Mosquito Diseases and the 2020 Mosquito Season

William Sames, Ph.D. - As you read this article, the world is working through the COVID-19, Coronavirus, pandemic. All of us have been affected in some form or fashion, and as we are seeing, COVID-19, like many other flu viruses, is spread through close human interactions. In the all-out effort to reduce the effects of this disease, social distancing has become the norm.

Going through this disease process may not be easy, and it will significantly impact the lives of many. However, in the aftermath of COVID-19, life will go on. It may not be what we had in the past, but we will recover from its effects and move forward with revised life goals and objectives. As humans, we tend to learn from adversity. Perhaps, what we learn from the challenges of COVID-19 will help us prevent or mitigate future pandemics.

Fortunately, vector-borne diseases such as those transmitted by mosquitoes are much more predictable in their transmission and disease cycle, and therefore, preventable. In the United States, many of us are so far removed from vector-borne diseases that we hardly think of them as a problem. Yet, less than 100 years ago, the US Public Health Service and state agencies worked to eliminate malaria from the United States. Every year, thousands of people died from disease and had been doing so for a long time. For example, Faust (1942) reported 1346 malaria deaths in 1940, which was the lowest on record in the United States at the time of the report. Malaria is still the number one fatal mosquito-borne disease in many countries (World Health Organization 2019).

In 1900 (120 years ago), the Walter Reed Commission was just starting the experiments in Havana, Cuba, that proved that a mosquito, specifically Aedes aegypti, was responsible for transmitting the yellow fever virus (Clements and Harbach 2017). They confirmed the virus could not be transmitted from human to human or by coming into contact with an infected, dying, or dead person's soiled clothing. The virus had to be transmitted through the mosquito. It was known that survivors of yellow fever did not get it again; they had life immunity.

Dr. William Gorgas, an Army medical doctor who survived yellow fever while stationed at Fort Brown, Brownsville, TX, was the medical officer in charge at the base in Havana and witnessed the experiments (Gorgas and Hendrick 1924). Later, he was appointed to direct the mosquito control efforts in Panama to mitigate yellow fever and malaria amongst Panama Canal workers. Through his leadership, yellow fever was eliminated, and the incidence of disease much reduced. With a healthier workforce, engineers could focus on the task at hand, and they completed the Panama Canal.

Dr. Gorgas was promoted to Major General and was the Army Surgeon General during World War I. He stressed medical readiness and applied for preventive medicine. Many of the procedures and standards that he initiated are still part of modern medical practices. Through his efforts, World War I was the first war in which the number of deaths caused by military action (bullets, bombs, gas, etc.) exceeded those of disease. In all previous wars, the disease killed more soldiers than military action (Gibson 1950/1989).

The first incidence of yellow fever in the Americas was thought to be in Spanish Florida in 1649-1650 (Patterson 1992), and until 1900, no one knew what caused yellow fever or how it was transmitted. Many concluded that there had to be some human to human transmission like was observed for flu viruses or when doctors learned to wash their hands before delivering a baby, which significantly increased post-partum survivability rates of women (NPR 2015). Sanitation efforts were tried to prevent yellow fever, but they did not work. So more intense sanitation efforts were employed, and those too did not work. Reed and Carrol (1901) said: "its ravages were to completely paralyze both the social and commercial interests of a given city and even of an entire section of our country." This sounds familiar to our current COVID-19 situation.

The general populace saw a person who could go from healthy to dead in about three days of an increasingly debilitating disease for no apparent reason. "The interval between 1793 and 1888 is almost one hundred years, but upon the appearance of yellow fever, we observe no difference of behavior on the part of the inhabitants of Jackson, Miss., in 1888, from that shown by the citizens of Philadelphia in 1793, except that the terror of the former was greater and their flight from their homes more precipitate than in the case of the latter. Therefore, the recurrence of succeeding epidemics has served to increase rather than lessen the public alarm" (Reed and Carrol 1901). A lack of knowledge caused much of this panic. Today, we know how yellow fever is transmitted and can take action to prevent it. We also have almost instant communications around the world and news updates every hour. Medical expertise on how to prevent and mitigate disease can be disseminated rapidly.

Aedes aegypti is a human-loving mosquito. It loves us so much that it is rarely found very far from human habitations, and their larvae develop, almost exclusively, in artificial containers. Over the centuries, Aedes aegypti became very good at transmitting several diseases to humans to include yellow fever, dengue fever, chikungunya, and Zika Virus. With the insight from the Walter Reed Commission's experiment, mosquito control against Aedes aegypti was implemented. In less than five years, yellow fever outbreaks in the United States were eliminated (Patterson 1992).

