We are all hearing more and more about the new epidemic of Methicillin-Resistant Staph Aureus (MRSA).  While the rate in the general public is still low,  it is rising and it seems especially so in pediatric age patients where a higher prevalence seems to occur in settings that enable close contact between individuals, such as day-care centers.

S aureus can colonize the nasopharynx, perineum, or skin.  The most common way to screen for colonization is culture of specimens from the nasopharynx. Although most colonized individuals remain asymptomatic,   small disruptions of the skin can lead to local infections. One-third of the general population and up to 50% of those with chronic medical conditions (renal failure, diabetes, etc) can be chronically colonized with S aureus.  In healthy persons, the rates of MRSA colonization were in the past low, but have been steadily increasing.  In children,  the colonization rate ranges from 0.8 to 3.0% but these too seem to be increasing.  A recent study in Texas children reported a 14-fold increase in cases in 1999-2001.

More importantly,  our hospitals are now reporting high rates of MRSA. My own hospital reports 54% of the strains of S aureus,  are methicillin-resistant. 

Recent reports of MRSA mastitis have been increasing, including infections in premature infants from MRSA infected milk. Numerous clinicians in the field report that they commonly see it in their practices. Many such clinicians are now culturing milk samples prior to prescribing antibiotics,  so that if the antibiotic fails, they’ll have a culture to guide their subsequent therapy.  This is probably wise.

But it is still important to remember, that the most likely cause of mastitis in breastfeeding mothers is still the old penicillin-resistant S. aureus,  not MRSA.   Because penicillin-resistant mastitis still commonly occurs in the breastfeeding population, and is still well-treated with dicloxacillin, cephalexin, trimethoprim-sulfamethoxazole, or even macrolide antibiotics,  clinicians are advised that in most instances these older antibiotics are still the best “first” choices.

Patients whose symptoms fail to resolve significantly within 24  hours of therapy with the older antibiotics may need to be referred back to their physician for a change.  Dicloxacillin or cephalexin work fast, and a mother who does not feel significantly better in 24-48 hours should be closely re-examined for abscess, or infection with methicillin-resistant Staph aureus.  Mastitis in nurses or other healthcare workers that work in healthcare facilities are probably at higher risk.

There are a number of antibiotics that are suitable for the treatment of MRSA infections.  In pediatric-aged patients we have rapidly converted to clindamycin,  which has minimal side effects in children and works well presently.   In adults,  good choices for breastfeeding mothers would be clindamycin, trimethoprim-sulfamethoxazole (a little slow),  or ciprofloxacin, ofloxacin, or levofloxacin.

Current Antimicrobial Susceptibility (MRSA)*

• Resistance to erythromycin is reported to range from 10-100%,  but averages about 50%.
• The clinical response to Clindamycin is still good.  Eighty to ninety percent of MRSA is still susceptible, but the duration of clindamycin is questionable due to the rapid emergence of resistance.
• 90% of MRSA isolates have been reported to be susceptible to fluoroquinolones (ciprofloxacin, levofloxacin, etc).  There are concerns that rapid emergence of resistance may occur with this family of antibiotics as well.
• Trimethoprim-sulfamethoxazole (TMP-SMX) still maintains good activity (>95%) against most strains of MRSA.  The excellent safety profile suggests it is a suitable candidate for patients with mastitis.
• Presents MRSA isolates are still uniformly susceptible to vancomycin, daptomycin, linezolid, and quinupristin-dalfopristin. In cases of severe infections, these intravenous preparations are ideal.

  * Subject to variation by region of country and even hospital.

  Tom Hale, PhD
  Professor of Pediatrics
  Texas Tech University School of Medicine