pinfish, eel, sea trout, tilapia, sturgeon, and striped bass (Inglis et al. 1993). Strep has also been
is a genus of bacteria containing
isolated from a variety of ornamental fish, including
some species that cause serious diseases in a
rainbow sharks, red-tailed black sharks, rosey
number of different hosts. A major identifying
barbs, danios, some cichlids including Venustus
feature of Streptococcus
is that they are Gram-
(Nimbochromis ("Haplochromis") venustus)
positive (they appear purple when stained using a
sp., and several species of tetras.
procedure called a Gram stain). By contrast, most of the common disease-causing bacteria of fish are
Strep infections in fish can cause high mortality
Gram-negative (appear pink with a Gram stain).
rates (> 50%) over a period of 3 to 7 days. Some
This is important when considering treatment
outbreaks, however, are more chronic in nature and
options, and will be discussed in greater detail
mortalities may extend over a period of several
weeks, with only a few fish dying each day. A typical history suggesting that Strep may be the
Streptococcal (Strep) diseases of fish are not
cause of disease in a group of fish might include
common; however, when they do occur, significant
reports of abnormal swimming behavior, often
losses can result. Some aquatic Strep species may
described as spiraling or spinning. Any time fish
cause disease in humans in unusual circumstances.
are observed behaving in an unusual manner, Strep
However, these latter species do not usually affect
should be considered as one of the possible causes;
healthy people. In addition to bacteria in the genus
however, not all infected fish show abnormal
there are several other closely
related groups of bacteria that can cause similar disease, including Lactococcus, Enterococcus,
Keep in mind that the keys to disease prevention
. For purposes of this fact sheet, each
in fish include: a) maintaining good water quality;
of these bacteria and the disease itself will be
b) providing proper nutrition; c) keeping the
referred to as Strep.
environment clean; and d) quarantining new fish before adding them to an existing collection.
Streptococcal disease in fish was first reported in
Knowing which species are susceptible to Strep and
1957, affecting cultured rainbow trout in Japan
seeking assistance for rapid diagnosis and proper
(Hoshina et al. 1958). Since then, numerous other
therapy if a disease outbreak should occur are
species of fish have been found susceptible to
important ways to decrease losses.
infection, including salmon, mullet, golden shiner,
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Waddill, Dean.
Streptococcal Infections of Fish 2
Streptococcal Disease in Fish
Clinical (Observable) Signs
Factors Contributing to Disease
Affected fish may exhibit one or more of the
following clinical signs, depending upon the
Most infectious diseases of fish are
species: erratic swimming (such as spiraling or
opportunistic. This means that the simple presence
spinning); loss of buoyancy control; lethargy;
of the pathogen in the environment of the fish is
darkening; uni- or bilateral exophthalmia ("pop-
inadequate to cause a disease outbreak. Other
eye" in one or both eyes); corneal opacity (whitish
factors usually come into play such that either the
eyes); hemorrhages (e,
pathogen has an advantage over the host, or the
the gill plate, base of the fins, vent/anus, over the
immune system of the host is compromised in some
heart, or elsewhere on the body; ascites
way, increasing its susceptibility to the pathogen.
(dropsy/bloating); and ulcerations. In some cases,
This phenomenon is often precipitated by "stress"
the fish may show no obvious signs before death.
and is discussed in greater detail in IFAS Circular 919, Stress − Its Role in Fish Diseases. Stress often plays a significant role in outbreaks of infectious disease in fish populations. Some stressors which have been associated with Strep outbreaks include high water temperatures (e.g., during the summer), high stocking densities, harvesting or handling, and
poor water quality such as high ammonia or nitrite
Bacteria that cause disease in fish are often
categorized based upon their perceived or
Of the signs listed above, hemorrhage, pop-eye,
documented virulence (ability to cause disease);
spinning, and rapidly progressing mortalities are
however, these are inexact characterizations. In
among the most frequent observations.
