CDC expecting more U.S. infections with emerging resistant staph strains
October 1, 1997 6 minutes read
CDC expecting more U.S. infections with emerging resistant staph strains
Vancomycin pressure in MRSA patients spurs independent VISA strains
The first two rapid-fire reports of vancomycin intermediate-resistant Staphylococcus aureus (VISA) in the United States are expected to be followed by similar cases, as the effectiveness of medicine’s most potent weapon against a common, virulent pathogen is now in question.
Within one month of identifying the first confirmed VISA case in a peritoneal dialysis patient in Michigan, the Centers for Disease Control and Prevention reported confirmation of what appears to be a different VISA strain in a patient with a bloodstream infection in New Jersey.1,2 (See related story, p. 147.) Both cases involved methicillin-resistant S. aureus (MRSA) infections that developed an intermediate resistance to vancomycin. The emerging VISA strain in each case had a minimum inhibitory concentration of 8 the same level of reduced vancomycin susceptibility in the first reported VISA patient in Japan in 1996.3 Initial reports indicate, however, that the two U.S. cases are unrelated epidemiologically to each other and to the strains emerging in Japan. Neither the strain genotypes nor the mechanisms of resistance have been determined in the U.S. cases, but the first case in Japan involved a strain with thicker cell walls that reduced its susceptibility to vancomycin. (See Hospital Infection Control, August 1997, pp. 113-118.)
While secondary transmission from the first two U.S. cases is a concern, it is more likely that subsequent, unrelated VISA cases will be confirmed in other patients undergoing prolonged, intermittent vancomycin therapy for MRSA infections, CDC officials tell HIC.
"I think it is a fair assumption that we are going to see more of these cases," says David Bell, MD, assistant director for antimicrobial resistance at the CDC national center for infectious diseases. "If the pattern holds up, it will be among people who have received vancomycin intermittently for a lengthy period."
There is some question about whether the rapid appearance of the first U.S. cases reflects enhanced surveillance efforts following the first case in Japan, or whether S. aureus is now finally expressing vancomycin resistance in a broad way.
"We think it is a little bit of both," says Michelle Pearson, MD, medical epidemiologist in the CDC hospital infections program. "It is certainly possible that some of these strains may have been detected in laboratories before. But because resistance to vancomycin was not known to occur in Staph aureus, it may not have been reported, or it was attributed to laboratory error. The report from Japan alerted people that it indeed can occur and people should be looking for it."
In addition to the first two cases, the CDC has received "a number of other reports" of VISA isolates in U.S. patients, but thus far has not confirmed any other cases, she adds.
For example, William Schaffner, MD, professor of infectious diseases and chairman of the department of preventive medicine at the University of Vanderbilt School of Medicine in Nashville, TN, reports there is an ongoing investigation of a possible VISA case in his area involving a patient with a similar clinical profile as the first reports. It appears after initial testing that the isolate is vancomycin-susceptible, but it is being retested, he notes. With clinical laboratories all over the country now actively looking for VISA, additional confirmations may soon follow, he adds.
"I’m not talking about clonal distribution. This evolution this development of resistance could occur in so many places in this country," Schaffner says. "There isn’t any doubt that clinicians and public health people are on tenterhooks. I’m sure there are a lot of people that are running down suspects."
Indeed, that emergence of VISA underscores the value of infection control programs and microbiological surveillance at a time when health care restructuring and managed care trends have been widely perceived as putting such programs in peril, notes Robert Haley, MD, director of the division of epidemiology in the department of internal medicine at the University of Texas Southwestern Medical Center in Dallas.
"We know in the last five years with the advent of financial pressures on hospitals to reduce expenditures, many hospitals have been de-emphasizing, reducing, or eliminating their infection control programs," he says. "What we are in danger of is a case like this not coming to our attention, going into a hospital, and becoming indigenous before it is recognized. Then we might have another MRSA scenario where we start seeing epidemics all over the country being transferred from one hospital to the next because they don’t have good surveillance programs. Infection control is not a luxury that you can cut out when your budget gets a little tight."
