Picture a dark and foggy night. Tall, nearly leafless trees loom around you, and newly fallen leaves whirl around your feet. The silhouette of a presumably uninhabited hut is barely distinguishable from the heavy darkness of the late October night. As you approach the structure, fear rising in your throat, the door swings open and out fly hundreds of bats! Not doves, not lovebirds, and not sparrows. It is almost inevitably bats—regardless of how likely the scenario is.
Bats have long been associated with haunted houses, vampires, and spooky caves, but their association with the spooky and haunted isn’t the only reason they are feared; bats are also reservoirs for infectious disease.
Reservoirs and vectors play important roles in the spread of infectious disease. Some infectious diseases are transmitted through vectors, such as mosquitoes or ticks, which themselves do not cause disease. These bugs convey the disease-causing pathogen from one host to another. Insects and arthropods are familiar vectors for disease, and their intent is often to bite or attack us in order to obtain a meal, or to propagate their young. Through this direct contact, vectors spread the infectious pathogen into another organism.
A disease reservoir, however, can be thought of as a source of pathogens. A reservoir, also called a nidus, represents a host that has co-evolved with a pathogen. The host does not always show clinical signs of infection, because the relationship between the two organisms is normally stable. The evolution of most pathogens with their reservoirs creates a mutually sustainable relationship, in which the pathogen can exist without causing too much distress and illness to its needed host. When a reservoir species comes in contact with another animal or human, there is a risk of “spillover,” or the infection of another species with the pathogen. This is how zoonotic disease outbreaks can begin. But these species, unlike vectors, do not normally seek out human contact.
Bats are widely recognized as reservoirs for emerging infectious diseases. Southeast Asian and Australian fruit bats of the genus Pteropus are the natural reservoirs for the paramyxoviruses that cause Hendra and Nipah viral infections. Ebola virus has recently been found in central Africa in fruit bats Hypsignathus monstrosus, Epomops franqueti, and Myonycteris torquata. The SARS coronoavirus has been detected in horseshoe bats of the genus Rhinolophus, and the new MERS coronavirus may be endemic to bats in the family Vespertilionidae (genus Neoromicia and Pipstrellus) on the Arabian Peninsula and Africa. The little yellow-shouldered bat (Sturnira lilium) in Guatemala is carrying a type of Influenza A virus that may be able to adapt to infect humans. Many people also know that bats carry rabies, a fact which is frightening and true, but not as prevalent as you might think.
Bats and Rabies
Every year, there are confirmed cases of bats carrying rabies. Indeed, this is a disease to be feared; the WHO estimates the rabies virus kills more than 55,000 people worldwide each year. Rabies is the most lethal human infectious disease known, with a fatality rate of nearly 100 percent. It causes horrific symptoms, including anxiety, aggression, hydrophobia, and hallucinations before death. But in the United States, between 1995 and 2009, only an average of two people per year have died of rabies associated with bats. Many more cases result from the bites of rabid dogs and cats. Humans rarely come close enough to a rabid bat to be bitten, and most people know to avoid any bat that seems sick or is unable to fly. Although it is possible to contract rabies if the saliva or other infectious material of a bat gets into a person's eyes, nose, mouth, or a wound, this kind of transmission is rare. The CDC estimates that 40,000 people in the U.S. receive post-exposure prophylaxis (PEP) every year due to a potential exposure to rabies. But bats are seldom the cause.
Some studies suggest that about ten per cent of bats tested by the public have rabies. However, a University of Calgary team found that the number of infected bats may be closer to only one per cent, regardless of geographic location or species. Maybe the reputation of bats spreading rabies is a little unjustified, in this light.
Bats and SARS-CoV
In 2002, the SARS coronavirus infected over 8,000 people around the world, and killed about ten percent of them. The race was on to find the reservoir of this deadly virus, and eventually the horseshoe bat (genus Rhinolophus) was identified as the source. Li et al. sampled 408 bats in China, and found antibody to SARS-CoV in three species: R. pearsoni (Pearson's horseshoe bat), R. pussilus (least horseshoe bat), and R. macrotis (big-eared horseshoe bat). The belief was that SARS required an intermediate host in which it amplifies before affecting humans. Because the palm civet cat and raccoon dog in Southeast Asia have been noted as carriers of a mammalian coronavirus that evolved from the bat coronavirus, these animals were considered the intermediate hosts.
New research, published in Nature on Oct. 30, suggests that the SARS virus does not require an intermediate host and can go directly from horseshoe bats to humans. While the results strengthen the evidence that bats are the source of SARS, researchers remain uncertain. The study was conducted by New York-based non-profit EcoHealth Alliance and a group of Chinese researchers from Wuhan, and funded in part by U.S. government funds and the USAID PREDICT project. In an interview with CIDRAP, Columbia University virologist Vincent Racaniello explains that these findings are “why it is important important to know the viruses infecting the animals around us."
Recently, a new coronoavirus called MERS-CoV (lineage C of the genus Betacoronavirus) has caused severe respiratory syndrome in the Middle East. Bat coronaviruses have also been found in this group, including the Tylonycteris coronavirus HKU4 and the Pipistrellus coronavirus HKU5. This family of viruses, Coronaviridae, is apparently capable of rapidly evolving, and is found in many diverse species of animal. Of the six coronaviruses known to infect humans, four are endemic and cause only mild respiratory symptoms. Only two, SARS-CoV and MERS_CoV, are emerging infections that cause severe symptoms and even death. Horseshoe bats are the known reservoirs for a similar strain, called SARS-Like CoV. Bats have likely existed for millions of years with SARS-Like CoV. Human contact with infected bats has only been a recent phenomenon, with devastating results.
Why are bats harboring so many potentially dangerous diseases? Bats are natural reservoirs for many viruses because of their ecology, genetics, and behavior. Bats are also tremendously diverse, representing almost 20 percent of all mammalian species on Earth. This diversity gives any pathogen a wide range of bat species to select and develop an endemic relationship with. Bats can fly over great distances, which may help facilitate the spread of viruses through intermediate hosts like sheep, horses, or monkeys. They also live in close quarters with many other species of bats, which also enables a virus to mutate and adapt to other bat species. This makes the disease difficult to detect until there is a spillover, and humans are exposed to a virus that was never intended to infect humans. Further, bats are nocturnal, so humans rarely come into contact with them (unless you are wandering around creepy woods in a horror movie-like setting at night on a frequent basis). Little was known about bats as reservoirs until recently, when human development began encroaching on bat territories around the world. Because of the difference in our temporal landscapes, viruses that have evolved to be endemic to bat species have previously not been likely to infect humans.
Knowing more about bat ecology and behavior is important, as humans must learn to exist with the threat of emerging diseases. Understanding how and where bats live, and how viruses adapt and potentially come into contact with humans will help, and will hopefully give us less to fear.
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