Evolution of Seventh Cholera Pandemic | CDC EID

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Volume 16, Number 7–July 2010
Dispatch
Evolution of Seventh Cholera Pandemic and Origin of 1991 Epidemic, Latin America
Connie Lam, Sophie Octavia, Peter Reeves, Lei Wang, and Ruiting Lan
Author affiliations: University of New South Wales, Sydney, New South Wales, Australia (C. Lam, S. Octavia, R. Lan); University of Sydney, Sydney (P. Reeves); and Nankai University, Tianjin, People's Republic of China (L. Wang)

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Abstract
Thirty single-nucleotide polymorphisms were used to track the spread of the seventh pandemic caused by Vibrio cholerae. Isolates from the 1991 epidemic in Latin America shared a profile with 1970s isolates from Africa, suggesting a possible origin in Africa. Data also showed that the observed genotypes spread easily and widely.
The seventh cholera pandemic began in 1961, and by 1966, it had affected most of Asia. Cholera incidence then decreased slightly until 1971, when an upsurge was observed in Africa and Europe, which had been free of cholera for >100 years (1). Cholera rates remained relatively low during the 1980s, with the disease confined to Asia and Africa. However, 2 major cholera outbreaks appeared in the 1990s: first, a resurgence of cholera in Africa, and, second, outbreaks that started in Peru became the first cholera epidemic in Latin America since 1895 (2). In addition, a novel serotype caused major outbreaks on the Indian subcontinent in 1992. That strain was referred to as O139 Bengal and was later shown to be a variant of the seventh pandemic clone with its replacement of the O antigen (1). Pulsed-field gel electrophoresis (3), amplified fragment length polymorphism analysis (4), and ribotyping (1) have been applied to seventh pandemic isolates but did not fully resolve the relationships of the various outbreaks. In this study, we used genome-wide single-nucleotide polymorphisms (SNPs) to track the evolution and spread of the seventh cholera pandemic, including the O139 Bengal strain.

The Study
The availability of complete genome sequences of a pre–seventh pandemic isolate, M66–2 (5), a seventh pandemic isolate, N16961 (6), and the partial genome sequence of an O139 Bengal isolate, MO10 (7), enabled identification and use of SNPs as evolutionary markers in Vibrio cholerae. A set of 18 SNPs was chosen from 125 N16961 SNPs (5) and 12 SNPs selected from 59 identified by comparison of the N16961 and MO10 genome sequences. The SNPs selected were mostly from genes with known function and were distributed throughout the 2 chromosomes for the N16961 SNPs and the large chromosome for the MO10 SNPs. We have previously shown that recombinant regions could be identified by the differences in distribution of SNPs in such regions (5); for this study, only mutational SNPs were selected.

The 30 SNPs (Technical Appendix [ 112 KB, 5 pages]) were used to type a collection of 64 seventh pandemic V. cholerae isolates. SNPs were detected by using hairpin primer real-time PCR. SNP data for 3 complete V. cholerae genomes (M66–2, N16961, MJ-1236) and 4 partially sequenced genomes (MO10, RC9, B33, CIRS 101) (7) were obtained from the National Center for Biotechnology Information (Rockville, MD, USA) and included in the analysis. The 71 isolates were divided into 10 SNP profiles by using the 30 SNPs (Appendix Table). Three profiles were represented by 1 isolate only, whereas the remaining profiles contained 4–17 isolates. The Simpson index of diversity for all SNPs combined was 0.929.

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Suggested Citation for this Article
Lam C, Octavia S, Reeves P, Wang L, Lan R. Evolution of seventh cholera pandemic and origin of 1991 epidemic, Latin America. Emerg Infect Dis [serial on the Internet]. 2010 Jul [date cited]. http://www.cdc.gov/EID/content/16/7/1130.htm

DOI: 10.3201/eid1607.100131