by Jim Murphy
Epidemics of yellow fever also struck numerous cities in South America, Europe, Russia, and West Africa. Wherever the climate was warm and large groups of people assembled, whether living in established cities or in tents during military campaigns, yellow fever took its toll.
Although millions of fever cases were studied and thousands of autopsies performed, not much new was learned about the disease during the entire nineteenth century. In 1878 the best medical advice a Memphis newspaper could offer frightened readers was: “Keep cool! Avoid patent medicines and bad whiskey! Go about your business as usual; be cheerful and laugh as much as possible.”
Doctors were thoroughly baffled by yellow fever, but curiosity and fear drove a number of them to continue to investigate and speculate on its cause, spread, and treatment. In 1848 Dr. Josiah Nott in Alabama noticed that yellow fever receded after swamps were drained off to kill mosquito infestations. Was the mosquito, Nott wondered, the cause of the fever? It’s very possible that Nott, in reading about yellow fever epidemics from the past, had come across Rush’s mention of those little red spots on patients called petechiae that “resembled moscheto bites.”
The idea that such a tiny creature could kill a human was considered preposterous in Nott’s day. Besides, Nott did not perform any experiments to prove his theory. It remained an educated guess based on logic and circumstantial evidence and was largely ignored by medical professionals.
One doctor familiar with Nott’s theory was intrigued enough to follow up on it. In 1880 Dr. Carlos Finlay of Havana, Cuba, captured mosquitoes and let them ingest the blood of patients suffering from yellow fever. Then, in an experiment that would be considered highly unethical today, he allowed these mosquitoes to feed on healthy humans. To his amazement, 20 percent of the healthy patients soon developed mild cases of the disease.
The following year Finlay presented his conclusions in a paper called “The Mosquito Hypothetically Considered as the Agent of Yellow Fever.” His idea certainly received a great deal of attention—almost all of it negative. Because his subjects had not gotten full-blown yellow fever, many scientists thought that Finlay had failed to prove the relationship between mosquitoes and the disease. Some even suggested that he might have seen yellow fever when it wasn’t really present in order to support his theory.
It would be more than twenty years before another doctor took Finlay and his work seriously. During those two decades remarkable discoveries were made that changed the entire science of medicine. The 1880s saw two scientists, France’s Louis Pasteur and Germany’s Robert Koch, isolate various bacteria—extremely small one-celled creatures—living in animals and humans and link them to specific diseases. Then, in the late 1890s, two Germans, Friedrich Löffler and Paul Frosch, discovered other disease-causing organisms even tinier than bacteria, called viruses.
None of these discoveries related specifically to yellow fever. They did, however, put an end to the notion of humors as a medical theory, and they established the possibility that many other diseases might be caused by creatures too small to be seen by the human eye.
It was in 1900 that a young doctor, Jesse Lazear, entered the picture as a member of the U.S. Army Yellow Fever Commission. The commission had been set up in Cuba following the Spanish-American War to discover the cause of yellow fever and develop a cure. Fewer than 400 American soldiers had been killed in the actual fighting on the island, while over 2,000 died of yellow fever. The United States government wanted this deadly enemy identified and eradicated.
Lazear had read Finlay’s paper and thought his experiments, while clearly inconclusive, showed promise and should be carried out more fully. In addition, he was aware that in 1898 two scientists working separately had announced that a mosquito was able to carry the malaria parasite and transmit it to humans. A mosquito, Lazear reasoned, might also be the carrier of yellow fever.
None of this impressed Walter Reed, the army doctor who headed the commission. Reed favored the idea that a bacterium, first identified by Italian scientists and usually found in swamps, was the culprit. But to his credit, Reed let Lazear proceed with his experiments on a limited basis.
Lazear’s early attempts to show that mosquitoes could transmit the disease from a sick person to a healthy one failed in all but one case. Tests to duplicate this one success also failed until a colleague of his on the commission, James Carroll, caught the disease from an infected insect and nearly died.
