With a wide host range, ability to undergo genetic recombination and cross species barrier, influenza A viruses (IAVs) continue to spread globally, causing huge economic losses to the poultry industry and threatening public health. Unlike the low pathogenic avian influenza viruses (LPAIVs), highly pathogenic avian influenza viruses (HPAIVs) cause very severe disease in gallinaceous poultry often leading to 100% mortality within 2–3 days1. HPAIVs of H5N1 subtype are of particular concern as certain contemporary Eurasian lineage H5N1 viruses can carry an alarming case fatality rate of up to 50% in humans2. Negative strand segmented RNA genomes contribute to the genetic variability of IAVs. In addition, IAVs can infect a wide range of avian and mammalian host species resulting in the emergence of novel subtypes with altered species tropism and/or virulence.

It is widely known that aquatic birds such as ducks, gulls, and shorebirds serve as a natural reservoir of most known IAVs3. IAVs are known to infect a wide range of avian and mammalian hosts, and it is highly likely that their host range could be broader than currently known, with more reservoirs to be revealed. For example, Northwest Atlantic gray seals have recently been suggested to be an endemically infected wild reservoir population for diverse IAVs4. From these natural reservoirs, IAVs can evolve into novel variants which can potentially lead to human pandemics. Influenza pandemics occur time to time with the most recent pandemic being in 2009. It is undisputed that the next influenza pandemic will happen, but the only question is when it will happen. Despite several reports investigating the basis of genetic variability of IAVs, the precise mechanism of pandemic IAV generation still remains an unresolved mystery. It is possible that additional IAV reservoirs are yet to be identified which will help to see the full picture of IAV ecology and evolution.

Bats (order: Chiroptera) are one of the ancient mammals, and their speciation occurred long before the development of most modern mammals. Bats are globally distributed, relatively long lived5 and represent approximately 24% of all known mammalian species. Further, bats are one of the most diverse families of mammals found in nearly every habitat/continent around the world except Antarctica. More importantly, certain Old World bat species are known to be natural reservoirs of zoonotic viruses that cause some of the deadliest diseases in humans including filoviruses (such as Ebola and Marburg viruses), lyssaviruses, severe acute respiratory syndrome (SARS)-related coronaviruses and henipaviruses (e.g. Hendra and Nipah viruses)6, 7. In addition, bats also act as a major natural reservoir for hepaciviruses and pegiviruses (hepatitis C virus and GB virus B)8. Notably, all the zoonotic viruses of bat-origin identified to date are RNA viruses5. However, it is believed that the actual diversity of viruses in bats could be much more than what is currently known, ﻿as most of the﻿ investigations have focused on searching for specific viruses of interest and many additional viruses must have been overlooked9.

The prospects for bats contributing to IAV epidemiology came to light after the identification of two novel influenza-like viruses in fruit bats by next generation sequencing10. These two viruses are genetically distinct from all previously known IAVs and hence are designated as novel subtypes, namely H17N10 and H18N11. These novel IAVs have been recently recovered in cell culture using synthetic DNA11. However, these HA and NA subtypes have not been identified in birds serologically or virologically. Consequently, the reservoir(s) of these novel IAV subtypes is still undefined. Phylogenetic studies raised a possibility that bats have the capacity to harbor more influenza virus genetic diversity than all the other mammalian and avian species combined10. In addition, it was demonstrated that little yellow-shouldered bats in Central America could constitute a potential sylvatic mammalian reservoir of influenza12. Susceptibility of bats to IAVs has been confirmed by recent serological evidence of AIV H9 subtype in about 30% of frugivorous bats from Africa13. It is worth noting that detection of antibodies against one AIV subtype in 30% of the bats tested is very significant.

Influenza viruses bind to sialic acid (SA) residues that are bound to glycans through α2,3 or α2,6 linkage on the host cells14. The expression of the appropriate host cell receptor to which viral haemagglutinin (HA) can bind is the key determinant of the ability of IAVs to infect a host15. Avian influenza viruses (AIVs) preferentially bind to SA receptors that are linked to galactose by an α 2,3 linkage (SA α2,3-Gal), while human and classical swine influenza viruses show preference to α2,6 linked SAs (SA α2,6-Gal). A key source of IAV genetic diversity could be from the replication of IAVs in a non-native host species that initiate evolution of new virus variants16. Hosts that co-express both SA α2,3-Gal and SA α2,6-Gal receptors such as chickens, ducks and pigs have been hypothesized to potentially support re-assortment of IAVs and hence play a major role in the evolution of IAVs14, 17.

While the role of bats in IAV evolution is not yet known, recent evidence raises a primary question, “can bats support co-infection of avian and human IAVs?”. Consequently a logical first unknown which needs to be addressed is whether bats express appropriate SA receptors compatible with avian and human IAV binding. To resolve this enigma, we investigated for the first time the distribution of SA receptors in little brown bats (LBBs) (Myotis lucifugus), a widely-distributed bat species in North America and their compatibility to support avian and human IAV binding.