Department of Virology1 and Department of Veterinary Medicine2, Istituto Superiore di Sanità, Rome, Italy
Istituto Zooprofilattico Sperimentale delle Venezie, Padua, Italy3
National Centre for Avian Influenza Viruses, Avian Pathology Section, University of Naples, Naples, Italy4
European Reference Laboratory for Avian Influenza Viruses, Central Veterinary Laboratory, New Haw, Addlestone, UK5
Department of Virology and Molecular Biology, St Jude Childrens Research Hospital, 332 N. Lauderdale, PO Box 318, Memphis, TN 38105, USA6
Author for correspondence: Robert Webster. Fax +1 901 523 2622. e-mail robert.webster{at}stjude.org
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Abstract |
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Introduction |
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Avian influenza viruses are rarely transmitted directly to humans. A notable exception is the avian virus that caused the 1997 influenza outbreak in Hong Kong (Yuen et al., 1998 ; de Jong et al., 1997
). All of the virus isolates were closely related to avian H5N1 strains that had previously caused outbreaks of influenza-like illness among poultry flocks in the new territories of Hong Kong, SAR, China (Shortridge et al., 1998
; Suarez et al., 1998
; Subbarao et al., 1998
). The transmission of an avian virus directly to humans raised serious concern over the emergence of an influenza pandemic.
From October 1997 to January 1998, eight outbreaks of highly pathogenic avian influenza (HPAI) occurred in poultry in northeastern Italy (Capua et al., 1999 ). An epidemiological survey was conducted to control the spread of avian influenza to neighbouring regions and to identify the origin and mechanism of transmission of infection among flocks. Here we present the results of antigenic and molecular characterizations of the isolated highly pathogenic and nonpathogenic H5 influenza viruses and the results of our comparison of these viruses with other avian and avian-like influenza viruses, including those isolated in Hong Kong.
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Methods |
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Viral HAs were characterized by haemagglutination inhibition (HI) assays with antisera to influenza virus (US Department of Health and Human Services, 1982 ). A more detailed antigenic analysis was performed with polyclonal antisera to representative H5 strains and a panel of monoclonal antibodies (MAbs) to the HA of Ck/PA/1370/83 (H5N2). These MAbs and antisera were prepared as previously described (Kawaoka et al., 1987
).
Ferret antiserum to HK/156/97 (H5N1) was kindly provided by the Centers for Disease Control (Atlanta, GA, USA). Viral NAs were characterized by using the NA inhibition test (US Department of Health and Human Services, 1982 ).
Pathogenicity testing in chickens.
The pathogenicity of the Ck/It/9097/97 virus was determined in 6-week-old chickens at the European Union (EU) Reference Laboratory according to the IVPI test described in the EU Directive (CEC, 1992 ). In this test, an IVPI of 3·00 indicates that all birds died within 24 h of infection, whereas an index of 0·00 indicates the absence of any clinical signs of illness in any birds during the 10 day observation period. The pathogenic potential of the isolate was also evaluated by sequencing the amino acids at the HA cleavage site (Senne et al., 1996
).
Serological analysis of human sera.
Sera obtained from people who had been in close contact with sick or dead birds were collected at the end of the outbreaks. Since the HI test is not considered a sensitive method for detecting antibodies against avian influenza viruses in humans (Lu et al., 1982 ; Zhou et al., 1996
), we analysed human sera by the new microneutralization test previously described (Rowe et al., 1999
), using the first avian H5N2 (Ck/lt/312/97) virus isolate as reference antigen. Neutralizing antibody titres below 80 were considered negative by Rowe et al. (1999)
.
The human sera were also tested by the single radial haemolysis (SRH) assay, using RT/DE/253/91 (H5N2) as a reference virus. This assay was performed by standard methods (Mancini et al., 1984 ) using chromic chloride-treated turkey erythrocytes, with minor modifications including 15% turkey erythrocyte suspension and adsorption of sera with both human H1N1 and H3N2 strains prior to testing with H5 antigens (J. Wood, personal communications).
Sequence and phylogenetic analyses.
Four influenza virus strains (Ck/It/312/97, Gf/lt/330/97, Ck/It/365/97, Ck/It/367/97) isolated in different areas (Fig. 1) and at different times were selected as representatives of the highly pathogenic virus isolates, and their HA genes were characterized by sequence analysis. The nonpathogenic Ck/It/9097/97 strain was also included in the molecular study. Viral RNA was extracted and amplified by RTPCR, as described previously (Campitelli et al., 1997
). The PCR products were subjected to cycle sequencing with an ABI dye terminator sequencing system.
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Accession numbers of the genes sequenced in this report are AF194169, AF19499092 and AF31964451.
