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Although the West Nile virus mainly infects birds, it can be transmitted to humans and other animals by certain species of mosquitoes that take blood meals from both birds and humans. The bite from just one infected mosquito is all that is needed to transfer the disease. In humans, symptoms vary from no visible effect to flu-like symptoms, paralysis, or even death. All age groups are susceptible, but the elderly and those with weakened immune systems are more likely to experience severe forms of the disease.

Since 1999, more than 60 mosquito species in the United States have been found in West Nile virus-positive mosquito pools. The Asian Tiger Mosquito, increasingly observed on the East Coast and southern states, is known to be an aggressive daytime biter and carrier of West Nile Virus.

Why Mosquitoes Bite

Only female mosquitoes bite — they need blood to produce eggs. Mosquitoes are mainly attracted to carbon dioxide in breath, body heat, and sweat. Individuals who produce more body heat, sweat, carbon dioxide, and lactic acid will be more attractive to mosquitoes. Arboviruses such as West Nile virus, western equine encephalitis virus, and Saint Louis encephalitis virus are among the most important microorganisms that mosquitoes transmit in the United States.

Transmission

West Nile virus is transmitted by mosquitoes from infected birds to other hosts such as humans or horses. The West Nile virus has not been shown to be transmitted from human to human, horses to humans, or horses to horses.

Personal Protection

Insect repellents are the best way to protect against mosquito bites:

• DEET and Picaridin — applied to skin; repel mosquitoes
• Permethrin — applied to clothing; kills mosquitoes on contact
• Using both skin repellent and permethrin together is more effective than either alone
• Alternative repellents (soybean oil, geranium oil, lemon eucalyptus combinations) repel mosquitoes but only for short periods

Always read and follow all label directions before using any repellent.

Home and Community Protection

• Mosquitoes lay eggs in standing water — eliminate items around your home that hold water
• Repair broken screens and windows
• Support your community mosquito control programs

Resources

• CDC West Nile Virus Maps and Data by Nation/State/County
• University of Florida — Featured Creatures (mosquito identification)
• University of Maryland — Controlling Mosquitoes
• Tracking Positive Mosquito Pools in the United States