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Dialysis patients at high risk for resistant S. aureus. (Worrisome Prospect).

SAN FRANCISCO -- Hemodialysis patients may be at high risk for infection by Staphylococcus aureus strains with intermediate-level resistance to vancomycin, Dr. Catherine Liu cautioned at the annual meeting of the Infectious Diseases Society of America.

This worrisome prospect raises the possibility that incurable staphylococcal infections will emerge in the nation's large and rapidly growing population of end-stage renal disease patients, added Dr. Liu of the University of California, San Francisco.

Since the first report in 1997 of S. aureus with intermediate-level resistance to vancomycin--strains known as VISA in infectious disease circles--there have been eight cases described in the United States. Five of them occurred in hemodialysis patients. This isn't altogether surprising, as dialysis patients are prone to infections and to colonization by S. aureus, she said.

"Furthermore, dialysis patients experience frequent and prolonged exposure to vancomycin because of the high prevalence of methicillin-resistant S. aureus in this group and to the pharmacokinetics of the drug, which has a half-life of up to 1 week in these patients," she noted.

Infection with VISA appears to be preceded by a subclinical state known as heterogenous VISA (hVISA), which refers to strains of S. aureus containing subpopulations of vancomycin-resistant cells that can't be detected using routine laboratory minimum inhibitory concentration tests.

The clinical significance of hVISA is a subject of debate. Dr. Liu subscribes to the working hypothesis that hVISA arises due to selective pressure generated by vancomycin exposure, and that hVISA represents an intermediate stage between vancomycin-susceptible S. aureus and VISA. If this hypothesis is correct, then patients with greater exposure to vancomycin should have a higher prevalence of hVISA.

To see if this is indeed the case, Dr. Liu and coinvestigators conducted a prospective study in 211 hemodialysis patients at four outpatient centers. All had nasal swabs obtained and cultured for S. aureus every 3 months. Nasal carriage of S. aureus is an important source of systemic staphylococcal infections.

After a median of 9 months, 44% of participants were colonized with a total of 239 S. aureus isolates. None of the isolates were homogeneous VISA. One-third of S. aureus-colonized patients were found to have hVJSA isolates. The hVISA isolates were found twice as frequently in methicillin-resistant S. aureus as in methicillin-susceptible strains.

One-quarter of study participants had received 1 g or more of vancomycin during the 6 months prior to the study or the follow-up period. The prevalence of hVISA isolates among these vancomycin-exposed patients was 20%, compared with 13% in those with less or exposure to vancomycin.

Thus, vancomycin exposure was associated with a 1.5-fold increased risk of colonization with hVISA, a difference that didn't quite reach statistic cal significance. But the trend suggests that if hVISA colonization predisposes to VISA infection, then hemodialysis patients are likely to constitute a high-risk population, Dr. Liu commented.
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Title Annotation:Staphylococcus aureus
Author:Jancin, Bruce
Publication:Internal Medicine News
Geographic Code:1USA
Date:Jan 15, 2002
Words:466
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