Abstract
We conducted a retrospective study of 40 case-patients and 58 controls as part of a nationwide investigation of a group B Streptococcus outbreak in Singapore in 2015. Eating a Chinese-style raw fish dish (yusheng) was a major risk factor for bacteremia, particularly caused by serotype III sequence type 283.
Keywords: group B Streptococcus, streptococci, Streptococcus agalactiae, sequence type 283, serotype III, bacteria, outbreak, raw fish, yusheng, raw foods, food-borne infections, food safety, Singapore
Group B Streptococcus (GBS) disease is caused by Streptoccocus agalactiae, a common commensal bacteria found in the gastrointestinal and genital tracts of 15%–30% of healthy adults (1). GBS can cause invasive infections, especially in elderly persons and those who have underlying medical conditions, such as diabetes mellitus (2). The serotype III sequence type 283 (ST283) strain has been associated with invasive disease in adults (3). GBS infection is not a reportable disease in Singapore, and positive culture isolates are not routinely serotyped.
GBS can also cause bovine mastitis (4) and disease in saltwater and freshwater fish (5–7). GBS serotypes Ia and III ST283 and ST491 have also been detected in tilapia from Southeast Asia (8).
In June 2015, the Ministry of Health (MOH) in Singapore was alerted to an increase in the number of GBS bacteremia cases among adults admitted to 6 acute-care public hospitals. Seven acute-care public hospitals in Singapore account for >80% of beds for acute-care patients; no increase in cases was observed at a women’s and children’s hospital (Figure).
Figure.
Timeline of group B Streptococcus bacteremia cases reported in 6 public hospitals, Singapore, epidemiologic weeks 1−52, 2015.
Anecdotal links between eating raw fish and illness were reported for some patients. Although 1 study in the United States found a link between eating fish and risk for GBS colonization in adults (9), and fish and humans can be infected with the same strains (5,8), links between eating fish and GBS infection in humans have not been established. To identify risk factors associated with this purportedly foodborne outbreak, the MOH conducted a case−control study in July 2015 as part of a national outbreak investigation.
The Study
The retrospective case−control study comprised 40 case-patients and 58 healthy contacts. Case-patients were defined as patients from the 6 hospitals who had GBS isolated from blood cultures during June 1−July 14, 2015, the period of highest GBS bacteremia incidence. Healthy household members or colleagues of case-patients were selected as controls because they were more likely to have had similar dietary patterns.
Using a standardized interviewer-administered questionnaire, we obtained demographic data, medical history, and 2-week food and exposure histories. Information was also obtained on consumption of raw fish, seafood, or unpasteurized dairy products; exposure to live fish; or invasive medical procedures. Data were collected over a period of 1 week.
Positive blood culture isolates from 36 of the 40 case-patients were obtained from hospital laboratories. Capsular typing was conducted by the National Public Health Laboratory (NPHL) of the MOH by using a PCR-based method (10). The NPHL was unable to obtain positive blood culture isolates from 4 case-patients for further typing. After the case selection period, the public hospitals were requested to report all cases of GBS bacteremia to the MOH and submit positive blood culture isolates for capsular typing by the NPHL. Sequence types were determined by using multilocus sequence typing (11,12).
Mean age of case-patients (61.5 years) was higher than that of controls (44.8 years) (Table 1). A larger proportion of case-patients (52.5%) were male than controls (31.0%). Most case-patients and controls were of Chinese ethnicity; none had occupational exposure to live animals. A total of 72.5% of case-patients reported underlying medical conditions, more than twice the total for controls (31.0%).
Table 1. Characteristics of case-patients and controls in group B Streptococcus outbreak investigation, Singapore, 2015*.
