Not Just Biology

Not Just Biology A Holistic Approach to Understanding the Emergence of Infectious Diseases in Developing Countries by Stephanie Oberfoell

H

ow do dramatic ecological shifts precipitate the outbreak of latent infectious diseases? Professor William Durham, the chair of the Anthropological Sciences Department at Stanford University, and his colleague, Assistant Professor of Anthropological Sciences James Jones, have begun to address this question. Using a holistic approach, they study how human-induced environmental change affects the transmission ecology of diseases. Durham’s current investigation on the malaria outbreak in Rôndonia, Brazil, in the 1980’s and the cholera epidemic in Peru in the 1990’s has revealed that cultural and social factors are also responsible for the upsurge of the diseases. Before 1980, Rôndonia, a state on Brazil’s western border with Bolivia, was covered in closed-canopy rainforest and only sparsely inhabited. The hostile headhunting tribes who occupied this region, numbering a few thousand people at most, decapitated opponents’ heads in a pattern of traditional warfare that dated back millennia. In times past, this practice was adaptive in that it resulted in the dispersal of local tribes, and increased the local game supply. The large distance between tribes reduced the prevalence of malaria in the region. Furthermore, the closed-canopy forest did not provide the standing puddles of warm water in direct sunlight that the malaria vector—the Anopheles mosquito—needs in order to breed. As a

result, endemic malaria remained at low transmission, low virulence, and low frequency before the construction of Brazilian Route 364. In 1980, the United States-supported World Bank granted a multi-million dollar loan to the Brazilian government to construct Brazilian Route (BR) 364, a highway connecting the state of Rôndonia to the rest of Brazil. BR 364 was intended to provide thousands of jobs in construction and to allow access to potential farmland to help the growing population of millions of unemployed and landless in the country. Because the government widely advertised the route’s development, millions of Brazil’s poor, landless farmers began to move into the surrounding area. The migrants received government-granted land titles if they built settlements and cleared forest for farming. They tried to grow annual crops like rice, manioc, maiz, and beans, but the soil was poor. As a result, the farmers slashed and burned more forest in hopes of finding better soil. Interactions between the westernized settlers and the indigenous headhunting tribes caused many problems. As the settlers continued to move into the depths of the Amazon, they continually encroached upon the land of the native headhunters. In retaliation, the headhunters burned the settlers’ homes and killed their children and livestock. But both settlers Layout designed by Candice Hsu

Volume IV 31

Not Just Biology Dr. Rita Colwell, former Director of the National Science Foundation, has done extensive research on the topic. Specifically, she relates sea temperature change to the bloom of the copepods and hence to the spread of cholera. Durham, however, causally implicates social conditions in the transmission of cholera as well. He argues that prior to 1991, there were international warnings that the overuse of the disinfectant, chlorine, could thin the earth’s pro-

http://ehp.niehs.nih.gov/docs/2003/111-7/waterglass.jpg

and natives lost lives: the settlers, for their part, brought new diseases to the region, killing as many as 60% of the indigenous people in some areas. Durham hypothesizes that these cultural clashes, along with ecological ones, contributed to the massive outbreak of malaria in Rôndonia. The dramatic ecological shift triggered by BR-364 created the perfect breeding ground for the Anopheles mosquito. In addition to the two to four meters of rainfall each year in Rôndonia, the network of roads led to thousands of kilometers of trenches and massive deforestation, creating warm, standing puddles for mosquito larvae. The immigration of millions of Brazilians into the region also created a high-density population for the rapid transmission of malaria. “You couldn’t have planned a better environment for Anopheles mosquito reproduction,” Durham claims. Durham devised a method to quantify the disease effects of the BR 364 construction by calculating the value of the “Basic Reproduction Number” using the “vectorial capacity” of the Anopheles mosquito in Rôndonia. The reproduction number, R0, considers the transmissibility of the disease, the average contact rate of people, and the duration of infectiousness. If R0<1, the disease will die out by itself. If R0>1, there is an epidemic. The vectorial capacity, VC, depends upon the number of blood meals per day per vector, the proportion of blood meals taken on humans, the time interval between blood meals, the vector’s average lifespan, and the vector’s development time. In Durham’s preliminary research, he found the Basic Reproduction Number for malaria dramatically increased with BR 364 because each

tective ozone layer. Although Peru used chlorine to make potable drinking water, the international community pressed Peru to decrease chlorine usage to save the ozone. With the reduction in chlorine, the public water

Durham’s multidisciplinary approach allows him

to draw links where few have drawn links before. variable in the numerator of the equation had increased significantly. He hypothesizes that before the construction of the route, R0<1.0, whereas now it is much greater than 1.0. In other words, this indicates that Rôndonia went in a decade or so “from a disease ecology with low transmission

http://www.cdc.gov/malaria/biology/mosquito/frame.htm

to almost perfect conditions for maximum transmission.” Paralleling the malaria scourge in Brazil a decade earlier, Peru in 1991 had a sudden upsurge of a previously quiescent disease—cholera. For over 50 years, R0 for cholera in the Americas was less than 1.0. But then along came El Niño weather conditions in 1991. Durham hypothesizes that global warming lies behind the “higher frequency and longer duration of El Niño events” in recent years. The warming of the sea allows for the proliferation of copepods, where Vibrio cholerae bacteria live. Vibrio cholerae produces a toxin called choleragin which is responsible for the disease symptoms.

32 Stanford Scientific

supply became a convenient means for cholera to spread among Peruvians. Hence, Durham hypothesizes that human-induced environmental change increased the prevalence of Vibrio cholerae in the seas, followed by an easy transfer to the water supply. Following the results of his work in Brazil and Peru, Durham hopes more broadly “to draw links between human-induced environmental change and the resurgence of some age-old pestilences—influenza, cholera, and malaria—and the emergence of some new ones, like HIV/AIDS and Ebola.” While the multiple variables of emerging infectious diseases are difficult to track down, Durham’s multidisciplinary approach is unique in that it allows him to “draw links where few have drawn links before.” He can begin to see the whole picture by linking not just biological phenomena, but cultural and social ones as well. Before relocating millions of people into the forest, cutting back on chlorine usage or making other large ecological changes, Durham hopes that people in power will take a more interdisciplinary approach to development and planning. In the future, he hopes that forestry officials and city planners will have policy agendas that factor in the effects of human-induced environmental changes on disease ecology. S Stephanie Oberfoell is a sophomore majoring in Human Biology and considering a minor in Art History. After her undergraduate career, she plans to pursue a Masters in Public Health and Doctor of Medicine. She loves to travel, snowboard, admire art and advocate conservation.