The Real Threat of Mosquitoes: What You Need to Know About Disease Transmission

Most of us think of mosquitoes as nothing more than annoying pests, but in reality, they are among the deadliest animals in the world. From West Nile virus to malaria, mosquitoes pose serious public health concerns, and the CDC considers mosquito-borne illnesses to be some of the most complex to control. Understanding these diseases and how to reduce risk is a critical first step in prevention.

West Nile Virus

First detected in New York in 1999, West Nile virus became the most prevalent mosquito-borne disease in the U.S. when a major outbreak occurred in 2002 and continued through 2003. Following that, West Nile systematically spread across the country and today remains a leading public health concern nationwide.

Many people infected with West Nile won’t experience symptoms, but around 20% can suffer from fever, joint pain, vomiting, or a rash. In severe cases, the virus can lead to neurological issues like encephalitis or meningitis, conditions that can result in coma, paralysis, or death.

The primary vectors for West Nile virus are Culex mosquitoes, which tend to thrive in water that stands for extended periods, such as ditches, rain barrels, catch basins, marshes, and irrigated agricultural areas. That’s why early intervention is key. In many of these habitats, source reduction is not possible, so proactive larvicide treatments with products such as Altosid^® and FourStar^® are important for reducing the risk of mosquito-borne diseases. If adult mosquito control is needed, Zenivex^® adulticides are EPA-classified as reduced risk and provide rapid population knockdown.

Dengue Fever

While dengue virus is more common in Latin America, Puerto Rico, and the Pacific Islands, its spread remains a growing reality in the continental U.S., especially in states like Florida, Texas, and Hawaii. According to the CDC, 2024 experienced a "record number of dengue cases" in U.S. travelers, an 84% increase compared to 2023. Health officials say this trend is expected to continue in 2025 with higher-than-normal cases being reported across multiple countries. Its symptoms—high fever, joint and muscle pain, rash, and nausea can be painful and debilitating, while severe cases can cause internal bleeding and even death. With no cure available, prevention is critical.

In the continental United States, dengue virus is transmitted primarily by the mosquito species Aedes aegypti, which is capable of breeding in containers holding as little as a tablespoon of water. Common in yards and urban areas, breeding grounds include tires, birdbaths, gutters, and even flower pots. Residents in high-risk areas should regularly eliminate standing water, use air conditioning and well-maintained screened windows, and apply EPA-approved repellents outside.

Zika Virus

Zika made headlines in 2016 when 21 cases were locally transmitted by mosquitoes in Miami. Although no new cases have been reported in the continental U.S. since 2018, Zika remains a concern in some regions and serves as a reminder of how quickly mosquito-borne illnesses can spread.

Like dengue, Zika virus is transmitted by Aedes aegypti mosquitoes, which are also the primary vectors for chikungunya. While most people infected with Zika experience only mild symptoms like fever, rash, joint pain, and red eyes, the virus is particularly dangerous during pregnancy due to its link to congenital disabilities. Zika can be sexually transmitted and can pass from mother to child during pregnancy.

Chikungunya Virus

Chikungunya has affected more than 1.7 million people globally and has made its way to the U.S., with the first local transmissions recorded in the Americas in 2013. While it’s rarely fatal, the symptoms—fever, rash, muscle aches, and especially joint pain—can be debilitating and sometimes last for months. Although local U.S. transmission has not been reported since 2019, travelers to high-risk areas should take precautions to avoid mosquito bites.

The Aedes aegypti mosquitoes responsible for spreading the virus, unlike some species, are active both day and night, increasing the chance of bites and transmission.

Eastern Equine Encephalitis (EEE)

Though less talked about, EEE is one of the deadliest mosquito-borne diseases in the U.S., with a 30% mortality rate in humans and up to 90% in horses. Of those people that do survive neurologic infection, approximately 50% will suffer from permanent debilitating neurological symptoms. There is no human vaccine or cure, making prevention from biting mosquitoes paramount.

EEE symptoms develop between 4–10 days after infection and include chills, fever, vomiting, and headaches. In more severe cases, the virus can lead to brain swelling, coma, seizures, and death.

EEE is spread by Aedes, Coquillettidia, and Culex mosquitoes, particularly those in and around freshwater hardwood swamps along the Atlantic and Gulf coasts. In horses, vaccination is available and highly recommended.

Malaria

Although malaria is no longer considered a significant public health issue in the U.S., about 2,000 cases are diagnosed here annually, mostly among travelers returning from regions where the disease is endemic, including sub-Saharan Africa and South Asia.

Malaria is transmitted by female Anopheles mosquitoes, which pick up the parasite from infected humans. The next time they bite, the parasite is transferred into the bloodstream of a new host. Symptoms include fever, chills, and flu-like illness, and if left untreated, malaria can lead to severe complications or death.

Globally, malaria continues to claim hundreds of thousands of lives each year, especially among children in sub-Saharan Africa. Combating malaria requires ongoing investment in mosquito control, public health initiatives, and access to health care.

Risk Reduction

For professionals in mosquito control, an Integrated Mosquito Management (IMM) approach is recommended. This strategy includes mosquito surveillance, larval and adult control, source reduction, public education, and biological controls. When implemented properly, IMM programs can significantly reduce disease risk.

Mosquito surveillance is the cornerstone for reducing the risk associated with mosquito-borne diseases. Different species of mosquitoes are vectors for various diseases, so knowing which ones need to be controlled allows people to adjust their control plans accordingly. For example, while managing the population of Culex mosquitoes to reduce the threat of West Nile virus might be best accomplished by applying larvicides such as Altosid^® and Fourstar^®, or even spraying Zenivex^® reduced risk adulticides. The successful abatement of Aedes aegypti or A. albopictus might depend more on source reduction and public education.

While every mosquito and mosquito-borne disease situation is different and may need a different combination of control strategies, taking personal precautions to avoid getting bitten is always advised. The American Mosquito Control Association recommends following the “Three D’s”:

1. Drain standing water—indoors and out.
2. Dress in long sleeves and pants when possible.
3. Defend yourself with EPA-registered mosquito repellents.

From nuisance bites to life-threatening illnesses, mosquitoes are far more dangerous than their small size suggests. Staying informed and adopting a layered approach to mosquito control through surveillance, source reduction, larvicides, adulticides, and public education can help protect communities and reduce the risk of disease transmission.