What Scares Bug
Experts?
The horrible disease that experts feared would
come to the United States has come to the United States.
If your
wife is a researcher in medical entomology, you’ll often hear odd tidbits
related to mosquito-borne diseases. For instance, did you know how cute malaria
parasites can look under a microscope? I didn’t either, until I met Cassandra
Urquhart. (Some other things I’ve heard described as “cute” since then include,
but are not limited to: cockroaches, nematodes, spiders, earwigs, and male
mosquitoes.) She’s fascinated by her own work with La Crosse virus, excited by
new papers on dengue fever, and interested in how many of the mosquitoes she’s
collected at sites around Knox County, Tennessee will test positive for West
Nile virus. In her spare time, she reads books on the history of yellow fever
and Chagas disease for fun. Don’t get me wrong—she cares about the human
toll of such diseases. But as a scientist, she’s usually more curious than
alarmed about them. However, when it comes to chikungunya virus, my cheerfully
bug-obsessed wife gets far more serious—and so do many entomologists. So why is
chikungunya different?
Chikungunya
virus seems, at first, to have a lot in common with dengue virus, another
mosquito-borne pathogen. Both cause extremely painful diseases—chikungunya’s
name comes from a Makonde word meaning “that which bends up,” referring to the
contortions sufferers put themselves through due to intense joint pain.
Dengue’s nickname is breakbone fever. Both
viruses are primarily transmitted by the Aedes aegypti mosquito, and
both have been moving slowly closer to the United States over the past decades,
with local cases of dengue fever already found in Florida and Texas.
Last
week the Centers for Disease Control and Prevention announced the first locally acquired
cases of chikungunya in the United States. A woman in Miami-Dade
County and a man in Palm Beach County, neither of whom had left the country
recently, both came down with the dreaded disease.
Chikungunya
doesn’t need a reservoir—it can be spread directly from one human host to another.
To
understand why it’s chikungunya, not dengue, that makes entomologists so
nervous, you’ll need to know a bit about another mosquito; Aedes albopictus
(more pronouncably known as the Asian tiger mosquito). The Asian tiger mosquito
is an invasive species that has spread over much of the eastern half of the
United States since its introduction in 1985. These back-and-white-striped
jerks are capable of spreading all sorts of diseases, including West Nile,
dengue, and yellow fever. However, they often do so pretty inefficiently, with
viruses found in only the tiniest minority of the mosquitoes tested. In the
case of chikungunya, however, at least one strain has been shown to spread as easily in tiger mosquitoes as in Aedes
aegypti. Ae. aegypti (which is also, confusingly, tiger-striped) is
mostly confined to the Gulf Coast. It’s common in parts of Florida, Texas, and
Louisiana. In most other places, however, Asian tiger mosquitoes have driven
down their numbers through competition—good
news for anyone concerned about dengue or yellow fever, but very bad news when
it comes to chikungunya.
Adding
to the reasons for alarm is the fact that chikungunya doesn’t need a
reservoir—it can be spread directly from one human host to another. This is in
contrast with several other mosquito-borne pathogens, including West Nile
virus, which needs to replicate inside a bird before it can pass from a
mosquito to a human. The special characteristics of tiger mosquitoes once again
exacerbate the problem—these particular mosquitoes prefer feeding off of, and
living close to, humans. (Many mosquitoes, in contrast, feed opportunistically
on humans, while primarily targeting other animals.) Tiger mosquitoes are also
daytime feeders, which means that while other species are taking a break,
preferring to feed at dawn or twilight, the tigers keep chomping during the
times of day when humans are most active.
Until
last week, the only cases of chikungunya found in the United States had been in
travelers, generally those returning from the Caribbean, where several
outbreaks are currently raging. Before we got confirmation last week, I asked
my wife’s adviser, Rebecca Trout Fryxell of the University of Tennessee
Institute of Agriculture, just how likely she thought it was that chikungunya
would eventually make it to the United States. “I think it’s inevitable” was
her reply. “Entomologists have been watching it since the introduction of Aedes
albopictus.”
Of
several strains of the virus, only one has been identified that is easily
transmitted by the Asian tiger mosquito. This strain, the one with the most
potential to spread far and wide in the United States, has been found in only
one place so far, the French island of RĂ©union in the Indian Ocean. This means
the virus (in the form of its other strains) may remain confined for now to
pockets of the southernmost United States where Aedes aegypti is common.
But there’s no reason to think that the situation will stay that way.
Chikungunya has shown its potential to spread more widely, and with no vaccine
and no treatment, that risk is likely to grow.
Of
course, that doesn’t mean that there is nothing we can do. According to Trout
Fryxell, personal awareness remains the most effective tool we have for
combating this disease. Travelers who think they may have contracted
chikungunya should not delay in seeking medical attention. Adults should take
measures to protect themselves and their families from mosquito bites by
checking window screens for holes, wearing effective bug spray when
outside, making sure kids apply and reapply bug spray regularly, and checking
for and eliminating any sources of standing water on their property to prevent
mosquitoes from finding human-adjacent breeding sites. (And be thorough—my wife
informs me that the larvae of Aedes mosquitoes can survive in less than
a tablespoon of water.) Visit the Control page of the American Mosquito
Control Association to learn more about the most effective methods of
prevention.
Above
all, if you’ve recently returned from the Caribbean or are living in the
Miami-Dade area and you’re feeling a bit feverish (or if you’re in so much pain
that you feel compelled to contort yourself into bizarre positions), do not
get bitten by any tiger-striped mosquitoes! Seriously, I know you’re
feeling terrible, I know you’re wondering if the pain will ever stop, and if
it’s worth even continuing to live, but spare a thought for the rest of us and
avoid mosquitoes altogether, will you?
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