'Social networks' may explain high prevalence of MRSA colonization seen in patients with HIV

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Infection with HIV per se does not increase the risk of colonisation with community-associated MRSA, according to US research published in the online edition of Clinical Infectious Diseases.

The investigators found that prevalence of nasal colonisation with community-associated MRSA (methicillin-resistant Staphylococcus aureus) was significantly higher in HIV-positive patients compared to HIV-negative patients.

However, they also found a hierarchy of risk among the HIV-positive patients. Moreover, after controlling for factors such as imprisonment and area of residence, infection with HIV did not have a significant association with MRSA colonisation.

Glossary

trend

In everyday language, a general movement upwards or downwards (e.g. every year there are more HIV infections). When discussing statistics, a trend often describes an apparent difference between results that is not statistically significant. 

multivariate analysis

An extension of multivariable analysis that is used to model two or more outcomes at the same time.

confounding

Confounding exists if the true association between one factor (Factor A) and an outcome is obscured because there is a second factor (Factor B) which is associated with both Factor A and the outcome. Confounding is often a problem in observational studies when the characteristics of people in one group differ from the characteristics of people in another group. When confounding factors are known they can be measured and controlled for (see ‘multivariable analysis’), but some confounding factors are likely to be unknown or unmeasured. This can lead to biased results. Confounding is not usually a problem in randomised controlled trials. 

antibiotics

Antibiotics, also known as antibacterials, are medications that destroy or slow down the growth of bacteria. They are used to treat diseases caused by bacteria.

not significant

Usually means ‘not statistically significant’, meaning that the observed difference between two or more figures could have arisen by chance. 

“Community exposures may be more important for predicting MRSA colonization than HIV status in certain groups,” comment the authors.

Earlier research by the same investigators showed that HIV-positive patients had a six-fold higher risk of community-acquired MRSA skin and soft tissue infections compared to HIV-negative individuals.

It is unclear why HIV-positive patients have an increased risk of the infection. Immune suppression and extensive use of antibiotics may be contributory factors.

But it is possible that factors beyond HIV infection may also have a role. For instance, sexual contact has been implicated in transmissions between gay men. Athletes also have a higher prevalence of the infection, as do individuals who have been recently incarcerated.

Given this uncertainty, investigators wished to establish a better understanding of the prevalence and risk factors for colonisation with community-associated MRSA in different groups of patients with HIV.

They therefore designed a study involving 458 HIV-positive patients, who received care at three different clinics, each of which served the needs of individuals from different risk groups (women, Hispanics, individuals recently released from incarceration). The study also involved 143 HIV-negative patients who received treatment at a nearby university hospital. The reseach was conducted between August 2010 and February 2011.

Overall, 9% of patients had nasal colonisation with community-associated MRSA. This included 50 HIV-positive patients (11%) and six of the HIV-negative patients (4%). The investigators calculated that prevalence of MRSA colonisation was a significant 2.6 times higher among HIV-positive patients compared to individuals who were HIV-negative (p = 0.03).

Prevalence of colonisation with the infection differed between the three groups of HIV-positive patients.

It was highest among those who had recently been released from incarceration (16%), followed by HIV-infected women (12%). Hispanic HIV-positive patients had the lowest prevalence of colonisation (3%).

Therefore, prevalence of MRSA colonisation was almost six times higher among recently incarcerated patients compared to Hispanic patients (p = 0.005). Prevalence of colonization was four times higher among HIV-infected women compared to HIV-positive Hispanic patients (p = 0.015).

The investigators’ first set of analysis showed that a number of factors increased the risk of colonization with MRSA. These included ethnicity (African American and White vs. Hispanic, p = 0.013); a prior history of MRSA infection (p = 0.033), a history of incarceration (p < 0.001); living in specific “high risk” geographic areas (p = 0.17); and infection with HIV (p = 0.016).

“CD4 cell count, [cotrimoxazole] prophylaxis, and antiretroviral therapy were not significantly associated with MRSA colonization among HIV-infected participants,” note the authors. This suggested to them that “factors beyond immune suppression contribute to the higher colonization prevalence.”

“Multivariate” anaylsis that controlled for potentially confounding factors continued to show an association between a history of incarceration and colonization with MRSA (p = 0.03). There was also a non-significant trend suggesting that area of residence was also a risk factor (OR = 1.6; 95% CI, 0.92-3.0, p = 0.097). Hispanic ethnicity was negatively associated with colonisation (p = 0.045).

After controlling for incarceration and residence, infection with HIV ceased to have a significant association with community-acquired MRSA colonization.

The investigators suggest “HIV status may be a marker for exposure to high-risk social networks rather than being the major factor contributing to high colonization and infection burden.”

They therefore conclude: “Social networks may need to be examined for both HIV-positive and HIV-negative individuals following release from correctional facilities into the community to differentiate community risk factors for community-associated-MRSA and inform prevention strategies.”

References

Popovich KJ et al. Community-associated methicillin-resistant Straphylococcus aureus colonization in high-risk groups of HIV-infected patients. Clin Infect Dis, online edition. DOI: 10.1093/cid/cis030, 2012 (click here for the free abstract).