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Interspecies transmission and emergence of novel viruses: lessons from bats and birds

2013; Elsevier BV; Volume: 21; Issue: 10 Linguagem: Inglês

10.1016/j.tim.2013.05.005

1878-4380

Jasper Fuk‐Woo Chan, Kelvin Kai‐Wang To, Herman Tse, Dong‐Yan Jin, Kwok‐Yung Yuen,

Virology and Viral Diseases

•Bats and birds are reservoirs of zoonotic viruses.•Their unique immune systems allow them to harbor a large variety of viruses.•Coronaviruses and influenza viruses are examples of interspecies transmission of viruses. As exemplified by coronaviruses and influenza viruses, bats and birds are natural reservoirs for providing viral genes during evolution of new virus species and viruses for interspecies transmission. These warm-blooded vertebrates display high species biodiversity, roosting and migratory behavior, and a unique adaptive immune system, which are favorable characteristics for asymptomatic shedding, dissemination, and mixing of different viruses for the generation of novel mutant, recombinant, or reassortant RNA viruses. The increased intrusion of humans into wildlife habitats and overcrowding of different wildlife species in wet markets and farms have also facilitated the interspecies transmission between different animal species. As exemplified by coronaviruses and influenza viruses, bats and birds are natural reservoirs for providing viral genes during evolution of new virus species and viruses for interspecies transmission. These warm-blooded vertebrates display high species biodiversity, roosting and migratory behavior, and a unique adaptive immune system, which are favorable characteristics for asymptomatic shedding, dissemination, and mixing of different viruses for the generation of novel mutant, recombinant, or reassortant RNA viruses. The increased intrusion of humans into wildlife habitats and overcrowding of different wildlife species in wet markets and farms have also facilitated the interspecies transmission between different animal species. Both mammalian and avian coronaviruses (CoV) have diverse host ranges. Phylogenetic dating of RNA-dependent RNA polymerase (RdRp) sequence divergence suggested that the most recent common ancestor (MRCA) of mammalian CoVs appeared around 7000–8000 years ago, whereas the MRCA of avian CoVs dates back to 10 000 years ago (Figure 1). These results are likely underestimations because they could not account for additional sequence diversity from undiscovered viruses. Nonetheless, the present estimates roughly coincide with the dispersal of the human population around the world about 50 000–100 000 years ago and greatly increased in the last 10 000 years during the first historic transition. During this transition, humans began various farming activities, such as forest clearing for agriculture and animal herding, leading to a significant shift in the ecology and population dynamics of viruses owing to the intrusion of wildlife habitats and intensive mixing of different animal hosts. Finally, the expansion of human travel and trading directly led to the spread of viruses to distant and isolated places. The migration of early humans over long distances was very limited and effectively a unidirectional 'rare' event. Eventually, improvements in transportation technology enabled distant trade missions in early Mesopotamia around 5000 years ago and possibly earlier in other regions [1Edens C. Dynamics of trade in the ancient mesopotamian "World System".Am. Anthropol. New Ser. 1992; 94: 118-139Crossref Google Scholar]. These periodic yet infrequent visits might have enabled transmission of various disease agents to previously segregated non-immune populations, leading to a serial founder effect associated with a boom and bust cycle. However, further technological improvements, especially the development of aviation and the flight industry in the last century, have allowed this type of travel to occur at such high frequencies that multiple segregated host populations effectively have become a single large population. These changes coincided with increased breeding between different host populations (for both humans and domestic animals), which can significantly impact the genetic and immunological make-up of the host populations. When these occurrences