Why does Ebola keep coming back?
Ebola outbreaks start with a small number of cases, grow, then fizzle. So how come Ebola keeps coming back?
Ebola was first identified in 1976. Since then, there have been not one, not two, but at least twenty-eight outbreaks of Ebola virus disease in African nations (including the ongoing one in Uganda). Each time, the outbreak first starts with a small number of cases, grows larger, and then fizzles out through the hard work of containment efforts and the heroism of health workers.
So how come it keeps coming back?
A brief history of Ebola outbreaks
Ebola virus disease (EVD) is a highly infectious and acutely lethal viral disease that has afflicted humans and animals primarily in equatorial Africa. Because the pathogens responsible for the virus are primarily found in Africa, outbreaks are most likely to originate there.
From 2014 through 2022, 32,311 people have contracted Ebola, of which 13,708 have died - a staggering 43% overall case fatality rate. The table below, from Statista, shows the breakdown of the outbreaks:
|Outbreak||Cases||Deaths||Case fatality (%)|
|2020 DR Congo||130||55||42|
|2018-2020 DR Congo||3,481||2,299||66|
|2018 DR Congo||54||33||61|
|2017 DR Congo||8||4||50|
|2014-2016 Sierra Leone*||14,124||3,956||28|
|2014 United States||4||1||25|
|2014 United Kingdom||1||0||0|
The outbreaks typically follow a classic "epidemic curve" - at first, a small number of people acquire the disease and spread it to others. As the number of sick people grows, the number of people they can infect also grows. Eventually, the replication of the virus is reduced by proactive public health measures like quarantines and by community behavior changes, such as modified burial practices. The disease "burns out" and runs out of new people to infect as prior patients recover and healthy people are kept isolated.
So how does Ebola keep coming back after it is seemingly defeated, time and time again?
Natural reservoirs keep Ebola alive in other animals
A zoonosis is an infectious disease that can spread from animals to humans. Zoonotic pathogens may be bacteria, viral or parasitic, and can spread to humans through direct contact or through food, water, or the environment. They represent a major public health problem around the world due to our close relationship with animals in agriculture, as companions, and in the natural environment. Zoonoses can also cause disruptions in the production and trade of animal products for food and other uses.
The CDC states it best: Scientists do not know where the Ebola virus comes from. But based on similar viruses, they believe it's a zoonotic disease, with bats or nonhuman primates being the most likely source. Infected animals carrying the virus can transmit it to other animals, like apes, monkeys, duikers, and humans.
Ebola keeps jumping from animals to humans
Animals, unlike humans, do not have a public health system. Therefore, the pathogens which cause Ebola are freely circulating in animal reservoirs in Africa, and are always ready to jump to a new host if the opportunity arises. And as human activity is increasingly bringing people into close contact with the animals that carry Ebola pathogens, Ebola keeps appearing in our human population.
Transmission of the ebolaviruses between natural reservoirs and humans is rare, but can occur if an individual comes into contact with the carcass of an infected animal. Once contracted, the virus then spreads person-to-person, especially within families, in hospitals, or during mortuary rituals where contact among individuals becomes more likely.
Ebola might also be spreading undetected
A report by labroots, written by Carmen Leitch, suggests that some people acquired Ebola from sources long-thought dormant:
Ebola has surprised researchers in recent years. It was thought that outbreaks of infection would start only when the virus jumped from an animal host to a human. But there have been cases in which Ebola has remained dormant in an infected but recovered or asymptomatic individual for an extended period of time, then reemerged to infect someone else.
For example, an outbreak of Ebola in Guinea in 2016 was found to have started when the virus was shed, 500 days after infection, into the semen of an infection survivor who was not showing symptoms. Their sexual partner was infected a year and a half after the man had originally gotten sick.
An article in Science states that an Ebola outbreak in Guinea in 2021 was spread by a person who had Ebola more than five years prior. Because the virus did not seem to change much, it is also possible that it can lie dormant in former patients for years and then reappear.
As Leitch points out in her article, this could also be explained by the virus circulating undetected: "Other researchers noted, however, that there was not enough evidence to rule out other possibilities, such as an unrecognized chain of transmission."
If indeed Ebola is spreading undetected, then that would be another possible explanation for how it keeps coming back in larger outbreaks - although the large gap between outbreaks would be difficult to explain.
Ebola has escaped from biological labs
Although fortunately, these outbreaks have been relatively small, Ebola has leaked from labs on three separate occasions:
- In 1976, a researcher in the United Kingdom caused an accidental infection by sticking themselves with a contaminated needle. No other cases were reported and the researcher recovered.
- In 1996, a Russian researcher died after being exposed to Ebola in a lab setting.
- In 2004, another Russian researcher had another accident with Ebola and also died.
These lab accidents are another reason Ebola keeps coming back. Reintroducing the disease into humans from national stockpiles is obviously quite dangerous, and we are lucky the spread from these cases was not more severe.
Will Ebola come back after the current outbreak ends?
Despite all our efforts, complete eradication of Ebola seems unlikely. The presence of animal reservoirs and the risk of lab leaks means there will always be a chance for Ebola to reappear.
However, there are many promising advances coming in vaccine development that could dramatically improve humanity's chances in the fight against EVD. Similar to Covid-19, Ebola is an RNA virus, and mRNA vaccines might be useful against it.
There is already a vaccine against the Zaire strain, although it does not protect against the currently-active Sudan strain. So, while Ebola will probably keep coming back, we might be able to invent new treatments and prevention methods to reduce the gravity of the Ebola threat.
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