Gay men are significantly more likely to become infected with the 'superbug' MRSA—methicillin-resistant Staphylococcus aureus—than are heterosexuals, according to a new study. Much of the transmission appears to be through sexual contact, though historically, transmission often is through non-sexual contact, generally via hands that touch infected skin or non-living surfaces contaminated with the bacteria. ( See accompanying article. ) .
Binh An Diep, Ph.D., a researcher at the University of California at San Francisco and the lead author of the study, said part of the reason why he conducted the study was to help empower gay men to take better care of their own health. It was published online in the Annals of Internal Medicine on Jan. 14.
However, given the hysteria in much of the media coverage of recent studies on MRSA, he is concerned with a possible backlash against the gay community because of how his study might be presented through the media. He made the comparison with the early days of the HIV epidemic in this country.
One part of the study retrospectively looked at the medical charts of patients with confirmed MRSA for the period 2004 to June 2006 at sites representing 98 percent of the all hospital beds in San Francisco and two public outpatient clinics. It randomly sampled 532 ( 21 percent ) of the 2495 cases for extensive review and the very time consuming analysis of the genetic sequences of the MRSA bacteria samples.
The USA300 strain of MRSA is dominant in community ( non-healthcare ) settings and often in more virulent in infecting young, healthy persons. It is resistant to some antibiotics but it is still relatively easy to treat. A subgroup of USA300 has acquired an additional genetic sequence, pUSA03, that makes it resistant to a greater number of drugs and hence more difficult to treat. It is known as multidrug-resistant ( MDR ) USA300.
Diep found the annual incidence of USA300 infection per 100,000 persons was 275 cases, while the incidence of the MDR variant was 26 cases in all of San Francisco. Geographically, 8 contiguous zip codes had an average incidence of 59 cases, compared with 4 cases in the rest of the city.
Overlaying that with census information, he found that higher rates of infection with USA300 correlated with higher numbers of self-reported male same-sex couples. In the first cluster of zip codes 10.3 percent of the population was male same-sex couples, compared with 2.2 percent in the rest of the city.
The Castro district ( zip code 94114 ) had the highest percentage ( 25.7 percent ) of male same-sex couple in the United States, and a MDR USA300 incidence rate per 100,000 of 170 cases. However, the total number of cases in an individual zip code is small and so the statistical confidence interval in large; one should we wary of drawing too many conclusions from the subset analysis.
SFGH HIV clinic study
An analysis of 183 consecutive patients with MRSA infection treated at the San Francisco General Hospital ( SFGH ) HIV Clinic found that most ( 179 ) were skin or soft tissue infections. The vast majority ( 170 ) was caused by USA300, and 30 of those were of the MDR variety. The later group of infections was more likely than other variants to be found on the buttocks, genitals, and perineum—the area between the anus and the scrotum—than other anatomical sites ( 30 percent vs. 14 percent ) .
Diep said they saw little difference among HIV-positive patients in terms of acquisition of MRSA, disease progression, or response to therapy. However, most of those patients had a CD4 count greater than 200. Significant risks of opportunistic infections often are not seen until the CD4 count drops below 100, and the number of patients in that category was too small for meaningful analysis.
Part of the study was conducted in Boston at Fenway Community Health, a clinic that primarily serves the GLBT community. It was drawn from data gathered as part of a larger, ongoing study. It involved 130 patients with MRSA; almost all ( 126 ) were infected with USA300, nearly half ( 60 ) the MDR variant.
The Fenway study actively screened patients for colonization with MRSA at four anatomic sites, something that is not done as part of normal care for the infection. So it was no surprise that it identified a greater presence of the bacteria on each patient.
The broader screening of all participants in that study, not just those with active MRSA infection, found that 4 percent carried USA300 in the nose and 2 percent in the perianal area—around the anus and over to the scrotum. Dr. Diep said, 'This is an extremely high rate of perianal colonization that is practically unheard of.'
Comparing the SFGH and Fenway groups, he found the risk for MDR USA300 on the buttocks, genitals, or perineum was 30 percent and 47 percent respectively. As the sites are where there is physical contact during anal sex, it strongly suggests that transmission is occurring during that activity.
The study also identified a Boston patient who regularly traveled to San Francisco; his medical chart specifically mentioned the 94114 zip code. Given the identical genetic sequences of the USA300 clones found in both cities, it seems likely that they shared a common origin and were disbursed by travelers from that site to other, perhaps many, locations.
Fenway's research director, Kenneth Mayer, MD, said the retrospective nature of the study made it impossible to ascertain the effect of multiple sexual partners on risk of acquisition of MRSA or link that acquisition to any particular venue or sexual activity.
He acknowledged that bathhouses and sex clubs, not to mention gyms, are all possible locations for acquisition of MRSA through contact with surfaces contaminated with the bacteria; sexual activity is not required for transmission.
Issues of infection control are likely to become more important in venues where sex occurs. Some cities banned bathhouses in the early days of the HIV epidemic in an attempt to control that infection. Sex clubs often sprang up to take their place. The main difference between the two is that the baths have shower facilities and the clubs often do not. Soap and water are the cornerstone of controlling MRSA infection.
Taken together, these findings add to the growing body of research showing that MRSA is a dynamic bacteria that is constantly evolving into a more fit pathogen that is more readily adapted to transmission during sexual contact and is more resistant to currently available antibiotics. It is a reminder that the best defense is prevention through regular bathing and liberal use of soap.