are considered in the context of the high mutation rate of RNA viruses, they become a driving force for speciation and subsequent evolution of new viruses. Although phylogenetic analysis and dating of individual influenza genes and lineages have been reported previously, large sequence divergence and frequent reassortment led to difficulties in precisely dating and phylogenetic positioning the common ancestor of modern influenza viruses [2Xu J. et al.Evolutionary history and phylodynamics of influenza A and B neuraminidase (NA) genes inferred from large-scale sequence analyses.PLoS ONE. 2012; 7: e38665Crossref PubMed Scopus (6) Google Scholar]. About 70% of the emerging pathogens infecting humans originate from animals. Most of these major outbreaks were due to RNA viruses as a result of their higher mutation rates compared with other types of microbes and their capability for unique genetic change, either by genetic recombination in positive-sense RNA viruses or genetic reassortment in RNA viruses with segmented genomes. Those with greatest impact on humans include the severe acute respiratory syndrome coronavirus (SARS-CoV), influenza virus, and HIV. Little was known about CoVs until the 2003 SARS epidemic, which caused 774 deaths among 8098 cases in over 30 countries [3Cheng V.C. et al.Severe acute respiratory syndrome coronavirus as an agent of emerging and reemerging infection.Clin. Microbiol. Rev. 2007; 20: 660-694Crossref PubMed Scopus (65) Google Scholar]. The natural reservoir of the more ancestral bat SARS-CoV is the Chinese horseshoe bat (Rhinolophus sinicus), which may have transmitted the virus to other game mammals including Himalayan palm civets (Paguma larvata), raccoon dogs (Nyctereutes procyonoides), and Chinese ferret badgers (Melogale moschata) in wildlife markets in South China [4Lau S.K. et al.Severe acute respiratory syndrome coronavirus-like virus in Chinese horseshoe bats.Proc. Natl. Acad. Sci. U.S.A. 2005; 102: 14040-14045Crossref PubMed Scopus (427) Google Scholar]. This finding sparked intense hunting for novel CoVs in humans and different animal species, especially in bats. The latest emerging novel human CoV, originally named human coronavirus EMC/2012 and later renamed Middle East respiratory syndrome coronavirus (MERS-CoV), which has caused 30 deaths among 54 cases in the Middle East, Europe, and Africa, is also phylogenetically closely related to the Tylonycteris bat CoV HKU4 (Ty-BatCoV-HKU4) and Pipistrellus bat CoV HKU5 (Pi-BatCoV-HKU5) discovered in bats in Hong Kong [5Chan J.F. et al.Is the discovery of the novel human betacoronavirus 2c EMC/2012 (HCoV-EMC) the beginning of another SARS-like pandemic?.J. Infect. 2012; 65: 477-489Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar, 6Woo P.C. et al.Comparative analysis of twelve genomes of three novel group 2c and group 2d coronaviruses reveals unique group and subgroup features.J. Virol. 2007; 81: 1574-1585Crossref PubMed Scopus (70) Google Scholar, 7Woo P.C. et al.Genetic relatedness of the novel human group C betacoronavirus to Tylonycteris bat coronavirus HKU4 and Pipistrellus bat coronavirus HKU5.Emerg. Microbes Infect. 2012; 1: e35Crossref Scopus (11) Google Scholar, 8Chan J.F. et al.The emerging novel Middle East Respiratory Syndrome Coronavirus: the "knowns" and "unknowns".J. Formos. Med. Assoc. 2013; 112: 372-381Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar] (http://www.who.int/csr/don/2013_06_05/en/index.html). The importance of birds as natural reservoirs of emerging influenza viruses is underscored by the persistent threat of avian influenza H5N1 since 1997 and the emergence of H7N9 in 2013 [9Wong S. et al.Bats as a continuing source of emerging infections in humans.Rev. Med. Virol. 2007; 17: 67-91Crossref PubMed Scopus (104) Google Scholar, 10Tong S. et al.A distinct lineage of influenza A virus from bats.Proc. Natl. Acad. Sci. U.S.A. 2012; 109: 4269-4274Crossref PubMed Scopus (178) Google Scholar, 11To K.K. et al.Avian influenza A H5N1 virus: a continuous threat to humans.Emerg. Microbes Infect. 2012; 1: e25Crossref Scopus (11) Google Scholar, 12Chen Y. et al.Human infections with the emerging avian influenza A H7N9 virus from wet market poultry: clinical analysis and characterisation of viral genome.Lancet. 