Box 1.6 Cholera returns

In 1991, a cholera epidemic swept down the west coast of South America – the first such outbreak in nearly a century in the New World. Between 1991 and 1995, a disease long thought to have been vanquished in the Americas had infected more than 1 million people and killed 11,000 (1). Africa experienced a similar cholera surge in 1991, with the number of cases rising fourfold in a single year (from 38,683 in 1990 to 153,367 in 1991) and deaths mounting to 14,000 (2)(3). Three years later, the cholera epidemic hit Russia and cholera cases jumped from just 23 the year before to 1,048 (4)(5).

Why has cholera reemerged as a global health threat, after virtually disappearing from the Americas and most of Africa and Europe for more than a century? (6) The answer may lie in how changing environmental conditions – from both natural and human causes – can affect the spread of an infectious disease.

Cholera is generally spread by contact with water or food contaminated with human waste containing cholera bacteria. That is why the disease has long been associated with the unsanitary conditions often found in urban slums, or in connection with war, natural disasters, and other dislocations. But cholera also has a traditional link with the sea. In nature, the cholera organism (Vibrio cholerae) thrives best in moderately salty waters such as coastal estuaries, though it can also tolerate the open ocean. It generally only inhabits rivers and other freshwater sources if nutrient levels from organic pollution such as human feces are quite high (7).

These two environmental links – with the sea and with unsanitary conditions – do much to explain the pattern of cholera epidemics throughout history. Global epidemics (pandemics) of cholera often hit first in coastal cities and have clearly been associated with unhygienic conditions. Originally restricted to the Indian subcontinent, cholera spread from India to Europe between 1817 and 1823, launching the first global cholera pandemic (8). Since then, six more pandemics have washed, wavelike, across the continents, receding for a time between each pulse.

By the end of the 19th Century, cholera appeared to retreat as a global health threat. After 1900, it disappeared from the Americas and most of Europe; and by 1950 it was largely confined to the Indian subcontinent, where it had originated, and the Asian countries west of India (9). But in 1961, a new pulse of the disease – the seventh pandemic – began to spread from Asia, eventually emerging with a vengeance in 1991 in Latin America and Africa. Though this latest pandemic has peaked, the disease remains endemic throughout these regions (10).

Part of the blame for the dramatic rise in cholera cases in Latin America and Africa rests with obvious causes: deteriorating water and sanitation systems, poor living conditions, malnutrition, crowding, and political and economic turmoil. For example, studies of the 1991 cholera epidemic in Peru suggest that the lack of effective water treatment measures contributed to the rapid spread of the disease. Engineers of the public water supply system in the coastal city of Trujillo believed that no water treatment would be necessary. The fear of the carcinogenic risk associated with chlorine disinfection byproducts superceded the fear of cholera infection (11). And, testimony of health workers in Iquitos, a jungle city in Peru, suggests that even home-based chlorination could have been key in arresting further dissemination of the disease (12). Rapid population growth and lack of investment in public services had led to serious declines in Lima’s sanitation coverage, and between 5 million and 6 million city residents had no access to acceptable latrines (13). In Africa, civil strife and drought in the 1980s had led to unusually large migrations and concentrations of people in urban slums and refugee camps (14). About 50,000 Rwandan refugees contracted cholera in such camps after the 1991 outbreak, and many thousands died (15).

References and notes

1. Integrated Management of Childhood Illness Communicable Disease Program, Division of Disease Prevention and Control, Pan American Health Organization (PAHO)/World Health Organization (WHO), “Cholera Situation in the Americas,” Update Number 14, (PAHO/WHO, Washington, D.C., April 1996), p. 1.

2. World Health Organization (WHO), unpublished data (WHO, Geneva, 1996).

3. World Health Organization (WHO), Weekly Epidemiological Record, Vol. 67, No. 34 (August 21, 1992), p. 258.

4. Op. cit. 2.

5.Op. cit. 2.

6. Robert Tauxe et al., “The Future of Cholera: Persistence, Change, and an Expanding Research Agenda,” in Vibrio cholerae and Cholera: Molecular to Global Perspectives, I. Kaye Wachsmuth, Paul A. Blake, and Orjan Olsvik, eds. (American Society for Microbiology, Washington, D.C., 1994), p. 443.

7. Rita R. Colwell, “Global Climate and Infectious Disease: The Cholera Paradigm,” Science, Vol. 274 (December 20, 1996), p. 2027.

8.Op. cit. 6.

10.Op.cit.7, pp. 2027