60 / 88
60
Therefore, as in humans, apes likely become infected either by
direct contact with the EBOV reservoir (presumed to include
different bat species)(Leroy
et al.
, 2004; Caillaud
et al.
, 2006),
via the touching of infectious other animals (Caillaud
et al.
,
2006; Walsh
et al.
, 2007) or via contact with bodily fluids of
an infected cohort (Rouquet
et al.
, 2005; Caillaud
et al.
, 2006;
Walsh
et al.
, 2007).
Determining total great ape morbidity and mortality due to
EHF is difficult. Great ape population surveys revealed de-
clines in great ape signs ranging from 95–98 % in Minkebé
National Park (Gabon), Lossi Sanctuary and Lokoué Bai (Re-
public of Congo) between 1994 and 2004. Additionally, Walsh
et al.,
2003) compared ape nest counts and concluded that Ga-
bon’s ape population had decreased by almost 50% (Walsh
et
al.
, 2003) over 2 decades. Considering the density of ape popu-
lations in these regions, and presuming that some epidemics
go unnoticed, it would not be unrealistic to consider that tens
of thousands of great apes may have been lost in recent years.
Based on the calculations, it seems likely, that EBOV is the ma-
jor driver of these losses (Huijbregts
et al.
, 2003; Walsh
et al.
,
2003; Bermejo
et al.
, 2006; Devos
et al.
, 2008). However, the
diagnostic data available for such calculations are scarce and as-
sumptions are mainly based on the fact that great ape declines
could be spatially or temporally linked with the few confirmed
EBOV outbreaks in wildlife and/or humans (Huijbregts
et al.
,
2003; Walsh
et al.
, 2003; Bermejo
et al.
, 2006; Wittmann
et
al.
, 2007; Devos
et al.
, 2008). The World Conservation Union
(IUCN) upgraded the western lowland gorilla (
Gorilla gorilla go-
rilla
) to a “critically endangered” status as a result of this alarm-
ing trend (IUCN, 2008), and lists infectious disease as one of
the top threats to the species. Indeed, while it is reasonable to
imagine EBOV is implicated in observed massive great ape de-
clines, it is obvious that baseline data on background mortality
caused by other pathogens are missing.
EBOV has been confirmed in carcasses of only 16 wild great
apes thus far (Wittmann
et al.
, 2007); a small number given the
thousands of animals presumed to have died from EHF. Pro-
ducing solid biological evidence of EBOV as the cause of great
ape population decreases is extremely challenging. Diagnostic
samples are difficult to acquire, due to the vastness and remote-
ness of the regions in question and the rapid decomposition of
carcasses. Samples that are collected from carcasses are often
of poor quality, making analyses prone to false-negative results
(Rouquet
et al.
, 2005).
Early detection of wildlife mortality events combined with
rapid sampling and diagnostic testing is key for understand-
ing threats to wildlife and needs to be enforced (Gillespie
et al.
,
2008; Gillespies and Chapman, 2008). Strengthening wildlife
disease surveillance systems in great ape range states, with the
involvement of local communities, represents an important
step towards obtaining more data. In addition, improving labo-
ratory capacity and employing field diagnostic techniques also
holds promise for identifying causes of mortality. Future EB-
OV-related research should strive to better understand EBOV
natural ecology and geographical distribution. This informa-
tion, combined with knowledge of infection risk factors and
length of immunity for great apes, may shed clues on which
ape populations are most at risk for future infections and be
used to develop timely, safe and ethically reviewed prophylac-
tic strategies and treatments for the mitigation of ape health
threats. For example, vaccination strategies are recommended
to reduce the infection rates of ape populations when consid-
ered critical for their survival. Several EBOV vaccines have been
developed for human use but identifying the ideal candidates
for wild great apes is challenging. Highly effective oral vaccines
may pose dangers for non-target species and injectable vaccines
pose major logistical challenges when considering the need to
dart vast numbers of elusive great apes. We must ensure that
the initiative is applied in a safe way consistent with the goals
and principles conservation.
Great ape health research must take a broad epidemiological
approach. Recent health studies have identified other patho-
gens as threats to the health of increasingly vulnerable great
ape populations (Leendertz
et al.
, 2006; Köndgen
et al.,
2008),
reminding us to be careful to avoid missing die offs due to a
“new” pathogen while we are hot on the trail of the one we
know best. The future of great ape health must be proactive
rather than reactive.