2013; 381: 1916-1925Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar]. The role of bats in the emergence of novel influenza viruses is less clear although influenza A H17 and H3N2 viruses have been discovered in Sturnira lilium recently and in Nyctalus noctula bats in Kazakhstan in 1970, respectively. We review the importance of bats and birds in the genesis of new virus mutants and interspecies jumping using CoVs and influenza viruses as examples. A number of unique ecological, biological, immunological, and genetic features make bats a favorable animal reservoir for the emergence of novel viruses. Bats have remarkable species diversity, with over 1240 species (20% of the nearly 5000 known species within Mammalia and only second to rodents) [9Wong S. et al.Bats as a continuing source of emerging infections in humans.Rev. Med. Virol. 2007; 17: 67-91Crossref PubMed Scopus (104) Google Scholar]. Because viruses are obligatory intracellular microbes, it is assumed that viruses have multiple independent evolutionary origins not separable from co-evolution of their hosts. This high biodiversity of bats makes them an important source of new viruses for interspecies jumping. Bats are widely distributed in all continents except the polar regions and a few oceanic islands. Their roosting or hibernation environment ranges from natural habitats such as caves, rock crevices, bird nests, and tree cavities, to man-made structures like mines, tombs, buildings, and bridges, which bring them closer to humans and companion animals or livestock. Their unique ability among mammals to fly long distances (up to 2000 km) to locate suitable habitats also allows them to acquire or disseminate viruses [9Wong S. et al.Bats as a continuing source of emerging infections in humans.Rev. Med. Virol. 2007; 17: 67-91Crossref PubMed Scopus (104) Google Scholar]. Their habit of roosting in large colonies ranging from 10–200 000 bats and relative longevity of up to 35 years also provide abundant mating opportunities and thus exchange of viral genetic material and further viral spread to other species. Exposures to infected urine and aerosols generated during defecation have been suggested as possible routes of intraspecies and interspecies transmission of viruses from bats [9Wong S. et al.Bats as a continuing source of emerging infections in humans.Rev. Med. Virol. 2007; 17: 67-91Crossref PubMed Scopus (104) Google Scholar]. Bats may also transmit viruses to human and other animals via bites and scratches as in the case of rabies. Consumption and handling of undercooked bat meat is still practiced in China, Guam, and some parts of Asia. Besides ecological and biological traits, bats also possess special immunological attributes that enhance their ability to serve as gene pools for emerging viruses (Table 1). Asymptomatic or persistent viral shedding with little evidence of pathology in bats is well reported [13Calisher C.H. et al.Bats: important reservoir hosts of emerging viruses.Clin. Microbiol. Rev. 2006; 19: 531-545Crossref PubMed Scopus (248) Google Scholar]. One possible mechanism is the very early control of viral replication by their innate immune response involving the early recognition by pattern recognition receptors (PRRs) and interferons (IFNs) followed by partial control by the adaptive immune response [14Baker M.L. et al.Antiviral immune responses of bats: a review.Zoonoses Public Health. 2013; 60: 104-116Crossref PubMed Scopus (6) Google Scholar]. Similar to other mammalian species, some bats possess the two major families of virus-sensing PRRs, namely the Toll-like receptors (TLRs), which are membrane-bound PRRs that detect single-stranded RNA (ssRNA) or double-stranded RNA (dsRNA), and retinoic acid inducible gene I (RIG-I)-like helicases (RLHs), which are cytosolic PRRs that detect dsRNA. Bat TLRs have been described in many fruit bat species and are highly similar to other mammalian TLRs [15Iha K. et al.Molecular cloning and expression analysis of bat toll-like receptors 3, 7 and 9.J. Vet. Med. Sci. 2010; 72: 217-220Crossref PubMed Scopus (6) Google Scholar, 16Cowled C. et al.Molecular characterisation of RIG-I-like helicases in the black flying fox, Pteropus alecto.Dev. Comp. Immunol. 