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.
The report, 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 includes illnesses reported in U.S. states and territories. During that period, 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.
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 the number reported in the CDC analysis. In 2016, the number of Lyme disease incidents 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.
The increase in disease cases caused by the bite of an infected mosquito, tick or flea in the United States is the result of many factors. Mosquitoes and ticks and the germs they spread are increasing in number and moving into new areas. West Nile virus, for example, was introduced to New York City in 1999 and then spread across the country. As a result of these increases, more people are at risk for infection. Overseas travel and commerce are also increasingly common, and someone infected with a mosquito-borne virus such as Zika in one country can unknowingly transport it home.
Climate change, which experts say can exacerbate many public health threats, also plays an important role, allowing mosquitoes and ticks to thrive in warmer temperatures, said Lyle Petersen, director of the CDC's Division of Vector-Borne Diseases, which produced the report. Warmer temperatures tend to make mosquitoes get infected faster and also more infectious, he said in an interview. “The amount of virus in the mosquito increases, and when it bites you, more virus gets into you and the chances of you getting infected and becoming sick goes up,” said Petersen, who has gotten sick from West Nile virus.
For ticks, the higher temperatures have allowed them to spread into new areas, allowing them to live farther north, and that puts more people at risk, he said. What's more, he said, “when the tick season is longer, people are exposed over longer periods.”
But other factors also play a role. The number of Lyme disease cases has increased rapidly because the population of deer — and deer ticks — has skyrocketed. “In New England 100 years ago, there were hardly any trees because they were all cut down for farmland, and the deer had been hunted to extinction,” he said. “All those forests have grown back, the deer population exploded, and the tick populations have also exploded, bringing more disease.”
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.
“The presence of vectors with proven or possible capacity to transmit a wide range of pathogens leaves the United States susceptible to outbreaks of exotic vector-borne diseases,” the report said.
Tick-borne diseases account for more than 75 percent of the reports and occur throughout the continental United States, but they are predominantly in the eastern part of the country and in areas along the Pacific Coast. Diseases spread by mosquitoes, such as dengue fever, chikungunya and Zika, 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, where it is the major mosquito-borne disease.
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 were also travel-related Zika cases and limited local spread of dengue and Zika viruses in Florida and Texas. The total number of reported Zika cases in the U.S. states and territories in 2016 was 41,680, accounting for the upsurge in disease cases from all vectors for that year.
The responsibility for detecting and responding to diseases spread by vectors such as mosquitoes and ticks is almost all funded locally and operated by local and state health departments. But their resources have been greatly reduced over the years. More than 80 percent of 1,083 local mosquito control organizations in a recent national survey reported some basic deficiencies.
But on a more basic level, Petersen said, more innovative ways are needed to control ticks and mosquitoes. When the first local spread of Zika in the continental United States was reported in Florida in 2016, authorities discovered that the Aedes aegypti mosquitoes that spread the virus there were “resistant to the most common pesticides that we have,” Petersen said.
In some ways, the Zika outbreak allowed state and local authorities to increase their capacity to fight these threats. But much more needs to be done, he said. “This is a long-term problem that's going to getting worse, and it requires a sustained response over time.”