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Results |
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To evaluate the antigenic characteristics of the isolates and to determine the diversity of their HAs, we expanded the analysis by using a panel of MAbs and goat, chicken and ferret polyclonal antisera to the H5 HA (Table 1). Because MAb CP46 can detect the presence of a carbohydrate at residue 158 near the receptor binding site on HA, this MAb has been used to distinguish two groups of Hong Kong H5N1 influenza virus isolates: one group has a carbohydrate at residue 158, and the other group lacks the carbohydrate (Shortridge et al., 1998
). Therefore, the HK/156/97 and Ck/HK/W162/97 (H5N1) viruses were included in the analysis as representatives of the antigenic group containing the carbohydrate, whereas Ck/HK/258/97 and HK/483/97 viruses were used as serological prototypes of the group lacking the carbohydrate. We also included two nonpathogenic H5 viruses, Ck/Chia/15224/97 (H5N2) and Dk/Sing-Q/F119-3/97 (H5N3).
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One of the nonpathogenic Asian viruses (Dk/Sing-Q/F119-3/97) reacted with all but one of the MAbs and with the reference polyclonal antisera (Table 1). The Italian Ck/It/9097/97 strain reacted with all of the antisera, but the titres of the reacting antibodies were low. This nonpathogenic strain did not react at all with CP24 MAb; thus, the antigenic pattern of the strain was distinguishable from that of the H5N2 group. Sequence analysis showed that HA1 of the Ck/It/9097/97 (H5N9) strain lacked the potential glycosylation site at amino acid residue 158; this characteristic is identical to that of the Italian H5N2 strains.
Sequence and evolutionary analysis of the H5 HAs
We sequenced the Italian H5 HA genes to investigate their relationships with the Hong Kong H5 HA genes and to test the hypothesis that the HAs of the highly pathogenic H5N2 viruses were derived from the HA of the nonpathogenic H5N9 virus after the acquisition of basic amino acids at the connecting peptide. This mechanism has already been observed in previous HPAI outbreaks (Horimoto et al., 1995 ). Table 2
reports the HA1 and HA2 nucleotide sequence identities among four highly pathogenic Italian isolates, the nonpathogenic strain and a group of antigenically related H5 viruses of the Eurasian avian lineage. The HA genes of the highly pathogenic Italian viruses were almost identical to each other (99·4% to 99·9% identity). The percentages of identities between the HA genes of the highly pathogenic H5N2 viruses and that of Ck/It/9097/97 (H5N9) were appreciably lower (94·3% to 94·6%). Comparison of the HA sequences of the Italian strains with the H5 HA sequences available in GenBank revealed that the Italian strains were most closely related to Ty/Eng/50-92/91, although the degree of identity differed for the highly pathogenic (95·4% to 95·6%) and nonpathogenic Italian strains (97·1%). Similar results were obtained in comparisons with HK/156/97 HA: the identities with HA genes of the highly pathogenic strains ranged from 93·7% to 93·9%, whereas the identity with the HA genes of the nonpathogenic strain was 92·2%.
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Regardless of their pathogenicity, the sequenced viruses showed an identical pattern of six potential glycosylation sites at positions 11, 23, 165, 286, 482 and 539. Only one glycosylation site (residue 165) appears to be located on the HA1 globular head of the highly pathogenic and nonpathogenic viruses isolated in Italy. This finding is similar to that for nonpathogenic H5 viruses isolated from aquatic birds (Matrosovich et al., 1999 ). Ty/Eng/50-92/91 virus, the most closely related, highly pathogenic strain in the Eurasian avian lineage, lacks a glycosylation site at the same position.
Evolutionary analysis of NP and NS genes
We analysed the nucleotide sequences of the genes encoding the nucleoprotein (NP) and the nonstructural (NS) protein to evaluate the phylogenetic relationships of the Italian H5 viruses as indicated by genes encoding internal proteins (Fig. 3). The Eurasian avian terminal branches are shown. Results of the phylogenetic analysis of the NS genes showed that the H5N2 Italian virus genes are clearly separated from those of the H5N9 strain; this separation is similar to that seen in the tree created from the HA gene data. The NS genes of the Italian isolates form a subgroup with that of Dk/HK/y439/97 (an H9N2 avian virus representing one of the three H9N2 lineages cocirculating in southern China), distinguishable from the Hong Kong H5N1 influenza viruses and from other H9N2 strains considered as possible donors of the internal protein genes to the HK H5N1 virus group (Guan et al., 1999
). In contrast, the NP phylogenetic tree indicated a closer genetic relationship between the highly pathogenic and the nonpathogenic Italian viruses, which clustered on the same branch, again together with Dk/HK/y439/97 (Fig. 3
).
