A study carried out in San Francisco and Boston, USA, found that sexually active gay men were many times more likely to acquire a new highly antibiotic-resistant strain of the MRSA superbug than the rest of the population.

The study is published in the January 15th early online issue of the Annals of Internal Medicine and was led by researchers at the University of California, San Francisco (UCSF).

Scientists have noticed that infection with the multidrug-resistant, community associated methicillin-resistant Staphylococcus aureus (MRSA) appears to occur in isolated pockets.

The new strain, called USA300, which is resistant to many more front line antibiotics, is a close relative of the MRSA strain that has begun to spread outside of hospitals and into the community in recent years (CA-MRSA, or community associated MRSA, but technically also known as USA300).

Both strains spread easily through skin to skin contact, and get into the skin and the underlying tissue, causing abscesses and ulcers that can become life-threatening quite quickly.

The UCSF researchers decided to investigate the risk factors for infection with the new USA300, which has gained a foothold in San Francisco and other US cities.

The study was in two parts: a population-based survey of 9 San Francisco hospitals and a cross-sectional study in 2 outpatient clinics in San Francisco and Boston. The data reviewed related to culture proven cases of MRSA infections spanning 2004 to 2006.

The researchers looked for: risk factors, annual incidence and spatial clustering for infection by multidrug-resistant USA300.

The strain of MRSA in the samples were identified using a range of methods such as: DNA sequencing (establishing the pattern of nucleotides in the DNA), polymerase chain reaction assays (amplifying DNA to help identify it), and pulse field gel electrophoresis (looking at very large DNA molecules).

The results for San Francisco showed that:

  • The overall incidence of USA300 infection in San Francisco was 26 cases per 100,000 of the population (ranging from 16 to 36).
  • The incidence was higher in 8 adjacent neighbourhoods (identified by ZIP codes) that had a higher proportion of male same-sex couples.
  • Men who have sex with men were 13 times more likely to be infected with USA300.
  • This risk was independent of previous history of MRSA infection or use of clindamycin (an antibiotic used to treat MRSA).
  • The risk also appeared to be independent of HIV infection.
  • USA300 infection mostly occurred in the buttocks, genitals, or perineum (the area between the anus and the penis).

The results for Boston showed that multi-drug resistant USA300 strains were recovered only from men who have sex with men.

The study concluded that:

“Infection with multidrug-resistant USA300 MRSA is common among men who have sex with men, and multidrug-resistant MRSA infection might be sexually transmitted in this population.”

In a separate press statement, the researchers expressed their concern that the new MRSA strain could soon spread to the general population. It can be spread through skin to skin contact but appears to be trasmitted more easily through intimate sexual contact, they said.

Lead author of the study, Dr Binh Diep, who is a UCSF postdoctoral scientist at San Francisco General Hospital Medical Center, said:

“These multi-drug resistant infections often affect gay men at body sites in which skin-to-skin contact occurs during sexual activities.”

“But because the bacteria can be spread by more casual contact, we are also very concerned about a potential spread of this strain into the general population,” he added.

He explained that the most effective way to protect oneself against infection, especially after sex, was to scrub the skin well with soap and water.

Diep said he was alarmed by the rapid rise in infections. In the figures they collected, they found that San Francisco’s Castro district, which has the highest proportion of gays in the country, the infection rate of MRSA was around 1 in 588 people. This compares with about 1 in 3,800 for the overall population of San Francisco, which is also high, said Diep.

Co-author Dr Henry Chambers, who is UCSF professor of medicine at San Francisco General Hospital Medical Center and lead scientist of a large multi-centered clinical trial recently funded by the National Institute of Health to study treatment of community-associated MRSA infections, said:

“Prompt diagnosis and the right treatment are crucial to prevent life-threatening infections and the spread of this bacteria to close contacts.”

The authors pointed out that their study was limited by the fact it was retrospective, and they had not looked at the link between sexual risk behaviours and infection. They recommended that:

“Further research is needed to determine whether existing efforts to control epidemics of other sexually transmitted infections can control spread of community-associated multidrug-resistant MRSA.”

“Emergence of Multidrug-Resistant, Community-Associated, Methicillin-Resistant Staphylococcus aureus Clone USA300 in Men Who Have Sex with Men.”
B. A. Diep, H. F. Chambers, C. J. Graber, J. D. Szumowski, L. G. Miller, L. L. Han, J. H. Chen, F. Lin, J. Lin, T. HaiVan Phan, H. A. Carleton, L. K. McDougal, F. C. Tenover, D. E. Cohen, K. H. Mayer, G. F. Sensabaugh and F.ço. Perdreau-Remington.
Ann Intern Med, early online 15 January 2008; 60520-204.
Print issue: 19 February 2008, Volume 148 Issue 4.

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Sources: Ann Intern Med journal article and press release.

Written by: Catharine Paddock