(Highly pathogenic) avian influenza as a zoonotic agent

Zoonotic agents challenging the world every year afresh are influenza A viruses. In the past, human pandemics caused by influenza A viruses had been occurring periodically. Wild aquatic birds are carriers of the full variety of influenza virus A subtypes, and thus, most probably constitute the natural reservoir of all influenza A viruses. Whereas avian influenza viruses in their natural avian reservoir are generally of low pathogenicity (LPAIV), some have gained virulence by mutation after transmission and adaptation to susceptible gallinaceous poultry. Those so-called highly pathogenic avian influenza viruses (HPAIV) then cause mass die-offs in susceptible birds and lead to tremendous economical losses when poultry is affected. Besides a number of avian influenza virus subtypes that have sporadically infected mammals, the HPAIV H5N1 Asia shows strong zoonotic characteristics and it was transmitted from birds to different mammalian species including humans. Theoretically, pandemic viruses might derive directly from avian influenza viruses or arise after genetic reassortment between viruses of avian and mammalian origin. So far, HPAIV H5N1 already meets two conditions for a pandemic virus: as a new subtype it has been hitherto unseen in the human population and it has infected at least 438 people, and caused severe illness and high lethality in 262 humans to date (August 2009). The acquisition of efficient human-to-human transmission would complete the emergence of a new pandemic virus. Therefore, fighting H5N1 at its source is the prerequisite to reduce pandemic risks posed by this virus. Other influenza viruses regarded as pandemic candidates derive from subtypes H2, H7, and H9 all of which have infected humans in the past. Here, we will give a comprehensive overview on avian influenza viruses in concern to their zoonotic potential.     

Avian and swine influenza viruses: our current understanding of the zoonotic risk

The introduction of swine or avian influenza (AI) viruses in the human population can set the stage for a pandemic, and many fear that the Asian H5N1 AI virus will become the next pandemic virus. This article first compares the pathogenesis of avian, swine and human influenza viruses in their natural hosts. The major aim was to evaluate the zoonotic potential of swine and avian viruses, and the possible role of pigs in the transmission of AI viruses to humans. Cross-species transfers of swine and avian influenza to humans have been documented on several occasions, but all these viruses lacked the critical capacity to spread from human-to-human. The extreme virulence of H5N1 in humans has been associated with excessive virus replication in the lungs and a prolonged overproduction of cytokines by the host, but there remain many questions about the exact viral cell and tissue tropism. Though pigs are susceptible to several AI subtypes, including H5N1, there is clearly a serious barrier to infection of pigs with such viruses. AI viruses frequently undergo reassortment in pigs, but there is no proof for a role of pigs in the generation of the 1957 or 1968 pandemic reassortants, or in the transmission of H5N1 or other wholly avian viruses to humans. The major conclusion is that cross-species transmission of influenza viruses per se is insufficient to start a human influenza pandemic and that animal influenza viruses must undergo dramatic but largely unknown genetic changes to become established in the human population.     

Influenza vaccines and vaccination strategies in birds

Although it is well accepted that the present Asian H5N1 panzootic is predominantly an animal health problem, the human health implications and the risk of human pandemic have highlighted the need for more information and collaboration in the field of veterinary and human health. H5 and H7 avian influenza (AI) viruses have the unique property of becoming highly pathogenic (HPAI) during circulation in poultry. Therefore, the final objective of poultry vaccination against AI must be eradication of the virus and the disease. Actually, important differences exist in the control of avian and human influenza viruses. Firstly, unlike human vaccines that must be adapted to the circulating strain to provide adequate protection, avian influenza vaccination provides broader protection against HPAI viruses. Secondly, although clinical protection is the primary goal of human vaccines, poultry vaccination must also stop transmission to achieve efficient control of the disease. This paper addresses these differences by reviewing the current and future influenza vaccines and vaccination strategies in birds.     

Avian influenza virus

Avian influenza viruses do not typically replicate efficiently in humans, indicating direct transmission of avian influenza virus to humans is unlikely. However, since 1997, several cases of human infections with different subtypes (H5N1, H7N7, and H9N2) of avian influenza viruses have been identified and raised the pandemic potential of avian influenza virus in humans. Although circumstantial evidence of human to human transmission exists, the novel avian-origin influenza viruses isolated from humans lack the ability to transmit efficiently from person-to-person. However, the on-going human infection with avian-origin H5N1 viruses increases the likelihood of the generation of human-adapted avian influenza virus with pandemic potential. Thus, a better understanding of the biological and genetic basis of host restriction of influenza viruses is a critical factor in determining whether the introduction of a novel influenza virus into the human population will result in a pandemic. In this article, we review current knowledge of type A influenza virus in which all avian influenza viruses are categorized.     

