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In the summer and fall of 2021, West Nile virus spread rapidly through Arizona’s Maricopa County and other areas of the state.

The outbreak, with more than 1,700 cases reported and 127 deaths. was the largest in the United States since the mosquito-borne virus first emerged in this country in 1999. But with the nation facing a far larger public health crisis with the Covid-19 pandemic, it went almost unnoticed.

Even before Covid-19 arrived, the public health response to diseases transmitted to humans by vectors like fleas, ticks and mosquitoes — including West Nile, Zika, dengue fever, Lyme disease, and others — was muted, perhaps because the number of reported cases has been relatively low, and the public largely unaware of the health risks such diseases pose.


With climate change accelerating, however, shifting the ranges of many disease-carrying species and sharply increasing infections, scientists and others warn that the nation’s public officials, as well as hospitals and doctors, are underprepared for a potentially devastating surge in infections. Research on vector-borne diseases and disease surveillance, they note, are underfunded by federal and local governments, leaving the country vulnerable to outbreaks.

“Without sustained funding in local vector control and surveillance, it ends up stymieing that response of looking for the threats before they become really huge causes for concern for local public health,” said Chelsea Gridley-Smith, director of environmental health for the National Association of County and City Health Officials (NACCHO).

In the United States, cases of 17 different vector-borne diseases have been reported to the Centers for Disease Control and Prevention, and nine pathogens new to the country have been identified since 2004, according to a 2020 report by the agency, which noted that the data for 2019 and 2020 might be incomplete due to underreporting during the Covid-19 pandemic.


Reported cases of vector-borne diseases more than doubled from 2004-2019, to more than 800,000 cases. But those figures are almost certainly an undercount, CDC officials said in a presentation to Congress last year. Only 2% to 3% of West Nile cases and about 10% of Lyme disease cases are reported, said Lyle Petersen, the director of the CDC’s Division of Vector-Borne Diseases in Fort Collins, Colo. Overall, cases of vector-borne diseases are probably underreported by 10-fold to 80-fold, according to Benjamin Beard, the CDC division’s deputy director.

Petersen noted that addressing vector-borne disease involves formidable challenges, including a lack of vaccines for diseases found in the continental United States; the difficulty in diagnosing some diseases in their early stages; and the sheer number of emerging pathogens.

Tick-borne diseases comprise the largest share of vector-borne diseases by far — over 80% of reported cases are caused by ticks. Longer summers, rising temperatures, and the expanding ranges of tick species such as Ixodes scapularis, the black-legged tick, and Amblyomma americanum, the lone star tick, are leading to an increased chance of human exposure to pathogens over a larger geographic area. The range of Ixodes scapularis, a tick that transmits Lyme and other diseases, expanded greatly over two decades, with the number of counties with established populations more than doubling from 1996 to 2015.

Over 80% of reported cases of vector-borne diseases in the U.S. are caused by ticks, including the species Ixodes scapularis, known as the black-legged tick. CDC via AP

Similarly, milder year-round temperatures mean that some mosquitoes may overwinter or emerge earlier in the spring. In the case of West Nile, this affects not just the mosquitoes carrying the virus but the virus itself, which replicates faster in warm temperatures. “So the mosquitoes actually are more infectious to people when they bite them,” Beard said.

Nelson Nicolasora, medical director for the infectious disease program at Banner University Medical Center in Phoenix, said that while the 2021 West Nile virus outbreak was “nothing like” the Covid-19 pandemic, the illness was “life-changing” for people who suffered debilitating neurological disease.

West Nile usually causes mild, flu-like symptoms, but about 1 in 150 people who are infected will develop severe neuroinvasive disease. “It can be devastating,” Nicolasora said. Two of his patients died during the 2021 outbreak, he said, and others faced serious short-term and longer-term effects: Some required a ventilator to breathe, or rehabilitation to regain the ability to walk.

Irene Ruberto, vector-borne and zoonotic disease program manager at the Arizona Department of Health Services, said that even though public health officials in the state were aware of the cyclical nature of West Nile virus infections from year to year, they had no idea the infection rate would be so high in 2021. It’s difficult to predict how many infections will occur in a given year, Ruberto said, because many factors are involved, including mosquito density, local environments, and the climate.

“We do know that birds play a role,” acting as an amplifying host for the virus, Ruberto said, which adds complexity to understanding virus transmission.

While West Nile is transmitted to humans by mosquitoes, mosquitoes get the virus through biting an infected wild bird. And different species of birds vary in their ability to transmit the virus once they’re infected.

Ruberto said the state health department in Arizona doesn’t on its own have the funding or the capacity to analyze the 2021 outbreak to understand the factors that drove it. Instead, she said, the department is working with the CDC and universities to study the 2021 data and develop a model to predict future outbreaks.

However, Ruberto said she’s even more concerned about the emergence in her state of another vector-borne disease: dengue fever. In 2022, two locally transmitted cases of dengue were discovered in Arizona, the first appearance of the disease in the state in modern times. Though dengue — known colloquially as “breakbone fever” because of the severe joint pain and muscle spasms it can cause — is also transmitted by mosquitoes, it differs from West Nile in an important way: The virus can be spread from one infected person to another person through a mosquito bite.

