We’ve seen recent spikes in measles infections.
Some European countries, including the UK, lost their measles-free status and many developing countries, especially parts of Africa, Asia and Oceania are seeing frequent outbreaks.
The Democratic Republic of the Congo (DRC) is experiencing a protracted outbreak of over a quarter of a million cases and more than 5,000 deaths, mainly in children under-five.
And the reason for this measles upturn?
Declining uptake of measles vaccination. You need to immunise over 90% of a population to protect it from measles outbreaks. In DRC immunisation rates are less than 60%.
And there’s a potential hidden danger of poor vaccine coverage.
Measles belongs to a group of highly related viruses called morbilliviruses, which can be found in various mammals, and these are adept at jumping from one host species to another.
The common ancestor of measles virus is thought to have been a virus circulating in cattle which, according to Louise Cosby, emeritus, honorary professor at the Wellcome Wolfson Institute for Experimental Medicine, “probably jumped into humans when cattle were domesticated thousands of years ago”.
“There are also historical records which suggest that canine distemper virus – or CDV – might have arisen from human measles in the Americas, following one or more human-to-dog spill-overs during extensive measles outbreaks in indigenous people, who were exposed to the virus for the first time when they came into contact with European explorers,” she explained.
A cross-species spill-over is the transmission of a pathogen from one vertebrate species to another.
Deadly to dogs
As CDV has spread around the world, there are many examples of it hopping into other species including seals, cats and even monkeys – often with devastating effects.
In the 1980s, this virus wiped out the last wild population of black-footed ferrets and is even putting some endangered big cat species in peril.
To be able to flit from one species to another, a virus often has to adapt in order to use the new host cell machinery.
We call these potential host-blocks to virus infection the species barrier.
The first barrier a virus must overcome is cell attachment and entry. According to Dr Dalan Bailey, a virologist based at the Pirbright Institute, what makes morbilliviruses so adept at cross-species transmission is that the proteins it commandeers to do this are very similar across different mammalian species, so the species barrier is low.
And this could pose a potential future risk to human health
“We’ve definitely got evidence that non-human morbilliviruses can easily adapt to enter human cells, and we’re confident that it can replicate in them too,” Dr Bailey said.
It takes just two simple mutations in one of CDV’s surface proteins to allow it to infect human cells.