Interspecies transmission of influenza viruses: H5N1 virus and a Hong Kong SAR perspective

This account takes stock of events and involvements, particularly on the avian side of the influenza H5N1 'bird flu' incident in Hong Kong SAR in 1997. It highlights the role of the chicken in the many live poultry markets as the source of the virus for humans. The slaughter of chicken and other poultry across the SAR seemingly averted an influenza pandemic. This perspective from Hong Kong SAR marks the coming-of-age of acceptance of the role of avian hosts as a source of pandemic human influenza viruses and offers the prospect of providing a good baseline for influenza pandemic preparedness in the future. Improved surveillance is the key. This is illustrated through the H9N2 virus which appears to have provided the 'replicating' genes for the H5N1 virus and which has since been isolated in the SAR from poultry, pigs and humans highlighting its propensity for interspecies transmission.     

Recent developments in avian influenza research: epidemiology and immunoprophylaxis

Influenza A viruses have been isolated from humans, from several other mammalian species and a wide variety of avian species, among which, wild aquatic birds represent the natural hosts of influenza viruses. The majority of the possible combinations of the 15 haemagglutinin (HA) and nine neuraminidase (NA) subtypes recognized have been identified in isolates from domestic and wild birds. Infection of birds can cause a wide range of clinical signs, which may vary according to the host, the virus strain, the host's immune status, the presence of any secondary exacerbating microorganisms and environmental factors. Most infections are inapparent, especially in waterfowl and other wild birds. In contrast, infections caused by viruses of H5 and H7 subtypes can be responsible for devastating epidemics in poultry. Despite the warnings to the poultry industry about these viruses, in 1997 an avian H5N1 influenza virus was directly transmitted from birds to humans in Hong Kong and resulted in 18 confirmed infections, thus strengthening the pandemic threat posed by avian influenza (AI). Indeed, reassortant viruses, harbouring a combination of avian and human viral genomes, have been responsible for major pandemics of human influenza. These considerations warrant the need to continue and broaden efforts in the surveillance of AI. Control programmes have varied from no intervention, as in the case of the occurrence of low pathogenic (LP) AI (LPAI) viruses, to extreme, expensive total quarantine-slaughter programmes carried out to eradicate highly pathogenic (HP) AI (HPAI) viruses. The adoption of a vaccination policy, targeted either to control or to prevent infection in poultry, is generally banned or discouraged. Nevertheless, the need to boost eradication efforts in order to limit further spread of infection and avoid heavy economic losses, and advances in modern vaccine technologies, have prompted a re-evaluation of the potential use of vaccination in poultry as an additional tool in comprehensive disease control strategies. This review presents a synthesis of the most recent research on AI that has contributed to a better understanding of the ecology of the virus and to the development of safe and efficacious vaccines for poultry.     

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.     

Novel human H7N9 influenza virus in China

Outbreaks of H7N9 avian influenza in humans in 5 provinces and 2 municipalities of China have reawakened concern that avian influenza viruses may again cross species barriers to infect the human population and thereby initiate a new influenza pandemic. Evolutionary analysis shows that human H7N9 influenza viruses originated from the H9N2, H7N3 and H11N9 avian viruses, and that it is as a novel reassortment influenza virus. This article reviews current knowledge on 11 subtypes of influenza A virus from human which can cause human infections.     

Highly pathogenic H5N1 avian influenza virus: Cause of the next pandemic?

Since 1997, when human infections with a highly pathogenic (HP) avian influenza A virus (AIV) subtype H5N1 – previously infecting only birds – were identified in a Hong Kong outbreak, global attention has focused on the potential for this virus to cause the next pandemic. From December 2003, an unprecedented H5N1 epizootic in poultry and migrating wild birds has spread across Asia and into Europe, the Middle East, and Africa. Humans in close contact with sick poultry and on rare occasion with other infected humans, have become infected. As of early March 2007, 12 countries have reported 167 deaths among 277 laboratory-confirmed human infections to WHO. WHO has declared the world to be in Phase 3 of a Pandemic Alert Period. This paper reviews the evolution of HP AIV H5N1, molecular changes that enable AIVs to infect and replicate in human cells and spread efficiently from person-to-person, and strategies to prevent the emergence of a pandemic virus.     