reality, individual bacterial strains often differ in their virulence, and many bacteria fall somewhere
Necropsy (Internal) Findings
within a wide spectrum of virulence, with true
Internal examination may reveal the presence of
opportunists being the least virulent, and primary
blood-tinged fluid in the body cavity, an enlarged
pathogens being considered the most virulent. For
reddened spleen, pale liver, as well as inflammation
example, the most common bacterial pathogen in
around the heart and kidney. Many Streps infect
freshwater fish, Aeromonas hydrophila, is
the brain and nervous system of fish, explaining the
considered a truly opportunistic pathogen, because
erratic swimming frequently observed in infected
it is relatively common in the aquaculture
environment (hence, the term "environmental bacteria") and typically does not cause disease in
Diagnosis and Treatment
healthy, well-maintained fish populations.
Most common opportunistic fish bacteria, when
viewed with a microscope at high power, appear
Strep, on the other hand, does not seem to be a
pink after a procedure known as Gram staining.
truly opportunistic pathogen, as it can be more
These pink-staining bacteria as a group are called
aggressive than many other environmental bacteria.
Gram-negative. Most of these are also rod-shaped.
In one experimental study (Ferguson et al. 1994),
By contrast, Streps appear purple after Gram
populations of zebra danios and white cloud
staining; purple-staining bacteria as a group are
mountain minnows exposed to high concentrations
considered Gram-positive. Streps are also typically
of Strep in the water experienced 100% mortality
more spherical or oval in shape (‘coccus' meaning
within 2-4 days of exposure. Consequently, it is
ball- or sphere-shaped).
important that infections be quickly identified and managed to prevent major losses.
Streptococcal Infections of Fish 3
A tentative diagnosis of Strep can be made from
Sanitation Practices for Aquaculture Facilities).
the history and clinical signs, necropsy findings,
Although Strep does seem to occur more frequently
and identification of Gram-positive bacteria from
at warmer temperatures, it can occur at any time of
stains of impressions (produced by blotting sections
of fresh tissues onto a glass slide) from the brain, spleen, kidney, or liver. Strep should be highly
Studies in marine systems in Japan (Kitao et al.
suspected if fish exhibit abnormal swimming
1979) indicate that Strep may be present in salt
behavior, pop-eye, hemorrhages, and rapid and
water and muds, with higher incidences in the water
severe mortalities, and Gram-positive cocci are
during summer months. This implies that, in
found in brain, kidney, and/or other organs.
aquaculture systems some Streps which occur naturally in the environment may become endemic
A confirmed diagnosis requires culture of
(established within the farm) and cause disease on a
internal organs, especially the brain and kidney,
periodic basis. As a result, Strep infections may be
followed by identification of the bacterium. Blood
a cyclic, recurring problem especially during
agar is the medium of choice for growing Strep.
periods of high stress.
Identification is important for determining treatment options. There are a variety of commonly
If you suspect that you have a group of fish with
used antibiotics that generally work best against
a Strep infection, the affected population should be
Gram-negative bacteria, and others that work best
isolated from all others. If possible, have dedicated
against Gram-positive bacteria, including Strep.
equipment (nets, siphon hoses) for these fish, and be sure to use a net dip with antibacterial action,
Ideally, after the bacterium has been identified
such as benzalkonium chloride (follow
from a sick fish, a sensitivity test should be
manufacturer's instructions), after using nets for
conducted to select the most effective antibiotic to
infected fish. Strep infection can spread rapidly in a
use. Typically, Gram-positive bacteria, including
group of fish exposed to water contaminated with
Strep, are susceptible to erythromycin. One oral
the bacteria. In addition, Strep can spread in a
dosage rate that has proven effective in the field is
group of fish through oral routes, including
1.5 grams of erythromycin per pound of food, fed
cannibalism of infected fish, or through feeding of
for 10 to 14 days. However, sensitivity testing will
infected fish food. Remove dead fish as soon as
confirm whether or not this is the antibiotic of
possible to minimize oral transmission. Live or
choice when an outbreak is in progress.
