Case-control study of shigellosis in San francisco:: The role of sexual transmission and HIV infection

Background. Shigella species infect similar to 450,000 persons annually in the United States. Person- to- person transmission 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 importance 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, California, 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.