10.23.17

Ebola Taï Forest: A Unique Emergence, and the Dawn of the Modern Age of Ebola

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“These woods are where silence has come to lick its wounds.”  – Samantha Hunt
 
In 1994, Ebola, a lethal virus that had been silent for fifteen years, awoke. Two separate outbreaks would shatter this silence, but one of these was unique, the likes of which has not been seen since.
Though the last known Ebola outbreak that affected humans had occurred in 1979 in Sudan, in 1989, Reston virus, a species of Ebolavirus, emerged in laboratory macaques in Virginia that had been imported from the Philippines. The lethal virus caused hemorrhagic fever symptoms in macaques, but was determined to be nonpathogenic to humans. The virus was a new type of Ebola; similar outbreaks would take place in Alice, Texas and again in Reston, Virginia in 1990, and in Sienna, Italy in 1992. The Reston outbreaks would later be dramatized in Richard Preston’s 1995 best-selling book, The Hot Zone, which captures the tension of an Ebola outbreak in a dense urban area, followed by the relief experienced after it was found to be harmless to humans.
The world once again took notice of human cases of Ebola in December 1994, when it reemerged from its fifteen-year slumber in Gabon. As with most mysterious febrile illnesses, the cases of hemorrhagic fever were initially assumed to be yellow fever. The epidemic took place in early December 1994 and was declared over on February 17, 1995; 52 cases were confirmed, 31 died from the virus. The outbreak originated near gold mining encampments near Makoukou, just 75 miles west of the border with Republic of Congo, and 160 miles north of the capitol city of Franceville. The causative virus was identified as Ebola Zaire on December 14, 1994.
However, unbeknownst to investigators in Gabon, the virus had already reemerged elsewhere a month prior. On November 16, 1994, a chimpanzee in the Taï Forest National Park, Côte d’Ivoire was discovered dead by a primate behavior researcher, and was dissected on the spot. The researcher developed dengue-like symptoms on November 24th, and was hospitalized in Abidjan on the 26th after developing further symptoms and not responding to anti-malarials. On December 1, she was evacuated to Switzerland for further treatment and monitoring, and would eventually make a full recovery after 6 weeks of illness.
The cause of infection was determined to be from the handling of the infectious blood and/or tissues from the necropsy of the dead chimpanzee. It was determined to be new species of Ebola, though not until February 1995, months after the Gabon outbreak identification. It was named Ebola Côte d’Ivoire (though its name was changed in 2002 to Ebola Taï Forest to conform with new naming conventions), and it was unlike any Ebolavirus that had been seen prior, or that has been seen since.
First, the infected researcher is the only known case of Ebola Taï Forest, ever. This is especially interesting given the timing of the infection, after a fifteen-year period of Ebola quiescence. The four other known species of Ebola have each had multiple outbreaks, but 1994 Taï Forest was a unique event.
Second, similar to the first outbreak of Ebola in 1976–which had since been discovered to be two separate outbreaks of non-concurrent, differing species of Ebola (Zaire and Sudan)[i]–the Ebola Taï Forest case happened within a month of the Ebola Zaire outbreak in Gabon. This is also interesting in the context of the fifteen-year period of Ebola going undetected.
Third, the Taï Forest outbreak is the first time that a human Ebola case had emerged outside of the central African area known as the Congo River Basin.  In fact, prior to 2014 it was the only time Ebola emerged outside of this region; it would be twenty years before Ebola would be discovered in West Africa again, and it would be under much different, catastrophic circumstances. However, the emergence from the Taï Forest is not inconceivable, given the environmental similarities to the tropical rainforests within Ebola’s known zone of endemicity in the Congo River Basin. The possibility exists that Ebola Taï Forest is a viral relic of times long ago, when the rainforests of Côte d’Ivoire and Central Africa were one and the same, stretching from coastal West Africa with the Congo River Basin.
Fourth, the Taï Forest case was the first documented human Ebola infection associated with naturally infected nonhuman primates in Africa. This discovery would shift research toward transmission of Ebola between primates and humans for years to come.  Today, epizootic Ebola transmission research is focused mostly on bats, but Ebola is still a significant issue for primates: recent research indicates that up to a third of the world’s chimpanzee and gorilla populations have been wiped out by Ebola.
Finally, the Taï Forest case, though unrecognized as such, is the first Ebola outbreak in what can be viewed as the modern era of Ebola; since 1994, an average of one Ebola outbreak has taken place every year. The questions remain, decades after the Taï Forest outbreak: What has precipitated this frequency of outbreaks, particularly after the fifteen-year lull? Why have we not seen Ebola Taï Forest again? When Ebola emerges again, will we be ready, or will it emerge someplace new?
 
[i] Cox NJ, McCormick JB, Johnson KM, Kiley MP. Evidence for two subtypes of Ebola virus based on oligonucleotide mapping of RNA. J Infect Dis. 1983;147:272–275.

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