2012; 36: 657-664Crossref PubMed Scopus (6) Google Scholar]. In addition, cytoplasmic RLHs including RIG-I, melanoma differentiation-associated protein 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2) have been found in Pteropus alecto and their homologs in Myotis davidii [16Cowled C. et al.Molecular characterisation of RIG-I-like helicases in the black flying fox, Pteropus alecto.Dev. Comp. Immunol. 2012; 36: 657-664Crossref PubMed Scopus (6) Google Scholar]. These PRRs allow bats to recognize a similar range of pathogens as other mammalian species and in turn these pathogens are controlled by IFNs. Type I IFNs have been described in numerous fruit bat species [17Kepler T.B. et al.Chiropteran types I and II interferon genes inferred from genome sequencing traces by a statistical gene-family assembler.BMC Genomics. 2010; 11: 444Crossref PubMed Scopus (12) Google Scholar], and type III IFNs have been found in Myotis lucifugus, P. alecto, and Pteropus vampyrus [18Zhou P. et al.Type III IFNs in pteropid bats: differential expression patterns provide evidence for distinct roles in antiviral immunity.J. Immunol. 2011; 186: 3138-3147Crossref PubMed Scopus (16) Google Scholar]. Notably, the type I IFNω family of genes are expanded in some bat species with up to a dozen members, and the type III IFN receptors are more widely distributed in tissues of P. alecto than in humans and mice [19Zhou P. et al.Type III IFN receptor expression and functional characterisation in the pteropid bat, Pteropus alecto.PLoS ONE. 2011; 6: e25385Crossref PubMed Scopus (7) Google Scholar]. Bats may also demonstrate a delayed or differential IFN response during in vitro infection by different kinds of viruses [17Kepler T.B. et al.Chiropteran types I and II interferon genes inferred from genome sequencing traces by a statistical gene-family assembler.BMC Genomics. 2010; 11: 444Crossref PubMed Scopus (12) Google Scholar, 18Zhou P. et al.Type III IFNs in pteropid bats: differential expression patterns provide evidence for distinct roles in antiviral immunity.J. Immunol. 2011; 186: 3138-3147Crossref PubMed Scopus (16) Google Scholar].Table 1Innate and adaptive immune systems in bats and birdsProteinFunctionBatsBirdsInnate immunityRIG-ICytosolic PRR; detects dsRNAPresent and functional in the black flying fox (Pteropus alecto; AEW46678); homolog also present in David's Myotis (Myotis davidii; ELK34300) 16Cowled C. et al.Molecular characterisation of RIG-I-like helicases in the black flying fox, Pteropus alecto.Dev. Comp. Immunol. 2012; 36: 657-664Crossref PubMed Scopus (6) Google ScholarPresent and functional in mallard duck (ACA61272) and present in goose (ADV58759), but absent in chicken 33Barber M.R. et al.Association of RIG-I with innate immunity of ducks to influenza.Proc. Natl. Acad. Sci. U.S.A. 2010; 107: 5913-5918Crossref PubMed Scopus (75) Google ScholarMDA5Cytosolic PRR; detects dsRNAPresent and functional in black flying fox (P. alecto; AEW46679); homolog also present in David's Myotis (M. davidii; ELK28159)Present in mallard duck (GU936632), goose (AGC51036), and chicken (ADD83027) 33Barber M.R. et al.Association of RIG-I with innate immunity of ducks to influenza.Proc. Natl. Acad. Sci. U.S.A. 2010; 107: 5913-5918Crossref PubMed Scopus (75) Google Scholar, 86Karpala A.J. et al.Characterization of chicken Mda5 activity: regulation of IFN-beta in the absence of RIG-I functionality.J. Immunol. 2011; 186: 5397-5405Crossref PubMed Scopus (22) Google ScholarLGP2Cytosolic PRR; detects dsRNAPresent and functional in black flying fox (P. alecto); homolog also present in David's Myotis (M. davidii)Present in chicken 34Liniger M. et al.Chicken cells sense influenza A virus infection through MDA5 and CARDIF signaling involving LGP2.J. Virol. 2012; 86: 705-717Crossref PubMed Scopus (14) Google ScholarTLR7Membrane-bound PRR; detects ssRNAPresent in black flying fox (P. alecto; ADO01609), Leschenault's rousette (Rousettus leschenaultia; BAH02556), and David's Myotis (M. davidii; ELK30184) 15Iha K. et al.Molecular cloning and expression analysis of bat toll-like receptors 3, 7 and 9.J. Vet. Med. Sci. 2010; 72: 217-220Crossref PubMed Scopus (6) Google Scholar, 85Zhang G.J. et al.Comparative analysis of bat genomes provides insight into the evolution of flight and immunity.Science. 