Shigella, gay men and HIV 'a perfect biological storm'

Most cases of shigella in gay men are sexually transmitted, and having HIV may increase the risk of acquiring or transmitting the infection, according to a study published in San Francisco and published in the February 1^st edition of Clinical Infectious Diseases (now online). An editorial accompanying the study calls the conjunction of shigella, gay men, and HIV a “perfect biological storm.”

There are an estimated 450,000 cases of shigella every year in the United States, resulting in 6,200 hospitalisations and 70 deaths. Shigella is transmitted via the faecal-oral route, with most infections transmitted person-to-person. Consuming contaminated food and drink, poor hand washing, inadequate sanitation, and sexual transmission are though to be the principal modes of infection.

In the 1970s, direct oral-anal contact (rimming) was recognised as a risk factor for shigella amongst gay men. A study, conducted in San Francisco in 1996, suggested that HIV may have a role in the epidemiology of shigella after it was found that there was a higher prevalence of the infection amongst HIV-positive individuals.

It has been theorised that infection with HIV is an important independent risk factor for shigella amongst gay men. Despite this, there has been no epidemiological examination of the roles which sexual behaviour and HIV infection have in the transmission of shigella.

Therefore, between 1998 and 1999, investigators from the California Emerging Infections Program (CEIP) conducted a case-controlled study to assess the roles of sexual behaviour and HIV infection in the transmission of shigella in a non-outbreak period.

A total of 76 shigella cases and 147 controls were recruited to the study. Among men, infection with shigella was associated with foreign travel (odds ratio, 20.0; p = 0.0001), being gay (odds ratio, 8.24; p = 0.0002), infection with HIV (odds ratio, 8.17; p = 0.0002), and direct oral-anal contact (odds ratio, 7.50, p = < 0.01).

The investigators then conducted a second set of analysis restricted to gay men. This confirmed that HIV infection (odds ratio, 8.59; p < 0.001) and direct oral-anal contact (odds ratio, 9.56; p = 0.01) were significantly associated with shigella.

When they restricted their analysis to women, they found that only foreign travel (odds ratio, 21; p = 0.0002) was a significant risk factor for shigella.

“Our epidemiological study…suggests that shigellosis is a sexually transmitted disease among MSM and that sexual practices among MSM that include anal contact are risky”, write the investigators. Although they admit that their findings “may have been generally assumed by some” nevertheless, they had not previously “been well documented in a population-based study of sporadic shigellosis.”

They suggest that infection with HIV could contribute to the epidemiology of shigella in two ways: individuals with HIV may shed more shigella for longer; and, may be more susceptible to infection with the bacteria.

They also suggest that sexual strategies such as “serosorting” may be contributing to the transmission of shigella in sexual networks of gay men.

Over 40% of cases of shigella in gay men could be prevented, the investigators suggest, by the avoidance of oral-anal contact.

The investigators acknowledge that gaps in the understanding of shigella transmission still exist. They write, “are HIV-infected persons more susceptible to shigella infection or more likely to have severe disease? Do HIV-infected persons with shigellosis shed more bacteria or shed bacteria for longer periods? Will barrier methods…reduce the risk of transmission from direct oral-anal contact?” They call for more research to answer these questions.

An accompanying editorial labels the study “a significant contribution to our knowledge of sexually transmitted enteric infections among MSM”, and says that shigella infection amongst gay men represents “a perfect biological storm: a highly infectious organism imported into a behavioural setting in which transmission can occur among a particularly susceptible population.”