The American Plague: The Untold Story of Yellow Fever, the Epidemic that Shaped Our History - Molly Caldwell Crosby (2006)
Part IV. United States, Present Day
Chapter 27. History Repeats Itself
Today, Aedes aegypti, the striped house mosquito, is blamed for any urban outbreak of yellow fever, but several other mosquitoes are known to carry the yellow fever virus as well. Those mosquitoes play a part in what’s known as jungle yellow fever. They live in the tree canopies of Africa and South America and pass the virus back and forth through wild monkeys. When a human becomes infected it is because he, and they are usually young men, is working in the jungle clearing forests. In fact, jungle yellow fever is considered more of an occupational hazard than anything else. It survives on a continual cycle between mosquito and monkey, with the occasional human getting caught in the crossfire.
An urban epidemic of yellow fever occurs when jungle yellow fever makes the jump into a large human population. The forest-dwelling mosquitoes, perhaps an Aedes africanus in Africa or a Haemagogus in South America, fly beyond the borders of the jungle into the territory of a city-dwelling Aedes aegypti. The two mosquitoes share a blood meal from a monkey, and suddenly, the virus is passed from a jungle mosquito to a city mosquito that spreads the virus to a human population. In Africa, the most common type of yellow fever is the intermediate one, in which yellow fever can survive in terrains between the jungle and urban cities. Whether it starts as a jungle outbreak in South America or an intermediate one on the African savannah, the worst-case scenario is the same: The fever moves into a large city, the virus builds more strength, and it infects thousands. An outbreak of urban yellow fever is always considered an epidemic.
Because the virus is not part of an urban cycle the way it is in the jungle, the virus is unleashed on a fresh population of nonimmunes. It is much the same as it was 400 years ago when Europeans first arrived in Africa during the slave trade. They landed on the shores of West Africa armed and ready to export human labor; instead, many served as nothing more than an import of nonimmunes for the yellow fever virus.
In just the same way, the cycle happened on this side of the world. It attacked nonimmune populations that had never before seen the virus. Aedes aegypti mosquitoes traveled on board the ships from Africa and then proliferated, most likely spreading yellow fever to native, forest-dwelling mosquitoes in South and Central America that settled back into the jungles to begin the cycle of mosquito and monkey transmission, harboring the fever in the sultry haunt of lush tropical life.
In the United States, the cycle had taken a different turn. There were no cases of jungle yellow fever, no forest-dwelling monkeys giving refuge to the virus; it was not a yearly occurrence. Instead, there was only a series of urban epidemics when the virus exploded on a population.
The cycle would not be broken until the mosquito’s breeding places were destroyed. In Memphis and elsewhere, it happened through the invention of the sewer system and elimination of private cisterns and privies.
Massive campaigns against Aedes aegypti essentially wiped out the mosquito from Central and South America, and the U.S. government promised to do the same. In their book Mosquito, Andrew Spielman and Michael D’Antonio wrote that there was a sense of irony to the situation: “After all, America had gone to war with Spain, in part, because of the danger of yellow fever spreading from Cuba into nearby lands.” Now that Latin America had the same concerns about the United States, there was opposition to the idea. North Americans did not welcome the intrusion of government employees trampling through yards and hunting mosquito larvae. What’s more, the Environmental Protection Agency banned the use of DDT. As a result, Aedes aegypti never fully left the United States; if its presence today was not already known, it was brought to our attention with recent outbreaks of dengue in Texas.
Over time, mosquitoes have proven their evolutionary dexterity, adapting to insecticides and building a resistance. Once again, the striped house mosquito now flourishes in cities throughout South America, Central America and the southern United States. Its lyre-marked body and striped legs swarm around potted plants, gutters and rain-filled watering cans. A homebody, the domestic aegypti prefers human habitations, houses, boats and fresh water. But this was not always the case.
Hundreds of years ago, Aedes aegypti lived only in the jungles of Africa, where it hovered around tree trunks to lay its eggs in pools of rainwater. Its range was short; the mosquito preferred to stay in one general place, close to the trees. As man traveled into the interior of Africa, the mosquito made its evolutionary leap: It adapted to human life. Rather than tree trunks, it first sought water casks, then standing water around homes, and in modern times, oddly enough, it has adapted to tires. The dark interior and the water that clings to the inside of a tire mimic the hollows of a tree trunk. Scientists believe that the world’s mass of discarded tires in urban settings has recreated the atmosphere of those ancient jungles in Africa.
