Generation of seal influenza virus variants pathogenic for chickens, because of hemagglutinin cleavage site changes

Hemagglutinin Subtype Specificity and Mechanisms of Highly Pathogenic Avian Influenza Virus Genesis

Highly Pathogenic Avian Influenza Viruses (HPAIVs) arise from low pathogenic precursors following spillover from wild waterfowl into poultry populations. The main virulence determinant of HPAIVs is the presence of a multi-basic cleavage site (MBCS) in the hemagglutinin (HA) glycoprotein. The MBCS allows for HA cleavage and, consequently, activation by ubiquitous proteases, which results in systemic dissemination in terrestrial poultry. Since 1959, 51 independent MBCS acquisition events have been documented, virtually all in HA from the H5 and H7 subtypes. In the present article, data from natural LPAIV to HPAIV conversions and experimental in vitro and in vivo studies were reviewed in order to compile recent advances in understanding HA cleavage efficiency, protease usage, and MBCS acquisition mechanisms. Finally, recent hypotheses that might explain the unique predisposition of the H5 and H7 HA sequences to obtain an MBCS in nature are discussed.

Training the domestic ferret to discriminate odors associated with wildlife disease

Recent avian influenza infection outbreaks have resulted in global biosecurity and economic concerns. Mallards are asymptomatic for the disease and can potentially spread AI along migratory bird flyways. In a previous study, trained mice correctly discriminated the health status of individual ducks on the basis of fecal odors when feces from post-infection periods were paired with feces from pre-infection periods. Chemical analyses indicated that avian influenza infection was associated with a marked increase of acetoin (3-hydroxy-2-butanone) in feces. In the current study, domesticated male ferrets (Mustela putorius furo) were trained to display a specific conditioned response (i.e. active scratch alert) in response to a marked increase of acetoin in a presentation of an acetoin:1-octen-3-ol solution. Ferrets rapidly generalized this learned response to the odor of irradiated feces from avian influenza infected mallards. These results suggest that a trained mammalian biosensor could be employed in an avian influenza surveillance program.

Biodetection of a specific odor signature in mallard feces associated with infection by low pathogenic avian influenza A virus

Outbreaks of avian influenza virus (AIV) infection included the spread of highly pathogenic AIV in commercial poultry and backyard flocks in the spring of 2015. This resulted in estimated losses of more than $8.5 million from federal government expenditures, $1.6 billion from direct losses to produces arising from destroyed turkey and chicken egg production, and economy-wide indirect costs of $3.3 billion from impacts on retailers and the food service industries. Additionally, these outbreaks resulted in the death or depopulation of nearly 50 million domestic birds. Domesticated male ferrets (Mustela putorius furo) were trained to display a specific conditioned behavior (i.e. active scratch alert) in response to feces from AIV-infected mallards in comparison to feces from healthy ducks. In order to establish that ferrets were identifying samples based on odors associated with infection, additional experiments controlled for potentially confounding effects, such as: individual duck identity, housing and feed, inoculation concentration, and day of sample collection (post-infection). A final experiment revealed that trained ferrets could detect AIV infection status even in the presence of samples from mallards inoculated with Newcastle disease virus or infectious laryngotracheitis virus. These results indicate that mammalian biodetectors are capable of discriminating the specific odors emitted from the feces of non-infected versus AIV infected mallards, suggesting that the health status of waterfowl can be evaluated non-invasively for AIV infection via monitoring of volatile fecal metabolites. Furthermore, in situ monitoring using trained biodetectors may be an effective tool for assessing population health.

Insertions of codons encoding basic amino acids in H7 hemagglutinins of influenza A viruses occur by recombination with RNA at hotspots near snoRNA binding sites

The presence of multiple basic amino acids in the protease cleavage site of the hemagglutinin (HA) protein is the main molecular determinant of virulence of highly pathogenic avian influenza (HPAI) viruses. Recombination of HA RNA with other RNA molecules of host or virus origin is a dominant mechanism of multibasic cleavage site (MBCS) acquisition for H7 subtype HA. Using alignments of HA RNA sequences from documented cases of MBCS insertion due to recombination, we show that such recombination with host RNAs is most likely to occur at particular hotspots in ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), and viral RNAs. The locations of these hotspots in highly abundant RNAs indicate that RNA recombination is facilitated by the binding of small nucleolar RNA (snoRNA) near the recombination points.

The Emergence of H7N7 Highly Pathogenic Avian Influenza Virus from Low Pathogenicity Avian Influenza Virus Using an in ovo Embryo Culture Model

Outbreaks of highly pathogenic avian influenza virus (HPAIV) often result in the infection of millions of poultry, causing up to 100% mortality. HPAIV has been shown to emerge from low pathogenicity avian influenza virus (LPAIV) in field outbreaks. Direct evidence for the emergence of H7N7 HPAIV from a LPAIV precursor with a rare di-basic cleavage site (DBCS) was identified in the UK in 2008. The DBCS contained an additional basic amino acid compared to commonly circulating LPAIVs that harbor a single-basic amino acid at the cleavage site (SBCS). Using reverse genetics, outbreak HPAIVs were rescued with a DBCS (H7N7_DB), as seen in the LPAIV precursor or an SBCS representative of common H7 LPAIVs (H7N7_SB). Passage of H7N7_DB in chicken embryo tissues showed spontaneous evolution to a HPAIV. In contrast, deep sequencing of extracts from embryo tissues in which H7N7_SB was serially passaged showed retention of the LPAIV genotype. Thus, in chicken embryos, an H7N7 virus containing a DBCS appears naturally unstable, enabling rapid evolution to HPAIV. Evaluation in embryo tissue presents a useful approach to study AIV evolution and allows a laboratory-based dissection of molecular mechanisms behind the emergence of HPAIV.

A Brief Introduction to Influenza A Virus in Marine Mammals

Influenza A infection has been detected in marine mammals going back to 1975, with additional unconfirmed outbreaks as far back as 1931. Over the past forty years, infectious virus has been recovered on ten separate occasions from both pinnipeds (harbor seal, elephant seal, and Caspian seal) and cetaceans (striped whale and pilot whale). Recovered viruses have spanned a range of subtypes (H1, H3, H4, H7, H10, and H13) and, in all but H1N1, show strong evidence for deriving directly from avian sources. To date, there have been five unusual mortality events directly attributed to influenza A virus; these have primarily occurred in harbor seals in the Northeastern United States, with the most recent occurring in harbor seals in the North Sea.There are numerous additional reports wherein influenza A virus has indirectly been identified in marine mammals; these include serosurveillance efforts that have detected influenza A- and B-specific antibodies in marine mammals spanning the globe and the detection of viral RNA in both active and opportunistic surveillance in the Northwest Atlantic. For viral detection and recovery, nasal, rectal, and conjunctival swabs have been employed in pinnipeds, while blowhole, nasal, and rectal swabs have been employed in cetaceans. In the case of deceased animals, virus has also been detected in tissue. Surveillance has historically been somewhat limited, relying largely upon opportunistic sampling of stranded or bycaught animals and primarily occurring in response to a mortality event. A handful of active surveillance projects have shown that influenza may be more endemic in marine mammals than previously appreciated, though live virus is difficult to recover. Surveillance efforts are hindered by permitting and logistical challenges, the absence of reagents and methodology optimized for nonavian wild hosts, and low concentration of virus recovered from asymptomatic animals. Despite these challenges, a growing body of evidence suggests that marine mammals are an important wild reservoir of influenza and may contribute to mammalian adaptation of avian variants.

