Community-associated methicillin-resistant Staphylococcus aureus, Switzerland

Two case-control studies evaluated the prevalence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) carriage at hospital admission and characteristics of patients with CA-MRSA. Among 14,253 patients, CA-MRSA prevalence was 0.9/1,000 admissions. Although 5 CA-MRSA isolates contained Panton-Valentine leukocidin, only 1 patient had a previous skin infection. No easily modifiable risk factor for CA-MRSA was identified.     

Methicillin-resistant Staphylococcus aureus in horses and horse personnel, 2000-2002

Methicillin-resistant Staphylococcus aureus (MRSA) infection was identified in 2 horses treated at a veterinary hospital in 2000, prompting a study of colonization rates of horses and associated persons. Seventy-nine horses and 27 persons colonized or infected with MRSA were identified from October 2000 to November 2002; most isolations occurred in a 3-month period in 2002. Twenty-seven (34%) of the equine isolates were from the veterinary hospital, while 41 (51%) were from 1 thoroughbred farm in Ontario. Seventeen (63%) of 27 human isolates were from the veterinary hospital, and 8 (30%) were from the thoroughbred farm. Thirteen (16%) horses and 1 (4%) person were clinically infected. Ninety-six percent of equine and 93% of human isolates were subtypes of Canadian epidemic MRSA-5, spa type 7 and possessed SCCmecIV. All tested isolates from clinical infections were negative for the Panton-Valentine leukocidin genes. Equine MRSA infection may be an important emerging zoonotic and veterinary disease.     

Community-associated methicillin-resistant Staphylococcus aureus isolates causing healthcare-associated infections

We noted a marked increase in healthcare-associated (HA) methicillin-resistant Staphylococcus aureus (MRSA) infections caused by isolates phenotypically consistent with community-associated (CA)-MRSA strains. To study this trend, we retrospectively examined all HA-MRSA isolates from patients in our institution during 1999-2004. An isolate was considered an SCCmecIV phenotype if it had antimicrobial drug susceptibilities consistent with typical CA-MRSA isolates. Our phenotypic definition was validated in a limited subset of isolates by SCCmec genotype, pulsed-field gel electrophoresis, and multilocus sequence typing. Among 352 patients with HA-MRSA isolates, SCCmecIV phenotype increased from 17% in 1999 to 56% in 2003 (p < 0.0001). Antimicrobial drug-susceptibility phenotype and genotype were consistent in 21 (91%) of 23 isolates. In a multivariate model, the SCCmec type IV phenotype was independently associated with wound culture source, later year of collection, and MRSA isolated earlier during hospitalization. In conclusion, MRSA isolates phenotypically similar to CA strains have become the predominant isolates associated with HA-MRSA in our hospital.     

Skin and soft tissue infections caused by methicillin-resistant Staphylococcus aureus USA300 clone

Until recently, methicillin-resistant Staphylococcus aureus (MRSA) has caused predominantly healthcare-associated infections. We studied MRSA infections and overall skin and soft tissue infections (SSTIs) in outpatients receiving care at the Baltimore Veterans Affairs Medical Center Emergency Care Service during 2001-2005. We found an increase in MRSA infections, from 0.2 to 5.9 per 1,000 visits (p < 0.01); most were community-associated SSTIs. Molecular typing showed that > 80% of MRSA infections were caused by USA300. In addition, SSTI visits increased from 20 to 61 per 1,000 visits (p < 0.01). The proportion of SSTI cultures that yielded MRSA increased from 4% to 42% (p < 0.01), while the proportion that yielded methicillin-sensitive S. aureus remained the same (10% to 13%, p = 0.5). The increase in community-associated MRSA infections and the overall increase in SSTIs in our population suggest that USA300 is becoming more virulent and has a greater propensity to cause SSTIs.     

