The emergence of yellow fever in the United States brought death and panic, but also initiated a cascade of research and discovery. The tireless work of scientists initiated theories of how the disease was spread, from miasmas to virus-harboring vectors, and led to the development of vaccines and diagnostic technology that are still commonly used today.
It was 1793 in Philadelphia and the summer was proving to be one of the hottest on record. A blanket of humidity covered the city, fed by putrid swamps that served as breeding grounds for the Aedes aegypti mosquito. While smaller outbreaks of yellow fever had occurred in the U.S. since 1693, this year would prove to be different. The capital city would be decimated. In just a matter of months, 10% of Philadelphia’s population was dead and a greater proportion had fled the city in fear for their lives.
Yellow fever is caused by a virus in the family Flaviviridae, and it is transmitted by the Aedes aegypti mosquito. The yellow fever virus most likely originated in Africa and arrived in the Western Hemisphere in the 1600s as a result of slave trade. The mosquito vector was likely introduced to the U.S. via water barrels on trade ships arriving from countries with endemic yellow fever. The virus followed when slaves from areas with endemic disease arrived. The hungry mosquitos then spread the virus from infected to uninfected in the New World. While many who are infected do not experience any symptoms or have mild symptoms, others have fever, muscle pain, headache, nausea and vomiting. A small percentage of people infected with the virus develop a life-threatening form of the disease that involves high fevers, internal bleeding, vomiting of blood and jaundice—which is where the “yellow” in yellow fever comes from. It has been estimated that for every 1 case of severe infection, there are between 1 and 70 infections that are asymptomatic or mild.
By the time shiploads of refugees from the French colony of St. Domingue arrived at the ports of Philadelphia, it was primed for a mosquito-driven epidemic. The city was surrounded by marshes and swamps, and human and animal waste was tossed into holes in the ground that captured and held rain runoff. While individuals who came to Philadelphia from areas where yellow fever was endemic had some level of immunity, most of the population was immunologically naive to the virus and therefore susceptible to its devastation.
Early Diagnostics
The discovery of the causative agent of yellow fever was a decades-long process. While Cuban physician Carlos Finlay first described the Aedes aegypti mosquito as the carrier of the disease in 1886, he was ridiculed for this theory. Finlay’s discovery was accepted 20 years later only after U.S. Army scientists working with Dr. Walter Reed confirmed that this was in fact correct. The work of Reed’s team also confirmed that yellow fever was caused by an agent small enough to pass through bacteria filters, but it was more than 25 years before the virus was first isolated.
In 1930, a Harvard virologist named Max Theiler discovered that white mice were susceptible to intracerebral inoculation with the yellow fever virus. This discovery led to the idea that mice could be used to test for the presence of protective antibodies. Researchers added mouse brain infected with yellow fever virus co*cktail to sera with antibodies against the yellow fever virus and subsequently injected this mixture into the brains of other mice. The level of protection against yellow fever disease was measured by the number of mice that lived or died during a specific incubation period following injection.
Drs. Sawyer, Lloyd and Theiler elaborated on this work by developing a method for determining the protective effects of sera against yellow fever called the intraperitoneal protection test in mice. Once optimized, this test allowed for the quantitation of protective antibodies in humans and became a critical tool in studying the epidemiology of the disease, as well as evaluating the performance of vaccines. Technology for detecting antibodies to yellow fever virus evolved over the years, and by 1960 hemagglutination-inhibition and complement fixation tests were available, which simplified the detection of antibodies to yellow fever.
Modern Diagnostics for Yellow Fever
The last major outbreak of yellow fever in the U.S. occurred in 1905 in New Orleans. Today, yellow fever is endemic in tropical and subtropical regions of South America and Africa. While the development of a yellow fever vaccine(Theiler won a Nobel prize for this work) has saved countless lives over the years, the global burden of this disease is still high. Urban transmission of the yellow fever virus can be controlled by public health interventions, but the enzootic/sylvatic cycle (where the disease is spread between mosquitoes and non-human primates) is much harder to control. This second cycle of transmission leads to large outbreaks due to spillover from non-human primate populations. Most recently, outbreaks of yellow fever have been reported in several states in Nigeria and Brazil. The true incidence is unknown due to limited diagnostic availability and ground surveillance in endemic areas, but is estimated to be between 200,000-300,000 cases per year.
Yellow fever can be difficult to diagnose because it mimics other severe diseases like malaria, leptospirosis and other hemorrhagic fevers. Today, the most commonly used yellow fever diagnostics includeenzyme-linked immunosorbent assay (ELISA), which tests for antibodies, and reverse transcription–polymerase chain reaction (RT-PCR), which tests for viral genetic material.RT-PCR testing of the blood can detect the virus within the first few days of infection, but may be unreliable after that point when the patient may becomesymptomatic. Because of this, negative RT-PCR results cannot rule out infection. Antibody testing is a commonly used methodology, but comes with its own challenges. According to the World Health Organization’s 2018 yellow fever report, there are no commercially available IgM kits for yellow fever available, pushing labs to validate in-house protocols using purified antigens. Additionally, cross-reactivity with other flaviviruses, such as dengue and Zika, is high in areas with co-circulation, and positive antibody results should be confirmed with a more specific test like plaque-reduction neutralization (PRNT). Finally, serologic testing can be complicated by vaccine-induced antibodies and test results from patients in areas with ongoing vaccine efforts should be interpreted with caution.
As is true with all diagnostic tests, clinical context is incredibly important for accurate interpretation of a yellow fever test result. In the U.S., apreliminary diagnosis is typically made based on presenting symptoms, vaccination status and travel history. All of these components must be taken into account when interpreting a yellow fever test result.
Modern Yellow Fever Research
Prevention of yellow fever primarily relies on vaccination, as there is no antiviral treatment available for the disease. While the live-attenuated vaccine has been demonstrated to be safe and effective (lifelong protection offered from one shot within 30 days of immunization in 99% of people who receive it), production can be slow and result in a low vaccine supply for areas in need. Since the sylvatic cycle of yellow fever is hard to control and can lead to large outbreaks, research that focuses on how yellow fever affects non-human primates in areas of South America and Africa, as well as the role of non-human primates in the spread of yellow fever is urgently needed.
New areas of research have begun to focus on intervention planning, which is vital to preventing large and deadly outbreaks. Some researchers are using available laboratory and epidemiology data to build models that assess vaccine impact, while others are developing methods to predict yellow fever epidemics. Climate change, particularly warming, has increased the spread of mosquitoes that can harbor the yellow fever virus to areas outside of where the disease is currently endemic. Additionally, large spillover outbreaks like the ones recently seen in Brazil and Nigeria demonstrate the potential for re-urbanization of the virus. Research that focuses on preventing large outbreaks is imperative to ensuring that yellow fever doesn’t become the next pandemic.
