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Shigella sonnei Outbreak Among Men Who Have Sex with Men --- San Francisco, California, 2000--2001

Shigella sonnei causes approximately 10,000 cases of gastroenteritis each year in the United States (1). These infections occur predominately among young children and usually are associated with poor hygienic conditions in child-care settings. Outbreaks of shigellosis among men who have sex with men (MSM) have occurred because of direct or indirect oral-anal contact (2,3) but usually are caused by Shigella flexneri (4). This report describes an investigation of S. sonnei cases that occurred among MSM in San Francisco during May--December 2000. Following efforts to heighten awareness, the number of cases has declined, but new cases continue to occur at low levels in this risk group (Figure 1). The increased incidence of sexually transmitted Shigella and other sexually transmitted diseases (STDs) in MSM require renewed and innovative prevention efforts.

During June--December 2000, 230 cases of culture-confirmed* S. sonnei infection were reported to the San Francisco Department of Public Health; an average of 21 cases (range: 13--29 cases) occurred during the same period from 1996 to 1999. Based on data obtained from 230 reported cases, the median age was 39 years (range: 16--77 years) and 211 (92%) patients were males. Of 199 males for whom information was available, 141 (71%) were non-Hispanic whites, 159 (80%) were residents of predominantly gay neighborhoods, and 121 (61%) were self-reported MSM. Sexual behavior was unknown for 62 (31%) patients, and 16 (8%) were self-reported heterosexuals. On the basis of denominator data obtained from the annual San Francisco HIV/AIDS epidemiology report, the rate of S. sonnei infection among MSM was 259 per 100,000 population. The rate among all other groups, including women and heterosexual men, was 16 (5).

Among persons aged >18 years with S. sonnei and symptom onset during May--December 2000, 106 were selected randomly for telephone interview; 35 (33%) could not be contacted and four (4%) refused to participate. Of the 67 (63%) who agreed to participate, 64 (96%) were male. Among the 64 male respondents, 62 (97%) were MSM, 42 (66%) were college graduates, and 29 (46%) had an annual income >$45,000. Of the respondents, 49 (78%) had health insurance coverage, 45 (70%) thought they became ill from a sexual partner, and 35 (55%) reported concurrent infection with human immunodeficiency virus (HIV).

The median duration of symptoms for male respondents was 7 days (range: 2--90 days); 62 (97%) reported diarrhea, 50 (78%) abdominal cramps, 49 (77%) fever, 47 (73%) weight loss, and 20 (31%) blood in stool.

In the week before illness, 50 (78%) of the 64 males reported being sexually active, including 34 (53%) who had multiple sex partners; 32 (50%) answered "yes" to, "The week before your illness did you put your tongue in a partner's anus?" Forty-seven (73%) answered "yes" to, "The week before your illness did you have a penis in your mouth?"

Of the 14 patients who reported sexual activity during the week of or the week following illness, three (21%) answered "yes" to, "During [or after] your illness did you have a tongue in your anus?" All 14 persons who were sexually active during and after illness reported diarrhea (duration: 3--23 days) for which they were prescribed antibiotics.

Local response to the outbreak included a press release, development of an Internet web site, and a media campaign with newspaper and Internet articles for the gay community. Approximately 2,000 notices were mailed to community agencies and providers, 10 presentations were conducted for community agencies, and 4,000 health alerts were distributed through a mass mailing to 40 acquired immunodeficiency syndrome-related agencies and their clients, several large gay and lesbian fairs, bars, sex clubs, and the city STD clinic.

Free Shigella screening was offered for 1 month at the city STD clinic. Of 119 patients screened, five reported having diarrhea at presentation to the STD clinic. Two of the five had S. sonnei isolated from their rectal swab samples; no Shigella species were isolated from the 114 remaining clients.

A convenience sample of S. sonnei from outbreak-related patients and controls (women and children with S. sonnei infection in the outbreak period and region) was subtyped by pulsed-field gel electrophoresis (PFGE). Of 26 outbreak-related isolates, 23 (88%) shared one of two closely related patterns, and only one (12%) of eight isolates from controls had a similar PFGE patterns.

Of 20 randomly selected isolates from outbreak-related patients, 19 were resistant to trimethoprim-sulfamethoxazole, tetracycline, ampicillin, sulfisoxazole, and streptomycin. All isolates were susceptible to ciprofloxacin, nalidixic acid, and ceftriaxone.

