Space travel caused herpes viruses to reactivate in more than half of crew aboard Space Shuttle and International Space Station (ISS), according to a NASA study, a finding that could jeopardise mankind's future missions to Mars and beyond.
While only a small proportion develop symptoms, virus reactivation rates increase with spaceflight duration and could present a significant health risk on future missions.
"NASA astronauts endure weeks or even months exposed to microgravity and cosmic radiation -- not to mention the extreme G forces of take-off and re-entry," said Satish K Mehta at NASA's Johnson Space Center.
"This physical challenge is compounded by more familiar stressors like social separation, confinement and an altered sleep-wake cycle," said Mehta.
To study the physiological impact of spaceflight, researchers analyse saliva, blood and urine samples collected from astronauts before, during and after spaceflight.
"During spaceflight there is a rise in secretion of stress hormones like cortisol and adrenaline, which are known to suppress the immune system," said Mehta.
"In keeping with this, we find that astronaut's immune cells --particularly those that normally suppress and eliminate viruses -- become less effective during spaceflight and sometimes for up to 60 days after," he said.
In the midst of this stress-induced amnesty on viral killing, dormant viruses reactivate and resurface, according to the research published in the journal Frontiers in Microbiology.
"To date, 47 out of 89 (53 per cent) astronauts on short space shuttle flights, and 14 out of 23 (61 per cent) on longer ISS missions shed herpes viruses in their saliva or urine samples," said Mehta.
"These frequencies -- as well as the quantity -- of viral shedding are markedly higher than in samples from before or after flight, or from matched healthy controls," he said.
Overall, four of the eight known human herpes viruses were detected.
These include the varieties responsible for oral and genital herpes (HSV), chickenpox and shingles (VZV) -- which remain lifelong in our nerve cells -- as well as CMV and EBV, which take permanent but uneventful residence in our immune cells during childhood.
CMV and EBV, are two viruses associated with causing different strains of mononucleosis or the "kissing disease".
So far, this viral shedding is typically asymptomatic, researchers said.
"Only six astronauts developed any symptoms due to viral reactivation. All were minor," said Mehta.
However, continued virus shedding post flight could endanger immunocompromised or uninfected contacts on Earth, like newborns.
"Infectious VZV and CMV were shed in body fluids up to 30 days following return from the International Space Station," he said.
As space agencies prepare for human deep-space missions beyond the moon and Mars, the risk that herpes virus reactivation poses to astronauts and their contacts could become more crucial.
Developing countermeasures to viral reactivation is essential to the success of these deep-space missions, said Mehta.
