities (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, isolation, abnormal swimming behaviour, floating impassively on the surface of the water, with concurrent loss of appetite. Poor growth, panophthalmitis and retarded sexual maturation may also occur. Affected fish populations may show chronic low-level mortality, and increased susceptibility to parasitic infection. Any group of rainbowfishes showing chronic, low-level mortality and spawning difficulties, regardless of whether they show external signs of the disease should be submitted to a fish-health laboratory for investigation. Mycobacteriosis probably predisposed the fish to other pathogens commonly found in the aquarium fishes.
Diagnosis
Unfortunately, there is no non-lethal method available to identify infected individuals, especially those in early to mid stages of disease. Slow mycobacterial growth rates contribute to the late onset and chronic effects of mycobacteriosis. By the time clinical signs and low-level mortality are observed, the disease may already be entrenched in the aquarium population. Methods for detection of infected individuals have yet to be developed. The techniques for diagnosing mycobacteriosis in fish are continually evolving, but clinical signs and gross pathology may give an initial indication of infection with mycobacterial species. Most cases of mycobacteriosis are not identified or more often, are simply misdiagnosed. It is therefore recommended that infected fish be submitted to a laboratory for identification.
Hobbyists Diagnose
The most common bacterial infections in aquarium fish are caused by organisms such as Aeromonas, Pseudomonas, Mycobacterium and Flavobacterium. Aeromonas has been found to be the most common. All of them cause opportunistic skin infections often caused by injury or parasitic infection. Mortality increases significantly once bacteria enter the circulatory system. Aeromonas, Pseudomonas and Flavobacterium generally have short incubation period and rapid progression of infection. Clinical signs are generally reached within one week of the initial infection of the disease. On the other hand, mycobacteriosis is a chronic disease and it may take a long time for infected fish to show any clinical signs. Aeromonas infections can cause 100% mortality amongst fish in 21 days. The average mortality rate of Pseudomonas can be as high as 50% during the first 21 days with continued mortality for another 7-14 days. Within 36 hours of infection with Flavobacterium, fish will show areas of greyish discoloration. Once established, the infection can spread quickly and cause high mortality rates. In contrast, mycobacteriosis infected fish populations generally show low-level mortality. Therefore, if you have an infected rainbowfish with a lesion that has not changed that much for more than 21 days, then I would suggest that in all probability it will be a case of mycobacteriosis.
Treatment
Control of mycobacteriosis in aquarium systems is extremely difficult once an infection has occurred. Unlike most other bacterial fish diseases, there is no cure for mycobacteriosis and it will progress despite your best efforts, resulting in chronic health problems and eventually, mortality in the whole population.
Currently there are no satisfactory treatments for mycobacteriosis and infections of aquarium fish should be considered non-treatable. There are several reasons why systemic mycobacteriosis should not be treated. There is a lack of information on the bioavailability of most chemotherapeutic agents in aquarium fishes, as are successful well-documented clinical trials. In fact, no chemotherapeutic agent is approved for the treatment of mycobacteriosis in aquariums or aquaculture. Efforts to eliminate infection in affected populations with antibiotics have not been successful as mycobacteria are mostly resistant to conventional antibiotics. Finally, mycobacteriosis has zoonotic potential.
In most situations, the customary treatment for infected fish or populations is euthanasia of the entire stock (especially in breeding facilities), and the disinfection of the aquarium before restocking. If several fish become infected in the same aquarium, it is usually assumed that the others are carriers and that they be treated accordingly. Fish that have survived an epizootic disease and have recovered may be latent carriers, posing a significant risk to the entire population. Following depopulation, the entire system, especially the filters and substrate, must be thoroughly disinfected with a biocide. 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 200-1000 mg/L available chlorine for 60 minutes 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).
However, disinfection is not always successful due in large part to the resistance of many species of mycobacteria to common disinfectants. Mycobacteria are resistant to many commonly used bactericidal agents at standard dosage rates, including chlorine and quaternary ammonium compounds. Mycobacteria can be highly resistant to chlorine disinfection. As much as 10,000 mg/L available chlorine has been reported necessary to kill some species of mycobacteria. Bacterial biofilm in an aquarium can harbour the organism even after aquariums and equipment are disinfected; indeed, biofilm bacteria appear to be more resistant to disinfection than free organisms.
Veterinarians at the National Aquarium in Baltimore, USA recommend using chlorine to clean the tank and substrate, etc., and then spray 65-90% 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).
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., of Aquatic Diagnostic Services International 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 mycobacteria transmitted from fish or the aquatic environment is quite common.
Mycobacteria have 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".
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 March, 2009
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