Decreased Susceptibility to Azithromycin Among Shigella Infections in New York City

BACKGROUND:   Shigellosis is the third most common enteric bacterial infection in the United States.  Risk groups include young children, men who have sex with men (MSM), and international travelers.  Shigellosis is generally self-limiting. When treatment is indicated, azithromycin may be recommended due to high rates of resistance to other antimicrobials.  Interpretive criteria for azithromycin susceptibility testing for Shigella are lacking.  In April 2013, the New York City Department of Health and Mental Hygiene (DOHMH) Public Health Laboratory (PHL) began testing Shigella isolates for azithromycin susceptibility. We examined the results to characterize the prevalence and epidemiology of decreased susceptibility.

METHODS:   Clinicians and laboratories are required to report laboratory-confirmed shigellosis to DOHMH and refer isolates to PHL for follow-up testing, including antimicrobial susceptibility testing.  Each isolate is tested for susceptibility to ampicillin, azithromycin, cefixime, ciprofloxacin, and trimethoprim-sulphamethoxazole (TMP) by E-test.  During April–December 2013, patients with decreased susceptibility to azithromycin (defined as minimum inhibitory concentration [MIC] >= 32 µg/mL) were interviewed to assess antecedent antibiotic use, international travel, sexual practices, and medical history. 

RESULTS:   Among 176 Shigella cases identified, 54% of isolates were resistant to ampicillin, 1% to cefixime, 6% to ciprofloxacin, and 79% to TMP. Isolates from 33 cases (19%) displayed decreased susceptibility to azithromycin. The median age of decreased susceptibility cases (n=33) was 36 years (range: 1-93), and 30 (91%) were male.  Of decreased susceptibility cases 12/33 (36%) identified as MSM, of whom six reported a positive HIV status and two reported antibiotic use prior to illness, though the antibiotic was not azithromycin.  Five cases (15%) were hospitalized, and no international travel was reported.  Two cases (6%) with decreased susceptibility to azithromycin also showed resistance to ciprofloxacin. Of those, one case was a man in his 30s whom we were unable to contact, and the other case was a woman in her 90s who reported frequent exposure to antibiotics to treat urinary tract infections.  Among 143 cases without elevated MICs to azithromycin, the median age was 25 years (range: <1-84 ); 99 (69%) were male.  A significantly higher proportion of patients with isolates displaying decreased susceptibility to azithromycin were male compared with patients with susceptible isolates (chi-square test: p<0.0001).

CONCLUSIONS:   Almost one in five Shigella infections in New York City have reduced susceptibility to azithromycin.  More study is needed to understand the implications for treatment and transmission and to elucidate why MSM are disproportionately infected.