Community-associated methicillin-resistant Staphylococcus aureus, Canada

A total of 184 methicillin-resistant Staphylococcus aureus (MRSA) strains were collected from patients who sought treatment primarily for skin and soft tissue infections from January 1, 1999, to March 31, 2002, in east-central Saskatchewan, Canada. Molecular subtyping analysis using pulsed-field gel electrophoresis showed 2 major clusters. Cluster A (n = 55) was composed of a multidrug-resistant MRSA strain associated with a long-term care facility and was similar to the previously reported nosocomial Canadian epidemic strain labeled CMRSA-2. Cluster B (n = 125) was associated with cases identified at community health centers and was indistinguishable from a community-associated (CA)-MRSA strain identified previously in the United States (USA400). Cluster B remained susceptible to a number of classes of antimicrobial agents and harbored the lukF-PV and lukS-PV toxin genes. Over 50% of both clonal groups displayed high-level resistance to mupirocin. This is the first report of the USA400 strain harboring the lukF-PV and lukS-PV toxin genes in Canada.     

Staphylococcus aureus infections in US veterans, Maryland, USA, 1999-2008

Trends in Staphylococcus aureus infections are not well described. To calculate incidence in overall S. aureus infection and invasive and noninvasive infections according to methicillin susceptibility and location, we conducted a 10-year population-based retrospective cohort study (1999-2008) using patient-level data in the Veterans Affairs Maryland Health Care System. We found 3,674 S. aureus infections: 2,816 (77%) were noninvasive; 2,256 (61%) were methicillin-resistant S. aureus (MRSA); 2,517 (69%) were community onset, and 1,157 (31%) were hospital onset. Sixty-one percent of noninvasive infections were skin and soft tissue infections; 1,112 (65%) of these were MRSA. Ten-year averaged incidence per 100,000 veterans was 749 (± 132 SD, range 549-954) overall, 178 (± 41 SD, range 114-259) invasive, and 571 (± 152 SD, range 364-801) noninvasive S. aureus infections. Incidence of all S. aureus infections significantly increased (p<0.001), driven by noninvasive, MRSA, and community-onset infections (p<0.001); incidence of invasive S. aureus infection significantly decreased (p<0.001).     

Pigs as source of methicillin-resistant Staphylococcus aureus CC398 infections in humans, Denmark

An emerging subtype of methicillin-resistant Staphylococcus aureus (MRSA), clonal complex (CC) 398, is associated with animals, particularly pigs. We conducted a matched case-control and a case-case study comparing 21 CC398 case-patients with 2 controls randomly selected from the Danish Civil Registry and 2 case-patients infected with MRSA other than CC398. On farms of case-patients, animals were examined for MRSA. Thirteen case-patients reported pig exposure. Living or working on farms with animals was an independent risk factor for CC398 in the case-control (matched odds ratio [MOR] 35.4, 95% confidence interval [CI] 2.7-469.8) and the case-case study (MOR 14.5, 95%CI 2.7-76.7). History of hospitalization was associated with an increased risk only in the case-control study (MOR 11.4, 95% CI 1.4-94.8). A total of 23 of 50 pigs on 4 of 5 farms were positive for CC398. Our results, corroborated by microbiologic testing, demonstrate that pigs are a source of CC398 in Denmark.     

Community-acquired methicillin-resistant Staphylococcus aureus carrying Panton-Valentine leukocidin genes: worldwide emergence

Infections caused by community-acquired (CA)-methicillin--resistant Staphylococcus aureus (MRSA) have been reported worldwide. We assessed whether any common genetic markers existed among 117 CA-MRSA isolates from the United States, France, Switzerland, Australia, New Zealand, and Western Samoa by performing polymerase chain reaction for 24 virulence factors and the methicillin-resistance determinant. The genetic background of the strain was analyzed by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). The CA-MRSA strains shared a type IV SCCmec cassette and the Panton-Valentine leukocidin locus, whereas the distribution of the other toxin genes was quite specific to the strains from each continent. PFGE and MLST analysis indicated distinct genetic backgrounds associated with each geographic origin, although predominantly restricted to the agr3 background. Within each continent, the genetic background of CA-MRSA strains did not correspond to that of the hospital-acquired MRSA.     