Keeping the genie in the bottle
The CDC has placed great emphasis on detecting VISA or fully resistant staph strains, urging clinicians to contain the pathogens at first appearance before they can establish the kind of endemic presence MRSA and vancomycin-resistant enterococci (VRE) have achieved in many health care settings. After the appearance of the Japanese case, the agency issued infection control guidelines for the pathogen and advised clinicians to search for isolates and report them promptly.4
"The investigations in both Michigan and New Jersey involve culturing contacts to see if anyone has been colonized," Bell says. "That is what we are worried about that is how it starts."
The first U.S. case, who is under home care in an undisclosed area of Michigan, has a history of multiple admissions to area hospitals under CDC contact isolation precautions for his MRSA infection. Cultures have been obtained from the hands and nares of the patient’s household contacts, hospital roommates, and health care providers. Although S. aureus was isolated from 13 (25.4%) of 51 hand cultures and eight (15.6%) of 51 nares cultures, none of the cultures were positive for VISA, the CDC reports. Culture results were not complete for the New Jersey case.
While posing the threat of infection to those found colonized or their contacts, the larger issue is whether VISA strains will move among hospitals and nursing homes, and even out into surrounding communities. The well-known ability of MRSA to move among such settings is disturbing, because VISA strains if established are expected to follow similar epidemiological lines as MRSA.
For example, consider a one-year review of 214 new MRSA cases among inpatients at East Orange (NJ) Veterans Affairs Medical Center conducted by Timothy Leach, MD, MPH, chief of infection control at the medical center. Overall, 50 (23.4%) of the patients had been hospitalized within one month prior to the positive culture for MRSA, and another 58 (27.1%) had been transferred from local nursing homes. In addition, MRSA was cultured within 48 hours from 65 (30.4%) of the patients. While 13 of those reported being hospitalized within the previous 30 days indicating possible nosocomial acquisition the other cases likely acquired MRSA in the community, he notes.
Community acquisition is more of a clinical issue than with VRE, for example, because S. aureus is a more virulent organism, he adds.
"VRE is a problem for patients who are very debilitated and very sick," Leach says. "But for otherwise healthy people, VRE is not a true pathogen. Boils and cellulitis things that people who are otherwise healthy can acquire would be susceptible to a Staph aureus isolate but not necessarily a VRE. If vancomycin-resistant Staph aureus starts circulating in the community, that would be bad news because this is a true pathogen. It is also bad news for hospitalized patients as well."
While underscoring the urgency of the situation to clinicians, the CDC is wary of fanning public fears about such scenarios. It is something of a balancing act, however, because at the same time the agency is trying to drive home the need for clinicians to use strict infection control measures with the first cases and eliminate inappropriate uses of vancomycin to preserve its efficacy.5
"We have to take a tempered approach," says William Jarvis, MD, acting director of the CDC hospital infections program. "We are obviously very concerned with the emergence for the first time of intermediate vancomycin resistance in this common pathogen. But we don’t want the lay public to think it is rampaging through the streets. With each of these [cases] we are hoping the clinicians, hospital administrators, and pharmacy directors will take this message very seriously. They really do need to re-examine antimicrobial use."
References
1. Centers for Disease Control and Prevention. Staphylococcus aureus with reduced susceptibility to vancomycin United States, 1997. MMWR 1997; 46:765-766.
2. Centers for Disease Control and Prevention. Update: Staphylococcus aureus with reduced susceptibility to vancomycin United States, 1997. MMWR 1997; 46:813-814.
3. Centers for Disease Control and Prevention. Reduced susceptibility of Staphylococcus aureus to vancomycin Japan, 1996. MMWR 1997; 46:624-626.
4. Centers for Disease Control and Prevention. Interim guidelines for prevention and control of staphylococcal infection associated with reduced susceptibility to vancomycin. MMWR 1997; 46:626-628.
5. Centers for Disease Control and Prevention. Recommendations for preventing the spread of vancomycin resistance: Recommendations of the Hospital Infection Control Practices Advisory Committee (HICPAC). MMWR 1995; 44:(no. RR-12)1-13.
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