On June 26, 2023, the Centers for Disease Control and Prevention issued an official Health Advisory. Four cases of malaria had been reported in Florida and one case in Texas over the preceding two months. While an estimated 2,000 or so cases of malaria are diagnosed in the U.S. in an average year, those are almost always travel cases, or cases reported in patients who have recently traveled to a country where malaria is common. 

These five cases were different. 

None of the patients had recently traveled. They had all been infected with malaria right here in the U.S. Locally acquired malaria cases had not been reported in the U.S. in 20 years.

A total of ten locally acquired cases were reported over the course of 2023. Seven of the cases were reported in Florida, one in Texas, one in Maryland, and one in Arkansas. Local health departments in these states, along with the CDC, stepped up their mosquito monitoring and control efforts to identify the source of these cases and prevent any further outbreaks. All patients were treated, none developed severe malaria, and all recovered. 

That same year, there were 263 million cases of malaria across the world, and an estimated 597,000 deaths. Malaria remains an extremely common and deadly disease, particularly in sub-Saharan Africa and India. In addition, following severe flooding in 2022, Pakistan saw a notable rise in malaria burden. While malaria is treatable, prompt diagnosis and care are critical. Left untreated, malaria can cause severe symptoms including seizures and organ damage, and many of these more serious cases are fatal. Children under five are the most likely to die.

While ten treatable cases of malaria may not seem like much compared to the enormous global burden of this deadly disease, the fact that this very small number of cases were identified and immediately acted upon is the sign of a highly sensitive and well-functioning public health system in the U.S. Malaria used to be as common and deadly here as it is in other countries today. But the U.S. set an ambitious goal of getting rid of malaria, and in 1951, decades of medical research and public health investment paid off. We successfully eradicated malaria.

So how did we manage this remarkable feat? And what do we need to do to keep our country malaria-free?

First, what is malaria?

It begins in the blood. A female mosquito bites a human being. As the mosquito draws blood up through its long, straw-like proboscis, it simultaneously spits out secretions that have anti-coagulant and anesthetic properties. The anti-coagulants help the mosquito draw fresh blood without clotting and the anesthetics help the mosquito take its bloodmeal without being detected.

Female mosquitoes need fresh blood to nourish their eggs. Many species of mosquitoes lay eggs more than once in a lifetime, meaning they need more than one bloodmeal. Without a recent bloodmeal, the female mosquito cannot reproduce. If the first person or animal it bites has an infection in their bloodstream, the mosquito can become infected as well and pass the infection along to the next person or animal it draws blood from. The mosquito digests all of the blood it draws from its first host before moving onto its second, so blood does not pass from host to host. But, like a contaminated syringe, the mosquito can still pass infections from one unwitting victim to the next. 

Not all diseases that mosquitoes carry can infect humans, and most human diseases cannot be carried by mosquitoes either, not even bloodborne illnesses like HIV. There is only so much overlap between mosquito and human biology, but right in the center of that overlap sits malaria.

Malaria is caused by a parasite, or an organism that lives by feeding off a host. It is transmitted by Anopheles mosquitoes, a group of mosquitoes with shimmering scales and a preference for biting in the evenings or at night. Anopheles mosquitoes are the only type of mosquitoes known to carry the malaria parasite.

When a human being gets bitten by an infected Anopheles female mosquito, about a hundred tiny parasites enter the human body through the skin. A portion of them pass into the bloodstream, and those that do quickly make their way to the liver. Once in the liver, the parasites quietly begin to reproduce. At this initial stage of infection, there are no symptoms. Then, about 7-10 days later, they make their next move.

Once the parasites have had time to replicate in the liver, they set out to explore the rest of the body. Taking advantage of human biology, the parasites hitch a ride inside little cellular transport vehicles known as vesicles. In a healthy body, these vesicles transport important materials or information from one cell to another. In a body infected with malaria, these same vesicles get hijacked by the parasites and used as free transportation, carrying them through the heart and into the lungs. The vesicles then disintegrate, and the parasites move again, this time into the blood cells themselves.

