What climate change means for vector-borne disease in NYC

Mosquitoes and ticks can cause serious diseases for New Yorkers

Bugs play an important role in our ecosystem: They pollinate plants, break down organic matter into soil, and are a source of food for animals. Some also spread diseases. We call bugs that spread disease vectors.

Some bugs, including certain types of mosquitoes and ticks, can pick up disease-causing germs from people and animals they bite. They can then spread them to the next person or animal they bite. Diseases that spread this way are called vector-borne diseases.  Examples you may have heard of in NYC are West Nile virus disease, spread by mosquitoes, and Lyme disease, spread by ticks. These diseases are sensitive to climate factors such as rainfall, humidity, and temperature.

Vector surveillance is critical for detecting and controlling vectors and vector-borne disease

Vector surveillance monitors bugs that spread disease and identifies new and emerging vectors. This process helps us understand where these vectors are in NYC, how many there are, and when they are active throughout the year.

The NYC Health Department collects mosquito and tick samples. At the NYC Health Department’s Public Health Laboratory, our researchers test them for several types of germs that can cause diseases.

Because of climate change, vector surveillance and testing is more important than ever. Warmer temperatures and changing weather are affecting how vectors reproduce, bite, carry germs, and spread, increasing risks to public health.

Vector-borne disease cases are increasing in NYC, likely due to climate change.

Most people infected with West Nile virus don’t get sick or are only mildly ill, so they may not get tested, and we don’t learn that they had the virus. The people that are more likely to get tested are those who are severely ill and hospitalized with West Nile Neuroinvasive Disease. This type of West Nile virus infection affects the central nervous system, including the brain, often causing encephalitis or meningitis. Since 1999, there have been between two and 45 people with West Nile Neuroinvasive Disease each year. Six of the nine years with more than 25 cases have been since 2010.

People can get West Nile virus when mosquitoes are active, usually between July and October. With climate change, we find people getting sick both earlier and later each year. People older than 60 have the highest risk of getting sick with West Nile virus. Most infections occur due to a mosquito bite within NYC.

Diseases transmitted by ticks are a growing health concern. The number of New Yorkers with tick-borne diseases such as Lyme disease, anaplasmosis, and babesiosis, has increased over time. These diseases can cause flu-like symptoms and sometimes more serious complications. The majority of New Yorkers who get sick with a tick-borne disease are infected while spending time outdoors outside of NYC, like in upstate New York or surrounding states.

The only vector that spreads tick-borne disease within NYC is the blacklegged tick. This tick can spread Lyme disease, anaplasmosis, and babesiosis. Within NYC, blacklegged ticks only live on Staten Island and in the northern Bronx. A smaller number of New Yorkers become sick from spending time outdoors in these areas.

Although the increase in Lyme disease cases reported that began in 2022 is due to a change in the way cases are defined, the slow but steady increase in tick-borne diseases overall is likely related to the climate change.

Climate variables like temperature and rainfall can affect vector-borne diseases

Climate change impacts vector-borne diseases as it does many other health issues. However, the direct impact of the changing climate on vector-borne diseases is more challenging to anticipate than it is for frequent heat waves and intense storms. This is because of the complex interactions between climate factors, like heat and rainfall, and the vectors themselves, their ecosystems, and the animals and people they bite.

Climate change is making summers hotter and wetter in NYC, and mosquito populations are increasing

The number of days per warm season where the average temperature exceeded 86 degrees F has been steadily climbing in NYC since 1975 due to human-caused climate change. Days under 74 degrees F have also been decreasing.

Summers have been getting hotter in NYC due to the Earth’s rising average temperature. Rainfall has also been increasing and becoming more intense. The number of mosquitoes that can spread West Nile virus and the number of batches (also called pools) of these mosquitoes testing positive for West Nile virus have also been increasing in NYC since tracking began in 1999. Mosquitoes tend to favor hot, wet, and humid weather, so these trends are likely related.

Human-led changes to natural areas and hot, humid summers mean ticks are active in more places for more of the year

As we keep developing and living in formerly natural areas and people move into habitats where ticks and their animal hosts live, people are more exposed to ticks and at increased risk of tick-borne diseases. Ticks thrive in warmer weather as long as there is humidity. Longer warm seasons and shorter, milder winters mean that ticks can thrive in more places for more of the year. New tick species have been found on Staten Island in recent years, including the Asian longhorned tick and the Gulf Coast tick. These ticks can spread the bacteria that causes a type of spotted fever. This disease can cause fever, rash, and aches.

However, ticks don’t survive well when it is too hot and dry. High temperatures and dry conditions, like during the drought in NYC in fall 2024, can reduce tick activity.

Preventing vector-borne diseases

Certain people, especially people 55 and older and those with weakened immune systems, may be at greater risk for getting sick from West Nile virus. The Health Department works to control mosquito populations. We treat marshlands, natural bodies of water, and catch basins with larvicide (a pesticide that kills mosquito larvae) during the season to prevent mosquito larvae from emerging as adult mosquitoes. We spray to control adult mosquitoes (adulticiding) using trucks when surveillance shows a high risk of disease to humans in certain areas.

How to protect yourself:

Wear long pants and long-sleeve shirts when possible and use an EPA-registered insect repellent when outdoors. This is especially important during dusk and dawn when the mosquitoes that transmit West Nile virus are most active and when in areas where ticks may live.

Do tick checks for yourself, your family and your pets after being outdoors in areas where ticks may live. You want to find and remove any ticks before or soon after they attach. Talk to your veterinarian about tick prevention products for your pets. Learn more about preventing and recognizing tickborne diseases.

Remove standing water on your property so mosquitoes don’t have a place to lay their eggs. People older than 55 or who have a weakened immune system should also minimize outdoor activities during dusk and dawn. Learn more about preventing and recognizing WNV.