The Aedes aegypti is responsible for infecting approximately 100 million people every year with dengue fever and other diseases. (DRAWING BY RICK PARKER/AUTHOR’S COLLECTION)
These two positive results suggested to Lazear that he was very close to proving Finlay’s theory. But even with success in sight, Lazear was extremely nervous about potential criticism. Both Nott and Finlay had been dismissed by their medical colleagues, and he did not want to suffer the same sad fate. In a letter to his wife, Lazear cautioned her that “nothing must be said as yet, not even a hint. I have not mentioned it to a soul.”
On September 13, 1900, Lazear was in a Havana hospital’s yellow fever ward letting mosquitoes feed on patients. As Lazear did his work that day, a mosquito that was not part of his experiment landed on his hand. He thought to shake the insect off, but did not want to interrupt the procedure he was performing. So he watched as the mosquito patiently probed his hand with the cutting part of her proboscis, then sucked his blood for over a minute before flying off.
Two days later Lazear felt ill; two days after this he was confined to bed as yellow fever wracked his body. For eleven days he suffered a high fever, agonizing sweats, and abdominal pain. Black vomit and delirium followed. Finally, on September 25, the thirty-five-year-old doctor died.
James Carroll and another colleague, Aristides Agramonte, felt that Lazear had established a connection between mosquitoes and yellow fever and sent a detailed report of his work to Walter Reed, who was in Washington, D.C., at the time. Reed’s initial reaction was anything but encouraging. “I cannot say that any of your cases prove anything,” he wrote back to Carroll.
Even so Reed was able once again to push aside his doubts and his own strongly held opinions about how humans contracted the disease. Besides, a dedicated colleague had died for his theory. The least Reed and his team could do was conduct careful tests that would either prove or disprove the idea conclusively.
On his return to Cuba. Reed initiated a series of experiments involving volunteers and insect-tight tents. Healthy subjects spent the night in one tent with a swarm of infected mosquitoes hungry for a meal. In a separate tent other healthy subjects slept wrapped in blankets soiled by the black vomit of patients. Those exposed to the mosquitoes eventually sickened (though, happily, none died); those who slept on the soiled blankets remained healthy. Less than one month after Lazear’s death, Walter Reed was able to announce that mosquitoes transmitted the disease and even named the culprit: the female Aedes aegypti mosquito (the male of the species prefers plant nectar to blood).
Despite the evidence provided by Reed’s commission, many people were still not convinced that the bite of a tiny mosquito could cause a fatal illness. The Washington Post denounced the findings in a November 2, 1900, editorial: “Of all the silly and nonsensical rigmarole of yellow fever that has yet found its way into print—and there has been enough of it to build a fleet—the silliest beyond compare is to be found in the arguments and theories generated by a mosquito hypothesis.”
Fortunately, scientists in other parts of the world were able to verify the commission’s experiments and prove the theory. Of course, establishing the Aedes aegypti mosquito as the disease carrier, or vector, did not answer all the questions about yellow fever. The actual source of the yellow fever virus—tree-dwelling monkeys in African and American rain forests—was not identified until 1929. And a safe and effective vaccine was not developed until 1937.
But knowing that a mosquito could spread the disease proved vital in Cuba. Patients with yellow fever w
ere isolated in rooms with screens on the windows so that mosquitoes couldn’t feed on their infected blood and then transmit the disease to healthy individuals. Next, the breeding areas for Aedes aegypti were systematically destroyed.
This mosquito is almost entirely dependent on humans for its breeding areas—the still water found in water barrels, cisterns, canals, ponds, sewers, gutters, and outhouses. Even a tin can with an inch of water in it can be the birthplace of hundreds of hungry and potentially dangerous mosquitoes.
This cartoon shows a very angry Uncle Sam charging “Amos Quito” with various crimes against humanity. (AUTHOR’S COLLECTION)
It was this dependence that made Aedes aegypti the perfect creature to carry yellow fever into Philadelphia in 1793. Its eggs were unwittingly brought aboard ships in water casks, where they hatched into larvae and grew into adult mosquitoes in seven days. These insects were a nuisance, but not dangerous—until they bit a person who’d come aboard with the virus.