Phosphoproteome Analysis of Cells Infected with Adapted and Nonadapted Influenza A Virus Reveals Novel Pro- and Antiviral Signaling Networks

Influenza A viruses (IAVs) quickly adapt to new environments and are well known to cross species barriers. To reveal a molecular basis for these phenomena, we compared the Ser/Thr and Tyr phosphoproteomes of murine lung epithelial cells early and late after infection with mouse-adapted SC35M virus or its nonadapted SC35 counterpart. With this analysis we identified a large set of upregulated Ser/Thr phosphorylations common to both viral genotypes, while Tyr phosphorylations showed little overlap. Most of the proteins undergoing massive changes of phosphorylation in response to both viruses regulate chromatin structure, RNA metabolism, and cell adhesion, including a focal adhesion kinase (FAK)-regulated network mediating the regulation of actin dynamics. IAV also affected phosphorylation of activation loops of 37 protein kinases, including FAK and several phosphatases, many of which were not previously implicated in influenza virus infection. Inhibition of FAK proved its contribution to IAV infection. Novel phosphorylation sites were found on IAV-encoded proteins, and the functional analysis of selected phosphorylation sites showed that they either support (NA Ser178) or inhibit (PB1 Thr223) virus propagation. Together, these data allow novel insights into IAV-triggered regulatory phosphorylation circuits and signaling networks.IMPORTANCE Infection with IAVs leads to the induction of complex signaling cascades, which apparently serve two opposing functions. On the one hand, the virus highjacks cellular signaling cascades in order to support its propagation; on the other hand, the host cell triggers antiviral signaling networks. Here we focused on IAV-triggered phosphorylation events in a systematic fashion by deep sequencing of the phosphoproteomes. This study revealed a plethora of newly phosphorylated proteins. We also identified 37 protein kinases and a range of phosphatases that are activated or inactivated following IAV infection. Moreover, we identified new phosphorylation sites on IAV-encoded proteins. Some of these phosphorylations support the enzymatic function of viral components, while other phosphorylations are inhibitory, as exemplified by PB1 Thr223 modification. Our global characterization of IAV-triggered patterns of phospho-proteins provides a rich resource to further understand host responses to infection at the level of phosphorylation-dependent signaling networks.

A viral race for primacy: co-infection of a natural pair of low and highly pathogenic H7N7 avian influenza viruses in chickens and embryonated chicken eggs

Highly pathogenic avian influenza virus (HPAIV) infection in poultry caused devastating mortality and economic losses. HPAIV of subtypes H5 and H7 emerge from precursor viruses of low pathogenicity (LP) by spontaneous mutation associated with a shift in the susceptibility of the endoproteolytic cleavage site of the viral hemagglutinin protein from trypsin- to furin-like proteases. A recently described natural pair of LP/HP H7N7 viruses derived from two spatio-temporally linked outbreaks in layer chickens was used to study how a minority of mutated HP virions after de novo generation in a single host might gain primacy. Co-infection experiments in embryonated eggs and in chickens were conducted to investigate amplification, spread and transmissionof HPAIV within a poultry population that experiences concurrent infection by an antigenically identical LP precursor virus. Simultaneous LPAIV co-infection (inoculum dose of 106 egg-infectious dose 50% endpoint (EID50)/0.5 mL) withincreasing titers of HPAIV from 101 to 105.7 EID50/0.5 mL) had a significant impeding impact on HP H7 replication, viral excretion kinetics, clinical signs and histopathological lesions (in vivo) and on embryo mortality (in ovo). LP/HP co-infected chickens required a hundredfold higher virus dose (HPAIV inoculum of 105 EID50) compared to HPAIV mono-infection (HPAIV inoculum of 103 EID50) to develop overt clinical signs, mortality and virus spread to uninfected sentinels. Escape and spread of HP phenotypes after de novo generation in an index host may therefore be highly precarious due to significant competition with co-circulating LP precursor virus.

Emergence and Adaptation of a Novel Highly Pathogenic H7N9 Influenza Virus in Birds and Humans from a 2013 Human-Infecting Low-Pathogenic Ancestor

Since its emergence in 2013, the H7N9 low-pathogenic avian influenza virus (LPAIV) has been circulating in domestic poultry in China, causing five waves of human infections. A novel H7N9 highly pathogenic avian influenza virus (HPAIV) variant possessing multiple basic amino acids at the cleavage site of the hemagglutinin (HA) protein was first reported in two cases of human infection in January 2017. More seriously, those novel H7N9 HPAIV variants have been transmitted and caused outbreaks on poultry farms in eight provinces in China. Herein, we demonstrate the presence of three different amino acid motifs at the cleavage sites of these HPAIV variants which were isolated from chickens and humans and likely evolved from the preexisting LPAIVs. Animal experiments showed that these novel H7N9 HPAIV variants are both highly pathogenic in chickens and lethal to mice. Notably, human-origin viruses were more pathogenic in mice than avian viruses, and the mutations in the PB2 gene associated with adaptation to mammals (E627K, A588V, and D701N) were identified by next-generation sequencing (NGS) and Sanger sequencing of the isolates from infected mice. No polymorphisms in the key amino acid substitutions of PB2 and HA in isolates from infected chicken lungs were detected by NGS. In sum, these results highlight the high degree of pathogenicity and the valid transmissibility of this new H7N9 variant in chickens and the quick adaptation of this new H7N9 variant to mammals, so the risk should be evaluated and more attention should be paid to this variant.IMPORTANCE Due to the recent increased numbers of zoonotic infections in poultry and persistent human infections in China, influenza A(H7N9) virus has remained a public health threat. Most of the influenza A(H7N9) viruses reported previously have been of low pathogenicity. Now, these novel H7N9 HPAIV variants have caused human infections in three provinces and outbreaks on poultry farms in eight provinces in China. We analyzed the molecular features and compared the relative characteristics of one H7N9 LPAIV and two H7N9 HPAIVs isolated from chickens and two human-origin H7N9 HPAIVs in chicken and mouse models. We found that all HPAIVs both are highly pathogenic and have valid transmissibility in chickens. Strikingly, the human-origin viruses were more highly pathogenic than the avian-origin viruses in mice, and dynamic mutations were confirmed by NGS and Sanger sequencing. Our findings offer important insight into the origin, adaptation, pathogenicity, and transmissibility of these viruses to both poultry and mammals.

VIRULENCE

Why do parasites harm their hosts? Intuition suggests that parasites should evolve to be benign whenever the host is needed for transmission. Yet a growing theoretical literature offers several models to explain why natural selection may favor virulent parasites over avirulent ones. This perspective first organizes these models into a simple framework and then evaluates the empirical evidence for and against the models. There is relatively scant evidence to support any of the models rigorously, and indeed, there are only a few unequivocal observations of virulence actually evolving in parasite populations. These shortcomings are surmountable, however, and empirical models of host-parasite interactions have been developed for many kinds of pathogens so that the relevant data could be acquired in the near future.

