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Climate change is shifting the ranges of many disease-carrying species like ticks and mosquitoes. Scientists warn that the U.S. is underprepared for a potentially devastating surge in infections. 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.
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...
