Mosquito and Tick Diseases a Large and Growing Threat in US

Megan Brooks

May 01, 2018

Over the last 13 years, illnesses from mosquito, tick, and flea bites have tripled in the United States and nine new vector-borne human diseases were discovered or introduced, according to a report released today by the Centers for Disease Control and Prevention (CDC).

"To effectively reduce transmission and respond to outbreaks will require major national improvement of surveillance, diagnostics, reporting, and vector control, as well as new tools, including vaccines," Lyle Petersen, MD, MPH, director, Division of Vector-Borne Diseases in the CDC's National Center for Emerging and Zoonotic Infectious Diseases, and colleagues wrote in their Vital Signs report published in the Mortality and Morbidity Weekly Report.  

"The data show that we're seeing a steady increase and spread of tickborne diseases, and an accelerating trend of mosquitoborne diseases introduced from other parts of the world. We need to support state and local health agencies responsible for detecting and responding to these diseases and controlling the mosquitoes, ticks, and fleas that spread them," Petersen added in a news release.

"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 US in recent years, making a lot of people sick. And we don't know what will threaten Americans next," CDC Director Robert Redfield, MD, said in the release. "Our Nation's first lines of defense are state and local health departments and vector control organizations, and we must continue to enhance our investment in their ability to fight against these diseases."

Upward Trend

CDC scientists analyzed trends in the occurrence of 16 nationally reportable vector-borne diseases during 2004–2016 using data reported to the National Notifiable Diseases Surveillance System.

During the study period, a total of 642,602 cases of vector-borne illness were reported, although cases are likely to be substantially underreported, the authors note. The total number of reported vector-borne disease cases was three times higher in 2016 (n = 96,075) than in 2004 (n = 27,388).

The number of reported cases of tick-borne disease more than doubled during the study period, from 22,527 in 2004 to 48,610 in 2016. Tick-borne diseases made up 77% of all vector-borne disease case reports. Lyme disease accounted for 82% of all tick-borne cases, but cases of spotted fever rickettsioses, babesiosis, and anaplasmosis/ehrlichiosis also increased.

Reported cases of mosquito-borne disease jumped from 4858 in 2004 to 47,461 in 2016. The most common mosquito-borne viruses were West Nile, dengue, and Zika. While rare, plague was the most common disease resulting from the bite of an infected flea.

Petersen said it's important to note that many infections are not reported or recognized, so it's hard to truly estimate the overall burden. "For example, we know that the number of Lyme disease cases that actually occur each year are approximately 300,000, or 10 times higher than what is nationally reported," he said during a media briefing.

Blame It on Climate Change?

During the study period, tick-borne diseases were seen largely in the eastern continental US and areas along the Pacific coast, although the area at risk for Lyme disease has been expanding.  Mosquito-borne dengue, chikungunya, and Zika viruses were almost exclusively transmitted in Puerto Rico, American Samoa, and the US Virgin Islands, where they were periodically epidemic. West Nile virus, also occasionally epidemic, was widely distributed in the continental United States, where it is the major mosquito-borne disease.

Redfield said the increase vector-borne diseases in the United States is likely due to many factors. "Mosquitos and ticks are moving into new areas nationwide. Overseas travel and commerce are more common than ever before. A traveler can get infected from a mosquito-borne disease like Zika in one country and then return home now infected, which puts other people at risk," he said at the briefing.

Climate change may also be a factor. "The mosquito-borne diseases tend to get worse during heat waves," said Petersen. For example, the major outbreaks of West Nile virus in the United States all happened during heatwaves. "Increasing temperatures tend to make mosquitoes more infectious and infectious faster, thus promoting outbreaks. For tick-borne diseases, increasing temperatures will tend to expand the range of these ticks further north as well as increase the length of the tick season, Petersen explained.

The report also notes that nine vector-borne human diseases were reported for the first time in the United States and its territories during the 13-year period. "The discovery or introduction of novel vectorborne agents will be a continuing threat," the authors note, as vector-borne diseases have been tough to prevent and control.

"These data indicate a persistent, locality-specific risks and a rising threat from emerging vectorborne disease, which have increasingly encumbered local and state health departments tasked with preventing, detecting, reporting, and controlling them," they conclude.

"Preventing and responding to vectorborne disease outbreaks are high priorities for CDC and will require additional capacity at state and local levels for tracking, diagnosing, and reporting cases; controlling vectors; and preventing transmission," they add.

IDSA Takes Action

In a statement, Paul Auwaerter, MD, president of the Infectious Diseases Society of America (IDSA), said IDSA is taking steps to address this "serious issue" by urging Congress to provide "increased funding for surveillance and prevention of vector-borne diseases, including resources to support research on the most effective methods for preventing tick-borne infections."

"We are also investigating how climate change may impact the spread of vector-borne diseases so that we can take appropriate actions to protect public health. We also advocate for federal investments in the research and development of new vaccines to prevent Zika, Lyme disease, and other serious vector-borne diseases," said Auwaerter.

"We support boosting resources for research and development leading to improved diagnostics for Lyme disease and other vectorborne diseases by the National Institutes of Health and the Biomedical Advanced Research and Development Authority. We also support funding for research to understand better how to safely and effectively treat symptoms that persist in some patients following initial antibiotic treatment for Lyme disease," said Auwaerter.

MMWR Morb Mortal Wkly Rep. Published online May 1, 2018. Full text

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