Increasing Relative Burden of Laboratory-Confirmed Shigella Flexneri Infections Among U.S. Men, 2003–2013: An Emerging Sexually Transmitted Disease

Background: Shigellosis causes an estimated 500,000 infections annually in the United States. Although most commonly reported in children <5 years old, many recent reports document outbreaks of antimicrobial-resistant shigellosis among men who have sex with men (MSM). We describe the epidemiology of US shigellosis cases from 2003–2013.    

Methods: We analyzed all cases of culture-confirmed shigellosis reported to CDC’s Laboratory-based Enteric Disease Surveillance system from January 2003–December 2013. Incidence rates were calculated as cases per 100,000 population per year, using census data. We defined men and women as persons ≥18 years old and children as persons <18 years old.

Results: From 2003–2013, 112,581 Shigella isolates were reported. The distribution of isolates by species was: S. sonnei, 74.5%; S. flexneri, 12.0%; S. boydii, 0.9%; S. dysenteriae, 0.3%; 12.3% were reported without species information. The median annual shigellosis incidence nationally was 3.3 (range 1.9–5.5) overall, 2.5 (range, 1.5–3.7) for S. sonnei, and 0.4 (range, 0.3–0.6) for S. flexneri.  In 2003, S. sonnei infection rates among women, children, and men were 1.7, 9.5 and 1.2; in 2013 they were 0.7, 3.9, and 0.5, respectively. In 2003, S. flexneri infection rates among women, children, and men were 0.3, 1.1, and 0.5; in 2013 they were 0.1, 0.3, and 0.5, respectively. Men-to-women incidence ratios were 0.9 for S. sonnei infections and 1.7 for S. flexneri infections overall.

Conclusions: The incidence of S. flexneri infections among men in 2013 equaled that of S. sonnei infections and was five times that of women in the United States.  The high relative burden of S. flexneri infections among men may signal increased transmission among MSM. Efforts to understand risk factors for, detect clusters of, and prevent shigellosis among men should be enhanced.