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MRSA incidence on the rise
MRSA is spreading in tertiary care and community hospitals, regardless of the size of the hospital, and also outside of the hospital.
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Methicillin-resistant Staphylococcus aureus (MRSA) infections today represent nearly 60% of nosocomial S. aureus isolates detected in hospital intensive care units and reported to the CDC, according to a panel discussion at the International Conference on Emerging Infectious Diseases in Atlanta.
“MRSA is spreading in tertiary care and community hospitals, regardless of the size of the hospital,” said Keith Kaye, MD, MPH, assistant professor of medicine at Duke University Medical Center. “MRSA is also becoming established in health care–associated settings or among patients with health care contact. These health care–associated MRSA isolates essentially have escaped from the hospital setting and have now colonized patients residing in the community.”
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| Source: National Nosocomial |
Non-nosocomial MRSA rates have increased considerably over the past several years. “In the year 2000, only about 73% of total MRSA isolates were non-nosocomial,” Kaye said. “But in 2002, that percentage had increased to over 84%. Interestingly, though, the nosocomial rates of MRSA have decreased each year, from 23% of total isolates in 2000 to 14.5% in 2003.” Non-nosocomial and health care–associated MRSA “is an increasing challenge and burden on hospitals,” Kaye said.
Data indicate that approximately 40% of these patients came from nursing homes and another nearly 40% had been hospitalized within the prior 90 days. About 10% had received home health care and about 10% had received dialysis. “The remaining 20% unknown category is currently being sorted out with chart reviews,” Kaye said.
MRSA also complicates therapy. “The standard of care is vancomycin, which must be administered intravenously,” Kaye said. “Vancomycin is also less effective than ß-lactam agents for methicillin-susceptible S. aureus [MSSA] infections.” Newer drugs such as linezolid (Zyvox, Pfizer), telithromycin (Ketek, Aventis) and quinupristin-dalfopristin (Synercid, Monarch) have limited track records for MRSA. “Obviously, emergence of resistance to these agents when used more frequently is a concern,” Kaye said.
MRSA is associated with adverse clinical outcomes. Published studies comparing nosocomial MRSA bacteremia to MSSA found that MRSA increased the likelihood of death almost twofold. “MRSA was also linked with eight additional, attributable hospital days per episode,” Kaye said. Moreover, MRSA was more expensive to treat than MSSA: nearly $80,000 extra in hospital costs.
“MRSA’s spread can be controlled to at least some degree,” Kaye said. “But this obviously requires great attention of resources.” Within hospitals, most spreading is from patient-to-patient by the hands of health care workers. “Hand hygiene is a critical preventative control to reduce the spreading of MRSA, particularly alcohol-based products,” Kaye said. Barrier precautions, including gowns and gloves, “are effective as well.” Active or targeted surveillance may be beneficial, too.
Community-acquired MRSA
Ruth Lynfield, MD, is head of the Emerging Infections Unit at the Minnesota Department of Public Heath in Minneapolis. “Reports of community-acquired MRSA [CA-MRSA] began in the 1980s,” Lynfield said. Patients included younger individuals, indigenous peoples and racial minorities. Outbreaks have been reported in players of close-contact sports, injection-drug users, prison/jail inmates, residents of group homes for the developmentally disabled and men who have sex with men. These individuals lacked established risk-factors for MRSA (hospitalization/long-term care over the past year, surgery, dialysis or invasive device use).
| Published studies comparing nosocomial MRSA bacteremia to MSSA found that MRSA increased the likelihood of death almost two-fold. |
“A number of patterns have emerged from these reports,” Lynfield said. “The strains also have distinct molecular and biological features compared with health care–associated MRSA.” Although most infections associated with CA-MRSA have been skin and soft-tissue, “some infections have been very severe, including necrotizing pneumonia and life-threatening infections,” Lynfield said.
A study conducted by the Minnesota Department of Public Heath found that 12% of MRSA cases were community-associated. The median age was much younger in the community group (23 years as opposed to 68 years) and 75% of the community isolates were from skin sources. In contrast, many cases of health care–associated MRSA were from respiratory sources (22%) or from urine (20%).