unprocessed (fresh or frozen) foods should be
Consequently, the assistance of a fish health
considered a possible source of exposure and
specialist or diagnostic laboratory is highly
should be submitted to a microbiology lab for
recommended. If you raise food or game fish, there
bacterial culture when an outbreak is being
are restrictions on which antibiotics can be used
legally to treat these animals. Consult with a fish health specialist before attempting to treat fish with
Species of Streptococcus that May Cause
a potentially illegal drug that could result in tissue
Disease in Fish
residues. A fish health specialist can also
There are many different species of Strep that
recommend the proper dosage regime for a
cause disease in fish. Many of these have not been
particular antibiotic and situation.
fully characterized. Identification features common to Streps that cause disease in mammals are
frequently not applicable to identification of those
Prevention of disease is always preferable and
species that cause disease in fish.
more profitable than treatment of disease outbreaks. Preventive medicine programs should be designed
Many of the Streptococcus species that are
to minimize stress (see IFAS Extension Circular
pathogenic (disease causing) to fish are found
919, Stress − Its Role in Fish Diseases); maintain
naturally in the environment and may become
the best water quality possible (see IFAS Extension
endemic to a farm. There is not enough information
Circular 715, Management of Water Quality for
available for most to determine which species or
Fish); and minimize exposure to infectious agents
strains of Strep are more pathogenic to fish.
by following appropriate disinfection and sanitation protocols (see IFAS Extension fact sheet VM-87,
Streptococcal Infections of Fish 4
Examples of Streps that have been associated
Although humans did become ill from a Strep
with disease in fish (see Austin and Austin 1999)
infection originating from the handling of diseased
include Streptococcus difficilis, isolated from fish in
tilapia, it is important to note that the people in this
Israel; S. milleri, isolated from the kidney samples
group were older (median age 69 years). Several
of koi observed with external ulcerations (Austin
were in debilitated health, and thus, as a group, had
and Austin 1999); and S. parauberis, isolated from
compromised immune systems. Also, the majority
farmed turbot in northern Spain (Austin and Austin
of affected individuals became ill only following
1999). Streptococcus iniae is another species that
puncture wounds and/or cuts to their hands. What
has been isolated from diseased fish and mammals
does all this mean? The risk of healthy humans
(Austin and Austin 1999), and will be discussed in
getting Strep from diseased fish is apparently very
the following section. Other Gram-positive bacteria
low. Fish health experts working with tilapia
that are closely related to Strep and which have
producers in several states have not seen any
been associated with disease in fish include,
problems resulting from farm workers handling
Lactococcus garvieae, isolated from diseased eels
and yellowtail in Japan (Austin and Austin 1999); L. piscium; and Vagococcus salmoninarum, isolated
People who are immunocompromised are at a
from diseased rainbow trout.
greater risk, especially if they suffer cuts or puncture wounds, and thus these individual should
Disease in Humans
follow appropriate precautions, such as wearing gloves and using antibacterial soap. Although it is
Streptococcus iniae is known to cause disease in
important to be aware of the potential risk posed by
both fish and mammals, including humans (Austin
S. iniae to immunocompromised workers,
and Austin 1999). Streptococcus iniae was first
information to date suggests that the risk is minimal
isolated from an Amazon River dolphin in the
to healthy individuals (Shoemaker and Klesius
1970s. In 1994, S. iniae was reported as the disease
agent from an outbreak in tilapia hybrids. However, it was later determined this Strep was
identical to S. shiloi, first isolated from tilapia from Israel in 1984.
