The number of illnesses caused by mosquito, tick, and flea bites have tripled in the United States over the last 13 years, according to new data from the Centers for Disease Control and Prevention. According to the report, more than 640,000 cases were reported across the country from 2004 through 2016, though officials say the actual number of illnesses is likely much higher. 

During this time period, nine new germs spread by mosquitoes and ticks were either discovered or introduced into the U.S.

The number of reported diseases from ticks more than doubled during the study period and accounted for more than 60 percent of all reported cases.

The most common tick-borne diseases in the United States were Lyme disease and ehrlichiosis/anaplasmosis. West Nile, dengue, and Zika accounted for the most mosquito-borne infections. While rare, plague was the most common disease resulting from the bite of an infected flea.

"Zika, West Nile, Lyme, and chikungunya — a growing list of diseases caused by the bite of an infected mosquito, tick, or flea — have confronted the U.S. in recent years, making a lot of people sick. And we don't know what will threaten Americans next," said CDC Director Robert R. Redfield, M.D., said in a statement.

The findings, published in the CDC's Vital Signs report, were based on data reported to the National Notifiable Diseases Surveillance System from 2004 through 2016.

What's behind the rise?

In a press briefing on Tuesday, CDC officials said the reasons behind the rise are complex and varied.

Lyle Petersen, M.D., MPH, director of the CDC's division of vector-borne diseases and one of the study's authors, noted that continued globalization has played an important role, especially when it comes to mosquito-borne diseases.

"Expanding global travel and trade, all of these diseases are basically a plane flight away," he said.

This was the case during the 2016 Zika outbreak, which began in Brazil and spread to other parts of South and North America after mosquitoes bit infected travelers who unknowingly brought the virus home.

Peterson also noted that an increase in deer populations in residential areas can also bring an increase of deer ticks, which can lead to more cases of Lyme disease and other tick-borne infections. This has been a problem in many communities as suburban development moves into formerly rural areas.

During the briefing, Peterson repeatedly declined to connect the rise in tick and mosquito-borne illnesses to climate change – an issue that has remained politically fraught – but he did say higher temperatures are a factor.

"If you increase temperatures in general, tick populations move further north," expanding the range of people affected, Peterson said. Warmer temperatures also increase the length of tick season.

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