Learn about the evolving Ebola epidemic and its various aspects including disease prevention, management and treatment, response to the epidemic, ethical considerations, and the post-Ebola global health landscape.
Ecology and the Emergence of Ebola and Other Tropical Forest-Associated Zoonosis
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來自 Emory University 的課程
埃博拉病毒：疫情发展
125 個評分
從本節課中
Introduction to Ebola
Upon completion of this module, learners will be able to:
Review the state of global public health prior to the 2014 Ebola epidemic.
Explore the etiology of Ebola and other viral hematic fevers.
Explain the epidemiology of the 2014 Ebola epidemic and why this outbreak is different than previous ones.
Explain why tropical forest-associated zoonotic pathogens like Ebola are emerging at an ever-increasing rate.
Recognize drivers of disease spillover from wildlife to people and discuss how to mitigate disease spillover.
與講師見面
Dabney P. Evans, PhD, MPHAssistant Professor
Hubert Department of Global Health. Rollins School of Public Health. Director, Center for Humanitarian Emergencies.
Carlos del Rio, MDChair
Department of Global Health at the Rollins School of Public Health
Hi, I'm Thomas Gillespie.
I'm a professor in the departments of Environmental Sciences and
Environmental Health at Emory University and the Rollins School of Public Health.
I work at the interface of global health and biodiversity conservation.
I've spent close to 20 years examining how land use change,
that's basically things like forest fragmentation and
logging, affect disease dynamics in wildlife, people, and
livestock in the tropical forests of Africa, Latin America, and Madagascar.
Today, I'm going to share with you our current understanding of where Ebola comes
from, how it spills over into humans, and what we can do
to reduce the frequency of future spillover of Ebola and similar diseases.
So let's get started.
Ebola is an emerging infectious disease, or EID.
EIDs can be caused by a novel pathogen, something like AIDS or SARS.
These are very good examples,
that arose from viruses that mutated in ways that gave rise to new viruses.
Or, an EID can emerge because a new pathogen strain results from two or
more different strains combining to create a new strain.
This is what we saw with avian and human influenza combining within pigs and
resulting in pandemic influenza.
In addition,
we can have EIDs emerge when a pathogen expands its geographic or species range.
West Nile virus is a good example of this in that it only occurred in East Africa
in the past and now has a global distribution.
And lastly, EIDs can, can occur through the development of drug-resistant
strains of known pathogens, for example, tuberculosis or MRSA.
We think Ebola is a novel pathogen, but
further research is needed to determine how long Ebola has been causing disease
in apes, duikers and other susceptible wildlife, to rule out Ebola
being a virus that has simply expanded its species range, in recent decades.
EIDs matter because they account for
a growing proportion of all infectious diseases impacting people.
Currently, they account for more than 10% of all diseases that affect us.
And they tend to be difficult and expensive to manage, because we don't know
very much about them, we haven't developed drugs or vaccines to respond to them.
Well over half of emerging infectious diseases are zoonotic, or
capable of jumping from animals to people.
And of these emerging zoonoses,
almost three quarters spill over from wildlife to people.
Our ability to respond to zoonotic disease emergence,
using the conventional approaches of drug and
vaccine development, is greatly hindered by the magnitude of unknown pathogens.
To put this in perspective, a little over a million species of eukaryotes
have been described by scientists.
Eukaryotes are the forms of life that have nucleated cells.
Basically everything except bacteria and viruses.
That may sound like a lot of diversity, but our best estimate of the number
of eukaryotic species is around 9 million, give or take a million.
In other words, about 86% of species in existence remain unknown to science.
And remember, that doesn't even include bacteria and viruses.
And these are the organisms that account for the majority of the pathogens.
This is a primary reason why it's critical that we better understand the ecological
and the epidemiological phenomena associated with broad groups of pathogens,
instead of specific pathogens, if our goal is to avoid spillover into humans.
Among zoonoses, there are three broad groups, each with a distinct pathway.
It's important to differentiate these pathways,
since each requires a different approach to mitigate human infection.
In the first,
human infection always results from primary infection from animals.
In other words, there's never human to human transmission.
These are things like anthrax, Lyme disease, rabies, and West Nile virus.
In the second pathway, we see an initial spillover from animals,
followed by human to human transmission.
Ebola falls into this category, along with bubonic plague,
Dengue fever, and some strains of influenza, as well as yellow fever.
The final zoonotic pathway occurs when human exposure to an animal
pathogen results in the evolution of a specialized human pathogen that's
only transmitted from human to human, after that first transmission event.
For this category, opportunities for cross-species transmission may be common,
but successful establishment of the pathogen may be rare.
Examples include AIDS, measles and smallpox.
Our ability to respond to emerging zoonoses is further complicated
by the fact that these pathogens are emerging at an ever-accelerating rate,
due to increased overlap between humans and wildlife.
