A recently-identified strain of methicillin-resistant Staphylococcus aureus (MRSA) resistant to most types of antibiotic is being seen more frequently amongst gay men, particularly in HIV-positive gay men, than in the general population, according to a new study released electronically yesterday and due to be published in the February 19th issue of Annals of Internal Medicine.
The investigators believe that this particular strain may sometimes be sexually transmitted and suggest that further studies examine the link between sexual behaviour and multidrug-resistant MRSA.
The mainstream press have widely reported the story today; however, at least one story in the London tabloid newspaper, Metro, has overstated both the risks of casual transmission and the seriousness of the infection, characterising it as the “new HIV”.
The link between community-acquired MRSA, HIV-positive gay men and sexual transmission is not new. As early as 2005, it was reported that community-acquired MRSA infections were being observed with greater frequency among HIV-positive patients in San Diego, and that community-acquired MRSA in HIV-positive gay men in Los Angeles was associated with sex, drug use and environmental exposure.
A 2006 study tracking MRSA infections in patients admitted to an HIV hospital ward in San Francisco between 1996 to 2005, found that one community-acquired strain, USA300, first identified in 2002 made up 93% of MRSA by 2005.
The current study has found a multidrug-resistant genotype of this MRSA strain that is resistant to treatment with pencillins, erythromycin, clindamycin, tetracycline, mupirocin, and Cipro-like antibiotics. However, it is still susceptible to older antibiotics, such as co-trimoxazole and may also resolve by simply draining the infected boil without the use of any antibiotics. Only in the most extreme – and rare – cases is the infection life-threatening.
In addition, infection with multidrug-resistant MRSA can usually be avoided by washing with soap and water. "Taking a shower after sexual contact may minimise contamination," study co-author Dr Chip Chambers, told the San Francisco Chronicle. "Ordinary soap will do. It dilutes the concentration of bacteria. You don't need antibacterial soap."
About the study
Investigators from California wanted to see how common multidrug-resistant MRSA infection is in San Francisco, and to identify risk factors for the infection. Of 2495 San Francisco residents found to have MRSA during a twelve month period in 2004-5 the investigators took a random sample of 532 (21%) in order to determine how many of these MRSA samples were multidrug-resistant.
In order to identify risk factors for multidrug-resistant MRSA, they studied two other groups of people with MRSA: 183 HIV-positive individuals attending an HIV clinic at a San Francisco hospital and 130 people, some of whom were HIV-positive, attending a health clinic in Boston.
Overall incidence of MRSA in San Francisco was estimated to be 275 cases per 100,000 people. However, overall the incidence of the newly-identified multidrug-resistant MRSA was much lower, at 26 cases per 100,000.
When the investigators examined the incidence of multidrug-resistant MRSA based on the area of residence within San Francisco, they found that the highest incidence of multidrug-resistant MRSA (170 cases per 100,000) was in the zip code that included the Castro district, which has a higher proportion of gay men than any other part of San Francisco.
They then examined the prevalence of multidrug-resistant MRSA in the 183 HIV-positive patients with MRSA at a San Francisco HIV clinic, and found that 30 (16%) were infected with multidrug-resistant MRSA. In multivariate analysis, sex between men was a highly statistically significant risk factor for having multidrug-resistant MRSA (relative risk, 13.2; p < 0.001).
Other statistically significant risk factors for multidrug-resistant MRSA included having had a previous MRSA infection (relative risk, 2.1; p = 0.007) or having previously used the antibiotic clindamycin (relative risk, 2.1; p = 0.007).
Finally, they examined the prevalence of multidrug-resistant MRSA in the 130 Boston clinic patients with MRSA infection, and found that 60 (46%) were multidrug-resistant MRSA. All 60 were gay men or other men who had sex with men, suggesting that the infection may have been acquired sexually.
A total of 45% of the patients with multidrug-resistant MRSA were HIV-positive, suggesting that although multidrug-resistant MRSA is not limited to people with HIV, it is more likely to be seen in HIV-positive people. When the investigators limited their analysis to the 121 gay men in the clinic with MRSA, 33 of 56 (59%) who were HIV-positive had multidrug-resistant MRSA infection compared with 27 of 65 (42%) who were HIV-negative (relative risk, 1.4; p = 0.056).
“These data suggest that although HIV infection is a risk factor for multidrug-resistant USA300 infection,” write the investigators, “having male–male sex is also a risk factor independent of HIV infection.”
They note that in San Francisco and Boston, multidrug-resistant MRSA most often manifested as an infection of the buttocks, genitals, or perineum, suggesting sexual transmission.
Previous studies have suggested that MRSA amongst HIV-positive gay men is associated with high-risk sexual behaviours, including use of methamphetamine and other illicit drugs, sex with multiple partners, participation in a group sex party, use of the internet for sexual contacts, skin-abrading sex, and history of sexually transmitted infections.
The investigators comment, however, that “it is not clear whether the behavior potentiating these infections among men who have sex with men is anal sex (that is, dissemination of rectal carriage of community-associated MRSA), skin-abrading sexual practices, or increased frequency of intimate skin-to-skin contact; prevention messages may therefore need to suggest caution in each of these practices.”
They also found a gay man from Boston with the same genotype of multidrug-resistant MRSA infection as the gay men in San Francisco. Since he said he had often visted San Francisco for sex, the investigators believe that “the multidrug-resistant USA300 epidemic probably started in San Francisco and has been disseminated by the frequent cross-coastal travel of men who have sex with men.”
However, there were several limitations to the study. Since relatively few multidrug-resistant MRSA infections were found, and because the researchers did not interview the people with multidrug-resistant MRSA, relying only on clinical records, their estimates of infection risk could be imprecise.
They conclude by summarising their findings thus: “we show that multidrug-resistant USA300 has emerged as an important source of disease among men who have sex with men in two geographically distinct communities."
“The high proportion of infection involving the buttocks, genitals, and perineum suggests that community-associated MRSA may be transmitted in the setting of sexual contact among men who have sex with men."
“The link among USA300, multidrug-resistant USA300, and unsafe sexual risk behaviors should be evaluated further in prospective studies.”
Diep BA. Emergence of multidrug-resistant, community-associated, methicillin-resistant Staphylococcus aureus clone USA300 in men who have sex with men. Annals of Internal Medicine 148 (4): 249-259, 2008.