H9N2 influenza viruses isolated from poultry in Korean live bird markets continuously evolve and cause the severe clinical signs in layers

H9N2 influenza viruses are endemic in many Asian countries. We demonstrated that H9N2 influenza viruses isolated from poultry in Korean live bird markets are genetically changing and could cause the clinical signs in layers. Genetic analysis showed that Korean avian H9N2 influenza viruses are distinct from H9N2 influenza viruses circulating in poultry in China and Hong Kong. When we infected layers with H9N2 isolates, layers showed about 30% mortality and the reduction of egg productions. Considering that H9N2 influenza virus is one of potential pandemic candidates, the continuous surveillance is needed to monitor avian H9N2 influenza viruses for the poultry industry and humans.     

"Constanze": a trinational project on avian influenza in wild birds at Lake Constance

When highly pathogenic avian influenza H5N1 (HPAI H5N1) arrived at Lake Constance in February 2006, little was known about its ecology and epidemiology in wild birds. In order to prevent virus transmission from wild birds to poultry, the adjacent countries initiated the tri-national, interdisciplinary research program ?Constanze? to investigate avian influenza infections in water birds at Lake Constance. In collaboration with government agencies scientists examined the prevalence of AI virus in the region of Lake Constance for a period of 33 months, compared the effectiveness of different surveillance methods and analysed the migration behaviour of water birds. Although virus introduction from regions as far as the Ural Mountains seemed possible based on the migration behaviour of certain species, no influenza A viruses of the highly pathogenic subtype H5N1 (HPAIV) was found. However, influenza A viruses of different low pathogenic subtypes were isolated in 2.2 % of the sampled birds (swabs). Of the different surveillance methods utilised in the program the sampling of so called sentinel birds was particularly efficient.     

Phylogenetic evolution of swine-origin human influenza virus: a pandemic H1N1 2009

The knowledge of the genome constellation in pandemic influenza A virus H1N1 2009 from different countries and different hosts is valuable for monitoring and understanding of the evolution and migration of these strains. The complete genome sequences of selected worldwide distributed influenza A viruses are publicly available and there have been few longitudinal genome studies of human, avian and swine influenza A viruses. All possible to download SIV sequences of influenza A viruses available at GISAID Platform (Global Initiative on Sharing Avian Influenza Data) were analyzed firstly through the web servers of the Influenza Virus Resource in NCBI. Phylogenetic study of circulating human pandemic H1N1 virus indicated that the new variant possesses a distinctive evolutionary trait. There is no one way the pandemic H1N1 have acquired new genes from other distinguishable viruses circulating recently in local human, pig or domestic poultry populations from various geographic regions. The extensive genetic diversity among whole segments present in pandemic H1N1 genome suggests that multiple introduction of virus have taken place during the period 1999-2009. The initial interspecies transmission could have occurred in the long-range past and after it the reassortants steps lead to three lineages: classical SIV prevalent in the North America, avian-like SIV in Europe and avian-like related SIV in Asia. This analysis contributes to the evidence that pigs are not the only hosts playing the role of "mixing vessel", as it was suggested for many years.     

Human‐infecting influenza A (H9N2) virus: A forgotten potential pandemic strain?

Continuously emergence of human infection with avian influenza A virus poses persistent threat to human health, as illustrated in H5N1, H7N9 and recent surge of H9N2 infections. Long‐term prevalence of H9N2 avian influenza A virus in China and adjacent regions favours the interspecies transmissions from avian to human. Establishment of multiple genotypes of H9N2 variants in this region contributes to the emergence of novel H7N9 and H10N8 viruses which caused human fatalities. Recent increasing human infection with H9N2 virus in China highlights the necessity to closely monitor the interspecies transmission events. Available human H9N2 sequences revealed that Y280/G9 lineage was responsible for the most of human cases. Presence of adaptive mutations beyond the human‐like receptor binding was indicative of the capacity of readily infecting new hosts without prior adaptation. Moreover, enlarged host range of H9N2 virus in this region substantially increased the transmission among mammals. Meanwhile, serological surveys implied human was more susceptible to H9N2 infection, compared with panzootic H5 and H7 subtype avian influenza virus. Thus, control at the source will be the ultimate and effective option for H9N2 pandemic preparedness. This review comprehensively summarized recent updates on H9N2 human infections, aiming to shed light on the prevention strategies against this strain with pandemic potential.     

