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The Mysteriously Shifting Distribution of Yellow Fever

I've been looking closely at the hemispheric distribution of yellow fever during the 19th century as part of research for an upcoming conference.  Using several secondary sources that list outbreaks of yellow fever in the United States, I created a map that shows yellow fever in the United States from 1790 to 1910.  I've done this before, but in less detail.

Yellow fever outbreaks in the US from 1790 to 1909 from Ian Read on Vimeo.

A few things stand out in this map.  First, yellow fever left its greatest mark on the eastern seaboard and, especially, in the northeastern ports between 1790 and 1820.  After this period, it lingered in this area, but also expanded southward, mostly in the Gulf Coast region. During the 1810s, it took root in and near the Mississippi delta.  Yellow fever appears to become endemic in New Orleans.  By the 1820s, it slowly but erratically spread up the Mississippi River.  The Texan Gulf also had a first outbreak.  By 1854, yellow fever was now a “southern” disease, with periodic outbreaks in the gulf and delta areas especially.  The 1878 epidemic was memorably severe in how far it spread inland (more on that in a minute) and high levels of mortality.  There were fewer epidemics during the 1880s, but yellow fever returned with virulence in the south in the 90s.  After 1901, health officials learned that yellow fever was transmitted by the (aedis aegypti) mosquito and in the next few years health boards attempted mosquito larva eradication programs.  Such programs diminished but did not halt the last major epidemic, in 1905.

I'm building a similarly detailed map for Brazil, but this is a more difficult task because far fewer historians have collected yellow fever statistics.  But from what we know, yellow fever and its mosquito vector shifted considerably in its range.  For instance, in São Paulo during the 1850s, the disease was almost entirely confined to the coast and only took its victims from an area not far from the international port at Santos.  Even though the coffee boom in the following decade brought many more ships, non-immune European sailors and immigrants, yellow fever had largely vanished.  It returned in 1870 and began to spread inland.  By 1889, yellow fever mosquitoes had crossed the tall mountain range and entered the Paulista highlands, where thousands of coffee trees were producing most of the world’s coffee, bringing new wealth to Brazil.  Epidemics occurred in coffee towns previously thought to be immune to the fever until 1903 when the last major outbreak occurred in Riberirão Preto.

This changing distribution of the disease in inland US and Brazil was largely caused by railroad cars that transported infected insects and people into areas with low levels of immunity.  In the two maps below, we can see that the outbreaks in Texas (1862-73) and São Paulo (1889-1903) usually occurred near the railroads.  In fact, Houston and Campinas were both hubs of two strikingly similar disease networks.  Residents were well aware of the threat that this new technology brought, and in both instances, the railroads were closed during (but not before) some of the worst epidemics.

Although there is little doubt that transportation technology like railroads and riverboats contributed greatly to the expanding and changing distribution of yellow fever outbreaks, it is much harder to explain why yellow fever shifted from the American east coast to the south and why yellow fever was not even present in Brazil between 1693 and 1849.  Furthermore, yellow fever epidemics were less common in both countries during the 1860s and 1880s than the 1850s, 1870s and 1890s.  I believe that these changes were caused not by something that people did, but by naturally shifting boundaries of the mosquito host.  


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