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Invasive Infection with Streptococcus iniae -- Ontario, 1995-1996

During December 1995-February 1996, four cases of a bacteremic illness (three accompanied by cellulitis and the fourth with infective endocarditis, meningitis, and probable septic arthritis) were identified among patients at a hospital in Ontario. Streptococcus iniae, a fish pathogen not previously reported as a cause of illness in humans (1-3), was isolated from all four patients. All four patients were of Chinese descent and had a history of preparing fresh, whole fish; three patients for whom information was available had had an injury associated with preparation of fresh, whole fish purchased locally. This report summarizes information about these cases and presents preliminary findings of an ongoing investigation by health officials in Canada (4), which suggests that S. iniae may be an emerging pathogen associated with injury while preparing fresh aquacultured fish. Case Reports

The first three cases occurred during December 15-20, 1995, among previously healthy women who ranged in age from 40-74 years. Each had a history of injury to the hand while preparing fresh, whole, aquacultured fish. The first case-patient reported a puncture wound to her hand with a fish bone while preparing a newly purchased tilapia (Oreochromis species) *, a freshwater fish marketed primarily as whole fish; the second lacerated the skin over her finger with a knife that had just been used to cut and clean a freshwater fish of unknown type; and the third punctured her finger with the dorsal fin while scaling a fresh tilapia.

The period from injury to onset of symptoms for the three cases ranged from 16 hours to 2 days. At the time of hospitalization, physical examination findings included fever (range: 100.4 F {38.0 C} to 101.3 F {38.5 C}) and cellulitis with lymphangitic spread proximate to the site of injury. Leukocyte counts ranged from 12,900/mm3 to 16,900/mm3 with an increased proportion of neutrophils. Blood cultures from all three patients were positive for S. iniae, and treatment with beta-lactam antibiotics or clindamycin resulted in complete resolution of all manifestations of illness.

The fourth patient, a 77-year-old man, was admitted to the hospital on February 1, 1996, because of a 1-week history of increasing knee pain, intermittent sweats, fever, dyspnea, and confusion. Past medical history included diabetes mellitus, hypertension, rheumatic heart disease, chronic renal failure, Paget's disease, and osteoarthritis. Approximately 10 days before admission, he had prepared a fresh tilapia, although it was unknown whether he incurred an injury while preparing the fish. Findings on examination included temperature of 96.1 F (35.6 C) and a large effusion and warmth of the right knee without overlying cellulitis. New murmurs of aortic insufficiency and mitral regurgitation were noted. While in the emergency department, he had a respiratory arrest and was intubated; treatment included administration of a beta-lactam agent and erythromycin. The leukocyte count on admission was 25,200/mm3 with 95% neutrophils. Ten hours following admission, his knee was aspirated, and a lumbar puncture was performed. Analysis of the joint fluid included a leukocyte count of 72,000/mm3 but no evidence of crystals. Analysis of the cerebrospinal fluid (CSF) included a leukocyte count of 87/mm3 (54% neutrophils), a glucose of 14 mg/dL, and a protein of 320 mg/dL. Cultures of samples of synovial fluid and CSF were negative, but blood cultures yielded S. iniae. Based on the clinical and laboratory findings, and a transesophageal echocardiogram that documented a mitral-valve vegetation, S. iniae endocarditis and meningitis were diagnosed. Treatment with beta-lactam antibiotics was continued, and he recovered. Microbiology

Isolates from all patients grew on sheep-blood agar incubated in room air at 95.0 F (35 C), appeared as gram-positive cocci in short chains or pairs, and were catalase-negative. During the first 18 hours of incubation, colonies were alpha-hemolytic and initially were identified as viridans streptococci. Further testing conducted by reference laboratories identified them as S. iniae. Three strains were resistant to bacitracin, and the fourth was susceptible. Pulsed-field gel electrophoresis patterns of chromosomal Sma1 digests of all four isolates were identical. Microbroth-dilution testing for susceptibility indicated that all isolates were susceptible to beta-lactams, macrolides, trimethoprim-sulfamethoxazole, and tetracycline. Follow-Up Investigation

All four patients had prepared fresh, whole fish, three of which were known to be tilapia, that had been purchased from different stores. In two cases, the fish were taken live from holding tanks in different fish markets. Surface cultures were obtained from four fresh tilapia purchased at selected fish markets in the community during March 1996. Cultures from three of the four fish yielded S. iniae; however, pulsed-field gel electrophoresis patterns were different for each, and none matched the outbreak strain. None of the vendors at the markets where the fish were purchased reported that the fish appeared to be sick. Fresh, whole tilapia sold in Ontario were imported from U.S. fish farms.