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Lack of evidence for transmission of Italian avian H5N2 viruses to humans
During the outbreak of highly pathogenic H5N2 avian influenza in Italy, there was no evidence of clinical respiratory infection of poultry workers or other humans working with the viruses. Despite the lack of even mild infection, analysis of a small number of human samples was done. Throat samples collected from intensively exposed poultry workers from the last outbreak failed to yield influenza viruses after three blind passages in chicken embryos and primary chick embryo fibroblast cells. Detailed serological analysis of 32 human sera from poultry workers and laboratory technologists (data not shown) by microneutralization and SRH gave negative results in both tests. Therefore, no evidence of anti-H5 antibodies in human sera was observed.
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Discussion |
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Because of the epidemiological features of the Italian avian influenza outbreaks, it is tempting to speculate that the pathogenic Italian viruses derived their HAs from the nonpathogenic Ck/It/9097/97 strain after polymerase stuttering (García et al., 1996 ) or acquisition of a mutation at the cleavage site (Röhm et al., 1995
). However, neither the nucleotide homology nor the phylogenetic relationship between these viruses supports such conjecture. Rather, the viruses in Italian poultry represent a distinct group that has a sister-group relationship to the 1997 H5N1 chicken and human isolates from Hong Kong. The nonpathogenic strain is more closely related to Ty/Eng/50-92/91 (97·1% homology) than to the pathogenic Italian strains (maximum identity 94·6%). These findings indicate the cocirculation of at least two separate H5 sublineages in Italian poultry.
Both the pathogenic and nonpathogenic Italian viruses had only one glycosylation site (residue 165) within the globular head region of the HA1 subunit. Recently, it has been postulated that the presence of more than one glycosylation site in this region and of deletions in the NA stalk is characteristic of several chicken H5 and H7 influenza viruses; these features are absent in viruses from aquatic birds (Matrosovich et al., 1999 ). However, the pathogenic relevance of this finding is still unclear, because several other highly virulent strains, including those in the present study, do not possess more glycosylation sites than do nonpathogenic viruses from wild aquatic birds. Future sequence analysis of the NA gene of the Italian viruses may provide additional clues about the interrelationships between the NA and the HA surface antigens.
Phylogenetic analysis of the NP genes of the Italian H5 viruses revealed that unlike the NS and HA genes, the NP genes of the two H5 sublineages are highly similar. Therefore, genetic exchange between the NP genes of viruses from the two sublineages (or their precursor virus) appears to have been possible. Also, all of the Italian NP genes are very similar to the NP gene of Dk/HK/y439/97 virus, a Hong Kong H9N2 virus isolated in 1997. Because migratory bird flyways cross northeastern Italy and because contact between poultry on the farms and migratory waterfowl may have occurred, the results of our NP genetic analysis suggest that a precursor virus with a similar NP gene was introduced by common waterfowl into Italian poultry. NP is part of the replicase complex of influenza viruses; therefore, it will be important to elucidate the origin of the rest of the genome, especially the polymerase genes.
Investigations of people who had been highly exposed to infected poultry showed no evidence of virus transmission to humans; the absence of even mild clinical signs of disease in humans was supported by the results of serological studies performed with a limited number of serum samples from persons directly involved in the Italian poultry industry in this region. Although the molecular determinants responsible for zoonotic transfer of H5N1 influenza viruses to humans in Hong Kong are largely unresolved, evidence suggests that the PB2, PA, NP and M genes are involved (Zhou et al., 1999 ).
Recent studies have shown that the Hong Kong H5N1 influenza viruses were reassortants that derived their HA gene from Goose/Guangdong/1/96 (H5N1) virus, whereas all of their genes encoding internal proteins were closely related to those of an H9N2 strain, Quail/HK/G1/97 (Xu et al., 1999 ; Guan et al., 1999
). These findings emphasize the role played by poultry species in the generation of new influenza virus strains with altered virulence and host range. Our results underscore the importance of increased surveillance of domestic birds to detect circulating nonpathogenic avian influenza viruses that could contribute to the emergence of influenza viruses potentially pathogenic in both birds and mammals.
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Acknowledgments |
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This research was partly funded by grant AI29680 and contract AI95357 to Dr R. G. Webster from the National Institute of Allergy and Infectious Diseases, by Cancer Center Support CORE grant CA-21765 and by the American LebaneseSyrian Associated Charities (ALSAC).
This paper was also supported by grant 1% D.Lgs 502-92 to Dr I. Donatelli.
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Footnotes |
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References |
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Received 29 August 2000;
accepted 22 November 2000.
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