Reported by: JD Klausner, MD, T Aragon, MD, WTA Enanoria, MPH, JK Mann, MPH, VM Zapitz, MS, D Portnoy, MPH, San Francisco Dept of Public Health, San Francisco; SA Shallow, MPH, California Emerging Infections Disease Program; K Israel-Ballard, MS Kim, MPH, Univ of California at Berkeley, School of Public Health, Berkeley; J O'Connell, DJ Vugia, MD, Acting State Epidemiologist, California Dept of Health Svcs. State Br, Div of Applied Public Health Training, Epidemiology Program Office, Foodborne and Diarrheal Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note:

This report indicates that S. sonnei can cause large community outbreaks through sexual transmission among MSM. The strains circulating among MSM were different from those circulating in the rest of the community, indicating unique transmission. The recent increases in STDs and enteric infections in MSM follow a 10-year decline (4). The rate of S. sonnei remained low in MSM until the summer of 2000 in San Francisco. These trends paralleled changes in sexual behavior that increased the risk for HIV and other STDs (6).

Approximately half of the patients in this report were infected with HIV compared with an estimated prevalence of 20% among MSM in San Francisco (7), suggesting that MSM with HIV infection are more likely to participate in sexual behaviors that place them at risk for shigellosis. Standard HIV management includes stool bacteria cultures of persons with diarrhea. However, HIV-infected persons with shigellosis might have more severe illness (8) leading to more frequent diagnosis and reporting.

The findings in this report are subject to at least two limitations. First, approximately a third of the selected cases could not be contacted, and those who were might have had difficulty accurately recalling events that occurred up to 6 months preceding the interview. Second, the magnitude of this outbreak probably was underestimated because reporting shigellosis in California is required of physicians but not of laboratories, and many cases probably were undiagnosed and unreported.

Because most patients in this outbreak were sexually active with multiple partners, the potential for ongoing transmission is high. In San Francisco and other communities with high rates of shigellosis in adult men, clinicians should obtain stool cultures and sexual orientation data from men with diarrhea and report suspected cases of shigellosis to the health department. Appropriate antimicrobial therapy will decrease the duration, transmission, and severity of symptoms and should be prescribed based on the severity of illness or the need to protect close contacts. Patients in certain occupations (i.e., foodhandlers, child-care providers, and health-care workers) and children who attend child care often are required to have a negative stool culture documented following treatment. The incubation period of shigellosis is 1--4 days, and shigellae are shed in stool from several days to several weeks after illness. Persons who receive appropriate antimicrobial therapy will be culture negative at 72 hours (9).

Patients with shigellosis should be counseled to abstain from sexual behavior that is likely to transmit infection for at least 3 days after starting an appropriate course of antimicrobial therapy (9). Because antimicrobial resistance is common, in cases in which antimicrobial susceptibility data are not available, patients should be counseled on abstaining from high-risk sexual behavior until at least one negative posttreatment stool culture is obtained. Patients also should be counseled on methods to avoid or reduce the risk for sexual transmission of enteric infections such as Shigella and hepatitis A, should be educated to avoid sexual practices that might result in fecal-oral transmission, and should be advised to wash with soap and water the perianal/perineal area, other body parts, and sex toys before and after sexual activity.


  1. CDC. Shigella surveillance: annual tabulation summary, 1999. Atlanta, Georgia: US Department of Health and Human Services, CDC, 2000.
  2. Bader M, Pedersen AHB, Williams R, Spearman J, Anderson H. Venereal transmission of shigellosis in Seattle-King County. STD 1977;4:89--91.
  3. Vugia DJ, Shallow S, Samuel MC, Aragon T, Reingold AL, Bradford WZ. Risk factors for shigellosis in San Francisco adults. 38th annual meeting of the Infectious Diseases Society of America, New Orleans, Louisiana, 2000, 507.
  4. Department of Public Health, City and County of San Francisco. Shigellosis in San Francisco, 1977--1985. San Francisco Epidemiologic Bulletin 1986;2:1--3.
  5. Department of Public Health, City and County of San Francisco. 2000 HIV/AIDS Epidemiology Annual Report. San Francisco Department of Public Health, HIV Seroepidemiology and AIDS Surveillance Section, 2000, 1--58. Available at <>. Accessed October 2001.
  6. CDC. Increases in unsafe sex and rectal gonorrhea among men who have sex with men---San Francisco, California, 1994--1997. MMWR 1999;48:45--8.
  7. Catania JA, Osmond D, Stall RD, et al. The continuing HIV epidemic among men who have sex with men. Am J Public Health 2001;91:907--14.
  8. Baer JT, Vugia DJ, Reingold AL, Aragon T, Angula FJ, Bradford WZ. HIV infection as a risk factor for shigellosis. Emerg Infect Dis 1999;5:820--3.
  9. Lolekha S, Vibulbandhitkit S, Poonyarit P. Response to antimicrobial therapy shigellosis in Thailand. Reviews of Infectious Diseases 1991 March--April;13 Suppl 4:S342--6.

*Defined as culture-confirmed S. sonnei infection in residents of San Francisco County aged >15 years.

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