Monday, April 21, 2014

The Spatial Epidemiology of Malaria - Part 2

In my previous post, I described the striking variation in the RDT positivity rate seen among different villages of the Bugoye sub-county. For example, from the village of Bugoye, we observed that 30% of RDTs were positive for malaria, while from the neighboring village of Izinga, the rate was more than twice as high. In fact, patients from Izinga accounted for nearly 20% of all positive RDTs at the health center. These differences suggest that local malaria transmission has a significant spatial component. In order to understand these trends, we must first understand the geography of the sub-county.

Cows crossing the Mubuku River near the Bugoye Power Station
For the last six months, my partners from the Mbarara University of Science and Technology (MUST) and I have been using GPS devices to construct a map of the sub-county. Guided by members of the local community, we record the locations of village health workers, community leaders, and health centers, in addition to outlining political boundaries. We have journeyed hundreds of miles up and down the hills and valleys of the sub-county. On the narrow paths, we have learned a lot about life in the villages in ways we had not been able to understand or appreciate before. In doing so, we have gained new insight into malaria risk factors, infrastructure challenges, and health-seeking behaviors.

Last week, we visited Izinga. At the bridge over the Mubuku River, we met the village health team that would lead us around the village. As we walked, we learned that Izinga was essentially a low-lying island situated between the Mubuku and Kitakena Rivers. In addition, a spillway for the hydroelectric plant supplying power to the nearby cobalt mines runs directly through the village. These features make the village particularly prone to seasonal flooding. The cold, fast-moving rivers are normally unsuitable for mosquitos, but when the water surges over its banks, it creates thousands of pools of warm, stagnant water that are ideal breeding sites.

Woman walking across damaged bridge over the Kitakena River
In May 2013, massive flooding devastated many areas of the Kasese District, which includes the Bugoye sub-county. Tons of mud and stone came crashing down through the valleys, destroying everything in its path. Thousands of residents from places like Kilembe, where the district referral hospital is located, were displaced. The sheer force of the floods cut new paths through the earth, changing the course of Mubuku River and washing away homes, livestock, and crops. As the water receded, once arable areas were turned to “swamps.” Unfortunately, the flooding has affected more than just agriculture. Compared to the previous two years, more patients from Izinga are being admitted to the health center for severe malaria. At this time last year, immediately before the flooding, patients from Izinga accounted for only 7% of inpatient admissions for malaria. This year, they account for 20%.

The town of Kilembe after massive flooding in May 2013
The case of Izinga is a classic example of how geography impacts human health. Geography, however, is not destiny. One need only to recall the devastation that Hurricane Katrina wreaked upon the city of New Orleans to know that Western Uganda is not unique in its experience with natural disasters. The difference is the resources. In New Orleans, Blackhawk helicopters, which cost about $6 million dollars each, quickly arrived on the scene to rescue victims from rooftops. Later, the Army Corps of Engineers rebuilt the levies, reportedly stronger than ever before, at a cost in the billions of dollars. In Izinga, there are no helicopters or engineers and certainly no billions of dollars.

Standing water in Izinga
How, then, do we respond to this epidemic? Minimizing the impact of future floods is the long-term solution, but this requires infrastructure investment that is not realistic given local resource constraints. In the meantime, malaria transmission will remain high as the mosquitos continue to flourish and the parasites infect more and more residents. Soon, we may see the RDT positivity rates in neighboring villages begin to climb. With relatively modest resources, such as bed-nets and insecticides, we can prevent the situation from getting worse. Even these interventions, however, are beyond the means of the community. But now that we understand the problem, we can begin to argue for the resources.

Ross M. Boyce MD, MSc
PGY-2, Internal Medicine
Global Primary Care Program
Massachusetts General Hospital

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