Vaccination

The lack of subsequent wild ape vaccination programs, despite mounting evidence of disease threat, is due in good part to opposition in some quarters of the ape conservation community. One argument against vaccination is that we should not interfere with “natural” phenomena such as disease. This argument clearly does not hold when the diseases that kill wild apes are being introduced by humans. Furthermore, we are intervening in Central Africa in so many negative ways (mechanized logging, mining, fuel-wood cutting, forest conversion to agriculture, poaching) that a little positive intervention seems the only ethical alternative.

A more serious argument against vaccination is the fear that its negative effects will outweigh its positive impact. Potential negative effects include infection or toxicity in vaccinated animals, “spillover” of live virus vaccines into non-target species, and stress to apes during vaccination. Each of these is a legitimate concern. But each can be addressed in a rigorous and scientific way.

Safety Testing

Most of the vaccines that might be used on wild apes have already been tested extensively for safety in trials on captive monkeys. Such trials are easily extended to captive apes. Thousands of apes held in zoos and other captive facilities are now routinely vaccinated against childhood respiratory diseases that are also threats to apes in the wild. In fact, the most common source of childhood respiratory infections, respiratory syncitial virus was first described in chimpanzees, which continue to be the model system of RSV vaccine development.

Non-Target Species

The fear that vaccine administered to one wildlife species will cause virulent infections in other species has been a major roadblock to the use of vaccination as a tool for the control of wildlife disease. There are several ways in which spillover risk can be minimized. First, efforts can be made to limit exposure of non-target species. For instance, vaccine can be delivered with a dart instead of with an oral bait. Second, vaccines can be safety tested on representative non-target species, as was the case for recombinant rabies vaccines. The safety of these vaccines has been confirmed by the fact that hundreds of millions of oral vaccine baits have virtually eradicated rabies from fox populations in Western Europe but resulted in no known rabies infections in humans. Third, one can use vaccines that are not “alive” and, therefore, pose no risk of spillover infection. This includes both classic “killed” vaccines and new engineered subunit vaccines that do not include a full, functioning virus. The disadvantage of these vaccines is that they typically do not produce as robust an immune response as live vaccines. However, this may be to some extent alleviated by recent advances in the use of adjuvants, chemicals that enhance immune response to vaccination.

Stress & Injury

One fear commonly expressed by primatologists is that vaccination will either injure apes or “stress them out”, thereby, making them more susceptible to disease or intolerant to the approach of researchers or tourists. Some of these fears appear to be driven by the mistaken assumption that vaccination would entail anestheticizing and handling apes. This is simply not the case. Vaccine can be delivered without “knocking down” the animal. This not only eliminates the risk of death under anesthesia, it limits stress to a brief window after the animal is struck by a dart. This very brief window of stress seems unlikely to have a major immunosuppressive effect. Negative effects on wild ape tolerance to humans may also be minimized by concealing the shooter, as wild gorillas and chimpanzees appear not to immediately associate the dart with the darter. One of the attractions of oral vaccination is that induces no stress other than, perhaps, that associated with the presentation of a novel item (i.e. neophobia). Furthermore, new darting technologies greatly reduce the potential for puncture wounds or tissue damage at the injection site.

Disease-Threats

Wild apes are now faced with two major disease threats: “naturally” occurring diseases and diseases introduced from humans. The former group includes both diseases that circulate persistently within ape populations and those that “spillover” from other wildlife reservoir hosts.

These diseases are of growing concern because threats such as poaching and habitat loss increasingly fragment apes into remnant populations that are too small to absorb the impact of disease. Magnifying the disease threat is the fact that both rising human populations on the edges of parks and the habituation of apes to human presence for tourism and research increase the rate at which wild apes are exposed to human diseases.

Ebola Virus

Currently the most worrying non-human disease threat is the Ebola virus. Over the last two decades gorilla and chimpanzee populations in Gabon and Republic of Congo have seen massive outbreaks of the Zaire strain of Ebola. Outbreaks have occurred in about half of the range of the western gorilla (Gorilla gorilla), by far the more abundant of the two currently recognized gorilla species.

About half of the National Parks that hold large gorilla populations have experienced serious crashes in gorilla numbers. Within each Park more than half of the habitat shows signs of Ebola outbreak, with gorilla mortality rates at about 95% in the local neighborhoods showing signs of outbreak. Years of investment in both tourism and research have also been lost when gorillas viewed by tourists and researchers were wiped out by Ebola. In all roughly one third of the world gorilla population and large numbers of chimpanzees have died. Projections of the spatial spread of Ebola outbreaks suggest that the death toll may rise to one half of the world gorilla population in the next decade.