| Characteristic | No. (%) case-patients, n = 40 | No. (%) controls, n = 58 | p value |
|---|---|---|---|
| Age group, y | <0.0005 | ||
| 0–4 | 0 | 0 | |
| 5–14 | 0 | 1 (1.7) | |
| 15–24 | 0 | 5 (8.6) | |
| 25–34 | 0 | 9 (15.5) | |
| 35–44 | 6 (15.0) | 13 (22.4) | |
| 45–54 | 8 (20.0) | 9 (15.5) | |
| 55–64 | 7 (17.5) | 11 (19.0) | |
| >65 | 19 (47.5) | 5 (8.6) | |
| Unknown |
0 |
5 (8.6) |
|
| Mean age, y | 61.5 | 44.8 | |
| Median age, y | 61.5 | 44.0 | |
| Age range, y |
33–68 |
14–95 |
|
| Sex | 0.038 | ||
| M | 21 (52.5) | 18 (31.0) | |
| F |
19 (47.5) |
40 (69.0) |
|
| Ethnicity | 0.527 | ||
| Chinese | 36 (90.0) | 47 (81.0) | |
| Malay | 3 (7.5) | 6 (10.3) | |
| Indian | 0 | 2 (3.4) | |
| Other |
1 (2.5) |
3 (5.2) |
|
| Housing type† | 7 (17.5) | 10 (17.2) | 0.991 |
| 3-room flats and smaller‡ | 17 (42.5) | 22 (37.9) | |
| 4-room HDB flats | 8 (20.0) | 13 (22.4) | |
| Executive/5-room HDB flats | 4 (10.0) | 7 (12.1) | |
| Private housing |
4 (10.0) |
6 (10.3) |
|
| Concurrent illness | <0.0005 | ||
| No | 11 (27.5) | 40 (69.0) | |
| Yes | 29 (72.5) | 18 (31.0) | |
| Diabetes mellitus | 15 (37.5) | 5 (8.6) | 0.001 |
| Cancer | 3 (7.5) | 0 | 0.065 |
| Cardiovascular disease | 22 (55.0) | 13 (22.4) | 0.001 |
| Renal disease | 5 (12.5) | 0 | 0.010 |
| Liver disease | 3 (7.5) | 0 | 0.065 |
| Gastrointestinal disease | 1 (2.5) | 0 | 0.408 |
| Respiratory disease | 3 (7.5) | 0 | 0.065 |
| Blood disease | 2 (5.0) | 1 (1.7) | 0.565 |
*HDB, Housing Development Board. †Housing type was used as a proxy for socioeconomic status; >80% of the population in Singapore live in public housing managed by the HDB. ‡HDB flats are considered public housing by the HDB.
Univariable logistic regression analysis showed that eating raw fish was associated with GBS bacteremia (Table 2). Eating yusheng, Chinese-style raw fish dish usually prepared with freshwater fish, such as Asian bighead carp or snakehead fish, or saltwater fish, such as wolf herring, was a major risk factor (odds ratio [OR] 5.11, 95% CI, 1.93–13.52). The odds of GBS bacteremia among case-patients >65 years of age were 17.73 times that of those 14–44 years of age (95% CI 4.73–66.52). We found no association between patient age and eating yusheng or between GBS and other exposures. However, diabetes mellitus (OR 6.36, 95% CI 2.08–19.45) and cardiovascular disease (OR 4.23, 95% CI 1.76–10.17) were major risk factors.
Table 2. Association between bacteremia and risk factors in a case−control study of a group B Streptococcus outbreak, Singapore, 2015*.
| Risk factor | No. (%) case-patients, n = 40 | No. (%) controls, n = 58 | Univariable model |
Multivariable model† |
|||
|---|---|---|---|---|---|---|---|
| Crude OR (95% CI) | p value | Adjusted OR (95% CI)† | p value | ||||
| Raw fish | 19 (47.5) | 11 (19.0) | 3.87 (1.57–9.54) | 0.003 | 8.58 (2.25–32.69) | 0.002 | |
| Yusheng‡ | 18 (45.0) | 8 (13.8) | 5.11 (1.93–13.52) | 0.001 | 11.38 (2.76–46.98) | 0.001 | |
| Sashimi/sushi | 5 (12.5) | 3 (5.2) | 2.62 (0.59–11.65) | 0.206 | 4.39 (0.74–25.91) | 0.103 | |
| Raw shellfish | 1 (2.5) | 1 (1.7) | 1.46 (0.09–24.07) | 0.791 | 5.18 (0.23–114.95) | 0.299 | |
| Fish-related activities§ | 2 (5.0) | 6 (10.3) | 0.46 (0.09–2.39) | 0.352 | 0.71 (0.10–5.07) | 0.736 | |
*OR, odds ratio. †Included age group (14–44, 45–54, 55–64, and >65 y), sex, and medical conditions as independent variables in a logistic regression model. ‡Chinese-style raw fish dish. §Included fishing, fish spas, fish rearing, and gutting of fish.
