S. aureus detected in specimens from vaginal and rectal cavities of pregnant women

November 2006

SAN FRANCISCO — Although methicillin-resistant Staphylococcus aureus is most commonly detected by looking for nasal carriage, vaginal and rectal cavities may serve as additional reservoirs for this bacterium and might contribute to the increased rates of community-acquired methicillin-resistant S. aureus infections occurring in nurseries.

According to new research presented at the 46th Interscience Conference of Antimicrobial Agents and Chemotherapy, 2% of pregnant women had vaginal-rectal colonization of the CA-MRSA strain that is now epidemic in the nation.

“Although screening the nose is a convenient method to look for carriage of S. aureus, it may not identify everyone carrying CA-MRSA,” said Henry S. Fraimow, MD, associate professor of medicine at the University of Medicine and Dentistry of New Jersey and with the division of infectious diseases at Cooper University Hospital in Camden, N.J. “Strategies that are developed to treat carriers of CA-MRSA need to take account all the sites where these bacteria may be hiding. Unsuspected transmission from colonized mothers to newborns may be important in causing CA-MRSA outbreaks that occur in hospital nurseries.”

Fraimow and colleagues also suggested that this finding might explain why many CA-MRSA infections occur on body locations below the waist and why surveys that look for people carrying CA-MRSA in their nasal passages may not accurately reflect CA-MRSA prevalence.

“There may be differences in the way these CA-MRSA strains colonize people that may be important as we think of strategies to try to treat people with recurrent infections and prevent transmission,” he said. “If we only think of the nose as potential reservoirs for these infections, we’re going to miss significant potential reservoirs.”

New carriage site

Since 2002, there has been a dramatic increase of CA-MRSA cases in Camden, according to Fraimow. Researchers have been tracking cases since 2002. As of 2004, they have found that most staph infections in healthy individuals that present in the emergency department are due to MRSA. Many infections occurred in areas below the waist, such as the groin, buttocks and upper thighs, and some of those patients lack nasal colonization. Furthermore, Fraimow and colleagues have documented several cases of MRSA infections in newborns, suggesting that the infants could have acquired these strains during the birth process.

“We’re seeing cases in everyone; infants children, young adults, people with no other medical problems and people without any of the traditional risks for MRSA,” he told Infectious Diseases in Children. “We wanted to look for other reservoirs for CA-MRSA other than the nares, which is the place where everyone looks, to see if some of the things we observed with the increase in prevalence of these infections could be explained by finding other sites of colonization.”

Fraimow and colleagues examined the bacterial cultures of S. aureus taken from specimens of vaginal and rectal cavities of 353 pregnant women who underwent routine prenatal screening for group B streptococcus bacteria from June 2005 through March 2006.

They found that 11% of the specimens contained S. aureus. The researchers then used PCR and DNA fingerprinting and determined that seven (18%) of the vaginal and rectal S. aureus strains were MRSA. Furthermore, all seven strains were susceptible to clindamycin and levofloxacin, according to the abstract. The researchers also identified the strains as USA 300, the CA-MRSA strain that is epidemic in the United States. All strains were different than those typically found in hospital settings. The carriage rate for CA-MRSA strains was 2%.

“That’s as high or higher than some of the surveillance we’ve done looking for nasal colonization with CA-MRSA,” he said.

The findings suggest that the vagina and rectum may be reservoirs for CA-MRSA and a potential source for perinatally acquired CA-MRSA. Although data are lacking, potential maternal-fetal transmission of CA-MRSA is a concern, and more information is needed to determine if mothers could be carriers and if that is a risk to the infant, Fraimow suggested.

“It raises the potential that this could be the source for some of these newborn infections,” he said.

Seasonal variation

Fraimow and colleagues also looked at seasonal differences in S. aureus carriage rates, since their data indicated higher rates of S. aureus colonization in women during summer months. They noted that carriage increased during warmer months from June through September (14.5%) compared with October through March (6.9%).

“Although no one has reported seasonal variation with staph colonization or infection, when we looked at our own hospital data for people reporting to the emergency room with CA-MRSA infections, every year for the last three years we’ve seen a big increase in July and August, as much as 50% higher than the rest of the year,” he said. “It may be that unlike the nose, where people have had a hard time showing any change in colonization at different times of the year, there may very well be a difference in the vaginal-rectal area and it correlates with the data we’re seeing for these infections.”

The researchers suggested that although seasonality differences in carriage rates have not been well studied, it may be important in understanding the epidemiology of these infections.

For more information:

• Farrow S, Knob C, Barry C, et al. Vaginal-rectal carriage of Staphylococcus aureus and community-acquired MRSA in pregnant women. Abstract C2-605. Presented at: 46th Interscience Conference on Antimicrobial Agents and Chemotherapy; Sept. 27-30, 2006; San Francisco.