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Exploring risks for MRSA infection-A tale of two studies

2 March 2012 -

A group of bacteria called S. aureus ( Staphylococcus aureus ) are commonly found on the skin of animals and people. Specifically, these bacteria can be found in 25% to 50% of healthy people in at least one of these places:

  • inside the nostrils
  • the skin between the anus and genitals
  • genitals
  • armpits
  • mouth and throat

For most people, exposure to S. aureus arises because of touching an infected surface or object. As people frequently touch parts of their body, especially the face, it is easy to imagine how S. aureus can spread from a contaminated surface to a person. Good hygiene and hand washing are important to help restrict the spread of S. aureus because attempts at creating a potent vaccine against this germ have not been successful. Further information on preventing S. aureus infections appears later in this CATIE News bulletin.

In otherwise-healthy humans, S. aureus lives on the skin and does not cause disease. When bacteria are resident on or inside the body and not causing harm, researchers say that the person has been "colonized" by these bacteria. However, because of cuts, punctures, wounds and abrasions, S. aureus can sometimes penetrate the skin. Once inside the skin, S. aureus can cause inflammation, boils and abscesses. In severe cases, the infection can affect and damage large areas of skin and other organs, causing serious complications. Some strains of S. aureus have become resistant to an antibiotic called methicillin and these strains are called MRSA-methicillin-resistant S. aureus .

Two kinds of MRSA

MRSA was originally a problem restricted to hospitalized patients. However, well-recognized cases of community-acquired MRSA have occurred. It is not just the location (health-care facility vs. community) that distinguishes these two types of MRSA. Health-care-associated MRSA has been associated with the following:

  • surgery
  • living in a nursing home
  • having a catheter

Health-care-associated MRSA can cause pneumonia.

In contrast, community-acquired MRSA is mostly associated with skin and soft tissue infections, infection of the lining of the heart, and, in some cases, a rapidly worsening form of pneumonia.

The rest of this report focuses on community-acquired MRSA.

Who is at risk?

Some outbreaks of community-acquired MRSA have affected people who may have been exposed to MRSA by close personal contact with other people in the following facilities:

  • military barracks
  • prisons
  • athletic facilities

Based on years of research, scientists have found that the following groups are at increased risk for S. aureus infection:

  • men
  • infants and young children
  • elderly people
  • people with severely damaged kidneys who need artificial blood filtration (dialysis)
  • diabetics
  • people receiving chemotherapy for cancer
  • people with rheumatoid arthritis
  • people who use street drugs
  • people who abuse alcohol

In most of the above cases, there is the issue of a weakened immune system from several potential causes. For instance, in both the very young and the elderly, the immune system is not at its prime and both populations are at increased risk for infections. Excess intake of alcohol and exposure to street drugs can also weaken the immune system.

As weakened immunity plays a role in a person's susceptibility to infections, it should not be surprising that there have been reports of MRSA causing complications in HIV-positive people, whose immune systems are also weakened because of HIV infection.

In separate studies in different American cities (Chicago and San Diego) two teams of researchers have investigated MRSA in the community and possible risk factors among HIV-positive people. In the Chicago study, researchers found that factors such as a history of being imprisoned and living in a neighbourhood with a relatively high proportion of former prison inmates seemed to confer a far greater risk for MRSA than being HIV positive.

In the second study, researchers in San Diego analysed data from many studies of MRSA among HIV-positive people. Their analysis suggests that engagement in high-risk behaviours seems to put some HIV-positive people at increased risk for MRSA.

Chicago-Prisons, shelter and geography

Researchers in Chicago recently investigated MRSA among 601 participants (76% were HIV positive). Participants were recruited from a clinic that provided care for HIV-positive women and from a clinic that provided care for people recently released from prison.

Analysis of swabs taken from the nostrils of participants revealed that HIV-positive people were nearly three times as likely to have MRSA as HIV-negative people. However, when researchers took many factors into account-such as whether participants were homeless or lived in a hostel for homeless people or in a shelter for people undergoing drug withdrawal or in public housing or a mental health facility-HIV infection was no longer a significant factor for having MRSA. According to the team, this finding "suggests that community exposures may be more important for predicting MRSA colonization than HIV status in certain populations."

Participants who lived in neighbourhoods where there were relatively many former prison inmates seemed to be at increased risk for having been colonized by MRSA. Indeed, the Chicago team found that 71% of people in its study who had been colonized by MRSA had formerly been imprisoned. The team theorized that "HIV status may be a [signifier] for exposure to high-risk social networks rather than being the major factor contributing to the high colonization and infection burden."

