ABCs Report: Methicillin-Resistant Staphylococcus aureus, 2008 (Update)

Last Updated: January 30, 2012; See methods and discussion for update explanation.

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Active Bacterial Core Surveillance (ABCs): Emerging Infections Program Network

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ABCs Areas

California (3 county San Francisco Bay area); Colorado (5 Denver area county); Connecticut; Georgia (8 county Atlanta area); Maryland (1 Baltimore area county); Minnesota (2 metro Twin City counties); New York (1 Rochester county); Oregon (3 county Portland area); Tennessee (1 Nashville county).
Note: the population under surveillance changed from 2007.

ABCs Population

The surveillance areas represent 18,300,643 persons.
Source: National Center for Health Statistics bridged-race vintage 2008 postcensal file.

ABCs Case Definition

Invasive methicillin-resistant Staphylococcus aureus (MRSA) disease: isolation of MRSA from a normally sterile site in a resident of the surveillance area in 2008. Cases of disease are classified into one of three epidemiologic classifications. A case is classified as hospital-onset (HO) if the MRSA culture was obtained on or after the fourth calendar day of hospitalization, where admission is hospital day 1; as healthcare-associated community-onset (HACO) if the culture was obtained in an outpatient setting or before the fourth calendar day of hospitalization and had one of more of the following: 1) a history of hospitalization, surgery, dialysis, or residence in a long term care facility in the previous year, or 2) the presence of a central vascular catheter within 2 days prior to MRSA culture; and as community-associated (CA) if none of the previously mentioned criteria are met.

ABCs Methodology

ABCs personnel routinely contacted all microbiology laboratories serving acute care hospitals in their area to identify cases. Standardized case report forms that include information on demographic characteristics, clinical syndrome, and outcome of illness were completed for each identified case. Convenience samples of isolates were collected and sent to CDC for routine testing, including: antimicrobial susceptibility testing, toxin testing and SCCmec typing. Pulsed field gel electrophoresis (PFGE) of all isolates was discontinued in 2008; an inferred PFGE algorithm was developed based on microbiologic and molecular characteristics of isolates. The algorithm has been validated for use with isolates collected though this surveillance only. Regular laboratory audits were performed to ensure completeness of case ascertainment.

Rates of invasive MRSA disease among all patients were calculated using population estimates for 2008. Cases with unknown race were assigned race based on distribution of known race and gender by EIP site. Methodology to make national estimates was modified in January 2012 to adjust for receipt of dialysis, as well as age, race, and gender. Previously reported national estimates were adjusted for age and race only. Confidence intervals for nationally estimated incidence rates of disease and mortality were calculated based on the gamma distribution (Stat Med, 1997 16:791-801).

ABCs Results

ABC Racial/Ethnic Profiles
Race	No.	(Rate^a)
White	3,375	(25.4)
Black	2,068	(62.1)
Other	232	(13.8)

Unknown race (n=776) distributed amongst known

^aCases per 100,000 population for ABCs areas (crude rates).

Distribution of cases, deaths and PFGE type by Epidemiological Classification
MRSA Class	No. (Rate) Cases^b	No. (Rate) Deaths ^c	Inferred PFGE Type (N,%)^d Tot N	Inferred PFGE Type (N,%)^d USA100	Inferred PFGE Type (N,%)^d USA300	Inferred PFGE Type (N,%)^d USA500 Iberian
CA	948 (5.2)	92 (0.5)	229	70 (30.6)	147 (64.2)	7 (3.1)
HCA^a	4,580 (25.0)	794 (4.3)	1,193	745 (62.5)	328 (27.5)	101 (8.5)
HCA-HO	1,298 (7.1)	308 (1.7)	338	237 (70.1)	71 (21.0)	28 (8.3)
HCA-HACO	3,282 (17.9)	486 (2.7)	855	508 (59.4)	257 (30.1)	73 (8.6)

^aHCA: Healthcare-associated invasive MRSA infections; sum of patients that are classified as either HO or HACO.
^bn=88; epidemiologic category unknown.
^cn=9; epidemiologic category unknown.
^disolates were eligible for testing at CDC

Incidence of Invasive MRSA by Epidemiological Class and Age Group 2008
Table data for this chart

Reported Clinical Syndrome by Epidemiologic Class
Syndrome^a	CA (n=948)	HACO (n=3,282)	HO (n=1,298)
Bloodstream Infection with other syndrome	223	1,187	597
Bloodstream Infection with no other syndrome	536	1,670	479
Pneumonia	155	468	234
Lower Respiratory Infection^b	50	105	94
Osteomyelitis	120	335	109
Endocarditis	92	200	46
Cellulitis	166	284	59
Wounds - Surgical^c	6	174	44
Wounds - Decubitus/Pressure Ulcers	18	101	40
Other wounds/abscesses^d	19	39	21
Traumatic Wounds	6	27	11

^aSome case patients had more than one syndrome.
^bLower Respiratory Infection is defined as: a patient with pneumonia documented in their discharge summary, who has a positive MRSA non-sterile respiratory specimen with accompanying chest radiology results documenting any of the following: bronchopneumonia/pneumonia, air space density/opacity, new or changed infiltrates.
^cCombines deep tissue/organ infection and infection of a surgical wound, post operatively.
^dCategory includes skin abscess, necrotizing fasciitis, gangrene, non-traumatic wounds.

National Estimates and Adjusted Incidence Rates of Invasive MRSA Infections
Epidemiologic Class	Estimated No.	Incidence Rate (Confidence Interval)^a
CA	15,343	5.05 (4.72-5.4)
HCA	77,138	25.65 (24.90-26.43)
HO	21,840	7.18 (6.79-7.6)
HACO	56,298	18.52 (17.87-19.2)
Overall^b	95,872	31.53 (30.7-32.39)

^aNational Estimates and Incidence (no. per 100,000 population per year) are adjusted for age, race, gender, and receipt of dialysis treatment using 2008 US Census Data.
^b88 cases could not be classified into an epidemiological category or category is unknown and therefore are counted in the overall estimate only.

National Estimates and Adjusted Incidence Rates for Mortality among Cases
Epidemiologic Class	Estimated No.	Mortality Rate (Confidence Interval)^a
CA	1,414	0.52 (0.37-0.71)
HCA	14,181	4.67 (4.35-5.02)
HO	5,416	1.78 (1.58-2.00)
HACO	8,775	2.89 (2.63-3.17)
Overall^b	16,096	5.29 (4.95-5.66)

^aNational Estimates and Mortality Rate (no. per 100,000 population per year) are adjusted for age, race, gender and receipt of dialysis treatment in the using 2008 US Census Data.
^b20 cases could not be classified into an epidemiological category or category is unknown and therefore are counted in the overall estimate only.

ABCs Discussion

Surveillance data from 2008 represent the fourth full year of performing population-based surveillance for invasive MRSA infections through the Emerging Infections Program/Active Bacterial Core Surveillance Activity. Several changes in operations, including addition of new variables to capture lower respiratory infections and implementation of an algorithm to infer PFGE type, resulted in some changes in the way data are presented in this 2008 Annual Summary.

National estimates and calculate incidence rates adjust for receipt of dialysis, as well as age, race, and gender. Previously reported national estimates were adjusted for age and race only.

Citation

Centers for Disease Control and Prevention. 2008. Active Bacterial Core Surveillance Report, Emerging Infections Program Network, Methicillin-Resistant Staphylococcus aureus, 2008.

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