MRSA Strains Muscle In on Kids

HOUSTON, Sept. 27 -- Children with acute bacterial infections of skeletal muscle caused by methicillin-resistant Staphylococcus aureus (MRSA) are showing up with increasing frequency in pediatric clinics and hospitals, researchers here reported.

Over the past six years Houston-area clinicians have seen a significant rise in pyomyositis and myositis cases in children caused by community-acquired MRSA, reported pediatrician Pia S. Pannaraj, M.D., of Baylor College of Medicine, and colleagues.

More than 75% of community-acquired staph infections seen at Texas Children's Hospital in Houston are caused by MRSA, they wrote in the Oct. 15 issue of Clinical Infectious Diseases.

The investigators also found evidence that compared with methicillin-susceptible strains, MRSA strains in general cause more serious disease, as do USA300 clones, and those carrying the Panton-Valentine leukocidin (pvl) gene.

Prompted by an increase in the frequency of bacterial infections of muscle they were seeing in children -- from four per year in 2000 to 12 per years in 2005 -- the investigators reviewed the medical records of all patients with pyomyositis and myositis hospitalized at Texas Children's over the six years.

They also obtained S. aureus isolates and tested them for the presence of a gene encoding for pvl, a staphylococcal virulence factor that has been associated with severe pneumonia, and serious infections of skin, soft-tissues, bones, and joints.

They identified and analyzed cases of bacterial myositis or pyomyositis (i.e., myositis with abscess). They found that in than 57.8% of the cases, the cause of infection was S. aureus, with Streptococcus pyogenes occurring in 2.2%. Cultures from the remaining 40% of children came back negative.

The authors also saw an increase in the number of cases from 2000 to 2005, corresponding with an increase in the prevalence of community-acquired MRSA.

Then children ranged in age from a little over eight months to 15 years (mean 5.5 years). Infection sites include the thigh in 40% of patients, and the pelvis in 28.9%. Other infection sites included the back, in 17.8%, distal lower extremity in 13.3%, neck and shoulder (6.7% each), and upper extremity (2.2%). One-fifth of patients had infections in more than one location.

The mean diameter of abscesses was 3.5 cm. In all 18 of the 45 children required at least one muscle-drainage procedure.

When they looked at the 24 available S. aureus isolates, 15 community-acquired MRSA isolates and nine community-acquired, methicillin-susceptible isolates, the authors found that 16 were the USA300 strain, and 17 were positive for pvl.

"Patients with community-acquired MRSA, USA300, and/or pvl-positive strains required more drainage procedures than did those with community-acquired methicillin-susceptible S. aureus, non-USA300, and/or pvl-negative strains, the authors wrote.

Specifically, they found that 81% of patients with community-acquired MRSA infections required muscle drainage, compared with 40% of those with susceptible strains (P=0.05). Similarly, 82% of those with the USA300 strain of MRSA required drainage, compared with 29% of those with other strains (P=0.02). Also, 81% of patients with infections positive for pvl needed muscle drainage procedures compared with 38% of those with pvl-negative infections, but this difference was not statistically significant (P=0.07).

"Empirical antimicrobial therapy for pyomyositis and myositis will depend upon local epidemiologic and susceptibility patterns," the investigators wrote. "However, as seen in our study, an increase in the prevalence of community-acquired MRSA is contributing to an increase in frequency of cases. Thus, one should strongly consider coverage for MRSA in the empirical treatment regimen, which, in our area, currently consists primarily of vancomycin or clindamycin."

The study was funded by a grant from Pfizer which markets clindamycin.