Within 2-3 days, the infected blood cells burst open. The parasites spill back out into the bloodstream and then invade more blood cells. Malaria symptoms tend to get stronger or weaker depending on what stage the parasites are in. Fever and chills are typical, and they may get worse whenever the next round of red blood cells ruptures, but temporarily recede when the parasites are comfortably ensconced inside a new set of host cells.

While most parasites reproduce inside human cells, some circulate freely throughout the bloodstream instead. If a person infected with malaria gets bitten by another mosquito, the new mosquito can pick up these circulating parasites, which then begin to mature and reproduce inside the mosquito, eventually spilling out of the mosquito’s gut and making their way into its saliva. When that mosquito bites another human, the cycle begins again. If enough parasites build up in the body, they can block blood flow or damage vital organs, including the brain. If the patient is not successfully treated before this stage, they die.

When did malaria first arrive in America?

Europeans likely carried two strains of the malaria parasite with them to the Americas during the 1500’s. Native Americans, who had never been exposed to the parasite before, were very susceptible to this new disease. Outbreaks got worse with the transatlantic slave trade, as a more deadly version of the parasite circulated in Western Africa, where slaves were being captured from. In addition, while native Africans may have had some immunity as well as some genetic protection from malaria, after being forced into slavery in a foreign environment, their advantage was lost. Immunity to malaria wanes quickly. Even people who grow up in highly endemic areas can lose any protection they have within months of leaving their home.

Environmental changes also contributed to malaria outbreaks during this time. Deforestation and certain agricultural practices created more hospitable breeding sites for Anopheles mosquitoes, which lay their eggs in water. The cultivation of rice, for example, may have been a factor in sustaining or increasing populations of Anopheles mosquitoes. Irrigation practices that are used to grow rice also happen to create very favorable conditions for Anopheles mosquitoes.

Control and prevention methods were limited during this time. Scientists still did not understand exactly what caused the disease or how it was transmitted. The malaria parasite was not discovered until the 1880’s, and even then, it took time for the early findings to be confirmed and shared with the world. A treatment method using quinine became popular in the 1800’s, as chemists discovered a way to isolate quinine from the bark of a particular kind of Peruvian tree, but there was still no reliable way to prevent people from getting the disease in the first place. 

From Massachusetts to Mississippi, outbreaks routinely plagued the country. During the Civil War, there were an estimated 1.3 million cases of malaria among the troops, and around 10,000 fatalities. Soldiers were more afraid of malaria, which they called “the shakes”, than they were of enemy bullets. Union soldiers had better access to quinine than their Confederate counterparts. In the early days of the war, an ounce of quinine cost around $5 in New York and $60 in South Carolina. By the end, that same amount sold in the Confederate states for around $600, or roughly 40 times the amount a Confederate soldier would have made in a year.

How did we get control of these deadly malaria outbreaks?

When construction on the Panama Canal began in 1904, research on malaria had advanced significantly. By this point, scientists knew which species of mosquito was responsible for spreading malaria and they understood that mosquitoes caused the disease by injecting a parasite into the bloodstream. They had also realized that standing water created ideal breeding habitats for the offending Anopheles mosquitoes.

In Panama, where the U.S. was leading the effort to build the canal, the U.S. Public Health Service was able to apply this new knowledge about malaria to develop a comprehensive mosquito control program. The USPHS focused its efforts on improving sanitation, removing standing water, and killing mosquito larvae in areas where water could not be drained (a process known as larviciding). The doctor in charge of mosquito control in Panama, Dr. William Gorgas, was so strict that even the holy water in the Panama cathedral had to be changed regularly to avoid becoming a mosquito habitat.

While larviciding methods today are rigorously tested for human, animal, and environmental safety, back then the USPHS created a cheap and toxic combination known as the “Panama Mix”, which consisted of sawdust, kerosene, pine rosin, caustic soda, and crude oil. The mix was first sprayed over an area, and then set on fire. (Note: do not try this at home!)

Many workers on the Panama Canal still died of malaria, but the death toll was far less than it had been just a few decades prior in the 1880’s when some 20,000 workers died, and efforts to build the canal failed. Mosquito control efforts had come a long way in just 20 years.

