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.