Why Do Bats Carry So Many Viruses Without Becoming Ill?
- 演化之聲
- Mar 15
- 4 min read
Updated: Apr 10
Bats occupy a unique position among mammals. They are the only mammalian group capable of sustained powered flight, and many species have also evolved echolocation to navigate and locate food. For decades, bats have been recognized as hosts of numerous zoonotic viruses. Rabies virus, Marburg virus, Nipah virus, and many coronaviruses have all been linked to bats. Yet infections in bats rarely produce obvious pathological symptoms. Instead, viruses can continue replicating and evolving within bat hosts, eventually giving rise to viral variants capable of infecting other species. In this sense, bats can function as natural reservoirs of viruses. In addition, studies have shown that bats develop cancer far less frequently than humans, suggesting that they possess biological mechanisms capable of counteracting both infectious diseases and tumor formation. These observations have sparked considerable scientific curiosity.
To understand the biological basis of these traits, researchers have sequenced the genomes of more than forty bat species over the past several years. Genomic studies have gradually revealed distinctive features of bats in both antiviral defense and tumor suppression. Analyses indicate that immune-related genes in bat genomes have experienced stronger evolutionary pressures than those in many other mammals. One comparative genomic study sequenced the genomes of the Jamaican fruit bat (Artibeus jamaicensis) and the Mesoamerican mustached bat (Pteronotus mesoamericanus), and compared them with the genomes of thirteen additional bat species and other mammals. The results showed that fourteen gene families had expanded in bats, whereas 105 gene families had contracted. Among these changes, one particularly notable category involved type I interferons, a group of proteins central to antiviral immunity.
Interferons are antiviral proteins found in vertebrates ranging from fish to humans. When a virus infects the body, certain cells release these molecules to inhibit viral replication and activate immune responses. Type I interferons are the most widespread group, and nearly all cell types can produce them. In most mammals, including humans, type I interferons consist of many IFN-α variants but only very few IFN-ω variants. Humans, for example, possess thirteen IFN-α genes but only one IFN-ω gene. Bats display the opposite pattern. IFN-α genes are relatively rare, whereas IFN-ω genes are comparatively expanded. The Jamaican fruit bat possesses four IFN-α genes and five IFN-ω genes, while the Mesoamerican mustached bat lacks IFN-α genes entirely but carries six IFN-ω genes. This shift in gene copy numbers may help explain how bats tolerate high levels of viral infection without developing severe disease.
Another study identified unusual changes in an antiviral gene called ISG15 within bats of the families Rhinolophidae and Hipposideridae. In humans, ISG15 is associated with excessive inflammatory responses and can promote inflammation during SARS-CoV-2 infection. In bats, however, the protein functions differently. Bat ISG15 exhibits stronger antiviral activity against SARS-CoV-2, and specific amino-acid changes in the gene appear to reduce harmful immune overreactions. These molecular modifications allow bats to mount antiviral defenses while avoiding damaging inflammatory responses.
In most mammals, viral infection triggers strong inflammatory reactions that can damage tissues and even lead to organ failure. Bats appear able to avoid this problem. Their immune systems suppress viral replication effectively while preventing excessive inflammation. For instance, studies have shown that Egyptian fruit bats (Rousettus aegyptiacus) infected with Marburg virus increase the expression of antiviral genes but show relatively low expression of pro-inflammatory genes. Similarly, the formation of inflammasomes—protein complexes involved in inflammation—is significantly reduced during viral infection in bats. These patterns indicate that bats possess highly regulated immune responses. Rather than completely eliminating viruses, their immune systems maintain viral levels at low concentrations, allowing viruses and hosts to coexist. This controlled equilibrium makes bats particularly suitable long-term hosts for many viruses.
Beyond their unusual immune responses, bats also appear to possess a remarkable resistance to cancer. Flight requires extremely high metabolic rates, which can increase the production of reactive oxygen species (ROS) inside cells. These molecules can damage DNA and potentially trigger cancer development. To counteract this risk, bats have evolved genetic mechanisms associated with tumor suppression and DNA repair. Several tumor-suppressor genes, including LATS2 and BIK, as well as DNA-repair genes such as PALB2, show signs of strong evolutionary selection in bats. These genetic adaptations may contribute both to the low incidence of cancer in bats and to their unusually long lifespans, which often exceed twenty years.
Scientists continue to investigate how bats can carry numerous pathogenic viruses while maintaining their own health, and whether their cancer-resistance mechanisms are related to these antiviral adaptations. If bats can eventually serve as model organisms for studying infectious diseases, understanding their biological strategies may open new avenues for antiviral therapies and medical research in humans.
Author: Shui-Ye You
References:
Morales AE et al. (2025). Bat genomes illuminate adaptations to viral tolerance and disease resistance. Nature.
Scheben A et al. (2023). Long-Read Sequencing Reveals Rapid Evolution of Immunity- and Cancer-Related Genes in Bats. GBE.
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