Understanding Public Health Threats Posed by Mosquitoes

Editor’s Note: All cited sources can be found in the digital edition version of this story on pctonline.com. This article originally appeared in the November print version of PCT under the headline "Danger Zone"

Mosquitoes continue to make headlines as disease outbreaks have cropped up in the United States and Europe. Accountable for more than 700,000 deaths worldwide every year, mosquitoes are considered the world’s deadliest animal (Gates, 2014). By comparison, sharks are responsible for only 10 deaths per year. Even elephants are more deadly than sharks, but I cannot remember the last time I saw groups of elephants freely roaming the streets. Andrew Spielman, in the 2001 book “The Mosquito,” summed up the little pest’s outsized role:

“No animal on earth has touched so directly and profoundly the lives of so many human beings,” he wrote. “For all of history and all over the globe she has been a nuisance, a pain, and an angel of death. Mosquitoes have felled great leaders, decimated armies, and decided the fates of nations. All this, and she is roughly the size and weight of a grape seed.”

WHAT MAKES THEM DEADLY? Mosquitoes are not only a backyard nuisance but also a public health menace. One of the first recorded cases of a mosquito-borne disease occurred in 1783 as the American Revolutionary War was coming to an end. Soon-to-be President George Washington wrote to his nephew that, “Mrs. Washington has had three of the ague and fever and is much with it — the better, having prevented the fit yesterday by a plentiful application of the bark — she is too indisposed to write to you.” This so-called “ague and fever” is better known today as malaria (Stewart, 2011). Eventually, both Martha and President Washington suffered from the disease. The treatment in those days was quinine, which was found in the bark of South American trees. Although quinine was widely used in Europe, Washington did not have access to it until later in life. This unfortunately led to severe hearing loss, a well-known side effect of quinine toxicity, during his second term (Stewart, 2011).

In addition to malaria, mosquitoes are also known for carrying other arboviruses, such as yellow fever, dengue, chikungunya and Zika. Although treatable (except for Zika), these diseases are still widely transmitted throughout the world (CDC 2015, 2016). Zika has been a concern in the past decade with the lack of specific medical treatment and its ability to cause microcephaly in newborns (Rasmussen et al. 2016). In 2017, the Centers for Disease Control & Prevention (CDC) reported that there were a total of 449 imported cases of Zika — seven of those were locally transmitted from Florida and Texas; however, since 2019, no new cases of Zika have been confirmed in the continental U.S. or its territories.

THAT’S NOT ALL. Those illnesses are not the only ones mosquitoes can carry. The Aedes genus of mosquitoes alone is responsible for more than 22 different arboviruses. That leads to discussion on other species of mosquitoes and the diseases they are responsible for. Recently in the Northeast U.S., there have been outbreaks of eastern equine encephalitis (EEE). EEE is spread to humans through the bite of an infected mosquito, specifically Culex mosquitoes. This disease can cause a wide variety of symptoms ranging from mild to severe. These symptoms include fever, headaches, GI symptoms, seizures, and coma, among others. One-third of these cases are often considered severe and lead to death. This disease is rare, responsible on average for 11 human cases each year in the U.S. However, as of September 2024, there have already been 11 human cases confirmed. More than likely, there are more, according to the CDC. One of these deadly cases occurred in New York, which was its first case of EEE since 2015. CDC says that only 4-5 percent of human EEE infections will lead to illness; however, when they do, it has the potential to be lethal, as was the case in New York.

Most patients that experience symptoms will recover completely from the illness in one to two weeks. Although, and I have seen this firsthand, EEE has the potential to impact a patient’s central nervous system, and when it does, it can lead to meningitis or encephalitis (inflammation of the brain).

So where is EEE typically found? Historically, it has been more prevalent in Gulf Coast states; however, this year is proving to be the exception to the rule. According to the CDC, as of Sept. 17, the following states had reported human cases of EEE: Massachusetts (four), New Hampshire (two), New Jersey (one), Rhode Island (one), Vermont (one) and Wisconsin (one). None of these are Gulf Coast states, and this count also did not include the most recent case resulting in death in New York.

This year also has seen significant cases of West Nile virus (WNV). CDC has reported that among 38 states, there have been 748 cases of WNV so far in 2024; last year, there were more than 2,500 cases recorded nationwide, which nearly doubled from 2022. WNV is another disease spread by Culex and Aedes mosquitoes and birds. Birds serve as the reservoir for the disease, and mosquitoes will spread it to humans after feeding on an infected bird.

THE THREAT. Aside from their role as disease carriers, mosquitoes pose such a great threat because they live and breed in close proximity to humans. Approximately 80 percent of people around the world live in urban areas, and because of the density within these urban communities, mosquitoes are more prevalent. As these mosquitoes multiply, so will the disease rates, because humans will become more likely to encounter mosquitoes.

Mosquito population numbers are both climate and habitat driven. Mosquitoes prefer warm, temperate environments, not something that we typically associate with the northern U.S. So historically, when you look at disease numbers as indicated by the CDC, counts are generally higher in the summer months, especially August, when it is uniformly warm nationwide.

However, weather is starting to trend to be warmer for longer now, and if these trends continue, the mosquito populations will not only grow, but will expand. Researchers from several universities are predicting that as the years march on, mosquito populations will begin spreading southeast up the Eastern Seaboard and west into the Midwest. As the mosquitoes travel, so too do the diseases they carry.

WHAT DO WE DO? As a general rule, PCOs should continue their mosquito control efforts. Providing mosquito control is becoming more and more necessary. Mosquito season is expanding, and in turn, the need for treatment is too. It is important not only to treat, but also to educate your customers. Educating customers on where mosquitoes are breeding and how best to protect themselves against them will help slow the spread of these diseases.

Finally, protect yourself and your employees. Although we treat and apply products to help reduce mosquito populations, that does not prevent us from being bitten. Wearing long sleeves and repellent can go a long way to protect you. Mosquitoes are here to stay, so it is important that we put up our best defenses.

The author is a board-certified entomologist.