Evolution of H5 highly pathogenic avian influenza: sequence data indicate stepwise changes in the cleavage site

The genetic composition of an H5 subtype hemagglutinin gene quasispecies, obtained from ostrich tissues that had been infected with H5 subtype influenza virus was analysed using a next generation sequencing approach. The first evidence for the reiterative copying of a poly (U) stretch in the connecting peptide region in the haemagglutinin cleavage site (HACS) by the viral RNA-dependent RNA polymerase (RdRp) is provided. Multiple non-consensus species of RNA were detected in the infected host, corresponding to likely intermediate sequences between the putative low pathogenic precursor nucleotide sequence of the H5 influenza strain and the highly pathogenic avian influenza virus gene sequence. In silico analysis of the identified RNA sequences predicted that the intermediary H5 sequence PQREKRGLF plays an important role in subsequent mutational events that relocate the HACS coding region from stable base-paired RNA regions to a single-stranded bulge, thereby priming the connecting peptide coding region for RdRp slippage.

The Influenza A Virus Genotype Determines the Antiviral Function of NF-κB

The role of NF-κB in influenza A virus (IAV) infection does not reveal a coherent picture, as pro- and also antiviral functions of this transcription factor have been described. To address this issue, we used clustered regularly interspaced short palindromic repeat with Cas9 (CRISPR-Cas9)-mediated genome engineering to generate murine MLE-15 cells lacking two essential components of the NF-κB pathway. Cells devoid of either the central NF-κB essential modulator (NEMO) scaffold protein and thus defective in IκB kinase (IKK) activation or cells not expressing the NF-κB DNA-binding and transactivation subunit p65 were tested for propagation of the SC35 virus, which has an avian host range, and its mouse-adapted variant, SC35M. While NF-κB was not relevant for replication of SC35M, the absence of NF-κB activity increased replication of the nonadapted SC35 virus. This antiviral effect of NF-κB was most prominent upon infection of cells with low virus titers as they usually occur during the initiation phase of IAV infection. The defect in NF-κB signaling resulted in diminished IAV-triggered phosphorylation of interferon regulatory factor 3 (IRF3) and expression of the antiviral beta interferon (IFN-β) gene. To identify the viral proteins responsible for NF-κB dependency, reassortant viruses were generated by reverse genetics. SC35 viruses containing the SC35M segment encoding neuraminidase (NA) were completely inert to the inhibitory effect of NF-κB, emphasizing the importance of the viral genotype for susceptibility to the antiviral functions of NF-κB.

IMPORTANCE

This study addresses two different issues. First, we investigated the role of the host cell transcription factor NF-κB in IAV replication by genetic manipulation of IAVs by reverse genetics combined with targeted genome engineering of host cells using CRISPR-Cas9. The analysis of these two highly defined genetic systems indicated that the IAV genotype can influence whether NF-κB displays an antiviral function and thus might in part explain incoherent results from the literature. Second, we found that perturbation of NF-κB function greatly improved the growth of a nonadapted IAV, suggesting that NF-κB may contribute to the maintenance of the host species barrier.

Identification of specific residues in avian influenza A virus NS1 that enhance viral replication and pathogenicity in mammalian systems

Reassortment of their segmented genomes allows influenza A viruses (IAV) to gain new characteristics, which potentially enable them to cross the species barrier and infect new hosts. Improved replication was observed for reassortants of the strictly avian IAV A/FPV/Rostock/34 (FPV, H7N1) containing the NS segment from A/Goose/Guangdong/1/1996 (GD, H5N1), but not for reassortants containing the NS segment of A/Mallard/NL/12/2000 (MA, H7N3). The NS1 of GD and MA differ only in 8 aa positions. Here, we show that efficient replication of FPV-NSMA-derived mutants was linked to the presence of a single substitution (D74N) and more prominently to a triple substitution (P3S+R41K+D74N) in the NS1MA protein. The substitution(s) led to (i) increased virus titres, (ii) larger plaque sizes and (iii) increased levels and faster kinetics of viral mRNA and protein accumulation in mammalian cells. Interestingly, the NS1 substitutions did not affect viral growth characteristics in avian cells. Furthermore, we show that an FPV mutant with N74 in the NS1 (already possessing S3+K41) is able to replicate and cause disease in mice, demonstrating a key role of NS1 in the adaptation of avian IAV to mammalian hosts. Our data suggest that (i) adaptation to mammalian hosts does not necessarily compromise replication in the natural (avian) host and (ii) very few genetic changes may pave the way for zoonotic transmission. The study reinforces the need for close surveillance and characterization of circulating avian IAV to identify genetic signatures that indicate a potential risk for efficient transmission of avian strains to mammalian hosts.

Influenza Virus Infection of Marine Mammals

Interspecies transmission may play a key role in the evolution and ecology of influenza A viruses. The importance of marine mammals as hosts or carriers of potential zoonotic pathogens such as highly pathogenic H5 and H7 influenza viruses is not well understood. The fact that influenza viruses are some of the few zoonotic pathogens known to have caused infection in marine mammals, evidence for direct transmission of influenza A virus H7N7 subtype from seals to man, transmission of pandemic H1N1 influenza viruses to seals and also limited evidence for long-term persistence of influenza B viruses in seal populations without significant genetic change, makes monitoring of influenza viruses in marine mammal populations worth being performed. In addition, such monitoring studies could be a great tool to better understand the ecology of influenza viruses in nature.

A novel eight amino acid insertion contributes to the hemagglutinin cleavability and the virulence of a highly pathogenic avian influenza A (H7N3) virus in mice

In 2012, an avian influenza A H7N3 (A/Mexico/InDRE7218/2012; Mx/7218) virus was responsible for two confirmed cases of human infection and led to the death or culling of more than 22 million chickens in Jalisco, Mexico. Interestingly, this virus acquired an 8-amino acid (aa)-insertion (..PENPK-DRKSRHRR-TR/GLF) near the hemagglutinin (HA) cleavage site by nonhomologous recombination with host rRNA. It remains unclear which specific residues at the cleavage site contribute to the virulence of H7N3 viruses in mammals. Using loss-of-function approaches, we generated a series of cleavage site mutant viruses by reverse genetics and characterized the viruses in vitro and in vivo. We found that the 8-aa insertion and the arginine at position P4 of the Mx/7218 HA cleavage site are essential for intracellular HA cleavage in 293T cells, but have no effect on the pH of membrane fusion. However, we identified a role for the histidine residue at P5 position in viral fusion pH. In mice, the 8-aa insertion is required for Mx/7218 virus virulence; however, the basic residues upstream of the P4 position are dispensable for virulence. Overall, our study provides the first line of evidence that the insertion in the Mx/7218 virus HA cleavage site confers its intracellular cleavability, and consequently contributes to enhanced virulence in mice.

