Two-thirds of humans are eternally infected by at least one of the two strains of herpes: the embarrassing mouth-inflicting cold sore variety or the awkward nether region-inhabiting sexually transmitted strain.
The duality of herpes strains makes us special in the primate kingdom, upon which herpesviridae has preyed for tens of millions of years. Humans are the only primates known to be infected by two distinct varieties. And researchers who set out to figure out where those strains came from have reached a conclusion that doesn't just challenge conventional wisdom—it seems straight-up creepy.
Researchers from San Diego and South Africa took advantage of recent scientific advances to analyze herpes strains using evolutionary models that simulate natural changes in genes over time, while taking into consideration the strengths of different selection pressures on those genes. They compared the three strains of herpes that affect chimpanzees and humans.
"Zoonotic diseases, resulting from pathogens jumping into humans from another species, are of immense importance to global health. Understanding how and when viral pathogens got into humans is relevant for preparing for future epidemics."
They found that herpes simplex virus 1, or HSV-1, which generally causes cold sores, has been with us since truly ancient times. Published in Molecular Biology and Evolution, the modeling results suggest that HSV-1's forebears infected our ancestors before they split off, evolutionarily, from the ancestors of modern chimpanzees some five to seven million years ago. Over time, the viruses evolved with their hosts, and those that infect modern chimps can no longer infect humans—and vice-versa. Scientists call this phenomenon host-virus co-divergence. Modern humans were left with HSV-1; while in chimpanzees the virus evolved into what we call ChHV.
But the story seems more convoluted for the sexually transmitted HSV-2, which, while often asymptomatic, can trigger many an awkward conversation with a new sexual partner. For the unluckiest of those who are infected, HSV-2 causes unsightly blisters and painful sores. The researchers concluded that this virus was the product of cross-species transmission. They think that it's the result of ChHV jumping over, 1.6 million years ago, from chimpanzee ancestors to one of the human-like species that preceded Homo sapiens. From there, it then evolved into today's virus.
That could suggest interspecies copulation—but not necessarily. In fact, the transfer might even have been the result of something decidedly non-amorous.
"HSV-1 and HSV-2 can be either oral or genital diseases in humans," says Joel Wertheim, an assistant research scientist at the University of California-San Diego who worked on the study. "We can’t say with any certainty what the route of transmission of HSV-2 to our ancestors was. It could have been sexual, but it also could have come through other types of physical contact such as predation. In fact, predation is how many current zoonoses make the jump from other animals into humans."
Wertheim doesn't think the discovery will lead to miracle cures for herpes. But the research could provide public health benefits.
"Zoonotic diseases, resulting from pathogens jumping into humans from another species, are of immense importance to global health," he says. "Understanding how and when viral pathogens got into humans is relevant for preparing for future epidemics."