Evil, blood-sucking pests,
mosquitoes are responsible for more human misery, disease, and death than any
other insect. Yellow fever, malaria,
dengue, encephalitis, and now West Nile virus are all transmitted by
mosquitoes. As someone who has suffered
a bout of malaria, I would shed no tears if mosquitoes were an endangered
species, but alas, they are thriving.
(To be fair, mosquitoes are an important source of food for bats and
birds, but we are not discussing that virtue today.)
Texas is currently the epicenter
of a small epidemic of West Nile disease; more than 1200 confirmed cases and 50
deaths in Texas this year. Forty percent
of all the cases in the US have been in Texas.
Mindy Powell, a registered
nurse, told me about her 80 year old relative in the Dallas area who recently
died of the most severe form of West Nile, called neuroinvasive because it
attacks the brain. One day this active
outdoor enthusiast and fisherman thought he might be getting a cold, but rapidly
worsened. He called his daughter to come
over. By the time she arrived, her
father was unable to walk. At the emergency
room he became confused and lost consciousness.
After three weeks in the ICU on a ventilator, he passed away.
Dr. Virginia Headley,
epidemiologist with the Williamson County and Cities Health District (WCCHD), stated
that six people in Williamson County have had the neuroinvasive form of West
Nile this year. Of those six, one died
and two formerly independent elderly adults are in nursing homes recovering. More commonly, people infected with West Nile
virus get a fever and other symptoms, but the brain is not affected, and they
eventually recover.
For every person who is
diagnosed with West Nile fever, at least 4 others are infected but never know
it because they have no symptoms. We
know this because donated blood from healthy people is checked for antibodies
to the virus. Between 2003 and 2008, the
CDC analyzed all the donated blood in the US and discovered that about 1% of
the population had already been infected.
A study in Houston found that among homeless men who had lived outside
for a year, more than 16% had developed antibodies to West Nile. Once you have had the virus and recovered,
you are probably immune, but it might be too soon to know if the immunity lasts
for life.
So what can we do to control
this situation? Pesticides can be
sprayed in an emergency, but pesticides only knock down the adult mosquito
population temporarily and are not practical in rural areas. Pesticides don’t get at the root cause which
is too many nice breeding places for mosquitoes. As Dr. Chip Riggins, executive director at
the WCCHD, explained, “Aerial spraying is an adjunct to other things. You can spray all day long and if you don’t
have the public helping, it won’t work for long.”
Last week’s rain provided
lots and lots of stagnant water to breed mosquitoes. Some kinds of mosquitoes can go from an egg
to a biting adult within 5 days, especially in warm weather, so we will start
to see mosquitoes now from the recent rains.
I decided to take a trip
around my house to see if I could find any breeding areas, supposing of course
that I would not, because who wants to think that her own home is a public
health hazard. Immediately upon stepping
out, I saw that the birdbath was teeming with wiggly mosquito larvae. Disgusted, I dumped it out. Next I found a bucket with water in it and a
saucer under a plant.
Deborah Marlow, deputy
director of environmental health services, has been interested in ecological
methods to control pests since her college days. Deborah studied mosquitofish, a minnow also
known as Gambusia affinis, as predators of mosquito larvae. A Gambusia minnow can eat its body weight in
larvae every day. A dozen mosquitofish
can keep an abandoned swimming pool mosquito free for months. My friends Bob and Janine Hall also use
mosquitofish in their aquaponics gardens and horse troughs. Janine said she bought the minnows at
McIntire’s nursery several years ago and doesn’t even have to feed them. They not only live off the mosquito larvae
but they reproduce prolifically. Whenever
the Halls build a new aquaponics bed, Janine just dips some more mosquitofish
out of the horse trough. The mosquitofish
are not sensitive about water quality either.
The only time the minnows ever died was the year it stayed below
freezing for three days straight and the horse trough froze into a solid block
of ice.
Mosquito dunks are another
way to control larvae in standing water that cannot be drained. The dunks contain spores of Bacillus
thuringiensis israelensis, called Bt for short.
When the spores are eaten by a mosquito larva, a toxin paralyzes the
larva’s intestine, causing it to die of starvation or septicemia. Luckily, birds, fish, and animals are
completely resistant to the toxin in Bt.
In 1959, an intrepid group of volunteers ingested a gram of Bt
daily. Some of the volunteers inhaled an
additional dose as well. After five days
of this regimen all the volunteers were completely unaffected. I don’t think you would be allowed to do an
experiment like that these days.
Mosquitofish in a pond at McIntire's Garden Center
Clint Hawes at The Feed Store demonstrating mosquito dunks
Bill Stump checking a gutter for standing water
Tips for Avoiding West Nile
Infections (from DSHS and CDC websites)
1. Use insect repellent containing DEET,
picaridin, or oil of lemon eucalyptus.
2. Dress in long sleeves and long pants when you
are outside.
3. Stay indoors at dusk and dawn, when mosquitoes
are most active.
4. Drain standing water where mosquitoes
breed. Common breeding sites include old
tires, flower pots, and clogged rain gutters.
Toys, buckets, and tarps that aren’t pulled taut can also collect water.
5. Make sure mosquitoes cannot enter rain
barrels. Use mosquito dunks or Gambusia minnows
in water features and ponds.
6. Install or repair screens on open windows.
7. Remove brush and debris where adult
mosquitoes hide.