Re: Large numbers of people in Gabon have antibodies to Ebola
And a ProMED post. If people are immune to Zaire ebolavirus in places where no outbreak has ever been reported, it seems obvious (IMO) that something other than that virus is giving people the immunity.
http://www.promedmail.org/pls/otn/f..._BACK_PAGE,F2400_P1001_PUB_MAIL_ID:1000,81339
Archive Number 20100212.0496
Published Date 12-FEB-2010
Subject PRO/AH/EDR> Zaire ebolavirus - Gabon: seroprevalence
ZAIRE EBOLAVIRUS - GABON: SEROPREVALENCE
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A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>
Date: Tue 9 Feb 2010
Source: PLoS (Public Library of Science) ONE [edited]
<http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0009126>
[Reference: Becquart P, Wauquier N, Mahlakoiv T, et al: High
prevalence of both humoral and cellular immunity to Zaire ebolavirus
among rural populations in Gabon. PLoS ONE 5(2): e9126.
doi:10.1371/journal.pone.0009126]
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[The authors are located at the Emerging Viral Diseases Unit,
International Centre of Medical Research, Franceville, Gabon; UMR190
Emergence of Viral Pathologies, Aix-Marseille II University &
Research Institute for Development, Marseille, France; and Mahidol
University at Salaya, Nakhonpathon, Thailand]
Abstract
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To better understand Zaire ebolavirus circulation and transmission to
humans, we conducted a large serological survey of rural populations
in Gabon, a country characterized by both epidemic and non epidemic
regions. The survey lasted 3 years and covered 4349 individuals from
220 randomly selected villages, representing 10.7 percent of all
villages in Gabon. Using a sensitive and specific ELISA method, we
found a Zaire ebolavirus-specific IgG seroprevalence of 15.3 percent
overall, the highest ever reported. The seroprevalence rate was
significantly higher in forested areas (19.4 percent) than in other
ecosystems, namely grassland (12.4 percent), savannah (10.5 percent),
and lakeland (2.7 percent). No other risk factors for seropositivity
were found. The specificity of anti-Zaire ebolavirus IgG was
confirmed by Western blot in 138 individuals, and CD8 T cells from 7
IgG+ individuals were shown to produce IFN-gamma after Zaire
ebolavirus stimulation.
Together, these findings show that a large fraction of the human
population living in forested areas of Gabon has both humoral and
cellular immunity to Zaire ebolavirus. In the absence of identified
risk factors, the high prevalence of "immune" persons suggests a
common source of human exposure such as fruits contaminated by bat
saliva. These findings provide significant new insights into Zaire
ebolavirus circulation and human exposure, and raise important
questions as to the human pathogenicity of Zaire ebolavirus and the
existence of natural protective immunization.
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
[The following extract from this paper provides the background to
this study and puts the data in perspective. Interested readers
should consult the full text and references via the URL at top of this report.
"Zaire ebolavirus is the most pathogenic ebolavirus species, with
reported case fatality rates of up to 90 percent. Zaire ebolavirus
has caused several outbreaks in Central Africa, Democratic Republic
of Congo (DRC), Republic of Congo (RC), and Gabon. North-east Gabon
experienced 4 outbreaks between 1994 and 2002, with a total of 259
confirmed human cases and only 79 survivors (case fatality rate: 69 percent)."
"Recently, significant advances have been made in our understanding
of filovirus ecology. Antibodies and nucleotide sequences specific
for Zaire ebolavirus have been detected in the liver and spleen of 3
fruit bat species in Gabon and RC (_Hypsignathus monstrosus_,
_Epomops franquetti_, and _Myonycteris torquata_), and antibodies and
nucleotide sequences specific for Marburg virus have been found in a
fruit bat species in Gabon (_Rousettus aegyptiacus_) and in 2
insectivorous bat species in DRC (_Rhinolophus eloquens_ and
_Miniopterus inflatus_). More recently, this virus was isolated for
the 1st time in cave-dwelling _Rousettus aegyptiacus_ in Uganda.
Together, these findings raise the possibility that these bats might
be a filovirus reservoir, but the mechanism of primary Zaire
ebolavirus transmission to humans, potentially leading to outbreaks,
remains unclear. Ebola hemorrhagic fever outbreaks that occurred in
Gabon and RC between 2001 and 2003 were also associated with major
outbreaks among wild-living large mammals (especially chimpanzees and
gorillas), devastating local animal populations. The primary human
cases involved hunters who became infected after handling animal
carcasses found in the forest. Similarly, the 1996 Mayibout outbreak
in Gabon started among children who handled a chimpanzee carcass. A
recent study showed that the 2007 Luebo outbreak in DRC was linked to
massive fruit bat migration, strongly suggesting for the 1st time
that humans could be infected directly by bats."
"It is generally accepted that Zaire ebolavirus is associated with a
case fatality rate of about 90 percent, but this may be an
overestimate. For example, 7 cases of asymptomatic infection were
identified during the 1996 Booue outbreak in Gabon, and some
ELISA-based serosurveys have shown
high antibody prevalence rates
among populations living in areas where no case of Ebola hemorrhagic
fever has ever been reported, suggesting that Zaire ebolavirus might
also be capable of causing mild illness or even asymptomatic
infection in humans. The IgG seroprevalence was 9.3 percent in
villages located in the 1995 outbreak area around Kikwit, DRC, where
no Ebola haemorrhagic fever cases were reported.
Likewise, a
seroprevalence of 13.2 percent was found in the Aka Pygmy population
of Central African Republic, where no Zaire ebolavirus outbreaks have
ever been reported."
"The source and significance of anti-Zaire ebolavirus IgG
seropositivity among people who have never had clinical signs of
haemorrhagic fever or who live in non epidemic areas are both
unclear, but they may have major implications for our understanding
of the epidemiology of Zaire ebolavirus primary transmission to
humans and outbreaks."
"This study provides important insights into Zaire ebolavirus
circulation, human exposure and pathogenicity, and outbreak
occurrence. The high frequency of 'immune' individuals with no
disease or outbreak history raises questions as to the real
pathogenicity of Zaire ebolavirus for humans in 'natural' conditions.
Added to the lack of identifiable risk factors, this points to bats
as the main source of human exposure, through handling and ingestion
of contaminated fruits. Past outbreaks in which no animal source was
identified may have been due to fruit bat activity close to the
villages concerned. Rural populations living in forested regions of
the central African forest block thus appear to be highly exposed to
the virus. More and more viruses are being detected in bats, some of
which are known to be pathogenic for humans."
The authors' conclusion that transmission of infection via fruit
contaminated by bat saliva has also been suggested as a route of
disease transmission in the case of Nipah virus infection in Asia
(such as, ProMED-mail post -- Nipah virus, fatal - Bangladesh: (FR)
20100122.0250).
The HealthMap/ProMED-mail interactive map of Gabon can be accessed at
<http://healthmap.org/r/0082>. - Mod.CP]
