Zanzibar shares global burden of cholera epidemic suffering annual outbreaks with case fatality rates (CFR) of 1.8%. This study aimed at determining the transmission chain of the outbreak through marine fish by molecular characterization of Vibrio cholerae isolates.

Cross sectional design was used to collect samples from fish, sewage sites and stool from clinical cases.

The study was carried out in Zanzibar municipality between November 2015 and May 2016.

Epidemiological data on the outbreak was provided by the Ministry of Health and Social Affairs, Zanzibar. Sixty samples from fish intestines, 23 samples from sewage drains and 38 human stools were investigated. Vibrio cholerae was identified biochemically and serology was done using Polyvalent, Inaba, Ogawa and O139 antisera. Kirby-Bauer disc diffusion was used for antibiotic sensitivity against seven antibiotics. DNA was extracted and PCR performed using ctxB forward and reverse ctxB primers. Gene sequences were interpreted by Mega 7.0 software.

Thirty stool samples (79%), 9 fish samples (15%) and 6 sewage samples (26%) were positive for Vibrio cholerae. All isolates were identified: serogroup O1, biotype El Tor and serotype Ogawa. Age category 16-30 yrs old had highest number of cases (37.6%). Case fatality rate (CFR) was 1.8%, more mortalities were in males and >5yrs old. None of the isolates was resistant to ciprofloxacin. High resistance was against nalidixic acid, erythromycin, co-trimaxozole, ampicillin and tetracycline. Multidrug resistance was observed in 40% of isolates. CtxB gene sequencing revealed that the current Vibrio cholerae strain was homologous to Haiti 2013-2015 and China 2016 strains, but distinct from Zanzibar 2013 strain.

Fish could be the source of Vibrio cholerae transmission in Zanzibar. Presence of rapidly emerging endemic reservoir of Vibrio cholerae in Zanzibar environment was suspected favouring horizontal gene transfer with resultant novel strains. High drug resistance and multidrug resistance are matters of public health concern.