Infection by Streptococcus pneumoniae accounts for more deaths than any other vaccine-preventable bacterial infection. The case fatality rate from pneumococcal bacteremia approaches 40 percent in elderly patients. Of great concern in the United States is the growing number of drug-resistant strains of S. pneumoniae. Resistant strains have been identified in certain settings, such as day care centers, hospitals and a pediatric long-term care facility, but epidemics of drug-resistant pneumococcal disease have not been previously reported among adults. Nuorti and associates of the Centers for Disease Control and Prevention (CDC) report the findings of a two-week outbreak of multidrug-resistant S. pneumoniae serotype 23F infection among unvaccinated residents of a nursing home.
The CDC investigation began in February 1996, after officials were notified that during a five-day period, three patients at a 100-bed nursing home in a rural community had been hospitalized because of pneumococcal bacteremia. Two of the patients died of rapidly progressive illness despite therapy with intravenous cefuroxime. Eleven of the 84 residents contracted pneumonia caused by the resistant strain, for an attack rate of 13 percent.
All isolates of S. pneumoniae were found to exhibit only intermediate sensitivity to penicillin and cefotaxime; the minimal inhibitory concentration was 1.0 μg per mL for both agents. The isolates were resistant to trimethoprim-sulfamethoxazole and erythromycin. They were susceptible only to vancomycin. The laboratory at the community hospital had not previously identified any resistant strains of S. pneumoniae since beginning routine screening of all isolates in 1995.
To evaluate the epidemiology of the outbreak, the investigators used swabs to obtain nasopharyngeal culture samples from all of the nursing home residents and employees. The resistant strain was identified in 17 (23 percent) of 74 residents and in two (3 percent) of 69 employees. One day after the investigation began, pneumococcal polysaccharide vaccine was administered to 71 unvaccinated residents, as well as 11 employees with chronic medical illnesses.
Because four additional cases of pneumonia occurred during the next three days, a seven-day course of oral penicillin or ofloxacin was initiated in all of the residents. Two employees who were colonized with the outbreak strain received combination chemoprophylaxis with rifampin and ofloxacin for one week. Results from testing of nasopharyngeal swabs after vaccination and chemoprophylaxis revealed colonization with the resistant strain in only three residents and in no employees. Five weeks later, only two residents were still colonized.
The patients who had pneumonia were similar to the other nursing home residents in mean age, race and sex. Multidrug-resistant S. pneumoniae serotype 23F was isolated from the blood of four patients and the respiratory tracts of three others. Three patients with bacteremia died. Only three (4 percent) of the 84 nursing home residents had received pneumococcal vaccination, although 60 (71 percent) residents had received influenza vaccination the previous fall.
The authors believe several factors contributed to this epidemic. Most of the nursing home residents had not received pneumococcal vaccination and thus were susceptible to S. pneumoniae. In addition, the prevalence of colonization with the virulent 23F strain was high (23 percent) in this population. (The reported rate of colonization with S. pneumoniae is usually less than 10 percent among adults.) The colonization and attack rates were higher among patients who were receiving antibiotics (or at the time nasopharyngeal samples were obtained, in residents who were not ill) when the illness developed. An association between recent antibiotic use and colonization or infection with drug-resistant S. pneumoniae has been reported in the literature.
The investigators believe the optimal strategy for controlling outbreaks of drug-resistant S. pneumoniae infection still needs to be determined. In the outbreak they investigated, administration of pneumococcal vaccine and, possibly, oral antibiotic chemoprohylaxis made a difference, as no additional cases of pneumonia developed after these interventions. In addition, the colonization rate decreased significantly.
editor's note: This outbreak serves as a frightening wake-up call to physicians; drug-resistant S. pneumoniae is a growing problem that may have significant implications in the next several years. The current treatment guidelines for community-acquired pneumonia in adults, from the American College of Physicians and the Infectious Diseases Society of America, recommend a second- or third-generation cephalosporin, with or without erythromycin. Yet these antimicrobial agents would have failed as empiric therapy for the patients in this outbreak. Widespread use of the pneumococcal vaccine is obviously needed as well as a more judicious use of oral antibiotics.—j.k.