Respect the mosquito, a sometimes deadly foe
By Gary Raham
Nature Writer and Illustrator
In one corner: a 165-pound primate armed with DEET, a rolled up newspaper,
and the largest brain-to-body-weight ratio on the planet. In the other
corner: a dusty-brown airborne vampire less than a quarter-inch long that
sounds like a dentist's drill.
In a direct confrontation, who will win? As often as not, as you can probably
testify, the latter creature--often referred to as "that !*$#!! mosquito" -- will
triumph. The mosquito, in fact, possesses the dubious distinction
of being one of the few creatures Homo sapiens has specifically tried (and
failed) to exterminate from the planet. And as sympathetic as I am to a
diverse biosphere, I would not be terribly offended if we had been successful
in global mosquitocide. This insect has served as handmaiden to some of
the deadliest diseases in human history.
It's hard to blame mosquitoes for their vampirism. They need blood to reproduce,
and only a few species find our kind suitable for blood sucking. The problem
is that many disease microorganisms, the true rulers of the planetary ecology
in many ways, use mosquitoes as middlemen in their own life cycles. Even
today, a full 10 percent of the world's population suffers from malaria
--only one of the diseases the mosquito exports. Every 12 seconds a malaria-infected
child dies.
The situation was once much worse.
In the 17th, 18th and 19th centuries, both world-traveling Europeans and
the peoples they visited suffered greatly from mosquito-borne diseases
--though it took a while before the mosquito was recognized as an accomplice
in the process. Yellow fever was blamed on dirt, sin and moral turpitude.
Blacks in Africa thought that Europeans were somehow weak and fragile as
they died in droves from malaria. In 1793, Philadelphia lost 5,500 people
(10 percent of its population) to yellow fever. In the 1800s, cities in
the southern United States routinely lost thousands of people to yellow
fever in at least a dozen outbreaks, including one in New Orleans in 1853
that killed 9,000 people in the city and 11,000 more in the lower Mississippi
Valley. One mosquito researcher said, "For much of the century, yellow
fever shaped migrations and economic development in New Orleans and throughout
the south."
Those who study contagious illnesses talk about the basic reproduction
number (BRN) of a disease, which is the number of additional infections
that an originally infected person will generate under ideal conditions.
Measles is very contagious with a BRN of 12 to 14, whereas AIDS only scores
1+ _just enough to propagate itself. Malaria, because the microorganism
gets flight and injection service from mosquitoes, scores over 100.
Four factors determine how effectively a vector like the mosquito is in
transmitting disease: vector food choices, vector abundance, vector lifespan
and whether the vector is working on an established or brand new population
of hosts.
The first factor is easy to explain: not all mosquitoes choose to bite
people. Many specialize in other animals and will only bite people as a
last resort, if at all. Mosquitoes also tend to be selective when they
bite. Most feed at dawn, dusk or during the night. Few will feed during
the hottest portion of the day.
Vector abundance refers to the density of mosquito populations. In 1935,
malaria infected about 130,000 Americans, many in the rural Southeast.
By 1950, only isolated cases were recorded. Most of the credit goes to
the Tennessee Valley Authority project (TVA), which disrupted mosquito-breeding
opportunities. TVA engineers removed vegetation from the banks of reservoirs
and angled the banks of waterways so that mosquito larvae would be stranded
when regulating water flow at dams lowered water levels. Wet areas were
drained or filled, and rigorous monitoring and response to malaria cases
were instituted. Screens placed on windows and doors kept night-feeding
mosquitoes away from people.
The insecticide DDT, easy and cheap to produce, proved especially effective
against mosquitoes. For a short period in the 1950s scientists and health
workers put a bull's eye on the mosquito, and DDT was the arrow that would
utterly destroy them. Unfortunately, DDT turned out to be deadly to a host
of other, quite necessary creatures in the environment--especially if
applied incorrectly--and mosquitoes showed a talent for evolving immunity
to the chemical. DDT also tended to shorten the lifespans of mosquitoes.
The older a vector gets the more likely it will have picked up and transmitted
disease.
Finally, vector-borne diseases are most devastating in new host populations
that haven't had the opportunity to evolve resistance. West Nile virus,
first identified in Uganda in 1937, produces a childhood disease there,
but can cause serious illness and even death to virgin populations, like
those of us now exposed in North America. International travel and commerce
and the mishandling of medicines and insecticides have increased the potential
for serious epidemics.
Here's a short list for protecting yourself against the wily and resilient
mosquito:
- Remove sources of water that can serve as breeding grounds. Clear out
clogged gutters. Remove barrels and old tires that collect rainwater. Change
the water in birdbaths, wading pools and flowerpot trays regularly.
- Avoid outdoor activities at dawn and dusk, prime mosquito feeding time.
If you are outside at that time, wear long sleeves and pants. Repair torn
screens on windows and doors.
- Avoid marshy, swampy or other breeding areas even during the day. Certain
daytime feeders can transmit encephalitis. When using DEET to deter mosquitoes,
avoid formulations with greater than 34 percent DEET. High concentrations
can cause nerve damage or other adverse reactions.
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