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2024 Mosquito-Borne Disease Year In Review

Exploring the Impact of Mosquito-Borne Diseases In 2024

In 2024, the U.S. was impacted by 27 individual weather and climate disasters—the second most on record, behind 28 disasters in 2023¹. Climate change appears to be increasing the frequency, intensity, and regional impact of hurricanes, storms, and heat waves. These events, in addition to other human-caused activities like deforestation, not only foster environments that enable mosquitoes to transmit diseases but also drive mosquito populations into regions that were once less hospitable or affected by certain vector-borne diseases.

This year-in-review examines six of the most widespread mosquito-borne diseases in the U.S., comparing five years of historical data compiled by the U.S. Centers for Disease Control and Prevention (CDC)². While some diseases such as St. Louis Encephalitis saw a decrease in cases, others like Eastern Equine Encephalitis (EEE) rose sharply. EEE made several headlines this past year when an increase in cases was reported in New England, specifically New Hampshire and Massachusetts. Not to mention an increase in dengue outbreaks in North, Central, and South America and the Caribbean which lead to a spike in travel-associated and locally acquired cases in the U.S. These insights can help shape your mosquito management plans and public education initiatives for the year ahead.  

*At the time of publishing this information, mosquito-borne disease-related deaths have not been reported by the CDC. Please note, the CDC counts the District of Columbia as a state

West Nile Virus

West Nile virus (WNv) remains the most deadly mosquito-borne illness in the continental U.S. Since its initial detection in 1999, it has become endemic (regularly occurring) in most regions. Although WNv cases significantly declined in 2024 compared to the previous year, the virus remains widespread—reported in every state except Alaska and Oregon. Of the 1,466 cases, more than 72% were classified as neuroinvasive.

Symptoms

While healthy individuals may not notice symptoms, they can range from mild flu-like symptoms such as fevers, headaches, or neck stiffness to severe neurological conditions such as meningitis and encephalitis (brain inflammation), particularly among older populations or those with compromised immune systems. Approximately 5-10% of cases are fatal.

Primary vectors 

WNv is most commonly transmitted by mosquitoes from the genera Culex, including Culex pipiens, Culex tarsalis, and Culex quinquefasciatus. Birds also play a notable role in the spread of the virus. When a mosquito bites an infected bird, it can contract the virus and become a carrier. The mosquito can then transmit the virus to humans through later bites.

Year-Over-Year Change in West Nile Virus Cases

2024
1,466 cases (1,063 neuroinvasive)
2023
2,406 cases (1,599 neuroinvasive)
2022
1,132 cases (827 neuroinvasive)
2021
2,911 cases (2,008 neuroinvasive)
2020
731 cases (559 neuroinvasive)

Top 5 states for West Nile Virus Human Case Count

2024 West Nile Virus Human Cases In the U.S.
Image Source: U.S. Centers for Disease Control and Prevention https://www.cdc.gov/west-nile-virus/data-maps/current-year-data.html
Cases
0
Neuroinvasive Cases
0
States Reporting Human Cases
0

Dengue

Dengue consists of four distinct viruses, referred to as dengue 1, 2, 3, and 4. It is most prevalent in tropical and subtropical regions. In 2024, over 12 million cases of dengue were reported in North, Central, and South America and the Caribbean. These outbreaks led to a major spike in travel-associated infections in the United States. Last year, the CDC reported 9,255 total cases, including both locally acquired and travel-associated infections. Combined, these cases spanned 53 U.S. jurisdictions, though local transmission occurred in only three states, California, Florida, and Texas.

Symptoms

Dengue symptoms can vary from mild joint pain, headaches, and fever to severe hemorrhagic fever, which can cause blood vessel damage. Contracting the virus a second time greatly increases the risk of severe illness, particularly in young children and older adults. Approximately 1% of cases are fatal.

Primary vectors 

Dengue is most often spread to humans by Aedes genus mosquitoes, particularly Aedes aegypti and Aedes albopictus. Additionally, the virus can be transmitted to fetuses during pregnancy.