During the next twelve days the yellow fever traveled through this mosquito’s body until it reached her salivary glands. After that, every time she fed on someone, she discharged some of the virus into her prey. At the time, a transatlantic sailing voyage might take anywhere from one to two months. This meant that passengers and crew were trapped on board with successive generations of diseased insects. When the ship finally docked, the infected mosquitoes flew off to establish new homes, and create new fever victims, near the open “sinks,” wells, water barrels, and privies that were everywhere in the city back then.
In Cuba in 1900, the Yellow Fever Commission sent out soldiers to patrol the city of Havana, going street by street, house by house, searching for open water containers that might be breeding spots for Aedes aegypti. Anything that could act as a breeding site was either emptied of water or smashed. Larger bodies of water, such as ponds, were treated with larva-killing oil.
A great many citizens felt such measures were harsh and unfair, especially after being fined when mosquito eggs were found on their property. But the campaign proved successful. Within six months yellow fever was all but gone from Havana. If people in 1793’s Philadelphia had only listened when “A. B.” had explained how to kill off mosquitoes breeding in water barrels, the fever there might have ended weeks sooner, and hundreds, if not thousands, of lives might have been saved.
The same thorough mosquito-control measures were instituted in the Isthmus of Panama, where work to dig a canal to connect the Atlantic and Pacific Oceans had bogged down. The French had initiated the canal project in 1881 but almost immediately encountered yellow fever and malaria (which is transmitted by a number of species of Anopheles mosquitoes). At one point 7,000 of their 19,000 workers were ill with the diseases; by the time the French company halted construction in 1889, more than 30,000 workers and engineers had died.
The United States took over the project in 1904 and encountered the same disease enemies as the French. The difference was that now the United States knew what to do. An aggressive antibreeding area campaign was launched and once again proved to be highly successful. Yellow fever was almost completely eliminated in Panama. Because Anopheles mosquitoes have slightly different breeding areas from Aedes aegypti, there were still hundreds of cases of malaria. But overall the campaign worked. Only 2 percent of the workers in the U.S.-led project were hospitalized at any one time, compared with 30 percent for the French.
The Havana and Panama campaigns controlled yellow fever and the Aedes aegypti mosquito in those regions, but they did not eliminate the disease completely. It continued to terrorize numerous cities, especially in Central and South America. Finally, in 1947, the Pan American Sanitary Bureau (later renamed the Pan American Health Organization) decided to eradicate the mosquito—and thus the disease—in the entire Western Hemisphere.
Along with destroying breeding areas, adult mosquito populations were also attacked with the widespread use of the pesticide dichloro-diphenyl-trichloroethane, better known as DDT, much of it sprayed from planes. By 1962 twenty-one countries declared themselves free of Aedes aegypti, and the world seemed very close to ending yellow fever forever. That was when problems began to develop in the United States.
First, experts in mosquito control complained that Congress had not budgeted enough money for the campaign to succeed. Virtually every southern state was infested with Aedes aegypti, these experts pointed out, but funds would run out before the job of eradicating the mosquitoes was half completed.
Second, concern about the health risks and environmental problems associated with the use of DDT increased during the 1960s. These fears were given a public platform with the publication of Rachel Carson’s Silent Spring in 1962. In this groundbreaking book, the author tackled many emerging ecological concerns, such as the environmental dangers associated with radiation. But it was the use of DDT, and its potential health risks to both animals and humans, that grabbed the public’s attention.
The book became a best seller and convinced many citizens and politicians of the dangers posed by indiscriminate spraying of DDT and other chemicals. The use of DDT would be banned in the United States in 1972, but the antimosquito campaign had died long before that.
