During the time covered in the study, nine new germs spread by the bites from infected mosquitoes and ticks were discovered or introduced in the United States, the report said.
The number of people who get diseases transmitted by mosquito, tick and flea bites has more than tripled in the United States in recent years, federal health officials reported Tuesday. Since 2004, at least nine such diseases have been newly discovered or introduced into the United States.
The warmer weather of spring and summer means the start of tick and mosquito season and the diseases they transmit, including Lyme disease, Rocky Mountain spotted fever, West Nile and Zika.
A new report from the Centers for Disease Control and Prevention, released Tuesday, shows that the number of reported cases of these diseases jumped from 27,388 cases in 2004 to more than 96,000 cases in 2016. The data include illnesses reported in the U.S. states and territories. During that period, a total of more than 640,000 cases of these diseases were reported to the CDC.
Officials say the actual number of people who have become sick is much higher, in part because many infections are not reported or recognized. Some patients may experience mild symptoms and not seek medical attention, and not all diseases were reported for the full 13-year analysis period or from all states and territories. The data “substantially underestimate disease occurrence,” the report said.
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For example, recent data from clinical and laboratory diagnoses estimate that Lyme disease infects about 300,000 Americans every year, which is eight to 10 times more than the number reported in the CDC analysis. In 2016, the number of Lyme diseases reported for the United States was 36,429.
As a group, these diseases in the United States are notable for their wide geographical distribution and resistance to control. Only one of the diseases, yellow fever, has a vaccine approved by the Food and Drug Administration.
For most of these diseases, the only way to stop outbreaks is through mosquito control, which is expensive and rarely effective. Miami, for instance, was the only city in the Western Hemisphere to stop a Zika outbreak with pesticides.
During the time covered in the study, nine new germs spread by the bites from infected mosquitoes and ticks were discovered or introduced in the United States, the report said.
“The pace of emergence of new or obscure vector-borne pathogens through introduction or belated recognition appears to be increasing,” the report said.
They include two previously unknown, life-threatening tick-borne viruses — Heartland and Bourbon — that were reported from the Midwest, and the chikungunya and Zika viruses transmitted by mosquitoes that were introduced to Puerto Rico in 2014 and 2015. In the United States, there was limited local spread of dengue and Zika viruses in Florida and Texas.
Environmental factors, such as rainfall and temperature, also affect the breeding and biting habits of these many different species. Most of the pathogens are transmitted to humans from animals, such as rodents or birds, “making them difficult or impossible to eliminate,” the report said.
Warmer weather is an important cause of the surge in cases reported to the Centers for Disease Control and Prevention, according to the lead author of a study in the agency’s Morbidity and Mortality Weekly Report.
But the author, Dr. Lyle R. Petersen, the agency’s director of vector-borne diseases, repeatedly declined to connect the increase to the politically fraught issue of climate change, and the report does not mention either climate change or global warming.
Many other factors are at work, he emphasized, while noting that “the numbers on some of these diseases have gone to astronomical levels.”
Ticks are thriving in regions previously too cold for them, and hot spells are triggering outbreaks of mosquito-borne diseases. Other factors, he said, include expanded human travel, suburban reforestation and a dearth of new vaccines to stop outbreaks.
In an interview, Petersen said he was “not under any pressure to say anything or not say anything” about climate change and that he had not been asked to keep mentions of it out of the study.
Tick-borne diseases account for more than 75 percent of the reports and occur throughout the continental United States, but are predominantly in the eastern part of the country and in areas along the Pacific Coast. Diseases spread by mosquitoes were almost exclusively transmitted in Puerto Rico, American Samoa, and the U.S. Virgin Islands. West Nile virus, also spread by mosquitoes, is widespread across the continental United States.
More jet travel from the tropics means that previously obscure viruses like dengue and Zika are moving long distances rapidly in human blood. (By contrast, malaria and yellow fever are thought to have reached the Americas on slave ships three centuries ago.)
A good example, Petersen said, was chikungunya, which causes joint pain so severe that it is called “bending-up disease.”
In late 2013, a Southeast Asian strain arrived on the Dutch Caribbean island of St. Maarten, its first appearance in this hemisphere. Within one year, local transmission had occurred everywhere in the Americas except Canada, Chile, Peru and Bolivia.
Ticks need deer or rodents as their main blood hosts, and those have increased as forests in suburbs have gotten thicker, deer hunting has waned, and rodent predators like foxes have disappeared.
Most disease outbreaks related to mosquitoes since 2004 have been in Puerto Rico, the Virgin Islands and American Samoa. But West Nile virus, which arrived in 1999, now appears unpredictably across the country.
The only flea-borne disease in the report is plague, the bacterium responsible for the medieval Black Death. It remains rare but persistent: Between two and 17 cases were reported from 2004 to 2016, mostly in the Southwest. The infection can be cured with antibiotics.