It’s October, 1793 in the capital city of Philadelphia. The stars equal the stripes on the flag of the fledgling United States and George Washington is at the onset of his second term. Almost ten percent of the city’s residents are dead or dying and a third of the population has fled. Thomas Jefferson writes to James Madison that everyone who could escape the city was doing so.1 An invisible and unknown scourge has plagued the city and the surrounding countryside.
Early theories by Benjamin Rush, a local physician who penned his name on the Declaration of Independence, supposed the problem lay with a pile of rotting coffee beans left on the docks. His bellicose treatment was bleeding the infected and administering copious amounts of mercury.2 Needless to say, neither the disposal of the hypothesized origin nor the crude treatment saw any reduction in new infections. It wasn’t until a cold snap late in the month that new cases began to slow, eventually subsiding just prior to the new year.
The American plague, as it was known at the time, was the result of an acute viral disease transmitted by infected mosquitoes – a disease that’s been known as Bronze John, Yellow Jack, Yellow Plague and Yellow Fever. The most likely cause was Caribbean migrants fleeing the same epidemic to Philadelphia earlier that summer.3 The migration of infected persons from the tropics, through the swamp-laden south into a densely populated Philadelphia resulted in the propagation of Urban Yellow Fever. This outbreak, one of several4 until a vaccine was developed in the 19th century, stole more than five thousand lives5 and caused economic chaos, its lingering effects haunting Philadelphia, and the nation, for decades.
The urban mosquito (A. aegypti), the primary vector, transmits the Yellow Fever Virus (YFV) from host to host in densely populated environments and can also transmit other diseases, including Chikungunya, Dengue and Zika Fever. The virus is picked up by a female mosquito when it ingests the blood of an infected human or primate. Once the virus reaches the stomach of the carrier there is a brief incubation and replication period. The virus then moves into the mosquito’s bloodstream and then into the salivary glands where it can be transmitted to a new host.
The transmission of YFV comes in different varieties, denoted primarily by the species of the winged hosts.6 The Sylvatic, or jungle, Yellow Fever occurs in tropical rainforests where the virus is transmitted from primates to humans working in jungle regions. Intermediate Yellow Fever, the most common in places like Africa, is transmitted by semi-domestic mosquitos who venture into small villages to feed, spreading the virus. Urban Yellow Fever is most frequently the cause of large epidemics, like that in Philadelphia, where the virus is spread in highly-populated areas with little or no immunity.
In 1927 YFV became the first human virus to be isolated.7 Vaccines followed soon thereafter in the 1930’s, the foremost of which was based on the 17D strain – the same strain Advanced Biotechnologies uses for the production of its Yellow Fever Quantitated Viral RNA. Even with a stable vaccine, it’s estimated that tens of thousands of people still die annually from YFV – most in central Africa.8
The World Health Organization expects that more than one billion people will be protected from Yellow Fever through vaccination by the year 2026.9 There is still an ever-present danger that an outbreak could occur in a new and unprepared geographic region due to the increase in global travel. So, whether you’re in Philadelphia or Angola, don’t be a Bronze John; make sure to don the DEET and swat those pesky, winged carriers of the YFV.