PB2 subunit of avian influenza virus subtype H9N2: a pandemic risk factor

Avian influenza viruses of subtype H9N2 that are found worldwide are occasionally transmitted to humans and pigs. Furthermore, by co-circulating with other influenza subtypes, they can generate new viruses with the potential to also cause zoonotic infections, as observed in 1997 with H5N1 or more recently with H7N9 and H10N8 viruses. Comparative analysis of the adaptive mutations in polymerases of different viruses indicates that their impact on the phylogenetically related H9N2 and H7N9 polymerases is higher than on the non-related H7N7 and H1N1pdm09 polymerases. Analysis of polymerase reassortants composed of subunits of different viruses demonstrated that the efficient enhancement of polymerase activity by H9N2-PB2 does not depend on PA and PB1. These observations suggest that the PB2 subunit of the H9N2 polymerase has a high adaptive potential and may therefore be an important pandemic risk factor.

Serial passage in ducks of a low-pathogenic avian influenza virus isolated from a chicken reveals a high mutation rate in the hemagglutinin that is likely due to selection in the host

A comparative study of the ability of three low-pathogenic avian influenza virus (LPAIV) isolates to be transmitted from duck to duck was performed. Pekin ducks were inoculated with two LPAIV isolates from chickens (A/Ck/PA/13609/93 [H5N2], H5N2-Ck; A/Ck/TX/167280-4/02 [H5N3], H5N3-Ck) and one isolate from a wild bird (A/Mute Swan/ MI/451072/06 [H5N1], H5N1-WB). During the establishment of the passage model, only two viruses (H5N1, H5N2) were able to be transmitted from duck to duck. Transmission of these isolates was dependent on the inoculation dose and route of infection. Analysis of swab samples taken from ducks revealed that the wild-bird isolate, H5N1-WB, was primarily shed via the cloacal route. The chicken isolate, H5N2-Ck, was isolated from cloacal as well as oro-pharyngeal swabs. Analysis of the amino acid sequences of the viral surface glycoproteins showed that the hemagglutinin (HA) of the H5N2-Ck isolate was under a stronger evolutionary pressure than the HA of the H5N1-WB isolate, as indicated by the presence of a larger number of amino acid changes observed during passage. The neuraminidase (NA) of both viruses showed either no (in the case of H5N1-WB) or very few amino acid changes.

Generation of a variety of stable Influenza A reporter viruses by genetic engineering of the NS gene segment

Influenza A viruses (IAV) pose a constant threat to the human population and therefore a better understanding of their fundamental biology and identification of novel therapeutics is of upmost importance. Various reporter-encoding IAV were generated to achieve these goals, however, one recurring difficulty was the genetic instability especially of larger reporter genes. We employed the viral NS segment coding for the non-structural protein 1 (NS1) and nuclear export protein (NEP) for stable expression of diverse reporter proteins. This was achieved by converting the NS segment into a single open reading frame (ORF) coding for NS1, the respective reporter and NEP. To allow expression of individual proteins, the reporter genes were flanked by two porcine Teschovirus-1 2A peptide (PTV-1 2A)-coding sequences. The resulting viruses encoding luciferases, fluorescent proteins or a Cre recombinase are characterized by a high genetic stability in vitro and in mice and can be readily employed for antiviral compound screenings, visualization of infected cells or cells that survived acute infection.

Virus pathotype and deep sequencing of the HA gene of a low pathogenicity H7N1 avian influenza virus causing mortality in Turkeys

Low pathogenicity avian influenza (LPAI) viruses of the H7 subtype generally cause mild disease in poultry. However the evolution of a LPAI virus into highly pathogenic avian influenza (HPAI) virus results in the generation of a virus that can cause severe disease and death. The classification of these two pathotypes is based, in part, on disease signs and death in chickens, as assessed in an intravenous pathogenicity test, but the effect of LPAI viruses in turkeys is less well understood. During an investigation of LPAI virus infection of turkeys, groups of three-week-old birds inoculated with A/chicken/Italy/1279/99 (H7N1) showed severe disease signs and died or were euthanised within seven days of infection. Virus was detected in many internal tissues and organs from culled birds. To examine the possible evolution of the infecting virus to a highly pathogenic form in these turkeys, sequence analysis of the haemagglutinin (HA) gene cleavage site was carried out by analysing multiple cDNA amplicons made from swabs and tissue sample extracts employing Sanger and Next Generation Sequencing. In addition, a RT-PCR assay to detect HPAI virus was developed. There was no evidence of the presence of HPAI virus in either the virus used as inoculum or from swabs taken from infected birds. However, a small proportion (<0.5%) of virus carried in individual tracheal or liver samples did contain a molecular signature typical of a HPAI virus at the HA cleavage site. All the signature sequences were identical and were similar to HPAI viruses collected during the Italian epizootic in 1999/2000. We assume that the detection of HPAI virus in tissue samples following infection with A/chicken/Italy/1279/99 reflected amplification of a virus present at very low levels within the mixed inoculum but, strikingly, we observed no new HPAI virus signatures in the amplified DNA analysed by deep-sequencing.

Control of avian influenza infections in poultry with emphasis on vaccination

Avian influenza is a World Organization for Animal Heath-listed disease that has become of great importance both for animal and human health. The increased relevance of avian influenza in the fields of animal and human health has highlighted the lack of scientific information on several aspects of the disease, which has hampered the adequate management of some of the recent crises. Millions of animals have died and there is growing concern over the loss of human lives and over the management of the pandemic potential. This special report will review the control methods for avian influenza infections in poultry that are currently available. The application of control policies, ranging from stamping out to emergency and prophylactic vaccination, are discussed on the basis of data generated from recent outbreaks, in the light of new regulations and also in view of the maintenance of animal welfare. Poultry veterinarians working for the industry or for the public sector represent the first line of defense against the pandemic threat and for the prevention and control of this infection in poultry and in wild birds.

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

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.

Virulence determinants of high-pathogenic avian influenza viruses in gallinaceous poultry

High-pathogenic avian influenza viruses (HPAIV) cause devastating outbreaks in domestic poultry worldwide. Moreover, they repeatedly lead to severe, even fatal disease in humans, raising concerns about their pandemic potential. HPAIV have evolved from circulating low-pathogenic precursors in several independent events by spontaneous acquisition of a polybasic cleavage site in the hemagglutinin (HA) envelope protein. Remarkably, in nature, HPAIV are confined to the HA serotypes H5 and H7 from the 16 HA serotypes known in birds. However, experimental introduction of a polybasic cleavage site into non-H5/H7 HA may result in a highly pathogenic phenotype, indicating that emergence of HPAIV with novel serotypes is conceivable, but requires further adaptation to the chicken host.

Inhibition of lung serine proteases in mice: a potentially new approach to control influenza infection

BACKGROUND

Host serine proteases are essential for the influenza virus life cycle because the viral haemagglutinin is synthesized as a precursor which requires proteolytic maturation. Therefore, we studied the activity and expression of serine proteases in lungs from mice infected with influenza and evaluated the effect of serine protease inhibitors on virus replication both in cell culture and in infected mice.