Although confirmed outbreaks of the Zaire strain of Ebola have been confined to Central Africa (Gabon, Republic of Congo, & Democratic Republic of Congo), human outbreaks of the Sudan and newly identified Budibungyo strains of Ebola as well as the closely related Marburg virus have occurred in or near gorilla and chimpanzee ranges in East Africa.

Human Respiratory Viruses

Wild apes are at risk from a wide range of human diseases, including everything from skin disease to intestinal parasites transmitted through contact with human feces. However, the most worrying class of human pathogens are respiratory viruses. During their evolutionary history, other apes have come in close enough contact with humans too rarely for transmission of human respiratory viruses to be a major problem.

Consequently, they have not evolved resistance to human respiratory pathogens. Now the habituation of wild apes to human presence for the purpose of research and tourism has suddenly brought humans into close enough proximity with wild apes. The potential impact of human respiratory pathogens on wild apes was already recognized twenty years ago, when a suspected measles outbreak killed mountain gorillas. However, it was only last year that diagnostic tests unambiguously identified two common human respiratory viruses, respiratory syncitial virus and meta-pneumovirus, as causing high morbidity and high mortality outbreaks in habituated chimpanzees.

Retrospective clinical and demographic analyses also suggest that similar human respiratory pathogens have occurred in many populations of habituated gorilla and chimpanzees, with several populations experiencing major declines as a consequence. These observations are particularly worrying in that habituation of apes for tourism is on the rise, with sites that already have ape tourism increasing the number of habituated ape social groups and many new sites coming online. Thus, we are faced with the perverse situation in which the very actions that are intended to protect apes in the wild are killing them.

Future Activities

Over the last two decades the Ebola virus has killed about one third of the world gorilla population and large numbers of chimpanzees. If this continues, the death toll may rise to one half of the world gorilla population. Human respiratory viruses are also a growing threat to wild apes. However, this need not happen. There now are at least six different vaccines that have successfully protected laboratory monkeys against the Ebola challenge and none have shown serious side effects. This is the rare case in which vaccine testing on non-human primates could actually help the conservation of critically endangered non-human primates, as the Ebola vaccines intended for human use would likely protect gorillas and chimpanzees.

VaccinApe

Since 2003, We have been agitating for one of the Ebola vaccines in development for humans to be used to protect wild apes. These efforts have gradually evolved into VaccinApe, a loose consortium of individuals and institutions. Because of concerns about the safety of vaccinating wild apes we have gone to great lengths to solicit input from a wide range of primatologists, virologists, vaccine labs, and conservationists, including large expert workshops in 2004 and 2008. The consensus of the experts is that if the process is approached in a rigorous, scientific manner, wild ape vaccination would be safe, effective, and affordable. They also recommend that, for reasons of safety, technical complexity, and cost, a darted vaccine should first be used to vaccinate the relatively small number of gorillas and chimpanzees in research and tourism programs. However, in the long term the most effective way to protect larger numbers of wild apes against Ebola and other disease threats such as human respiratory viruses would be vaccines delivered in oral baits.

To meet this high standard of safety and scientific rigor, VaccinApe is now embarked on an incremental program that will lead eventually to the vaccination of wild apes against the Ebola virus. The first step of the process is to test a vaccine on captive chimpanzees. This testing will not involve exposure of chimpanzees to Ebola virus, but simply evaluate whether: 1) the vaccine produces negative side-effects or 2), the vaccine evokes serum antibodies correlated with immunity to Ebola, and 3) antibody correlates of immunity can also be assayed from non-invasively sampled dung. The vaccine used in these trials has been chosen specifically because it has low potential for serious side-effects and no potential for spread between animals (the vaccine is not “alive”). In parallel, a pilot measles vaccination study will be used to validate that wild gorillas can be safely darted and that non-invasively collected dung sampling can be used to assay for immune response to vaccination. Measles vaccine has been chosen both because of its high safety level (it has been safely given to hundreds of millions of human children) and because measles is a known threat to apes in tourist programs (it is the suspected cause of death in an outbreak that killed numerous mountain gorillas).

Results from these captive and field studies will be included, with extensive results from the human vaccine development program, in an application for veterinary licensing of the Ebola vaccine by the US Department of Agriculture. Once this license has been obtained, a field pilot study will then be used to deliver the vaccine using a hypodermic dart to wild gorillas habituated to human presence. After the pilot study is completed and vaccination success is evaluated, the next step will be a larger program in which gorillas and chimpanzees in multiple research and tourism programs are dart vaccinated. In parallel to these darted vaccination programs, research on oral vaccine delivery will be conducted, with the long term objective of vaccinating large numbers of gorillas and chimpanzees against Ebola and other pathogens; particularly human respiratory viruses transmitted to habituated apes in tourism and research programs. A working group at the National Center for Ecological Analysis and Synthesis is also examining efficient strategies for vaccinating apes and comparing the cost-effectiveness of Ebola vaccination to that of other ape conservation strategies.