Bat virome

The bat virome is the group of viruses associated with bats. Bats host a diverse array of viruses, including all seven types described by the Baltimore classification system: (I) double-stranded DNA viruses; (II) single-stranded DNA viruses; (III) double-stranded RNA viruses; (IV) positive-sense RNA viruses; (V) negative-sense RNA viruses; (VI) positive-sense RNA viruses that replicate through a DNA intermediate; and (VII) double-stranded DNA viruses that replicate through a single-stranded RNA intermediate. The greatest share of bat-associated viruses identified as of 2020 are of type IV, in the family Coronaviridae.

Bats harbor many viruses and a few of them are zoonotic, or capable of infecting humans and causing disease, thus, are considered important emerging viruses. These zoonotic viruses include the rabies virus, SARS-CoV, MERS-CoV, Marburg virus, Nipah virus, and Hendra virus. While research strongly indicates that SARS-CoV-2 originated in bats, it is unknown how it was transmitted to humans, or if an intermediate host was involved. It has been speculated that bats may have a role in the ecology of Ebola virus, though this is unconfirmed. A closely-related ebolavirus, Bombali ebolavirus, has been detected in insectivorous bats. While transmission of rabies from bats to humans usually occurs from bites or scratches, most other zoonotic bat viruses are transmitted by direct contact with infected bat fluids like urine, guano, or saliva, or through contact with an infected, non-bat intermediate host. There is no firm evidence that butchering or consuming bat meat can lead to viral transmission, though this has been speculated because such activity causes aerosols that may facilitate respiratory transmission.

Despite the abundance of viruses associated with bats, they rarely become ill from viral infections, and rabies and Tacaribe viruses are the only viruses known to kill bats. A limited amount of research has been conducted on bat virology, particularly bat immune response. Bat immune systems are mostly similar to other mammals; however they differ in their lack of several inflammasomes, which activate the body's inflammatory response, as well as a dampened stimulator of interferon genes (STING) response, which helps control host response to pathogens. Some research indicates that bats have limited affinity maturation in their antibody responses to infection, whereas others show they make high-titer and durable antibody responses. Collectively, studies suggest that bats may be more tolerant of viruses than other mammals, although the basis of this tolerance is unknown. Suppression of bat immune responses can lead to disease from viral infection, suggesting that immune tolerance is not a mechanism of tolerance to viruses.

While much research has centered on bats as a source of zoonotic viruses, reviews have found mixed results on whether bats harbor more zoonotic viruses than other groups. A 2015 review found that bats do not harbor more zoonotic viruses than primates or rodents, though the three groups harbored more than other mammal orders. In contrast, a 2020 review found that bats do not have more zoonotic viruses than any other bird or mammal group when viral diversity is measured relative to host diversity, as bats are the second-most diverse order of mammals.