Aeromonas infections are caused by bacteria which are present in the
water all of the time. Usually, when fish get sick with an Aeromonas
infection, something has happened to make them susceptible to bacterial
invasion. There are several species of Aeromonas which can infect fish.
The first is Aeromonas salmonicida, which causes a disease called
furunculosis in salmon and trout. This bacteria is not usually of concern
for producers of warmwater fish and will not be discussed further in this
publication. The two species of Aeromonas which do cause disease in
warmwater fish are Aeromonas hydrophila and Aeromonas sobria. The
difference between these two bacteria is of greater interest to scientists
than of practical importance to producers; thus, they will be referred to
collectively as Aeromonas infections or Motile Aeromonas Septicemia (MAS).
Aeromonas infections are probably the most common bacterial disease
diagnosed in cultured warmwater fish. Usually, mortality rates are low
(10% or less) and losses may occur over a period of time (2 to 3 weeks or
longer). In these instances, some factor; usually stress, has caused the
fish to become more susceptible to the bacteria. Common sources of stress
are poor water quality, overcrowding, or rough handling.
Some strains of Aeromonas are more virulent, which means that they
possess special properties which enable them to cause more serious disease
outbreaks. If these more damaging strains become endemic in a population
of fish (which means that they are there all of the time and the fish
develop an immunity to them), it becomes difficult to introduce new fish
into the water body without suffering major losses of newly-stocked fish.
Signs of Aeromonas infection
There is no single physical or behavioral sign specific for Aeromonas
infections. Infected fish frequently have: small pinpoint hemorrhages at
the base of the fins or on the skin, distended abdomens, and protruding
eyes. Internal signs include: fluid in the abdomen, swollen liver and
spleen, and the intestines are distended and fluid-filled.
Submission of suspect fish to a diagnostic laboratory
It is important to submit fish suspected of being infected with Aeromonas
to a diagnostic laboratory to confirm the disease, and to determine the
antibiotic sensitivity of the strain of Aeromonas causing the problem. In
addition, because Aeromonas is a stress-mediated disease, it is not
unusual to find that infected fish are heavily parasitized or concurrently
infected with another systemic disease agent. Contact your county
extension agent for assistance and information on where and how to submit
samples for diagnostic services.
Management of an Aeromonas outbreak
When MAS, or any bacterial infection, is suspected in your fish, you
should immediately submit a live, sick fish to the nearest diagnostic
facility. If Aeromonas is diagnosed, you need to know what legal drug the
isolate is sensitive to and whether or not other infectious agents are
present. There are two antibiotics legal for the treatment of bacterial
diseases of channel catfish. Both of these are administered in the feed.
The first, Terramycin, has been available for many years and many strains
of Aeromonas are resistant to it. If the bacteria is resistant to the drug
then there is no benefit attained by feeding that medication. Terramycin~,
an oxytetracycline product, is available in sinking feed only, and is fed
for 10 days followed by a 21-day withdrawal time. The other product,
Romet-30 , a potentiated sulfonamide, has only been available since 1985.
It is available in a floating feed, and is fed for 5 days, followed by a
3-day withdrawal period. The withdrawal period is the time you need to
wait after feeding the medicated feed for the last time until the fish can
be sold for human consumption.
In many cases, it may not be necessary to treat Aeromonas infections
with medicated feeds. For example, if fish are heavily parasitized, they
may resist the bacterial disease if the parasites are removed. Similarly,
if disease susceptibility is attributed to poor water quality, then
correction of the basic husbandry problem could result in a resolution of
the bacterial disease outbreak. Keep in mind that the purpose of
antibiotics is to keep disease-causing bacteria at bay long enough for the
fish to heal itself. In addition, if the affected system is an indoor or
closed system, good sanitation is essential to decrease the number of
bacteria in the system.
Motile Aeromonas Septicemia (MAS) is a common bacterial disease, caused by
Aeromonas, which affects warmwater fish, both in commercial production
systems and in natural waters. Frequently, MAS outbreaks are
stress-related and elimination of the underlying stress factor may be
sufficient to resolve the disease outbreak. It is important to run
antibiotic sensitivity tests prior to using antibiotics for controlling
Aeromonas outbreaks. Many strains of Aeromonas are resistant to
commonly-used antibiotics, and it is important to determine which drug
should be used before spending time and money on an ineffective product.
Anytime an Aeromonas infection persists as a chronic problem, it is
important to make an effort to determine if an underlying stress factor is
causing the fish to have insufficient immune protection from the bacteria.
Evaluate the water quality, nutrition, handling, and use of drugs and
chemicals. Deficiencies in any of these areas could predispose fish to
Aeromonas infections. Cleanliness and good management practices will
reduce Aeromonas outbreaks in a fish production unit.
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). date first printed june