Infections of Fish
Peggy A. Reed and Ruth Francis-Floyd
Vibrio infections usually occur in fish from marine and estuarine
environments, and have been reported throughout the world. Occasionally,
vibriosis is reported in freshwater fish. The disease can cause
significant mortality (=>50%) in fish culture facilities once an
outbreak is in progress. Common names for Vibrio infections of fish
include "red pest" of eels, "salt-water furunculosis",
"red boil", and "pike pest". Vibrio infections
can spread rapidly when fish are confined in heavily stocked, commercial
systems and morbidity may reach 100% in affected facilities.
The disease is caused by gram negative bacteria in the family
Vibrionaceae. This group of bacteria includes two important genera which
can be significant fish pathogens. The genus Aeromonas includes
several species which are important pathogens of freshwater fish, although
they occasionally cause disease in marine species (see IFAS Extension Fact
Sheet FA-14 for more information on Aeromonas infections of fish).
Bacteria in the genus Vibrio are important pathogens of marine and
brackish water fish, although they occasionally are reported in freshwater
species. Seven species of Vibrio have been associated with disease
A new, extremely pathogenic Vibrio infection of cold-water marine
fish (i.e., salmon) is caused by V. salmonicida and is referred to
as "cold-water vibrio" or "hitra" disease. Cold-water
vibrio has not been reported in warm-water fish and will not be discussed
further in this publication.
- V. anguillarum (isolated most commonly from marine and
brackish water fish);
- V. ordalli (an atypical strain of
- V. anguillarum , sometimes referred to as Biotype 2);
- V. damsela (isolated from damsel fish);
- V. carchariae (isolated from sharks);
- V. vulnificus (reported in Japanese eels); and
- V. alginolyticus (reported from cultured seabream in Israel).
Vibrio species are also known to cause disease in humans, most
often following the consumption of contaminated shellfish. Most serious
illness is usually limited to individuals with a suppressed immune system,
such as those with liver disease or Acquired Immune Deficiency Syndrome
(AIDS). However, it is always wise to wear gloves while examining sick
fish, and to wash your hands thoroughly with a bactericidal soap
Signs of Infections
The signs of vibriosis are similar to many other bacterial diseases of
fish. They usually start with lethargy and a loss of appetite. As the
disease progresses, the skin may become discolored, red and necrotic
(dead). Boil-like sores may appear on the body, occasionally breaking
through the skin surface resulting in large, open sores. Bloody blotches (erythema)
are common around the fins and mouth. When the disease becomes systemic,
it can cause exopthalmia ("pop-eye"), and the gut and rectum may
be bloody and filled with fluid. It should be noted that all of these
"signs" can be caused by other bacterial diseases, and are not
proof of a Vibrio infection.
Although a Vibrio infection can be suspected given certain case
histories and clinical signs, proper diagnosis requires isolation of the
bacteria and its identification. If you are unable to perform these tasks
yourself, live, diseased fish should be delivered to a diagnostic
laboratory familiar with fish diseases to confirm the infection, identify
the species of Vibrio , and perform an antibiotic sensitivity test.
Contact your county extension agent for assistance on where and how to
submit samples for diagnostic services.
For those who are capable of culturing bacteria, Vibrio spp.
prefer a blood agar supplemented with 3% salt, but enriched media such as
trypticase soy agar with 5% ovine blood is adequate for initial isolation.
Vibrio spp. can be differentiated from closely related bacteria by
its specific sensitivity to Novobiocin and 0/129, two commercially
available vibriostatic agents. Despite the unique "comma-shape"
of Vibrio bacteria, microscopic examination of infected tissues
cannot be used in place of culture and isolation techniques.
The precise route of Vibrio infection is unclear, but oral
transmission is suspected. It is possible to isolate Vibrio spp.
from the intestinal tract of clinically normal fish. Under certain
conditions, the bacteria may be capable of crossing the intestinal wall,
resulting in systemic disease. Once an outbreak is in progress, the number
of infectious particles in the environment rises dramatically, increasing
the chance that exposed fish will get sick.
