Latent Viral Reactivation During Short and Long Term Spaceflight
To better understand the medical significance of the immune changes
associated with space flight, we at the NASA Johnson Space Center,
studied the relationship between astronauts and the latent herpes
viruses they harbor. Astronauts experience various stresses that may
result in inhibition of their cell-mediated immunity and increased
reactivation of latent viruses during space flight, potentially increasing
the risk of disease among crew members. Risks associated with many
infectious agents are reduced by restricting pre-flight contact of the flight
crews with high-risk populations.
However, the risk of latent virus reactivation
is unaffected by such precautions. Virus reactivation could pose an important
health risk for astronauts, as well as for people living and working in other
extreme environments. The most commonly recognized latent viruses in humans
are herpes viruses. There are currently eight known herpes viruses specific
for humans. These viruses cause specific primary diseases (e.g. chickenpox),
but may remain in the body (sensory ganglia, blood cells, epithelial cells)
in a latent or quiescent state for decades. Elements of the immune system maintain
this state of latency. When, however, the immune system is diminished by an
underlying cause such as stress, aging, or cancer chemotherapy, latent viruses
reactivate and proliferate, leading to disease (e.g. shingles). Thus, reactivation
of viruses is a tool to predict changes in the immune status.
Our laboratories and collaborators have found evidence for latent virus
reactivation in astronauts by examining the shedding of viral DNA and
the titer of antibodies to the same viruses, and in many of the same
experiments we have determined the concentration of stress hormones.
We began with a ground-based model studying expeditioners, and demonstrated
that Epstein-Barr virus (EBV), one of the most common latent herpes viruses,
reactivates with increased incidence, coincident to decreased immunity.
Subsequently, this non-invasive, sensitive, and specific assay was utilized
to study shuttle astronauts. To determine if increased EBV appearance
in saliva was unique, we expanded the study to include Cytomegalovirus
(CMV), another latent virus, by examining astronauts urine before, during,
and after space flight. Healthy control subjects rarely shed CMV in urine.
Our focus shifted to Varicella-Zoster virus (VZV), the causative agent
of chickenpox and shingles, since this virus also represents an infectious
disease risk among astronauts. Asymptomatic VZV reactivation and shedding
in saliva of astronauts was demonstrated (Mehta et al, 2004) and subsequently
the virus present in saliva was found to be live and infectious.
The Surface, Water, and Air Biocharacterization (S.W.A.B.) Project
The presence of human in space, whether permanent or transient,
is accompanied by the presence of microorganisms. Currently, microbial
analysis only identifies microorganism that will grow in culture,
omitting greater than 90% of all microorganisms due to the limitations
of traditional cultivation techniques. The SWAB study will use modern
molecular techniques that are culture-independent to identify microorganisms.
One of the goals of the SWAB experiment is to use advanced technologies
to better understand the types of microorganisms that the crew could
encounter, their sources, and assess the associated potential risks.
The SWAB sampling will consist of 8 in-flight dockings as well as preflight
sampling of all crew and logistics modules.
However, the risk of latent virus reactivation
is unaffected by such precautions. Virus reactivation could pose an important
health risk for astronauts, as well as for people living and working in other
extreme environments. The most commonly recognized latent viruses in humans
are herpes viruses. There are currently eight known herpes viruses specific
for humans. These viruses cause specific primary diseases (e.g. chickenpox),
but may remain in the body (sensory ganglia, blood cells, epithelial cells)
in a latent or quiescent state for decades. Elements of the immune system maintain
this state of latency. When, however, the immune system is diminished by an
underlying cause such as stress, aging, or cancer chemotherapy, latent viruses
reactivate and proliferate, leading to disease (e.g. shingles). Thus, reactivation
of viruses is a tool to predict changes in the immune status.
