Comparison of Community and Health Care-Associated MRSA Infection

JAMA says that, looking back, the year 2003 will probably be recognized as the year that methicillin-resistant Staphylococcus aureus (MRSA) really started to take off in the community. Rather than be confined to the institutionalized, MRSA appeared to now attack those in the prime of life, even the most athletic among us. These investigators from the Minnesota Department of Health (Minneapolis), French Reference Centre for Staphylococci (Lyon), and the Centers for Disease Control and Prevention (Atlanta) performed a prospective study of MRSA cases in Minnesota during calendar year 2000.

Using 12 laboratories, they were able to identify sufficient numbers of community-associated cases (CA) to make valid demographic, clinical, and microbiological comparisons with healthcare-associated cases (HCA).

The 12 referring laboratories were dispersed throughout the state and serviced representative populations. They identified MRSA cases from clinical cultures rather than systematic survey. Facility infection control personnel then performed chart review to gather appropriate demographic and clinical information in order to determine whether the case was CA or HCA. HCA was prospectively defined by conventional criteria. CA was determined by exclusion of HCA, so all suspected CA cases after chart review subsequently underwent interview. Overall, 13% of such suspected CA cases were reclassified to HCA after the interview revealed information missing from the record. All CA and some HCA isolates were subjected to pulsed-field gel electrophoresis (PFGE), as well as standard microbiological investigation to determine speciation and resistance patterns, and PCR was used to confirm persistence of the mecA gene. Finally, samples of 26 CA and 26 HCA isolates were sent to the French reference laboratory, where blinded assays were performed for a compendium of toxin-associated genes in the 2 sets of organisms.

During 2000, there were 4612 patients with S aureus isolates from the reference facilities. Of these, 1100 were MRSA, with 937 (85%) classified as HCA, 131 (12%) as CA, and 32 (3%) not classified because of insufficient information. The proportion of CA cases was higher in areas of the state outside of the greater Minneapolis-St. Paul area (OR, 1.66; CI, 1.24-2.15). CA patients were younger (median age, 23 vs 68 years), even after exclusion of data from 2 pediatric hospitals (median age, 30 vs 70 years). Also, CA-MRSA patients were more likely to be nonwhite (OR, 3.13) and have lower median household income compared with HCA-MRSA and especially the general population.

Underlying conditions were uncommon among children < 18 years of age with CA-MRSA, with only 9% having previous dermatologic illness. Most adults, on the other hand, had some health risk, with tobacco use (19%), diabetes (17%), dermatologic illness (13%), chronic obstructive pulmonary disease (6%), and hypertension (6%) most notable.

CA-MRSA was more likely than HCA-MRSA to be manifested as cutaneous or soft-tissue disease (OR, 4.25). Respiratory (OR, 0.22) and urinary (OR, 0.04) disease were much less common in the community. There was no significant difference in the incidence of bacteremia (4% among CA, 9% among HCA). Remarkably, 61% of CA patients were initially treated with ineffective antibiotics. Unfortunately, detailed outcome analysis was not possible.

Resistance patterns differed between CA and HCA strains, with the former generally susceptible to a wider range of non beta-lactam antibiotics, particularly ciprofloxacin (OR, 1.90) and clindamycin (OR, 1.46) or all 4 of the following: ciprofloxacin, clindamycin, gentamicin, and trimethoprim-sulfamethoxazole (OR, 5.88). A single PFGE clonal group accounted for 62% of CA-MRSA, with similar rates for all demographic subgroups. A second clone was also associated with a lesser number of CA cases. The following virulence factors were significantly more common in CA-MRSA: Panton Valentine leukocidin (PVL); enterotoxins A, C, and K; accessory gene regulator 3; and SSCmecIV).

PVL was present in 77% of CA, but only 4% of HCA isolates tested (OR, 5.01), and was almost always (90%) associated with skin infection in community patients. This cytotoxin has been found in different CA strains on 3 continents[1] and seems to enhance tissue necrosis and leukocyte dysfunction by membrane injury.

The investigators, since they are from public health agencies, offered important clinical suggestions based on their data. It is important to know the prevalence of CA-MRSA in your community, and be particularly alert to children with aggressive skin infection. All such children should be cultured for MRSA. If there is a history of contact with a patient with CA-MRSA or sufficient risk factors to suggest epidemic spread, empiric therapy with clindamycin or ciprofloxacin should be considered, rather than vancomycin. Surgical drainage should be performed when appropriate. Public quarantine and nasal or skin decolonization procedures are not necessary. Rather, physicians should educate patients and their families, as well as the general public, that CA-MRSA is an emerging threat and that suspected infections need prompt management.

JAMA December 10, 2003 (Volume 290, Number 22)
Vandenesch F, Naimi T, Enright MC, et al. Community-acquired methicillin resistant Staphylococcus aureus carrying Panton-Valentine leukocidin genes; worldwide emergence. Emerg Infect Dis. 2003;9:978-984.