Plan It for the Apes: Sound Science Must Inform Any Plans to Vaccinate Gorillas or Chimps Against Ebola

Vaccinating wildlife is risky business. With vaccines to prevent Ebola virus disease (EVD) in people becoming available for the first time, the wildlife conservation community faces the obvious question regarding whether there is also a prospect for protecting great apes from this scourge. It’s incumbent upon wildlife health professionals to be sure of the safety and actual efficacy of any vaccine we might ultimately want to deploy to try to protect endangered wildlife. If something goes wrong (including if the wild animal subjects in question subsequently and coincidentally die for totally unrelated reasons post-vaccination), the impacts on governments’ willingness to allow conservationists to help manage wildlife when needed can be long lasting. As for much of what we do as wildlife conservationists, sound cost / benefit analysis needs to be part of any proposed vaccination strategies.

There is clearly much work to be done to gain a better understanding of Ebola’s mysterious life cycle, information that is required to inform any potential intervention strategies conservationists might wish to attempt at a large scale. We cannot skip steps or completely disregard important knowledge gaps when developing potential interventions we hope will be sustainable. That is not to say we “let the perfect be the enemy of the good.” We cannot necessarily have every puzzle piece in place before developing a plan. But we owe it to our closest living relatives to abide by basic precautionary standards, to use sound scientific principles, and to utilize genuine peer review before putting any captive or wild great apes (or any non-target species) at risk.

Ebola virus disease, caused by members of the genus Ebolavirus, has of course been on the world stage given its tragic human toll in West Africa. For years, Zaïre ebolavirus (EBOV) has also posed a very real and present danger to great apes in Central Africa. With vaccines to prevent Ebola in people becoming available for the first time, the wildlife conservation community is faced with determining whether there is also a prospect for protecting great apes from the deadly virus. The elevated importance of EVD to great ape conservation in Africa follows numerous EVD outbreaks from 1994 to 2005 linked to unprecedented western lowland gorilla and chimpanzee population declines. The discovery of Taï Forest ebolavirus in 1994 involved the disappearance or death of 12 chimpanzees from a known community of 43 living within Taï National Park, Côte d’Ivoire. In late 1994, gorilla and chimpanzee deaths associated with human EVD outbreaks were reported in a region of mining camps in northeastern Gabon. Researchers later documented a ‘catastrophic decline’ of gorillas and chimpanzees living in Minkébé Forest, Gabon, between 1994 and 2000 and suggested EVD was a significant contributor. In 2003, over 90% gorilla mortality was observed in two study groups with known individuals in Lossi Gorilla Sanctuary, Republic of Congo. The last confirmed EVD-associated great ape deaths, in June 2005, were two gorillas found dead in Odzala-Kokoua National Park, roughly 70 km northeast of Lossi Gorilla Sanctuary. There, transect-based population surveys found a statistically significant drop in gorilla numbers within the park, from 41,000 in 2005 to 22,000 in 2012, mostly within the park’s interior, suggesting that habitat loss and poaching were not factors.

First, do no harm

Now that human trials of several Ebola vaccines have proven extremely encouraging, we believe the conservation community has the responsibility to establish consensus-based guiding principles for potential Ebola vaccine trials and eventual vaccine deployment in free-ranging great apes (gorillas and chimpanzees) in Africa. Any vaccine-based intervention simply must be based on sound science and an awareness of legal and political realities at local and international scales. Perhaps most importantly, the safety of both the target as well as non-target species, including people, must be taken into consideration. The precautionary principle “First, do no harm” is a foundation of both human and veterinary medicine. Despite that, best practices to ensure the safety of experimental vaccine field trials and eventual potential deployment in free-ranging great apes have yet to be established. The poor state of knowledge of Ebola virus ecology makes such a cautious approach both imperative and challenging. High-stakes decisions about interventions may well need to be made based on less than perfect knowledge, but it is critical that we in the conservation science community, in our efforts to protect Africa’s great apes from this potentially devastating disease, first do no harm.

