Fall Mosquitoes and Mosquito-Borne Diseases

When summer transitions into fall, it's natural to assume that concerns about mosquitoes, ticks, and the diseases they carry will fade with the season. While cooler temperatures can reduce the activity of many biting insects, mosquitoes and the viruses they transmit remain a threat well into the autumn months if it remains warm enough. Even as other pests become less active, some mosquito species continue to thrive in fall conditions, maintaining their ability to spread diseases such as West Nile virus (abbreviated as WNV) and Eastern equine encephalitis (often abbreviated as EEE or "triple E"). This means protective measures shouldn't be abandoned just because the calendar has turned to September or October.

In the northeastern United States, including Pennsylvania, WNV and EEE viruses are transmitted by mosquitoes and can affect both humans and horses. These viruses spread through a natural cycle between birds and mosquitoes. Some birds, primarily crows and jays, may become severely ill and potentially die as a result of WNV infection. When mosquitoes bite infected birds carrying EEE virus or WNV, the mosquitoes become infected and can transmit the virus to humans, horses, and other animals through their bites. However, humans and horses are considered "dead-end" hosts — meaning they cannot pass the virus to mosquitoes or spread it to others, which helps limit further transmission.

Mosquitoes can transmit the EEE virus to birds, humans, and horses (CDC, 2024b). Both humans and horses can become sick due to infection with the virus, yet cases of EEE are relatively rare. The most recent human case in Pennsylvania was reported to the CDC in 2018; however, several human cases were reported in nearby states in 2024 (CDC, 2025a). Cases of EEE tend to follow a seasonal trend, with cases typically appearing in mid-summer before peaking in late summer and early fall, usually starting in August and declining as fall progresses (Lindsey et al., 2018). Animals, particularly horses, are more frequently affected by EEE than humans (Lindsey et al., 2018). While many people infected with EEE virus remain asymptomatic, some infections can progress to severe illness.

A more common mosquito-borne disease, West Nile virus, is historically the leading cause of mosquito-borne illnesses in the United States (Padda et al., 2025). Most people infected with WNV do not become sick. Only a small percentage of people who become ill experience severe symptoms or illness (CDC, 2024a; Padda et al. 2025). Like EEE, cases of WNV begin to rise in July, usually reaching a peak in August. The number of people sick with this virus may remain high in September and October. Usually, fewer people are diagnosed with WNV by mid-October (CDC, 2025b).

While mosquitoes may persist through early fall, you can protect yourself and your family by taking simple steps to reduce the chance of being bitten. All mosquitoes rely on water to complete their life cycle. Adult mosquitoes lay their eggs directly on or near water, and earlier mosquito life stages, such as the larvae, will remain in the water until they emerge as adults. The aquatic portion of the life cycle can happen in as little as a week. The larval stages can be the easiest to manage since they are confined to water. For items such as bird baths, tarps, buckets, water troughs, or children's toys, simply dumping the water from them once a week or removing them can reduce the number of mosquitoes in an area. Once emptied, cleaning these items can also help remove any eggs that may be stuck to them. For rainwater barrels, cover any openings with a screen that has a mesh size of 1/16 inch or smaller to prevent mosquitoes from entering.

Larvicide treatments can be used to treat ponds, irrigation ditches, or other larger water features that cannot be emptied. Bacillus thuringiensis israelensis, also known as Bti, is a treatment that can be applied to large water sources. This bacterium is naturally found in soil and emits spores that are toxic only to certain fly larvae, including mosquitoes.  Bacillus thuringiensis israelensis is an approved option for pest management in organic farming operations and is not known to be toxic to humans or other animals (EPA, 2025). Other treatments exist for mosquito larvae and adults, but remember to follow the instructions on the label of any pesticides used to ensure the most effective and safest application.

Once mosquitoes are out and biting, you can reduce the chances of being bitten by wearing long-sleeved, baggy clothing if feasible and wearing EPA-registered insect repellents. Although different mosquitoes may be active at all times of the day, limit your time outside after sunset to reduce the possibility of being bitten by the mosquitoes that may transmit WNV or EEE. Ensure that window and door screens do not have any holes that may allow mosquitoes to enter your home. If you believe you may have WNV, EEE, or any other mosquito or tick-borne disease, consult a healthcare professional for a diagnosis or treatment. Vaccines for WNV and EEE do not exist for humans; however, there are vaccines available for horses. Following these tips can help reduce the chances of you and your family being bitten by mosquitoes throughout the year.

Further Reading

Mosquito-Borne Diseases EEE and WNV

West Nile Encephalitis: What You Need to Know

Tick and Mosquito Repellent Guide

Pennsylvania Department of Health

References

Centers for Disease Control and Prevention. (2024a). About West Nile virus. Retrieved July 25, 2025.

Centers for Disease Control and Prevention. (2024b). Eastern equine encephalitis: Causes and how It spreads. Retrieved July 18, 2025.

Centers for Disease Control and Prevention. (2025a). Eastern equine encephalitis virus historic data (2003-2024). Retrieved September 30, 2025.

Centers for Disease Control and Prevention. (2025b). West Nile virus historic data (1999-2024). Retrieved July 25, 2025.

Lindsey, N. P., Staples, J. E., & Fischer, M. (2018). Eastern equine encephalitis virus in the United States, 2003–2016. The American Journal of Tropical Medicine and Hygiene, 98(5), 1472–1477. doi.org/10.4269/ajtmh.17-0927

Padda, H., Jacobs, D., Gould, C. V., Sutter, R., Lehman, J., Staples, J. E., & Lyons, S. (2025). West Nile virus and other nationally notifiable arboviral diseases—United States, 2023. MMWR, 74(21).

United States Environmental Protection Agency. (2025). Bti for mosquito control. Retrieved September 30, 2025.

Disclaimer: This publication was supported by the Centers for Disease Control and Prevention of the U.S. Department of Health and Human Services (HHS) as part of award NU50CK000637. The contents are those of the author(s) and do not necessarily represent the official views of, nor an endorsement by CDC/HHS, or the U.S. Government.