Flavobacteriosis
Columnaris (Flavobacteriosis) is the common name for disease caused by the bacterium Flavobacterium columnare, which often gives the appearance of white fungal growth on the fish, but is actually a bacterial infection. It is characterised by gill necrosis, greyish white spots on the body, skin erosion, and finrot. It is mostly an external infection but the bacterium is capable of entering the blood stream and is often isolated from the internal organs. Fish may die however, without any clinical symptoms. The biology of the disease is poorly understood even though it is one of the oldest known fish diseases. Flavobacterium is a genus of gram-negative, aerobic or facultatively anaerobic bacterium that consists of at least ten recognised species. They are highly heterogeneous and comprised of pathogenic and non-pathogenic species. Several species are known to cause disease in freshwater fish; Flavobacterium columnare and F. psychrophilum are the main species involved. Although other species belonging to the Flavobacterium genus could be involved in several cases. Flavobacterium psychrophilum is more frequently diagnosed in a wider temperature range especially in temperate water.
Flavobacterium columnare is ubiquitous (i.e., found in water and soil), and on the skin of healthy fish worldwide. It was first described by Herbert Spencer Davis in 1922. It has been a significant problem in many warm water fish species for decades and is among the most common pathogens in aquaculture, ornamental, and wild fish populations. Its name is derived from columnar shaped bacteria, which are present in virtually all aquarium environments. It may be opportunistic or of a secondary infectious nature. References to the disease can be confusing. It has been referred to by different names including Bacillus columnaris, Chondrococcus columnaris, Flexibacter columnaris, Cytophaga columnaris, and most recently Flavobacterium columnare (Bernardet et al. 1996).
Flavobacterium columnare is generally of low pathogenicity although the pathogenesis of the disease is not all that clear. In acute infections, hypoxia and death may result from extensive damage to the gills. Columnaris disease is usually transmitted by direct contact with infected fish or contaminated water. Young fish are more susceptible to Flavobacterium columnare than older fish, but healthy, older fish with previous columnaris infection can be carriers of the disease. The infection can be expected to spread most rapidly if water conditions are less than ideal making fish more susceptible to infections by various pathogens.
In most instances, bacterial infection occurs in fish that are exposed to stressful conditions, such as high water temperature (~28°C), overcrowding, excessive handling, and poor water quality, especially high ammonia or nitrite concentrations or increased organic waste content (~2 g/L) in the water. However, spontaneous, natural infections with Flavobacterium columnare have been reported in the absence of any obvious stressors. Spontaneous infections usually involve highly virulent strains and are associated with high mortality. Mortality rates can be extremely high, with 60–100% mortality common. Several days elapse before mortality results from infection by low virulence strains. High virulence strains cause death within 24–48 hours post exposure to the pathogen. High mortality (~100%) has been reported with acute Flavobacterium columnare infections in the absence of obvious clinical signs or lesions. The morbidity, mortality, and course of disease depend mainly on the water temperature and virulence of the bacterial strain.
In an experimental infectivity study, mortality did not occur at 5°C or 10°C. In contrast, 25% mortality was observed at 15° C, with a mean death time of 7 days, whereas 100% mortality was recorded at 20°C and higher, with a mean death time of 1 to 3 days. This study demonstrated that increasing temperature is associated with increased mortality during bacterial infection. Sudden changes in water temperature of 5°C or more pose significant stress, predisposing fish to infection by Flavobacterium columnare. The ideal temperature for the growth of Flavobacterium columnare is 20−25°C, but some strains are known to grow at temperatures ranging from 4−37°C. Good survival of the bacterium occurs over a wide range of water pH and hardness. Flavobacterium columnare are known to form biofilms, which might also serve as an infection source as well and influence prolonged and repeated infections.
The initial clinical signs of columnaris disease are nonspecific and include listlessness, lethargy, inappetence, swimming near the water surface, and accelerated opercular movement. The disease can have different clinical manifestations, with various combinations of gill, skin, or fin lesions. Columnaris disease associated with primary gill involvement is acute, with mortalities occurring in the range from 2–5 days. Fish with peracute columnaris disease may be observed lying on their sides.
