By Ronald Baker, PhD

Shigella species are a significant cause of bacterial dysentery worldwide, with approximately 165 million cases resulting in more than 1 million deaths each year. In the U.S., Shigella species infect approximately 450,000 persons annually. Person-to-person transmission of Shigella species, which have a low infectious dose, occurs frequently, particularly in areas with poor sanitation and hygiene.

The current case-control study, published in the February 1, 2007 issue of Clinical Infectious Diseases, focused on the sexual transmission of Shigella species among men who have sex with men (MSM) in San Francisco. Sexual transmission of Shigella has been inferred from outbreaks of shigellosis among the MSM population, and limited studies have suggested the importance of HIV infection as a risk factor for shigellosis.

No population-based studies of sporadic shigellosis have evaluated the role of sexual practices (especially among MSM) and HIV infection along with other established risk factors for shigellosis.

Researchers with the University of California at Berkeley and at the San Francisco Department of Public Health conducted a population-based case-control study of shigellosis among adults in San Francisco during the period 1998-1999. Cases of Shigella infection were identified through laboratory-based active surveillance. A total of 76 case patients in this study were matched by sex with 146 control subjects. Exposure data were collected on established risk factors, sexual practices, and HIV infection status.

Results

• In a multivariable analysis, for men, shigellosis was associated with being MSM, HIV infection, direct oral-anal contact, and foreign travel.

• For women, shigellosis was associated only with foreign travel.

Discussion

This study reveals that - at least among MSM in San Francisco - shigellosis is predominantly a sexually transmitted disease. In this population, direct oral-anal sexual contact conferred the highest risk for transmission of Shigella infection. HIV infection likely contributes to transmission through increased host susceptibility.

"Given the continuing outbreaks of shigellosis among MSM," wrote the study authors, "we believe that there is enough evidence…to recommend that MSM avoid direct oral-anal sexual contact, especially if sex partners are ill or if there are community outbreaks of enteric infection."

A number of scientific questions remain unanswered by this study: Does HIV infection in MSM cause increased susceptibility to Shigella infection? Do HIV positive individuals with shigellosis experience more severe HIV disease? Is there a positive role for the use of barrier methods, such as dental dams, to prevent sexual transmission? Are HIV positive women also at higher risk for shigellosis? What, if any, is the impact of CD4 T-cell count on the development of shigellosis?

Despite the limitations of the study, it clearly establishes an association between shigellosis, oral-anal sex between men, and HIV status. These results warrant the issuance of public health recommendations for HIV positive MSM to avoid anal-oral sex.

San Francisco Department of Public Health; School of Public Health, University of California at Berkeley, Berkeley, CA; Department of Health Services, Richmond, and California Emerging Infections Program, Oakland, CA; Centers for Disease Control and Prevention, Atlanta, GA.

01/16/07

References

T J Aragón, D J Vugia, S Shallow, and others. Case-Control Study of Shigellosis in San Francisco: The Role of Sexual Transmission and HIV Infection. Clinical Infectious Diseases 44(3): 327-334. February 1, 2007.

D C Daskalakis and M J Blaser. Another Perfect Storm: Shigella, Men Who Have Sex with Men, and HIV (editorial). Clinical Infectious Diseases 44(3): 335-337. February 1, 2007.

K L Kotloff, J P Winickoff, B Ivanoff, and others. Global burden of Shigella infections: implications for vaccine development and implementation of control strategies. Bulletin of the World Health Organization 77: 651-666. 1999.