Coronavirus: Why bats, pangolins are ideal disease hosts
Charli Shield
March 26, 2020
Pangolins are now confirmed carriers of viruses closely related to SARS-CoV-2. But as with SARS and MERS, bats are the most likely original source. Scientists believe bats' unique genetic quirks make them ideal hosts.
As the deadly coronavirus sweeps across the globe, killing tens of thousands, prompting border closures and a worldwide lockdown, scientists race to find out exactly how the outbreak began.
Now, a new study published in Naturesuggests that endangered pangolins — a scaly anteater trafficked for food and medicine — are a likely link between the coronavirus, bats and humans.
While initial speculation pointed to seafood, snakes and another bat-borne coronavirus from Yunnan province in southwestern China, researchers from Hong Kong, China and Australia have found that genetic sequences of the novel coronavirus in pangolins are 85.5% to 92.4% identical to the coronavirus currently infecting hundreds of thousands people. That means, before reaching humans, the virus could have passed from bats to the pangolin, the most illegally traded animal in the world.
More than 25,000 people have now died from the fast-moving coronavirus, known as SARS-CoV-2, which was first detected in December 2019 in Wuhan, China.
What is it about bats?
It isn't the first time the world has witnessed an outbreak of a bat-borne virus. Ebola is thought to have originated in bats, as well as two other types of coronavirus — SARS (Severe Acute Respiratory Syndrome) which emerged in Asia in 2003 after moving from bats to civets to humans, and MERS (Middle East Respiratory Syndrome), which has infected around 2,500 people since 2012 after being transferred from camels.
This isn't necessarily surprising considering the great size and spread of the bat population, says Yan Xiang, a professor of virology at the University of Texas. Bats are the second most common mammal after rodents, making up nearly 20% of all species of mammals — there are more than 1,300 species of bat and some can live up to 40 years.
But experts believe it's the bats' unique immune system that allows it to harbor so many viruses.
Image: Narayan Prasad Koju
Bats are a common source of viruses: Ebola, SARS, MERS, and Nipah can all be traced back to themImage: Getty Images/AFP
While Xiang says scientists "don't yet have a complete picture" of this system, he points to two key elements of the mammal's immune response, called "innate immunity" — their high body temperatures and higher levels of interferon, which signals the activation of an antiviral state.
Bats are the only mammal with the ability to fly, which increases their body temperature and metabolic rate, and puts their bodies into a constant state of "fever." Some scientists believe that bats have suppressed their immune systems to cope, which allows them to tolerate more viruses.
Intermediate species
Coronaviruses are zoonotic viral diseases, meaning they are passed from animals to humans, and while in the animal, the virus goes through a series of genetic mutations that allows it to infect and multiply inside humans. Although the coronavirus is thought to have originated in bats, this doesn't mean it was directly passed from bats to humans.
Pangolins are the most illegally traded animal in the world, and used in traditional Chinese medicineImage: picture-alliance/dpa/H.H. Young
As researchers involved in the latest Nature study suggest in a SARS-CoV-2 explainer, the distinct ecological differences between bats and humans means it is likely that the novel coronavirus needed to pass through another mammal in order to "amplify," or further mutate, so that it could be transmitted to humans.
"The virus was probably unable to infect humans directly through bats, so it had to go through an intermediate animal to further mutate in order to infect humans," Xiang told DW. In the case of previous coronaviruses transmitted to humans, SARS and MERS, civets and camels acted as these intermediate hosts.
The link between the novel coronavirus and pangolins was previously suggested by a study from South China Agricultural University researchers in early February, who studied more than 1,000 samples from wild animals.
Xiang says the evidence for those claims actually "already exists" in a paper from October 2019, which published genome sequences of sick pangolins smuggled from Malaysia to China, finding evidence of coronaviruses. Although there's a strong similarity between the coronaviruses found in pangolins and SARS-CoV-2, further research is needed to establish exactly how the virus jumped from animal to human.
Human defense mechanisms keep us safe — mostly
While the devastation of such outbreaks is difficult to predict, Stuart Neil, head of virology at King's College London, says "in the grand scheme of things," events like this "don't happen very often."
