2023’s Most Vulnerable Counties for Mosquito-Borne Diseases

Which U.S. counties are most susceptible to swarms of pesky mosquitoes and the viruses they carry?

To mark World Mosquito Day on Aug. 20, Pest Gnome ranked 2023's Most Vulnerable Counties for Mosquito-Borne Diseases.

We compared nearly 800 counties based on four categories. We looked at the number of mosquito species in each state, recent cases of diseases like West Nile and Eastern equine encephalitis viruses, and mosquito-friendly climate, among 12 total metrics.

See if your county’s at risk in our ranking below. To learn how we ranked the counties, see our methodology.

Contents

Rankings

See how each county fared in our ranking:

Infographic showing the Most Vulnerable Counties for Mosquito-Borne Diseases, a ranking based on the number of mosquito species in each state, recent cases of mosquito-borne diseases, and mosquito-friendly climate
Note: For presentation purposes, not all ties may be displayed for some metrics above.

Top 5 Close Up

Check out the slideshow below for spotlights on our five most vulnerable counties.

Map of North Carolina with a mosquito icon over Pitt County indicating it as one of the most vulnerable counties to mosquito-borne diseases
No. 1: Pitt County, North Carolina | Overall Score: 34.44

Mosquito Prevalence: 186
Disease Prevalence: 32
Mosquito-Friendly Climate: 52
Mosquito Control: 434
Map of Arizona with a mosquito icon over Maricopa County indicating it as one of the most vulnerable counties to mosquito-borne diseases
No. 2: Maricopa County, Arizona | Overall Score: 34.17

Mosquito Prevalence: 596
Disease Prevalence: 1
Mosquito-Friendly Climate: 505
Mosquito Control: 771
Map of North Carolina with a mosquito icon over Buncombe County indicating it as one of the most vulnerable counties to mosquito-borne diseases
No. 3: Buncombe County, North Carolina | Overall Score: 33.22

Mosquito Prevalence: 181
Disease Prevalence: 2
Mosquito-Friendly Climate: 547
Mosquito Control: 496
Map of Texas with a mosquito icon over Brazoria County indicating it as one of the most vulnerable counties to mosquito-borne diseases
No. 4: Brazoria County, Texas | Overall Score: 32.94

Mosquito Prevalence: 15
Disease Prevalence: 276
Mosquito-Friendly Climate: 15
Mosquito Control: 566
Map of Georgia with a mosquito icon over Liberty County indicating it as one of the most vulnerable counties to mosquito-borne diseases
No. 5: Liberty County, Georgia | Overall Score: 32.82

Mosquito Prevalence: 121
Disease Prevalence: 32
Mosquito-Friendly Climate: 262
Mosquito Control: 356

Key Insights

The Big Picture

Four Texas counties finish among the top 10 most vulnerable, with high Mosquito Prevalence and a hot, humid, Mosquito-Friendly Climate. North Carolina counties Pitt (No. 1) and Buncombe (No. 3), land at the top alongside Maricopa County, Arizona (No. 2) — each with high Disease Prevalence.

Counties in North and South Dakota landed at the bottom of our ranking — meaning they’re the least vulnerable for mosquito-borne diseases. This region has low Mosquito Prevalence and low Disease Prevalence in addition to a climate that is not ideal for mosquito activity.

While most Northern counties have fewer mosquitoes to swat away, that may change in coming years due to climate change.

Standout Stats

Pitt Full of Pests

  • Pitt County, North Carolina, places first overall — in other words, it’s the most vulnerable. Although Pitt reported fewer mosquito-borne diseases than other counties over the past few years, it has the perfect conditions that allow mosquitoes to thrive. Pitt reported one case of Eastern equine encephalitis (EEE) virus between 2020 and 2022, which is far deadlier than the more common West Nile virus.

Disease in the Desert

  • Despite having low Mosquito Prevalence and lacking a Mosquito-Friendly Climate, Disease Prevalence is highest in Maricopa County, Arizona (No. 2). Maricopa saw the highest number of West Nile virus cases1,515 over the past three years — due to higher-than-average precipitation leading to an unprecedented outbreak.

