Susceptibility to Fluoroquinolones of Vibrio cholerae O1 Isolated From Diarrheal Patients in Zimbabwe FREE

To the Editor: Cholera is endemic in many countries in Asia, Latin America, and Africa. Sub-Saharan African countries, particularly Angola, Congo, Mozambique, and Zimbabwe, have been strongly affected by cholera in recent years, with a large epidemic occurring in Zimbabwe in 2008. From August 2008 until February 2009, 70 640 patients were reported with cholera in Zimbabwe, of whom 3467 died.¹ On request from the World Health Organization, a team from the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDRB), went to Zimbabwe to assess the cholera situation and provide assistance to control the outbreak. Preliminary data on the antibiotic resistance of Vibrio cholerae O1 strains that were isolated from the patients in Zimbabwe are reported.

Stool and rectal swab specimens were collected from randomly selected patients at 4 cholera treatment centers in Zimbabwe, including Beatrice Road Infectious Diseases Hospital and Budiriro in Harare province, Chinhoyi in Mashonaland West, and Binga in Matebeleland North provinces. V cholerae O1 was isolated and identified following standard procedures.² The presence of virulence-associated genes (including genes encoding cholera toxin) and the major colonization factor (toxin coregulated pilus) were tested by specific polymerase chain reaction assays.² The strains were also subjected to ribotyping to assess clonal relationship.³ The minimum inhibitory concentrations (MICs) of ciprofloxacin, tetracycline, and azithromycin were determined using Etest (AB bioMérieux, Solna, Sweden) following standard procedures. The measured MICs were rounded to the nearest higher 2-fold dilution. The strains were also tested for sensitivity using the disk diffusion method. Protocols were approved by the Ministry of Health and Child Welfare and the World Health Organization, Harare, Zimbabwe, and patients provided oral consent for participation.

A total of 64 stool and rectal swab samples were collected from January 16 to January 27, 2009. V cholerae O1 was isolated in 38 samples (59%). All isolates belonged to the El Tor biotype; 31 and 7 isolates belonged to Ogawa and Inaba serotypes, respectively. Of the 38 isolates, 31 strains were available for further testing. Two distinct ribotype patterns (A and B) were observed among the isolates.

All strains were susceptible to azithromycin and tetracycline, with MIC₉₀ (MIC required to inhibit the growth of 90% of organisms) of 0.125 mg/L and 0.5 mg/L, respectively. The MIC₉₀ of ciprofloxacin was 0.5 mg/L, with a distinction between the ciprofloxacin susceptibility of ribotype A and ribotype B. The ciprofloxacin MIC of ribotype A ranged from 0.125 to 0.5 mg/L. The ciprofloxacin MIC of type B was 0.004 mg/L (Figure). Thus, the ciprofloxacin MICs of clonal type A were 30 to 125 times higher than those of clonal type B and within 1 to 3 two-fold dilutions of the susceptibility breakpoint of 1 mg/L. The strains of clone B were susceptible to nalidixic acid by disk diffusion, but clone A strains were resistant. All strains were resistant to furazolidone and trimethoprim-sulfamethoxazole.

To our knowledge, this is the first description of an outbreak of cholera with reduced susceptibility against fluoroquinolones on the African continent. Nalidixic acid resistance has been reported in Ghana and Mozambique, but no quantitative susceptibility tests or MICs were performed and no resistance to fluoroquinolones was detected.⁴

Ciprofloxacin resistance may be associated with progressive multiple mutations in the gyrA gene⁵; nalidixic acid resistance may therefore have facilitated the emergence of resistance to the fluoroquinolones. Reduced susceptibility of V cholerae against ciprofloxacin (MIC, 0.25 mg/L) has been reported previously in patients hospitalized in Bangladesh and was associated with therapy failure in these patients.⁶ The reported reduced susceptibility is therefore reason for concern.

Antimicrobial susceptibility is often not determined in cholera, and qualitative antimicrobial susceptibility tests (eg, disk diffusion) may fail to detect strains with reduced susceptibility. Longitudinal surveillance of antimicrobial susceptibility of V cholerae O1 using quantitative (MIC) methods is therefore recommended for early detection of emergence of resistance.