before scientists traced the transmission of malaria to mosquitoes, people knew
and feared the seasonal fevers that developed in the marshes of Europe. In Book
VII of the History of the Peloponnesian War, Thucydides recounts the
destruction of the Athenian expedition at Syracuse in 413 B.C. After a particularly
devastating battle Thucydides wrote, the Athenian generals consulted upon
the disaster which had happened, and upon the general weakness of the army.
They saw themselves unsuccessful in their enterprises, and the soldiers disgusted
with their stay; disease being rife among them owing to its being the sickly
season of the year, and to the marshy and unhealthy nature of the spot in which
they were encamped.
Mosquitoes of the genus Anopheles can infect people with malaria, a disease that causes intense fevers, chills and other symptoms. Image courtesy of WHO/TDR/Stammers.
Thucydides statement identifies two prominent features of malaria in
Europe: its seasonality and its association with swampy habitats. The word malaria
literally means bad air in Italian. It was formerly thought that
the vapors emanating from marshes and swamps brought on the malarial cycle of
chills, fever and sweating. References to seasonal intermittent fevers abound
in the literature of the Roman Empire, as well as in texts of ancient Assyria,
China and India. In England, malaria was referred to as the ague,
meaning sharp fever in Latin. Chaucer, Shakespeare and Defoe all
make reference to this ailment.
In the 16th to 19th centuries, the recognized differences in malaria incidence among lowland swamp and estuarine residents and upland hilltop residents led medical geographers and physicians to speculate on the benefits and detriments of different habitats and climates. The contrasts between the pestilence of reeking, noxious marsh airs and putrid stagnant waters, and the healthfulness of fragrant country air and fresh, running mountain waters, were generally acknowledged.
Not until the end of the 19th century did Ronald Ross definitively demonstrate that mosquitoes of the genus Anopheles (or anopheline mosquitoes) transmit malaria, work for which he was awarded the Nobel Prize in medicine. Most female mosquitoes need a blood meal for egg production. Generally, a female mosquito takes a blood meal and then rests while her eggs develop; she then oviposits the mature eggs in a suitable habitat, and if she survives, repeats these actions. By repeatedly feeding, anopheline mosquitoes are able to acquire and then transmit the malaria parasite.
But despite this understanding of the diseases transmission, much confusion remains as to how and why differences in climate, weather, ecosystem type and land management practices manifest as noisy patterns of disease and health. Today, scientists from various fields, including the geosciences, are contributing to the fight against malaria and other infectious diseases.
Following a path
Four species of malaria infect humans: Plasmodium falciparum, P.
vivax, P. ovale and P. malariae. P. falciparum is the
most prevalent and by far the most deadly, accounting for almost all malaria
deaths worldwide. Today, approximately 1 to 2 million people, mostly children,
die from malaria each year. More than 90 percent of this mortality occurs in
The life cycle of the malaria parasite, genus Plasmodium, is quite complex and requires an anopheline mosquito, a human host and their interaction (see image, page 21, print exclusive). A host is infected with malaria sporozoites, which enter the cells of the liver. Between six to 16 days later, the parasite bursts from the infected liver cells and enters the bloodstream in a new form, called merozoites, which invade the red blood cells of the host and reproduce asexually, eventually bursting the infected cells. Released into the blood stream, the merozoites infect still more red blood cells, increasing their numbers further, and within red blood cells, they may also sexually differentiate and produce gametocytes, which fill the whole of the host blood cell.
Anopheline mosquitoes acquire the malaria parasite by biting an infected host and ingesting gametocytes. Within the gut of the mosquito, the gametocytes sexually reproduce and form an oocyst on the lining of the mosquito stomach. The oocyst grows over many days and eventually ruptures, releasing sporozoites that then fill the salivary glands of the mosquito. Mosquito saliva acts as an anticoagulant and prevents clotting during feeding. (The mosquito saliva also produces the localized reaction and annoying itch.) When a mosquito inserts its proboscis into a host for blood feeding, it first injects saliva into the area and thus infects the host.
More than 70 species of Anopheles can transmit human malaria. Some feed indoors, others outdoors; some feed almost exclusively on humans, making them very effective malaria vectors, while others feed on a number of vertebrate hosts, and thus are less effective vectors. Each of these species has its own breeding site preferences and will respond differently to changing environmental conditions.
For instance, Anopheles gambiae is the dominant transmitter of malaria in much of sub-Saharan Africa. Its larvae are found in temporary bodies of water such as the pools and puddles that form in tire tracks, hoof prints and naturally low-lying areas. Increased rainfall creates standing waters that favor this mosquito.
In contrast, Anopheles culicifacies is the dominant transmitter of malaria in South Asia. Its larvae are found in a great variety of habitats including rice fields, irrigation ditches, stream edges and wells. This mosquito species flourishes in drought, exploiting the pools that form in drying streambeds.
Repeated exposure to malaria, if survived, confers immunity, meaning that the individual grows less susceptible to the severe clinical manifestations of the disease. Kenyan colleagues of mine who have been exposed to malaria throughout their lives state that they feel nothing but a mild headache or slight fever when infected. In essence, the repeated infectious mosquito bites act as inoculations, keeping the bodys immune system in prime malaria fighting shape. In many areas of sub-Saharan Africa, there is perennial, measurable incidence of malaria. In these endemic populations, adults have a high degree of conferred immunity, but mortality is high among children under five years of age.
is inversely associated with prosperity and development. Wealthy societies drain
their marshlands, control their water supplies, mass manage their livestock,
and air-condition and screen their homes and offices. These actions reduce contact
between anopheline mosquitoes and the human population. Without sufficient interaction
between Anopheles and humans, the malaria parasite cannot survive. This
control of malaria, of course, benefits the society. By reducing the incidence
of the disease through effective and continued malaria control, productivity
and prosperity increase further.