A multivariable model that controlled for age group, sex, and concurrent medical conditions showed that the odds of GBS bacteremia among those who had eaten raw fish were 8.58 times that of those who had not eaten raw fish (95% CI 2.25–32.69). Eating yusheng remained a major risk factor in separate analysis by type of raw fish food items consumed (adjusted OR 11.38, 95% CI 2.76–46.98) (Table 2).
Mean and median durations between eating raw fish and onset of illness were 3.5 and 4.5 days, respectively. Mean and median durations between eating yusheng and onset of illness were 3.7 and 4.0 days, respectively.
Of 36 isolates tested, all from case-patients who ate raw fish (n = 19) were serotype III ST283. Isolates from case-patients who did not eat raw fish had various serotypes: Ia (n = 1), II (n = 4), III (n = 7), VI (n = 4), and VII (n = 1). Restricted analysis of 25 case-patients with serotype III ST283 bacteremia and 58 controls showed a strong association between eating yusheng and serotype III ST283 bacteremia (adjusted OR 25.92, 95% CI 5.38–124.96).
Mean age of case-patients infected with ST283 was significantly younger than patients infected with non-ST283 (57.4 vs. 68.6 years; p = 0.014). A significantly lower proportion of patients infected with ST283 than non-ST283 reported having underlying medical conditions (56% vs. 100%; p = 0.003).
Conclusions
We found a strong association between eating yusheng and GBS serotype III ST283 bacteremia. These findings are further supported by the presence of GBS serotype III ST283 in fish samples tested by the Agri-Food and Veterinary Authority of Singapore and the National Environment Agency (NEA) (13). Other major risk factors for GBS bacteremia, such as age >65 years, diabetes mellitus, and cardiovascular disease, were consistent with known risk factors.
After a public advisory was issued on July 24, 2015, to halt sales of raw fish dishes made with Asian bighead carp and snakehead fish (14), the weekly number of invasive GBS cases decreased rapidly in <2 weeks, from a peak of 26 cases/week to an average of <5 cases/week. This decrease supported the hypothesis of a short incubation period after eating raw fish. The proportion of serotype III ST238 cases also decreased rapidly after the advisory (Figure).
This study might be limited by recall bias, in which case-patients were more likely to recall exposure to raw fish, given the increased public awareness at the time of the study. Not all patients given a diagnosis within the case selection period were interviewed because some were not available or too ill to be interviewed. We obtained a case:control ratio of only 1:1.45 because of difficulties in recruiting participants who were willing to be interviewed. Case-patients detected before June 1 were not recruited because they would have been unlikely to provide an accurate food history. Nonetheless, we still obtained statistical power >80% in detecting a 4-fold increased odds of GBS bacteremia after eating yusheng. Although we found no strong association between GBS bacteremia and consumption of sashimi, sushi, or raw shellfish or exposure to fish-related activities, power to detect statistical significance was limited because of small sample sizes.
Although we found a strong epidemiologic link between eating raw fish, particularly yusheng, and GBS serotype III ST283 bacteremia, further studies are needed to understand the precise mechanism of transmission from raw fish to humans and the pathologic process that takes place for infection to occur. In the interest of public health, the NEA has since banned use of freshwater fish in all ready-to-eat raw fish dishes sold by retail food establishments (15). The NEA and the Agri-Food and Veterinary Authority of Singapore have also increased measures to ensure that raw saltwater fish intended for consumption are closely monitored and tested for safety.
Acknowledgments
We thank the infectious disease physicians, microbiologists, and epidemiologists at the public and private healthcare institutions in Singapore for their contributions to this study.
Biography
Dr. Tan is a medical officer at the Ministry of Health, Singapore. Her primary research interests are public health and disease prevention.
