Transmission Of Avian Influenza Viruses Research Articles

Preliminary study for evaluation of avian influenza virus inactivation in contaminated poultry products using electron beam irradiation

Spread of avian influenza is associated with movement of infected poultry and poultry products, and, as a result, strict international trade restrictions are in place. In addition, the possibility of transmission of avian influenza virus (AIV) strains to mammalian species by the consumption or handling of infected poultry, although a rare occurrence, is an important trade concern. Traditional methods for poultry product decontamination, such as thermal processing, are effective in inactivating AIV. However, alternative technologies such as electron beam (E-beam) irradiation offer some advantages in maintaining organoleptic properties of fresh poultry products. This study was designed to evaluate the applicability of high-energy (10 MeV) E-beam irradiation to inactivate AIV and reduce virus loads in egg products and poultry meat. Commercially available egg-white and ground turkey meat samples were spiked with the low-pathogenic A/chicken/TX/2002 H5N3 AIV and exposed to varying doses of high-energy E-beam irradiation in increments ranging from 0 kGy to 8 kGy. The viral titres in irradiated samples showed a linear dose-dependent reduction. The dose required to achieve 90% reduction (i.e. the D 10 value) of viable AIV loads was 2.3 kGy in phosphate buffer, 1.6 kGy in egg-white and 2.6 kGy in ground turkey meat samples. The effectiveness of E-beam irradiation to inactivate AIV was similar to the previously reported effectiveness of the E-beam against poliovirus and rotavirus. These results illustrate the potential applicability of high-energy (10 MeV) E-beam irradiation as a poultry product decontamination technology to inactivate AIV.

Avian Influenza Virus Glycoproteins Restrict Virus Replication and Spread through Human Airway Epithelium at Temperatures of the Proximal Airways

Transmission of avian influenza viruses from bird to human is a rare event even though avian influenza viruses infect the ciliated epithelium of human airways in vitro and ex vivo. Using an in vitro model of human ciliated airway epithelium (HAE), we demonstrate that while human and avian influenza viruses efficiently infect at temperatures of the human distal airways (37°C), avian, but not human, influenza viruses are restricted for infection at the cooler temperatures of the human proximal airways (32°C). These data support the hypothesis that avian influenza viruses, ordinarily adapted to the temperature of the avian enteric tract (40°C), rarely infect humans, in part due to differences in host airway regional temperatures. Previously, a critical residue at position 627 in the avian influenza virus polymerase subunit, PB2, was identified as conferring temperature-dependency in mammalian cells. Here, we use reverse genetics to show that avianization of residue 627 attenuates a human virus, but does not account for the different infection between 32°C and 37°C. To determine the mechanism of temperature restriction of avian influenza viruses in HAE at 32°C, we generated recombinant human influenza viruses in either the A/Victoria/3/75 (H3N2) or A/PR/8/34 (H1N1) genetic background that contained avian or avian-like glycoproteins. Two of these viruses, A/Victoria/3/75 with L226Q and S228G mutations in hemagglutinin (HA) and neuraminidase (NA) from A/Chick/Italy/1347/99 and A/PR/8/34 containing the H7 and N1 from A/Chick/Italy/1347/99, exhibited temperature restriction approaching that of wholly avian influenza viruses. These data suggest that influenza viruses bearing avian or avian-like surface glycoproteins have a reduced capacity to establish productive infection at the temperature of the human proximal airways. This temperature restriction may limit zoonotic transmission of avian influenza viruses and suggests that adaptation of avian influenza viruses to efficient infection at 32°C may represent a critical evolutionary step enabling human-to-human transmission.

