Other factors played a role as well in cholera’s resurgence. Some cholera cases in Latin America were traced to the growing use of wastewater to irrigate crops near urban areas (16). In addition, food-handling practices, especially by street vendors, may have added to the global outbreak. Street vendors are a central feature of poor urban communities throughout the developing world, but their lack of refrigeration and clean water often increases the risk of contamination (17). In Latin America, uncooked seafood, such as ceviche, was also an important route of cholera transmission because seafood is often caught or processed in unhygienic conditions (18).
But these factors don’t fully explain how cholera reemerged in Latin America so suddenly after more than a century’s absence. Moreover, the 1991 pandemic struck nearly simultaneously over a wide area, appearing in ports from the Chilean to the Ecuadorean border within a few weeks. What event or series of events had so effectively reintroduced cholera over huge stretches of open coastline?
One possibility is that the cholera organism was carried by ship from Asian to Latin American ports in ballast water – a well-known vehicle for transporting foreign organisms, ranging from microscopic bacteria and viruses to mollusks and small crabs (19). DNA testing of the Latin American cholera strain shows that it is genetically similar – although not identical – to a cholera strain common in Bangladesh (20), and this strain has been isolated in samples of ballast, bilge, and sewage from cargo ships active in the area (21). Still, the speed with which the epidemic spread to points so widely dispersed casts some doubt on whether shipping traffic alone can explain the disease’s reemergence (22).
A second theory is that cholera never really disappeared from the Americas at all, but merely went into a dormant, noninfective state in coastal waters, from which it reemerged when the right combination of favorable environmental conditions appeared. Evidence for this theory of ocean waters as a reservoir of cholera comes from the recent discovery that some species of plankton can act as hosts for the dormant cholera organism, allowing the organism to persist in coastal waters for long periods, and then “reappear” after years of seeming absence (23). This theory also helps explain why cholera epidemics in Bangladesh occur seasonally, often coinciding with plankton blooms in the Bay of Bengal.
The cholera-plankton connection probably also offers the disease a means of long-distance travel, hitchhiking with the plankton on ocean currents across thousands of kilometers and over periods of months and years. This might explain how an Asian cholera strain could find its way to several points along the coast of South America without stowing away in ballast water.