News Story

November 15, 2011

Managing Malaria, Beating the Mosquito in the Amazon Jungle

By Steve Bradt

In the developed world, malaria can seem like a relic of a bygone era. But in Brazil, home to 70 percent of the Amazon rainforest, it’s a scourge that in recent decades has been resurgent.

The Brazilian Amazon has been steadily settled—and deforested—since the 1970s, when the government created incentives to support agriculture, mining, and human settlement in this vast but once sparsely populated region. The massive environmental impact of this human influx is widely known: one-sixth of the Amazon Forest has been lost, much of it over the last 30 years.

But a less-recognized side effect of the clearing of the Amazon is that it has opened up massive new breeding grounds for mosquito species that are the primary carriers of malaria in South America. Because malaria in the Amazon takes a very different form than it does elsewhere in the world, there is a distinct name for the disease in Brazil and neighboring countries: “frontier malaria."

The effect on public health has been catastrophic in the large swath of Brazilian territory officially classified as Amazon (the Amazônia Legal, or “Legal Amazon”), which has seen a tenfold increase in malaria since 1970. The number of cases peaked at more than 635,000 in 1999, before sliding to about half that since the implementation of comprehensive government programs to control the disease.

Marcia C. Castro, an assistant professor of demography at the Harvard School of Public Health, has been studying the thicket of factors contributing to frontier malaria—and complicating the response to the disease in this area of the world—for more than 10 years.

“Malaria transmission in Brazil is extremely complex, characterized by the poorly understood interplay of social, environmental, economic, political, and behavioral factors,” she says. “Among the most troubling aspects of our limited understanding of frontier malaria is that we know very little about how climate change will impact transmission patterns. Infrastructure projects intended to promote further development of the region may worsen the burden of malaria if we fail to anticipate and mitigate the impacts.”

There’s long been an inherent tension between the environmental and public health implications of Amazon frontier expansion. Environmentalists would prefer to see as little of the Amazon clear-cut as possible. So when settlers move in to establish a homestead and farm, environmental concerns would dictate clearing as small a lot as possible.

But this, Castro says, flies in the face of what scientists know about malaria transmission: Namely, that proximity of poorly constructed houses to tropical forest is a surefire recipe for the spread of malaria.

“Malaria control is optimized when there’s a buffer of at least 100 meters between houses and the forest fringe, where mosquito density is highest,” Castro says. “When you have great proximity between man and the edge of the forest, the exposure to mosquito bites is maximized.”

Furthermore, the ramshackle homes in many newly settled parts of the Amazon—only about three-fifths of which offer their occupants sanitation and clean water—don’t offer much protection from the primary carriers of malaria, the Anopheles genus of mosquito. And compared to the malarial vectors prevalent in Africa and Asia, the Amazon species’ behavior seems optimized to infect people.

The mosquito carriers “are most active at 5 to 6 a.m., and at 5 to 6 p.m. They are also exophilic – they prefer to bite humans outdoors,” Castro notes. “Unfortunately, dawn and dusk are exactly the times when settlers are most likely to be outdoors, on their way to and from their work and school.”

Most of the migrants who’ve sought economic opportunity in the Amazon during the past four decades, tripling its population to 23.6 million, came from malaria-free areas in Brazil, and therefore have no immunity against the disease. They also know little to nothing about malaria protection or transmission.

But Castro’s research has helped develop new ways of modeling the determinants of malaria transmission in the Amazon. Specifically, her work supports a temporally and spatially targeted approach to combating malaria in newly settled areas.

In the initial years after an area is settled, she says, the Brazilian government needs to focus on environmental management to minimize malarial transmission. This does not necessarily include large scale vector and larval control, an almost impossible task given the sheer size of the region. But it does include strategies to improve the quality of houses and to reduce human-mosquito contact.

