Vibrio cholerae O1 Hybrid El Tor Strains, Asia and Africa

Abstract

Vibrio cholerae is a water-borne pathogen that causes a severe watery diarrhea disease known as cholera. On the basis of variable somatic O antigen composition, >200 serogroups of V. cholerae have been recognized. Classical and El Tor are 2 well-established biotypes within the V. cholerae O1 serogroup, and they can be distinguished by differences in their biochemical reactions or phenotypic traits (1). In addition to phenotypic traits, genetic markers have recently been used in the identification of the biotypes of V. cholerae. For example, the major toxin-coregulated pilus (TCP) gene, tcpA, of the TCP cluster possesses classical- and El Tor–specific alleles that encode identical functions but differ in their DNA sequence composition; however, the rtxC gene of the repeat in toxin (RTX) cluster is present in El Tor strains only and absent in classical strains (2,3). The cholera toxin, encoded by the ctxA and ctxB genes, is the principal toxin produced by V. cholerae O1 and O139 and is responsible for the disease cholera. Heterogeneity within the ctxB gene and protein was first reported in the early 1990s, and, on this basis, 3 ctxB genotypes of the V. cholerae O1 strains have been identified. Based on amino acid residue substitutions at positions 39, 46, and 68, all classical and US Gulf Coast El Tor strains have been categorized as genotype 1, the Australian El Tor strains as genotype 2, and the El Tor strains of the seventh pandemic and the Latin American epidemic as genotype 3. Genotyping of ctxB has indicated that the classical strains harbor a unique cholera toxin gene that is not in the El Tor strains except for the US Gulf Coast El Tor clone (4). The US Gulf Coast hybrid El Tor strains that harbor the classical cholera toxin have been associated with sporadic outbreaks in the United States (5) and, until recently, had not been reported anywhere else in the world. Then in 2004, hybrid El Tor strains that encode the classical cholera toxin were isolated from cholera patients in Matlab, Bangladesh (6), and in Beira, Mozambique (7). In 2006, Nair et al. reported that the current seventh pandemic prototype El Tor strains had been replaced by hybrid El Tor strains in Bangladesh (8). We now report how far the hybrid El Tor strains have spread in Asia and Africa.