Friday, October 12, 2007

Vibrio Infections of Fish

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.
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 in fish:

• 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).

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.

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

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.


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 therapy.
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 salmonid.)

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.