RESULTS

Two different inbred mouse strains were investigated: DBA/2J as a highly susceptible and C57Bl/6J as a more resistant strain to influenza virus infection. The serine proteases from lung homogenates of mice exhibited pH optima of 10.00. Using the substrate Bz-Val-Gly-Arg-p-nitroanilide or in zymograms, the intensities of proteolysis increased in homogenates from both mouse strains with time post infection (p.i.) with the mouse-adapted influenza virus A/Puerto Rico/8/34 (H1N1; PR8). In zymograms at day 7 p.i., proteolytic bands were stronger and numerous in lung homogenates from DBA/2J than C57Bl/6J mice. Real-time PCR results confirmed differential expression of several lung proteases before and after infecting mice with the H1N1 virus. The most strongly up-regulated proteases were Gzma, Tmprss4, Elane, Ctrl, Gzmc and Gzmb. Pretreatment of mouse and human lung cell lines with the serine protease inhibitors AEBSF or pAB or a cocktail of both prior to infection with the H1N1 or the A/Seal/Massachusetts/1/80 (H7N7; SC35M) virus resulted in a decrease in virus replication. Pretreatment of C57Bl/6J mice with either AEBSF or a cocktail of AEBSF and pAB prior to infection with the H1N1 virus significantly reduced weight loss and led to a faster recovery of treated versus untreated mice while pAB alone exerted a very poor effect. After infection with the H7N7 virus, the most significant reduction of weight loss was obtained upon pretreatment with either the protease inhibitor cocktail or pAB. Furthermore, pretreatment of C57BL/6J mice with AEBSF prior to infection resulted in a significant reduction in the levels of both the H1N1 and H7N7 nucleoproteins in mice lungs and also a significant reduction in the levels of the HA transcript in the lungs of the H1N1--but not the H7N7-infected mice.

CONCLUSION

Multiple serine protease activities might be implicated in mediating influenza infection. Blocking influenza A virus infection in cultured lung epithelia and in mice by the used serine protease inhibitors may provide an alternative approach for treatment of influenza infection.

Molecular mechanisms of interspecies transmission and pathogenicity of influenza viruses: Lessons from the 2009 pandemic

The emergence of the 2009 H1N1 virus pandemic was unexpected, since it had been predicted that the next pandemic would be caused by subtype H5N1. We also had to learn that a pandemic does not necessarily require the introduction of a new virus subtype into the human population, but that it may result from antigenic shift within the same subtype. The new variant was derived from human and animal viruses by genetic reassortment in the pig, supporting the concept that this animal is the mixing vessel for the generation of new human influenza viruses. Although it is generally believed that the 2009 outbreak was mild, there have been severe cases particularly among the young and the middle-aged. Pathogenicity and host range are determined to a large extent by the polymerase, the haemagglutinin and the NS1 protein of influenza A viruses. There is evidence that mutations of these proteins may change the pathogenicity of the new virus.

TMPRSS2 and TMPRSS4 facilitate trypsin-independent spread of influenza virus in Caco-2 cells

Proteolysis of influenza virus hemagglutinin by host cell proteases is essential for viral infectivity, but the proteases responsible are not well defined. Recently, we showed that engineered expression of the type II transmembrane serine proteases (TTSPs) TMPRSS2 and TMPRSS4 allows hemagglutinin (HA) cleavage. Here we analyzed whether TMPRSS2 and TMPRSS4 are expressed in influenza virus target cells and support viral spread in the absence of exogenously added protease (trypsin). We found that transient expression of TMPRSS2 and TMPRSS4 resulted in HA cleavage and trypsin-independent viral spread. Endogenous expression of TMPRSS2 and TMPRSS4 in cell lines correlated with the ability to support the spread of influenza virus in the absence of trypsin, indicating that these proteases might activate influenza virus in naturally permissive cells. Indeed, RNA interference (RNAi)-mediated knockdown of both TMPRSS2 and TMPRSS4 in Caco-2 cells, which released fully infectious virus without trypsin treatment, markedly reduced the spread of influenza virus, demonstrating that these proteases were responsible for efficient proteolytic activation of HA in this cell line. Finally, TMPRSS2 was found to be coexpressed with the major receptor determinant of human influenza viruses, 2,6-linked sialic acids, in human alveolar epithelium, indicating that viral target cells in the human respiratory tract express TMPRSS2. Collectively, our results point toward an important role for TMPRSS2 and possibly TMPRSS4 in influenza virus replication and highlight the former protease as a potential therapeutic target.

Replication and pathogenesis associated with H5N1, H5N2, and H5N3 low-pathogenic avian influenza virus infection in chickens and ducks

A comparative study examining replication and disease pathogenesis associated with low-pathogenic H5N1, H5N2, or H5N3 avian influenza virus (AIV) infection of chickens and ducks was performed. The replication and pathogenesis of highly pathogenic AIV (HPAIV) has received substantial attention; however, the behavior of low-pathogenic AIVs, which serve as precursors to HPAIVs, has received less attention. Thus, chickens or ducks were inoculated with an isolate from a wild bird [A/Mute Swan/MI/451072/06 (H5N1)] or isolates from chickens [A/Ck/PA/13609/93 (H5N2), A/Ck/TX/167280-4/02 (H5N3)], and virus replication, induction of a serological response, and disease pathogenesis were investigated, and the hemagglutinin and neuraminidase (NA) gene sequences of the isolates were determined. Virus isolated from tracheal and cloacal swabs showed that H5N1 replicated better in ducks, whereas H5N2 and H5N3 replicated better in chickens. Comparison of the NA gene sequences showed that chicken-adapted H5N2 and H5N3 isolates both have a deletion of 20 amino acids in the NA stalk region, which was absent in the H5N1 isolate. Histopathological examination of numerous organs showed that H5N2 and H5N3 isolates caused lesions in chickens in a variety of organs, but to a greater extent in the respiratory and intestinal tracts, whereas H5N1 lesions in ducks were observed mainly in the respiratory tract. This study suggests that the H5N1, H5N2, and H5N3 infections occurred at distinct sites in chicken and ducks, and that comparative studies in different model species are needed to better understand the factors influencing the evolution of these viruses.

Morpholino oligomers targeting the PB1 and NP genes enhance the survival of mice infected with highly pathogenic influenza A H7N7 virus

Peptide-conjugated phosphorodiamidate morpholino oligomers (PPMO) are single-stranded nucleic acid-analogue antisense agents that enter cells readily and can reduce gene expression by steric blocking of complementary RNA (cRNA) sequences. Here, we tested a panel of PPMO designed to target conserved sequences in the RNA genome segments encoding polymerase subunits of a highly pathogenic mouse-adapted influenza A virus (SC35M; H7N7). Three PPMO, targeting the translation start site region of PB1 or NP mRNA or the 3'-terminal region of NP viral RNA (vRNA), potently inhibited virus replication in MDCK cells. Primer extension assays showed that treatment with any of the effective PPMO led to markedly reduced levels of mRNA, cRNA and vRNA. Initially, the potential toxicity of a range of intranasally administered PPMO doses was evaluated, by measuring their effect on body weight of uninfected mice. Subsequently, a non-toxic dosing regimen was used to investigate the effect of various PPMO on SC35M infection in a mouse model. Mice administered intranasal treatment of PPMO targeting the PB1-AUG region or NP vRNA, at 3 mug per dose, given once 3 h before and once 2 days after intranasal infection with 10xLD(50) of SC35M, showed a 2 log(10) reduction of viral titre in the lungs and 50 % survival for the 16 day duration of the experiment, whereas the NP-AUG-targeted PPMO treatment resulted in 30 % survival of an otherwise lethal infection. These data suggest that PPMO provide a useful reagent to investigate influenza virus molecular biology and may constitute a therapeutic strategy against highly pathogenic influenza viruses.

