Ulcerative Bacterial Dermatitis of Koi (Cyprinus carpio) and Ornamental Goldfish (Carassius auratus auratus )
Shanna L. Siegel;1 T. Lindsay Lewis, BS;2 Niraj K. Tripathi, BVScAH;3 Victoria V. Burnley, BS;4 and Kenneth S. Latimer, DVM, PhD3
- 1Undergraduate Honors Biology, The University of Georgia, Athens, GA 30602
- 2Class of 2002, College of Veterinary Medicine, The University of Georgia, Athens, GA 30602
- 3Department of Pathology, College of Veterinary Medicine, The University of Georgia, Athens, GA 30602
- 4Department of Medical Microbiology and Parasitology, College of Veterinary Medicine, The University of Georgia, Athens, GA 30602
Introduction
Koi (Cyprinus carpio) and fancy goldfish (Carassius auratus auratus) have been symbolic figures in the paintings, pottery, and oral history of Asian and Far Eastern countries for centuries. They also are popular and intensively reared ornamental fishes for pets, display, and show. Intensive rearing of these fishes usually results in high population densities, which subsequently may result in a high incidence of disease.
Ulcerative skin disease of koi and goldfish is of special concern because such disease may result in death or permanent disfigurement. Loss of market value of diseased fish is obvious and may be a cause for considerable economic concern. The causes of skin disease in fish are diverse and include trauma (including netting injuries), poor water quality, stress, parasitism, viral infection, fungal infection, and bacterial infection. The purpose of this electronic publication is to present a brief overview of bacterial skin disease of fish.
The Skin as a Defensive Barrier
The skin provides an important barrier against infection. Microscopically, the skin is composed of the epidermis, dermis, and hypodermis (subcutis). The quality of these layers can vary between species and among individuals. Other factors influencing the quality of the skin include age, sex, season, and environment.
The outermost layer of the skin is the epidermis. The epidermis is a nonkeratinized epithelium that can range from 3-4 to 20 cells in thickness. These epithelial cells composing the epidermis cover the scales as they grow out of the dermis and provide an osmotic barrier in the aquatic environment. Furthermore, the epidermis also produces mucus that helps to protect the skin surface. Since the epidermis is the surface layer of skin, it is within this region that skin disease often develops.
The middle layer of the skin is the dermis. Chromatophores (pigment cells) and scale pockets are two important cellular components within the dermis. Chromatophores provide color and some reflective quality of the skin. The scale pockets are epidermal invaginations from which the scales originate. Scales, an important characteristic of many teleost fishes, generally grow in an overlapping pattern like clapboards on a house. The tough character and overlapping position of the scales provide a dense defensive barrier. Some koi may lack scale coverage, and these fish are called leather skinned.
The subcutis or hypodermis is the deepest layer of the skin. The subcutis consists of loosely arranged connective tissue and contains many capillaries. These capillaries are responsible for transport of oxygen and nutrients to the skin. In addition, this vascular pathway allows for the distribution of leukocytes (white blood cells) and antibodies to sites of cutaneous inflammation and infection. However, these same blood vessels may also provide a channel for the movement of bacterial pathogens from the skin into the systemic circulation.
Clinical Signs of Bacterial Dermatitis
Many pathogens such as parasites, fungi, viruses, and bacteria can cause skin disease in fish, but bacteria are of particular interest because infections are common and often deadly if untreated. Some of the most obvious signs of bacterial dermatitis (bacterial skin infection) include development of reddened lesions, sores, or ulcers on the body; reddening of the base of the fins; and dulling or darkening of skin color (Fig. 1). The distribution of the skin lesions is quite variable and may include the head, face, operculum, mouth, back, trunk or body wall including the lateral line, and caudal peduncle (tail base) (Fig. 1). Other nonspecific signs of ill thrift in infected fish may include anorexia (loss of appetite), weight loss, and decreased activity.
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| Fig 1a. Goldfish, Flavobacterium columnaris infection. Necrosis of fins is accompanied by white, cotton-like accumulations of bacteria and detritus. | Fig 1b. Koi, Aeromonas hydrophila infection. Ulcers are present around the mouth. |
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| Fig. 1c. Koi, Aeromonas hydrophila infection. Ulcers are present below the dorsal fin and on the caudal peduncle. | Fig 1d. Koi, Aeromonas hydrophila infection. Ulcers are scattered over the trunk, base of the fins, and caudal peduncle. |
Bacteria Associated with Skin Infection in Fish
There are numerous species of bacteria, many of which infect fish. Presented below are the major bacterial pathogens that have been associated with dermatitis in ornamental freshwater fishes. This basic list is not meant to be all inclusive. Microbiology and fish medicine textbooks can be consulted to obtain a more extensive listing of bacterial pathogens in fishes (see references).
