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The number of vector-borne illnesses in the U.S. tripled from 2004 to 2016, rising to nearly 650,000 cases, CDC researchers reported.

Cases of tick-borne disease more than doubled, and accounted for more than three-quarters of all vector-borne disease, reported Ronald Rosenberg, ScD, of the CDC in Atlanta, and colleagues.

Also, Lyme disease accounted for 82% of tick-borne diseases during this time period, they wrote in "," in the Morbidity and Mortality Weekly Report.

Tick-borne diseases in particular are not reported or recognized, so it is difficult to estimate their cost and burden, noted Lyle Petersen, MD, director, vector-borne diseases at the CDC, on a media call. He pointed out that the Lyme disease burden is 300,000, which is 10 times higher than what is nationally reported.

"Lyme patients are treated in an outpatient setting, so outpatient disease is underreported to a certain extent," Petersen said, but added that even though all the cases are not reported, it's not necessary that every case is counted.

"In a rainstorm, you don't have to count every drop to know how much rain there actually is," he stated.

Tick-borne diseases and mosquito-borne diseases also seem to follow different patterns. While Petersen noted that tick-borne diseases are going up steadily, with an increase in cases every single year, he described mosquito-borne diseases as more "episodic," going up and down over time.

West Nile virus was the most common form of mosquito-borne illness, Petersen said, but there has been a recent "accelerating trend" of mosquito-borne disease being introduced to the U.S., with West Nile in 1999, chikungunya in 2014, and Zika in 2016.

"Mosquito-borne diseases are sensitive to how we travel and how we live. They are spread by movements of people or animals or vectors, and with expanding global travel and trade, all diseases are basically a plane flight away," he said.

But Petersen pointed out that surveillance for these types of illnesses is at the state level, and that states control their own surveillance systems. He highlighted the 1,900 vector control organizations, and said that improving them requires "a long-term sustainable effort."

He cited a 2017 survey that found that "84% of vector control organizations report needing improvement in one or more of five core competencies," adding that they are looking to expand the number of prevention and control programs, as well as looking to rebuild comprehensive vector control programs.

"Zika funding helped prepare us for these types of diseases, but more funding is necessary," he said.

Rosenberg's group examined data from the for 16 notifiable vector-borne diseases. Overall, there were 642,602 cases reported, with the number of tick-borne "bacterial and protozoan diseases" rising from >22,000 in 2004 to >48,000 in 2016.

The authors also noted there were nine vector-borne human diseases that were reported for the first time in the U.S. from 2004 to 2016.

Though Petersen stopped short of saying that climate change was responsible for an increase in these types of illnesses, he did acknowledge that "temperature is important" for vector-borne diseases. Warmer weather has not only moved the tick population further north and expanded tick season, but hotter temperatures "make mosquitoes more infectious," which promotes outbreaks, he said.

CDC Director Robert Redfield, MD, acknowledged the "growing burden" of these vector-borne illnesses, but he expressed optimism about the ability of state and local health departments, vector control organizations, and industry to help protect the public from these types of threats.

"Our nation is better prepared today than we were a year ago," Redfield said. "We have improvements in vector control, better diagnostics and research, and better methods of treating and preventing human infection. It sounds ambitious, but I would encourage everyone not to underestimate the possible."

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