Only 44% of CA-MRSA isolates were susceptible to erythromycin; 83% were susceptible to clindamycin and 79% to ciprofloxacin (Cipro, Bayer). Ninety-five percent were susceptible to trimethoprim-sulfamethoxazole, and 92% were susceptible to tetracycline. Between 1996 and 2001, susceptibility to erythromycin and ciprofloxacin significantly decreased. Also, 84% of clindamycin-susceptible, erythromycin-susceptible CA-MRSA isolates had inducible resistance to clindamycin. Certain staphylococcal virulence factors and toxins were more likely found in CA-MRSA isolates than in health care-associated isolates including Panton-Valentine leukocidin and enterotoxins A, C, H and K.
A recently published study from the University of Texas Southwestern Medical Center at Dallas found that incision and drainage was effective management for skin and soft tissue abscesses < 5 cm. Only 7% of the 69 healthy children with MRSA had been given an initial antibiotic active against their isolate. Results appeared in The Pediatric Infectious Disease Journal.
“We are starting to see reports of hospital transmissions of CA-MRSA,” Lynfield said. In recent reports, several postpartum women developed skin and soft-tissue infections: five women had to be rehospitalized and three underwent drainage. “Pulsed-field gel electrophoresis [PFGE] showed that the isolate was indistinguishable to the CA-MRSA strain MW2,” Lynfield said.
Additional evaluation is needed for treatment options for nonsevere infections, such as incision and drainage.
“CA-MRSA susceptibility trends and other antimicrobial classes should also be monitored,” Lynfield said. Likewise, “more information is needed on the clinical impact of inducible clindamycin-resistant strains. More data are also needed on the impact of bacterial virulence factors on the severity of CA-MRSA infections.” In addition, risk factor studies are necessary to identify people at risk and optimal methods of prevention.
Understanding the molecular epidemiology of MRSA will make it much easier to compare outbreaks and control them. “DNA sequencing technology is now considered a viable genotyping method,” said Barry Kreiswirth, PhD, director of the Tuberculosis Center at the Public Health Research Institute in Newark, N.J.
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| INFECTIOUS DISEASE NEWS | |
| Source: Keith S. Kaye, MD, MPH |
Sequencing strategies
Two different strategies are currently being employed to provide genotyping data. “Multilocus sequencing involves seven fragments of housekeeping gene targets,” Kreiswirth said. Spa-typing is the other technique. “This method looks at one gene protein A, which has a variable repeat. Both these tools provide you with powerful data,” said Kreiswirth, who developed spa-typing.
| Two different DNA sequencing strategies are being employed to provide genotyping data: multilocus sequencing and spa-typing. | ||
Kreiswirth also validated spa-typing. “We demonstrated that spa-typing could genotype the S. aureus isolates from two different collections in congruence with established procedures,” he said. “This disproves the belief that the repeat region is too unstable for epidemiologic studies.” Kreiswirth acknowledged that spa-typing does not have the resolving power of PFGE subtyping. “But it is fast, easy to use and interpret and compatible for building relational databases,” he said. “Most importantly, DNA sequence analysis of the protein A repeat region provides an unambiguous, portable dataset that simplifies information sharing between laboratories and facilitates, thus creating a large-scale database for studying global and local epidemiology. In short, sequence typing permits the widespread use of a proactive approach to investigate suspected outbreaks of MRSA.”
Kreiswirth said this promises to be a useful molecular tool. He said using the objective sequence approach is definite, as opposed to giving arbitrary names to clones.
Five major MRSA clones appear to be spreading. “There is something very successful about these five clones that we are observing all over the world, whereas, the minor clones don’t seem to be spreading,” Kreiswirth said. “However, the real question about these strains is not their genetic backgrounds and, presumably, not their Sccmec-type. The virulence factor is likely much more important. But at this point, we really don’t have a handle on virulence.” – by Bob Kronemyer
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| INFECTIOUS DISEASE NEWS | |
| Source: Keith S. Kaye, MD, MPH |
For more information:
- Kaye K, Lynfield R, Kreiswirth B. Methicillin-resistant Staphylococcus aureus. Presented at the International Conference on Emerging Infectious Diseases. Feb. 29-March 3, 2004. Atlanta.