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.