The incidence of West Nile Virus (WNV), which is spread by the bites of mosquitoes, increases through the late summer months. According to statistics from the Centers for Disease Control and Prevention (CDC), reports of infection follow an annual pattern in which the number of cases increases each month from July through September.

This year, cases reported through August 14 totaled 444, up from just 15 cases reported through mid-July.

Cases of WNV infection rise during these months because mosquitoes are most abundant during this time of year. Mosquito populations can be influenced by summer weather patterns, so unusually wet or dry conditions in different parts of the country may have an effect on the number of cases counted. However, any wet environment such as areas around lakes, streams, ponds and even swimming pools can experience an increase the number of mosquitoes and the prevalence of the disease. Since first documented in 1999, WNV has been reported in every state in the U.S. except Hawaii and Alaska as well as in Canada. So far this year, cases of the disease seem to be concentrated in the West and South and not in the eastern half of the country.

It should be noted that, historically, statistics for WNV infections tend to be incomplete and actual numbers of cases may be much higher. Many are never reported because most people who are infected with WNV have no symptoms and are never tested. When symptoms are present, they are most often mild and resemble the flu. Signs of infection may include headache, fever, chills, nausea, vomiting, body aches and/or rash. Less than 1% of all WNV cases cause serious brain infections that can be fatal.

Health officials agree that the best way to protect yourself from becoming infected is to avoid mosquito bites by staying way from areas with high mosquito populations and by taking a few simple precautions:

Use mosquito repellent and wear light-colored clothing with long sleeves and pants when spending time outdoors.

Avoid the outdoors during dusk and dawn when mosquitoes are most active.

Ensure that window screens fit properly and have no holes.

Clear any standing water from your property. Change the water often in bird baths, flower pots and pet water dishes. Make sure that pools and ponds have circulating water sources.

For more information and current statistics, visit the West Nile Virus page on the CDC website.

Sources

S1
Centers for Disease Control Morbidity and Mortality Weekly Reports (MMWR). All accessed August 2007:

Current volume, July 27, 2007 / Vol. 56 / No. 29 West Nile Virus Update --- United States, January 1--July 24, 2007. See http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5629a4.htm

Current volume, August 17, 2007 / Vol. 56 / No. 32 West Nile Virus Update --- United States, January 1--August 14, 2007. See http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5632a4.htm

Past volume, July 21, 2006 / Vol. 55 / No. 28 West Nile Virus Activity --- United States, January 1--July 18, 2006. See http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5528a4.htm

Past volume, August 18, 2006 / Vol. 55 / No. 32 West Nile Virus Activity---United States, January 1—August 15, 2006. See http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5532a3.htm

S2
CDC Division of Vector-Borne Infectious Diseases. West Nile Virus, Statistics, Surveillance and Control, 2007 West Nile Virus Activity in the United States (Online information, accessed August 2007). See http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount07_detailed.htm

S3
Center for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases. West Nile Virus: What You Need to Know, CDC Factsheet (Online information, accessed August 2007). See http: www.cdc.gov ncidod dvbid westnile wnv_factsheet.htm

S4
Capital Health. Edmonton area, 2007 Features. Tips to Avoid West Nile Virus (Online information, accessed August 2007).