Seaside Swarms

  • Miami-Dade County, Florida (No. 13), Ascension Parish, Louisiana (No. 24), and Mobile County, Alabama (No. 87), attract mosquitoes with the top three Mosquito-Friendly Climates. These coastal regions feature hot weather, high rates of precipitation, flooding, and drought

Perfect Storm

  • Many mosquito-vulnerable regions are also among the most vulnerable to hurricanes, including Miami-Dade County, Florida (No. 13), Palm Beach County, Florida (No. 39), Horry County, South Carolina (No. 96), and Harris County, Texas (No. 116). These areas become susceptible to booming mosquito populations post-hurricane flooding

DEET in the Heart of Texas

  • Grab the bug spray: 59 Texas counties claim the highest Mosquito Prevalence. The five counties covering the Texas TriangleHarris County (No. 116), Dallas County (No. 86), Bexar County (No. 105), Tarrant County (No. 109), and Travis County (No. 98) — take up the top five spots, respectively. 

Lone Star Species

  • Texas deals with the most species of mosquitoes, 85 five more than in Florida, the state with the 2nd-highest number. Harris County, Texas (No. 116), fights back with the most exterminators and pest control providers out of the counties in our ranking. Cottle County (No. 83) and Reeves County (No. 321) unfortunately tied for the fewest exterminators, despite having high Mosquito Prevalence

Northeastern Invasion

  • Two Northeastern countiesMerrimack County, New Hampshire (No. 21), and Plymouth County, Massachusetts (No. 28) — stand out with high Disease Prevalence. Merrimack had the highest occurrence of Jamestown Canyon virus, with six cases between 2020 and 2022. Plymouth reported the most cases of Eastern equine encephalitis virus. Both diseases historically are most common in this region.

Distant from Danger

  • Neither Alaska nor Hawaii has recorded a case of West Nile virus since the disease first reached the U.S. in 1999. Anchorage Municipality County, Alaska (No. 758), finished as the 15th-least vulnerable county, lacking mosquitoes, their diseases, and mosquito-friendly conditions in general. 

Scrapping Skeeters

  • Hawaii was once completely mosquito-free, and Hawaiian counties were excluded from our ranking due to a lack of data from the Centers for Disease Control and Prevention for human disease cases in the past three years. Today, there is a campaign to restore a mosquito-free Hawaii as avian malaria is threatening endangered Native bird species.

Bugging Out

  • Los Angeles County, California (No. 211), residents Googled “mosquito repellent” the most times over the past year, indicating an infestation. In fact, LA County registered the 2nd-highest number of West Nile virus cases — 166 over the past three years. Maricopa County, Arizona (No. 2) and Orange County, California (No. 287) tied for the 2nd-highest number of searches.

Ask The Experts

We turned to a panel of experts to learn about the dangers of mosquitoes and how to keep them at a distance. Read their insights below.

  1. Mosquitoes are deemed the world’s deadliest animal. How concerned should Americans be about the West Nile virus, locally transmitted malaria, or other mosquito-borne diseases?
  2. What types of environments attract mosquitoes into people’s homes, and how can they be avoided?
  3. What are your top three mosquito repellant recommendations?
  4. What are your top three tips for managing itchiness or pain from mosquito bites?
  5. What are the most concerning symptoms from mosquito bites?
  6. Which animal species are most vulnerable for mosquito-borne diseases? Should pet owners be worried?
Eva Buckner, Ph.D.
Assistant Professor & State Extension Specialist
Patricia Pietrantonio
Professor and Texas A&M AgriLife Research Fellow
Sharon P. Lawler
Professor Emeritus Entomology and Nematology
Eva Buckner, Ph.D.
Assistant Professor & State Extension Specialist
University of Florida, UF/IFAS Florida Medical Entomology Laboratory

Mosquitoes are deemed the world’s deadliest animal. How concerned should Americans be about the West Nile Virus, locally transmitted malaria, or other mosquito-borne diseases?

West Nile is the mosquito-borne disease that Americans should be the most concerned about because West Nile virus (WNV) is the leading cause of mosquito-borne disease in the continental US.

West Nile virus and the mosquitoes that can transmit the virus are found throughout the entire continental US. For that reason, there is always a risk of exposure to West Nile virus for all Americans, especially during mosquito season (usually May – October depending on location).

Since 1999, there have been over 56,000 human cases, 25,000 hospitalizations, and 2,000 deaths due to West Nile virus. Here’s a great CDC website that shows West Nile cases by county over the past 20 years.

Local cases of malaria are very rare. The most recent local malaria cases prior to the ones this year were in 2003. There was a total of 8 cases then.

Depending on where you live, you can also be at risk of additional mosquito-borne diseases. Areas with Aedes aegypti mosquitoes like Florida, Hawaii, Texas, Arizona, California, and US territories are at risk for local dengue infection. Most dengue outbreaks in the continental United States have been relatively small and limited. People in eastern or Gulf Coast states are at more of a risk of Eastern equine encephalitis (EEE) virus infection. Although rare, EEE is very serious. Approximately 30% of people with EEE die and many survivors have ongoing neurologic problems.

