PHILADELPHIA — Infections caused by methicillin-resistant Staphylococcus aureus occur in a bimodal pattern in the first year of life, with peaks in the first 5 months and again shortly before the first birthday. The peaks reflect two separate reservoirs of methicillin-resistant S. aureus (MRSA)—the first possibly from a maternal source and the latter from the community, Dr. Ana Krishnan and Dr. Karen Carpenter said in a poster presentation at a meeting of the Eastern Society for Pediatric Research.
The physicians conducted a 10-year retrospective review of MRSA among children less than age 12 months treated in a large Northern Virginia birthing center. The review identified 85 MRSA-positive cultures, which occurred with increasing frequency as the years progressed. Only two cases were identified in 1997, the first year of the study. Cases remained infrequent, between 1 and 8 per year, until 2004, when they doubled to 15. Infections have continued to rise each year since then, with 25 cases recorded by the end of 2006.
Most infections (64%) occurred in infants less than 5 months old, with 25% occurring in children less than 1 month old, said Dr. Krishnan, a pediatric resident at Inova Fairfax Hospital for Children, Falls Church, Va.
“In the first 30-60 days of life, there are more [hospital-acquired] than community-acquired MRSA infections,” she said in an interview. “This correlates with the greater number of babies in the neonatal intensive care unit. About 50% of the babies who had the infections in the first 60 days were in the [neonatal] ICU.”
The ratio of hospital-acquired (HA) and community-acquired (CA) infections began to change after the patients reached about 2 months of age, she said. “At this point, we found a shift toward more infections being acquired in the community. But this difference disappears toward the end of the first year.” In months 11 and 12, there are about equal numbers of community- and hospital-acquired infections.
She also noted that a second peak of infection occurred just before the first birthday, with the highest incidence (71%) during the fall and winter months. Additionally, most patients in this age group (73%) presented with a concurrent upper respiratory infection. “This apparent association needs further investigation,” Dr. Krishnan said at the meeting, which was cosponsored by Children's Hospital of Philadelphia.
CA-MRSA also was significantly more likely than HA-MRSA to cause pustulosis (28% vs. 13%) and abscesses requiring drainage (33% vs. 10%). Invasive infections were similar between the groups (22% and 26%), but among invasive infections, bacteremia was more common in HA-MRSA (18% vs. 7%) and nonbacteremic invasive infections more common in CA-MRSA (17% vs. 8%).
The two types of MRSA infections display distinct antibiotic susceptibilities, Dr. Krishnan said. Most CA-MRSA infections were susceptible to clindamycin (76%), with none of the tested cultures displaying inducible resistance. Only 37% of HA-MRSA cultures were sensitive to clindamycin, however, and 25% of those tested did display inducible resistance.
The researchers plan additional studies to explore the possible relationship between early-infancy infections and a maternal reservoir of MRSA, Dr. Krishnan said. “We plan a prospective study looking at mother-infant couplets. We want to culture mothers before delivery, identify those with MRSA, and follow the pairs for a year to assess MRSA colonization and disease.”
Most infections (64%) occurred in infants less than 5 months old, with 25% occurring in children less than 1 month old. DR. KRISHNAN