2013; 339: 456-460Crossref PubMed Scopus (25) Google ScholarPresent in chicken (ACR26250) and mallard duck (ABK51522) 35MacDonald M.R. et al.The duck toll like receptor 7: genomic organization, expression and function.Mol. Immunol. 2008; 45: 2055-2061Crossref PubMed Scopus (24) Google Scholar, 87Downing T. et al.The differential evolutionary dynamics of avian cytokine and TLR gene classes.J. Immunol. 2010; 184: 6993-7000Crossref PubMed Scopus (12) Google Scholar, 88Volmer C. et al.Immune response in the duck intestine following infection with low-pathogenic avian influenza viruses or stimulation with a Toll-like receptor 7 agonist administered orally.J. Gen. Virol. 2011; 92: 534-543Crossref PubMed Scopus (8) Google ScholarTLR3Membrane-bound PRR; detects dsRNAPresent in black flying fox (P. alecto; ADO01605), Leschenault's rousette (R. leschenaultia; BAH02555), and David's Myotis (M. davidii; ELK23529) 15Iha K. et al.Molecular cloning and expression analysis of bat toll-like receptors 3, 7 and 9.J. Vet. Med. Sci. 2010; 72: 217-220Crossref PubMed Scopus (6) Google Scholar, 85Zhang G.J. et al.Comparative analysis of bat genomes provides insight into the evolution of flight and immunity.Science. 2013; 339: 456-460Crossref PubMed Scopus (25) Google ScholarPresent in chicken (ABG79022) and muscovy duck (AFK29094) 89Jiao P.R. et al.Molecular cloning, characterization, and expression analysis of the Muscovy duck Toll-like receptor 3 (MdTLR3) gene.Poult. Sci. 2012; 91: 2475-2481Crossref PubMed Scopus (3) Google ScholarTLR8Membrane-bound PRR; detects ssRNAPresent in black flying fox (P. alecto; ELK17709) and David's Myotis (M. davidii; ELK30183) 85Zhang G.J. et al.Comparative analysis of bat genomes provides insight into the evolution of flight and immunity.Science. 2013; 339: 456-460Crossref PubMed Scopus (25) Google ScholarDisrupted in chicken and duck genome 90MacDonald M.R. et al.Genomics of antiviral defenses in the cuk, a natural host of influenza and hepatitis B viruses.Cytogenet. Genome Res. 2007; 117: 195-206Crossref PubMed Scopus (9) Google ScholarIFN-αType I IFN; inducible cytokine that regulates the antiviral responseVariable number of members present in multiple bat species, including black flying fox (P. alecto; α1: ELK15818, α5: ELK06973) and David's Myotis (M. davidii; α3: ELK29335); may be absent in other Myotis sp. 17Kepler T.B. et al.Chiropteran types I and II interferon genes inferred from genome sequencing traces by a statistical gene-family assembler.BMC Genomics. 2010; 11: 444Crossref PubMed Scopus (12) Google ScholarAt least 13 putative IFN-α genes identified in the chicken genome 91Kaiser P. et al.A genomic analysis of chicken cytokines and chemokines.J. Interferon Cytokine Res. 2005; 25: 467-484Crossref PubMed Scopus (99) Google Scholar; also identified in the mallard duck (ADU60335) and Gerylag goose (AFU54612), but exact number of members is not knownIFN-βType I IFN; inducible cytokine that regulates the antiviral responsePresent in multiple bat species, including black flying fox (P. alecto; ELK06976) and David's Myotis (M. davidii; ELK29334)Present in chicken (NP_001020007) and mallard duckIFN-ωType I IFN; inducible cytokine that regulates the antiviral responseMultiple members present in bats, including black flying fox (P. alecto; ELK06971, ELK06975, ELK08131, ELK15817, and ELK15819) and David's Myotis (M. davidii; ELK26494, ELK26493, and ELK26495)AbsentIFN-λType III IFN; inducible cytokine that regulates the antiviral responseTwo members identified in black flying fox (P. alecto; λ1: AEF33950, λ2: AEF33949); also present in the closely related Malaysian flying fox (Pteropus vampyrus) 18Zhou P. et al.Type III IFNs in pteropid bats: differential expression patterns provide evidence for distinct roles in antiviral immunity.J. Immunol. 2011; 186: 3138-3147Crossref PubMed Scopus (16) Google ScholarAt least one member present in chicken (ABU82742) 92Han X. et al.Molecular cloning and characterization of chicken interferon-gamma receptor alpha-chain.J. Interferon Cytokine Res. 2008; 28: 445-454Crossref PubMed Scopus (5) Google ScholarAdaptive immunityIg VHVariable region of immunoglobulin heavy chain; contributes to diversity of antibody repertoireAt least 23 genes classified into five families were identified in the black flying fox (P. alecto; GQ427153:GQ427172) 18Zhou P. et al.Type III IFNs in pteropid bats: differential expression patterns provide evidence for distinct roles in antiviral immunity.J. Immunol. 