First, improved construction techniques would keep malarial mosquitoes at bay. Second, homes should be built with buffers of at least 100 meters from the woods and 500 meters from bodies of water, she says. The mosquitoes that carry the malarial pathogen in the Amazon exist in particularly dense numbers at the forest fringe, where the species find the partially shaded habitat they prefer for breeding. Such habitat is also found alongside lakes, rivers, and streams.

Unfortunately, the environmental transformation undertaken by many new arrivals to the Amazon often ends up creating a perfect mosquito-breeding habitat. Settlers generally leave taller trees standing when they slash-and-burn a parcel of land, maintaining the partial shade Anopheles finds desirable. Burning increases soil pH, and the mosquitoes prefer breeding in alkaline standing water.

Malaria outbreaks in the first two years after an area is settled tend to be especially severe. Castro points to the experience of a settlement called Machadinho, where migrants first arrived in late 1984. By 1986, more than 90 percent of the population had malaria at least once, and 56 percent of residents were sickened in at least five months of the year.

Longer-term, after an area has been settled for about 10 years, urbanization and a degree of community cohesion tend to supplant erratic migration and highly variable land clearance practices. Paving and improved drainage create environments that are less hospitable to mosquito larvae, and health clinics become more prevalent.

Castro’s research suggests that at this later stage of settlement the government could manage malaria by focusing on human behavior. Key contributors to the virulence of frontier malaria, she says, include the many asymptomatic carriers of the disease – essentially, outwardly healthy people who are unlikely to seek medical treatment for symptoms but who can nonetheless infect mosquitoes when bitten.

“We really think the two critical reasons transmission remains stubbornly high in the Amazon are the large number of asymptomatic carriers and the high rate of human mobility,” she says. “Put these two pieces together and you have a recipe for sustained transmission of malaria.”

The distinction between frontier malaria and malaria elsewhere in the world was first recognized in the 1980s, as the opening of the Amazonian frontier accelerated during the Brazilian government’s launch of aggressive efforts to encourage settlement there. Frontier malaria’s distinctive characteristics include low levels of immunity, lack of community and social isolation, and exacerbation of problems by poor or absent government planning.

A way for the Brazilian government to respond, according to Castro, would be frequent testing of everyone in frontier areas of the Amazon, treating those found to be asymptomatic. But this approach is not without its drawbacks, and its feasibility is questionable.

There are also practical and logistical hurdles: How do you find and treat everyone in a vast area where the population is highly transient? It’s proved very difficult to map and track the asymptomatic population, Castro notes.

“The cost here is in searching for and finding settlers, and in monitoring their long-term health,” she says. “Once you’ve done that, the cost of actual treatment is very manageable.”

But in a vast jungle spread across nine nations—not only Brazil but also Bolivia, Colombia, Ecuador, French Guiana, Guyana, Peru, Surinam, and Venezuela—borders are permeable and a single government simply can’t coordinate testing and treatment all by itself.

All across the Amazon basin, rivers rise dramatically during the rainy season, flooding areas all along their banks. When the rains end and the rivers subside, pools of water—ideal for mosquito breeding—are left all along the Amazon and its many thousands of tributaries.

“The sheer size of the region, and its unique climate and hydrology, are important challenges to controlling malaria,” Castro says. “Control of mosquito larvae is not feasible, given the extremely difficult task of identifying, or even reaching, most breeding habitats.”

During the first half of the 1900s in coastal areas of Brazil, engineering projects—such as the construction of drainage systems and the filling of marshes—were successfully used to combat malaria, which is thought to have come to South America with Europeans in the sixteenth century. But these projects never extended inland to the vast Amazon region. The construction of ditches to foster drainage along Amazonian rivers was proposed in 1916, but never implemented.

These early engineering projects were later augmented by nationwide anti-malaria campaigns starting in 1940, when some four to five million Brazilians—then one-tenth of the population—contracted malaria annually. Beginning in 1947, the insecticide DDT was used to spray the interior of houses across the country every six months, reducing the number of cases by 99 percent in just six years. (Interestingly, the tremendous success of this approach may explain why the Anopheles mosquitoes that transmit Amazon malaria have now flipped to become an almost exclusively outdoor species.)

