Study Suggests How Cholera Returned to Latin America After 100-year Hiatus

After a century of absence, Vibrio cholerae -- the microbe responsible for cholera -- invaded the Western Hemisphere by way of Peru in late January 1991. Although a number of theories about the origin of the epidemic have been proposed, ten years later the cause of the disease's return is still debated. Now a study by scientists at the Johns Hopkins School of Public Health, the University of Maryland, and their colleagues in Peru suggests that V. cholerae was present in the coastal waters of Peru for at least four months before the recognized onset of the epidemic. The study appeared in the April 2000 issue of the American Journal of Tropical Medicine and Hygiene (published in February 2001).

While many aspects of cholera are well known, such as its routes of transmission, diagnosis, pathophysiology, and treatment, how cholera arises in continents where the disease has previously been unreported, and where the reservoir of cholera microbes resides between epidemics, remains poorly understood.

Between February 1998 and February 1999, the investigators visited the emergency departments of hospitals in seven cities along the northern Pacific coast of Peru -- all were hospitals that provide medical care to people of low socioeconomic status and that treated the majority of cholera patients in the region during the 1991 epidemic. The scientists reviewed the medical records of all persons who had entered those hospitals during the 17 months leading up to the epidemic and found a total of 3,640 medical charts of patients with diarrhea.

From these, they identified seven patients, each admitted during the four months preceding the cholera outbreak, whose symptoms fit the World Health Organization's clinical definition of cholera: acute, watery diarrhea and evidence of severe dehydration (shock, generalized cramps, and signs of severe fluid loss) in patients older than five years of age. The investigators found no such cases during the whole of the previous year.

According to the authors, this finding -- that scattered cholera patients had been appearing at hospitals in at least five coastal cities along a 1,000-kilometer coastline for some months before the epidemic hit -- contradicts a prevalent theory that the Peruvian outbreak was due to contamination of sea water by excreta from cholera patients arriving on a ship from Asia.

"One could conclude that the pathogens were imported from an endemic area, possibly from Asia or Africa," said senior author R. Bradley Sack, MD, ScD, professor, International Health, Johns Hopkins School of Public Health. "But this begs the question: How could cholera have occurred in several places at the same time along the northern Pacific coast of Peru?" A more likely hypothesis, based on studies done by the researchers at the University of Maryland, is that the cholera microbe, which occurs naturally in the aquatic environment and is known to be associated with marine plankton, was carried by zooplankton to multiple coastal locations before the epidemic. Moreover, the 1991 El Niño would have raised seawater temperatures, a factor that would have encouraged plankton growth until the microbe population reached concentrations infectious to humans.

The authors speculate that future cholera epidemics may be averted through an early warning system that uses marine biology, satellite imagery, and clinical surveillance, thus permitting public health measures to be swiftly and fully implemented.

Support for this study was provided by a STAR grant from the U.S. Environmental Protection Agency Office of Research and Development.