Actually, Carson had predicted that the campaign would fail even if the spraying continued as mosquito-control experts wanted. “Spraying kills off the weaklings,” she explained. “The only survivors arc insects that have some inherent quality that allows them to escape harm. These are the parents of the new generation, which, by simple inheritance, possess all the qualities of toughness inherent in its forebears.” In other words, supermosquitoes were being created that were capable of resisting DDT.
Careful testing established that it takes about seven years for this new mosquito to emerge and replace the old one. In addition, the same evolutionary process happens when newer pesticides, such as malathion, Sevin, or permethrin, are used.
As this new pesticide-resistant Aedes aegypti gradually reestablished itself in Central and South America, another problem was noted. Because the old mosquito—and with it the disease—had been absent so long, hardly anyone had built up an immunity to yellow fever. As a result, hundreds of millions of people were now susceptible to getting yellow fever and other deadly diseases carried by Aedes aegypti.
An even more alarming problem was that several mosquito-borne diseases had begun to change. Malaria was the first in which a change was observed. Prior to the 1960s a number of drugs, such as Atabrine and chloroquine, had been developed that effectively treated this illness. Unfortunately, patients would often use only enough of these medicines to reduce the symptoms, saving the rest for future bouts of the disease. Many of the microscopic parasites that produced malaria would survive the sublethal dose and produce offspring capable of withstanding a full dose of the medicine.
This drug-resistant type of malaria began to appear among U.S. troops during the Vietnam War, in which more soldiers were incapacitated by the disease than by battle wounds. Despite the introduction of different, more powerful drugs, the new kind of malaria spread across Asia, then to Africa, and eventually to South America. Today 10 percent of the world’s population suffers from malaria every year, resulting in almost three million deaths. In the time it takes to read this sentence, another person has died of malaria.
It’s clear now that mosquitoes, animals, and human disease go together. We know that the virus that causes West Nile encephalitis is carried by birds that travel up and down the east coast of the United States and that mosquitoes feed on them and then give the disease to humans. Aedes albopictus, better known as the Asian tiger mosquito, sucks the blood of both animals and humans and is capable of carrying a wide variety of viruses, including dengue fever, eastern equine encephalitis. West Nile encephalitis, and LaCrosse encephalitis, all serious illnesses and all potentially lethal to humans. In fact, of the 2,500 kinds of mosquitoes that infest the world, almost 400 of them are capable of transmitting diseases to humans.
“No ani
mal on earth,” assert mosquito experts Andrew Spielman and Michael D’Antonio, “has touched so directly and profoundly the lives of so many human beings. . . . With their glassy wings, delicate legs, and seemingly fragile bodies, mosquitoes are nevertheless a powerful, even fatal, presence in our lives.”
Which brings us back to Aedes aegypti and yellow fever. The disease exists anywhere there are monkey populations, as does the pesticide-resistant mosquito that can transport the disease to humans. As new roads are cut into virgin rain forests, more and more people enter areas where they can become infected. A car ride takes that newly infected person to a major city, where more Aedes aegypti mosquitoes wait to feed on him, then carry the disease to another and another and another person. A plane ride carries one of these infected persons to a new country, where still more Aedes aegypti wait to feed and fly off.
Two factors make the situation especially dire in the United States. First, no company here has produced the vaccine in recent years. If the disease invaded a large city and a call went out for hundreds of thousands of doses of the vaccine, it would take months to produce it. The U.S. Institute of Medicine studied the situation in 1992 and estimated that an outbreak of yellow fever in a city like New Orleans would infect 100,000 people and kill at least 10,000 of them before it could be brought under control.
Second, despite years of research, there is still no cure for yellow fever. While modern medicines can lessen the impact the disease has on the human body, once a person has yellow fever, he or she will have to endure most of the horrible symptoms that Philadelphia’s people suffered in 1793.
“Once urban transmission begins in the American region,” Duane Gubler, a director at the Centers for Disease Control, warns, “it’s probably going to spread very rapidly throughout the region to other urban centers and then from there to Asia and the Pacific.” In other words, yellow fever is a “modern-day time bomb. We’re just sitting here waiting for it to happen.”