Characteristic skin discoloration and ulcers are not usually observed. Sudden and vehement onset of columnaris disease is usually observed in younger fish that die within 1–4 days without visible lesions. Fish with established columnaris disease usually have lesions on the external body surface and gills.
The distribution of these lesions tends to vary with the species of the fish. In scaleless fish, the skin lesions begin as areas of discoloration, primarily at the base of the dorsal fin. However, lesions may also be seen on the head and craniodorsal part of the body. As skin lesions spread from the base of the dorsal fin, a pale white band extends laterally and encircles the body to form a characteristic, pale white, “saddleback” lesion. A yellowishwhite ulcer often develops in the centre of the “saddle” as the lesion progresses. In advanced disease, extensive and deep skin ulcers may develop, exposing underlying muscle and bone. In scaled fish, prominent gill and fin necroses are usually observed; however, skin ulcers may also be present in the absence of gill and fin lesions. Necrosis of gills and fins begins at the outer margins and extends from the distal end toward the body. Gill necrosis is observed as yellowish-white spots on the tips of the primary lamellae of the gills. Initially, the skin lesions are less prominent in scaled fish but become obvious as the skin lesions advance from mild hyperaemia to deep skin ulcers. The scales become loosened and slough off as the skin disintegrates. Grossly, bacterial mats can be seen attached to skin and/or fins and have a typical “cotton wool” appearance.
In aquarium fish species, inflammation of the mucous membrane of the mouth is common. The mouth and inner walls of the oral cavity may be covered with a yellowish-brown mucoid-like growth. This condition is popularly called cotton-wool mouth, and fungi are frequent secondary invaders. The infection may involve the opercula, teeth, maxillae, mandibles, and the spongy bones of the head.
The diagnosis of columnaris disease is based on cytological examination of smears from skin, gills, and fins; histopathology examination of tissues obtained during necropsy; and microbiologic culture, which is the most reliable technique for the definitive diagnosis of Flavobacterium columnare infection. The lesions are characterised by the presence of long, thin rods that exhibit flexing movement and are able to form columns. To verify the diagnosis, isolation of Flavobacterium columnare is required. However, isolation is often problematic, because the disease usually presents as a mixed infection with numerous other opportunistic bacteria belonging to the normal skin flora.
Treatment Columnaris disease can be treated successfully, provided it is diagnosed early. Avoidance of exposure to the disease is the primary method of prevention. An aquarium that has had a columnaris outbreak should be completely disinfected before restocking if all the fish have been lost. In addition, all equipment that has been in contact with the infected fish should be disinfected. This disease can be spread easily between tanks from contaminated nets, shared equipment, etc.
Because Flavobacterium columnare primarily attacks the skin and gills, infections in the early stages usually respond to treatment using surface-acting disinfectants. The in vitro growth of F. columnare is reported to be inhibited at 10 g/L
Sodium chloride (NaCl). A salt bath at 10 g/L of water for 20– 30 minutes can be used. Stop the treatment earlier if the fish show signs of stress.
When treating systemic (internal) infections, medicated antibiotic feed or injection, are the preferred methods. In such cases an appropriate antibiotic can be given as an in-feed preparation. The U.S. Food and Drug Administration have approved the use of Florfenicol as a medication feed additive for the treatment of Flavobacterium columnare. 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. However, the effectiveness of oral antibiotic therapy has been inconsistent and, as a consequence, mortalities continue to occur. Several trials have been undertaken with alternative antibiotics such as doxycycline and enrofloxacine; however they have shown uneven results.
Antibiotics are only effective in treating bacterial diseases if treatment is applied very early during the course of the disease. Flavobacterium have shown resistance to chloramphenicol, streptomycin, ampicillin, tetracycline, chlortetracycline, oxytetracycline, neomycin, nitrofurazone, nalidixic acid, kanamycin and penicillin G. The resistance of Flavobacterium columnare to polymyxin and neomycin has also been reported by Fijan, Griffin, Bullock et al., Bernardet and Grimont. When bacteria become resistant to a specific antibiotic, even high concentrations of that drug will not be effective.