Exponential increase in community-acquired methicillin-resistant Staphylococcus aureus infections in South Texas children.

Abstract

To The Editors: We recently reported in this journal on our experience with the emergence of community-acquired methicillin-resistant Staphylococcus aureus (CAMRSA) infections at Driscoll Children’s Hospital in Corpus Christi, TX, between October 1, 1990 and December 31, 2000. 1 We have updated our data and added the methicillin-resistant S. aureus (MRSA) cases from 2001 (Fig. 1). The annual rate of MRSA isolation, which gradually rose from 2.9% of all S. aureus isolates in 1990 to 10.6% in 1999 and jumped to 19.0% in 2000, increased substantially to 40.3% in 2001. During this 11-year period, the annual number of nosocomial MRSA cases was relatively constant, ranging from 4 to 12. The exponential increase in MRSA isolates is accounted for by an explosion of CAMRSA infections.Fig. 1: MRSA cases from 1991 to 2001 at Driscoll Children’s Hospital.In 2001 we observed 97 cases of CAMRSA (86 without identified risk factors), which was more than the total number of cases seen from 1990 through 2000 (60; 53 without identified risk factors). This represented a 2.8-fold increase in the number of CAMRSA cases compared with those in 2000 (35; 33 without known risk factors) and a 13.9-fold increase compared with cases in 1999 (7; 6 without known risk factors). Of the CAMRSA isolates in 2001 from children without known risk factors, 100% were susceptible to trimethoprim-sulfamethoxazole and rifampin, 99% to gentamicin, 94% to clindamycin, 69% to tetracycline, 67% to levofloxacin and 11% to erythromycin. Of the CAMRSA isolates from children with identified risk factors, 100% were susceptible to trimethoprim-sulfamethoxazole, rifampin and gentamicin; 91% to tetracycline; 55% to clindamycin (P = 0.001); 18% to levofloxacin (P = 0.002); and 9% to erythromycin. Of the nosocomial MRSA isolates 100% were susceptible to trimethoprim-sulfamethoxazole, rifampin and gentamicin; 57% to tetracycline; 43% to clindamycin (P = 0.001); 29% to levofloxacin; and 0% to erythromycin. We continue to see significant differences between the antibiotic susceptibility pattern for isolates obtained from children without an identified risk factor with community-acquired infections compared with the patterns for isolates obtained from children with an identified risk factor with community-acquired infections and from children with nosocomial infections. Because of this finding the increased number of CAMRSA infections in the children with no known risk factors is difficult to explain by simply saying that a history of contact with the health care system (e.g. through a relative with chronic disease or a family friend who works in a health care facility) was overlooked. However, the cases of CAMRSA infections in children with a known risk factor are likely to be an expression of nosocomial acquisition because of the similar antibiotic susceptibility patterns. Also if the increase in the number of CAMRSA infections was only a reflection of transmission from the health care setting, we would have expected to see an initial increase in nosocomial MRSA infections, which at least in our hospital has not been the case. New strains of MRSA arising in the community as a result of increased selection pressure from antimicrobial usage may be a more plausible reason for the exponential increase in CAMRSA infections in South Texas children. Kevin Purcell, M.D. Pharm. D. R. Ph. Jaime E. Fergie, M.D.