West Nile Virus Fact Sheet

ORIGINS:  West Nile virus is a type of virus that has been commonly found in humans, birds, and other animals in Africa, Eastern Europe, and the Middle East.  Until 1999 it had not previously been detected in the United States.

TRANSMISSION:  West Nile virus exists in nature through a transmission cycle involving mosquitoes and birds.  Mosquitoes become infected with West Nile virus when they feed on infected birds, which may carry the virus in their blood for a few days.  Infected mosquitoes can then transmit West Nile virus to humans and animals when biting to take a blood meal.

SYMPTOMOLOGY:  Most individuals infected with West Nile virus will not have any symptoms or signs of illness.  People who do develop illness may experience mild symptoms such as fever, headache, and body aches; occasionally a skin rash and swollen lymph glands may be noticed.  These symptoms generally appear 3 - 15 days following the bite of an infected mosquito.  Less than 1% of persons infected with the virus will develop more severe disease with symptoms such as high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and rarely, death.  People who are over 50 years of age have the highest risk of developing severe disease.

RISK REDUCTION:  Maryland citizens can reduce their risk of becoming infected with West Nile virus by taking the following steps to avoid mosquitoes:

1.  Staying indoors at dawn or early in the evening.
2.  Wearing long-sleeved shirts and long pants when going outdoors.
3.  Spraying clothing with repellents containing permethrin or DEET, since mosquitoes may bite through thin clothing.
4.  Apploying insect repellent sparingly to exposed skin and following package instructions.  An effective repellent will contain 30% DEET (N, N-diethylmeta-toluamide).  Higher concentrations of DEET do not provide additional protection.  Contact your local health care provider before applying DEET products to children.

PRECAUTIONS:  Mosquitoes can breed in a as little as 1/4 inch of water.  In addition to personal protective measures listed above,  Maryland citizens should take the following precautions to reduce mosquito-breeding sites:

• Remove all discarded tires from your property.
• Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers.
• Make sure roof gutters drain properly.  Clean clogged gutters in the spring and fall.
• Clean and chlorinate swimming pools, outdoor saunas, and hot tubs.  If not in use, keep empty and covered.
• Drain water from pool covers.
• Change the water in birdbaths at least once a week.
• Turn over plastic wading pools and wheelbarrows when not in use.
• Eliminate any standing water that collects on your property.
• Remind or help neighbors to eliminate breeding sites on their property.
• Check for trapped water in plastic or canvas tarps used to cover boats, pools, etc.  Arrange the tarp to allow water to drain.
• Pump out bilges in boats.  Store canoes and small boats upside down.
• Remove outdoor pet food and water dishes that are not being used.
• Flush livestock water troughs twice a week.
• Do not leave garbage can lids lying upside down.  Be sure water does not collect in the bottom of garbage cans.
• Check around construction sites or do-it-yourself improvements to ensure that proper backfilling and grading prevent drainage problems.

**The Maryland Department of Health and Mental Hygiene is collaborating with the Maryland Department of Agriculture and the Maryland Department of Natural Resources to monitor bird, mosquito, horse, and human populations for evidence of West Nile virus and other arboviruses (those viruses transmitted by biting insects such as mosquitoes).