Bat-borne virusWikipedia open wikipedia design.
A bat-borne virus is any virus whose primary reservoir is any species of bat. The viruses include coronaviruses such as severe acute respiratory syndrome coronavirus (SARS-CoV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); hantaviruses; lyssaviruses such as rabies virus and Australian bat lyssavirus; henipaviruses such as nipah virus and Hendra virus; Lassa virus; Ebola virus; and Marburg virus. Several bat-borne viruses are considered important emerging viruses.
Bats are especially tolerant of viruses compared to terrestrial mammals. A single bat can host several different kinds of viruses without becoming ill, though some like the rabies virus do cause illness in bats. Most of the viruses harbored by bats are RNA viruses, though they are also known to have DNA viruses. 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. Additionally, bats lack several inflammasomes found in other mammals. While inflammation is an immune response to viruses, excessive inflammation is damaging to the body, and viruses like severe acute respiratory syndrome coronavirus (SARS-CoV) are known to kill humans by inducing excessive inflammation. The authors stated, "The nature of 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."
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.
Bats and their viruses may be the subject of more research than viruses found in other mammal orders, an example of research bias. A 2015 review found that from 1999 to 2013, there were 300–1200 papers published about bat viruses annually, compared to 12–45 publications for marsupial viruses and only 1–9 studies for sloth viruses. The same review found that bats do not have significantly greater viral diversity than other mammal groups. Bats, rodents, and primates all harbored significantly more zoonotic viruses (which can be transmitted to humans) than other mammal groups, though the differences among the aforementioned three groups were not significant.
There are four main transmission routes for bat-borne viruses: droplet, airborne, contact, and oral-fecal transmission.
The 2002 outbreak of severe acute respiratory syndrome (SARS), and the 2012 outbreak of Middle East respiratory syndrome have been traced to have an origin in bats. Coronaviruses are positive-sense, single-stranded RNA viruses with four genera: Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus. Of these four, alphacoroanviruses and betacoronaviruses are bat-borne.
In 2019, a food market that sold live wild game (called ye wei Chinese: 野味) (a "wet market") in Wuhan, China was linked to the outbreak of SARS-CoV-2. Through genetic analyses, scientists initially found the virus resembles those typically found in bats. Subsequent genetics studies suggest the virus may have been transmitted to people from pangolins, since the sequences of coronavirus from these animals have high similarity with SARS-CoV-2, and these species were sold in the market. However, there are also concerns from the scientific community about the validity of the genetics technique used (codon usage bias).
Lyssaviruses include the rabies virus, Australian bat lyssavirus, and other related viruses, many of which are also harbored by bats. Unlike most other viruses in the family Rhabdoviridae, which are transmitted by arthropods, lyssaviruses are transmitted by mammals, most frequently through biting. All mammals are susceptible to lyssaviruses, though bats and carnivores are the most common natural reservoirs. The vast majority of human rabies cases are a result of the rabies virus, with only twelve other human cases attributed to other lyssaviruses as of 2015. After transmission has occurred, the average individual is asymptomatic for two months, though the incubation period can be as short as a week or as long as several years. Italian scientist Antonio Carini was the first to hypothesize that rabies virus could be transmitted by bats, which he did in 1911. This same conclusion was reached by Hélder Queiroz in 1934 and Joseph Lennox Pawan in 1936. Bats have an overall low prevalence of rabies virus, with a majority of surveys of apparently healthy individuals showing rabies incidence of 0–0.5%. Sick bats are more likely to be submitted for rabies testing than apparently healthy bats, known as sampling bias, with most studies reporting rabies incidence of 5–20% in sick or dead bats.
Bats are the most common source of rabies in humans in North and South America, Western Europe, and Australia. In the United States, there were 19 cases of human rabies from 1997–2006, 17 of which were attributed to bats. In North America, about half of human rabies instances are cryptic, meaning that the patient has no known bite history. While it has been speculated that rabies virus could be transmitted through aerosols, studies of the rabies virus have concluded that this is only feasible in limited conditions. These conditions include a very large colony of bats in a hot and humid cave with poor ventilation. While two human deaths in 1956 and 1959 had been tentatively attributed to aerosolization of the rabies virus after entering a cave with bats, "investigations of the 2 reported human cases revealed that both infections could be explained by means other than aerosol transmission". It is instead generally thought that most instances of cryptic rabies are the result of an unknown bat bite. Bites from a bat can be so small that they are not visible without magnification equipment, for example. Outside of bites, rabies virus exposure can also occur if infected fluids come in contact with a mucous membrane or a break in the skin. Rabies virus has also been transmitted when an infected human unknowingly dies of rabies, and their organs are transplanted to others.
Hantaviruses, usually found in rodents and shrews, were discovered in two species of bats. The Mouyassué virus (MOUV) was isolated from banana pipistrelle bats captured near Mouyassué village in Cote d'Ivoire, West Africa. The Magboi virus was isolated from hairy slit-faced bats found near the Magboi River in Sierra Leone in 2011. They are single-stranded, negative sense, RNA viruses in the Bunyaviridae family.
Henipavirus is a genus of RNA viruses in the family Paramyxoviridae, order Mononegavirales containing five established species. Henipaviruses are naturally harboured by pteropid fruit bats (flying foxes) and microbats of several species.
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