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The world's ocean temperatures influence global climate variability and are important to infectious disease dynamics, according to Mercedes Pascual. This global ocean grid shows the correlation between sea surface temperature anomalies in January and cholera cases in September in Matlab, Bangladesh. The large orange area in the Pacific Ocean indicates a significant positive correlation between these variables (the color bar at the bottom gives the values of the correlations). Interestingly, this is the same area in the Pacific that warms during El Niño events.
She turned to Bangladesh, where cholera is a fact of life. Bordered by India and the Bay of Bengal, Bangladesh is home to 150 million people, many of them forced by population pressure to the low-lying coast. Inundated by rivers that flow south from the Himalayas and perched on a landscape just a few feet above sea level, Bangladeshis face routine flooding with water contaminated by fecal matter.
Cholera bacteria thrive in this setting; once ingested by a human, the microbes proliferate in the gut, doubling their numbers every eight minutes. Within hours, the disease produces an explosive, clear diarrhea, speckled with rice-like shreds of intestinal lining. Untreated, it can kill in a day.
Cholera cases in Bangladesh spike twice yearly, according to infectious disease specialist Gary Schoolnik at Stanford University School of Medicine. The first spike occurs just after the monsoon rains that pour torrentially from June to September, overwhelm sanitation systems, and liberate V. cholerae into water used for drinking and bathing. The second occurs during the hot, dry spring, when shrinking pools of standing water concentrate the bacteria, unleashing another round of infections. Yet, the intensity of these seasonal outbreaks also varies on interannual timescales that, Pascual noticed, seemed to correspond to ENSO-generated escalations in ocean temperature.
Pascual had found valuable collaborators at the International Center for Diarrheal Disease Research (ICDDR) in the capital city, Dhaka, who had been monitoring cholera in different locations in Bangladesh since 1966. Their “time-series” data for Dhaka describing cases since 1980 was a crucial resource for Pascual's investigation. By considering those data against ENSO sea-surface temperature changes in a nonlinear model, she found what she was looking for: quantitative evidence tying ENSO to cholera dynamics. “In the end, we discovered a lag of 9 to 11 months between ENSO and an increase in cases,” Pascual says.
The finding—reported by Pascual, Colwell, and colleagues in Science in 2000—made international headlines. By linking cholera to global climate cycles, Pascual had fueled hopes for an ENSO-driven warning system that might avert outbreaks in Bangladesh and elsewhere altogether. By that time, Pascual had received 10 years of research funding (in 1999) from the James S. McDonnell Foundation; in 2001, she moved to the University of Michigan to become an assistant professor of ecology and evolutionary biology.
The ability to forecast epidemics in advance would be an important breakthrough for public health, Colwell says. Cholera—a symptom of poverty that's been virtually extinguished from the developed world—is an imminently treatable disease. Most patients recover with antibiotics and an oral rehydration solution containing salts, sugar, and clean water. “Better outbreak prediction would make it possible to gather the needed treatments and resources more efficiently,” Colwell says. “If you've got a few months to prepare, you can be more effective in dealing with the problem.”
Image: Climate Research, Vol 36: 131–140m 2008, “Predicting endemic cholera: the role of climate variability and disease dynamics.”