What types of environments attract mosquitoes into people’s homes, and how can they be avoided?

Mosquitoes need water. Half of their life cycle is aquatic. A home with a yard containing lots of containers holding water is a great place for producing lots of mosquitoes. Dumping water-holding containers once a week can reduce mosquitoes around your home. Also make sure all windows have screens with no holes to prevent mosquitoes from coming indoors.

What are your top three mosquito repellant recommendations?

1. DEET

2. Oil of lemon eucalyptus

3. Picardin

What are your top three tips for managing itchiness or pain from mosquito bites?

Tips from CDC:

1. Place an ice pack on the bite for 10 minutes to reduce itchiness and inflammation.

2. Use over-the-counter anti-itch or antihistamine cream.

3. Mix one tablespoon of baking soda with just a little bit of water to make a paste. Leave the paste on the bite for 10 minutes, then wash off.

What are the most concerning symptoms from mosquito bites?

The most concerning symptoms of mosquito bites are neurological symptoms of mosquito-borne diseases like West Nile and Eastern equine encephalitis, which include:

  • Seizures
  • Paralysis
  • Coma

Which animal species are most vulnerable for mosquito-borne diseases? Should pet owners be worried?

Horses and dogs are the pets most vulnerable to mosquito-borne diseases. Yes, pet owners should be concerned and take action to protect their pets.

  • Horses are vulnerable to West Nile and Eastern equine encephalitis. In 80-90% of Eastern equine encephalitis horse cases, the infection is fatal. Vaccinations are available for horses for West Nile and Eastern equine encephalitis viruses. Horse owners should make sure that their horses stay up to date with vaccinations.
  • Dogs are vulnerable to heartworms, which are only transmitted by mosquitoes. Heartworms cause heartworm disease. Without proper treatment, most dogs will die from heartworm disease. Ask your vet about FDA-approved heartworm prevention products, which all require prescriptions.
Patricia Pietrantonio
Professor and Texas A&M AgriLife Research Fellow
Texas A&M University

Mosquitoes are deemed the world’s deadliest animal. How concerned should Americans be about the West Nile Virus, locally transmitted malaria, or other mosquito-borne diseases?

West Nile is the highest risk overall as malaria cases are localized and fewer. Zika has not been a problem recently.

What types of environments attract mosquitoes into people’s homes, and how can they be avoided?

Yes, mosquito populations of Culex quinquefasciatus the vector of West Nile Virus are resistant to pyrethroids and some survive the applications performed by Harris County public health treatments. This is why it is important to dress in long pants and sleeves in the evening when these mosquitoes are active; use repellents and/or stay inside.Eliminate any standing water and keep birds away from the house including chickens and bird feeders as their smell ( feces) attracts mosquitoes.

What are your top three mosquito repellant recommendations?

Of course, Deet is the gold standard ” repellent ” in quotes because it really does not repel it is more of an “invisibility cloak” Mosquitoes cannot smell us but also mosquitoes can taste Deet so it may act as a feeding deterrent.

Other products are important in that there are not flammable as the Deet products and may be more suitable for children for example.

Sharon P. Lawler
Professor Emeritus Entomology and Nematology
University of California, Davis

Mosquitoes are deemed the world’s deadliest animal. How concerned should Americans be about the West Nile Virus, locally transmitted malaria, or other mosquito-borne diseases?

Americans should be moderately concerned about mosquito-borne illness, to the extent of protecting themselves from bites and preventing accumulation of stagnant water where mosquitoes breed (other than natural wetlands and ponds, which will support beneficial predators). Serious mosquito-borne illness is rare in the Continental U.S.A., thanks to public and private Mosquito Abatement organizations, and certain environmental factors. However, some warm, humid regions, like Puerto Rico, American Samoa, and the US Virgin Islands are prone to outbreaks of mosquito-transmitted pathogens, such as Dengue viruses. West Nile Virus is widespread and can be harmful to those with weak immune systems. Other encephalitis-causing viruses may be more localized, but can occasionally be serious, such as Eastern Equine Encephalitis. Malaria is only found in few places in the South, but it has some capacity to spread if mosquitoes are not controlled.

What types of environments attract mosquitoes into people’s homes, and how can they be avoided?

