May 1, 2018 -- Diseases transmitted by ticks, fleas, and mosquitoes are becoming more dangerous and far more numerous: Between 2004 and 2016, they tripled to nearly 650,000 cases, a new CDC report finds.

The report published Tuesday says there are likely many more cases that go unreported or unrecognized.

New CDC Director Robert R. Redfield, MD, said the growing public health problem will only worsen without “major improvement” in how local, state, and federal levels work together to track, report, and control the diseases.

“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,” Redfield said.

Ticks, which transmit most of the diseases spread by organisms, are gaining ground in the United States. Ticks transmit Lyme infection and Rocky Mountain spotted fever, which appear to be on the rise, along with newer and deadlier infections, including Heartland and Bourbon viruses. The CDC has identified seven new germs transmitted by ticks.

Mosquitoes are a growing threat, too, causing periodic epidemics of dengue, chikungunya, West Nile, and Zika. These diseases are spread as mosquitoes expand into new territory and are brought home by travelers. Unlike tick-borne diseases, which are not infectious, mosquito-borne diseases are.

Warm weather brings out mosquitoes and triggers the northward movement of ticks, which are most prevalent in the Northeast and Upper Midwest.

These diseases are found throughout the U.S. and are resistant to control. Only one vaccine is available against yellow fever, a mosquito-borne disease that hasn’t been seen in the U.S. but could show up again, according to the CDC report.

Getting control of these tiny creatures is a challenge. And because they are carried by wild animals like rodents and birds, it is difficult if not impossible to eliminate them.

The blacklegged or deer tick is responsible for Lyme disease, which makes up more than 80% of tick-borne disease, along with anaplasmosis, babesiosis and Powassan virus. The tick’s range is expanding south and westward, although it is still concentrated in the Northeast and Upper Midwest.

The diseases transmitted by ticks in 2016, the most recent year for which statistics are available, and the numbers of cases are:

  • Lyme disease: 36,429 (experts believe the annual number is around 300,000, based on surveillance)
  • Anaplasmosis/ehrlichiosis: 5,750
  • Spotted fever rickettsiosis (Rocky Mountain spotted fever): 4,269,
  • Babesiosis: 1,910
  • Tularemia: 230
  • Powassan virus: 22

Ehrlichiosis, which is spread by the lone star tick, is grouped with anaplasmosis as the second most common tick-transmitted disease in the U.S. Without prompt treatment, both diseases can kill.

Babesiosis is a life-threatening parasitic disease, and Powassan virus is a tick-borne disease that does not respond to antibiotics.

Heartland and Bourbon viruses, transmitted by ticks, are emerging diseases. Both have led to a handful of deaths, mainly in the Midwest.

Fleas pass along plague, which is also rare.

Rickettsial diseases, those marked by a spotted rash, are also becoming more common. They are also known as Rocky Mountain spotted fever and are transmitted by the American dog or brown dog tick, which is more common in the “tick belt” stretching across five states from Oklahoma to North Carolina. They are also being seen more in poorer areas of eastern Arizona, where free-roaming dogs are bringing the tick closer to humans.

CDC epidemiologist Paige Armstrong, MD, says she is concerned about the increase in tick-borne diseases. “The fact that these exist and they’re increasing is concerning to me.”

Better diagnostics and prompt treatment with antibiotics have kept the death toll down from tick-borne diseases. The CDC report notes that West Nile Virus outbreaks depend on mosquitoes feeding on infected birds and that despite the presence of the Aedes aegypti mosquito in 38 states, the incidence of Zika is low.

Lyle Petersen, MD, director of the CDC’s Division of Vector-Borne Diseases, said 84% of the 1,900 vector control groups working to prevent and control threats told the CDC in 2017 that they need better monitoring and pesticide-resistance testing.

Petersen said a $12.2 million increase in the CDC’s budget this year will go to vector control efforts in 9 states.

“By taking steps, we as a nation will become more resilient in the fight against these diseases,” he said.