A few years ago, scientists developed a desperately needed Ebola virus vaccine designed for use in wild ape populations residing in areas affected by the disease. Although the vaccine proved to be effective in preventing mice from infection in lab settings, how long the animals remained protected was not determined. Now, building on this previous work, scientists have demonstrated that this vaccine provides long-lasting immunity against Ebola virus and could therefore be a useful agent to curb infection in wild African ape species. The study has been published in Vaccine.
The current Ebola crisis has received a deserved amount of attention, but most of us are probably unaware of the fact that it is also a serious threat to the survival of other great ape species, namely chimpanzees and gorillas. Not only is the virus more deadly in these animals, with mortality rates as high as 95% for gorillas, but it is estimated to have slashed their populations by a third since the 1990s.
Although fruit bats are thought to be natural hosts of the virus, great apes are an important source of Ebola virus transmission to humans due to man hunting them for bush-meat. An effective vaccine would therefore not only benefit chimps and gorillas, but it could also reduce the incidence of future outbreaks in humans. But scientists are faced with a problem: how do we achieve good coverage in wild animals residing in inaccessible populations? Attempting to individually vaccinate a sufficient number of animals is not feasible, which is why researchers decided to fight virus with virus by enlisting the help another pathogen: Cytomegalovirus (CMV).
Not only is this virus capable of provoking a strong immune response, but it can also re-infect and spread, or disseminate, through target populations, regardless of whether they have been infected with CMV before. Armed with this knowledge, scientists from Plymouth University designed a potential vaccine by engineering CMV to express part of the Ebola virus that could be recognized by the immune system.
Next, the team administered a single dose to susceptible mice and investigated whether it could protect the animals from infection, and for how long. They found that the vaccine not only induced high levels of immune cells specific to Ebola virus, but it also protected all animals against a lethal dose of Ebola virus. Furthermore, the immune responses were maintained for more than 14 months, which is the equivalent to half the average life span of mice, and they were protected from infection for four months post-vaccination.
Although these findings are promising, an important limitation of this study is that it did not investigate the level of protection offered by disseminated vaccines since mice were directly injected with the vaccine. Furthermore, it remains unclear as to whether the same long-lasting protection would be provided in chimps and gorillas. Because medical research on great apes is highly restricted, it is not possible to expose vaccinated animals to live virus. However, some valuable insight into its efficacy in primates should be yielded through trials on macaque monkeys, which are currently underway.
