Interpretation
Prevalence of MRSA carriers can vary across time and
regions6,11,12. Our hospital is the only perinatal
medical center within 2,000 km2, and approximately 90% of deliveries in
this area are performed in our facility. Therefore we anticipate that
this study almost accurately reflects the proportion of MRSA carriers
among pregnant women living in this area. As of 2018, the prevalence
rate of community-acquired MRSA can be estimated to be approximately 6%
in Japan.
Among the clinical background characteristics related to MRSA carriers
in pregnant women, being multiparous and being a healthcare provider
were found to be independent risk factors. Several other studies have
also shown that multiparous women and healthcare providers are at
greater risk for being MRSA carriers7,13,14. It is
still unclear why the proportion of MRSA carriers is higher among
multiparous women. We assume that these women had been to the hospital
more frequently for obstetrical and neonatal issues, which might
increase the probability of acquiring MRSA.
We observed that the vertical transmission rate of MRSA from the mother
to the neonate during delivery was significantly higher in pregnant
women who were MRSA carriers compared to those who were not. While MRSA
vertical transmission was observed even among pregnant women with MRSA
detected only in the nasal cavity, the vertical transmission rate was
much higher in women with MRSA detected in the vagina. Considering that
MRSA was transmitted at a high rate when detected in the maternal vagina
and that no neonates delivered by C/S were MRSA positive, most MRSA
transmissions are highly likely to be due to direct vertical
transmission by vaginal delivery.
Consistent with several previous reports, our study found that maternal
MRSA carrier status had little influence on maternal and neonatal
adverse outcomes15,16. Interestingly, however,
maternal MRSA carrier status was found to be significantly associated
with SSTIs in the neonatal period. Although MRSA infections in preterm
neonates are known to be associated with poor prognostic
outcomes3, the clinical significance of MRSA infection
in full-term neonates has not been well studied thus far. We revealed
for the first time that the vertical transmission of MRSA was associated
with increased occurrence of SSTIs in full-term birth neonates.
Given that MRSA is generally found in the epidermis, we think this is a
plausible observation and consistent with other
reports7,17. MRSA was the causative bacterium in the
neonates delivered by the three mothers who were MRSA carriers (Table
4). These three patients showed moderate to severe symptoms such as
abscess. Moreover, one patient needed an incision and drainage. On the
other hand, all patients with SSTIs who were delivered by non-MRSA
carriers showed relatively mild symptoms. There was a tendency for MRSA
infections to occur shortly after birth, sometimes on the same day they
were born. When an SSTI with moderate to severe symptoms, such as
abscess, occurs immediately after birth, we may consider that MRSA is
the causative bacterium and that his or her mother is a MRSA carrier.