Footnotes
Suggested citation for this article: Tan S, Lin Y, Foo K, Koh HF, Tow C, Zhang Y, et al. Group B Streptococcus serotype III sequence type 283 bacteremia associated with consumption of Chinese-style raw fish dish, Singapore. Emerg Infect Dis. 2016 Nov [date cited]. http://dx.doi.org/10.3201/eid2211.160210
References
- 1.Landwehr-Kenzel S, Henneke P. Interaction of Streptococcus agalactiae and cellular innate immunity in colonization and disease. Front Immunol. 2014;5:519. 10.3389/fimmu.2014.00519 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Farley MM. Group B streptococcal disease in nonpregnant adults. Clin Infect Dis. 2001;33:556–61. 10.1086/322696 [DOI] [PubMed] [Google Scholar]
- 3.Ip M, Cheuk ES, Tsui MH, Kong F, Leung TN, Gilbert GL. Identification of a Streptococcus agalactiae serotype III subtype 4 clone in association with adult invasive disease in Hong Kong. J Clin Microbiol. 2006;44:4252–4. 10.1128/JCM.01533-06 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Oliveira IC, de Mattos MC, Pinto TA, Ferreira-Carvalho BT, Benchetrit LC, Whiting AA, et al. Genetic relatedness between group B streptococci originating from bovine mastitis and a human group B Streptococcus type V cluster displaying an identical pulsed-field gel electrophoresis pattern. Clin Microbiol Infect. 2006;12:887–93. 10.1111/j.1469-0691.2006.01508.x [DOI] [PubMed] [Google Scholar]
- 5.Ye X, Li J, Lu M, Deng G, Jiang X, Tian Y, et al. Identification and molecular typing of Streptococcus agalactiae isolated from pond-cultured tilapia in China. Fish Sci. 2011;77:623–32 . 10.1007/s12562-011-0365-4 [DOI] [Google Scholar]
- 6.Evans JJ, Klesius PH, Pasnik DJ, Bohnsack JF. Human Streptococcus agalactiae isolate in Nile tilapia (Oreochromis niloticus). Emerg Infect Dis. 2009;15:774–6. 10.3201/eid1505.080222 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Ruane NM, Collins EM, Geary M, Swords D, Hickey C, Geoghegan F. Isolation of Streptococcus agalactiae and an aquatic birnavirus from doctor fish Garra rufa L. Ir Vet J. 2013;66:16. 10.1186/2046-0481-66-16 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Delannoy CM, Crumlish M, Fontaine MC, Pollock J, Foster G, Dagleish MP, et al. Human Streptococcus agalactiae strains in aquatic mammals and fish. BMC Microbiol. 2013;13:41. 10.1186/1471-2180-13-41 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Foxman B, Gillespie BW, Manning SD, Marrs CF. Risk factors for group B streptococcal colonization: potential for different transmission systems by capsular type. Ann Epidemiol. 2007;17:854–62. 10.1016/j.annepidem.2007.05.014 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Yao K, Poulsen K, Maione D, Rinaudo CD, Baldassarri L, Telford JL, et al. ; Members of the DEVANI Study Group. Capsular gene typing of Streptococcus agalactiae compared to serotyping by latex agglutination. J Clin Microbiol. 2013;51:503–7. 10.1128/JCM.02417-12 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.MLST databases. Streptococcus agalactiae (group B streptococcus, GBS) [cited 2016 Feb 4]. http://pubmlst.org/sagalactiae/
- 12.Jolley KA, Maiden MC. BIGSdb: Scalable analysis of bacterial genome variation at the population level. BMC Bioinformatics. 2010;11:595. 10.1186/1471-2105-11-595 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.National Environment Agency. Singapore. Advisory issued on consumption of raw fish, November 27, 2015. [cited 2016 Jan 12]. https://www.ava.gov.sg/docs/default-source/default-document-library/gbs-media-release-nov-27_final_upload.pdf
- 14.Ministry of Health. Singapore. Update on investigation into group B streptococcus cases. July 24, 2015. [cited 2016 Jan 12]. https://www.moh.gov.sg/content/moh_web/home/pressRoom/pressRoomItemRelease/2015/update-on-investigation-into-group-b-streptococcus-cases.html
- 15.National Environment Agency. Singapore. Freshwater fish banned from ready-to-eat raw fish dishes. December 5, 2015. [cited 2016 Jan 12]. http://www.nea.gov.sg/corporate-functions/newsroom/news-releases/freshwater-fish-banned-from-ready-to-eat-raw-fish-dishes