Gram-negative bacilli: Most of the bacteria that are commonly isolated from fish are Gram-negative bacilli (Fig 2).
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| Fig 2. Koi, Aeromonas hydrophila infection, Lillie-Twort stain. Photomicrograph of infected, necrotic skin and muscle. Gram-negative bacilli (red rods) are present. |
Aeromonas salmonicida is a Gram-negative, non-motile, bacillus (rod-shaped bacterium) that usually measures 2-3 mm in length. This bacterium primarily affects salmonids but it can also infect other freshwater fish, including carp and goldfish. Infection with Aeromonas salmonicida may produce "furunculosis" (skin nodules) and ulcers in goldfish, koi, and other carp. The ulcers are generally surrounded by reddened tissue from which scales can be easily removed. Muscle infection and necrosis are common, allowing infection to penetrate to the depth of the underlying bone. Secondary septicemia is relatively common. Histologically, infected tissues exhibit necrosis with many bacterial colonies but few inflammatory cells. These observations are the result of protease, hemolysin, and leukocidin production by the bacteria. The proteases and hemolysins digest tissue and destroy erythrocytes. Leukocidin, as the name suggests, is an exotoxin that kills inflammatory leukocytes.
Aeromonas hydrophila is a motile, Gram-negative bacillus. This bacterium is free living and is always present in the water. As an opportunist, this bacterium may infect many species of freshwater and brackish water fishes. Synonyms for A. hydrophila infection include "motile aeromonas septicemia," "infectious abdominal dropsy," and "bacterial hemorrhagic septicemia." Infection with this organism is thought to be at least partially associated with overcrowding and high levels of stress in the fish population. This bacterium is associated with hemorrhagic septicemia (blood-borne bacteria). The clinical and histological signs of disease are similar to those associated with A. salmonicida (please see above).
Flavobacterium columnare (previously known as Flexibacter columnaris) is a long, slender, Gram-negative bacillus. This bacterium measures 7 to 10 mm in length, being approximately 10 to 20 times longer than it is wide. Synonyms for F. columnare infection include "columnaris disease," "bacterial gill disease," and "saddle back disease." F. columnare causes erosion of the gills as well as skin lesions. Primary infection of the gill may cause acute death of fish, whereas primary skin infection may have a more protracted clinical course. Cartilage- and protein-degrading enzymes generated by the bacterium erode gill tissue, resulting in death. Skin lesions caused by infection range from shallow, white erosions (early disease) to yellowish-brown ulcers (more advanced disease). Skin lesions typically are found in the "saddleback region" of the fish. Because this bacterium is a natural resident of aquatic habitats where healthy fish are found, it is thought that disease occurs under high stress conditions. The virulence of the bacterium differs between strains. F. columnare infection can be diagnosed using cytological and histological methods, as well as traditional bacterial culture. Cytology is an easy and economical technique for the presumptive diagnosis of columnaris disease. A scraping is taken of the affected area, the material is placed on a glass slide, a coverslip is applied, and the specimen is examined microscopically. This "wet mount" (unstained cytologic preparation) may reveal the characteristic "haystack" formations of bacteria. In addition, the characteristic flexing and sliding motion of the bacterium, extended length, and/or size and shape of the colonies may be appreciated. A related bacterium, Flavobacterium psychrophilum, causes fin rot that progresses to involve the caudal peduncle (tail base).
Pseudomonas fluorescens is a motile, Gram-negative, bacillus. It is associated with subacute ulcerative dermatitis and is the causative agent of lesions similar to those due to Aeromonas sp. as described above. This bacterium may produce a pigment that fluoresces yellow-green under a dark (ultraviolet) lamp.
Edwardsiella sp. are motile, Gram-negative, gas-producing bacilli. There are two main bacterial species, designated Edwardsiella tarda and Edwardsiella ictaluri. The former species causes "putrefactive disease of catfish" (EPDC) or "fish gangrene," while the latter species is associated with "enteric septicemia" of catfish. These bacteria primarily affect channel catfish, but bacterial infection also may be observed in gold fish, golden shiners, and large mouth bass. Edwardsiella sp. also presents a serious threat to eel culture in Asia. The lesions produced by Edwardsiella tarda include small cutaneous ulcers and hemorrhages in the skin and muscles. In advanced disease, large abscesses with malodorous, gas-filled cavities may be observed in muscles. Edwardsiella ictaluri may be associated with petechial (pinpoint) hemorrhages in the skin. Other lesions caused by these organisms may appear similar to those caused by Aeromonas sp. (see appropriate section above).