Although not as common as other bacterial
diseases, Strep infections do occur in fish, and can
In 1991, the first recognized case of S. iniae in
result in significant financial loss to the fish farmer.
humans occurred in Texas (MMWR 1996). In
Streps are Gram-positive bacteria. Fish populations
1994, a second case occurred in Ottawa, Canada.
with Strep infections typically experience moderate
However, the source of infection for both cases was
to heavy losses. Because Strep frequently infects
not determined (MMWR 1996).
the brain, abnormal behavior, such as spinning, is often noted. Other signs include lethargy,
In 1995, a group of Canadian doctors described
darkening, exophthalmia ("pop-eye"), hemorrhages,
Streptococcal disease in a group of older fish
ascites (dropsy/bloating), and ulcerations, although
handlers, of whom at least four individuals had
these are not necessarily specific to Strep.
additional underlying ailments, including diabetes
However, in many cases, there may be no obvious
and/or rheumatic heart disease (Weinstein et al.
signs other than rapid mortality. Other potential
1997). They were known to have injured their
causes of disease should be ruled out as part of the
hands while preparing whole raw fish, primarily
diagnostic process. If you suspect you may have a
tilapia, and subsequently became ill. The most
Strep infection in your fish, consult a fish health
common type of infection (8 of 11 patients) was
specialist who can isolate the bacteria from the fish,
cellulitis (infection of the connective tissue) of the
properly identify the disease-causing organism, and
hand, causing fever in these patients. One patient,
recommend appropriate antibiotic therapy.
who had other health risks (heart disease, kidney disease, diabetes, and arthritis), developed a more
Streptococcus iniae, known to infect certain fish
serious infection. All cases were treated with
species, has caused disease in humans, including
antibiotics and resolved.
one group of people in Canada who handled live tilapia. However, these people were considered much more susceptible to disease than the general population because of their advanced age and
Streptococcal Infections of Fish 5
underlying health problems. Additionally, puncture wounds or cuts were required to initiate infection in this group. Normal, healthy persons are at minimal risk of acquiring this disease from sick fish.
Austin, B. and Austin, D.A. 1999. Chapter 2 -
Characteristics of the diseases. In Bacterial Pathogens: Diseases of Farmed and Wild Fish. pp 13-15.
Ferguson, H.W., Morales, J.A. and Ostland,
V.E. 1994. Streptococcosis in aquarium fish,
Diseases of Aquatic Organisms, 19(1): 1-6.
Hoshina, T., Sano, T. and Morimoto, Y. 1958.
A Streptococcus pathogenic to fish. Journal of
Tokyo University of Fisheries, 44: 57-68.
Inglis, V., Roberts, R.J. and Bromage, N.R.
1993. Chapter 12, Streptococcal Infections. In Bacterial Diseases of Fish, Halsted Press, John Wiley & Sons, Inc., NY. pp. 196-97.
Kitao, T., Aoki, T. and Iwata, K. 1979.
Epidemiological Study on streptococcosis of
cultured yellowtail (Seriola quinqueradiata) — I.
Distribution of Streptococcosis sp. in seawater and
muds around yellowtail farms. Bulletin of the
Japanese Society of Scientific Fisheries, 45: 567-
MMWR. 1996. 2 August, 45(30): 650-53.
Invasive Infection with Streptococcus iniae, Ontario, 1995-1996. Morbidity and Mortality Weekly Report available on-line from the Centers for Disease Control's website at www.cdc.gov/mmwr.
Shoemaker, C. and Klesius, P. 1997.
Streptococcal Disease Problems and Control: A Review. In Tilapia Aquaculture, K. Fitzsimmons, Editor, Vol. 2, pp 671-80, NREAES 106, Ithaca, NY.
Weinstein, M.R., Litt, M. and Kertesz, D.A.,
Wyper, P., Rose, D., Coulter, M., McGeer, A.,
Facklam, R., Ostach, C.,Willey, B.M., Borczyk, A.
and Low, D.E. 1997. Invasive infections due to a
fish pathogen, Streptococcus iniae. The New
England Journal of Medicine, August 28, 337(9):
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