At its peak in 2014, Ebola was wreaking havoc in Africa, derailing healthcare systems, economies, and lives, ultimately placing several countries within the continent into a state of emergency. In addition to the recorded 11,312 lives the virus claimed, Ebola also saw to the death of a third of the world’s gorillas and chimpanzees. The Ebola virus, or EBOV for short, travels species to species, so it is crucial to stop the spread in wild animals in order to prevent future infection in humans.
Typically, vaccines given to animals are delivered through hypodermic darts. However, this method of delivery is highly inefficient, since primates fear humans and are difficult to target in dense forest settings. Thus, both scientists and conservationists are looking towards oral vaccinations as the key of eradicating infectious diseases in primate populations. The concept of oral vaccinations isn’t new. Edible vaccines incorporated into chicken heads were previously used in 1983 to eliminate fox rabies, ultimately reducing the number of wild rabid foxes by 75%.
The study was led by Peter Walsh at the University of Louisiana in Lafayette’s New Iberia Research Center, where Walsh and his team orally administered the vaccine to six of the chimps and vaccinated the other four chimps via intravenous injection. To create the vaccine, the inactive rabies virus (RABV) was vectored against EBOV in order to allow EBOV to pass easily into the bloodstream. Blood samples were drawn on over a period of four weeks in order to determine the amount of circulating EBOV antibodies. Results revealed that the oral vaccine was just as effective as the injection, in terms of antibody count in the blood after the fourth week. However, due to ethical restrictions, Walsh and his team were prevented from exposing the vaccinated chimpanzees to Ebola to test the effectiveness of the vaccine. Creating a successful vaccination campaign also limits the implementation of the oral vaccine. For example, primates are usually wary about baits. Additionally, other animals may consume the edible vaccine first.
Image source: CDC Public Health Image Library
The team also analyzed the effects of stress on the chimpanzees, as animal welfare advocates claimed that the research was torturous, due to the invasive procedures and isolation of the chimpanzees. In an effort to dispute this claim, Walsh and his team analyzed both acute and chronic stressors. Alkaline phosphatase is an enzyme that responds quickly to acute stress, such as invasive blood assays. The concentration of alkaline phosphatase peaked at the beginning of the experiment when the chimpanzees were initially sedated and analyzed, but decayed exponentially by the end of the trial. Chronic stressors, such as social isolation, are correlated with higher serum glucose levels, which peaked during the second week and then returned to a baseline by the end of the study. Ultimately, homeostatic mechanisms were able to regulate the chimpanzees’ stress.
According to Walsh, “a minority of chimps behave like young children when they get vaccinated, screaming and carrying on. This causes a stress level comparable to college students studying for exams. Otherwise, the major downside is boredom.” However, it is suggested that experimental processes be modified to diminish stress levels all together.
Source: Walsh, P. D., Kurup, D., Hasselschwert, D. L., Wirblich, C., Goetzmann, J. E., & Schnell, M. J. (2017). The Final (Oral Ebola) Vaccine Trial on Captive Chimpanzees. Nature, 7(43339).