Pattern Of Human Ebola Outbreaks Linked To Wildlife And Climate
Pastor Dale Morgan
Pattern Of Human Ebola Outbreaks Linked To Wildlife And Climate*
San Diego CA (SPX) Nov 16, 2006
A visiting biologist at the University of California, San Diego and her
colleagues in Africa and Britain have shown that there are close
linkages between outbreaks of Ebola hemorrhagic fever in human and
wildlife populations, and that climate may influence the spread of the
disease.
The decade-long study, published this month (with a cover date of
January) in the journal Transactions of the Royal Society of Tropical
Medicine and Hygiene, tracked animal disease outbreaks and human
exposure to the Ebola virus in Gabon and adjoining northwestern Republic
of the Congo (RoC). The researchers found that many additional wildlife
and human populations within and outside of known epidemic zones have
been exposed to the virus. When they considered disease outbreaks in all
mammals, not just humans, the spread of Ebola no longer seemed erratic
and inexplicable.
"Some researchers have hypothesized that outbreaks of Ebola are
randomly-spaced periodic outbursts, while others have suggested that
Ebola has spread like a wave surging over the Central African
landscape," said Sally Lahm, a visiting scholar in UCSD's Division of
Biological Sciences and the primary investigator of the study. "Our
results are intermediate between these two views. There is a perceived
pattern to the way the virus spreads, but it is not simply a wave
affecting everything in its path, since apparently healthy mammal
communities thrived in close proximity to Ebola epidemic sites."
Lahm has been a research associate at the Institute for Research in
Tropical Ecology in Makokou, Gabon since 1982. She was conducting
unrelated ecological studies when outbreaks of Ebola virus in humans
prompted her to explore how the disease was affecting animal populations
in the region. Between 1994 and 2003, she collected reports of animal
illness and deaths from wildlife survey teams, villagers, hunters,
fishers, loggers, miners, Ebola survivors and families of victims from
across Gabon and into northwestern RoC.
Despite the low probability of finding dead animals in the humid forests
that cover most of the region, due to the scavenging by animals and
insects and rapid decomposition, Lahm received and verified reports of
397 dead animals. The carcasses, which were found at 35 different sites
in Gabon and RoC, included gorillas, chimpanzees, mandrills, bush pigs,
porcupines and four species of antelope. Tests on 14 samples from the
decomposed carcasses did not detect the Ebola virus, but at 12 sites,
observers also saw sick or dying animals with symptoms consistent with
Ebola infection. In addition, 16 reported wildlife mortality incidents
coincided with known Ebola epidemics.
"The transmission of Ebola within animal populations is much more
widespread than previously believed," explained Lahm. "Ebola appears to
spread both within species and between different species of animals."
To determine the extent of human exposure to Ebola within Gabon, Lahm
collaborated with Maryvonne Kombila, the director of the Department of
Tropical Medicine and Parasitology at the University of Health Sciences
in Libreville, Gabon and with Robert Swanepoel, the director of the
Special Pathogens Branch of the National Institute of Communicable
Diseases in Sandringham, South Africa. Swanepoel tested for antibodies
to the Ebola virus in more than one-thousand human blood samples that
had been collected by Kombila and her colleagues for other research in
Gabon between 1981 and 1997.
Fourteen of the blood samples tested positive for antibodies to Ebola.
Some people had been exposed at least three years before the first known
Ebola outbreak in Gabon, while others lived in regions where no known
epidemics had occurred. In 2003, Lahm was able to track down six of the
people whose blood samples indicated that they had been exposed to the
Ebola virus. Life history interviews revealed that some of the
antibody-positive people had never visited a region where known Ebola
outbreaks occurred in humans. Therefore people have been exposed to the
Ebola virus where it has not been recognized.
Based on their findings, the researchers were able to identify
relationships among previously documented Ebola outbreaks in humans and
wildlife in Gabon and RoC that initially seemed disparate and unrelated.
They proposed that the virus first spread southwest across Gabon. It
then looped back toward the northeast from sites in western or central
Gabon and caused the most recent outbreaks in RoC.
"If the spread of the Ebola virus follows its current northeastward
path, the next outbreak would be expected to occur in northern Republic
of the Congo towards Cameroon and the Central African Republic,"
predicted Lahm.
However, according to the findings, the spread of Ebola also depends on
climate factors. Illness and deaths among animals were most prevalent
during periods of prolonged drought-like conditions in the rainforest,
which indicates that severe environmental stress may facilitate disease
transmission.
In the study, the researchers urge that public education is needed to
decrease human contact with potentially infected wildlife by
discouraging people from scavenging dead animals and by promoting safe
hunting and trapping practices, especially because the results show that
outbreaks in wildlife populations have been much more frequent than
previously believed. They emphasize that monitoring wildlife in
collaboration with rural African residents could provide information
essential for protecting public health as well as comprehending the
ecology of the disease.
Lahm points out that there remain many unanswered questions about Ebola
including how the virus spreads within and between mammal species.
"Our study provides more pieces of the puzzle, but at the same time it
is enlarging the puzzle," she noted.
Richard Barnes from the Environmental Sciences Research Center at Anglia
Ruskin University, Cambridge, England, who is currently a visiting
scholar in UCSD's Division of Biological Sciences, also contributed to
the study.