Introduction to Geographic Information Systems

Lab 7: Medical Geography and Cholera in Perú

The materials in this exercise have been modified from the originals prepared by Dr. Sonia Arbona and Shannon Crum, The Geographer's Craft Project (http://www.Colorado.EDU/geography/gcraft/warmup/cholera/cholera_f.html). All commercial rights reserved. Copyright 1996 Sonia Arbona.

The Research Problem

Early in 1991 a number of cases of cholera were reported in coastal areas of Perú. Within a few weeks, the disease spread throughout Perú and into other countries in South, Central, and North America. The goal of this project is to map the spread of cholera through Perú during the January 1991 epidemic. We know that cholera is a disease associated with poverty and that, given adequate sanitation and access to medical care, the disease is not a major health threat.

Introduction to Medical Geography

Medical geography uses the concepts and techniques of the discipline of geography to investigate health-related topics. One of the predominant concepts of geography as a science is that it examines relationships between peoples and their environments in holistic terms. In the particular subfield of medical geography the focus is upon those interactions that bear specifically upon human health within a variety of cultural systems and a diverse biosphere. Regarding geographic method, the main tool is spatial analysis. Where disease occurs is a matter of importance. The map on the right illustrates the spread of an influenza epidemic in Glasgow. Comparison of disease rates in different places may provide clues to causation or serve as a starting point for further investigation. Maps showing distributional patterns of health-related phenomena with their accompanying associative analyses bring insight and strategy for better public health. A famous example of an old partnership between geography and epidemiology is the work of Dr. John Snow in the middle of the 19th century, who backed up his hunch that cholera was spread by infected water supplies by making a map of victims in a part of 19th century London (shown at right). This was during the days when medical opinion ascribed the disease to 'miasmas' and other emanations from the swamps and mud of the Thames River. His map showed a cluster of cases centered on a public water pump. This spatial association was used to eliminate the source of infection. The causative agent of cholera, cholera Vibrio, was not identified until several decades later by Robert Koch. One of the most exciting things about new developments in geography is that they have produced cooperation with people working in other fields. The medical person brings the skills for prevention, diagnosis and treatment, but an understanding of why and how a disease spreads rests at the core of modern geography with its concern for spatial structure. There are several books on medical geography. A basic reference from which some of the ideas above have been derived is Medical Geography by Melinda Meade, John Florin and Wilbert Gesler. The book was published in 1988 by The Guilford Press, New York. For a more detailed overview of the connections between geography and epidemic diseases, see the following site: Spatial and Temporal Analysis of Epidemiological Data

Health and Disease Statistics

The International Classification of Diseases (ICD) was developed to ensure that when a particular disease is referred in a statistical source all the different countries in the international community are describing the same phenomena . The global surveillance of disease is articulated by the World Health Organization (WHO) through its regional offices. However, international statistics are dependent for completeness upon the quality and coverage of the national records. Geographers need to be aware of the problems associated with disease reported rates which might be unavoidable.

Cholera

The cholera bacteria produces a toxin that inhibits the absorption of liquids by the body. It kills because it dehydrates the body. Mortality resulting from severe dehydration may be above 50% if the symptoms are untreated. Oral rehydration treatment (ORT) and rapid intravenous rehydration therapy (RIT) avoid deaths in most cases. Chemoprofilaxis and vaccination are not recommended. Mass treatment with antibiotics may lead to emergence of drug resistance and vaccination has been effective in only 50% of those who receive it and only for a few months. In addition, being vaccinated may give a false sense of protection. The key to effective control is environmental sanitation.

During the 19th century six cholera pandemics (epidemics of global proportions) originated in the Indian subcontinent. Cholera reached the Americas during these pandemics. The disease reached epidemic proportions in the United States three times during the last century. These epidemics have been studied by geographer Gerald Pyle. He pointed out that when the three epidemics are compared, differences in spread are related to changes in transportation and urban environments. Cholera disappeared from the industrialized world in this century.

Cholera Vibrio 01 occurs as two biotypes; they are the classical and El Tor biotypes. Each biotype also occurs as two serotypes, Ogawa and Inaba. El Tor biotype was first identified in 1960 among pilgrims to Mecca who fell ill in the El Tor quarantine station on the Saudi Arabian coast of the Red Sea. El Tor biotype has a lower case fatality rate and produces more asymptomatic carriers than the classic biotype (one in eight cases is severe with El Tor and one in two with the classic type). The relatively milder El Tor replaced the more severe classic type throughout the world in the 1960s. The 19th century pandemics were caused by the classic cholera biotype. This 20th century pandemic is thus, the seventh and the first one caused by El Tor biotype. The displacement of the classical type by El Tor has been explained in terms of El Tor's greater duration of excretion from infected hosts and its greater ability to survive on surfaces and in night soil. However, other considerations are being explored. An endemic focus of an unique western hemisphere strain of biotype El Tor, serotype Inaba exists along the coast of Louisiana, Texas, and possibly northern Mexico. Cholera infections with the classical biotype still occur on the Indian subcontinent.