Interaction of polymerase subunit PB2 and NP with importin alpha1 is a determinant of host range of influenza A virus

We have previously reported that mutations in the polymerase proteins PB1, PB2, PA, and the nucleocapsid protein NP resulting in enhanced transcription and replication activities in mammalian cells are responsible for the conversion of the avian influenza virus SC35 (H7N7) into the mouse-adapted variant SC35M. We show now that adaptive mutations D701N in PB2 and N319K in NP enhance binding of these proteins to importin α1 in mammalian cells. Enhanced binding was paralleled by transient nuclear accumulation and cytoplasmic depletion of importin α1 as well as increased transport of PB2 and NP into the nucleus of mammalian cells. In avian cells, enhancement of importin α1 binding and increased nuclear transport were not observed. These findings demonstrate that adaptation of the viral polymerase to the nuclear import machinery plays an important role in interspecies transmission of influenza virus.

The natural hosts of influenza A viruses are aquatic birds. On rare occasions these viruses may be transmitted to humans and then give rise to an influenza pandemic. Human influenza is therefore a typical re-emerging infection. Evidence is increasing that the viral polymerase, an enzyme that has to enter into the nucleus of the infected cell in order to promote replication and transcription of the viral genome, is a major determinant of host range. Thus, in a comparative study of an avian influenza strain and its mouse adapted variant we have previously shown that adaptation to mice depended exclusively on mutations in the polymerase proteins. These findings supported the concept that adaptation of the polymerase to host factors is an important mechanism underlying interspecies transmission. In the present study, we have identified importin α1, a component of the nuclear pore complex, as such a host factor. We show that adaptive mutations in polymerase subunits improve binding to importin α1 in mammalian, but not in avian cells. As a result, nuclear transport of these proteins and efficiency of replication are enhanced in mammalian cells. These observations demonstrate that the interaction of the viral polymerase with the nuclear import machinery is an important determinant of host range.

Ecology, epidemiology and human health implications of avian influenza viruses: why do we need to share genetic data?

Avian influenza (AI) is a listed disease of the World Organisation for Animal Health (OIE) that has become a disease of great importance both for animal and human health. Until recent times, AI was considered a disease of birds with zoonotic implications of limited significance. The emergence and spread of the Asian lineage highly pathogenic AI H5N1 virus has dramatically changed this perspective; not only has it been responsible of the death or culling of millions of birds, but this virus has also been able to infect a variety of non-avian hosts including human beings. The implications of such a panzootic reflect themselves in animal health issues, notably in the reduction of a protein source for developing countries and in the management of the pandemic potential. Retrospective studies have shown that avian progenitors play an important role in the generation of pandemic viruses for humans, and therefore these infections in the avian reservoir should be subjected to control measures aiming at eradication of the Asian H5N1 virus from all sectors rather than just eliminating or reducing the impact of the disease in poultry. Collection and analysis of information in a transparent environment and close collaboration between the medical and veterinary scientific community are crucial to support the global AI crisis.

Influenza A virus strains differ in sensitivity to the antiviral action of Mx-GTPase

Interferon-mediated host responses are of great importance for controlling influenza A virus infections. It is well established that the interferon-induced Mx proteins possess powerful antiviral activities toward most influenza viruses. Here we analyzed a range of influenza A virus strains for their sensitivities to murine Mx1 and human MxA proteins and found remarkable differences. Virus strains of avian origin were highly sensitive to Mx1, whereas strains of human origin showed much weaker responses. Artificial reassortments of the viral components in a minireplicon system identified the viral nucleoprotein as the main target structure of Mx1. Interestingly, the recently reconstructed 1918 H1N1 "Spanish flu" virus was much less sensitive than the highly pathogenic avian H5N1 strain A/Vietnam/1203/04 when tested in a minireplicon system. Importantly, the human 1918 virus-based minireplicon system was almost insensitive to inhibition by human MxA, whereas the avian influenza A virus H5N1-derived system was well controlled by MxA. These findings suggest that Mx proteins provide a formidable hurdle that hinders influenza A viruses of avian origin from crossing the species barrier to humans. They further imply that the observed insensitivity of the 1918 virus-based replicon to the antiviral activity of human MxA is a hitherto unrecognized characteristic of the "Spanish flu" virus that may contribute to the high virulence of this unusual pandemic strain.

Pathogenesis of avian influenza (H7) virus infection in mice and ferrets: enhanced virulence of Eurasian H7N7 viruses isolated from humans

Before 2003, only occasional case reports of human H7 influenza virus infections occurred as a result of direct animal-to-human transmission or laboratory accidents; most of these infections resulted in conjunctivitis. An increase in isolation of avian influenza A H7 viruses from poultry outbreaks and humans has raised concerns that additional zoonotic transmissions of influenza viruses from poultry to humans may occur. To better understand the pathogenesis of H7 viruses, we have investigated their ability to cause disease in mouse and ferret models. Mice were infected intranasally with H7 viruses of high and low pathogenicity isolated from The Netherlands in 2003 (Netherlands/03), the northeastern United States in 2002-2003, and Canada in 2004 and were monitored for morbidity, mortality, viral replication, and proinflammatory cytokine production in respiratory organs. All H7 viruses replicated efficiently in the respiratory tracts of mice, but only Netherlands/03 isolates replicated in systemic organs, including the brain. Only A/NL/219/03 (NL/219), an H7N7 virus isolated from a single fatal human case, was highly lethal for mice and caused severe disease in ferrets. Supporting the apparent ocular tropism observed in humans following infection with viruses of the H7 subtype, both Eurasian and North American lineage H7 viruses were detected in the mouse eye following ocular inoculation, whereas an H7N2 virus isolated from the human respiratory tract was not. Therefore, in general, the relative virulence and cell tropism of the H7 viruses in these animal models correlated with the observed virulence in humans.

