A resistant staph strain comes in from community

Strain became more resistant in hospital

In an unusual case with ominous implications, epidemiologists have discovered that a strain of methicillin-resistant Staphylococcus aureus (MRSA) with emerging resistance to vancomycin was transmitted in the community between family members. A 77-year-old woman, who apparently acquired it from her 35-year-old son in the home, died after developing an MRSA infection that showed increased resistance to vancomycin after prolonged exposure to the drug in an intensive care unit, reported Linda A. Greene, RN, MPS, CIC, an infection control professional at Rochester General Hospital in New York.

Patient came in colonized

The patient came in colonized with an MRSA strain that barely registered for any reduced susceptibility to vancomycin, with lab tests showing it had less than one MIC (minimum inhibitory concentration) for the drug. However, the level of resistance increased to four MIC after the patient was administered vancomycin for 45 days during hospitalization. "This is the unique thing about these strains," Greene told Hospital Infection Control. "Basically, had she never received vancomycin, this would have never happened. This is a strain that will evolve only under the pressure of vancomycin."

First discovered in Japan in 1996, strains of MRSA with reduced susceptibility to vancomycin have a MIC in lab tests of at least four. More resistant strains with a MIC of eight are called vancomycin intermediate-resistant S. aureus (VISA).^1-3 Four documented cases of VISA infection have occurred in the United States, and many experts expect a fully resistant strain to emerge eventually. In addition to the VISA strains, the Centers for Disease Control and Prevention is receiving reports from clinicians such as Greene about infections occurring with the reduced-susceptibility strains with a MIC of four. "I know that the CDC is looking at 20 or so [similar] cases, and they are doing case-control studies to determine whether or not there may be some inherent risk factors," she says.

Losing a weapon against MRSA

Though new drugs may be important in fighting such strains, the matter is of concern because vancomycin has long been the most steadfast weapon against MRSA strains resistant to everything else. Moreover, reports have been increasing of MRSA transmission in the community to people who lack traditional risk factors like previous hospitalization.

The Rochester case began in March 1999 when the woman was admitted to an ICU at the hospital, Greene reported recently in Minneapolis at the 27th annual conference of the Association for Professionals in Infection Control and Epidemiology.⁴ The patient had not been hospitalized in more than three years. Nine days after admission, her blood and sputum cultures grew MRSA. Vancomycin was started, with the assumption that it was a nosocomial infection with one of the circulating hospital strains. After 10 days of therapy, a MIC of one to vancomycin was reported. MRSA bacteremia was again detected after 45 days of vancomycin therapy, and the MIC had increased to four. Recognition of a strain of MRSA with reduced vancomycin susceptibility prompted a full investigation, and isolation efforts were intensified, Greene and co-investigators reported. Cultures from staff and personnel were obtained.

The woman’s son was previously known to be colonized with MRSA after a lengthy hospitalization. A culture from the son, the index case, and representative MRSA samples from the hospital were sent to the CDC. However, surveillance in the ICU failed to identify a similar isolate in staff or patients. Pulse gel electrophoretic patterns from mother and son were identical, but different from endemic hospital strains. Because the son had no contact with his mother during her hospitalization, the investigators hypothesized that colonization with a potentially vancomycin-resistant MRSA strain occurred in the household setting.

Exogenous flora not always to blame

"One of the things [of concern] is that it was pretty innocuous until she became hospitalized," Greene tells HIC. "And the other thing in terms of infection control is, regardless of the fact that she became colonized in the community setting, she developed the infection in the hospital. So she infected herself from her own endogenous flora. How do we as health care workers, when we are caring for lines or when we are taking care of patients, realize that infection doesn’t always occur with exogenous flora? It occurs because maybe there hasn’t been appropriate line care, and [the patients] infected themselves with their own flora."

As a result of the case, patients are now screened for MRSA before admission to ICUs at the hospital. Decisions about drug therapy for MRSA remain a case-by-case clinical choice. "If the MIC stays below point-five, I would never say do not use vancomycin," Greene says. "But I am no ID physician. But maybe at the point of one [MIC], you may start looking at this and [asking] is my patient clinically improving?’ Clearly, this lady was not clinically improving, and so we ended up putting her on Synercid. It was a little too late."

References

1. Centers for Disease Control and Prevention. Reduced susceptibility of Staphylococcus aureus to vancomycin — Japan, 1996. MMWR 1997; 46:624-626.

2. Centers for Disease Control and Prevention. Staphylococcus aureus with reduced susceptibility to vancomycin — United States, 1997. MMWR 1997; 46:765-766.

3. Centers for Disease Control and Prevention. Update: Staphylococcus aureus with reduced susceptibility to vancomycin — United States, 1997. MMWR 1997; 46:813-814.

4. Greene L, Chodoff A, Hollick G, et al. Evidence for community transmission of methicillin-resistant Staphylococcus aureus with a propensity for reduced vancomycin susceptibility. Presented at the Association for Professionals in Infection Control and Epidemiology. Minneapolis; June 18-22, 2000.