2011; 186: 3138-3147Crossref PubMed Scopus (16) Google Scholar; diverse genes in the VH3 family repertoire also reported for other bat species 93Bratsch S. et al.The little brown bat, M. lucifugus, displays a highly diverse V H, D H and J H repertoire but little evidence of somatic hypermutation.Dev. Comp. Immunol. 2011; 35: 421-430Crossref PubMed Scopus (8) Google ScholarSingle functional VH gene in chicken; 58 ψVH pseudogenes located upstream in IgH locus also contribute to repertoire diversity through gene conversion; similar organization in ducks and other birds 37Magor K.E. Immunoglobulin genetics and antibody responses to influenza in ducks.Dev. Comp. Immunol. 2011; 35: 1008-1016Crossref PubMed Scopus (11) Google ScholarIg DHDiversity region of Ig heavy chain; contributes to diversity of antibody repertoireDiverse DH elements in the little brown bat (Myotis lucifugus) 93Bratsch S. et al.The little brown bat, M. lucifugus, displays a highly diverse V H, D H and J H repertoire but little evidence of somatic hypermutation.Dev. Comp. Immunol. 2011; 35: 421-430Crossref PubMed Scopus (8) Google ScholarApproximately 15 DH elements in chickenIg JHJoining region of Ig heavy chainAt least 13 elements identified in the little brown bat (M. lucifugus) 93Bratsch S. et al.The little brown bat, M. lucifugus, displays a highly diverse V H, D H and J H repertoire but little evidence of somatic hypermutation.Dev. Comp. Immunol. 2011; 35: 421-430Crossref PubMed Scopus (8) Google ScholarSingle JH element in chickenIg VLVariable region of immunoglobulin light chain; contributes to diversity of antibody repertoireλ and κ loci present in black flying fox (P. alecto) 94Papenfuss A.T. et al.The immune gene repertoire of an important viral reservoir, the Australian black flying fox.BMC Genomics. 2012; 13: 261Crossref PubMed Scopus (11) Google Scholar, representative of megabats; λ locus present in microbats, but κ locus is absent 94Papenfuss A.T. et al.The immune gene repertoire of an important viral reservoir, the Australian black flying fox.BMC Genomics. 2012; 13: 261Crossref PubMed Scopus (11) Google ScholarSingle functional VL gene in single (λ) IgL locus in chicken; approximately 25 ψVL pseudogenes contribute through gene conversion 95Ratcliffe M.J. Antibodies, immunoglobulin genes and the bursa of Fabricius in chicken B cell development.Dev. Comp. Immunol. 2006; 30: 101-118Crossref PubMed Scopus (83) Google ScholarIg JLJoining region of Ig light chain(Not studied)Single JL element in chickenIg CδConstant region of IgDAbsent in black flying fox (P. alecto) and probably other megabats; present in microbats, such as the little brown bat (Myotis lucifugus; ADI96045, ADI96044) 96Butler J.E. et al.The two suborders of chiropterans have the canonical heavy-chain immunoglobulin (Ig) gene repertoire of eutherian mammals.Dev. Comp. Immunol. 2011; 35: 273-284Crossref PubMed Scopus (13) Google ScholarAbsent Open table in a new tab However, important differences between the adaptive immune response of bats and other mammals have been observed. In the humoral response, bats have an antibody repertoire as diverse as those of humans and mice. At least 23 genes for immunoglobulin (Ig) VH and the λ and κ loci of Ig VL have been identified in P. alecto, and diverse Ig DH and Ig JH elements and the λ locus of Ig VL have been found in M. lucifugus and other microbats (Table 1). However, the amino acid sequence composition of the antigen-binding site (CDR3 region) of the expressed VH region in bats is different from those of humans and mice with a higher proportion of arginine and alanine residues, and lower proportion of tyrosine residues, and thus possibly a lower poly-reactivity [20Baker M.L. et al.Immunoglobulin heavy chain diversity in Pteropid bats: evidence for a diverse and highly specific antigen binding repertoire.Immunogenetics. 