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Methicillin-Resistant Staphylococcus
aureus1
Fred C. Tenover*
and Michele L. Pearson*
*Centers for Disease Control and Prevention, Atlanta, Georgia, USA
Suggested
citation for this article
Methicillin-resistant Staphylococcus aureus (MRSA) is first and
foremost a pathogen of healthcare settings. It is the most common pathogen
associated with nosocomial infections in the United States, particularly
nosocomial pneumonia and surgical site infections. It is also a frequent
cause of bloodstream and skin and soft tissue infections. The percentage
of S. aureus isolates resistant to oxacillin/methicillin in U.S.
intensive care units increased from 30% to 40% in the mid-1990s to 57%
in 2002.
Data from a recent Duke Infection Control Outreach Network survey indicate
that of patients with healthcare-associated MRSA infections, 39% were
from nursing homes, 37% had been hospitalized in the previous 90 days,
10% had received home health care, and 10% received dialysis. Data suggest
that MRSA bacteremia is associated with an increased likelihood of death,
longer hospital stays, and increased cost of hospitalization, when compared
with bacteremia levels caused by methicillin-susceptible strains. Increasing
resistance to vancomycin among MRSA also complicates therapy, which is
already difficult because of multidrug resistance among healthcare-associated
MRSA. Because spread of MRSA in healthcare settings is often clonal, hand
hygiene and barrier precautions are often effective in interrupting spread.
Targeted surveillance for MRSA is also a useful aid for infection control.
Data from the Duke network indicate that the spread of MRSA can be curtailed
in healthcare settings, given vigilance and adequate funding of infection
control activities.
MRSA is now spreading in community settings. Reports from the early 1980s
indicate that patients in the community without established risk factors
for MRSA (i.e., recent hospitalization, residence in a long-term care
facility, or dialysis) sought medical care with MRSA infections. In the
late 1990s, four children in Minnesota and North Dakota died from community-associated
MRSA infections. The isolates were susceptible to most non-β-lactam
drugs, had pulsed-field gel electrophoresis (PFGE) profiles that differed
from typical healthcare-associated MRSA, and contained the Panton-Valentine
leukocidin toxin. Prospective surveillance for MRSA in Minnesota at 12
sentinel hospitals (6 in metropolitan areas and 6 in rural areas) indicated
that community-associated MRSA patients were significantly younger than
healthcare-associated MRSA patients and more likely to have skin and soft
tissue infections than respiratory or urinary tract infections. A study
in Texas showed that incision and drainage of abscesses due to community-associated
MRSA was more effective management than administering antimicrobial agents
alone, particularly since many patients were given ineffective antimicrobial
agents (i.e., β-lactam agents).
Molecular analysis of the community-associated MRSA strains showed that
the methicillin resistance gene mecA is typically carried on a
much smaller genetic element than is seen in healthcare-associated MRSA.
Four distinct elements, called staphylococcal chromosome cassette mec
(or SCCmec), have been described. In the United States, SCCmec
type II, which is approximately 60 kb in size and also carries an erythromycin
resistance determinant, predominates among healthcare-associated MRSA,
while SCCmec type IV, which is only ยป23 kb in length and carries no other
resistance determinants, is typically associated with community-associated
MRSA. Three major strain typing methods, PFGE, multi-locus sequence typing
(MLST), and staphylococcal protein A typing (spa typing), are used to
study the spread of MRSA. MLST identified a series of five major lineages
(also called clonal complexes) of MRSA globally, while spa typing and
PFGE subdivide this group into approximately a dozen epidemic clones.
Virulence determinates for MRSA include a series of enterotoxins, toxic
shock toxin, and the Panton-Valentine leukocidin toxin.
1Presented at the International Conference
on Emerging Infectious Diseases, Atlanta, Georgia, February 29 –
March 3, 2004, by Keith Kaye, Duke University; Ruth Lynnfield, Minnesota
State Department of Heath; and Barry Kreiswirth, New York University Public
Heath Research Institute.
Suggested citation
for this article:
Tenover FC, Pearson
ML. Methicillin-resistant Staphylococcus aureus. Emerg Infect Dis
[serial on the Internet]. 2004 Nov [date cited]. Available from
http://www.cdc.gov/ncidod/EID/vol10no11/04-0797_10.htm
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