Fortunately, the risk of neonatal transmission or colonization in these cases is very low, and preliminary data suggest there's no increased risk of adverse neonatal outcomes even if the mother initially is given the wrong treatment for community-associated methicillin-resistant S. aureus (CA-MRSA), Dr. Natali Aziz said at a conference on antepartum and intrapartum management sponsored by the University of California, San Francisco.
In general, as many as one in three breastfeeding women in the United States develops postpartum mastitis, with approximately 10% of these developing breast abscesses. Studies of breast milk cultures have found S. aureus present in 37%-50% of mastitis cases.
A case-control study of 48 cases of S. aureus-associated postpartum mastitis in 1998-2005 found that 17 (81%) of 21 cases that were resistant to methicillin occurred in 2005 (Emerg. Infect. Dis. 2007;13:298-301).
Genetic analyses also suggested that 20 of the 21 MRSA cases were due to community-acquired MRSA, which may reassure clinicians that mastitis associated with MRSA should be susceptible to oral antibiotics, added Dr. Aziz of the university.
What few data exist on postpartum MRSA infection suggest that most cases involve mastitis or soft tissue infection, and that mastitis commonly leads to abscesses, she said. In one series of 10 postpartum MRSA infections, 4 affected the breast, 3 were associated with incisions, and 3 were in soft tissue. In another series of eight postpartum MRSA infections, half were in the breast, and three of these four cases progressed to abscesses.
The 21 cases of MRSA were less likely than the methicillin-susceptible cases to receive timely and appropriate treatment, but there were no significant differences in clinical outcomes in this small study, she noted.
In the largest study to date of hospitalized women with puerperal mastitis, cultures from 35 women who had both mastitis and breast abscesses found that CA-MRSA was the most common organism in breast abscesses, with MRSA in approximately two-thirds of cases. MRSA was much less likely in 54 women who had mastitis alone, growing in only one culture. As in the smaller study, a majority of women with CA-MRSA did not receive an appropriate antibiotic, but empiric use of an ineffective antibiotic did not adversely affect outcomes (Obstet. Gynecol. 2008;112:533-7).
At San Francisco General Hospital in 2005, S. aureus was cultured in the breast milk of 8 of 15 cases of mastitis; only 2 had MRSA, but three women with breast abscesses all had MRSA, Dr. Aziz said.
The data so far suggest that clinicians can continue to treat routine cases of mastitis with conventional first-line medications, and that it's reasonable to start treatment for CA-MRSA before cultures are completed in patients with abscesses or recurrent failure on conventional mastitis therapy. Consider getting cultures for recurrent disease, in areas with a high prevalence of CA-MRSA, or in patients with risk factors for CA-MRSA.
“Be aware of your local epidemiology for your antibiotic choice” for CA-MRSA, Dr. Aziz advised, and remember that abscesses with CA-MRSA usually will require adjunct drainage.
Women whose breast milk is colonized with CA-MRSA without mastitis can continue to breastfeed or pump breast milk for term infants, but this may put preterm infants at higher risk of conjunctivitis or other problems, case reports suggest.
It is not cost effective to universally screen for MRSA or to decolonize women with MRSA in obstetric populations, a recent decision-analysis study concluded (Obstet. Gynecol. 2009;113:983-91).
Dr. Aziz said she has no conflicts of interest.