2010; 62: 173-184Crossref PubMed Scopus (16) Google Scholar]. Such antibodies have lower avidity and form a weaker association with antigens. Furthermore, the primary antibody response may be delayed from reaching a peak and the secondary antibody response may also be slow or delayed in bats. In adverse environmental situations such as prolonged exposure to low temperature at 8 °C simulating the state of hibernation, bats may temporarily fail to mount an antibody response but resume this ability 1 week after transferring to 24 °C. Importantly, bats may clear viral infection even in the absence of neutralizing antibody by other unknown mechanisms [14Baker M.L. et al.Antiviral immune responses of bats: a review.Zoonoses Public Health. 2013; 60: 104-116Crossref PubMed Scopus (6) Google Scholar]. Bats appeared asymptomatic during bat–SARS-CoV infection despite a high viral load in their anal swabs with absent or low serum neutralizing antibody although their body weights were generally lower than the uninfected bats [4Lau S.K. et al.Severe acute respiratory syndrome coronavirus-like virus in Chinese horseshoe bats.Proc. Natl. Acad. Sci. U.S.A. 2005; 102: 14040-14045Crossref PubMed Scopus (427) Google Scholar, 21Lau S.K. et al.Ecoepidemiology and complete genome comparison of different strains of severe acute respiratory syndrome-related Rhinolophus bat coronavirus in China reveal bats as a reservoir for acute, self-limiting infection that allows recombination events.J. Virol. 2010; 84: 2808-2819Crossref PubMed Scopus (43) Google Scholar]. As for a cell-mediated immune response, those of bats are generally slower to peak after stimulation by T-cell mitogens. Environmental changes such as roost ecology and physiological alterations such as pregnancy also affect cell-mediated immunity of bats [14Baker M.L. et al.Antiviral immune responses of bats: a review.Zoonoses Public Health. 2013; 60: 104-116Crossref PubMed Scopus (6) Google Scholar]. Further studies should be performed to ascertain if these quantitative and qualitative differences in the adaptive immune response of bats may account for their unique interaction with viruses leading to asymptomatic infection and viral persistence. The discovery of influenza virus in bats challenges the notion that aquatic birds are the only source of all influenza A gene segments [9Wong S. et al.Bats as a continuing source of emerging infections in humans.Rev. Med. Virol. 2007; 17: 67-91Crossref PubMed Scopus (104) Google Scholar, 10Tong S. et al.A distinct lineage of influenza A virus from bats.Proc. Natl. Acad. Sci. U.S.A. 2012; 109: 4269-4274Crossref PubMed Scopus (178) Google Scholar]. The recently discovered H17 influenza virus from Guatemala bats is unique in that all eight gene segments appear to be distinct from any known influenza A gene segments [10Tong S. et al.A distinct lineage of influenza A virus from bats.Proc. Natl. Acad. Sci. U.S.A. 2012; 109: 4269-4274Crossref PubMed Scopus (178) Google Scholar]. The hemagglutinin (HA) of this virus has unique structural features and exhibits receptor binding and fusogenic activities that are distinct from its counterparts in other influenza viruses [22Zhu X. et al.Hemagglutinin homologue from H17N10 bat influenza virus exhibits divergent receptor-binding and pH-dependent fusion activities.Proc. Natl. Acad. Sci. U.S.A. 2013; 110: 1458-1463Crossref PubMed Scopus (17) Google Scholar]. Likewise, the neuraminidase (NA) of this virus, called N10, is phylogenetically distinct from the NAs of all influenza A and B viruses. The N10 is also structurally distinct with no enzymatic activity [23Zhu X. et al.Crystal structures of two subtype N10 neuraminidase-like proteins from bat influenza A viruses reveal a diverged putative active site.Proc. Natl. Acad. Sci. U.S.A. 2012; 109: 18903-18908Crossref PubMed Scopus (16) Google Scholar]. Additional surveillance is necessary to understand the ecology of influenza viruses in bats. The high biodiversity of birds (over 10 000 species) allows avian influenza viruses to evolve in different host environments. Birds living near wetlands or aquatic environments, especially in the orders Anseriformes (aquatic waterfowls) and Charadriiformes (shorebirds), are considered to be the sources or gene pool for all influenza viruses [24Olsen B. et al.Global patterns of influenza a virus in wild birds.Science. 2006; 312: 384-388Crossref PubMed Scopus (648) Google Scholar, 25Webster R.G. et al.Evolution and ecology of influenza A viruses.Microbiol. Rev. 1992; 56: 152-179Crossref PubMed Google Scholar]. Waterfowl feed on submerged water plants in marsh water that are readily contam