MAJOR ARTICLE Case-Control Study of Shigellosis in San Francisco: The Role of Sexual Transmission and HIV Infection Toma´s J. Arago´n, 1,2 Duc J. Vugia, 3,4 Sue Shallow, 4 Michael C. Samuel, 4 Arthur Reingold, 2,4 Frederick J. Angulo, 5 and Williamson Z. Bradford 4 San Francisco Department of Public Health, City and County of San Francisco, San Francisco, 2 School of Public Health, University of California at Berkeley, Berkeley, 3 California Department of Health Services, Richmond, and 4 California Emerging Infections Program, Oakland, California; and 5 Centers for Disease Control and Prevention, Atlanta, Georgia (See the editorial commentary by Daskalakis and Blaser on pages 335–7) Background. Shigella species infect ∼450,000 persons annually in the United States. Person-to-person trans- mission of Shigella species, which have a low infectious dose, occurs frequently, particularly in areas with poor sanitation and hygiene. Sexual transmission of Shigella species among men who have sex with men (MSM) has been inferred from outbreaks of shigellosis among that population, and limited studies have suggested the im- portance of human immunodeficiency virus (HIV) infection as a risk factor for shigellosis. No population-based study of sporadic shigellosis has evaluated the role of sexual practices (especially among MSM) and HIV infection along with other established risk factors for shigellosis. Methods. We conducted a population-based case-control study of shigellosis in adults in San Francisco, Cal- ifornia, during the period 1998–1999. Cases of Shigella infection were identified through laboratory-based active surveillance conducted by the California Emerging Infections Program. Seventy-six case patients were matched by sex with 146 control subjects. Exposure data were collected on established risk factors, sexual practices, and HIV infection status. Bivariable and multivariable analyses were conducted. Population-attributable fractions were calculated. Results. From the multivariable analysis, for men, shigellosis was associated with MSM (odds ratio [OR], 8.24; 95% confidence interval [CI], 2.70–25.2), HIV infection (OR, 8.17; 95% CI, 2.71–24.6), direct oral-anal contact (OR, 7.50; 95% CI, 1.74–32.3), and foreign travel (OR, 20.0; 95% CI, 5.26–76.3), with population-attributable fractions of 0.72, 0.42, 0.31, and 0.18, respectively. For women, shigellosis was associated only with foreign travel (OR, 21.0; 95% CI, 2.52–899), with a population-attributable fraction of 0.37. Conclusions. Among MSM, shigellosis is predominantly a sexually transmitted disease, with direct oral-anal contact conferring the highest risk and HIV infection likely contributing to increased host susceptibility. Each year, an estimated 450,000 persons in the United States are infected with Shigella species, resulting in 6200 hospitalizations and 70 deaths [1]. Humans and other primates are the only natural reservoirs for Shi- gella species [2]. Shigella species are transmitted by the fecal-oral route, and most infections are transmitted from person to person, reflecting the low infectious dose [3]; as few as 10 viable organisms can result in clinical infection [4]. In areas in which infection is prev- Received 6 September 2006; accepted 27 September 2006; electronically published 29 December 2006. Reprints or correspondence: Dr. Toma´s J. Arago´n, School of Public Health, Div. of Epidemiology, University of California at Berkeley, 140 Warren Hall, MC-7360, Berkeley, CA 94720-7360 (aragon@berkeley.edu). Clinical Infectious Diseases 2007; 44:327–34 2006 by the Infectious Diseases Society of America. All rights reserved. alent, risk for Shigella infection increases with poor hand hygiene, ingestion of contaminated food or water, inadequate sanitation and toileting, overcrowding, and sexual contact [5–13]. Among reported culture-confirmed infections in the United States during the period 1989–2002, 72% were due to Shigella sonnei, 18% were due to Shigella flexneri, 1.6% were due to Shigella boydii, and 0.7% were due to Shigella dysenteriae [3]. Most reported cases occurred in children ! 10 years of age, followed by women 20– 39 years of age; the high rate of infection in the latter group was presumably related to caring for infected children. In the mid-1970s, shigellosis was recognized as a potentially sexually transmitted disease among men who have sex with men (MSM) [7, 14, 15]. In the late 1970s and early 1980s, the increased incidence of S. flexneri infection in men was attributed to the sexual Sexual Transmission of Shigellosis • CID 2007:44 (1 February) • 327