Community-associated methicillin-resistant Staphylococcus aureus infections in Pacific Islanders--Hawaii, 2001-2003

Methicillin-resistant Staphylococcus aureus (MRSA) is emerging as a cause of skin and soft-tissue infections in persons who have little or no contact with health-care settings. The majority of these infections are mild, involving skin and soft tissue; however, certain cases can progress to invasive tissue infections, bacteremia, and death. Transmission of MRSA has been reported most frequently in certain populations (e.g., children, sports participants, or jail inmates). Persons in the American Indian or Alaska Native population in the United States and aboriginals and Pacific Islanders (PIs) in Australia have high rates of MRSA colonization and infection. In 2003, clinicians reported an increased number of skin abscesses caused by MRSA among patients examined in ambulatory care settings. This report summarizes the findings of a retrospective study of community-associated MRSA (CA-MRSA) infections in Hawaii that identified a higher proportion of cases among PIs than were identified among Asians, compared with their respective proportions in the Hawaii population. Efforts to prevent CA-MRSA in Hawaii should focus on identifying factors causing the disproportionate number of infections among PIs.     

Methamphetamine use and methicillin-resistant Staphylococcus aureus skin infections

Methicillin-resistant Staphylococcus aureus (MRSA) infections and methamphetamine use are emerging public health problems. We conducted a case-control investigation to determine risk factors for MRSA skin and soft tissue infections (SSTIs) in residents of a largely rural southeastern community in the United States. Case-patients were persons >12 years old who had culturable SSTIs; controls had no SSTIs. Of 119 SSTIs identified, 81 (68.1%) were caused by MRSA. Methamphetamine use was reported in 9.9% of case-patients and 1.8% of controls. After we adjusted for age, sex, and race, patients with MRSA SSTIs were more likely than controls to have recently used methamphetamine (odds ratio 5.10, 95% confidence interval 1.55-16.79). MRSA caused most SSTIs in this population. Transmission of MRSA may be occurring among methamphetamine users in this community.     

Community-acquired methicillin-resistant Staphylococcus aureus among military recruits

We report an outbreak of 235 community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infections among military recruits. In this unique environment, the close contact between recruits and the physical demands of training may have contributed to the spread of MRSA. Control measures included improved hygiene and aggressive clinical treatment.     

Community-associated methicillin-resistant Staphylococcus aureus and healthcare risk factors

To determine frequency of methicillin-resistant Staphylococcus aureus infections caused by strains typically associated with community-acquired infections (USA300) among persons with healthcare-related risk factors (HRFs), we evaluated surveillance data. Of patients with HRFs, 18%-28% had a "community-associated" strain, primarily USA300; of patients without HRFs, 26% had a "healthcare-associated" strain, typically USA100.     

Community-associated methicillin-resistant Staphylococcus aureus in pediatric patients

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections increased from 2000 to 2003 in hospitalized pediatric patients in Houston. CA-MRSA was associated with greater illness than was infection with methicillin-susceptible strains. Children with CA-MRSA were younger and mostly African American. Of MRSA isolates, 4.5% had the inducible macrolide-lincosamide-streptogramin B phenotype.     

Co-trimoxazole-sensitive, methicillin-resistant Staphylococcus aureus, Israel, 1988-1997

Among bloodstream methicillin-resistant Staphylococcus aureus (MRSA) isolates from adult patients in a single hospital, susceptibility to co-trimoxazole increased progressively from 31% in 1988 to 92% in 1997 (p<0.0001). If also observed in other institutions, these findings should encourage the performance of a clinical trial of sufficient size to compare co-trimoxazole to vancomycin in treating MRSA infections.     

Molecular epidemiology of methicillin-resistant Staphylococcus aureus, rural southwestern Alaska

USA300 is the dominant strain responsible for community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA) infections in most of the United States. We examined isolates from outbreaks of MRSA skin infections in rural southwestern Alaska in 1996 and 2000 (retrospective collection) and from the hospital serving this region in 2004-2006 (prospective collection). Among 36 retrospective collection isolates, 92% carried Panton-Valentine leukocidin (PVL) genes; all carried staphylococcal chromosomal cassette mec (SCCmec) type IV. None belonged to clonal complex (CC) 8, the CC associated with USA300; 57% were sequence type (ST) 1, and 26% were ST30; 61% were clindamycin resistant. In the prospective collection, 42 isolates were PVL+ and carried SCCmec type IV; 83.3% were ST1, 9.5% were ST30, and 7.1% were ST8. Among 120 prospective isolates, 57.5% were clindamycin resistant. CA-MRSA epidemiology in southwestern Alaska differs from that in the lower 48 states; ST8 strains were rarely identified and clindamycin resistance was common.     

