Electron micrograph of an Ebola virus virion
A genetics research team led by
Pardis Sabeti of Harvard University has published a paper in the journal Science,
suggesting that some infectious diseases that are thought to be relatively new,
may in fact have been around for thousands of years.
They
claim they have genetic evidence that shows that some diseases such as Ebola
and Lassa, which some believe first began infecting people in just the past
half century, likely have been around for hundreds and in some cases thousands
of years.
Recent field evidence has
suggested that some people that live in areas where such diseases are common
show rates of exposure to Ebola and Lassa that are high enough to indicate that
some have developed an immunity to them. Last year, for example, blood tests
showed that up to 55 percent of the population in a community in Guinea had
been exposed to Lassa and 22 percent had been exposed to Ebola. Generally it
takes many generations to build up such a level of immunity.
Additionally, in studying the
genetic structure of Ebola and Lassa, the researchers found that Lassa appears
to have diverged from a family of hemorrhagic diseases approximately 500 hundred
years ago. Ebola appears to go back even further, having diverged from the Marburg virus approximately
10,000 years ago.
In studying field reports
regarding Ebola and Lassa the researchers also found that both can show
symptoms that differ from those that have been commonly associated with them,
namely mucosal and internal bleeding. Sometimes, they found, victims with the diseases
show no signs of bleeding – instead they have a fever, cough or sore throat. In
such situations, it's likely others believed they had contracted another
disease. Such cases they say, might explain why so many researchers have come
to conclude that both are among those classified as emerging infectious
diseases. Instead they suggest, it's more of an emerging diagnosis trend.
The team suggests that if Lassa
and Ebola are diseases that
are newly diagnosed, rather than emerging, it makes sense that others likely
are as well. And if that is the case, then research that focuses on which
populations are harboring them might turn up data to better predict when an
outbreak might occur.
More information: Emerging Disease or Diagnosis? Science,
9 November 2012: Vol. 338 no. 6108 pp. 750-752 DOI: 10.1126/science.1225893
Abstract
Outbreaks this year of the deadly and highly contagious Ebola and Marburg viruses in the Democratic Republic of Congo and Uganda and Lassa virus in Nigeria raised concerns about possible epidemic spread of these hemorrhagic fevers. These pathogens seemed to appear out of nowhere around the middle of the 20th century: Marburg virus in 1967, Lassa virus in 1969, and Ebola virus in 1976.
By the early 1990s, public health
concerns were crystallized in a landmark report (1) that was the first to
popularize the concept of "emerging pathogens". But could
"emerging diagnosis" explain the rise in appearance of hemorrhagic
fevers caused by these pathogens? Recent epidemiologic and genetic studies of
Lassa and Ebola fevers suggest that these diseases may have widespread prevalence
and ancient origins. They raise the possibility that some viral infections may
reflect "emerging diagnoses" of diseases that are circulating more
widely than thought, with an emerging character primarily a matter of improved
detection of the culprit pathogens.
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