An estimated 1 to 2 million people die from malaria each year. More than 90 percent of these deaths occur in sub-Saharan Africa, such as in Kenya, shown here. Copyright of Jerome Wyckoff.
Conversely, areas with endemic malaria tend to be impoverished, lacking sanitation, clean water supplies and modern construction; livestock live within villages and households, and mosquito-breeding habitats abound. Malaria infections in these areas restrict economic development and widen the disparity between the developed and underdeveloped worlds. According to World Health Organization estimates from 1999, economic growth is 1.3 percent lower per year in countries affected by malaria.
Effective malaria control in endemic areas is also complicated by conferred immunity. Without repeated exposure to the pathogen, adults once again become susceptible to the severe clinical effects of malaria. By increasing their vulnerability, effective malaria control puts the population at much greater risk, should the control measures fail. Such failures have occurred a number of times with disastrous consequences.
While the segregation of human and anopheline populations has proven very effective at reducing malaria transmission, mosquito control through use of residual insecticides, such as DDT, has also proved enormously valuable. DDT remains one of the more effective insecticides available and is still used today, though a number of developed countries, including the United States, refuse to fund its use. (It was the liberal use of DDT in crop pest control, not malaria control, which led to its damaging environmental effects in the United States.)
In low dosages, DDT can be applied to the interior walls of homes. Following an indoor blood meal, female Anopheles mosquitoes often rest on nearby walls. The mosquitoes absorb the DDT through their feet and die. The treatment thus kills Anopheles females after a single blood meal; by only feeding once they are never able to transmit malaria sporozoites.
Many other insecticides and larvicides, which target the mosquitoes as larvae, are also used. Because mosquitoes are highly prolific, insecticide resistance can develop rapidly in areas where a single insecticide is used continuously. As a result, mosquito control often alternates among several insecticide types.
At present, there is no malaria vaccine; however, curative and prophylactic drugs, which are not readily and cheaply available in the developing world, are effective in the elimination and prevention of most malaria. Like the mosquito, the malaria protozoa is highly fertile; drug resistance in some regions has developed to some of the older anti-malaria drugs.
In recent decades, there has been a renewed interest in understanding the effects
of climate and weather variability on malaria transmission. Researchers have
attempted to quantify the impact of the El Niño-Southern Oscillation,
monsoons, rainfall, temperature, drought, floods and hurricanes on malaria incidence
in different regions of the world. These analyses are often complicated by a
lack of good public health data, as many parts of the developing world have
only limited hospital records, which paint an incomplete picture of the disease
burden in the local population. The effects of other diseases, such as AIDS,
tuberculosis, respiratory illness and diarrheal disease, further confound identification
of causes of disease and death.
Medical geographers, epidemiologists, geoscientists and malariologists employ a variety of tools to help monitor and forecast malaria transmission. Using satellite and other remotely sensed data, they can now document over large areas and at high resolution many of the factors affecting malaria that were previously poorly quantified or simply ignored. These factors include the spatial distribution of houses in villages, standing bodies of water, surface temperatures, rainfall rates, and the abundance, location and type of agriculture. Such data can then be compared with records of malaria transmission, mosquito abundances or other factors among local communities.
The resulting datasets can show a variety of relationships: whether village children are more prone to malaria when living within 100 meters of a catch basin; if numbers of infectious mosquitoes are associated with deforestation or irrigated agriculture; and if the health or greenness of vegetation may be used as a proxy for the amount of pooled water at the surface beneath the canopy. Public health officials may then be able to use this information to monitor and control malaria transmission and mosquito numbers.
Computer models also offer a means of understanding malaria transmission by elucidating the basic processes governing it in a region, filling in missing data, or forecasting conditions favorable to the increase of mosquito abundance and mosquito-host interactions. Information from both computer models and satellite imaging can be combined using geographic information systems (GIS) technologies to provide further analysis of the distribution of malaria.
GIS technologies are software systems that give researchers the ability to manipulate, visualize and analyze multiple geospatial data forms simultaneously. For malaria, a GIS system may pull data from satellites, computer models, digitized maps, hospital records and vector control agencies, allowing for the analysis of malaria transmission and mosquito abundance in both space and time.
Researchers at the NASA Ames Research Center, the University of California at Davis and the Mexican Ministry of Health, for example, have used a combination of remote sensing and GIS software to characterize the landscape in and around villages in southern Mexico. Comparing these landscape characteristics to local mosquito abundance data, they found that villages surrounded by more transitional swamp and unmanaged pasture support greater numbers of anopheline mosquitoes. Using quantitative models based on these findings, the researchers were then able to accurately predict anopheline abundances in other areas of Mexico and thus identify regions of increased vector-human contact risk.
In Africa, the MARA/ARMA (Mapping Malaria Risk in Africa) project seeks to use a range of public health, climate, weather, satellite and model data to develop continent-wide maps of malaria risk. The MARA project began in the late 1990s. Their maps, which are developed using GIS and spatial statistical analysis, provide estimates of the regional and district-level malaria burden throughout Africa, including many areas where transmission levels remain poorly documented. The MARA maps have been disseminated throughout Africa, where government and public health officials are using them to help determine the distribution of resources for combating malaria.
These efforts are promising and beneficial, but much work remains to be done. With four species of Plasmodium, more than 70 Anopheles transmitters and a wealth of factors affecting human resistance, the transmission dynamics of malaria are both complex and variable. Our understanding of the scope of these dynamics is incomplete, and the geosciences have much to contribute to this understanding and to malaria control. In addition, similar tools can be brought to bear on other vector-borne diseases, such as West Nile virus, filariasis, Lyme disease and Chagas disease.