Methicillin resistant staphylococcus aureus epidemiology and demographics
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Overview
Worldwide, an estimated 2 billion people carry some form of S. aureus. Of these 2 billion, up to 53 million (2.7% of carriers) are thought to carry MRSA. In the United States, 95 million carry S. aureus in their noses; of these 2.5 million (2.6% of carriers) carry MRSA. A population review conducted in 3 communities in the US showed the annual incidence of CA-MRSA during 2001–2002 to be 18–25.7/100,000 ; most CA-MRSA isolates were associated with clinically relevant infections, and 23% of patients required hospitalization.
Epidemiology and Demographics
It has been difficult to quantify the degree of morbidity and mortality attributable to MRSA. Patients with S. aureus infection had, on average, 3 times the length of hospital stay (14.3 vs 4.5 days), 3 times the total charges ($48,824 vs $14,141), and 5 times the risk of in-hospital death (11.2% vs 2.3%) than inpatients without this infection. Cosgrove et al, in a meta-analysis of 31 studies, conclude that bacteremia as a result of MRSA is associated with an increased mortality compared with MSSA bacteraemia with an odds ratio of 1.93 (95% CI, 1.54±2.42; In addition, Wyllie et al. report a death rate of 34% within 30 days among patients infected with MRSA, while among MSSA patients the death rate was similar at 27%.
Because cystic fibrosis patients are often treated with multiple antibiotics in hospital settings, they are often colonized with MRSA, potentially increasing the rate of life-threatening MRSA pneumonia in this group. The risk of cross-colonization has led to increased use of isolation protocols among these patients. In a hospital setting, patients who have received fluoroquinolones are more likely to become colonized with MRSA. This is probably because many circulating strains of MRSA are fluoroquinolone-resistant, which means that MRSA is able to colonize patients whose normal skin flora have been cleared of non-resistant S. aureus by fluoroquinolones.
In the USA, reports have been increasing of outbreaks of MRSA colonization and infection through skin contact in locker rooms and gymnasiums, even among healthy populations. MRSA also is becoming a problem in pediatrics, including hospital nurseries. A 2007 study found that 4.6% of patients in US healthcare facilities were infected or colonized with MRSA.
MRSA causes as many as 20% of Staphylococcus aureus infections in populations that use intravenous drugs. These out-of-hospital strains of MRSA, now designated as community-acquired, methicillin-resistant Staphylococcus aureus, or CA-MRSA, are more easily treated than hospital-acquired MRSA (although more virulent than MSSA). CA-MRSA apparently did not evolve de novo in the community, but represents a hybrid between MRSA which escaped from the hospital environment and the once easily treatable community organisms. Most of the hybrid strains also acquired a virulence factor which makes their infections invade more aggressively, resulting in deep tissue infections following minor scrapes and cuts, and many cases of fatal pneumonia as well.
As of early 2005, the number of deaths in the United Kingdom attributed to MRSA has been estimated by various sources to lie in the area of 3000 per year. Staphylococcus bacteria accounts for almost half of all UK hospital infections. The issue of MRSA infections in hospitals has recently been a major political issue in the UK, playing a significant role in the debates over health policy in the United Kingdom general election held in 2005.
During the summer of 2005, researchers in The Netherlands discovered that three pig farmers or their families were infected by MRSA bacteria that was also found on their pigs. Researchers from Radboud University Nijmegen are now investigating how widespread the MRSA bacteria is in pigs, and whether it will become characterized among the zoonoses.
Recently, it has been observed that MRSA can replicate inside of Acanthamoeba, increasing MRSA numbers 1000-fold. Since Acanthamoeba can form cysts easily picked up by air currents, these organisms can spread MRSA via airborne routes. Whether or not control of Acanthamoeba in the clinical environment will help to contain MRSA remains an area for research.
Strains
In the UK, the most common strains are EMRSA15 and EMRSA16. EMRSA16 is the best described by epidemiology: it originated in Kettering, England, and the full genomic sequence of this strain has been published. This has been recognized as being identical to the ST36:USA200 strain which circulates in the USA, and carries the SCCmec type II, enterotoxin A and toxic shock syndrome toxin 1 genes. Under the new international typing system, this strain is now called MRSA252, and the entire genome sequence of this strain has been published. It is not entirely certain why this strain has become so successful, where previous strains have failed to persist: one explanation is the characteristic pattern of antibiotic sensitivities. Both the EMRSA-15 and -16 strains are resistant to erythromycin and ciprofloxacin. It is known that Staphylococcus aureus can survive intracellularly, and these are precisely the antibiotics that best penetrate intracellularly. It may be that these strains of S. aureus are therefore able to exploit an intracellular niche.
In the USA, the epidemic of community-associated MRSA is due to a CC8 strain designated ST8:USA300, which carries mec type IV, Panton-Valentine leukocidin, and enterotoxins Q and K. Other community-associated strains of MRSA are ST8:USA500 and ST59:USA1000.
MRSA: a Growing Problem in the Healthcare Setting, But One with a Cure
MRSA is becoming more prevalent in healthcare settings. According to CDC data, the proportion of infections that are antimicrobial resistant has been growing. In 1974, MRSA infections accounted for two percent of the total number of staph infections; in 1995 it was 22%; in 2004 it was some 63%.
The good news is that MRSA is preventable. The first step to prevent MRSA, is to prevent healthcare infections in general. Infection control guidelines produced by CDC and the Healthcare Infection Control and Prevention Advisory Committee (HICPAC) are central to the prevention and control of healthcare infections and ultimately, MRSA in healthcare settings. To learn more about infection control guidelines to prevent infections and MRSA go to http://www.cdc.gov/ncidod/dhqp. CDC welcomes the increased attention and dialogue on the important problem of MRSA in healthcare. CDC, state and local health departments and partners nationwide are collaborating to prevent MRSA infections in healthcare settings. For example, CDC
- Monitors trends in infections and MRSA through surveillance systems such as the National Healthcare Safety Network, formerly the National Nosocomial Infection Surveillance System and the Dialysis Surveillance Network to identify which patients are at highest risk and where prevention efforts should be targeted.
- Works with multiple prevention partners including state health departments, academic medical centers, and regional and national collaboratives to identify and promote effective strategies to prevent MRSA transmission.
- Developed an overarching strategy to help guide healthcare facilities to control antibiotic resistance called The Campaign to Prevent Antimicrobial Resistance in Healthcare Settings. This campaign includes specific strategies for various healthcare populations, including hospitalized adults and children, dialysis patients, surgical patients, and long-term care patients.