As the number of Covid-19 victims continues to climb globally, another related virus still causes fatalities years after it first emerged.
Figures released this month by the World Health Organisation show that five people died from Middle East Respiratory Syndrome in February, some eight years after it materialised in 2012.
Though tragic for its victims and their families, the overall death toll from Mers is far smaller than what might have been expected.
In 2013, the disease was branded “a threat to the entire world” by WHO’s then director-general, Dr Margaret Chan.
Since then, however, experts have been able to better understand its ability to spread between hosts, recognising that “sustained person-to-person transmission” was low.
“The coronavirus Covid-19 has the ability to cause disease and to be highly infectious,” said Dr Bharat Pankhania, a senior consultant in communicable disease control and senior clinical lecturer at the University of Exeter in the United Kingdom.
“Mers is different. If you get infected, it’s difficult to be infectious.”
Seven strains of coronaviruses are known to infect humans, with four leading to common cold symptoms largely restricted to the upper airways.
The other three - which can cause Covid-19, Mers and Severe Acute Respiratory Syndrome (Sars) - are more harmful because they affect lower airways, including the lungs, where they can cause pneumonia.
New WHO figures show there have been 871 fatalities from Mers since 2012, from a total of 2,538 infections.
Ten of the deaths occurred in the UAE, with additional victims also recorded in 20 other countries.
Meanwhile, between 2002 and 2004, Sars is known to have killed 774 people globally.
Key to the way these pathogens infect human cells are their spike proteins, which stand out from the main surface of the virus.
Spike proteins attach to receptor proteins on the surface of human cells and enable the virus to pass through the cell membrane.
Once inside, the virus takes over the cell’s machinery and starts producing multiple copies of itself, which then infect other cells.
Different coronaviruses have different spike proteins, and this affects which cells the viruses target.
Spike proteins on the coronaviruses that cause Covid-19 and Sars attach to a receptor called ACE2, found on cells in the lungs.
The Mers coronavirus, however, attaches instead to a receptor called DPP4, which is more prevalent on cells further down in the lung’s airways, so the virus must travel for longer to pass on the infection.
As a result, people need a larger dose of the Mers coronavirus to contract the disease. This helps explain why the virus does not spread so easily person-to-person.
Many cases of Mers are know to have begun with someone who works on a farm.
The infection can be caught from camels after sustained exposure, while person-to-person transmission happens mostly in hospitals.
Camels' ability to infect people is the chief cause behind continued new cases of Mers.
The differences in where cell receptors are distributed is also thought to explain why it, unlike Covid-19, infects and replicates in the intestines, where it can cause pain, vomiting and nausea.
“You need prolonged contact with an infected animal [to be infected with Mers],” said Dr Pankhania.
“If you’re unlucky [enough to be infected], any human will need prolonged contact to be infected [by you].”