Puddles and other wet areas that hold water for five or more days can let mosquitoes breed, because their larvae are aquatic. Avoid over-watering such that water accumulates in ditches, empty plant saucers and anything else that collects water (old tires and similar), and drain unused pools. Treehole water can support the mosquitoes that carry heartworm to pets. A hole drilled into the base of the treehole can drain it, while still leaving habitat for nesting birds. Maintain good screens on the house.

What are your top three mosquito repellant recommendations?

I really only have two: repellants based on DEET, and lemon-eucalyptus oil. Some botanicals other than lemon eucalyptus also work, such as geraniol, but might need to be applied more often.

What are your top three tips for managing itchiness or pain from mosquito bites?

Prevention first- wear light-colored, long-sleeved shirts and long pants if you can, and use repellant. If bitten, you can use a topical antihistamine, or witch hazel. A band-aid can be helpful for compulsive scratchers.

What are the most concerning symptoms from mosquito bites?

I am not a medical doctor, so the following is not official medical advice, and should not be presented as such in any context. Most of the mosquito-borne diseases take a couple of days or more to incubate. They can cause headaches and fever. Consider consulting a medical doctor if these symptoms arise in the days after bites occur. Secondary infections caused by scratching are also concerning. These are frequently indicated by discharge, excessive swelling and pain, and sometimes red streaks near the bite.

Which animal species are most vulnerable for mosquito-borne diseases? Should pet owners be worried?

I am also not a veterinarian, so the following is not comprehensive. Owners of some kinds of pets need to be concerned about pet exposure to mosquitoes. Heartworm is carried by mosquitoes. This can affect dogs and sometimes cats. These should be vaccinated, especially where heartworm is common (most of the USA). Horses are quite vulnerable to some of the encephalitis viruses, such as West Nile Virus and Eastern Equine Encephalitis, and should be vaccinated. Pet birds can contract West Nile virus and some of the other viruses. This often isn’t serious, but young birds should be protected as they may have less immunity, and outdoor aviaries should be screened.

Behind the Ranking

First, we determined the factors (metrics) that are most relevant to rank the Most Vulnerable Counties for Mosquito-Borne Diseases. We then assigned a weight to each factor based on its importance and grouped those factors into four categories: Mosquito Prevalence, Disease Prevalence, Mosquito-Friendly Climate, and Mosquito Control. The categories, factors, and their weights are listed in the table below.

For each of the 772 U.S. counties that reported human cases of mosquito-borne diseases from 2020 to 2022, we then gathered data on each factor from the sources listed below the table. 

Finally, we calculated scores (out of 100 points) for each county to determine its rank in each factor, each category, and overall. A county’s Overall Score is the average of its scores across all factors and categories. The highest Overall Score ranked “Most Vulnerable” (No. 1) and the lowest “Least Vulnerable” (No. 772).

  • The “Least Vulnerable” among individual factors may not be No. 772 due to ties.
  • We considered only counties with available data from the Centers for Disease Control and Prevention for human cases of West Nile virus, La Crosse Disease, Eastern equine encephalitis disease, and Jamestown Canyon virus between 2020 and 2022.
  • We did not include cases of malaria, dengue fever, chikungunya, or Zika due to a lack of data for the same period.

Sources: Centers for Disease Control and Prevention, Federal Emergency Management Agency, Google Ads, National Centers for Environmental Information, World Population Review, and Yelp

Final Thoughts: Prevent Mosquito Mayhem

Mosquitoes are a growing nuisance as climate change amplifies the risk of contracting a mosquito-borne disease.

Locally transmitted cases of malaria — an illness considered “eliminated” in the U.S. and typically acquired abroad — have been recently detected in Florida and Texas but currently are not considered a threat. There is also a history of locally transmitted dengue fever in several states, including Florida, Hawaii, Arizona, and Texas.

West Nile virus is another concern due to growing rates of cases across the nation. Many who contract the virus carry it asymptomatically, but the disease can cause brain infections and neurological issues. 

With the Strengthening Mosquito Abatement for Safety and Health (SMASH) 2.0 Act recently reintroduced, Americans might see better control of mosquitoes and vector-borne diseases in coming years. 

Scientists emphasize not to worry about contracting malaria or West Nile virus. You can still prevent transmission by taking precautions to deter mosquitoes from biting you in the first place. 

If you live in a mosquito-infested area or just have the type of blood they’re attracted to, consider the following precautions: 

What is Pest Gnome?

Pest Gnome — part of the Home Gnome family of home service providers — puts local pest control experts at your fingertips. We connect you to the best local pros to get the job done quickly.

Media Resources

Sav Maive

Sav Maive is a writer and director based in San Antonio. Sav is a graduate of the University of Virginia and is a loving cat and plant mom.