An overview of the epidemiology of avian influenza

Only viruses of the Influenzavirus A genus have been isolated from birds and termed avian influenza [AI] viruses, but viruses with all 16 haemagglutinin [H1-H16] and all 9 neuraminidase [N1-N9] influenza A subtypes in the majority of possible combinations have been isolated from avian species. Influenza A viruses infecting poultry can be divided into two groups. The very virulent viruses causing highly pathogenic avian influenza [HPAI], with flock mortality as high as 100%. These viruses have been restricted to subtypes H5 and H7, although not all H5 and H7 viruses cause HPAI. All other viruses cause a milder, primarily respiratory, disease [LPAI], unless exacerbated. Until recently HPAI viruses were rarely isolated from wild birds, but for LPAI viruses extremely high isolation rates have been recorded in surveillance studies, with overall figures of about 11% for ducks and geese and around 2% for all other species. Influenza viruses may infect all types of domestic or captive birds in all areas of the world, the frequency with which primary infections occur in any type of bird usually depending on the degree of contact there is with feral birds. Secondary spread is usually associated with human involvement, either by bird or bird product movement or by transferring infective faeces from infected to susceptible birds, but potentially wild birds could be involved. In recent years there have been costly outbreaks of HPAI in poultry in Italy, The Netherlands and Canada and in each millions of birds were slaughtered to bring the outbreaks under control. Since the 1990s AI infections due to two subtypes have been widespread in poultry across a large area of the World. LPAI H9N2 appears to have spread across the whole of Asia in that time and has become endemic in poultry in many of the affected countries. However, these outbreaks have tended to have been overshadowed by the H5N1 HPAI virus, initially isolated in China, that has now spread in poultry and/or wild birds throughout Asia and into Europe and Africa, resulting in the death or culling of hundreds of millions of poultry and posing a significant zoonosis threat.

The challenge of controlling notifiable avian influenza by means of vaccination

Avian influenza (AI) is an Office International des Epizooties listed disease that has become a disease of great importance both for animal and human health. The increased relevance of AI in animal and human health has highlighted the lack of scientific information on several aspects of the disease, which has hampered the adequate management of some of the recent crises. Millions of animals have died, and there is growing concern over the loss of human lives and over the management of the pandemic potential. The present article reviews the currently available control methods for notifiable AI infections in poultry. The application of control policies, ranging from stamping out to emergency and prophylactic vaccination, is discussed on the basis of data generated in recent outbreaks and in light of new regulations, also in view of the maintenance of animal welfare. Poultry veterinarians working for the industry or for the public sector represent the first line of defense against the pandemic threat and for the prevention and control of this infection in poultry and in wild birds.

Differential polymerase activity in avian and mammalian cells determines host range of influenza virus

As recently shown, mutations in the polymerase genes causing increased polymerase activity in mammalian cells are responsible for the adaptation of the highly pathogenic avian influenza virus SC35 (H7N7) to mice (G. Gabriel et al., Proc. Natl. Acad. Sci. USA 102:18590-18595, 2005). We have now compared mRNA, cRNA, and viral RNA levels of SC35 and its mouse-adapted variant SC35M in avian and mammalian cells. The increase in levels of transcription and replication of SC35M in mammalian cells was linked to a decrease in avian cells. Thus, the efficiency of the viral polymerase is a determinant of both host specificity and pathogenicity.

Inactivation of avian influenza viruses by chemical agents and physical conditions: a review

The recent outbreaks of avian influenza (AI) worldwide have highlighted the difficulties in controlling this disease both in developed and in developing countries. Biosecurity is considered the most important tool to prevent and control AI. In certain areas of the world, AI has become endemic and the recent outbreaks in Europe and Africa show that the epidemiological situation is evolving in an unprecedented way. The consequences of this situation are economic losses to the poultry industry, food security issues in developing countries and a serious threat to human health, due to the direct consequences of AI infection in humans, and more alarmingly due to the risk of the generation of a new pandemic virus from the animal reservoir. In this paper, the physical and chemical methods of inactivating AI viruses are reviewed, with particular emphasis on the practicalities of using such methods in the poultry industry.

The use of vaccination to combat multiple introductions of Notifiable Avian Influenza viruses of the H5 and H7 subtypes between 2000 and 2006 in Italy

Since 1999, Italy has been challenged by several epidemics of Notifiable Avian Influenza (NAI) of the H5 and H7 subtypes, occurring in the densely populated poultry areas of northern part of the country. Vaccination with a conventional vaccine containing a seed strain with a different neuraminidase subtype to the field virus was used to complement biosecurity and restriction measures as part of an overall eradication strategy. This vaccination technique, known as the "DIVA-Differentiating Infected from Vaccinated Animals" system, enabled, the identification of field exposed flocks and ultimately the eradication of H7N1, H7N3 and H5N2 infections. A bivalent H5/H7 prophylactic vaccination programme of defined poultry populations was introduced subsequently to increase their resistance to field infection. Retrospective analysis of the outbreaks identified important reservoir species such as quail, and demonstrated clearly the higher susceptibility of turkeys to infection. Data generated during 6 years of experience with vaccination against Avian Influenza (AI) indicate that it is a useful tool to limit secondary spread and possibly prevent the introduction of AI viruses in a susceptible population. The Italian AI control programme including vaccination was managed in a flexible manner and enabled the continuation of international trade. It is imperative that if vaccination is to be used to combat the current H5N1 epidemic it is used in conjunction with other measures and under official supervision. An extraordinary effort is required from international organisations to accredit control strategies so that harmonised and validated programs can be implemented. Transparency and sharing of field results from countries that are practising such programmes is crucial to the progressive control and ultimately the eradication of NAI infections in the animal reservoir.

Control and prevention of avian influenza in an evolving scenario

Continuing outbreaks of highly pathogenic avian influenza (HPAI) across Eurasia and in Africa, caused by a type A influenza virus of the H5N1 subtype appear out of control and represent a serious risk for animal and public health worldwide. It is known that biosecurity represents the first line of defence against AI, although in certain circumstances strict hygienic measures appear to be inapplicable for social and economic conditions. The option of using vaccination against AI viruses of the H5 and H7 subtypes, has made its way in recent times--primarily as a tool to maximise the outcome of a series of control measures in countries that are currently infected, but also as a means of reducing the risk of introduction in areas at high risk of infection. In developing countries vaccination programmes in avian species have been recommended recently, however it will require concurrent management of local husbandry practices and industry compliance to eradicate the disease rather than the establishment of an endemic situation. Other key deliverables expected for this control strategy include maintaining a major source of food for rural communities and the preservation of the commercial viability of the local poultry industry. In developed countries vaccination is being used as a means of increasing resistance of susceptible animals to reduce the risk of introduction from the reservoir host or to reduce secondary spread in densely populated poultry areas. The recent joint OIE/FAO summits recommended applying vaccination, using the differentiating infected from vaccinated animals (DIVA) strategy when there is risk of major spread and depopulation is not feasible or desirable. Particularly in developing countries, stamping out of infected animals does not seem to be an appropriate means of reducing the spread of infection, if food supplies are to be guaranteed and economic consequences minimised. Crucial points to the success of a vaccination campaign are the implementation of complex territorial strategy involving upgraded biosecurity, monitoring vaccine efficacy, identification of field exposure and the appropriate management of infected flocks, regardless of vaccination status. Granting financial support for the compensation of farmers is also a key part of this strategy. Poultry veterinarians working for the industry or for the public sector represent the first line of defence against the pandemic threat and for the prevention and control of this infection in poultry and in wild birds.

Control of avian influenza in poultry

Avian influenza, listed by the World Organization for Animal Health (OIE), has become a disease of great importance for animal and human health. Several aspects of the disease lack scientific information, which has hampered the management of some recent crises. Millions of animals have died, and concern is growing over the loss of human lives and management of the pandemic potential. On the basis of data generated in recent outbreaks and in light of new OIE regulations and maintenance of animal welfare, we review the available control methods for avian influenza infections in poultry, from stamping out to prevention through emergency and prophylactic vaccination.

