Every year
there seems to be a new virus that just popped up out of nowhere to cause us a
great deal of pain and suffering. Is it the work of a mad scientist vying for
global domination? Are these viruses
coming back to life after being frozen for millennia? Are they hitching a
ride to Earth via meteorites?
The truth is many of these viruses are not so new – but we are creating new
opportunities for them to infect us. Many viruses jump from other animals into people – a
process known as “zoonotic transmission” – and some of our actions roll out the red carpet for the virus. Let’s take a closer look at where
some of these “new” viruses may have originated and how they spiral out of control.
Zika
Microcephaly
is a term used to describe babies born with much smaller head size than normal,
which is indicative of incomplete brain development. In Brazil, this birth
defect occurs about 150 times per year. However, in the past 4 months, nearly
4,000 babies have been born with microcephaly - a dramatic spike that has set
off alarm bells.
Photo of a child born with microcephaly, which has been linked to the Zika virus. |
Global
warming and increased travel have conspired to create excellent opportunities
for viruses like Zika to spread. It only takes one infected person to attend a
major spectacle (for example, the 2014 FIFA World Cup in Brazil) to start a
chain reaction of viral transmission. Viruses need no passports and can jet set
around the world in unprecedented time. Global warming is an issue because it
has allowed the species of mosquito that carries these viruses to thrive in areas
that used to be too cold. Even El
Niño has been catching some of the blame for helping to spread Zika.
Ebola
While Zika
jumped to humans from other primates, the African filovirus Ebola is thought to
have originated in fruit bats. Bats can transmit a number of other deadly
viruses, including rabies. Bats happen to be a source of food in several of the
areas where Ebola outbreaks have occurred, consistent with the idea that bats
are the culprits. Once Ebola infects a human, it can spread quite easily to
other people through bodily fluids.
Bats like this one are now considered to be a major carrier capable of spreading the Ebola virus to people. |
The 2014 Ebola outbreak started with a toddler who fell sick in Meliandou village in Guinea. Source. Credit: Live Science |
MERS, Middle
East Respiratory Syndrome, first made headlines in 2012. This life-threatening
respiratory virus reared its ugly head in Saudi Arabia first, but has since
been reported in 25 other countries, including those not in the Middle East
(due to unwitting travelers carrying more than their luggage). MERS is caused
by a coronavirus, so the causative agent is typically referred to as MERS-CoV.
Like many other respiratory viruses, coughing in close proximity can spread
MERS-CoV between people.
But how did
MERS-CoV get into people in the first place? According to the World Health
Organization: “It is believed that
humans can be infected through direct or indirect contact with infected
dromedary camels in the Middle East. Strains of MERS-CoV have been identified
in camels in several countries, including Egypt, Oman, Qatar and Saudi Arabia.”
It is easy to understand the respect and admiration one can have for a noble creature like the camel. But getting a little too intimate with a camel may literally leave you breathless. |
UPDATE (3/1/16): A new study suggests that we have bats to thank once again for spreading MERS-CoV to camels.
HIV
Human
Immunodeficiency Virus (HIV), which causes AIDS, wasn’t on anyone’s radar until
an unusually large number of people starting suffering from rare diseases with
strange names like Kaposi’s sarcoma, toxoplasmosis, and pneumocystis. These diseases
are hardly ever seen in people with normal, healthy immune systems. Turns out
they were secondary infections – the primary infection was HIV, which was
destroying the very immune cells that are needed to keep those other illnesses
at bay.
Historical
records have placed the earliest cases of HIV infection to the 1950s, which
suggests it has been moving through humans slowly through the decades prior to its
explosion in the early 1980s. An increase in international travel, unsafe
sexual practices, and intravenous drug use are all factors that have
contributed to accelerating the epidemic.
HIV (yellow particles) is a cunning foe that destroys the immune cells (blue) designed to protect us from foreign invaders. |
It’s a virus world after all
As you can
see from these examples, many “new” viruses were actually pre-existing in other
animals and just made a “species jump” into humans. But how did these viruses
get into the other animals in the first place? That question is a lot harder to
answer.
Viruses are
little more than a fragment of DNA or RNA, perhaps
rogue genes that escaped a cell and became independent, infecting other
cells in order to replicate and spread. Richard Dawkins coined the term, “the
selfish gene”, and that is a very accurate description of viral DNA/RNA. What
we do know is that viruses have been around a long, long time, perhaps before the dawn of life
itself. There are even viruses
that infect bacteria.
Once inside
host cells, viruses replicate quickly, which means they are very adaptable.
Their ability to evolve quickly is likely to be a key factor explaining why
these selfish genes can make a reproductive factory out of a wide variety of
different hosts…and why “new” viruses can appear to spring out of nowhere.
While
viruses are a nuisance to us now, they may have been important drivers of
evolutionary change in the past. It has been proposed that RNA viruses may have led to the
formation of DNA and DNA replication mechanisms, without which we would not
even be here to complain about them!
Contributed
by: Bill Sullivan, Ph.D.
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