Scientists
have pretty much ruled out the possibility that mosquitoes can spread the virus
that causes AIDS. No documented case of HIV has ever been linked to the hated
bloodsucker. While lack of evidence cannot by itself disprove a hypothesis, the
chances of a mosquito transmitting HIV are so slim that the idea has faded out
of scientific discussion as researchers face the real challenges of the immense
predicament of AIDS.
However, when scientists were first learning about HIV, the insect
transmission question was yet another unknown about the new disease. Some experiments and unexplained
cases in the 1980s led to finger-pointing at mosquitoes, although scientists
already had strong doubts that insects could transmit the disease.
In 1987, the now-defunct U.S. Office of Technology Assessment
held a workshop to address concerns about a possible HIV threat from
mosquitoes, bedbugs, ticks and cockroaches. Besides room for “a rare and
unusual event” of possible insect transmission, the report states
that it is almost impossible for the insects to pass along HIV.
The discussion has almost fizzled out, although a few
investigations scattered over the years have continued to look for connections
between HIV transmission and insects such as bedbugs and flies. In 2006, the
United States Army Center for Health Promotion and Preventive Medicine issued a
definitive report that
outlined why there is no reason to worry about contracting HIV from a mosquito
bite.
But why can’t you get HIV from a mosquito when it’s clearly
the culprit in malaria, yellow fever and dengue fever? It’s all about the bug. There are two methods by which
bloodsucking insects typically transmit disease: the biological method and the
mechanical method.
The biological route is how malaria infects
more than half a billion people each year. Its disease agent, the Plasmodium parasite,
relies on the mosquito as a go-between to settle in human hosts.
Every mosquito bite involves a female mosquito looking for a
blood meal to nourish her eggs. She injects saliva to keep the blood from
clotting, and an allergic reaction to the saliva makes our skin annoyingly
itchy and red after the bite. If the mama mosquito happens to bite a
malaria-infected person, she ingests the parasites, which end up invading her
cells and replicating. They then migrate to the salivary glands from where they
can infect another human host in her next bite.
If the blood
that she sucks up contains HIV, though, the virus can’t follow the same path as
the malaria parasite. Instead of multiplying and eventually heading for the
salivary glands, the viruses get digested, and meet their death in the insect’s
gut.
The mechanical method is the other way for bloodsucking insects to
pass along disease. Suppose a feeding mosquito is slapped away but is still
hungry. Since
insects don’t use napkins, blood remains on its mouthparts as it flies over to
bite another victim. Theoretically, if Victim 1 had HIV circulating in his
bloodstream, some could end up in Victim 2.
However, the
probability of the transaction is almost zero. For one thing, the mosquito needs
a healthy victim within quick buzzing distance of the HIV-positive one. Even in
these conditions, the mosquito’s eating habits and the nature of HIV’s presence
in the bloodstream still make it difficult to pick up viruses to transmit.
In a typical meal, a mosquito eats just a
thousandth to a hundredth of a milliliter out of the average person’s 5.5
liters of blood. That’s like drinking a two-liter soda bottle of water
out of an Olympic-sized pool.
From its tiny snack, the mosquito has hardly a chance of ingesting
HIV. While the amount of the virus in blood varies from a few dozen to several
hundred thousand viruses per milliliter, usually the levels are low. Blood left
on the sloppy mosquito’s mouth is highly unlikely to have any HIV in it. If the
mosquito bit someone with 1,000 viruses per milliliter, for example, there
would be a 1 in 10 million chance of injecting just one virus body into
another victim.
By now, scientists have a clear understanding of the ways HIV is spread, and insects are not one of them. With HIV’s estimated
annual cost of around
$20 billion and immeasurable effects on its victims, we’re lucky that the pesky
mosquito’s bite isn’t another weapon in the disease’s arsenal.
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