Modeling analysis revealed the distinct global transmission patterns of influenza A viruses and their influencing factors

Influenza A virus has caused huge damage to human health and poultry production worldwide, but its global transmission patterns and influencing factors remain unclear. Here, by using the Nearest Genetic Distance Approach with genetic sequences data, we reconstructed the global transmission patterns of 4 most common subtypes of influenza A virus (H1N1, H3N2, H5N1, and H7N9) and analyzed associations of transmission velocity of these influenza viruses with environmental factors. We found that the transmission patterns of influenza viruses and their associations with environmental factors were closely related to their host properties. H1N1 and H3N2, which are mainly held by humans, are transmitted between regions at high velocity and over long distances, which may be due to human transportation via airplane; while H5N1 and H7N9, which are mainly carried by animals, are transmitted locally at short distances and at low velocity, which may be facilitated by poultry transportation via railways or high ways. H1N1 and H3N2 spread faster in cold seasons, while H5N1 spread faster in both cold and warm seasons, and H7N9 spread faster in wet seasons. H1N1, H3N2, and H5N1 spread faster in places with both high and low human densities. Our study provided novel insights into the global transmission patterns, processes, and management strategies for influenza under accelerated global change.     

Different infection routes of avian influenza A (H5N1) virus in mice

The continuing outbreaks of avian influenza A H5N1 virus infection in Asia and Africa have caused worldwide concern because of the high mortality rates in poultry, suggesting its potential to become a pandemic influenza virus in humans. The transmission route of the virus among either the same species or different species is not yet clear. Broilers and BABL/c mice were inoculated with the H5N1 strain of influenza A virus isolated from birds. The animals were inoculated with 0.1 mL 10^6.83 TCID₅₀ of H5N1 virus oronasally, intraperitoneally and using eye drops. The viruses were examined by virological and pathological assays. In addition, to detect horizontal transmission, in each group, healthy chicks and mice were mixed with those infected. Viruses were detected in homogenates of the heart, liver, spleen, kidney and blood of the infected mice and chickens. Virus antigen was not detected in the spleen, kidney or gastrointestinal tract, but detected by Plaque Forming Unit (PFU) assay in the brain, liver and lung without degenerative change in these organs (in the group inoculated using eye drops. The detection results for mice inoculated using eye drops suggest that this virus might have a different tissue tropism from other influenza viruses mainly restricted to the respiratory tract in mice. All chicken samples tested positive for the virus, regardless of the method of inoculation. Avian influenza A H5N1 viruses are highly pathogenic to chickens, but its virulence in other animals is not yet known. To sum up, the results suggest that the virus replicates not only in different animal species but also through different routes of infection. In addition, the virus was detection not only in the respiratory tract but also in multiple extra‐respiratory tissues. This study demonstrates that H5N1 virus infection in mice can cause systemic disease and spread through potentially novel routes within and between mammalian hosts.     

Novel reassortant H5N6 highly pathogenic influenza A viruses in Vietnamese quail outbreaks

Avian influenza A H5N6 virus is a highly contagious infectious agent that affects domestic poultry and humans in South Asian countries. Vietnam may be an evolutionary hotspot for influenza viruses and therefore could serve as a source of pandemic strains. In 2015, two novel reassortant H5N6 influenza viruses designated as A/quail/Vietnam/CVVI01/2015 and A/quail/Vietnam/CVVI03/2015 were isolated from dead quails during avian influenza outbreaks in central Vietnam, and the whole genome sequences were analyzed. The genetic analysis indicated that hemagglutinin, neuraminidase, and polymerase basic protein 2 genes of the two H5N6 viruses are most closely related to an H5N2 virus (A/chicken/Zhejiang/727079/2014) and H10N6 virus (A/chicken/Jiangxi/12782/2014) from China and an H6N6 virus (A/duck/Yamagata/061004/2014) from Japan. The HA gene of the isolates belongs to clade 2.3.4.4, which caused human fatalities in China during 2014–2016. The five other internal genes showed high identity to an H5N2 virus (A/chicken/Heilongjiang/S7/2014) from China. A whole-genome phylogenetic analysis revealed that these two outbreak strains are novel H6N6-like PB2 gene reassortants that are most closely related to influenza virus strain A/environment/Guangdong/ZS558/2015, which was detected in a live poultry market in China. This report describes the first detection of novel H5N6 reassortants in poultry during an outbreak as well as genetic characterization of these strains to better understand the antigenic evolution of influenza viruses.     

