Bats and infectious disease
Bats are flying mammals that belong to the order Chiroptera, which encompasses over 920 species, making them a significant portion of the mammalian population. They are unique in their ability to fly and utilize echolocation for navigation and hunting. Despite their ecological benefits, such as pest control and pollination, bats are often linked to infectious diseases, raising public health concerns. They are recognized as hosts for a variety of zoonotic diseases, which are diseases that can be transmitted from animals to humans. Bats have been identified as reservoirs for several strains of coronaviruses, including those implicated in recent epidemics like SARS, MERS, and COVID-19.
The anatomical and behavioral traits of bats, such as their long lifespans and social living in dense colonies, enhance their potential to harbor and transmit pathogens. Notably, while rabies is a well-known viral disease associated with bats, the overall risk of transmission to humans is relatively low compared to other wildlife sources. However, the increasing interaction between humans and bat habitats, exacerbated by factors like climate change, raises the possibility of emerging infectious diseases. Consequently, while bats are often viewed through a lens of risk, they also play crucial roles in maintaining ecological balance, and understanding their interactions with human health is essential.
Bats and infectious disease
Definition
Bats are flying mammals belonging to the order Chiroptera (meaning “hand-wing”). With over 1,400 described species, bats make up about one-fifth of all classified mammals. The order is one of the most widespread on Earth, inhabiting all continents except Antarctica. Bat wingspans range in size from 150 millimeters to 2 meters. The first bats evolved about 52 million years ago, making the order ancient. The genome of many species is highly conserved, meaning the species has not changed significantly over time.
In addition to being the only mammal that flies, bats also have a highly specialized sensory specialization of echolocation. Echolocation requires considerable energy and neuronal capacity. Bats are relatively small, but because they fly, they can inhabit large ranges; some migrate hundreds of miles. They also tend to spend a great deal of time living in large, multispecies colonies at densities as high as three hundred bats per square foot.
Zoonotic Disease
Historically, bats have been associated with disease and danger to humans in many cultures. This is perhaps in part because a few species, known as vampire bats, are hematophagous (blood-eating). Notably, they have also long been known for their capacity to transmit rabies to both humans and domestic animals (even more so if they are hematophagous). However, their contribution to the transmission of rabies is relatively small worldwide.
Bats have chiefly become a concern in the twenty-first century because they are increasingly associated with the emergence of previously unknown viruses. Studies have indicated that bats are hosts to more zoonotic diseases than any other animal order, and that those diseases are more fatal in humans than diseases from other hosts. In particular, many strains of coronavirus have been traced to bats, including types that have caused epidemic and even pandemic-level outbreaks in the human population. Strains of lyssavirus and paramyxoviruses also arose in the early 2010s from virus reservoirs in bats. In addition, bats are potential sources of bacterial and fungal pathogens such as histoplasmosis. Research continues to isolate previously unknown viruses from bats, although most are not dangerous to humans.
Natural History and Risk Factors
Knowledge of the natural history of bats helps explain their fitness as reservoirs for potential pathogens for other species. Bat species are ancient; their genome contains conserved information likely to be shared by many mammalian species. Thus, cellular receptors recognized by bat viruses may be homologous and identical to receptors conserved in many other mammalian species. An article in The Scientist in 2024 also suggested that bats' evolution equipped them with uniquely powerful immune systems, which over time contributed to the development of viral strains that prove particularly deadly when transmitted to other species.
Bat behavior also promotes the long-term harboring of pathogens. Bats often spend much of their lives in a state of torpor or hibernation. In this state of lowered metabolism, pathogens may remain latent, and depressed immune systems may fail to clear them from their systems. As bats are flying mammals, their range is large, increasing the probability of transmission of pathogens over a large area. Some bats migrate, some as far as eight hundred miles, increasing the range for dispersal of pathogens significantly.
Bats often rest en masse in caves, in colonies that often run in the millions, often including more than one species. Direct contact can increase transmission rates for pathogens. In addition, bats use echolocation and make high-pitched sounds that are emitted with great pressure, resulting in aerosolization of droplets from their respiratory tracts. The droplets could carry pathogens and be easily passed among bats.
