April 19, 2012

Swab study shows MRSA in pregnancy may not mean much

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Natalia Jimenez-Truque, MSCI, and Buddy Creech, M.D., MPH, are studying the ramifications of expectant mothers passing MRSA to their children. (photo by Anne Rayner)

Swab study shows MRSA in pregnancy may not mean much

Vanderbilt pediatric infectious disease researchers studying methicillin-resistant staphylococcus aureus (MRSA) say fears that mothers carrying the germ may set their newborns up for infection are unfounded.

Buddy Creech, M.D., MPH, senior author of the recent study in Pediatrics, said the research is directed at a recent trend of swabbing the noses of pregnant women, and even more frequently, newborn babies in neonatal intensive care units, for the presence of MRSA.

“Labs were finding a substantial amount of MRSA. Even in our study, we found 20 percent of pregnant women will have it, as will 20 percent of babies at eight weeks. But there is tremendous anxiety about what that means — and for physicians, what do you do?” Creech said.

Creech said the concern stems in part from knowledge of another bacteria that pose a serious risk to babies, Group B strep.

This bacteria is transmitted from a carrier mother to her baby, either in pregnancy or during birth, a process called “vertical” transmission. In most states, women are tested for Group B strep in their third trimester because eradicating it at that time can save infants.

Recently, labs have begun reporting back to physicians when MRSA is detected in a woman’s Group B strep test. The concern was mothers might vertically transmit MRSA as well, exposing babies to an increased risk of illness.

In older populations of children and adults, there is evidence that colonization with a certain strain of MRSA, called USA300, increases the risk of illness: most commonly skin boils, but occasionally serious blood and joint infections.

The study enrolled more than 500 pregnant women in Nashville and Memphis. Nasal and vaginal swabs were collected and tested for the presence of bacteria at regular intervals, including at the time of delivery. Babies were swabbed right after birth and at 2 and 4 months of age.

The results show little vertical transmission of MRSA from mother to child. However, by age 2 months, babies closely matched their mother’s carrier status. This suggests a mother who carries S. aureus bacteria in her nose will give it to her baby and her baby will become colonized from close contact within six to eight weeks after birth. This is called horizontal transmission.

Creech said describing the timing and mode of transmission may be important, but the most critical finding in this research is that babies rarely became ill with MRSA infections.

“We don’t want to overreact to carriage when incidence of disease is low. A lot of babies are colonized. Twenty percent at 2 months of age is the highest rate we’ve ever seen, but in our study only two babies got disease,” Creech said.

The Vanderbilt research did find that of the MRSA carriers, about 30 percent of mothers and babies share the USA300 strains. But in this newborn population, many of the genes responsible for the increased virulence of USA300 were not present; suggesting that not all MRSA are created equal.

So in answer to the question what should be done when MRSA colonization is detected in a pregnant woman, Creech said the best action may be no action at all. He said the next step in research is to determine if mother-to-child transmission of MRSA in infancy might provide benefits, like greater protection against more serious MRSA illnesses later in life.

The study’s first author is Natalia Jimenez-Truque, MSCI, an Epidemiology graduate student in Pediatric Infectious Diseases.