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The New York City Health Department has identified West Nile virus in mosquitoes throughout the city, and there’s growing concern about cases of dengue, another mosquito-borne disease, among travelers returning to New York.
There have been no confirmed human infections of West Nile virus in the city or state as of late July, but the city Health Department has reported 703 positive results in mosquito pools across all five boroughs.
In 2022, the city hit a record high of 1,555 positive mosquito pools and 36 diagnoses of West Nile neuroinvasive disease. Last year, the Health Department reported 1,146 positive mosquito pools and 31 cases of West Nile neuroinvasive disease.
The vast majority of West Nile virus infections are subclinical or asymptomatic, according to the Health Department. Symptoms of West Nile fever, or mild disease, include headache, weakness, muscle aches and joint stiffness, gastrointestinal issues and a rash. Less than 1% of infections lead to West Nile neuroinvasive disease, which includes encephalitis, meningitis, or acute flaccid paralysis (severe muscle weakness), and can be fatal.
This summer, the Health Department first detected West Nile virus in the city’s mosquito population on June 5, the earliest identification on record, according to an advisory. But early detection doesn’t necessarily mean a worse year for outbreaks, said Dr. Nicholas DeFelice, an associate professor of environmental medicine and climate science at the Icahn School of Medicine at Mount Sinai, whose research focuses on forecasting diseases including West Nile.
“Every year is different. There’s a lot of moving parts to it, and a lot of things that need to line up well for spillover events to occur,” DeFelice said, referring to the cross-species transmission of the virus.
In New York City, mosquitoes with West Nile virus are present from July through October, with peak activity in August and September. This month marks a “critical point” in the transmission cycle of the virus, when it’s most important to wear mosquito repellent, DeFelice said.
“During this time of the year, the thing that you can do is wear bug spray when you go out,” he said. “Dawn and dusk is when the mosquito that transmits West Nile bites.”
The Health Department also encourages New Yorkers to eliminate standing water around their homes, which can become breeding grounds for mosquitoes.
Climate change may increase risk
The first cases of West Nile virus in the United States were identified in New York City in the summer of 1999, when Dr. Deborah Asnis, an infectious disease specialist in Queens, alerted the city health department about strange symptoms in two of her patients. As the outbreak spread in the city, seven people died and dozens of others fell sick.
In the 25 years since, New York has seen significant fluctuations in annual West Nile virus cases. But climate change could lead to increased mosquito activity, particularly given the city’s temperate climate, said Dr. Yesim Tozan, an assistant professor of global and environmental health at the NYU School of Global Public Health, who researches infectious disease prevention and control.
Climate change increases the risk of human exposure to the virus, as warmer temperatures accelerate mosquito development, biting rates, and disease incubation periods, according to the U.S. Environmental Protection Agency.
Still, it’s hard to know precisely how climate change will shape the transmission of mosquito-borne diseases, DeFelice said. While warmer, wetter conditions might encourage transmission, extreme heat waves could also kill off mosquito populations, he said.
Tozan stressed the importance of collective responsibility when it comes to mitigating outbreaks, from the city’s pesticide-spraying efforts to New Yorkers using bug spray and eliminating standing water.
“There are various points, in this big, complex cycle that is embedded in the environment, where we could go and do things to decrease this transmission force,” she said.
New York, Florida lead dengue count
The United States is also facing an increase in dengue cases among travelers returning from countries where the mosquito-borne viral disease is endemic. Outbreaks of dengue frequently occur in the Caribbean, Central America, South America, Southeast Asia and the Pacific Islands. Earlier this year, Puerto Rico declared a public health emergency after recording a rapid rise in dengue cases.
New York and Florida lead the dengue case count among the 50 states, according to data from the U.S. Centers for Disease Control and Prevention. All of New York’s cases are from returning travelers, not local transmission.
Travelers to countries where dengue is endemic should inform themselves about the risk before departure and take care to protect against mosquito bites while visiting, Tozan said. Swift diagnosis and treatment of the disease is key, she added. Symptoms of dengue may include a fever, aches and pains, nausea, and a rash, according to the CDC. Severe cases can be life-threatening.
In a June alert about the increased risk of dengue among travelers, the Health Department noted that a local outbreak was unlikely. Aedes aegypti, the type of mosquito that primarily transmits dengue, is not typically found in the city, although Aedes albopictus, a close relative, is.
There remains a chance that a viral disease like dengue could start to spread locally, particularly if ideal conditions for rain, temperature and infectious reservoirs are met, Tozan said.
“It is a real possibility,” she said. “It’s just that it may not necessarily cause very big outbreaks, but it will jump around through these chance encounters.”
Eliza Fawcett is a reporter covering public health in New York City for Healthbeat. Contact Eliza at efawcett@healthbeat.org.