For more information on cholera, examine the following items:

• WHO Cholera Fact Sheet

Cholera in Perú

Officials say that Perú's poor water supply and overcrowding of the shanty towns that surround the coastal cities helped to spread the disease. The rapid population growth in Lima and other coastal cities of Perú during recent decades exceeded the infrastructure available to deal with fecal contamination of water supplies. Chlorination is not kept at proper levels and the water pressure is not maintained for twenty-four hours a day, so waste water can flow into pipes that are cracked. Where population growth has been rapid water supplies have become overstreched.

The coastal area of Perú has desert conditions, but rivers that flow down from the Andes mountains are used for irrigation and create oases where the combination of stagnant waters and high population densities are favorable for the spread of a waterborne disease. A traditional practice in the dry coastal plain is to use sewage to fertilize fields when water is scarce. The farmers need to grow the kind of crops that have high cash yields and short growing seasons, and these are often vegetables that are eaten raw. The large fishing industry of Perú was also blamed. A local dish called seviche, which is raw fish and is a national favorite was particularly denounced. However, further studies indicated no correlation. The fish is marinated in lemon and other acidic substances that kill the vibrio.

Cholera is an infection associated with poverty. It is also important to note that blood group O is detected in up to 75% of the Peruvian population, typical of Latin American populations with a high proportion of indigenous individuals. This fact represents another factor in explaining the epidemic proportions reached in this and other Latin American countries.

Cholera was detected in the Peruvian population near the beginning of 1991, brought to the country perhaps from the bilge water of a Chinese freighter. Another interesting observation, and a possible explanation for the appearance of cholera in the South American country, is that the sea current El Niño is a hot current coming from the north to the south along the South American coast in the Pacific Ocean. This phenomenon takes place at the end of December and the beginning of January. In 1991, this current produced a higher temperature than usual in that part of the Pacific Ocean. According to Rita Colwell at the University of Maryland, when a vibrio is submerged in cold water it can shrink to be 300 times smaller than its usual size. Zooplankton that inhabit cold waters may carry large number of cholera vibrios on their bodies. Zooplankton feed by grazing on phytoplankton which bloom with sunshine and warm conditions. Thus, a phytoplankton bloom leads to an increase in the population of zooplankton which carry the vibrios. A warmer than usual El Niño may have created the right kind of conditions along the Peruvian shores for human to become infected with cholera. Zooplankton live in the water of ponds and rivers that people may drink. Also, fish and shellfish eat zooplankton and raw fish. Shellfish consumption also leads to cholera if it is contaminated with the bacteria.

From Perú the disease quickly spread to other Latin American countries. In general Latin America did remarkably well with 1% case fatality rate (the proportion of those that died among the infected). Public consciousness and prompt rehydration treatment made the difference. Diffusion of the disease, however, has continued in Central and South America. Recently, the Pan American Health Organization (PAHO) reported that:

Now, as the epidemic enters its eighth year in the hemisphere, cholera has become established in a number of countries, serving as a stark reminder of deficiencies in water quality, sanitation, and hygiene that will continue to challenge governments and health agencies into the next century.

For more information on cholera in Perú and Latin America, see the following sites:

• Cholera Situation in the Americas (PAHO)
• Country Health Profile: Peru (PAHO)

For background information on Peru and other socio-economic statistics, see the following:

• PERU - A Country Study
• PERU - CIA World Factbook

Map Gallery

The following maps of the cholera epidemic are from the PAHO:

• The Geographic Spread of Cholera in Latin America, 1991-1995
• The Year of Peak Incidence of Cholera in Latin America, by country, 1991-1995
• Cholera Cumulative Incidence Rates, by country, 1991-1995

Statistics of the Epidemic

Further Reading

Mata, Leonardo. "Cholera El Tor in Latin America, 1991-1993" in Disease in Evolution, Global Changes and Emergence of Infectious Diseases, Mary E. Wilson, Richard Levins, Andrew Spielman (editors). The New York Academy of Sciences, New York. pp. 55-68.

Glass, Roger I., et al. 1991. Cholera in Africa: Lessons on Transmission and Control for Latin America. The Lancet. Vol. 338, Sept 28, 791-795.

Reyna, Carlos and Antonio Zapata. 1991. Crónica sobre el cólera en el Perú. Desco: Centro de Estudios y Promoción del Desarrollo.

Tauxe, Robert V. and Paul A. Blake. 1992. Epidemic Cholera in Latin America. Journal of the American Medical Association. Vol. 267 no. 10: 1388-1390.

Webb, Richard and Graciela Fernández Baca de Valdez. 1990[?]. Peru en numeros, 1990. Cuánto S.A.

Webb, Richard and Graciela Fernández Baca de Valdez. 1992. Peru en numeros, 1992. Cuánto S.A.

Wolford, Kathryn. 1991. Peru in the Time of Cholera. The Christian Century. October 23, 1991.

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Modified from the original 27 January 1999 by JTK. Last revised 18 October 2000.