Bats have remarkably long lives for such small mammals. Life expectancy for some species is commonly twenty-five years. A bat with a latent infection could continue in a carrier state and transmit pathogens for decades. This may contribute to the apparent capability of bats to carry pathogens in a subclinical state for long periods.
Bat-Associated Diseases
Rabies. Bats transmit rabies, a type of viral encephalitis that is nearly 100 percent fatal once signs have set in. A number of different bat species propagate their own rabies variant, and genetic sequencing identifies specific strains. Globally, the number of rabies cases caused by exposure to bats is negligible in comparison with those transmitted by dogs and terrestrial wildlife. In 2024, the CDC reported that about 60,000 individuals receive treatment following exposure to rabies each year. Human rabies deaths in the United States are rare, but seven out of ten Americans who die from rabies were exposed to bats. In South America, vampire bats transmit rabies to humans and domestic animals during their normal feeding behavior.
Lyssavirus. A genus of the family Rhadoviridae, which includes rabies, lyssavirus also causes severe forms of encephalitis that are indistinguishable from rabies. Between 1996 and 1998, a novel lyssavirus was isolated from a flying fox in New South Wales, Australia; two women died of the virus after exposure to sick bats.
Coronaviruses. In late 2002, a new disease emerged from the wildlife meat markets of China. In nine months, severe acute respiratory syndrome (SARS) had spread from southern China to cause a near pandemic around the world, infecting eight thousand people in thirty-seven countries and killing nearly eight hundred persons. The disease was caused by a coronavirus that was isolated from masked palm civets and raccoon dogs, but, ultimately, the origin of the virus was believed to be in Chinese horseshoe bats.
Other coronaviruses have also been traced back to bat hosts. The disease known as the Middle East respiratory syndrome (MERS), first identified in 2012, is believed to have originated with bats, although human cases most commonly come from camels. Early evidence also suggested that coronavirus disease 2019 (COVID-19) likely developed in bats. The outbreak of COVID-19 was labeled a pandemic by the World Health Organization (WHO) in 2020, and within months had infected millions of people around the world. By February 2023, the WHO had confirmed 758,390,564 total cases and 6,859,093 deaths.
Henipavirus. Flying foxes were implicated as sources for another novel pathogen, the paramyxoviruses known as hendra virus and nipah virus (a family of viruses that includes measles, rinderpest, and canine distemper viruses), which emerged to cause outbreaks of acute respiratory syndromes, respectively, in horses and humans in Hendra, Australia, and in pigs and humans in Malaysia from 1994 to 2004. The diseases were often fatal, killing 105 of 265 people in Malaysia and resulting in the culling (slaughter) of more than one million pigs. Nipah virus has also been found in flying foxes in Bangladesh, India, and Cambodia. It is considered unlikely that people can contract these viruses from bats directly.
Ebola virus. The Ebola virus causes hemorrhagic disease that is fatal in 80 percent of cases. The disease has occurred as localized outbreaks in Africa and has been exported to Europe and North America with nonhuman primates. The natural reservoir has not yet been discovered, but Ebola virus RNA has been detected in bat tissues.
Histoplasmosis. Humans exposed to dust from large bat colonies may contract the pulmonary infection known as fungal histoplasmosis. The disease is generally mild, flu-like, and self-limiting, but it can be severe if exposure is significant or is suffered by immunocompromised persons.
Impact
While bats constitute a unique threat as reservoirs for emerging viruses, the magnitude of the threat may be exaggerated, particularly if weighed against the positive contributions of bats to the world’s ecosystems. As effective insectivores, bats save millions of dollars in damage to crops and probably prevent more vector-borne diseases in humans than they cause. Some bat species are also important pollinators.
However, experts have warned that factors such as climate change and habitat destruction that can put humans in closer contact with bats raise the risk of more frequent transmission of infectious disease. The COVID-19 pandemic showed how such diseases can quickly have massive impacts on humans, both from a direct public health perspective and through secondary effects such as economic disruption.
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