What do we currently know about the types of vaccines being proposed for use in wild great apes? Many of the proposed vaccines have shown some degree of efficacy in protecting animals (usually mice, sometimes laboratory macaques) against subsequent challenge with Ebola virus. The different vaccine approaches currently of most focus are all recombinant, meaning that Ebola proteins or glycoproteins are attached to another unrelated virus ‘platform.’ Such recombinant vaccines do not contain actual Ebola virus, so are unable to cause Ebola Virus Disease.

• All of the candidate vaccines are self-replicating, meaning they are live and continue to replicate in the host (vaccine recipient) for a time, thus continuing to stimulate the host’s immune system to produce antibodies against Ebola. By doing so, they should ideally avoid the need for a booster dose.
• Some are also self-disseminating, meaning that the virus is shed and is transmitted from individual to individual. This self-disseminating vaccine approach seems to be gaining traction as a potential means of vaccinating large numbers of free-ranging great apes as an alternative to reliance on injectable or orally (bait)-administered vaccines. There are legitimate biosafety and environmental concerns when it comes to self-disseminating vaccines, concerns which merit additional scrutiny. Before a genetically modified organism that can spread from animal to animal is released into the environment, we need to be as sure as we can be that it will not have adverse consequences in target (i.e. gorillas and chimps) or non-target species (i.e. other wildlife or people).
• Even if a vaccine has been demonstrated as safe and efficacious in people, further testing would be required to confirm the vaccine performs similarly, for a meaningful period of time, in gorillas or chimps. Monitoring vaccine-generated immunity, as through non-invasive fecal antibody assays we’ve helped develop, would be a critical part of vaccine testing prior to any use in wild apes. A vaccine that does not generated lasting immunity (meaning it would require periodic booster dosing) would be unlikely to be practical or cost-effective over the long-term.
• The modified live virus platforms used to carry Ebola antigens in candidate vaccines have not been well characterized or studied in target great ape populations, so we currently lack information about potential illness that could be inadvertently caused by a given vaccine platform virus.
• Actually deploying a “good” vaccine, once we have proven we have one, remains a significant practical challenge in terms of free-ranging great apes. The most likely pilot efforts would only be able to be undertaken in habituated animals, a tiny portion of Africa’s gorillas and chimps – those which have been habituated for tourism or research and could thus be approached closely enough to be darted with a vaccine (one which would ideally have been proven to induce protective immunity with just one dose). Distributing baits laden with vaccine (the way we vaccinate skunks and raccoons, for example, against rabies in North America and Europe) would be extremely challenging in the dense forests of the Congo Basin. And even if such baits could be widely distributed, making sure that apes are the animals that consume them is a separate, equally tricky issue. Furthermore, given how incomplete our understanding of Ebola epidemiology is, we would need to ask ourselves a priori if there is perhaps another species involved in the maintenance or spread of the virus (such as a bat) that would potentially be a more practical and effective target for vaccination.
• The “should we be vaccinating wild great apes at all” question is an important philosophical and ethical one, given that great apes and Ebolavirus have evolved in African forests over the millennia. Should we intervene when a pathogen is a natural part of an ecosystem’s biodiversity? Perhaps intervention is justifiable if the scales have been tipped against the animals susceptible to the virus, as is happening in Central Africa due to pressures from deforestation and hunting? These are issues worth discussing and debating.

Given the multiple threats such as habitat loss and hunting facing the declining populations of our gorilla and chimpanzee cousins, we believe the threat of diseases like Ebola merit our ongoing attention. In considering disease threats, however, proposed interventions must be based on sound biomedical and ecological science – as well as on pragmatic assessments of the legal and political realities that come into play when one talks about releasing a genetically modified organism (GMO- which, again, is what most Ebola vaccines are) into the wilderness of sovereign nations. The global community has developed international treaties focused on biosafety to address these very real types of concerns, and any proposed uses of modified live vaccines in free-ranging wildlife need to be thoroughly vetted accordingly. First, do no harm.

Steven A. Osofsky, Sarah H. Olson and Kenneth Cameron

The authors are members of the Wildlife Conservation Society’s Wildlife Health & Health Policy Program. We are planning on holding an international forum on these issues later in the year, as part of our ongoing "Vaccines for Conservation" working meeting series.