Aedes aegypti is not the only mosquito to have made the journey from tree holes to tires. The Asian tiger mosquito, Aedes albopictus, is relatively new to the United States. The tiger mosquito arrived in 1983 in a shipment of tires from Asia. The large, striped mosquito is very similar in appearance to its aegypti cousin, but true to its name, the tiger mosquito is a more voracious feeder. As a vector the tiger mosquito has been known to carry dengue, encephalitis and yellow fever in other countries. It has yet to transmit disease in the United States. The tiger mosquito, hardy and determined, has proliferated in North America, even crowding out some of its aegypti neighbors. Nature has a dark sense of humor though: The first Asian tiger mosquito on this continent was found hovering in Elmwood cemetery in Memphis, Tennessee.
In recent years, vaccine usage for yellow fever has fallen off. The problem is a lack of education and funds sufficient for the dissemination of the vaccine. Surveillance is also minimal. As a result, in the 1970s, outbreaks of yellow fever began once again, creating what is known as the “yellow fever belt” in Africa. The World Health Organization described the 1980s and 1990s as an extraordinarily active period for yellow fever in Africa. But that activity continues even today, and Nigeria has reported more epidemics than any other African country.
Ninety-three percent of the countries in West Africa now have cases of yellow fever—up 30 percent just since the mid- 1990s. Yellow fever, it seems, is making a comeback; it is also spreading to areas that have never before seen the virus. Thirty-three countries in Africa and nine in South America are now known to house the virus. The number of deaths vary, but in South America, mortality rates from yellow fever have been as high as 80 percent. Two hundred thousand people worldwide are infected each year, but the number of actual cases is thought to be 10- to as much as 250-fold higher due to underreporting or misdiagnosis.
At one time in history, there were several factors in play that led to recurring, explosive epidemics of yellow fever. They were modes of transportation, populations of vulnerable hosts, warm weather cycles and the colonization of Aedes aegypti in urban environments. The circumstances today in Africa and parts of South America mirror those of Memphis in 1878—there is poverty, people living in shanty houses, poor sanitation, containers of water in place of plumbing, a tropical atmosphere and a high population of Aedes aegypti mosquitoes. As the human population increases rapidly, so does the number of nonimmune people. And just like the paddleboats and trains of the nineteenth century, we now have shipping containers and airplanes. Global warming has broadened the range of disease-carrying mosquitoes. And we have a new threat: Yellow fever is listed among the pathogens that might be used during a bioterrorist attack.
Will urban outbreaks of yellow fever become more widespread and more deadly? If recent statistics are any indication, they will. Africa and South America are already moving in that direction. And unlike a virus such as smallpox, yellow fever is passed between insects and animals—it can never be eradicated because nature itself gives the virus sanctuary. Migration of humans closer to jungles and forests will place people and the virus at even closer range. But with education and routine vaccine use, the outbreaks could be contained and infect fewer people. Programs are already under way to include yellow fever in childhood vaccines in Africa.
Science is also looking for ways to understand the virus better. Recent studies have unraveled some of the mystery as to how a virus like yellow fever interacts with the human immune system. In one study, scientists identified the protein on the virus coating that interferes with the immune response. In another, scientists located part of the viral protein that human antibodies lock onto to defeat it. Isolating and reproducing that protein could lead to a safer vaccine against yellow fever.
The likelihood of the American plague returning to the United States is anyone’s guess. In a 1996 article in the Journal of the American Medical Association the author wrote: “Because A aegypti mosquitoes are once again established in urban areas . . . there is widespread concern that yellow fever could erupt in explosive outbreaks, which could also occur in the southeastern United States.” But we are certainly better off than people were 150 years ago. We have a vaccine. We have modern amenities like air-conditioning instead of open windows, cars instead of open-air wagons. We have insect repellent. And best of all, we have the knowledge that the virus is spread by mosquitoes.
Still, viruses have taught us one thing throughout history, and it is this: That their will and ability to survive may be stronger than ours.