Malaria was a different story. The pathogen for malaria is a parasite, and there are several species of these parasites. The most common are Plasmodium falciparum and Plasmodium vivax. Neither are good, but vivax malaria tends to have few fatalities, and falciparum malaria, if untreated, has a high fatality rate. Both circulated in the United States before 1900, with the deaths primarily attributed to falciparum malaria (Moreno-Madrinana and Turell 2018).

Malaria is a mosquito-borne disease, but mosquitoes can only carry it in the genus Anopheles. Not all Anopheles can transmit malaria, but several species in the United States can transmit it. Anopheles mosquitoes live in swamps and wetlands and are widespread compared to Aedes aegypti, which love to live near humans. In the early 20th century, massive efforts to drain swamps and wetlands were undertaken, and methods to kill mosquito larvae with Paris Green and oil were implemented. Later, DDT and other insecticides were developed and thought to be the cure-all for mosquito-borne diseases. By the mid-1950s, endemic malaria in the United States was eliminated.

Since then, the United States has dealt with travel cases of malaria and other mosquito-borne diseases. However, some mosquito-borne diseases are endemic or were imported into the United States, and periodic outbreaks still occur. These diseases include West Nile virus, St. Louis encephalitis, Eastern Equine encephalitis, Western Equine encephalitis, Cache Valley virus, and others. In 1971, there was a surprise outbreak of Venezuelan Equine encephalitis in Texas (Olson and Grimes 1974), then it disappeared and has not been detected in the United States since then.

We are fortunate to have exceptionally talented and dedicated medical personnel helping us through the COVID-19 outbreak. Whether it be the service and guidance of those at the Centers for Disease Control or the Public Health Service or those medical personnel who are on the front lines treating patients and working to prevent further infections or those who are working to develop diagnostic testing, vaccines, or treatment protocols, they are doing their part to bring us through this pandemic. Similarly, service personnel from many disciplines work hard to keep food and supplies moving, utilities working, and maintain normalcy.

Similarly, there are mosquito surveillance and control personnel who are getting ready to protect you and your community from the potential effects of mosquito-borne disease. In 2019, the CDC received 38 confirmed reports of human Eastern Equine Encephalitis virus. Also, the West Nile virus has been a concern since its introduction in 1999. While the numbers for most years have decreased, the West Nile virus is still present, and through funded and effective programs, its effects can be further reduced.

Researchers at universities and private companies conduct studies to understand better these diseases' nature, the biology and distribution of the vectors, and how to improve diagnostics, treatments, surveillance, and control methods. There has been much improvement in these areas since the Walter Reed Commission's study's results were released in 1901, and each year, new products and methods are released or implemented that are even better than those from the not so distant past.

As we go into this mosquito season, like the COVID-19 outbreak, we don't know exactly how it will play out, but you can do things to prevent mosquito-borne disease in yourself and the ones you love. First, support local mosquito control and surveillance efforts. Ensure they do a good job and ensure they receive the funding, training, and supplies they need. Second, use personal protective measures to avoid being bitten by any mosquito. Third, if you have a yard and are interested in creating a barrier around your home to ward off mosquitoes and ticks, contact your FlyFoe representative to assess your situation and recommended treatments.


Clements AN, Harbach RE. 2017. History of the discovery of the mode of transmission of yellow fever virus. J Vector Ecol 42:208-222.

Faust EC. 1942. In the southern United States for the year 1940, malaria mortality with supplementary data on malaria in other states. J Nat Malaria Soc 1:125-131.

Gibson JM. 1950/1989. The physician to the world: the life of General William C. Gorgas. Tuscaloosa, Alabama: The University of Alabama Press.

Gorgas MD, Hendrick BJ. 1924. William Crawford Gorgas: his life and work. Garden, New York: Doubleday, Page, and Company.

Moreno-Madrinan MJ, Turell J. 2018. History of mosquito-borne diseases in the United States and implications for new pathogens. Emerg Infect Dis 24: 821-826.

NPR. 2015. The doctor who championed hand-washing and briefly saved lives. Accessed March 29, 2020.

Olson JK, Grimes JE. 1974. Mosquito activity in Texas during the 1971 outbreak of Venezuelan equine encephalitis (VEE). II. Virus incidence in mosquito samples from the South Texas Plains. Mosq News 34:178-182.

Patterson KD. 1992. Yellow fever epidemics and mortality in the United States, 1693-1905. Soc Sci Med 34:855-865.

Reed W, Carroll J. 1901. The prevention of yellow fever. Pub Health Pap Rep 27:113-129.

World Health Organization. 2019. World malaria report, 2019. Accessed March 29, 2020.

Posted | 07.28.2020

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