This increased overlap can be facilitated by human encroachment into wilderness
areas, that can be associated with commercial ventures such as logging,
mining, oil exploration, and industrial agriculture.
Or with the movement of wild animals into areas where people are already living.
This often happens as natural habitat is lost and animals expand their range in
search of food and mates, often into areas that are dominated by human activity.
Many of the activities that are leading to greater human wildlife overlap
are occurring in or on the boundaries of tropical forests.
This is especially important when we think about Ebola,
because Ebola only occurs naturally in tropical forest systems.
All four strains of Ebola, as seen here in red, green, blue, and yellow,
have spilled over from wildlife to people, in tropical forested regions of Africa.
Tropical forests have the greatest diversity of species on the planet.
And each of those species has its own unique set of pathogens,
some of which are zoonoses.
This is one of the reasons why tropical forests have such a high
rate of zoonotic disease emergence.
Now, let's take some time to talk specifically about the ecology of Ebola.
Ebola spillover can occur by contact with blood or other body fluids of
an infected animal, and may also occur by contact with a recently contaminated item.
Only frugivorous and insectivorous bat species have shown virus replication and
developed high circulating virus titers without developing illness.
This suggest that these are the potential natural reservoirs of the virus.
Interestingly, despite major efforts,
Ebola virus has never been isolated from these bats in the wild,
suggesting that the dynamics of this interaction are likely complex.
Historically, many human infections have been associated with the hunting and
butchering of African apes, both gorillas and
chimpanzees, as well as duikers, a type of forest deer.
Such bush meat, or
wild game, has the potential to serve as a source of zoonotic disease emergence.
So lets back up for a minute.
If fruit bats are the most likely reservoir for
Ebola, why are people getting infected via these other animals?
Well ripe fruit is an important part of the diet of all of these species.
We hypothesize that apes and duikers are getting infected when feeding on
fruits that have been contaminated by the saliva or feces of fruit bats.
Interestingly, climate change may be playing a role
increasing the chance that apes and duikers become infected with Ebola.
You see, fig trees are a keystone species in tropical forest ecosystems.
As climate becomes more unpredictable and extreme,
the fruiting of tropical trees also becomes unpredictable, but
figs have coevolved with specific pollinators, the fig wasps.
And because of their complex interaction, there is always a fig tree fruiting
somewhere in the forest, providing a critical, consistent food source for
fruit bats, apes and duikers.
Unfortunately, this valuable food source also has the potential
to serve as a focal point for zoonotic transmission.
Apes and duikers become ill and die from Ebola in a similar way to people.
In fact, the impact of previous Ebola outbreaks on wild gorilla populations
is the reason western lowland gorillas are listed as critically endangered
on IUCN red list.
This is the body of the United Nations responsible for
defining the risk of extinction of species.
Thus Ebola is a biodiversity conservation threat, as well as a global health threat.
Interestingly, in the West African Ebola outbreak,
there is no evidence of apes or duikers as the source of spillover.
In fact, surveys of such animals near the human index case suggest that if anything,
these species have increased instead of declined in relation to the outbreak.
During the human epidemic, many fruit bats were captured and
screened for evidence of infect, infection or exposure to Ebola.
Surprisingly, all were negative.
Fortunately, the multidisciplinary team conducting the investigation
also included an anthropologist who discovered through interviews that
the two-year-old boy who was initially infected with Ebola routinely played
within the hollow of a dead tree that served as a roost for insectivorous bats.
The tree had been burned by the time the researchers arrived, but they were able to
confirm that the bat described was in fact a species capable of shedding Ebola.
Considering the lack of evidence of Ebola in local fruit bats, and
the lack of an impact of Ebola on resident susceptible wildlife,
our best guess is that the initial spillover for the West African Ebola
epidemic resulted from transmission from these insectivorous bats to the child.
Either through contact with a live or dead bat, or through exposure to bat feces or
saliva, as has been seen with nepovirus,
another tropical forest-associated emerging zoonotic.
The end of armed conflict in Sierra Leone and Liberia in the past decade
has allowed for rapid expansion of commercial extractive activities,
such as logging and the clearing of forests for oil palm plantations.
This has resulted in extreme land use change.
Only 10% of the original forest cover still remains in this region.
Such dramatic environmental changes likely forced bats to seek food and
shelter in close proximity to people, and
provide new opportunities for direct exposure to infected bats.
Preventing future emergence will require spillover risk to be assessed and
mitigated for corporate and governmental enterprises that involve large scale land
use change, as well as effective education to reduce spillover risk for
those living in or visiting potential transmission zones.
As the human population grows and the demand for
resources pushes us into new areas, we're going to see more diseases emerge.
Any time we alter a pristine natural system,
there are going to be unintended consequences.
