While the number of astronauts who've experienced a total reactivation of dormant viruses is relatively small, the fact that it seems to be directly caused by their time in space is a concern for the US space agency NASA, which put out a warning following the February publication of a study in Frontiers in Microbiology.
Of the 89 astronauts they tested, "only six astronauts developed any symptoms due to viral reactivation," said senior author Dr. Satish K. Mehta of KBR Wyle at the Johnson Space Center in a Friday press release — a rate of roughly 7 percent. However, 47 "shed herpes viruses in their saliva or urine samples" but remained asymptomatic, signaling that the virus had reactivated.
This condition persisted for a month after the astronauts returned to Earth, the study noted.
"Reactivation of latent viruses during long-duration spaceflight could increase risk for adverse medical events during exploration-class deep-space missions," warns the study, conducted by scientists at the University of Colorado in Denver and several institutions in Houston, Texas, associated with NASA's Johnson Space Center.
"Maintenance of viral latency requires a vigorous and vigilant immune system, highly dependent upon competent cytotoxic T-cells, and any changes in immune status tend to promote viral reactivation," the study says.
"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," Mehta said. "This physical challenge is compounded by more familiar stressors like social separation, confinement and an altered sleep-wake cycle."
This "unique set of stressors" causes the body to produce hormones like cortisol and adrenaline, both of which "play a crucial role in the modulation of the human immune response," the study notes.
This results in fewer virus-fighting white blood cells being produced and, as the saying goes, when the cats are away, the mice will play, and the most common dormant viruses in the body, normally suppressed by the immune system, can come roaring back.
"Herpes viruses have co-evolved with humans for millennia and subsequently employ sophisticated strategies to evade the host immune response," the study notes. "Consequently, after primary infection, they persist lifelong in a latent or dormant phase, and are generally asymptomatic in immunocompetent individuals. However, they may reactivate during periods of increased stress, isolation and during times of immune challenge."
The most common types of herpes viruses the scientists recorded in the skin and urine samples of astronauts were Epstein-Barr virus (EBV), responsible for infectious mononucleosis; Varicella Zoster virus (VZV), which causes both chickenpox and shingles; and Herpes Simplex virus-1, commonly referred to as "oral herpes."
Further, it was often not only one type of herpes virus, but many.
The scientists also noted the re-emergence of Cytomegalovirus, a type of herpes best known as a killer of patients with Acquired Immunodeficiency Syndrome (AIDS). A type of beta herpes that's evolved to survive in the human body without causing infection, when the immune system is weakened, it can break through the body's defenses, causing encephalitis, gastroenteritis, pneumonia and chorioretinitis, among other ailments.
The study found reactivation in astronauts who endured both long (up to 180 days) and short (10 to 16 days) flights. The scientists noted that it's not just in space flight that these conditions can emerge, though: "virus reactivation has also been observed in ground-based models of spaceflight including Antarctica, undersea habitat, artificial gravity and bed rest studies, though not to the extent seen during spaceflight studies," they wrote.
"The occupational hazards for astronauts are profound, but research into the causes and mechanics of viral reactivation not only benefit the astronaut but also the general patient population," the scientists wrote in the study, noting their research consequently has "tremendous clinical relevance."
"Ultimately, the information gleaned from these space studies will shape the way we prepare for and design exploration-class missions, beyond the moon and Mars, where reactivation of latent viruses could result in increased risk for wide-ranging adverse medical events," the study concluded. "Partial-gravity environments, e.g., on Mars, might be sufficient to curtail serious viral reactivation, but this needs to be addressed in future research."
With humanity entering a new race for manned space missions beyond low-Earth orbit, these are important risks to consider. Next month, SpaceX is expected to test its Starship vehicle, which founder Elon Musk says will one day take humans to Mars. The private space flight firm will also begin use of its Dragon 2 reusable spacecraft in what will be the first manned launch from US soil since 2011, when the Space Shuttle program was discontinued. Several space programs, including those in the US, China and Russia, as well as the European Space Agency, have announced their intention to send manned missions to Mars in the next few decades, while those countries plus Japan also intend to undertake manned lunar missions before then.