Emerging infectious diseases frequently originate from animals, a pattern that has been well established through the study of viruses such as influenza, hantaviruses, and HIV. These pathogens often reside in specific animal species, known as reservoir hosts, before spilling over into human populations. Identifying these reservoirs is critical to understanding how viruses transfer to humans and to developing strategies to prevent future outbreaks.

For example, investigations into the Nipah virus outbreak in Malaysia in 1998 traced the pathogen to fruit bats. The outbreak primarily affected pig farmers in western Malaysia, where fruit trees planted above pig enclosures attracted bats. The bats’ saliva-contaminated fruit was consumed by pigs, which subsequently infected farmers. The encroachment of deforestation disrupted bat habitats, pushing them closer to human activity. Measures like separating fruit orchards from pig farms have been implemented to reduce the risk of spillover events.

In contrast, the origins of ebolaviruses remain more elusive despite extensive research over the past five decades. Ebola virus disease has caused numerous outbreaks, predominantly in Central and West Africa, with the Democratic Republic of Congo currently experiencing its second-largest outbreak to date, reporting over 7,400 cases and 3,600 deaths. While bats are often presumed to be the reservoir hosts, definitive evidence confirming this has not been obtained. Unlike Nipah virus, attempts to isolate active Ebola virus from healthy animals have consistently failed.

Ebolaviruses are a group of five viruses that infect humans, including Ebola virus, Bundibugyo virus, Sudan virus, Tai Forest virus, and Reston virus. Of these, Ebola virus and Bundibugyo virus have led to the most severe outbreaks. The term “reservoir host” refers to the species that maintains the virus over time without succumbing to disease, a status that certain animals such as chimpanzees and gorillas cannot fulfill since they are fatally affected by Ebola infection.

First identified in 1976 during an outbreak in northern Zaire (now Congo), Ebola virus reappeared sporadically with cases often linked to contact with dead wildlife. For instance, outbreaks in Gabon during the mid-1990s were associated with reports of deceased chimpanzees, gorillas, and bush pigs. However, these animals are considered victims rather than reservoirs because the virus kills them. The true reservoir remains unidentified, although bats remain the leading but unconfirmed candidate.

Understanding the reservoir host would be instrumental in shaping public health interventions, such as restricting hunting or trade of the implicated species and issuing community warnings, thereby reducing human exposure to the virus. It would also enhance outbreak investigations by enabling scientists to trace infections to their initial source.

Currently, Ebola outbreaks are understood to begin through a spillover event, after which human-to-human transmission sustains the epidemic until either containment or population immunity ends it. Unlike some viruses, such as SARS-CoV-2, which have been documented to spill back from humans into animal populations, no evidence suggests that Ebola virus re-enters its reservoir from human hosts.

Preventing future spillovers hinges on minimizing human disturbance of reservoir hosts and their habitats. When humans refrain from hunting, trading, or encroaching on these species, the virus remains confined to the animal population, posing less risk to people. However, ongoing environmental change and human activity continue to create opportunities for viruses to cross species barriers and cause outbreaks.