The ongoing epidemiologic and microbiologic investigation includes the establishment of surveillance for cases of upper-extremity cellulitis in patients visiting the emergency departments of 10 Toronto-area hospitals and use of a standardized questionnaire for interviewing patients. In addition, to better characterize the prevalence of S. iniae in fish, samples from live, aquacultured fish imported into Canada are being collected and tested by Canadian health officials for S. iniae.

Reported by: M Weinstein, MD, DE Low, MD, A McGeer, MD, B Willey, Mount Sinai Hospital and Princess Margaret Hospital, Univ of Toronto, and Canadian Bacterial Diseases Network, Toronto; D Rose, MD, M Coulter, P Wyper, Scarborough Grace Hospital, Scarborough; A Borczyk, MSc, Public Health Laboratory of Ontario, Toronto; M Lovgren, National Reference Center for Streptococcus, Laboratory Center for Disease Control, Edmonton, Alberta, Canada. Childhood and Respiratory Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Because of recent increases in aquaculture, the occurrence of infections caused by a variety of streptococcal species is increasing among some salt-water and freshwater fish. S. iniae was first recognized in 1972 as a cause of disease in an Amazon freshwater dolphin, Inia geoffrensis. In 1986, S. iniae (reported as S. shiloi) was identified as a cause of meningoencephalitis among tilapia and trout in Israel; the organism was identified subsequently among tilapia in the United States and Taiwan. Infections with S. iniae may be asymptomatic or may cause disease associated with death rates of 30% to 50% in affected fishponds (2).

The first recognized case of S. iniae infection in humans occurred in Texas in 1991, and a second case occurred in Ottawa, Canada, in 1994; however, potential sources for both cases were not determined. The pulsed-field gel electrophoresis digest from the isolates causing both of these infections was identical to the isolates of the cases described in this report, except for a one-band shift.

Whether the recent cases of S. iniae infection represent the emergence of a new human pathogen or previously unrecognized disease is unclear. S. iniae infection may not be recognized because cultures rarely are obtained from patients with wound infections or cellulitis and, if cultured, viridans streptococcus isolates may be considered contaminants and not be further characterized. In addition, it is unclear whether human infections may be caused by any S. iniae strain or whether the strain implicated in all six of the cases is more virulent than other strains. Finally, because all four persons described in this report were of Chinese descent, potential racial/ethnic associations with risk for this infection should be further considered. Additional culture surveys and laboratory studies of tilapia should assist in characterizing the diversity and virulence among S. iniae.

To more clearly define the role of S. iniae as a human pathogen, physicians are encouraged to obtain blood and wound cultures from persons with upper-extremity cellulitis and to seek a history of recently having prepared a fresh, whole fish. Microbiology laboratories should be able to make a preliminary identification of S. iniae based on several distinguishing phenotypic characteristics. ** Possible S. iniae isolates can be confirmed at the CDC Streptococcal Reference Laboratory and tested to determine whether they are the same strain as identified from the six cases of human disease.

References

  1. Eldar A, Frelier P, Assenta L, et al. Streptococcus shiloi, the name for an agent causing septicemic infection in fish is a junior synonym of Streptococcus iniae. Int J Syst Bacteriol 1995;45:840-2.

  2. Eldar A, Bejerano Y, Bercovier H. Streptococcus shiloi, and Streptococcus difficile: two new streptococcal species causing a meningoencephalitis in fish. Curr Microbiol 1994;28:139-43.

  3. Perera R, Johnson S, Collins M, et al. Streptococcus iniae associated with mortality of Tilapia nilotica and T. aurea hybrids. Journal of Aquatic Animal Health 1994;6:335-40.

  4. Weinstein M, Low D, McGeer A, et al. Invasive infection due to Streptococcus iniae: a new or previously unrecognized disease -- Ontario, 1995-1996. Canada Communicable Disease Report 1996;22:129- 32.

  5. Pier GB, Madin SH. Streptococcus iniae sp. nov., a beta-hemolytic streptococcus isolated from an Amazon freshwater dolphin, Inia geoffrensis. Int J Syst Bacteriol 1976;26:545-53.

  6. Pier GB, Madin SH, Al-Nakeeb S. Isolation and characterization of a second isolate of Streptococcus iniae. Int J Syst Bacteriol 1978;28:311-4.

* Tilapia is one of the fastest growing aquaculture industries in the United States and the world. 

** S. iniae is beta-hemolytic; however, some strains may appear to be alpha-hemolytic because a narrow zone of beta-hemolysis is surrounded by a larger zone of alpha-hemolysis (5,6). Beta-hemolysis always is observed under anaerobic incubation and in the area of stabs in the agar. S. iniae is nongroupable with Lancefield group A through U antisera. In addition, the pyrrolidonylarylaminase and leucine aminopeptidase tests are positive, the Voges-Proskauer test is negative, and the organism may have variable susceptibility to bacitracin.




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