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Even today as Covid spreads, the World Health Organisation – now armed with a vaccine – is fighting a new Ebola outbreak in the north west of the Democratic Republic of the Congo. In just four weeks, there have been 56 confirmed cases and at least 20 deaths.
Experts suspect the “natural reservoir” for Ebola is also bats – but they are far from the only animal to carry zoonotic viruses. The Spanish Flu of 1918 is thought to have started in North American poultry.
From spark to conflagration
Middle East respiratory syndrome (Mers), another coronavirus, comes from camels and has killed 858 people since it was first discovered in Jordan in 2012.
The case numbers reported for these and other zoonotic diseases are almost certainly just the tip of the iceberg. The vast majority of spillover events go unreported, say experts.
“We’re continually exchanging viruses with animals, that’s what happens”, says Dr David Redding, from the Centre for Biodiversity and Environment Research at UCL.
“These conspiracy theories about labs misunderstand the basics of virology. We know that all species are sharing pathogens all the time. It is through this process that viruses naturally mutate and evolve.”
It is HIV that best illustrates the point, a virus now known to date back to the early 1900s. It’s simian version (SIV) is thought to have jumped from monkeys to humans through hunters and butchers in Africa. Cities like former Belgian colony Leopoldville, which were rife with prostitution and the ulcerating venereal disease syphilis, are thought to have provided the ideal environmental conditions for the virus to mutate.
Dr Daszak describes HIV, which has killed an estimated 32 million people, as the “ultimate” example of spillover. After many decades of repeated small scale flare ups (all unnoticed at the time) it exploded as a pandemic in the early 1980s. What had changed was not so much the virus itself – the spark – but the society it landed in. The population boom in Africa, the globalisation of air travel, the sexual revolution in the west – they all played a part.
“Changes to human behavior increase the transmission of viruses between people, for example sexual contact or injected drug use,” says Dr Daszak. “These changes alter the ‘R’ or reproduction rate of a virus and may assist in driving their emergence.”
Sars-Cov-2 may also have been circulating longer than thought. The virus has mutated very little since first being discovered in humans. This may be because it is a stable virus which faces little pressure to adapt. But it may also be because it has already adapted.
“The evolution of this virus to become a human pathogen may have already happened and we missed it,” Angela Rasmussen, a virologist at Columbia University told Science magazine last week.
Michael Novakhov – SharedNewsLinks℠