"We're probably exposed to these viruses from other species much more often than we get transfers of new viruses from animals and these sustained epidemics," he told DW.
The reason for that, Neil says, is "due to our intrinsic defensive mechanisms." There is no such thing as an inherently deadly virus, he points out, because what may be harmless to one species, as shown by the numerous coronaviruses that circulate in bats, may be deadly to another.
"It's entirely dependent on the defense mechanisms of the host species and whether they can live in harmony with a virus or not."
Such epidemics are becoming more likely, though, as humans increasingly encroach on the habitats of wild animals, he warns, saying that "humans are exposed to these viruses because of how they behave and interact with animals."
Coronavirus: Timeline of the global spread of COVID-19
Since the first cases were confirmed in December 2019, the flu-like COVID-19 virus exploded into a global pandemic, killing tens of thousands of people and infecting around 800,000. Scientists scramble for a vaccine.
Image: picture-alliance/dpa/SOPA Images/A. Marzo
Pneumonia-like virus hits Wuhan
On December 31, 2019, China notifies the World Health Organization of a string of respiratory infections in the city of Wuhan, home to some 11 million people. The root virus is unknown and disease experts around the world begin working to identify it. The strain is traced to a seafood market in the city, which is quickly shut down. Some 40 people are initially reported to be infected.
Image: Imago Images/UPI Photo/S. Shaver
First death in China
On January 11, China announces the first death from the coronavirus — a 61-year-old man, who had shopped at the Wuhan market, dies from complications with pneumonia. Like SARS and the common cold, scientists identified that the new virus is in the coronavirus family. It is temporarily named 2019-nCoV. Symptoms include fever, coughing, difficulty breathing, and pneumonia.
Image: Reuters/Str
Virus reaches neighboring countries
In the following days, countries such as Thailand and Japan begin to report cases of infections in people who had visited the same Wuhan market. In China, a second fatality is confirmed in the city. By January 20, three people have died in China and more than 200 are infected.
Image: Reuters/Kim Kyung-Hoon
Millions under lockdown
China places Wuhan on quarantine on January 23 in an attempt to limit the spread of the virus. Transportation is suspended and workers attempt to quickly build a new hospital to treat infected patients, which total over 830 by January 24, as the death toll climbs to 26. Officials eventually extend the lockdown to 13 other cities, affecting at least 36 million people.
Image: AFP/STR
A global health emergency?
More and more cases are confirmed outside of China, including in South Korea, the US, Nepal, Thailand, Hong Kong, Singapore, Malaysia and Taiwan. As the number of infections rises, the World Health Organization on January 23 determines that it's "too early" to declare a global public health emergency.
Image: Getty Images/X. Chu
Coronavirus reaches Europe
On January 24, French authorities confirm three cases of the new coronavirus within its borders, marking the disease's first appearance in Europe. Hours later, Australia confirms four people have been infected with the respiratory virus.
Image: picture-alliance/dpa/S. Mortagne
First cases confirmed in Germany
On January 27, Germany announces its first known case of the virus — a 33-year-old in Bavaria who contracted it during a workplace training with a visiting Chinese colleague. He is put under quarantine and observation at a Munich hospital. The following day, three of his colleagues are confirmed infected. The death toll in China reaches 132, with around 6,000 infected worldwide.
Image: Reuters/A. Uyanik
WHO declares global health emergency
On January 30, the UN's World Health Organization (WHO) declares coronavirus a public health emergency of international concern in a bid to protect countries with "weaker health systems." However, WHO Secretary-General Tedros Adhanom Ghebreyesus does not recommend trade and travel restrictions, saying these would be "an unnecessary disruption."
Image: picture-alliance/KEYSTONE/J.-C. Bott
First death outside China
The first death linked to the novel coronavirus outside of China is reported in the Philippines on February 2. A 44-year-old Chinese man had traveled from Wuhan to Manila before falling ill and being taken to hospital, where he later died of pneumonia.
Image: Getty Images/AFP/T. Aljibe
Bad ending to a cruise
Also on February 3, the cruise ship Diamond Princess is quarantined off Yokohama in Japan after cases of the new coronavirus were found on board. As of February 17, the number of people infected has grown to more than 450, the largest cluster of cases outside of China. Several of the 3,700 passengers and crew onboard the ship are being or have been flown back to their home countries.
