| Killer on the Wing
By Andrew Curry
Special to the Washington
Post
August 11, 1999, Horizon
Section
Most Americans
think of mosquitoes as little more than an annoyance. But they are the
world's most common disease carrier, or "vector," responsible for transmitting
sickness from infected hosts to others. Microscopic parasites such as Plasmodium
falciparum, which causes malaria, and the tiny wuchereria bancrofti worm
responsible for elephantiasis, live and grow inside mosquitoes. Many viruses
-- including those that cause yellow fever, dengue (aka "breakbone") fever
and equine encephalitis -- also hitchhike on mosquitoes to move from host
to host.
"Mosquitoes
aren't just passive transmitters of these viruses and
parasites," explains David
Brandling-Bennett of the Pan-American Health Organization. "They have complex
life cycles, and several stages of development are inside the mosquito.
[The parasites] go through different forms and migrate to the salivary
glands," where the mosquito injects them into a host. This active development
and transmission process is why most scientists believe that mosquitoes
cannot transmit the blood-borne AIDS virus, which is presumed to be digested
along with the blood meal and thus never reaches the salivary glands.
All of this
knowledge is fairly recent. A century ago, experts believed that malaria
(from the Italian mala aria, or "bad air") was caused by breathing unwholesome,
swampy air. Doctors guessed yellow fever was spread by the vomit or clothes
of sick people.
Ignorance helped
the spread of disease. Yellow fever, which was common in Africa, killed
entire crews of white slave traders on their way to the Americas, leaving
the resistant African slaves untouched and introducing the disease to the
Caribbean in the 1600s. When France sent an army of 33,000 men to conquer
Haiti and occupy the Mississippi Valley in 1802, 29,000 of them succumbed
to yellow fever -- prompting Napoleon to sell the disease-ridden Louisiana
Territory to the United States for a song in 1803.
It would take
another century for scientists to grasp the connection
between mosquitoes and tropical
disease. In 1897, British scientist Ronald Ross, working at a remote hospital
in India, dissected an Anopheles mosquito that had just fed on a malaria
patient. In its stomach, he noticed the P. falciparum cells other scientists
had spotted in the blood of malaria patients. That night he wrote a short
poem, which ended: "I know this little thing/A myriad men will save/O Death,
where is thy sting?/Thy victory, O Grave?"
Ross's discovery
-- which won him a Nobel Prize in 1902 -- was fleshed out by U.S. Army
doctor Walter Reed, who helped demonstrate that malaria and yellow fever
pathogens both incubate and mature inside the mosquito. Their work inspired
a second Army doctor, William Gorgas, to free Havana (in American hands
after 1898's Spanish-American War) from the mosquito Aedes
aegypti, which spread yellow
fever.
Commanding a
huge force of fumigators and building inspectors, Gorgas rid the city of
standing-water breeding sites. Within months, yellow fever in the Cuban
capital -- a hotbed of the disease for almost three centuries -- plummeted,
dropping from 1,400 cases in 1900 to 37 in 1901. In 1904, Gorgas repeated
his efforts in Panama, making possible the construction of the Panama Canal
through more than 40 miles of dense tropical jungle.
Thereafter,
optimistic scientists lavished attention on the tiny mosquito. When Ross
made his discovery, "he assumed he had found the way to eliminate malaria,"
Brandling-Bennett says.
This logic prompted
massive, world-wide mosquito eradication campaigns in the 1950s and '60s.
The World Health Organization helped local governments use DDT and other
insecticides to kill adult mosquitoes. One authority, noting the spectacular
success of the intense spraying, wrote in 1955: "This is the DDT era of
malariology. For the first time it is economically feasible for nations,
however undeveloped and whatever the climate, to banish malaria completely
from their borders."
The efforts
succeeded in eradicating malaria in some areas, such as Europe and North
America. But in most places mosquito populations were reduced but not eliminated.
When interest in control programs declined and funds dried up, mosquitoes
made a comeback. Liberal spraying also made many insect populations resistant
to common insecticides. Today, authorities focus on selectively controlling
populations, not eradicating them.
There's more!
Check out "Skeeters" Part I, "The Horrible
Truth About Mosquitoes," and Part III, "The
Circle of Life."
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