Yersinia ruckeri is a short, Gram-negative rod of variable motility. This bacterium infects a variety of fishes, including goldfish and carp. Synonyms for the disease include "enteric redmouth," "Hagerman redmouth," and "yersiniosis." Disease is usually manifested as low grade mortality in fingerlings. Clinical signs include lethargy (sluggishness), anorexia (inappetance), dark skin pigmentation, and hemorrhages in and around the mouth, base of the fins, and eyes.
Gram-positive, filamentous bacteria:
Nocardia sp. are Gram-positive, filamentous rods, which can be positively stained by a modified acid-fast staining technique. Aquarium fishes are more prone to the infection but these bacteria also may infect cultured salmonids. Nocardia sp. infection causes a chronic disease characterized by raised masses in mouth, gills, and skin. The dermal masses eventually ulcerate and numerous granulomas are often observed in visceral organs. Skin wounds and abrasions are thought to be the portal of entry for bacterial infection.
Aerobic, Gram-positive bacilli:
Mycobacterium fortuitum and Mycobacterium marinum are Gram-positive, pleomorphic, non-motile bacilli that stain red using an acid-fast staining technique. Mycobacteriosis in fish usually is characterized by protracted development of disease characterized by wasting or emaciation and the formation of granulomas in many tissues and organs, including the skin. Treatment of the disease is largely ineffective. Zoonotic transmission of Mycobacterium sp. from fish or aquatic environments to people has caused skin granulomas referred to as "fish handler’s disease" or "swimming pool granuloma."
Gram-positive cocci:
Streptococcus iniae and Streptococcus dysgalactiae are facultatively anaerobic, Gram-positive cocci, that are usually arranged in short chains. These organisms may cause skin abscesses and shallow ulcers in freshwater fishes. Streptococcus iniae has zoonotic potential in that bacterial infection has been documented in a few people who cut their hands while cleaning fish that harbored the bacterium.
Diagnosis of Bacterial Dermatitis (Bacterial Skin Disease)
Culture and sensitivity testing: Definitive diagnosis of bacterial dermatitis requires culture and sensitivity testing. This procedure allows identification of the bacteria involved with the skin lesions as well as determining their sensitivity or resistance to antibiotics. Because aquatic environments are not sterile, questions may arise whether a given bacterium is a primary pathogen or an opportunist.
Cytology: Cytology is the study of individual cells. This technique can provide useful information as both wet mounts preparations and stained specimens. Wet mount preparations are unstained, but bacterial morphology and arrangement (i.e., haystacks with Flavobacterium columnare infection) may be helpful in making a presumptive diagnosis. Cytology specimens that are stained with traditional (Wright, Giemsa, or Leishman stains) or rapid modifications (Diff-Quik stain) of Romanowsky stains reveal bacteria as blue-staining cocci or bacilli (Fig 3). An exception is Mycobacteria sp. which fail to stain with Romanowsky stains and appear as refractile, unstained bacilli. However, acid-fast techniques will stain Mycobacteria sp. bright red to facilitate their identification.
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| Fig 3a. Koi, cytology specimen, Flavobacterium columnaris infection, Wright-Leishman stain. A uniform population of slender bacteria is present. | Fig 3b. Koi, cytology specimen, Flavobacterium columnaris infection, Wright-Leishman stain. Leukocytes, erythrocytes, and bacteria are present. |
Histopathology: Histopathology is the microscopic study of diseased tissues that have usually been preserved in formalin, processed and infiltrated with paraffin wax, sectioned at 3-4 µm, and stained with hematoxylin and eosin dyes. Tissue specimens obtained while the fish is alive (surgical biopsy) or after the fish has died (postmortem tissue specimens) reveal alterations in tissue architecture associated with bacterial infection (Fig 4). Bacteria can be demonstrated within the tissue by Giemsa staining. Gram staining characteristics can be determined by application of Brown and Brenn, Brown and Hopps, or Lillie-Twort staining. Kinyoun’s acid-fast staining can be used to demonstrate mycobacteria. In addition, a modified acid-fast staining technique can be used to assist in the identification of Nocardia sp.
