Home/ Specials / See How Ticks and Mosquitoes Are Carrying Diseases to More of the U.S.

Diseases are riding ticks and mosquitoes to new pastures.

Lyme disease and West Nile virus are among the ailments that ticks and mosquitoes are spreading in more of the U.S. Reported infections after tick, flea and mosquito bites have surged over the past two decades, according to data from the Centers for Disease Control and Prevention.

Changing land use and climate have allowed ticks to expand their habitat. Warming temperatures can encourage mosquito-borne outbreaks, and more global travel risks moving the bugs and their diseases to new areas. That can challenge doctors and communities unaccustomed to dealing with these diseases.

Lyme disease is the most common tick or mosquito–borne disease in the U.S. It is most prevalent in the Northeast and Midwest, where the blacklegged tick that carries Lyme-causing bacteria has expanded its range, leading to more infections.

West Nile virus and other mosquito-related infections spread cyclically but unpredictably, in part because of the mosquito’s short lifespan.

The virus first appeared in the U.S. in New York City in 1999 and reached most of the country within five years. It is now the leading cause of mosquito-borne illness in the continental U.S.

Why Lyme disease is spreading

The razing and regrowth of forest in the Northeast has facilitated a boom in the deer population, which have brought blacklegged ticks back with them. White-footed mice also host the Lyme-causing bacteria.

Building homes and infrastructure deeper into the woods can increase the chances people encounter ticks, and shorter, warmer winters are lengthening the time the ticks are active and allowing them to move north.

How West Nile spread across the U.S.

The spread of West Nile virus often relies on infected migrating birds. There also tend to be more outbreaks when it’s warmer, partly because the virus can replicate faster within the mosquitoes and make them infectious earlier.

Long sleeves, bug-spray, tick checks and showers after being outdoors help prevent tick and mosquito bites. People should also dump standing water and put air conditioners or screens in windows during the summer. If bitten, people should watch for rash, fever or body aches and consult a doctor if symptoms appear.

Write to Emma Brown at Emma.Brown@wsj.com, Josh Ulick at josh.ulick@wsj.com and Brianna Abbott at brianna.abbott@wsj.com

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