Harvard Health Publications

Threat level high for West Nile infection

Posted By Howard LeWine, M.D. On August 24, 2012

The United States is in the midst of the largest outbreak of West Nile virus since it was first discovered here in 1999, the Centers for Disease Control and Prevention announced this week. So far this year, more than 1,100 cases of human West Nile infection have been reported, about half of them in Texas, and at least 40 people have died from the virus. More cases are expected, since West Nile virus infections generally peak in late August and September.

“We’re in the midst of one of the largest West Nile outbreaks ever seen in the United States,” said Dr. Lyle Petersen, director of the CDC’s mosquito-borne illness programs, during a press conference to discuss the West Nile outbreak.

Experts can only speculate about the reasons for this dramatic increase in cases. It probably has some relation to the record warm temperatures this past winter and spring. And this was followed by a very hot early summer. The breeding cycles of mosquitoes speed up in hotter weather. Also the virus seems to replicate faster in warmer temperatures.

What is West Nile virus?

West Nile virus, first discovered in Uganda in 1937, has since spread to many parts of the world and almost every state in the U.S. The virus is transmitted to humans by mosquitoes. Mosquitoes pick it up by feeding on birds carrying the virus. Very rarely, West Nile virus infection occurs due to transfusion of infected blood. It can’t spread from human to human by casual contact.

Most people bitten by a mosquito carrying the West Nile virus won’t get sick. First, the virus has to get from the mosquito’s salivary gland into the person’s blood. Even then, 3 in 4 people fight off the virus. When symptoms do occur, they start three to 15 days after the mosquito bite. The symptoms almost always resemble a mild case of the flu, with fever, headache, and body aches.

Approximately 1 in 150 people who become infected with the West Nile virus develop a severe illness with

  • high fever
  • confusion
  • severe headache
  • stiff neck

People over age 50 and those with altered immune systems have a higher risk of developing severe problems from the virus.

Protect yourself

None of the antiviral drugs currently available kill West Nile virus. And no vaccine is available. In some communities where the West Nile virus is discovered, spraying programs are used to kill mosquitoes.

You can do two things to prevent infection: guard yourself and your family, and wage your own personal battle against mosquitoes:

  • Stay indoors when mosquitoes are most likely to bite – between dusk and dawn, but especially in the early evening.
  • Wear long pants and shirts with long sleeves to protect you from mosquito bites.
  • Make sure you have window and door screens to keep out mosquitoes.
  • Use insect repellant when you go outside. Those containing the active ingredients DEET or picaridin provide more effective and longer-lasting protection than other repellents. Check the insect repellent you will use to see how long it will protect you. Generally, the more active ingredient a repellent contains, the longer it can protect you from mosquito bites. Spray your clothing with repellents or with permethrin products for extra protection. (Read the label and follow the directions.)
  • Apply insect repellant on children. Don’t have them do it themselves. Choose the lowest concentration of DEET needed for the length of time you need protection.
  • Don’t use DEET products on infants less than 2 months old. Use mosquito netting to protect infants from mosquitoes.
  • Check around your home for standing water. Dump out any water where mosquitoes may breed.

You can read more about West Nile virus on the CDC’s West Nile and Stop Mosquitoes pages.

Related Information: Viruses and Infectious Diseases: Protecting yourself from…

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