Mosquitoes and the Diseases They Carry: What is Your Risk of Infection?

The association between human disease and mosquito bites was suspected for millennia. The symptoms of malaria are described in an Indian Sanskrit text titled Susruta Samhita dating from around 600 BCE. The text attributes the source of infection to the bites of insects. A scientifically-based association between mosquitoes and human disease was proposed in 1881 by Carlos Finlay, who worked with the yellow fever virus in Cuba. The association was conclusively confirmed in 1897 by the Scottish physician Sir Ronald Ross, who described the complete life cycle of avian malaria in India.

Mosquito-borne disease has played an important role in U.S. history. Yellow fever epidemics decimated port cities from Boston to Pensacola between 1668 and 1905. Building on Finlay’s work, the physician Colonel William Gorgas designed a mosquito control program targeting Aedes aegypti, the species known to transmit the yellow fever virus. Gorgas was appointed as the chief sanitation officer for the U.S. Panama Canal construction project in 1904 after the U.S. purchased the project from the French in 1903. It is generally believed that the failure of the French canal project (1881 to 1894) was due in part to mosquito-borne disease. Under Gorgas’ leadership mosquito-borne diseases (mainly malaria and yellow fever) were virtually eliminated from the construction zone by 1906, allowing the successful completion of the U.S. Panama Canal project (1904–1914).

Malaria had a major dampening effect on the economic development of the U.S., especially in the south and mid-Atlantic regions, from 1850 through the early 1900s. Major epidemics caused by the St. Louis encephalitis virus affected many regions of the continental U.S. during the mid-20th century. West Nile virus has disrupted the economies of many U.S. states since its invasion of North America in 1999. The threat of emerging and re-emerging mosquito-borne diseases such as malaria, dengue, chikungunya, and even yellow fever has resulted in public health emergencies around the globe. Recently, the Zika virus made the jump from Africa through Asia and Micronesia into the New World. The threat of the Zika virus on our doorstep has become an important part of the U.S. political landscape in the 21st century.

How Do Mosquitoes Become Infected?

The process through which a mosquito becomes infected with a disease agent (or pathogen) is complex. Briefly, an adult female mosquito that is susceptible to infection must first blood-feed on a vertebrate host that has a pathogenic agent (such as West Nile virus) circulating in its blood. The pathogen is swallowed with the blood, which moves into the mosquito’s stomach for digestion, giving the mosquito that swollen red look that we know so well! When the blood enters the stomach, the pathogen must quickly escape by passing through the stomach wall and into the mosquito’s body cavity. As the pathogen circulates throughout the mosquito’s body, it infects a variety of organs including the salivary glands. This process can take up to several weeks and is sensitive to changes in temperature: the warmer it is, the faster mosquito infection occurs. This is why mosquito-borne diseases are most common during the summer. When a pathogen infects the salivary glands, the female mosquito remains infected for the rest of her life and she can pass the pathogen on to a new host every time she blood-feeds.

The Risk of Mosquito-Borne Disease Transmission in the U.S.

Mosquito-borne diseases were prevalent throughout early U.S. history. Malaria was widespread in America east of the Rocky Mountains during all of the 19th century. There were several reasons for the high malaria transmission rates. First, houses lacked window and door screens, allowing mosquitoes the free access into the homes. Second, there were no organized mosquito control programs and existing public health efforts were not directed toward identifying and controlling malaria parasites. This would change in the early 20th century. For example, in 1919 the malaria infection rate for humans living in rural Taylor County, Florida was 65 percent. There was an intensive effort that year to identify all of the malaria transmission hotspots in Taylor County and to control the Anopheles mosquitoes responsible for transmitting malaria in those areas. In 1920, the transmission rate dropped to 6.5 percent as a result of the Anopheles control effort. By the middle of the 20th century, mosquito control and public health efforts had drastically reduced the incidence of mosquito-borne disease throughout most of the country. Screens and air conditioning effectively eliminated indoor biting mosquitoes in most areas of the country.

However, outbreaks of mosquito-borne disease in the U.S. continued into the 21st century. The well-documented march of West Nile virus through the continental U.S. indicated that many people still had a significant mosquito exposure risk. In the 18th and 19th centuries, mosquitoes living in our homes and entering through unscreened windows carried with them the greatest threat of disease transmission. In the 21st century, it is the mosquitoes that we encounter in our backyards and during our evening outdoor activities that pose the greatest risk.

When weighing the risk of acquiring a mosquito-borne disease, it is important to recognize the difference between locally-acquired infections and imported infections. Imported infections result when individuals acquire a disease while traveling away from home and are diagnosed after they return home. Imported infections usually result in a reduced risk for individuals living in the traveler’s community because the infected mosquitoes responsible for the imported infection are hundreds or thousands of miles away. For example, the spread of Zika virus from the Old to New Worlds was facilitated by the ease and speed of modern travel that allows individuals infected in one area to travel to another part of the world carrying the virus with them. At their new destination, some infected individuals may encounter Aedes aegypti mosquitoes, which blood-feed, acquire the virus, become infected, and start a new outbreak of locally acquired infections. These locally acquired outbreaks are of great concern because once they begin, they are very difficult to control. One of the greatest concerns about the ongoing Zika pandemic is that a person infected outside of the U.S. may return with virus in their blood and be bitten by susceptible mosquitoes, resulting in the start of a locally acquired human transmission cycle.

How Can We Protect Ourselves Against Mosquito-Borne Diseases?

Mosquito-borne diseases continue to pose a significant global human health risk. Throughout much of modern America, mosquitoes are infrequent visitors in our homes. However, they are all around us. Some important species, such as Ae. aegypti, were virtually eradicated by mosquito control programs in the 1960s, but their populations have recovered to record numbers, especially in the deep south. Most of us know from personal experience that mosquitoes are nearly impossible to avoid, but there are several tried and true ways that individuals can protect themselves from mosquitoes and the diseases they carry:

Be Aware. Public health officials and mosquito control organizations track and disseminate information about local mosquito-borne disease outbreaks, including information about recently imported cases. If the threat of local disease transmission is high, mosquito control and public health officials will alert the public through radio, TV, print, and social media outlets. Be aware and take advantage of these information sources.

Avoid Mosquitoes. Many mosquito species are most active at dawn and dusk. Mosquitoes prefer high humidity and low wind, which is why they are so abundant in heavily-vegetated habitats. Adjust your activities to avoid peak mosquito biting times and preferred mosquito habitats.

Cover Up. When venturing into prime mosquito country, cover as much skin as possible with breathable and tightly woven fabrics that prevent mosquito blood-feeding.

Use Repellents. The Centers for Disease Control and Prevention recommends the use of repellents containing one of three active ingredients: DEET, oil of lemon eucalyptus, or picaridin. The use of a repellent that contains one of these active ingredients should provide at least 90 minutes of protection with a single application.

Clean Up. Most yards contain hundreds of mosquito breeding sites, many of which are well hidden. Get rid of or empty water-filled natural and artificial containers that may provide mosquito egg-laying habitats.

Travel Safe. Be aware of the mosquito-borne disease risk reported in your destination city and country. Remember that malaria, dengue, chikungunya, West Nile, Zika, and even yellow fever are unwelcome diseases that may be present at your next vacation destination. Don’t be afraid to change plans to avoid a mosquito-borne disease epidemic.