Many deadly diseases that afflict humans were originally acquired through our contact with animals. But pathogens can also jump the species barrier to move from humans to animals: a new study shows that green monkeys in The Gambia acquired Staphylococcus aureus from humans.
But there’s more. Genetic analysis showed the monkeys have acquired S. aureus strains from humans on multiple occasions. And most of the S. aureus found in monkeys were part of a clade (a group with common ancestors) which appeared to have resulted from a human-to-monkey transmission event that occurred 2,700 years ago.
Two of the most recent human-to-monkey transmission events appear to have taken place about 30 years ago and about 7 years ago. These both appear to be the result of human encroachment into the monkeys’ natural habitat, and probably resulted from transfer of human bacteria from hands to food that was then fed to monkeys. According to the report the monkeys are wild but very acclimated to humans – who often feed them peanuts.
Interestingly, investigators found no evidence of transmission of S. aureus from monkeys to humans. It seems that the strains that jump from humans to monkeys lose the genes that help them adapt to human hosts.
Humans historically acquired many of their deadliest diseases from livestock domesticated in the early years of civilisation. In the last few generations, the combination of increasing human encroachment on wild ecosystems and increasing human travel has led to acquisition and spread of diseases ranging from HIV to Lyme.
And as humans encroach ever more steadily into natural ecosystems, the risk increases that pathogens will be transmitted from humans to animals, as well as vice versa.