It has been a bad West Nile virus (WNV) season, the worst
since the virus arrived on the continent in 1999, and it isn’t over yet. But
why? Why Texas, and even more curious, why South Dakota? WNV is a mosquito-born
virus, but so is malaria and we don’t have malaria in the US anymore.
To understand the epidemiology of WNV we have to look at
nature. It turns out that humans are “dead end” hosts, meaning we aren’t the
intended target. That’s because we don’t sustain a significant viremia (meaning
there is lots of virus circulating in the blood as happens with malaria for
instance). Mosquitoes don’t pick up virus from us to transmit it to others. WNV
is a virus adapted for birds and the normal cycle is that an infected mosquito
bites a bird. The bird becomes viremic, other mosquitoes bite the bird and
perpetuate the cycle. This is how the virus “amplifies” over the summer months. Most of these mosquitoes specifically feed on birds and don’t
bite humans, but there are crossover mosquito species, omnivores if you will,
with a taste for bird and human blood. They come into the picture late in the
season when the cycle has amplified the virus to infect many birds. Crossover
occurs, the omnivores bite birds, pick up the virus then bite humans. This is why we see most of the cases of
WNV in late August and September. Amplification takes time.
Believe it or not WNV mostly causes an inapparent human
infection. Only two out of ten get sick, and most of those get a flu-like
illness. About one in a hundred get the more serious version, called
“neuroinvasive disease.” This is a brain infection that causes weakness,
confusion, coma and even death.
Age is a risk factor with 55 years seemingly where the increased risk
begins, but anyone with a health condition that impairs the immune system
should consider themselves at risk and take precautions when outside during
mosquito season.
Mosquito control includes the use of larvacides-chemicals
that kill the non-flying stage of mosquito – early in the season and
adulticides throughout the season. Adulticides are chemicals that kill the flying
adults. We all remember spraying from
when we were kids and chased the fogger down the block on our bicycles (well,
some of us remember.)
So what’s a person to do? If you own a home make sure there
are no items in your yard that could collect water and serve as a breeding
sites for mosquitoes. Ensure that all windows are secure and screens intact.
Don’t leave your door open. Peak mosquito biting hours are dusk and dawn but also
anytime in between. Permethrin-based repellents can be applied to clothing but
not skin. Long sleeved shirts and
pants, if made of thick material can deter bites, but it has been really hot
throughout most of the US this summer.
The best put-on-the-skin repellents are DEET and Picaridin and the
duration of effectiveness depends on the concentration. DEET can be toxic to
small kids, so apply with caution. Oil of lemon eucalyptus is another
alternative. CDC’s advice on mosquito repellent use is here:
http://www.cdc.gov/ncidod/dvbid/westnile/qa/insect_repellent.htm
Back to why is Texas and South Dakota having so much West
Nile? The answer is nature. The right combination of rain and sun increases
food supply for birds. If there are more birds, there is more food for
mosquitoes and then more mosquitoes. Amplification of the virus is increased. The weather and rain also impact on mosquito breeding and hot
weather means a higher amount of WNV in mosquitoes. If there are a lot of juvenile birds of
the right species, birds that haven’t had been infected with the virus before,
then that is the perfect storm. I haven’t been to Texas or South Dakota but I
bet there has been an increase in young, naïve birds capable of sustaining the
WNV and a bumper crop of all mosquitoes, including the crossover biting ones.
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