title: Unexpected Interfarm Transmission Dynamics during a Highly Pathogenic Avian Influenza Epidemic.
abstract: Next-generation sequencing technology is now being increasingly applied to study the within- and between-host population dynamics of viruses. However, information on avian influenza virus evolution and transmission during a naturally occurring epidemic is still limited. Here, we use deep-sequencing data obtained from clinical samples collected from five industrial holdings and a backyard farm infected during the 2013 highly pathogenic avian influenza (HPAI) H7N7 epidemic in Italy to unravel (i) the epidemic virus population diversity, (ii) the evolution of virus pathogenicity, and (iii) the pathways of viral transmission between different holdings and sheds. We show a high level of genetic diversity of the HPAI H7N7 viruses within a single farm as a consequence of separate bottlenecks and founder effects. In particular, we identified the cocirculation in the index case of two viral strains showing a different insertion at the hemagglutinin cleavage site, as well as nine nucleotide differences at the consensus level and 92 minority variants. To assess interfarm transmission, we combined epidemiological and genetic data and identified the index case as the major source of the virus, suggesting the spread of different viral haplotypes from the index farm to the other industrial holdings, probably at different time points. Our results revealed interfarm transmission dynamics that the epidemiological data alone could not unravel and demonstrated that delay in the disease detection and stamping out was the major cause of the emergence and the spread of the HPAI strain.
  
  
  
  The within- and between-host evolutionary dynamics of a highly pathogenic avian influenza (HPAI) strain during a naturally occurring epidemic is currently poorly understood. Here, we perform for the first time an in-depth sequence analysis of all the samples collected during a HPAI epidemic and demonstrate the importance to complement outbreak investigations with genetic data to reconstruct the transmission dynamics of the viruses and to evaluate the within- and between-farm genetic diversity of the viral population. We show that the evolutionary transition from the low pathogenic form to the highly pathogenic form occurred within the first infected flock, where we identified haplotypes with hemagglutinin cleavage site of different lengths. We also identify the index case as the major source of virus, indicating that prompt application of depopulation measures is essential to limit virus spread to other farms.
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy afusaro@izsvenezie.it.
authors.forename: Alice
authors.initials: A
authors.lastname: Fusaro
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Tassoni
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Adelaide
authors.initials: A
authors.lastname: Milani
authors.affiliation: MRC-University of Glasgow Center for Virus Research, Glasgow, United Kingdom.
authors.forename: Joseph
authors.initials: J
authors.lastname: Hughes
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Annalisa
authors.initials: A
authors.lastname: Salviato
authors.affiliation: MRC-University of Glasgow Center for Virus Research, Glasgow, United Kingdom.
authors.forename: Pablo R
authors.identifier: http://orcid.org/0000-0002-4352-394X
authors.initials: PR
authors.lastname: Murcia
authors.affiliation: Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna, Brescia, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Massi
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Gianpiero
authors.initials: G
authors.lastname: Zamperin
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Lebana
authors.initials: L
authors.lastname: Bonfanti
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Marangon
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Cattoli
authors.affiliation: Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy.
authors.forename: Isabella
authors.initials: I
authors.lastname: Monne
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002645:Chickens / Q000235:genetics / Q000821:virology*; D058872:Epidemics / Q000662:veterinary*; D014644:Genetic Variation / Q000235:genetics*; D059014:High-Throughput Nucleotide Sequencing; D053126:Influenza A Virus, H7N7 Subtype / Q000235:genetics*; D005585:Influenza in Birds / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D007558:Italy / Q000453:epidemiology; D010802:Phylogeny