Influenza pandemic planning

In this article the most important properties of influenza A viruses are described to understand influenza pandemics. There are at least three possibilities: (1) By reassortment between an avian and the prevailing human influenza A virus viruses with a new surface are created, against which no neutralizing antibodies are present in the human population. Such a virus can spread immediately worldwide. (2) Viruses, which have been present in the human population some time ago, reappear and infect the new generation, which has not been in contact with this virus before. (3) An avian influenza virus crosses the species barrier to humans and forms there a new stable lineage. In relation to pandemic planning, the first possibility can be more or less excluded, since the now-a-days human influenza A viruses have evolved so far away from their original source, the avian influenza viruses, that the formation of a well-growing and well-spreading reassortant is practically not possible anymore. Point two is a dangerous possibility, in that, e.g., a human H2N2 virus could reappear, which had disappeared in 1968 from the human population. The third possibility is at the moment the most dangerous situation, if, e.g., a highly neurotropic H5N1 virus from Southeast Asia crosses the species barrier to humans. An infection with such a pandemic virus presumably cannot be treated efficiently by antivirals. In such a situation only a rapid vaccination would be successful. In this respect in the last year important results have been obtained by using the reverse genetics. Meanwhile in about 50 countries there have been drawn up pandemic-preparedness plans.     

禽流感感染人的分子机制研究进展

禽流感病毒可以感染多种动物,包括人、猪、鸟、马、海豹、鲸和雪貂等。流感病毒在不同的宿主存在一定的屏障,但禽流感毒株能突破宿主屏障直接感染人,造成死亡。因此流感病毒的变异和病毒如何选择物种跨越物种流行的机制,对预防和控制流感的爆发是非常重要的。本文综述病毒毒力的分子生物学基础、禽流感染人的分子机制研究进展及在控制禽流感方面的研究。     

Characterization of H5N1 influenza viruses isolated from migratory birds in Qinghai province of China in 2006

Avian influenza H5N1 viruses pose a significant threat to human health because of their ability to infect humans directly. In the paper, three highly pathogenic H5N1 influenza viruses were isolated from three species of migratory birds in Qinghai Province of China in 2006. The analysis of the genome sequences indicated that the three isolates shared high homology with each other (94% to 99%). Three isolates shared a common ancestor and were closest to strains isolated from Qinghai and Siberia in 2005, but distinct from poultry viruses found in Southeast Asia. In experimental infection, all three viruses were highly pathogenic to chickens and mice. The results suggest that highly pathogenic avian influenza H5N1 viruses still exist in the migratory birds and could spread to other regions with wild bird migration.     

Pandemic influenza is a zoonosis, as it requires introduction of avian-like gene segments in the human population

Human influenza viruses manage to cause epidemics almost every year. The circulating viruses change their surface glycoproteins by accumulating mutations (antigenic drift) which results in variant viruses of the same subtype that are able to evade the immune pressure in the population. Every now and then, a completely new subtype of influenza A virus is introduced in the human population, which can result in an influenza pandemic. Pandemic human influenza viruses have been emerging for many centuries. Based on the genetic information of influenza viruses that have been isolated in this century, introduction of genes of the avian influenza virus reservoir obviously is required. Interspecies transmission, via another mammalian host and reassortment of avian and human influenza viruses are potential mechanisms for such an introduction. A summary of the cases in which influenza viruses containing avian-like gene segments were introduced into the human population is presented. In three cases, such infections resulted in conjunctivitis. Influenza-like illness and even pneumonia was reported in some other infections. Finally, a mortality rate of 33% was observed in the avian influenza A (H5N1) viruses that infected 18 people in Hong Kong in 1997. Although some of these viruses fulfilled some criteria of pandemic influenza viruses, they lacked the ability to rapidly spread through the human population.