Recent Cases of Mosquito-Borne Diseases

West Nile virus (WNV) and Eastern equine encephalitis (EEE) cases have been reported in the northeastern United States this year [2024], including just over a dozen cases of WNV in Pennsylvania (CDC, 2024e). No human cases of EEE have been reported in Pennsylvania yet this year, but there have been cases reported in nearby states, including New Jersey, Massachusetts, New Hampshire, and Vermont (CDC, 2024b).

What are West Nile virus and eastern equine encephalitis?

West Nile virus disease and EEE are caused by the West Nile virus and eastern equine encephalitis virus, respectively, and these viruses are transmitted through the bite of mosquitoes. These viruses are naturally transmitted among birds by bird-biting mosquitoes. Some mosquitoes bite both birds and mammals and can transmit the virus to humans or horses. Many people infected may not show any symptoms (CDC, 2024a, 2024d). However, about 20% of humans can develop symptoms from WNV, which can be severe (CDC,2024d). Nearly 30% of all those who develop severe symptoms of EEE die (CDC, 2024a).

When is the greatest risk for WNV and EEE?

Although the number of cases can vary from year to year, late August and early September tend to be the peak time for WNV and EEE cases in humans, but cases can occur into late fall and early winter, depending on temperature (Lindsey et al., 2018; McDonald et al., 2021). Cases of both viruses may occur regularly in the northeastern United States, but WNV tends to be the most reported mosquito-borne disease in the Northeast and throughout much of the rest of the United States (Fagre, 2023).

How can I prevent mosquito bites?

Preventing mosquito bites is the most effective way to protect yourself from these viruses because there are no vaccines for humans that prevent WNV or EEE (CDC, 2024c; Fagre, 2023). When heading outside, wear EPA-registered mosquito repellents, wear long-sleeved and loose-fitting clothing, and treat clothing with permethrin. Be sure to follow the product label when using mosquito repellents or treating clothing with permethrin.

You can also prevent mosquito bites by eliminating mosquito habitat. Prevent mosquitoes from entering your home by ensuring any window or door screens are sealed and do not contain any holes. Since mosquitoes require water for portions of their life cycle, removing or tightly covering items that contain standing water, such as tires and buckets, will prevent mosquitoes from multiplying near the house. Some water features, such as bird baths, should be emptied every few days to remove developing mosquitoes. If a water feature cannot be emptied or drained at least weekly, consider covering it with a fine netting to prevent mosquitoes from accessing it. Homeowners can treat water features with a bacteria called BTI (Bacillus thuringiensis subspecies israelensis) if water cannot be emptied or eliminated. BTI will kill developing mosquito larvae before they emerge as adults (Lacey, 2007). Follow the product label for any repellents or pesticides used, including BTI.

More Penn State Extension Resources for Preventing Mosquito Bites:

• Tick and Mosquito Repellent Guide
• Fight the Bite: Treating Clothes with Premethrin to Prevent Tick and Mosquito Bites
• Mosquito Borne Diseases EEE and WNV

References

Centers for Disease Control and Prevention. (2024a, August 29). Eastern equine encephalitis: Symptoms, diagnosis, and treatment. Retrieved September 10, 2024.

Centers for Disease Control and Prevention. (2024b, September 4). Eastern equine encephalitis virus current year data (2024). Retrieved September 10, 2024.

Centers for Disease Control and Prevention. (2024c, August 28). Preventing Mosquito Bites. Retrieved September 12, 2024.

Centers for Disease Control and Prevention. (2024d, May 17). West Nile: Symptoms, diagnosis, & treatment. Retrieved September 10, 2024.

Centers for Disease Control and Prevention. (2024e, September 3). West Nile virus current year data (2024). Retrieved September 10, 2024.

Fagre, A. C. (2023). West Nile Virus and Other Nationally Notifiable Arboviral Diseases—United States, 2021. MMWR. Morbidity and Mortality Weekly Report, 72.

Lacey, L. (2007). Bacillus thuringiensis serovariety israelensis and Bacillus sphaericus for mosquito control. Journal of the American Mosquito Control Association, 23(sp2), 133–163.

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.

McDonald, E., Mathis, S., Martin, S. W., Erin Staples, J., Fischer, M., & Lindsey, N. P. (2021). Surveillance for West Nile virus disease—United States, 2009-2018. American Journal of Transplantation: Official Journal of the American Society of Transplantation and the American Society of Transplant Surgeons, 21(5), 1959–1974.

Disclaimer: This project was supported by Cooperative Agreement Number NU50CK000637 from the Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention.