Articles, Blog

Where do new diseases come from?

March 2, 2020


At the time of making this
video, it’s mid-February, 2020 , and the coronavirus has
infected more than 40,000 people, killing more than 1000. Scientists are rushing to
develop a vaccine, but there is currently no
medicine available to treat or prevent it, and the fear is that it will
spread to countries without resources to handle a mass
outbreak. The coronavirus first appeared
in December , 2019 and in only a couple of months,
the number of those infected has exploded. So where did it come from? Well, like SARS, Ebola, HIV, swine flu, and many other recently
emerging diseases, the 2019 coronavirus came from
animals.
The U.S. Centers for Disease Control
estimates that three out of every four emerging
diseases are zoonotic infections, or
zoonoses, meaning they are passed from
animals to humans. China has vast wildlife and
exotic meat industries, both legal and
illegal, and much of this trade is done
in open air wet markets where live animals and all
sorts of meats are sold in large quantities, often
without regulation. 49 out of 99 early coronavirus
patients were linked to a wet market in
Wuhan that specialized in the wild animal
trade. It was identified as the source
of the virus and has since been shut down. China was also ground zero for
the 2003 SARS outbreak that killed 774 people. SARS is another example of a
zoonotic disease, and it also originated in a wet
market. So what are zoonotic diseases,
and how do they work? Zoonotic diseases include
viruses, bacteria, and parasites that can pass
from animals to humans. It is thought that 61% of all human pathogens came from
other species, and 75% of new diseases that emerged in
the last decade were passed to humans from
animals. But of course, you don’t have
to worry about passing your cold or flu to your pet,
and you won’t catch parvo or kennel
cough from your dog. So not all diseases are
zoonotic. Why is that? Host tropism is the specificity
of a pathogen like a virus or bacteria to
infect particular hosts and host
tissues. Some pathogens can infect
multiple species, while others are limited to
one. Pathogens are also localized to
specific tissues like the respiratory
tract or the stomach. For a pathogen to effectively
cause disease , it must be able to survive
and replicate in the host tissue. Tropism, whether or not the
pathogen can live and
replicate, is determined by a few
factors. Firstly, the host must be
susceptible. Pathogens often must be able to
adhere to or enter into host cells to cause
an infection. In order to do this, the host
cell must have compatible surface receptors. All of our cells have surface
receptors that fit with the shape of certain
nutrients. When a nutrient comes into
contact with the cell, the surface receptors
recognize it, bind to it and pull it inside the
cell. Many viruses have surface
proteins that are uniquely compatible
with certain surface receptors. The virus tricks the cell into
allowing it to enter by mimicking the shape
of nutrients. Like a lock and key, the virus
must be able to fit into the host cell’s
surface receptors in order to bind to the cell
and infect it. Bacteria can replicate without
entering the cell, but must be able to find
nutrients. Once the bacteria finds a
suitable, nutrient rich environment, it will
colonize the cell’s surface. Secondly, the host must be
permissible. Pathogens must get adequate
nutrients in order to
replicate, and so the host tissue must
have the necessary specific nutrients that the
pathogen needs. Even if a virus is able to
enter or attach to a cell, it cannot replicate if the
cell’s environment does not
provide the right stuff, like
polymerases, which are enzymes necessary to
synthesize DNA and RNA. And lastly, the pathogen must
overcome or evade the host cell’s
defense mechanisms. Animals, and humans especially,
have complex and effective immune
systems with multiple lines of defense
against pathogens. Anatomical barriers like the
skin, saliva and mucus aim to prevent
pathogens from entering the body and tissues. If the pathogen makes it
through those barriers, the
immune system has a number of tools
to fight it off. Inflammation is one of the
first responses to an invading
pathogen, and if the pathogen overcomes
this, the acquired immune system steps
in. As the name suggests
, the acquired immune system is
built up over the organism’s
lifetime to respond to specific pathogens. This is why vaccines work. They immunize your body against
specific infections. If your body has seen the
pathogen before, it will recognize it and make
antibodies that bind to the pathogen and summon up macrophages and
killer cells that swoop in and attack
the pathogen. One reason new diseases are so
dangerous is that humans have not built
up immunity towards them, and so the acquired
immune system is much less effective. So, to sum it up, for a
pathogen to be able to infect a host, the host cells must be both
susceptible, having compatible receptors, and permissive, having the
necessary nutrients to allow it to replicate and
spread. Because mammals and, say
, reptiles, have very different anatomies, it’s more unlikely that a virus
would be compatible in both mammals and reptiles
, although there are some exceptions
, like salmonella. However, the anatomies of
humans and bats or cows or primates are similar enough that a
pathogen may evolve to be compatible and show
tropism in both species. Also, viruses and bacteria evolve rapidly. One virus particle can
replicate to produce as many as 10 million virus
particles in just 24 hours. Bacteria generally replicate a
bit more slowly, but one bacterium can generate
2 million bacteria in seven hours. This means viruses and bacteria
can rapidly develop mutations that allow
them to either suppress the host’s
immune system , become more infectious, infect new tissue or new host
species, or become resistant to
medications. In a large population of
animals, like a factory farm or a cave
full of thousands of bats, pathogens are constantly
mutating. When another animal, like a
human, comes into contact with that
population, its pathogens can try out a
new host species. If one of them expresses
tropism for this unlucky organism, the pathogen
can now be spread throughout the new
species’ population. In a cave in China, most likely
in the Yunnan province, there is a population of bats
in which the 2019 coronavirus evolved. How the virus got from the cave
to the Wuhan market is unclear, but we can
hypothesize. In 1998, the Nipah virus broke
out in Malaysia. Nipah virus causes inflammation
of the brain, fever and headaches, mental
confusion, and disorientation. Within 24 to 48 hours, symptoms
may progress into a coma. Fortunately, only 265 people
were infected with Nipah virus in
the 1998-1999 outbreak, but 40% of them died, a
frighteningly high death rate. It was discovered that N
ipah virus originated in fruit bats. The bats were eating mangoes
off trees that were overhanging pig
enclosures. The bats dropped feces and
saliva, covered mangoes into the pig pens, infecting
the pigs , and the pigs passed the
virus on to people. A similar situation likely
happened in Wuhan. Infected bats passed the virus
on to wildlife, which was hunted and brought to
the Wuhan market. Scientists donning full body
suits regularly visit caves all over China to collect samples of bat
droppings, urine, and saliva to test for and monitor
pathogens. Scientists around the world are
always monitoring pathogens in bat, rodent and livestock
populations and at least 200 different
coronaviruses, which is the name given to a
group of viruses that have a
crown shaped structure, have been identified in bat
populations globally, including 12 recently
documented in Mexico. You may be wondering why bats
are so especially disease-ridden. Well, they live in huge groups,
allowing pathogens to spread between many
animals, but they also have very high
body temperatures when they fly, 104 degrees Fahrenheit. So any virus that evolves to
withstand bat ‘s high temperatures can naturally withstand human
fevers, making them more dangerous. Also, bats have very effective
immune systems that prevent
them from getting sick and dying
from pathogens like corona viruses. But they act as a perfect
reservoir for the viruses to thrive, mutate and
potentially evolve to become more deadly
to humans. Several of the most infamous
deadly diseases that recently emerged were
passed to humans from animals. I’ll mention a few examples. The consensus among HIV
researchers is that the virus evolved from its
close relative, simian immunodeficiency virus, or
SIV. “Simian” refers to apes and
monkeys, and SIV affects non-human primates in
Western Africa. At some point in the late 19th
or early 20th century, SIV was transmitted from an ape
or monkey to humans, where it evolved into human
-specific HIV. This transfer was most likely
the result of the bushmeat practice. A hunter or bushmeat vendor was
bitten or cut while butchering an
infected primate. HIV has caused an estimated 32
million deaths and has spread to every
corner of the globe. Then there’s Ebola. Ebola was discovered in 1976
and is one of the most dangerous known infections
, with a death rate of up to 90%. While it came from the
Democratic Republic of Congo, its specific origin is still
unclear, but it most likely came from
fruit bats. The largest Ebola outbreak was
in 2014 to 2016, and there have been smaller
outbreaks since. The total death toll is thought
to be around 13,000. The last example I’ll mention
is the swine flu, which was first detected in
pigs in the United States in 1998. Within a year it spread
throughout the continent’s
livestock pig populations. Most strains of the virus
cannot be spread to humans. But one particular
strain became zoonotic, and it was transmitted to
humans for the first time in
Mexico in 2009, quickly leading to a global
pandemic that resulted in between 151,000 and 575,000 deaths in just the first year alone, according to the CDC. The news media seems intent on
causing a worldwide panic over corona
virus. It’s likely the worst
predictions are exaggerated, but only time will tell. What the coronavirus does,
however, is highlights our vulnerability to a global
pandemic. Our world is so interconnected
, a new disease could spread around the globe
in a matter of weeks or months. While it’s unlikely a disease
could emerge that would wipe out nearly all
of humanity, like in a zombie movie, it’s not out of the question
that we could experience another pandemic like the 1918
Spanish flu. The most severe pandemic in
recent history, it infected one third of the
world’s population, 500 million people, and killed at least 50 million. If a similar crisis occurred
today, the human and economic costs would be
enormous. Of course, such a worst case
scenario is highly unlikely, but it’s still
something we should be prepared for and try to
prevent. The coronavirus illustrates one
less-obvious consequence of the wildlife
trade and of human wildlife conflict. The Wildlife Conservation
Society is calling on China to shut down its exotic meat
markets, warning that if they don’t, new
outbreaks will occur every few years. I fully agree with that, but it’s important to point out
that even if China did follow such advice, there
are plenty of places and opportunities
for zoonotic diseases to
emerge. A 2016 study published in the
journal , Trends in Parasitology, looked at zoonotic diseases
around the globe and mapped out where they most frequently
emerge. What they found is that
pathogens are not most common in tropical
regions with lots of biodiversity, but that temperate regions are
hotspots as well. This is partly because species
in temperate regions are more likely to carry
zoonoses. And also, animal agriculture
provides a perfect breeding ground for
zoonotic diseases, and large livestock producing
nations are at risk of seeing new diseases like the swine
flu, mad cow disease, and the bird flu. Animal agriculture is relevant
for another reason as well. All of the recent deadly
diseases have been viruses , because while we have
antibiotics to fight bacterial infections, we do not have cures for
viruses. However, more and more
antibiotic resistant bacteria are appearing. Every year in the U.S. alone, 2.8 million people become sick from
antibiotic resistant bacteria, and 35,000 of them die. Antibiotic resistance is the
result of the overuse of antibiotics, and 80% of all antibiotics sold
in the U.S. are for use in animal
agriculture, where they are given to
livestock to prevent
infections and improve growth rates. Farms with hundreds or
thousands of animals on antibiotics provide the perfect environment
for bacteria to develop mutations that allow
them to overcome antibiotics. Several European nations have
already banned the use of antibiotics in livestock,
but the U.S. has been slow to follow. Well, this has been a lot of
doom and gloom, but there are some reasons to
be optimistic. Scientists are always searching
for new medicines to treat bacterial and viral
infections, and there are some promising solutions
on the horizon. And while our society may not
be as prepared for a pandemic as it possibly
could be, there are many organizations
like the CDC and WHO working around the clock
to prevent a worst case scenario. I wanted to talk about this
topic because I think it’s another great
reason to get serious about regulating
the wildlife trade and animal agriculture. And I think it’s very important
to be informed about serious issues like this. If you enjoyed this video and
want more content like this, please like and subscribe. This is a new channel where
I’ll be talking about current events, history and topics I
find important or interesting. My name is Sarah Bethea, and
again, thanks so much for watching!

2 Comments

  • Reply Jorge Sáenz February 28, 2020 at 1:28 am

    I did not remember when did I suscribe to this channel, I'm glad I still was,

  • Reply R Garcia February 28, 2020 at 6:53 am

    This isn’t your first video. What happened to your travel vlogs?!

  • Leave a Reply