A TALE OF MANY PANDEMICS – PART 2: ZOONOTIC VIRUSES

Pandemics caused by zoonotic viruses

During lockdown, I watched the movie Contagion against my better judgement. At the end of the movie a montage depicts how the novel virus enters the human population. Essentially, it jumps from bats (which are disturbed by human activity), uses pigs as an intermediary and finds its way to humans due to a chef with poor personal hygiene. This fictional novel virus is an example of a zoonotic virus. Not all zoonotic viruses cause deadly pandemics. Most viruses are endemic, meaning they circulate in a human population in one geographical location. The coronavirus that causes the common cold is an example of an endemic virus. Other viruses are epidemic, whereby they are restricted to a small geographic location, but cause infections at higher- than- normal rates. A classic example is Ebola, which is thought to have spilled over from fruit bats, although this is still unconfirmed. There have been several outbreaks since the 1970s, with the largest Western African outbreak being declared an epidemic (2013- 2016). When zoonotic viruses acquire a passport, cross international borders and cause abnormally high rates of infection, the disease is declared a pandemic. Zoonotic viruses have been responsible for some of the worst pandemics to hit humanity, not including the current one we’re living through. 

1. HIV/AIDS– People don’t usually think of Acquired Immunodeficiency Syndrome (AIDS) as a pandemic and it is sometimes designated an epidemic by WHO. However, the causative virus Human Immunodeficiency Virus (HIV) has infected roughly 38 million people globally as of 2019. AIDS is thought to have started in the Democratic Republic of Congo around 1920 when HIV jumped from chimpanzees to humans. However, the current pandemic is only estimated to have started in the mid- to late- 1970s. Recent advances in anti- retroviral therapy mean that HIV infection is no longer a death sentence, as it is possible to suppress the virus with drugs and avoid getting AIDS. However, there is still no vaccine or permanent cure. 
2. Influenza– There have been four influenza pandemics in the last 100 years, with the Spanish Flu of 1918 being the worst. The causative organism for all is the influenza A virus, which is predominantly found in birds. This is classified into several subtypes based on the presence of two identifying proteins (haemagglutinin {H} and neuraminidase {N}) on its surface. Each Influenza A subtype is named for the number of these surface proteins. For instance, H1N1, the causative virus of the 1918 pandemic, has one of each. Flu pandemics are different from the seasonal flu as they are caused by a virus that humans have never encountered before. Influenza A and B that cause the seasonal flu are simply the latest iteration of the same virus, modified by spontaneous genetic mutations. There is a vaccine for the seasonal flu, which should be taken at this time to reduce strain on our healthcare systems. 
3. SARS/MERS– These two diseases were caused by coronaviruses, named for the glycoproteins on their surface that give them a crown- like appearance. Severe Acute Respiratory Syndrome (SARS) emerged in February 2003. The causative SARS-CoV virus is thought to have emerged from bats in 2002. It then jumped to civet cats before finding a home in humans. Middle East Respiratory Syndrome (MERS) is not classified as a pandemic, but I will mention it as it was a novel coronavirus with pandemic potential. It was first reported in Saudi Arabia in 2012 and is caused by the MERS-CoV virus. Like SARS-CoV, this virus is thought to originate in bats, but was transmitted to humans from camels. Both diseases were not as infectious as COVID 19, as the number of cases was in the low to high thousands. However, the death rate was extremely high (10% for SARS and 35% for MERS). There are currently no vaccines for either disease. 

Crossing the species barrier: Why does this happen? 

Why do these viruses spill over to humans from animals? The simple answer is proximity. Research into virus ‘spill- over’ events has revealed two stark truths: One, the majority of zoonotic viruses with pandemic potential arise from wildlife. Two, they can jump to humans because we have the unfortunate habit of exploiting wild animals and destroying their natural habitats. It should be noted that zoonotic viruses have also jumped from domesticated animals (swine flu) and birds (avian influenza), but the pandemics in the last 100 years all had wildlife origins. Once a virus jumps to humans, it still needs to establish itself and propagate through human- human transmission. Interestingly, many novel viruses cannot make it past the first human host. For a virus to cause an outbreak, several factors need to be satisfied. These include but are not limited to: (1) Frequency of contact between animal- host and humans, (2) mechanism of spread, for example airborne, waterborne or sexually transmitted, (3) ability of the virus to evade the host immune system and (4) efficacy of gaining entry into human cells. If a novel virus is able to easily infect humans and spread without being detected, it has pandemic potential. SARS-CoV2 is causing the deadliest pandemic since the Spanish influenza because it is one of those rare viruses that has satisfied all the above criteria. Through the illegal wildlife trade/ wet markets, the virus had ample opportunity to encounter human hosts. It is easily spread through droplets released by coughing, sneezing and talking. It can evade the host immune system by suppressing the first line of defence against an infection and finally, it can easily enter human cells using the ACE2 receptor in lung cells.