Published in UAB Insight, Spring 2006
Community-Associated MRSA: Infection Widespread, Often Misdiagnosed
ABSTRACT: Methicillin-resistant Staphylococcus aureus is increasingly present not only in hospitals but also in the community. It requires accurate diagnosis and treatment to break the cycle of recurrent infections.
CME OBJECTIVE: Readers will be familiar with common presentations for community-associated methicillin-resistant S aureus and appropriate prevention methods.
Craig J. Hoesley, MD, grants and research support Cubist Pharmaceuticals, Inc, Pfizer
Methicillin-resistant Staphylococcus aureus (MRSA) first emerged more than 40 years ago and is now endemic in hospitals, accounting for nearly 60% of S aureus isolates, according to the most recent National Nosocomial Infections Surveillance report (Am J Infect Control. 2004;32[8]:470-485).
Until the late 1990s, MRSA infection was rarely reported outside health care settings, but antibacterial-resistant S aureus strains are now occurring with increasing frequency in community-dwelling individuals without conventional risk factors, which include recent hospitalization or surgery, residence in a long-term care facility, dialysis, and indwelling catheters or percutaneous devices.
“Community-associated (CA) MRSA strains are different from hospital isolates,” explains UAB infectious diseases expert Craig J. Hoesley, MD. “Community strains are genetically distinct, more likely to cause skin and soft tissue infections instead of the invasive staphylococcal disease often seen with hospital–associated (HA) MRSA, and may carry different virulence factors. Although patients with HA-MRSA are unlikely to infect family members once they leave the hospital, it appears individuals can pass community-associated isolates to their close contacts.”
Most cases of CA-MRSA present as skin and soft tissue infections such as abscesses and
furunculosis,and less frequently, as invasive infections such as necrotizing pneumonia, fasciitis, and septicemia.
Genetics of Community Association
MRSA’s universal resistance to β-lactam antibiotics is due to an alteration in the penicillin-binding protein PBP 2a that reduces affinity for this class of antimicrobials. PBP 2a is encoded by the chromosomal gene mecA, which is present in all MRSA strains and carried on a genetic element called the staphylococcal cassette chromosome (SCC). Molecular analysis of MRSA strains circulating in hospitals worldwide found almost all were one of three SCC types (I-III). CA-MRSA is strongly associated with a recently identified fourth type, SCC mec type IV (Antimicrob Agents Chemother. 2002;46:1147-1152).
“This fourth type differs from types I through III in several significant aspects,” Hoesley says. “The smaller type IV cassette increases mobility and allows greater ease of transfer. While many hospital-associated S aureus isolates are resistant to multiple antimicrobials, CA-MRSA is often susceptible to non-β-lactam antibiotics. CA-MRSA is further distinguished from nosocomial isolates by distinct virulence factors, notably the bacterial toxin Panton-Valentine leukocidin, which is strongly associated with skin and soft tissue infections.” The genetics of CA-MRSA continue to evolve rapidly, and resistance to non-β-lactam antibiotics, as well as changes in virulence genes, are likely to occur over time.
Prevalence
The Centers for Disease Control and Prevention has reported clusters of CA-MRSA in children who attend daycare, prisoners, men who have sex with men, military recruits, and individuals participating in team sports at high school, college, and professional levels. “People who have recently or frequently used antibiotics or who live in crowded conditions may also be at increased risk for CA-MRSA,” Hoesley says, noting that it also can occur in individuals without any risk factors.
A recent study comparing CA-MRSA with the nosocomial pathogen found that after excluding pediatric cases, the median age of patients infected with the CA strain was 30 years, compared with 70 years for those with HA-MRSA. Results also showed HA-MRSA is likely to cause infection at a range of sites, such as the respiratory system, urinary tract, and bloodstream, while 75% of CA-MRSA infections involve the skin and soft tissues (JAMA. 2003;290:2976-2984).
UAB studies indicate CA-MRSA is embedded in the Birmingham community. “Approximately 20% of MRSA isolates identified at University Hospital in 2004 were community-associated,” Hoesley says. “Our prospective analyses demonstrated CA-MRSA accounted for approximately 15% of all S aureus isolates — both methicillin-susceptible and resistant strains — suggesting a substantial prevalence in central Alabama. This finding reflects national trends, and community physicians should be aware CA-MRSA infections are an increasingly common and serious problem.”
Not a Spider Bite
The skin abscesses and soft tissue infections commonly seen with CA-MRSA are frequently misdiagnosed as spider bites, Hoesley says. “Because CA-MRSA affects people without traditional risk factors, and physicians may see multiple infected individuals from a single household, an exogenous source, such as a spider bite, is often pinpointed as the causative factor. Physicians should always treat any skin and soft tissue infection as MRSA until culture proves otherwise, selecting antibiotics active against these strains.”
Hoesley advises physicians to drain and culture all furuncles and abscesses and counsel patients on appropriate wound care to prevent recurrence and spread, stressing proper coverage of draining lesions. He notes first-generation cephalosporins such as cephalexin and amoxicillin are no longer reasonable options for patients with suspected CA-MRSA. “Possible outpatient treatments include trimethoprim-sulfamethoxazole [Bactrim], clindamycin, and linezolid, although this drug is expensive, and no studies suggest it is more effective than other agents.” Patients who do not improve or show systemic symptoms of invasive disease may require hospitalization and treatment with vancomycin or newer parenteral agents such as daptomycin and quinupristin-dalfopristin.
Preventing Recurrence
Although most individuals experience a single CA-MRSA infection, some people are susceptible to recurrence, Hoesley says. “Antibiotics probably do not eliminate MRSA colonization in these individuals. First, consider any immunodeficiency that might predispose patients to recurrent infection — hyperimmunoglobulin E syndrome, Ig abnormalities, cyclic neutropenia, or HIV. Decolonization strategies such as sterilization of the nasal carriage with mupirocin ointment and use of chlorhexidine soap may help break the cycle of recurrent infection, as can simple personal hygiene practices.”
CA-MRSA is embedded in communities throughout the nation, he concludes. “Through rapid recognition and appropriate treatment and prevention, primary care physicians can help limit spread of CA-MRSA in their local communities.”
For information
Dr. Craig Hoesley
Dr. Mukesh Patel
1.800.UAB.MIST
mist@uabmc.edu