It’s Peak Mosquito Season: A Refresher on Mosquito-borne Infections

“We’ve never had to fight malaria in cities like this before. This mosquito is rewriting the rules of malaria transmission.”

Gary Frandsen, CEO, Oxitec

“As summer ushers in peak mosquito season, health and vector control officials are bracing for the possibility of another year of historic rates of dengue.”

Phillip Reese, journalist, KFF News

The mosquito is widely regarded as the most deadly animal in the world. It transmits pathogens that cause at least a dozen infections that kill an estimated 700,000 people globally per year. Members of the estimated 3,600 mosquito species are found on every continent except Antarctica. (It is just too cold and devoid of standing water for mosquitoes to survive there.)

There are 113 different genera of mosquito. The three that stand out as our biggest enemies are:  Anopheles (carriers of malaria); Aedes (carriers of dengue viruses); and Culex (carriers of West Nile virus). In this week’s post, I highlight some new developments regarding these three mosquito genera.

Anopheles mosquitos. Anopheles mosquitoes are found throughout the tropics and subtropics of Africa. These mosquitoes are capable of transmitting Plasmodia, the parasite that causes malaria.

Malaria is the leading cause of preventable illness and death in the world. A vast majority of malaria cases and deaths occur in Sub-Saharan Africa. In 2023, the World Health Organization African Region accounted for 95% of all global malaria deaths.  To make matters worse, Anopheles stephensi, the Asian malaria vector, recently emerged in Africa presenting a growing challenge as it flourishes in cities and is resistant to insecticide-impregnated bed nets.

Fortunately, malaria is no longer endemic in the U.S. (It’s estimated, however, that 30,000 soldiers died of malaria in the Civil War.) Nonetheless, there are Anopheles mosquitoes in the U.S., and the mere presence of these mosquitoes means there is a risk of recurrence.

Recent breakthroughs in malaria research include the introduction of first-generation vaccines and ongoing research on mRNA vaccines. The mainstay drug for treatment of malaria is artemisinin. But because of the emergence of artemisinin resistance, novel drugs with new modes of action are currently undergoing clinical trials. In addition, new diagnostic tools as well as innovative methods for vector control are being rolled out.

Last week, Novartis announced a new malaria treatment for newborns and infants under 11 pounds. (See, “First Malaria Drug Approved for Newborns and Small Babies,” HealthDay News, July 10, 2025). “Coartem Baby” (also known as “Riamet Baby”) contains artemether and lumefantrine, two drugs that were previously approved for use in patients weighing over 11 pounds that work together to fight the malaria parasite. Novartis plans to offer the new treatment on “what is essentially a non-profit basis in places where malaria is common.” Its CEO  commented, “[We’re] proud to have gone further to developing the first clinically proven treatment for newborn and young babies, ensuring even the smallest and most vulnerable can finally receive the care they deserve.”

Reducing malaria cases and averting deaths from this preventable disease is a global priority. “Since its inception in 2005, the U.S. President’s Malaria Initiative (PMI) has played a major role in the reductions in malaria morbidity and mortality observed across Africa.” (Symons, T.L., et al., “Estimating the potential malaria morbidity and mortality avertable by the US President’s Malaria Initiative in 2025: a geospatial modeling analysis,” The Lancet, June 23, 2025). According to the article, continued investment in the PMI, “would directly translate into millions of malaria cases averted and approximately 100,000 lives saved across its focus geographies in Africa throughout 2025.”

One hopes that the U.S. will continue its investment in PMI. But, as the current U.S. administration seems to be cutting anything and everything that deals with global health, the fate of the PMI is unclear. 

Aedes mosquitoes. A. aegypti and A. albopictus are the two species of Aedes mosquitoes that transmit human pathogens. Both can transmit the four recognized dengue viruses, designated DENV-1, DENV-2, DENV-3, and DENV-4. These viruses cause up to 400 million cases and 40,000 deaths per year, in over 100 countries, mainly in Central and South America and South-East Asia.

Most cases of dengue in the continental U.S. occur in travelers from other countries. Nonetheless, local spread is increasing. Florida, Hawaii, Texas, Arizona, and California have reported indigenous dengue cases. (See, “Dengue Virus Descends on Florida: Why and So What?,” Germ Gems, August 23, 2023 and “Dengue Fever, a Barometer of Climate Change,” Germ Gems, July 3, 2024, highlighting the consequences of the arrival of the dengue virus in the U.S. and the development of indigenous cases of dengue).

Since climate change supports the expansion of mosquito vectors, dengue isn’t going away any time soon. It therefore seems likely that many areas of North America will be joining the “dengue deluge.” (See, “As Mosquito Season Peaks, Officials Brace for New Normal of Dengue Cases,” KFF News, July 6, 2025).

There are no specific antiviral drugs to treat dengue. Dengue vaccines are, however, available. But decisions regarding if and when they should be administered are complex and need to be guided by a well-informed health professional.

Culex mosquitoes. West Nile virus (WNV) is a neurotropic flavivirus spread by Culex mosquito species. Currently, WNV is by far the leading cause of mosquito-borne disease in the contiguous U.S. In 2024, the U.S. reported 1,466 cases of WNV with 134 deaths. Older adults and people who are immunocompromised are at highest risk of severe disease and death.

Birds serve as reservoirs for WNV. More than 300 bird species have been identified as targets for WNV, almost double the 173 wild bird species targeted by H5N1, the bird flu virus. Infected birds amplify the virus, and infected mosquitoes transmit the virus to humans. We humans, along with horses, are “dead-end hosts,” meaning we don’t transmit the virus to other susceptible hosts.

While numerous advancements in understanding the pathogenesis of WNV have occurred, an effective vaccine and antiviral therapy remain elusive goals. Nevertheless, one thing that is clear: the best way to prevent WNV, as well as all other mosquito-borne diseases, is to avoid being bitten by mosquitoes.

Combatting mosquitoes. It is important to do everything you can to minimize mosquito bites. Using screens on windows and doors can help keep mosquitoes out of your home. The Centers for Disease Control and Prevention (CDC) recommends wearing protective clothing, eliminating standing water where mosquitoes breed, and using insect repellent. For more detailed information regarding what type of mosquito repellent to use, etc., visit the CDC’s website “Preventing Mosquito Bites.”

Additionally, if you are planning a trip to a country with known mosquito-borne diseases, visit the CDC’s website, Travelers’ Health.  And, if you are one of the people that mosquitoes “love,” read Connie Chang’s National Geographic article “You really are a mosquito magnet. Here’s what you can do about it.” (Originally published, June 16, 2023, updated May 30, 2025). It may help you understand why these thirsty bloodsuckers can’t seem to get enough of you.