In confined, heavily stocked, commercial systems, Vibrio disease
outbreaks can proceed rapidly. Therefore, prevention is essential to any
management scheme. As Vibrio species are believed to be
opportunistic, conditions which favor a disease outbreak are often caused
by environmental stress which can be avoided. Poor nutrition or water
quality, improper handling, overcrowding, and the presence of other
disease-causing agents will all increase your fish's chances of
contracting a Vibrio infection. Parasites are of special concern,
as they often cause damage to fish tissue, creating an ideal location for Vibrio
infections to begin. Chemical treatments, including the use of copper
compounds, can be harsh on fish and have been reported to precipitate Vibrio
Quarantine of new fish and good sanitation practices should be used at
all times, and will minimize the spread of Vibrio infection from
infected to uninfected fish, should a disease outbreak occur. New fish
should always be kept away from existing fish. Tanks and culture
facilities should be kept clean and free of any unnecessary wastes.
Before any treatment with antibiotics, a thorough investigation of water
quality and husbandry practices should be conducted. Removal of underlying
problems is essential to successful resolution of the problem.
Occasionally, removal of contributing factors (i.e., poor water quality)
will be all that is required to control the infection, but in most cases
it is prudent to treat an active Vibrio outbreak with antibiotic
The selection of an antibiotic should be based on results of an in
vitro sensitivity test. There are two antibiotics which have been
approved by the Food and Drug Administration (FDA) for use in food fish
(catfish and salmonids) in the United States. Terramycin contains the
antibiotic oxytetracycline. It is sold for fish in a sinking feed and
should be fed for 10 days. Fish which have been fed Terramycin should not
be eaten for at least 21 days following treatment (the legal withdrawal
time) to ensure complete elimination of drug residue from edible tissue.
Romet is a potentiated sulfonamide which contains two drugs,
sulfadimethoxine and ormetoprim. It is sold for fish in a floating feed
and should be fed for 5 days. The withdrawal time of Romet for catfish is
only 3 days because the drug is bound in the skin of the fish which is
removed when catfish are cleaned. In salmonids, however, the withdrawal
time is 6 weeks because the fish are not skinned during processing. Either
drug will be effective if the strain of Vibrio is sensitive to it
and if sick fish ingest enough medication to maintain the drug in the
bloodstream throughout the treatment period.
In pet fish, the traditional treatment for bacterial disease has been
the addition of antibiotics to tank water. This practice should only be
pursued as a last resort. Antibiotics should be delivered to fish in
medicated feeds or by injection. Flake foods which contain Terramycin or
Romet are commercially available through pet retail outlets for use in
aquarium fish. Because there is no FDA-approved antibiotic available for
use in pet fish, veterinary supervision of antibiotic therapy is
recommended. If fish do not respond to antibiotic therapy within 48 hours
a sample of sick fish and water should be sent to a fish disease
diagnostic laboratory to confirm the original diagnosis and determine
whether additional problems, such as parasitism, may also be present.
Commercial vaccines are available to prevent vibriosis in salmonids. In
the United States, animal vaccines are regulated by the United States
Department of Agriculture (USDA). In species other than salmonids,
veterinary supervision will be required for vaccine access and use.
Vaccinated fish appear to grow and survive better than their unvaccinated
counterparts, however the exact nature of the immunity provided is not
clear. Most commercial products are bivalent vaccines; this means that
they provide protection to two different organisms, in this case V.
anguillarum and V. ordalli . Vaccines are usually administered
to the fish by immersion, although injectable and oral products are also
available. Efficacy of oral vaccines has not been as good as injectable or
immersion products.Vaccines made from sonicated, heat-killed bacteria are
also available and effective.
Vibriosis is primarily a disease of marine and estuarine fish, both in
commercial production systems and natural waters throughout the world.
Stress and overcrowding often are associated with disease outbreak.
Although a presumptive diagnosis can be made based on history and
examination of the fish, a definitive diagnosis can only be made following
bacterial isolation and identification. Antibiotic therapy should be based
on results of in vitro sensitivity tests. Two antibiotics,
Terramycin (an oxytetracycline compound) and Romet (a potentiated
sulfonamide) have been approved by the Food and Drug Administration for
use in catfish and salmonids. Vaccination, which can be administered by
injection, immersion, or orally, is used by the salmon industry to
minimize the impact of vibriosis.
Peggy A. Reed, Biological Scientist,
Fisheries and Aquatic Sciences;
Ruth Francis-Floyd, IFAS Extension
Veterinarian, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville, 32611.
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). Published June 1996.