Long
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 disease’s 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
sub-Saharan Africa.
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 body’s 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.
Controlling transmission
Malaria
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.
Expanding possibilities
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.
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