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| Fig 4a. Koi, histologic specimen, normal gill, hematoxylin & eosin stain. The primary (longitudinal orientation) and secondary (vertical orientation) lamellae are clearly defined for optimal oxygen exchange and waste excretion. | Fig 4b. Koi, histologic specimen, Flavobacterium columnaris infection of gill, hematoxylin & eosin stain. The secondary lamellae are obliterated by detritus, bacteria, and inflammatory cell infiltrates. |
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| Fig 4c. Koi, histologic specimen, normal muscle (body wall), hematoxylin & eosin stain. The myocytes and their nuclei are clearly visible. | Fig 4d. Koi, histologic specimen, necrotic muscle (body wall), Aeromonas hydrophila infection, hematoxylin & eosin stain. The myofibers are necrotic (dead) as a result of severe bacterial infection. |
Treatment
Treatment of bacterial skin disease depends upon the species of bacteria involved and the severity of the disease process. Antibiotics, 8% salt dip, and cleansing wounds with 7.5% betadine solution (an organic iodine preparation) have been used to successfully treat skin ulcers and erosions in fish. Selection of antibiotics should always be made with knowledge of the culture and sensitivity data. Some bacterial infections, particularly those caused by Gram-negative bacilli, may be rapidly fatal despite attempted treatment. Conversely, skin disease secondary to chronic mycobacteriosis may be slowly progressive but not very amenable to treatment.
Good water quality, good diet, and lack of crowding also are important in treating diseased fish. Frequent water changes (with the addition of 0.3% NaCl to water), a clean filtration system, and proper maintenance insure that water conditions are optimal and help to eliminate unwanted waste products and organic detritus. Attention to water quality has proven to be one of the most effective actions to slow or eliminate the progression of bacterial infection. Good nutrition also is important to promote the immune system and tissue healing. Lastly, individual fish with skin lesions should be isolated in a hospital tank during treatment to prevent the infection of other clinically healthy fish. If many fish are infected, the population may have to be distributed among several designated tanks to reduce crowding and promote optimal response to treatment.
Selected References and Links
Textbooks:
Noga EJ: Fish Disease: Diagnosis and Treatment. Mosby-Year Book, Inc., St. Louis, 1996.
Stoskopf MK: Fish Medicine. W. B. Saunders Co., Philadelphia, 1993.
Ferguson HW: Systemic Pathology of Fish. A Text and Atlas of Comparative Tissue
Responses in Diseases of Teleosts. Iowa State University Press, Ames, 1989.
Selected Refereed Journal Articles:
Bader JA, Shotts EB: Determination of phylogenetic relationships of Flavobacterium psychrophilum (Flexibacter psychrophilus) and Flavobacterium columnare (Flexibacter columnaris), and Flexibacter maritimus by sequence analysis of 16s ribosomal RNA genes amplified by polymerase chain reaction. J Aquat Animal Health 10:320-327, 1998.
Decostere A, Haesebrouck F, Charlier G, Ducatelle R: The association of Flavobacterium columnare strains of high and low virulence with gill tissue of black mollies (Poecilia sphenops). Vet Micro 67:287-298, 1999.
Decostere A, Haesebrouck F, Devriese LA: Characterization of four Flavobacterium columnare (Flexibacter columnaris) strains isolated from tropical fish. Vet Micro 62:35-45, 1998.
Noga EJ, Wright JF, Pasarell L: Some unusual features of mycobacteriosis in the cichlid fish Oreochromis mossambicus. J Com Pathol 102:335-344, 1990.
Internet Resources:
Anders BB, Burnley VV, Ritchie B, Poet SE: Identification of the etiologic agent for ulcerative disease in koi (Cyprinus carpio). Second International Virtual Conference in Veterinary Medicine: Diseases of Exotic Animals and Wildlife, Athens, GA, 1999. http://www.vet.uga.edu/ivcvm/1999/anders/anders.htm
Aquatic Pathobiology Center, University System of Maryland, Baltimore, MD: Normal Histology of the Fathead Minnow provides excellent general guidance in fish histology, including the integumentary system (skin). http://medschool.umaryland.edu/AquaticPath/fhm/index.html
AquaVet: Links to various aquatic related pages, including species, journals, and regulatory organizations. http://lama.kcc.hawaii.edu/praise/aquavet.html
Colorado Koi, Denver, CO: Overview of koi history, husbandry, maintenance, diseases, etc. http://www.coloradokoi.com/
FishDoc, United Kingdom: A general information website related to fish diseases and based in the United Kingdom. http://www.fish.disease.btinternet.co.uk/koi_inf.htm
International Virtual Conferences in Veterinary Medicine, The University of Georgia, Athens, GA: Yearly virtual conference with a themes on some aspect of (exotic) animal health. The topic for the 2001 conference will be disease of aquatic organisms. http://www.vet.uga.edu/ivcvm/
Moeller RB: Diseases of fish. Armed Forces Institute of Pathology, Washington, DC. http://www.aquaworldnet.com/awmag/diseases.htm
Photo Gallery of Bacterial Pathogens: Photomicrographs of pathogenic bacteria, including Gram-staining characteristics. http://www.geocities.com/CapeCanaveral/3504/gallery.htm
Acknowledgement
"Goldfish and Iris" © Victoria Fine Art. http://www.genesisnet.com/victoria/