The viral polymerase mediates adaptation of an avian influenza virus to a mammalian host

Mammalian influenza viruses are descendants of avian strains that crossed the species barrier and underwent further adaptation. Since 1997 in southeast Asia, H5N1 highly pathogenic avian influenza viruses have been causing severe, even fatal disease in humans. Although no lineages of this subtype have been established until now, such repeated events may initiate a new pandemic. As a model of species transmission, we used the highly pathogenic avian influenza virus SC35 (H7N7), which is low-pathogenic for mice, and its lethal mouse-adapted descendant SC35M. Specific mutations in SC35M polymerase considerably increase its activity in mammalian cells, correlating with high virulence in mice. Some of these mutations are prevalent in chicken and mammalian isolates, especially in the highly pathogenic H5N1 viruses from southeast Asia. These activity-enhancing mutations of the viral polymerase complex demonstrate convergent evolution in nature and, therefore, may be a prerequisite for adaptation to a new host paving the way for new pandemic viruses.

Avian influenza: recent developments

This paper reviews the worldwide situation regarding avian influenza infections in poultry from 1997 to March 2004. The increase in the number of primary introductions and the scientific data available on the molecular basis of pathogenicity have generated concerns particularly for legislative purposes and for international trade. This has led to a new proposed definition of 'avian influenza' to extend all infections caused by H5 and H7 viruses regardless of their virulence as notifiable diseases, although this has encountered some difficulties in being approved. The paper also reviews the major outbreaks caused by viruses of the H5 or H7 subtype and the control measures applied. The zoonotic aspects of avian influenza, which until 1997 were considered to be of limited relevance in human medicine, are also discussed. The human health implications have now gained importance, both for illness and fatalities that have occurred following natural infection with avian viruses, and for the potential of generating a reassortant virus that could give rise to the next human influenza pandemic.

Recombination resulting in virulence shift in avian influenza outbreak, Chile

Influenza A viruses occur worldwide in wild birds and are occasionally associated with outbreaks in commercial chickens and turkeys. However, avian influenza viruses have not been isolated from wild birds or poultry in South America. A recent outbreak in chickens of H7N3 low pathogenic avian influenza (LPAI) occurred in Chile. One month later, after a sudden increase in deaths, H7N3 highly pathogenic avian influenza (HPAI) virus was isolated. Sequence analysis of all eight genes of the LPAI virus and the HPAI viruses showed minor differences between the viruses except at the hemagglutinin (HA) cleavage site. The LPAI virus had a cleavage site similar to other low pathogenic H7 viruses, but the HPAI isolates had a 30 nucleotide insert. The insertion likely occurred by recombination between the HA and nucleoprotein genes of the LPAI virus, resulting in a virulence shift. Sequence comparison of all eight gene segments showed the Chilean viruses were also distinct from all other avian influenza viruses and represent a distinct South American clade.

Should we change the definition of avian influenza for eradication purposes?

The current definitions of high-pathogenicity avian influenza (HPAI), formulated over 10 years ago, were aimed at including viruses that were overtly virulent in in vivo tests and those that had the potential to become virulent. At that time the only virus known to have mutated to virulence was the one responsible for the 1983-84 Pennsylvania epizootic. The mechanism involved has not been seen in other viruses, but the definition set a precedent for statutory control of potentially pathogenic as well as overtly virulent viruses. The accumulating evidence is that HPAI viruses arise from low-pathogenicity avian influenza (LPAI) H5 or H7 viruses infecting chickens and turkeys after spread from free-living birds. At present it can only be assumed that all H5 and H7 viruses have this potential and mutation to virulence is a random event. Therefore, the longer the presence and greater the spread in poultry the more likely it is that HPAI virus will emerge. The outbreaks in Pennsylvania, Mexico, and Italy are demonstrations of the consequences of failing to control the spread of LPAI viruses of H5 and H7 subtypes. It therefore seems desirable to control LPAI viruses of H5 and H7 subtype in poultry to limit the probability of a mutation to HPAI occurring. This in turn may require redefining statutory AI. There appear to be three options: 1) retain the current definition with a recommendation that countries impose restrictions to limit the spread of LPAI of H5 and H7 subtypes; 2) define statutory AI as an infection of birds/poultry with any AI virus of H5 or H7 subtype; 3) define statutory AI as any infection with AI virus of H5 or H7 subtype, but modify the control measures imposed for different categories of virus and/or different types of host.

Human health implications of avian influenza viruses and paramyxoviruses

Among avian influenza viruses and avian paramyxoviruses are the aetiological agents of two of the most devastating diseases of the animal kingdom: (i). the highly pathogenic form of avian influenza, caused by some viruses of the H5 and H7 subtypes, and (ii). Newcastle disease, caused by virulent strains of APMV type 1. Mortality rates due to these agents can exceed 50% in naïve bird populations, and, for some strains of AI, nearly 100%. These viruses may also be responsible for clinical conditions in humans. The virus responsible for Newcastle disease has been known to cause conjunctivitis in humans since the 1940s. The conjunctivitis is self-limiting and does not have any permanent consequences. Until 1997, reports of human infection with avian influenza viruses were sporadic and frequently associated with conjunctivitis. Recently, however, avian influenza virus infections have been associated with fatalities in human beings. These casualties have highlighted the potential risk that this type of infection poses to public health. In particular, the pathogenetic mechanisms of highly pathogenic avian influenza viruses in birds and the possibility of reassortment between avian and human viruses in the human host represent serious threats to human health. For this reason, any suspected case should be investigated thoroughly.

Pathogenesis of avian influenza A (H5N1) viruses in ferrets

Highly pathogenic avian influenza A H5N1 viruses caused outbreaks of disease in domestic poultry and humans in Hong Kong in 1997. Direct transmission of the H5N1 viruses from birds to humans resulted in 18 documented cases of respiratory illness, including six deaths. Here we evaluated two of the avian H5N1 viruses isolated from humans for their ability to replicate and cause disease in outbred ferrets. A/Hong Kong/483/97 virus was isolated from a fatal case and was highly pathogenic in the BALB/c mouse model, whereas A/Hong Kong/486/97 virus was isolated from a case with mild illness and exhibited a low-pathogenicity phenotype in mice. Ferrets infected intranasally with 10(7) 50% egg infectious doses (EID(50)) of either H5N1 virus exhibited severe lethargy, fever, weight loss, transient lymphopenia, and replication in the upper and lower respiratory tract, as well as multiple systemic organs, including the brain. Gastrointestinal symptoms were seen in some animals. In contrast, weight loss and severe lethargy were not noted in ferrets infected with 10(7) EID(50) of two recent human H3N2 viruses, although these viruses were also isolated from the brains, but not other extrapulmonary organs, of infected animals. The results demonstrate that both H5N1 viruses were highly virulent in the outbred ferret model, unlike the differential pathogenicity documented in inbred BALB/c mice. We propose the ferret as an alternative model system for the study of these highly pathogenic avian viruses.