By 1970 cases of malaria nationwide reached their low point: just 52,371 individuals, about 60 percent of them in the Amazon. But throughout the 1960s, trends were building that would soon send the rate of infection skyrocketing anew. In 1960 the Brazilian capital was moved inland from Rio de Janeiro to Brasília, marking the launch of a broad effort to integrate the Amazon region with the rest of the nation. Construction of highways into the rainforest intensified during the late 1960s and early 1970s, bringing with it an influx of impoverished migrants seeking land and employment.

With this effort to link the coast with Brazil’s interior, to promote national development, and to increase the nation’s industrial power, the modern Amazon frontier expansion began. But it didn’t take long for the government’s strategy to upset the ecosystem balance of the once-thinly settled region. By the early 1980s, the Brazilian Amazon was already experiencing high rates of deforestation, conflicts with indigenous populations, and severe malaria outbreaks.  The lack of acquired immunity among most settlers, combined with precarious living conditions—houses with partial walls, or with walls and roofs made of tree leaves—led to a resurgence of the disease.

Unlike in Africa, where mortality rates are high, the disease causes relatively few deaths in the Amazon. Primarily due to improved clinical treatments, fewer than 100 people a year die of malaria in Brazil. The hundreds of thousands of cases annually in the Amazon result mostly in missed work and school, due to flulike symptoms that commonly include fever, chills, headache, sweats, fatigue, nausea, and vomiting. Despite the low mortality rate, the disease imposes a heavy social and economic burden, since most residents of the Amazon are farmers who are unable to hire others to work on the land.

“Government permits to settle should be based, in part, on the risk of malaria in those areas,” Castro says. “In many parts of the Amazon, people should not have been settled in the first place.”

One of the biggest question marks in the ongoing control of malaria in the Amazon is what role climate change will play in future transmission patterns. While in temperate North America heavy rains usually promote mosquito reproduction by creating standing pools of water for breeding, the opposite can actually be the case in the humid tropics.

During periods of drought, Castro says, river and lake levels fall in the Amazon, creating areas of stagnant water along their margins that are optimal mosquito breeding grounds. For example, 2005 saw a drought, followed by a jump in cases of malaria. But, she cautions, the relationship between precipitation and malarial outbreaks is probably more complicated than this, an issue she is currently exploring.

“The variation in water levels, combined with irregular river and stream margins, helps to create pools of water,” Castro says. “But if a drought gets really severe and the area dries out, there are no breeding habitats. It all depends on how fast the process is, and if the water stays there long enough to allow larvae to mature into adult mosquitoes.”

At least half a million cases of malaria were recorded annually in the Amazon between the late 1980s and the early 2000s. Since a 2005 spike—the last time cases of malaria numbered more than 600,000—the incidence of the disease has been on a steep downward trajectory: only about 300,000 cases were documented in both 2008 and 2009.

Whether Brazil and its neighbors can sustain this progress remains to be seen. A malaria control program implemented by the Brazilian government in 2000 and bolstered in 2003—targeting the communities that are home to the largest numbers of cases—has cut outbreaks dramatically. Castro says the government has improved its monitoring of pathogen resistance to existing malaria drugs, as well as its tracking of water levels so as to step up mosquito control when needed. She notes that increased hospital capacity in vulnerable areas could also help control the disease. But it’s possible, Castro says, that the government might instead curtail the program in an economic slowdown, as has happened during previous malaria control efforts.

“Despite accounting for half the cases of malaria in the Western Hemisphere, Brazil has been cited in recent years as a country where malaria eradication should, in theory, be feasible,” Castro says. “I’m not sure I agree with that optimistic assessment, given the likely high prevalence of asymptomatic individuals. Aggressive and active surveillance—and treatment—of these carriers might be essential to containing malaria in the Brazilian Amazon.”

Note: This article originally appeared in Environment@Harvard Volume 3, Issue 1.


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