Dengue had been absent from the continental United States for decades before reappearing in 2009. But cases of local transmission of dengue fever are now being documented in three continental U.S. states: Arizona, Texas and Florida. In 2022, 57 people in Florida were infected by locally transmitted dengue.

Studies find that the ranges of mosquito species that harbor dengue, Aedes aegypti and Aedes albopictus, are undergoing a significant expansion. According to CDC maps released in 2017, at least 25 states may host one or both of these mosquito species. And one study estimated the rate of spread of the Aedes aegypti mosquito at 155 miles per year. (While maps for the ranges of Aedes aegypti and Aedes albopictus mosquitoes have not been updated since 2017, a CDC spokesperson said the agency plans to release current maps in the next several months).

The increased range of disease-carrying vector species is bringing them into new areas where health care providers may be unfamiliar with the risks they pose and unprepared to diagnose and treat the illnesses they cause, said Beard.

Other human impacts on natural systems, like habitat loss, also affect the emergence and spread of vector-borne and other zoonotic diseases. A loss of biodiversity has been found to lead to a greater abundance of species that host zoonotic pathogens. In the case of Lyme disease, spread by ticks, for example, changes in land use and habitat fragmentation have increased the incidence of the disease. And with increased development of forests, wetlands, and other natural areas, humans are coming into greater contact with animals, increasing the opportunity for exposure to such pathogens.

John Gittleman, a professor of ecology at the University of Georgia, noted that a majority of diseases in humans — at least 60% — are zoonoses, having their origins in other animal species. The urgency of addressing zoonotic disease, he added, is “huge,” and the approach needs to include an environmental perspective as well as a medical perspective.

Urbanization is also a factor driving range expansion in species of mosquitoes and ticks.

“We know that mosquitoes that really like people and like people’s habitats are likely to be the ones that thrive in those newer, urban, expanding environments,” said Sadie Ryan, an associate professor of medical geography and the co-director of the Florida Climate Institute at the University of Florida, adding that this leads to selection for mosquitoes that may pose a particular threat to humans. In Miami, a recent study found that two types of mosquito  — Aedes aegypti, which transmits several other dangerous viruses, including chikungunya and Zika, in addition to dengue, and a species of Culex mosquitoes that transmits West Nile virus — were more abundant than other mosquito species in urban areas.

Ryan said she’s concerned about Aedes aegypti since it’s been a vector for novel pathogens, like Zika. But she’s especially worried about Culex mosquitoes, she said, which transmit viruses that need animal hosts, often birds, to reproduce. These “spillover” diseases, like eastern equine encephalitis virus and West Nile virus, which primarily infect wild animals, can lead to severe illness in humans, a threat she called “alarming.”

Yet federal funding for vector-borne diseases, with the exception of malaria, has remained mostly flat since targeted funding was provided several years ago to address Zika, said Erin Cadwalader, director of strategic initiatives for the Entomological Society of America, during a congressional briefing last year.

The Arizona West Nile outbreak did nothing to change that. In fact, Cadwalader said, funding from the National Institutes of Health for West Nile has been “trending down.”

Beard, of the CDC’s vector-borne diseases division, said high-risk states received funding to sample and map the ranges of Aedes mosquitoes after the Zika outbreak in 2015. “But as it is now, we don’t have the funding support to do that,” he said.

Mosquitoes swarm on the east shore of California’s Salton Sea, one of the first places where West Nile Virus arrived to the state. David McNew/Getty Images

In a paper published in January 2022 in the Journal of Medical Entomology, a team of scientists who study vector-borne diseases warned that the “cyclical and reactive” nature of federal funding leaves the United States vulnerable to outbreaks.

The authors of the study, led by scientists from the University of South Carolina, called the lack of tick surveillance a “critical issue,” given the high incidence of tick-borne disease and the recent discovery of the invasive Asian longhorned tick. Insecticide resistance is another “significant concern,” the authors said, calling for funding for local vector control programs to develop strategies to address it. They also pointed to a “lack of communication and cooperation among U.S. federal agencies” and fragmentation at the state and local levels.

Gridley-Smith, of the NACCHO, noted that federal funding “ebbs and flows,” increasing in response to new threats, like Zika, then falling again. A 2020 member survey by her organization found that the vast majority of local, city and county health departments were in need of improvement in terms of their capacity to conduct mosquito surveillance and control.

Gridley-Smith also pointed to a “huge disparity” in funding for local and regional entities that address vector-borne disease. According to the NACCHO survey, local health departments had dedicated budgets of $254,000 per year on average, while mosquito-control districts, which manage mosquito-related activities for counties, received an average of $3 million.

The CDC has proposed a national strategy to address vector-borne disease, as mandated by the 2020 Kay Hagan Tick Act. The strategy includes five goals with underlying priorities like “modernize and maintain surveillance systems for vectors, reservoirs and [vector-borne diseases],” and “better understand vectors, the pathogens they transmit, and the potential effects of a changing climate.” The federal government has allocated $300,000-$500,000 a year to develop the strategy. Last November, the Department of Health and Human Services released the proposed plan for public comment.

Beard said the new framework will provide a “roadmap” for 17 federal agencies to develop plans to address concerns about vector-borne disease. But, he said, since the measures are implemented by states, counties, and cities, “at the end of the day, it’s out of our control.”

This story is part of ongoing coverage of climate change and health, supported by a grant from The Commonwealth Fund.

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