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Bat Influenza (Flu)

Questions & Answers

What is bat influenza (flu)?

Bat influenza refers to influenza A viruses found in bats. Bat influenza was first discovered in “little yellow-shouldered bats” in Guatemala during a study conducted in 2009 and 2010 by experts from CDC and the Universidad del Valle (University of the Valley) in Guatemala*. Bat influenza viruses have only been detected in little yellow-shouldered bats at this time. Additional testing is needed to determine if bat influenza viruses are found in other types of bats. Laboratory research at CDC suggests these viruses would need to undergo significant changes to become capable of infecting and spreading easily among humans. Little yellow shouldered bats are not native to the continental United States, but are common in Central and South America.

Does bat influenza pose a threat to human health?

Preliminary laboratory research at CDC suggests that human cells do not support growth of the bat influenza viruses in the test tube. This suggests that bat influenza viruses cannot grow or replicate in humans and would need to undergo significant changes to become capable of infecting and spreading easily among humans. However, testing of the bat influenza virus’s genome suggests the that its internal genes are compatible with human influenza viruses, so CDC scientists cannot rule out the possibility of these viruses eventually becoming capable of infecting humans. For more information, see the question below entitled “How could bat influenza viruses become capable of infecting and spreading among humans?”

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Yellow-shouldered bat found in Guatemala. Photo credit: CDC/OID/NCEZID – Amy T. Gilbert.
Yellow-shouldered bat found in Guatemala. Photo credit: CDC/OID/NCEZID – Amy T. Gilbert.

How could bat influenza viruses become capable of infecting and spreading among humans?

Because the internal genes of bat influenza viruses are compatible with human influenza viruses, it is possible that these viruses could exchange genetic information with human influenza viruses through a process called “reassortment.” Reassortment occurs when two or more influenza viruses infect a single host cell, which allows the viruses to swap genetic information. Reassortment can sometimes lead to the emergence of new influenza viruses capable of infecting humans.

However, the conditions needed for reassortment to occur between human influenza viruses and bat influenza virus remain unknown. A different animal (such as pigs, horses or dogs) would need to serve as a “bridge,” meaning that such an animal would need to be capable of being infected with both this new bat influenza virus and human influenza viruses for reassortment to occur. Additional studies are needed to determine the likelihood that reassortment would occur in nature between bat and human influenza viruses.

Why is the discovery of bat influenza important for public health?

The discovery of bat influenza is important for public health because bats represent a new animal species that may act as a source of influenza viruses. Influenza viruses are already known to cause disease and spread in other animals, including domestic and wild birds, pigs, horses, and dogs, with sporadic outbreaks in seals, whales, ferrets and cats. CDC and disease experts around the world monitor influenza viruses that circulate in animals because the previous pandemics of the 20th century as well as the 2009 H1N1 pandemic were caused by influenza viruses in animals that gained the ability to infect and spread easily in humans.

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Could bat influenza viruses cause severe illness in humans?

Additional studies are required to better understand the severity of illness that could be associated with infection with a bat influenza virus. In its current form, bat influenza does not appear well suited to infecting and causing illness in humans. In fact, there were no signs of disease noted among the infected bats.

What has the discovery of bat influenza taught us about influenza viruses?

The discovery of bat influenza has shed light on the evolution of influenza A, B and C viruses. It is possible that many of the internal genes of this new bat influenza virus are descendants of influenza virus families that once circulated more widely in previous centuries and that are now extinct – or yet to be discovered.

How is the bat influenza virus different from other influenza viruses?

The bat influenza virus discovered in Guatemala is very different from other influenza viruses in humans and animals. All influenza A viruses have hemagglutinin (HA) surface proteins, and until the discovery of this virus, there were only 16 different classes (or “subtypes”) of HA proteins known to exist in nature. This new bat virus is distinct enough from those pre-existing subtypes that CDC scientists have classified it as a new subtype, denoted as “H17”. The other surface protein-coding gene of the virus, neuraminidase (NA), is extraordinarily different from that of known influenza viruses, as well. It is possible this gene came from ancient bat influenza viruses that are extinct or yet to be discovered. For additional information on influenza virus biology and pictures of influenza viruses, see Influenza (Flu) Viruses and Images of Influenza Viruses.

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How was this bat influenza virus discovered?

The discovery of the bat influenza virus was made possible by field work conducted by CDC’s Global Disease Detection (GDD) Regional Center in Guatemala in collaboration with CDC’s Pathogen Discovery Program, CDC’s Rabies program and the Universidad del Valle in Guatemala.

A total of 316 healthy bats from 21 different species were captured from eight locations in southern Guatemala during two consecutive years, 2009 and 2010. As part of the study, 180 bats were collected in May 2009 and 136 bats were collected in September 2010. With the assistance of the GDD Guatemala Regional Center and the Universidad del Valle, the CDC Rabies program within CDC’s Office of Infectious Diseases and GDD took the lead in swabbing the bats. Rectal swabs and tissues were collected from each of the bats. (Videos showing how specimens are collected from bats in Guatemala are available here and here.)

Swabs that were negative for rabies were analyzed to detect other pathogenic viruses by the Pathogen Discovery Program, which is located in CDC’s Division of Viral Diseases. Initial viral screening conducted by the Pathogen Discovery Program determined that three of the 316 bats tested positive for influenza. All were from a single species known as little yellow-shouldered bats.

CDC’s Pathogen Discovery Program was able to identify and sequence the complete genome of the new virus using high-throughput sequencing instruments in the laboratory (specifically 454 pyro sequencing and Illumina GAIIx) at CDC’s laboratories at its Atlanta headquarters and Emory University’s sequencing core, respectively.

* "A distinct lineage of influenza A virus from bats” by Suxiang Tong et al. PNAS 2012.

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