Clinical implications of varying degrees of vancomycin susceptibility in methicillin-resistant Staphylococcus aureus bacteremia

We conducted a retrospective study of the clinical aspects of bacteremia caused by methicillin-resistant Staphylococcus aureus (MRSA) with heterogeneously reduced susceptibility to vancomycin. Bloodstream MRSA isolates were screened for reduced susceptibility by using brain-heart infusion agar, including 4 mg/L vancomycin with and without 4% NaCl. Patients whose isolates exhibited growth (case-patients) were compared with those whose isolates did not (controls) for demographics, coexisting chronic conditions, hospital events, antibiotic exposures, and outcomes. Sixty-one (41%) of 149 isolates exhibited growth. Subclones from 46 (75%) of these had a higher MIC of vancomycin than did their parent isolates. No isolates met criteria for vancomycin heteroresistance. No differences in potential predictors or in outcomes were found between case-patients and controls. These data show that patients with vancomycin-susceptible MRSA bacteremia have similar baseline clinical features and outcomes whether or not their bacterial isolates exhibit growth on screening media containing vancomycin.     

Methicillin-resistant Staphylococcus aureus in Europe, 1999-2002

We explored the variation in proportions of methicillin-resistant Staphylococcus aureus (MRSA) between and within countries participating in the European Antimicrobial Resistance Surveillance System and temporal trends in its occurrence. This system collects routine antimicrobial susceptibility tests for S. aureus. We examined data collected from January 1999 through December 2002 (50,759 isolates from 495 hospitals in 26 countries). MRSA prevalence varied almost 100-fold, from <1% in northern Europe to >40% in southern and western Europe. MRSA proportions significantly increased in Belgium, Germany, Ireland, the Netherlands, and the United Kingdom, and decreased in Slovenia. Within countries, MRSA proportions varied between hospitals with highest variance in countries with a prevalence of 5% to 20%. The observed trends should stimulate initiatives to control MRSA at national, regional, and hospital levels. The large differences between hospitals indicate that efforts may be most effective at regional and hospital levels.     

Vancomycin susceptibility within methicillin-resistant Staphylococcus aureus lineages

Methicillin-resistant Staphylococcus aureus (MRSA) with reduced vancomycin susceptibility vancomycin-intermediate S. aureus (VISA) has been reported from many countries. Whether resistance is evolving regularly in different genetic backgrounds or in a single clone with a genetic predisposition, as early results suggest, is unclear. We have studied 101 MRSA with reduced vancomycin susceptibility from nine countries by multilocus sequence typing (MLST), characterization of SCCmec (staphylococcal chromosomal cassette mec), and agr (accessory gene regulator). We found nine genotypes by MLST, with isolates within all five major hospital MRSA lineages. Most isolates (88/101) belonged to two of the earliest MRSA clones that have global prevalence. Our results show that reduced susceptibility to vancomycin has emerged in many successful epidemic lineages with no clear clonal disposition. Increasing antimicrobial resistance in genetically distinct pandemic clones may lead to MRSA infections that will become increasingly difficult to treat.     

Staphylococcus aureus bacteremia, Australia

Staphylococcus aureus bacteremia (SAB) is common and increasing worldwide. A retrospective review was undertaken to quantify the number of cases, their place of acquisition, and the proportions caused by methicillin-resistant S. aureus (MRSA) in 17 hospitals in Australia. Of 3,192 episodes, 1,571 (49%) were community onset. MRSA caused 40% of hospital-onset episodes and 12% of community-onset episodes. The median rate of SAB was 1.48/1,000 admissions (range 0.61-3.24; median rate for hospital-onset SAB was 0.7/1,000 and for community onset 0.8/1,000 admissions). Using these rates, we estimate that approximately 6,900 episodes of SAB occur annually in Australia (35/100,000 population). SAB is common, and a substantial proportion of cases may be preventable. The epidemiology is evolving, with >10% of community-onset SAB now caused by MRSA. This is an emerging infectious disease concern and is likely to impact on empiric antimicrobial drug prescribing in suspected cases of SAB.