Saturday, September 22, 2012

West Nile Virus and how to avoid it:  published in the Sun Sept. 22, 2012

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.


1 comment:

  1. Mosquito dunks and mosquito fish... I was about to put a lid on the aquaponics system we have here, but it occurs to me that's probably a waste of time, since it wouldn't be very well sealed.