Image: picture-alliance/dpa/kyodo
Italy under quarantine
Cases in Italy rise dramatically, with 77 deaths and thousands of confirmed cases by March 3. Many countries instigate travel restrictions to northern Italy and tourist numbers plummet. On March 8, the Italian government put the entire Lombardy region into quarantine, affecting 16 million people. March 10 sees 168 fatalities in Italy, the highest in a single day.
Image: Reuters/R. Casilli
Economic woes
European and US stock markets slump on March 6, leading to the worst week since the 2008 financial crisis. The effect on global business has been significant, with many companies reporting losses and the tourism industry and airlines badly hit. The EU pledge €7.5 billion ($8.4 billion) on March 10 in an investment fund to try to stop the Eurozone falling into a recession.
Image: picture-alliance/Jiji Press/M. Taguchi
WHO declares outbreak as pandemic
As worldwide cases top 127,000 and deaths pass 4,700, the World Health Organization designates the global outbreak as a "pandemic" on March 11. US President Donald Trump announces a travel restriction on people coming from the Schengen Zone in Europe, annoying the EU. German Chancellor Angela Merkel announces that in Germany, 70% of the population could get the virus.
Image: picture-alliance/Photoshot
Public life on hold in Europe
On March 14, Spain joins Italy in imposing a near-total nationwide lockdown to prevent the virus spreading. The population of 46 million is told not to leave their homes unless for essential tasks. In France, cafés, restaurants and non-essential shops are closed as of March 15. Many public events in Germany are cancelled and schools close.
Image: picture-alliance/dpa/AAB. Akbulut
International travel severely restricted
As of March 15, many countries impose strict travel bans or restrictions in an attempt to stop the spread of Covid-19. For example, New Zealand and Australia require all international passengers to self-isolate for 14 days after arrival in the country. The US extends a European travel ban to include the United Kingdom and Ireland.
Image: picture-alliance/Photoshot
Germany imposes partial lockdown
In a landmark televised address German Chancellor Angela Merkel announces far-reaching restrictions on everyday life on March 22, banning meetings between more than two people not from the same household outside of the workplace. The country has a surprisingly low death rate, a phenomenon attributed to a high level of testing, and a high number of intensive care beds.
Image: picture-alliance/EibnerT. Hahn
Virus strikes at top as UK locks down
On March 23rd Britain becomes the latest country to impose restrictions on personal freedoms, with people only allowed to leave their homes in a limited number of circumstances. Prime Minister Boris Johnson is diagnosed with the viruson March 27, as well as heir to the throne Prince Charles on March 25. Meanwhile, there are complaints that not everyone is taking social distancing seriously.
Image: picture-alliance/R. Pinney
Grim milestone for the US
On March 27 the US overtakes China in terms of the number of people infected, making it the country with the most cases of COVID-19. This came as President Donald Trump claimed that the nation would get back to work "pretty quickly." At the same time, it emerged that more than 3 million Americans had lost their jobs due to the pandemic. New York is worst-hit, with a hospital ship sent to help out.
Image: picture-alliance/Photoshot/J. Fischer
Spain's surging death toll
Spain also overtakes China in the number of COVID-19 cases on March 30, as the government toughens the severity of its lockdown. All non-essential activities are halted. Only Italy has a higher death toll than Spain. Most affected is the capital, Madrid. With funeral services overwhelmed, officials turn the Palacio de Hielo ice skating rink into a temporary morgue.
Image: picture-alliance/Geisler-Fotopress
More than a million
On April 2nd the Johns Hopkins University announced on Thursday that there were more than a million confirmed coronavirus cases around the world. The US is the most affected with three times the number than China, where the virus emerged in December. Over 50.000 people have died — and the outlook remains grim.
Image: Reuters/J. Redmond
UK PM Boris Johnson hospitalized
The 55-year-old was admitted to the intensive care unit at London's St Thomas hospital on Monday evening (6.4.) and was given oxygen treatment after his condition worsened. He had been diagnosed with COVID-19 on March 27.
