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 single-stranded RNA viruses; (V) negative-sense single-stranded RNA viruses; (VI) positive-sense single-stranded 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 several viruses that are zoonotic, or capable of infecting humans, and some bat-borne viruses 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 clearly 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 the Ebola virus, though this is unconfirmed. While transmission of rabies from bats to humans usually occurs via biting, 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.
Despite the abundance of viruses associated with bats, they rarely become ill from viral infections, and rabies is the only viral illness known to kill bats. Much research has been conducted on bat virology, particularly bat immune response. Bats' immune systems differ from other mammals 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. Preliminary evidence indicates bats are thus more tolerant of infection than other mammals. While much research has centered on bats as a source of zoonotic disease, 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.
Viral diversity
Viruses have been found in bat populations around the world. Bats harbor all groups of viruses in the Baltimore classification, representing at least 28 families of viruses. Most of the viruses harbored by bats are RNA viruses, though they are also known to have DNA viruses. Bats are more tolerant of viruses than terrestrial mammals. A single bat can host several different kinds of viruses without becoming ill. Bats have also been shown to be more susceptible to reinfection with the same viruses, whereas other mammals, especially humans, have a greater propensity for developing varying degrees of immunity. Their behavior and life history also make them "exquisitely suitable hosts of viruses and other disease agents", with long lifespans, the ability to enter torpor or hibernate, and their ability to traverse landscapes with daily and seasonal movement.
Though bats harbor diverse viruses, they are rarely lethal to the bat host. Only the rabies virus and a few other lyssaviruses have been confirmed to kill bats. Various factors have been implicated in bats' ability to survive viral infections. One possibility is bats' use of flight. Flight produces a fever-like response, resulting in elevated temperature (up to ) and metabolic rate. Additionally, this fever-like response may help them cope with actual fevers upon getting a viral infection. Some research indicates that bats' immune systems have allowed them to cope with a variety of viruses. A 2018 study found that bats have a dampened STING response compared to other mammals, which could allow them to respond to viral threats without over-responding. STING is a signaling molecule that helps coordinate various host defense genes against pathogens. The authors of the study concluded that "the weakened, but not entirely lost, functionality of STING may have profound impact for bats to maintain the balanced state of 'effective response' but not 'over response' against viruses."