Year-Over-Year Change in Dengue Cases

2024
6,076 locally acquired cases (3,179 travel-related)
2023
1,104 locally acquired cases (1,452 travel-related)
2022
1,044 locally acquired cases (1,494 travel-related)
2021
609 locally acquired cases (205 travel-related)
2020
983 locally acquired cases (354 travel-related)

Top 5 states for Dengue Human Case Count

2024 Dengue Human Cases In the U.S.
Dengue​ cdc map 2024
Image Source: U.S. Centers for Disease Control and Prevention, Accessed on January 1, 2025 https://www.cdc.gov/dengue/data-research/facts-stats/current-data.html
Locally Acquired Cases
0
Travel-associated Cases
0
States Reporting Locally Acquired Human Cases
0

Eastern Equine Encephalitis

Eastern Equine Encephalitis (EEE) is a rare but potentially deadly mosquito-borne disease primarily found in Northeast, Mid-Atlantic, and Gulf Coast states. In 2024, EEE made several headlines with reports reaching a five-year high with 19 confirmed cases—the third-highest count since 2005. 

Symptoms

Symptoms include high fever, headache, brain inflammation, seizures, and paralysis. Severe cases are most likely to occur among children and older populations. Approximately 30% of cases are fatal, but evidence suggests survivors develop lifelong immunity to the virus. Horses, pigs, rodents, and some deer species are also susceptible to the disease. 

Primary vectors 

EEE primarily circulates between birds and the primary mosquito vector species, Culiseta melanura. However, transmission to humans and animals occurs through mosquitoes from the Aedes, Coquillettidia, and Culex genera, which feed on infected birds before biting mammals.

Year-Over-Year Change in Eastern Equine Encephalitis Cases

2024
19 cases (19 neuroinvasive)
2023
7 cases (7 neuroinvasive)
2022
1 case (1 neuroinvasive)
2021
5 cases (5 neuroinvasive)
2020
13 cases (13 neuroinvasive)

Top 5 states for Eastern Equine Encephalitis Human Case Count

2024 Eastern Equine Encephalitis Human Cases Reported In the U.S.
Image Source: U.S. Centers for Disease Control and Prevention, Accessed on January 1, 205 https://www.cdc.gov/eastern-equine-encephalitis/data-maps/current-year-data.html
Cases
0
Neuroinvasive Cases
0
States Reporting Human Cases
0

La Crosse Encephalitis

In 2024, La Crosse Encephalitis cases continued their upward trend, with most reports coming from North Carolina and Tennessee. Nearly all reported infections progressed to neuroinvasive disease.

Symptoms

This mosquito-borne disease primarily impacts children, causing symptoms like fever, headache, and nausea. In neuroinvasive cases, it can lead to severe complications such as seizures, disorientation, vision loss, and other serious issues. Approximately 1% of cases are fatal.

Primary vectors 

The La Crosse virus is transmitted through the bite of an infected Aedes triseriatus, commonly known as the Eastern treehole mosquito.

Year-Over-Year Change in La Crosse Encephalitis​ Cases

2024
35 cases (34 neuroinvasive)
2023
31 cases (30 neuroinvasive)
2022
22 cases (19 neuroinvasive)
2021
40 cases (39 neuroinvasive)
2020
88 cases (84 neuroinvasive)

Top 3 states for La Crosse Encephalitis Human Case Count

2024 La Crosse Encephalitis Human Cases In the U.S.
Image Source: U.S. Centers for Disease Control and Prevention, Accessed on January 1, 2025 https://www.cdc.gov/la-crosse-encephalitis/data-maps/current-year-data.html
Cases
0
Neuroinvasive Cases
0
States Reporting Human Cases
0

St. Louis Encephalitis

The St. Louis encephalitis virus was first identified in 1933 during an epidemic in St. Louis, Missouri, which led to over 1,000 cases of encephalitis. In 2024, St. Louis Encephalitis cases saw a dramatic decline, with only one reported case in South Carolina—the lowest case count since 2013 when just a single case was also recorded.

