For patients entering hospitals and long term care facilities, Methicillin-Resistant Staphylococcus aureus, also known as MRSA, can become a larger health threat than their admitting diagnosis. The organism Staphylococcus aureus is normally carried in the nose of 20-40% of healthy people (5), in the lower colon (9), and on the skin of most people without causing health problems (7). If the organism invades the body, it can cause boils or pneumonia (7). S. aureus is an "aerobic or facultative anaerobic, coagulase-positive organism" which appears as "gram positive clusters on a gram stain" (10). Some "staph" infections have become resistant to commonly used antibiotics and have been termed MRSA (7). MRSA poses a significant threat to elderly and immunodeficient patients and has become a challenge to the health care providers concerned with treating and eliminating the spread of this infection.
Methicillin was developed about two years after Kirby's first report of a penicillin-resistant strain of S. aureus in 1944 (10, 4). Resistance to methicillin, the drug of choice at that time for S. aureus infections, was first noted in 1968 (2). Oxacillin is now used to test for and treat S. aureus infections, but the name MRSA is still used (2).
The rate of MRSA has gradually increased in hospital settings and is becoming increasingly difficult to treat. MRSA is now one of the leading causes of nosocomial pneumonia and blood stream infections (10). An estimated 80,000 patients a year get MRSA after entering the hospital (3). A recent study in the United Kingdom revealed alarming rates of MRSA infections, up to 0.69 cases per 1,000 patient hospital days and 40% of staph infections caused by MRSA (8). The CDC has also collected data through National Nosocomial Infections Surveillance (NNIS) in the United States. This data has shown a drastic increase in the rate of methicillin-resistant strains of S. aureus in medical settings, from 5-10% rates in in early 1980's, mainly limited to large, urban hospitals, to up to 40% in the 1990's, with up to 20% rate in small community hospital settings (4). The current prevalence rate of MRSA in United States hospitals is now believed to exceed 50% (4). Worldwide, rates of methicillin resistant S. aureus strains vary dramatically. In 1999, Canada reported a 6% rate, while Japan's rate exceeded 80% (11). Most European countries had a greater than 6% rate of S. aureus strains be MRSA in 1999, but the Netherlands reported less than 1% (11). Not only are these trends very risky for patients, they are costly for hospitals due to vancomycin use and isolation procedures (10).
Not all MRSA infections occur in health care settings. An outbreak which occurred in Detroit in 1980-1981 was associated with injection drug-use rather than recent hospitalization and believed to be spread by needle-sharing (4). Since MRSA in non-drug users is highly associated with hospitalization, researchers felt that some drug-users had become colonized with MRSA during a prior hospitalization, which may have occurred even years earlier (4). Rates of community acquired MRSA are believed to be growing, though no systematic surveillance exists (4). A 1998 study of children in two Dallas, Texas day care centers revealed that 3% and 24% of children were colonized with MRSA (4). In 1999, four children died of community-acquired MRSA infections in rural Minnesota and North Dakota (4).
MRSA infections are resistant to beta-lactamase-resistant penicillins, cephalosporins and carbapenems (6). When a "Unique, low-affinity penicillin binding protein" called PBP 2a is encoded by a chromosomal gene called mec-A, methicillin resistance occurs (10). S. aureus infections are very common and, according to Arvidson's report, virulence is due to "a plethora of bacterial cell surface proteins that that mediate adherance to host matrix and plasma proteins, and a large number of secreted toxins and enzymes that promote tissue damage and invasion" (1a). Arvidison's study further reports that two "global regulators, agr and sar, seem to be involved in transision of S. aureus cells from the adhesive to the invasive phenotype" (1a). Certain factors increase a person's chance of acquiring a MRSA infection. These include being an elderly or extremely ill patient, having an open area such as a surgical wound or decubitus ulcer (3, 10), having an external tube such as a catheter (3), staying in ICU or a burn unit (10), using broad spectrum antibiotics or multiple antibiotics for a long period of time (9, 10) and being in close proximity to or having contact with another MRSA patient (10).
MRSA diagnosis is only through laboratory examination of a sample, such as a tissue swab, urine or wound culture (5). The "Screening Test for Oxacillin-resistant S. Aureus" is recommended by the National Committee for Clinical Laboratory Standards to test for MRSA (2, 10). This test uses an "agar plate containing 6 g/ml of oxacillin and Mueller-Hinton agar supplemented with NaCl" (2). It should be noted that unless cultures are incubated at 35 degrees Celsius for 24 hours, heteroresistance can be problematic in the accurate detection of MRSA because oxacillin/methicillin resistant cells may grow more slowly than susceptible cells in a culture (2). An amplification test to detect the mecA gene can also confirm MRSA infection (2).
