Ulcerative Bacterial Dermatitis of Koi (Cyprinus carpio) and
Ornamental Goldfish (Carassius auratus auratus)
Shanna L. Siegel; T. Lindsay Lewis, DVM; Niraj K. Tripathi,
BVScAH; Victoria V. Burnley, BS;and Kenneth S. Latimer, DVM, PhD
Undergraduate Honors Biology (Siegel), The University of Georgia, Athens, GA 30602; Class of 2002 (Lewis), Department of Pathology (Tripathi, Latimer), and Department of Medical Microbiology and Parasitology (Burnley), 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.
 |
 |
| 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. |
 |
 |
| 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).
 |
| 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.
 |
 |
| 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.
 |
 |
| 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. |
 |
 |
| 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/ |