The incidence of methicillin-resistant Staphylococcus aureus (MRSA) infections has decreased in children in recent years, but resistance to clindamycin has increased over the same period, a study showed.

“The epidemic of skin and soft tissue infections and invasive MRSA led to modifications of antimicrobial prescribing practices for suspected S. aureus infections,” reported Dr. Deena E. Sutter of the San Antonio Military Medical Center in Fort Sam Houston, Tex., and her associates. “Over the study period, erythromycin susceptibility among methicillin-susceptible S. aureus (MSSA) remained stable, suggesting that declining clindamycin susceptibility is a result of an increase in inducible resistance.”

The steady decline in clindamycin susceptibility “may lead to some concern about the continued reliance on clindamycin for the empirical treatment of presumptive S. aureus infections, although it is probably premature to abandon this effective antibiotic choice,” they wrote (Pediatrics. 2016 Mar. 1. doi: 10.1542/peds.2015-3099). “It is crucial that clinicians remain knowledgeable about local susceptibility rates as it would be prudent to consider [alternative] antimicrobial agents for empirical use when the local clindamycin susceptibility rate drops below 85%.”

The researchers retrospectively analyzed lab results from 39,209 patients under age 18 who were treated for S. aureus infections at one of the 266 U.S. facilities of the Military Health System from 2005 to 2014. The data included 41,745 S. aureus isolates, classified as MRSA if found resistant to cefoxitin, methicillin, or oxacillin and as methicillin susceptible (MSSA) if susceptible to those antimicrobials. The isolates had also been tested for susceptibility to ciprofloxacin, clindamycin, erythromycin, gentamicin, oxacillin, penicillin, rifampin, tetracycline, and trimethoprim/sulfamethoxazole (TMP/SMX).

During that decade, overall S. aureus susceptibility to clindamycin, ciprofloxacin, and TMP/SMX decreased – although susceptibility to TMP/SMX in 2014 stayed high at 98% – while overall susceptibility to erythromycin, gentamicin, and oxacillin increased. Specifically, 59% of S. aureus isolates were susceptible to oxacillin in 2005, which dropped briefly to 54% in 2007 before climbing to the 2014 rate of 68%.

Meanwhile, overall susceptibility to clindamycin dropped from 91% in 2005 to 86% in 2014, and MSSA susceptibility to clindamycin dropped from 91% in 2005 to 84% in 2014. “Erythromycin susceptibility remained stable among MSSA isolates throughout the study period at 63.5%, whereas MRSA susceptibility to erythromycin increased from 12.1% to 20.5%,” Dr. Sutter and her associates reported. “Ciprofloxacin susceptibility significantly decreased overall, although an initial decrease of 10.6% over the first 7 years of the study was subsequently followed by an increase of 6% between 2011 and 2014.”

Most of the isolates came from patients with skin and soft tissue infections, which were less likely to be susceptible to oxacillin than were other infections. Infections in children aged 1-5 years also were less likely to be susceptible to oxacillin, compared with infections in children of other age groups.

If the local clindamycin susceptibility rate falls below 85%, “beta-lactams, TMP/SMX, or tetracyclines may be used for less severe infections with intravenous vancomycin employed in severe cases,” the investigators said. “If overall MRSA rates continue to decline and clindamycin resistance among MSSA continues to increase, we may see a return to antistaphylococcal beta-lactam antimicrobial agents such as oxacillin or first-generation cephalosporins as preferred empirical therapy for presumed S. aureus infections.”

The research did not use external funding, and the authors reported no relevant financial disclosures.