Epidemic cholera in the new world: translating field epidemiology into new prevention strategies.

Cholera, a devastating diarrheal disease, has swept through the world in recurrent pandemics since 1817. The seventh and ongoing pandemic began in 1961 when the El Tor biotype of Vibrio cholerae O1 emerged in Indonesia. This pandemic spread through Asia and Africa and finally reached Latin America early in 1991 (1). After explosive epidemics in coastal Peru, it spread rapidly and continues throughout Latin America (Figure 1). Because of underreporting, the more than 1,000,000 cholera cases and 10,000 deaths reported from Latin America through 1994 (Table 1) (2) represent only a small fraction of the actual number of infections. Molecular characterization of V. cholerae O1 strains from Peru has shown that they do not match strains from anywhere else in the world; therefore, the source of the Peruvian epidemic strains remains unknown (3). Moreover, other strains have since appeared in Latin America. At least one of these, a strain resistant to multiple antimicrobial drugs, was first identified in Mexico and elsewhere in the world in mid-1991 and has since spread widely throughout Central America (4). The introduction of strains into new areas illustrates the rapid global transfer of pathogens. V. cholerae O139 Bengal, which emerged as a new cause of epidemic cholera in Asia in 1992, could also appear in Latin America (5). Such introductions are not easy to prevent, because they may follow the arrival of travelers who are not aware of their infection or of ships carrying contaminated ballast water. The key to controlling epidemic cholera lies in limiting its spread by using measures that prevent sustained transmission. One measure might be using an inexpensive and effective vaccine to provide lasting protection; however, no such vaccine yet exists, although progress in vaccine development is being made (6-8). Another measure is interrupting transmission so that the causative organism never reaches the human host. This approach to prevention successfully controlled many epidemic diseases in the industrialized world, including cholera, typhoid fever, plague, and malaria, before vaccines or antibiotics were developed. Over the last century, a large engineering infrastructure, built in industrialized nations, has provided safe water and sewage treatment for nearly all people in these nations and has made sustained transmission of cholera in those countries extremely unlikely. Despite sporadic cases along the U.S. Gulf Coast and repeated introduction of the epidemic organisms by travelers, epidemic cholera has not occurred in the United States since the nineteenth century (9,10). To prevent cholera by interrupting transmission of the organism to the host, it is important to understand precisely how the bacteria are transmitted. John Snow demonstrated waterborne transmission of cholera during a large epidemic in London in 1856 (11). He and many others since have suspected that other routes of transmission are also important. Epidemiologic investigations during the seventh pandemic have documented a variety of specific food and water pathways by which the bacteria reach the host, some of which were new and unsuspected (12). The El Tor biotype of V. cholerae O1, for example, multiplies rapidly in moist foods of neutral acidity (13). This bacterium also persists in the estuarine environment in niches that are poorly understood but may involve the plankton on which shellfish feed. This means