Scientists speculate that the virus was then either transmitted directly to humans from bats, or passed through an intermediate host species, with SARS-like viruses isolated from Himalayan palm civets found in a live-animal market in Guangdong. The virus infection was also detected in other animals (including a raccoon dog, Nyctereutes procyonoides) and in humans working at the market.
The MERS coronavirus is a betacoronavirus that was first reported in Saudi Arabia in 2012. It turned out to be far more deadly than either SARS or the Wuhan virus (at least as far as current estimates of the new coronavirus’s behavior). The MERS genotype was found to be closely related to MERS-like viruses in bats in Saudi Arabia, Africa, Europe, and Asia. Studies done on the cell receptor for MERS showed an apparently conserved viral receptor in both bats and humans. And an identical strain of MERS was found in bats in a nearby cave and near the workplace of the first known human patient.
Wikimedia Commons/Mickey Samuni-Blank
Baby Egyptian fruit bat (Rousettus aegyptiacus), known carrier species of deadly Marburg virus.
However, in many of the other locations of the outbreak in the Middle East, there appeared to be limited contact between bats and humans, so scientists looked for another vector species, perhaps one that was acting as an intermediate. A high seroprevalence of MERS-CoV or a closely related virus was found in camels across the Arabian Peninsula and parts of eastern and northern Africa, while tests for MERS antibodies were negative in the most-likely other species of livestock or pet animals, including chickens, cows, goats, horses, and sheep.
In addition, the MERS-related CoV carried by camels was genetically highly similar to that detected in humans, as demonstrated in one particular outbreak on a farm in Qatar where the genetic sequences of MERS-CoV in the nasal swabs from 3 of 14 seropositive camels were similar to those of 2 human cases on the same farm. Similar genomic results were found in MERS-CoV from nasal swabs from camels in Saudi Arabia.
Other mixing-vessel zoonoses
HIV, the viral cause of AIDS, provides an almost-textbook origin story of the rise of a zoonotic supervillain. The virus was genetically traced to have a chimpanzee-to-human origin, but it was found to be more complicated than that. The virus first emerged in the 1920s in Africa in what is now the Democratic Republic of the Congo, well before its rise to a global pandemic in the 1980s.
Researchers believe the chimpanzee virus is a hybrid of the simian immunodeficiency viruses (SIVs) naturally infecting two different monkey species: the red-capped mangabey (Cercocebus torquatus) and the greater spot-nosed monkey (Cercopithecus nictitans). Chimpanzees kill and eat monkeys, which is likely how they acquired the monkey viruses. The viruses hybridized in a chimpanzee; the hybrid virus then spread through the chimpanzee population and was later transmitted to humans who captured and slaughtered chimps for meat (becoming exposed to their blood). This was the most likely origin of HIV-1.
HIV-1 also shows one of the major risks of zoonotic infections. They can continue to mutate in its human host, increasing the risk of greater virulence, but also interfering with the production of a universally effective vaccine. Since its transmission to humans, for example, many subtypes of the HIV-1 strain have developed, with genetic differences even in the same subtypes found to be up to 20%.
Colorized transmission electron micrograph (TEM) revealing some of the ultrastructural morphology displayed by an Ebola virus virion.
CDC/Frederick A. Murphy
Ebolavirus, first detected in 1976, is another case of bats being the potential culprit. Genetic analysis has shown that African fruit bats are likely involved in the spread of the virus and may be its reservoir host. Further evidence of this was found in the most recent human-infecting Bombali variant of the virus, which was identified in samples from bats collected from Sierra Leone.
It was also found that pigs can also become infected with Zaire ebolavirus, leading to the fear that pigs could serve as a mixing vessel for it and other filoviruses. Pigs have their own forms of Ebola-like disease viruses, which are not currently transmissible to humans, but could provide a potential mixing-vessel reservoir.