The specific factors that occur in prison that could place people at risk for MRSA colonization were not explored in the Chicago study.

San Diego-Links to behaviour

A research team in San Diego conducted a review of scientific literature published between 1996 and January 2011 on MRSA and HIV. The review found that HIV-positive people are at increased risk for colonization by MRSA.

It is important to note that while many people are colonized by S. aureus (some of which is MRSA), most people, including HIV-positive people, will not develop infections unless they have risk factors for MRSA. To gain an understanding of possible risk factors, the San Diego researchers scoured the scientific literature for MRSA risk factors among HIV-positive people. They found that studies linked a heightened risk for MRSA colonization to known risk factors such as weakened immunity (low CD4+ cell counts), recent hospitalization, recent use of antibiotics and having chronic skin disease.

When the researchers focused on recent research, they found that MRSA colonization was linked to certain "high-risk behaviours," particularly among HIV-positive men who have sex with men (MSM) regardless of age who engaged in the following:

  • unprotected anal intercourse
  • multiple sexual partners
  • recent diagnosis of a sexually transmitted infection (including syphilis)
  • visiting a public bathhouse or sauna
  • substance use (including crystal meth)

Exactly how unprotected sex can help spread MRSA deserves further study, but here is one possibility: MRSA can live in the intestine and anything inserted into the anus-penises, fingers, sex toys-may become contaminated with these bacteria.

The widespread availability of potent combination therapy for HIV, commonly called ART or HAART, has meant that there are now less people with HIV who have severely weakened immune systems compared to the time before ART was available. Therefore, behavioural factors (rather than low CD4+ cell counts) likely play a more prominent role in the spread of and risk for MRSA infections today.

Types of MRSA infections among HIV-positive people

The literature review revealed that abscesses were the most common type of skin and soft tissue infection caused by MRSA among HIV-positive people. According to the research team, such infections were "usually mild and associated with low rates of complications." Most of these infections were in the lower extremities-arms and hands, feet and legs. However, infections could occur elsewhere and recent reports suggest that MRSA in the ano-genital area is increasing. The researchers suggest that this is associated with high-risk behaviours (as previously described).

More serious MRSA infections-such as those that had spread to the bloodstream-tended to occur among people who had one or more of the following risk factors:

  • injected street drugs
  • severe kidney dysfunction
  • low CD4+ cell counts (less than 200 cells)

The research team also found that MRSA could cause other complications, such as infecting the lining of the brain and the heart, bones, the sinuses and vital organs (liver, lungs and kidneys). However, such serious infections were uncommon particularly among people using ART.

Trends in MRSA

Among HIV-negative people, MRSA infections now seem to be less common both in health-care settings and in the community than they were several years ago. A similar general trend appears to be occurring among HIV-positive people.

Preventing MRSA

Several steps can be taken to reduce the risk of exposure to MRSA:

  • Practice good hygiene-wash hands regularly with warm water and soap or use an alcohol-based hand sanitizer.
  • Shower with soap after sexual contact.
  • Avoid sharing personal items such as towels, washcloths, razors, clothes (including uniforms) and sex toys.
  • Cover wounds and avoid contact with other people's wounds or bandages.
  • Reduce illicit substance use and seek professional help for quitting.
  • See a physician for accurate advice about treating skin infections.

The San Diego teams calls for further research to evaluate the impact of "high-risk behaviours" on MRSA colonization and infection. High-risk behaviour is a relatively broad term but can include the following:

  • engaging in unprotected intercourse with or without multiple partners
  • engaging in substance use (not only injecting drugs but inhaling or otherwise ingesting street drugs, as these weaken the immune system)

Future research on MRSA risk will hopefully also explore the following issues:

  • clinical trials with antibiotics to decolonize large groups of people in a community
  • studies that take swabs not only from inside the nostrils but from other body parts such as the ano-genital region
  • the impact of ART  on MRSA colonization risk in HIV-positive people

Although reports of MRSA infections among HIV-positive people in high-income countries appear to be waning, overall, the risk of MRSA colonization and infection is still greater for HIV-positive people than it is for HIV-negative people. Therefore, further research -focusing on the immune system, use of antibiotics and behaviour-is necessary to understand why this is the case.

Until the results of such research become available, the San Diego researchers urge HIV-positive people to reduce their risk of acquiring MRSA by engaging in good hygiene practices, taking ART and minimizing episodes of unprotected sex and substance use.


-Sean R Hosein


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CATIE-News is written by Sean Hosein, with the collaboration of other members of the Canadian AIDS Treatment Information Exchange, in Toronto.

From Canadian AIDS Treatment Information Exchange (CATIE). For more information visit CATIE's Information Network at


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