Symptoms

Common symptoms include fever, headache, and confusion, with severity often increasing with age. On average, 90% of infected older adults develop brain inflammation. The fatality rate ranges from 3% to 30%.

Primary vectors 

The main mosquito vectors for the virus are Culex pipiens, Culex quinquefasciatus, Culex tarsalis, and Culex nigripalpus. Wild birds, such as blue jays, sparrows, robins, and pigeons serve as amplifying hosts. 

Year-Over-Year Change in St. Louis Encephalitis​ Cases

2024
1 case (0 neuroinvasive)
2023
19 cases (13 neuroinvasive)
2022
33 cases (27 neuroinvasive)
2021
17 cases (11 neuroinvasive)
2020
16 cases (14 neuroinvasive)

Top states for St. Louis Encephalitis Human Case Count

2024 St. Louis Encephalitis Human Cases In the U.S.
Image Source: U.S. Centers for Disease Control and Prevention, Accessed on January 24, 2024 https://www.cdc.gov/sle/data-maps/current-year-data.html
Case
0
Neuroinvasive Cases
0
States Reporting Human Cases
0

Jamestown Canyon virus

In 2024, the number of Jamestown Canyon virus cases remained consistent with the previous year, with 22 reported cases—15 of which became neuroinvasive. The majority of cases were concentrated in the Great Lakes region. Historically, more than half of all cases have been reported in Wisconsin and Minnesota.

Symptoms

Many people infected with Jamestown Canyon virus do not have symptoms. Those who do may experience fever, headache, muscle soreness, and respiratory issues. Severe infections can lead to disorientation, seizures, and other neurological complications.

Primary vectors 

The virus can be spread by different mosquito species, with deer often serving as the amplifying host. Humans are deemed “dead-end” hosts for the virus, as their blood lacks the virus levels necessary to infect mosquitoes and propagate the disease further.

Year-Over-Year Change in Jamestown Canyon Virus Cases

2024
22 cases (15 neuroinvasive)
2023
21 cases (15 neuroinvasive)
2022
12 cases (11 neuroinvasive)
2021
32 cases (21 neuroinvasive)
2020
13 cases (10 neuroinvasive)

Top 3 states for Jamestown Canyon Virus Human Case Count:

2024 Jamestown Canyon virus Human Cases Reported In the U.S.
jamestown
Image Source: U.S. Centers for Disease Control and Prevention, Accessed on January 24, 2025 https://www.cdc.gov/jamestown-canyon/data-maps/current-year-data.html
Locally Acquired Cases
0
Neuroinvasive Cases
0
States Reporting Human Cases
0

Utilizing Data to Drive Effective Integrated Mosquito Management Programs

The 2024 data underscore the growing threat of mosquito-borne diseases and the need for adaptive solutions and strategies, such as Integrated Mosquito Management (IMM) programs. IMM programs incorporate adult and larval mosquito control, surveillance and disease testing, and public education to stay ahead of evolving environmental challenges, reduce the risk of insecticide resistance, and keep our communities safe from vector-borne diseases. Through a proactive approach, we can build a strong foundation for long-term mosquito prevention and control.

SOURCES

1 NOAA National Centers for Environmental Information (NCEI) U.S. Billion-Dollar Weather and Climate Disasters (2025). https://www.ncei.noaa.gov/access/billions/ 

2 U.S. Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/ 

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Since 1992, Vector Disease Control International (VDCI) has taken pride in providing municipalities, mosquito abatement districts, industrial sites, planned communities, homeowners associations, and golf courses with the tools they need to run effective mosquito control programs. We are determined to protect the public health of the communities in which we operate. Our mosquito control professionals have over 100 years of combined experience in the field of public health, specifically vector disease control. We strive to provide the most effective and scientifically sound mosquito surveillance and control programs possible based on an Integrated Mosquito Management approach recommended by the American Mosquito Control Association (AMCA) and Centers for Disease Control and Prevention (CDC). VDCI is the only company in the country that can manage all aspects of an integrated mosquito management program, from surveillance to disease testing to aerial application in emergency situations.