In 1933, the Tennessee Valley Authority was established under the New Deal to help improve living conditions in Tennessee and surrounding states. One of its objectives was to help monitor and control malaria in the region. A door-to-door survey in northern Alabama in 1934 found that on average 30 percent of blood samples were positive for malaria. The disease was endemic in the Tennessee Valley region.

Early efforts through the TVA focused on removing timber and other debris from areas with standing water to help improve drainage, and identifying areas for larvicide treatment. Subsequent efforts grew more sophisticated, including water level management and application of new types of larvicide.

In addition, the TVA partnered with state and local health officials on a robust outreach campaign, teaching at-risk communities about the risk of malaria. Elementary school and high school education included lessons on malaria, as did teacher-training curriculums. By 1950, these efforts paid off. Surveys in the Tennessee Valley showed no parasites in any of the blood samples, even in the highest risk areas. The program was a success.

Around the same time, a branch of the USPHS called the Centers for Disease Control began getting involved in the anti-malaria effort as well. The newly formed CDC focused on treating homes in rural areas with insecticides. In just two years, they reached over 4.6 million homes. CDC also supported efforts to improve drainage wherever possible and even ran aerial spraying missions to control the most persistent mosquito populations. Cases of malaria around the country continued to drop.

In 1951, malaria was officially eradicated from the U.S.

How do we keep malaria from returning?

The U.S. public health system continues to monitor both mosquitoes and patients alike for any signs of malaria. Doctors carefully monitor all patients who are diagnosed with malaria, and work with local and state health officials to confirm what kind of malaria the patient has and where they got it from.  Any locally acquired case will prompt immediate action to target the infected mosquitoes and keep surrounding communities safe.

Since 1951, the definition of disease eradication has been updated and malaria would now be considered an “eliminated” disease, under control within the country but requiring active efforts to keep it that way. Eradicated is now used to mean there is no risk for the disease anywhere across the globe. Yet, the change in terminology does not diminish this incredible American accomplishment in any way. Thanks to decades of hard work to get malaria under control, and a very sensitive disease monitoring system, the risk of widespread malaria outbreaks coming back to the U.S. is very low.

However, there are other mosquito-borne diseases that do routinely cause outbreaks. In 2021, Arizona experienced a deadly outbreak of West Nile Virus, the largest ever recorded in this country. Over a hundred people died. In 2024, the island of Puerto Rico experienced an epidemic of dengue, which has still not been fully brought under control over a year later. Most of the patients who have been hospitalized throughout this epidemic are children or teenagers. New diseases can also emerge at any time and even old viruses can start to cause new and worrying symptoms if they mutate. An outbreak of Zika in Brazil in 2015 resulted in severe birth defects in children born to mothers who contracted the virus while pregnant.

Most mosquito-borne diseases have no treatment and no vaccine. In the case of Zika, there is no known way to prevent birth defects in a developing fetus if the infection is present. Without additional research to find a cure, there is little else to be done except monitor the outbreaks when they happen, control the mosquito populations that are carrying these viruses, and hope for the best for any patients who have contracted the disease already.

Local governments do most of the day-to-day mosquito management across the country. Surveys of these local mosquito control programs consistently show that many lack funding and full-time staff. This can prevent them from being able to effectively control the mosquito populations in their area, and limits their ability to keep their communities comfortable, healthy, and safe. Resources tend to flow in after an outbreak has already been detected, but are not always available beforehand to prevent outbreaks in the first place. Moreover, the money is often temporary. When the outbreak ends, the funding usually does too.

Our success in eradicating malaria did not happen overnight. It was the result of centuries of research and scientific progress across the world, and the result of decades of dedicated public health efforts here in the U.S. to put that hard-won scientific knowledge into action. The good news is that our incredible progress also cannot be undone overnight. It would take years, if not decades of neglect and disinvestment in our public health and medical system to make widespread malaria outbreaks a possibility again. But the risk is not zero.

As long as humans and mosquitoes are forced to share this planet, we will need to find ways to manage the risk that comes with living alongside these deadly creatures.

References available upon request. Email [email protected].