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Circular 57
pinfish, eel, sea trout, tilapia, sturgeon, and striped bass (Inglis et al. 1993). Strep has also been Streptococcus 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) and positive (they appear purple when stained using a Pelvicachromis 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 Streptococcus, there are several other closely related groups of bacteria that can cause similar disease, including Lactococcus, Enterococcus, and Keep in mind that the keys to disease prevention Vagococcus. 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 Prevention
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 diseased tilapia. 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. References
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-
72.
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):
589-94.

Source: http://www.twra.us/FishDiseasesParasites_TWRA/files/Streptococcal.pdf