MRSA can be on the skin, in the nose, blood and urine (3). Patients are considered either "colonized" or "infected" (1b). Colonized patients carry the MRSA organism but have no signs or symptoms of infection (2). Both colonized and infected patients or healthcare workers can transmit MRSA (1b). This most typically occurs on the hands of healthcare workers, which can become contaminated through contact with an affected person or surface/device (1b). While MRSA is usually transmitted by physical contact, it can be spread in droplets from the nose and mouth when MRSA is contained in the lungs or nose (7).
MRSA is dangerous due to its limited treatment options (2). IV Vancomycin is the drug of choice when treating MRSA (2, 6, 10). IV therapy can last 2-4 weeks, depending on the severity of infection (6). Special care must be used in patients with impaired renal function when using vancomycin (6). Alternative antibiotics to treat MRSA include linezolid and quinupristin/dalfopristin (10). Rifampin can only be used in conjunction with other drugs because of a "rapid emergence of rifampin resistance" (2). Also of note, MRSA strains with resistance to vancomycin have recently been detected in Japan and the USA (6).
Due to the growing difficulty of treatment, infection control has become the primary focus of MRSA containment. Handwashing is the single-most important means of infection control (10). Chlorhexidine (Exidine) soap and hospital disinfectant is recommended (9). Frequent and thorough handwashing may prevent transfer of infections between patients and cross-contamination of different body sites (1b). Further precautions include using masks and gowns, appropriate device/equipment handling and appropriate handling of laundry using universal precautions (1b). Facilities may also choose to culture personnel who appear to be connected with MRSA transmission based on epidemiologic data (1b). To assist in the control of outbreaks, the CDC and local or state health departments are important resources available to individuals and facilities (1b).
References
1a. Arvidson, Staffan. Regulation of virulence genes in Staphylococcus aureus. 14 January 1999. Available from: http://research.kib.ki.se/e-uven/show_project.cfm?projects_no=C11248. Accessed 22 February 2002.
1b. Centers for Disease Control (CDC). MRSA- Information for Healthcare Providers. Available from: http://www.cdc.gov/ncidod/hip/aresist/mrsahcw.htm. Accessed 6 February 2002.
2. Centers for Disease Control (CDC). MRSA- Laboratory Detection of Oxacillin/Methacillin-resistant Staphylococcus aureus. Available from: http://www.cdc.gov/ncidod/hip/Lab/FactSheet/mrsa.htm. Accessed 6 February 2002.
3. Centers for Disease Control (CDC). MRSA- Information for Patients. Available from: http://www.cdc.gov/ncidod/hip/aresist/mrsafaq.htm. Accessed 6 February 2002.
4. Chambers, Henry F. The Changing Epidemiology of Staphylococcus aureus? Vol. 7, No. 2 March-April 2001. Available from: http://www.cdc.gov/ncidod/eid/vol7no2/chambers.htm#Figure%201. Accessed 13 February 2002.
5. Chudley, Pauline. The Dorset Health Authority Website. MRSA. Available from: http://www.dorset.swest.nhs.uk/comm_dis/mrsa.html. Accessed 6 February 2002.
6. The Merck Manual. Section 13. Chapter 157. 1995-2001. Available from: http://www.merck.com/pubs/mmanual/section13/chapter157/157a.htm. Accessed 10 February 2002.
7. MRSA. Available from: http://www.link.med.ed.ac.uk/RIDU//Mrsa.htm. Accessed 6 February 2002.
8. Reuters Health. UK Hospitals Have High 'Superbug' Rate. 8 February 2002.Available from: http://story.news.yahoo.com/news?tmpl=story&u=/nm/20020208/hl_nm/superbug_1. Accessed 10 February 2002.
9. UC Davis Medical Center. Epidemiology and Infection Control Fact Sheets. Methicillin Resistant Staphylococcus Aureus (MRSA). Available from: http://www.pcs.ucdmc.ucdavis.edu/epi/mrsa.htm. Accessed 6 February 2002.
10. Wise, Stephanie. The Johns Hopkins Website. Methicillin-Resistant Staphylococcus aureus (MRSA). Available from: http://hopkins-heic.org/infectious_diseases/mrsa.htm. Accessed 10 February 2002.
11. Verhoef, Jan. The Canadian Medical Association Journal. Stopping the spread of methicillin-resistant Staphylococcus aureus. Available from: http://www.cma.ca/cmaj/vol-165/issue-1/0031.asp. Accessed 22 February 2002.