Emergence of highly trimethoprim-sulfamethoxazole-resistant Shigella in a native American population: an epidemiologic study.

Resistance to trimethoprim-sulfamethoxazole (TMP-SMX) emerged among Shigella isolates from the Navajo Reservation in the southwestern United States in 1985, years after this antimicrobial agent came into common use. In the study area, TMP-SMX resistance increased dramatically from 3 per cent in 1983 to 21 per cent in 1985. Resistance was polyclonal and occurred in both S. sonnei and S. flexneri. No single plasmid was common to all resistant strains. However, all 28 TMP-SMX resistant isolates examined were resistant to ampicillin and streptomycin and had minimum inhibitory concentrations to sulfamethoxazole of greater than or equal to 4,096 micrograms/ml and to trimethoprim of greater than or equal to 1,024 micrograms/ml. The authors found that 28 of 101 Navajo children with gastrointestinal symptoms who were not taking antimicrobials had TMP-SMX-resistant aerobic fecal flora. To determine risk factors for acquiring resistant strains, they compared 40 case-patients with TMP-SMX-resistant Shigella to 66 controls with TMP-SMX-sensitive Shigella. Case-patients were more likely than controls to have used antimicrobials recently (p = 0.004) and to be hospitalized for shigellosis (p = 0.05). These findings suggest that polyclonal highly TMP-SMX-resistant Shigella emerged by transfer of trimethoprim resistance genes from aerobic bowel flora to endemic Shigella strains, that use of antimicrobials can lead to symptomatic shigellosis and thus the persistence of trimethoprim-resistant Shigella, and that appropriate therapy of shigellosis on the reservation is now a major challenge.