Interspecies transmission and host restriction of avian H5N1 influenza virus

Long-term endemicity of avian H5N1 influenza virus in poultry and continuous sporadic human infections in several countries has raised the concern of another potential pandemic influenza. Suspicion of the avian origin of the previous pandemics results in the close investigation of the mechanism of interspecies transmission. Entry and fusion is the first step for the H5N1 influenza virus to get into the host cells affecting the host ranges. Therefore receptor usage study has been a major focus for the last few years. We now know the difference of the sialic acid structures and distributions in different species, even in the different parts of the same host. Many host factors interacting with the influenza virus component proteins have been identified and their role in the host range expansion and interspecies transmission is under detailed scrutiny. Here we review current progress in the receptor usage and host factors.

Water-borne transmission drives avian influenza dynamics in wild birds: The case of the 2005–2006 epidemics in the Camargue area

Transmission and persistence of avian influenza viruses (AIV) among wildlife remains an unresolved issue because it depends both on the ecology of the host ( e.g. population density, migration) and on the environment ( e.g. AIV persistence in water). We have developed a mathematical model that accounts for both AIV epidemics and bird community dynamics. The model is parameterized using bird counts and AIV prevalence data. Results suggest that the transmission patterns driving the dynamics of infection at our study site (Camargue, South of France) involved both a density-dependent and a water-borne transmission processes. Water-borne transmission is, however, the main determinant of the disease dynamics and observed prevalence level. This pattern of transmission highlights the importance of the persistence of viral particles in water in AIV dynamics in wild birds.

The role of environmental transmission in recurrent avian influenza epidemics.

Avian influenza virus (AIV) persists in North American wild waterfowl, exhibiting major outbreaks every 2–4 years. Attempts to explain the patterns of periodicity and persistence using simple direct transmission models are unsuccessful. Motivated by empirical evidence, we examine the contribution of an overlooked AIV transmission mode: environmental transmission. It is known that infectious birds shed large concentrations of virions in the environment, where virions may persist for a long time. We thus propose that, in addition to direct fecal/oral transmission, birds may become infected by ingesting virions that have long persisted in the environment. We design a new host–pathogen model that combines within-season transmission dynamics, between-season migration and reproduction, and environmental variation. Analysis of the model yields three major results. First, environmental transmission provides a persistence mechanism within small communities where epidemics cannot be sustained by direct transmission only (i.e., communities smaller than the critical community size). Second, environmental transmission offers a parsimonious explanation of the 2–4 year periodicity of avian influenza epidemics. Third, very low levels of environmental transmission (i.e., few cases per year) are sufficient for avian influenza to persist in populations where it would otherwise vanish.

The ecology of avian influenza virus in wild birds: What does this mean for poultry?

As natural hosts for avian influenza virus (AIV), wild birds, particularly aquatic birds, are the primary reservoir for transmission of AIV to domestic poultry. Therefore, understanding the dissemination and maintenance of AIV in wild birds is important for understanding the factors that contribute to transmission of AIV from wild birds to poultry. However, relatively little is known about the ecology of the virus in wild birds, and the depth of data are inconsistent worldwide. Also, the biology of the virus itself is very important, because AIV is highly adaptable to different hosts and likely to the environment as well. Some insight may be gained from the Asian H5N1 highly pathogenic AIV lineage, because surveillance for this virus has increased considerably in wild birds worldwide since 2005. Also, numerous species that have not previously been represented in AIV testing have been included in surveillance for the Asian H5N1 highly pathogenic AIV, allowing for a more complete understanding of the distribution of AIV in wild birds.

The public health impact of avian influenza viruses

Influenza viruses with novel hemagglutinin and 1 or more accompanying genes derived from avian influenza viruses sporadically emerge in humans and have the potential to result in a pandemic if the virus causes disease and spreads efficiently in a population that lacks immunity to the novel hemagglutinin. Since 1997, multiple avian influenza virus subtypes have been transmitted directly from domestic poultry to humans and have caused a spectrum of human disease, from asymptomatic to severe and fatal. To assess the pandemic risk that avian influenza viruses pose, we have used multiple strategies to better understand the capacity of avian viruses to infect, cause disease, and transmit among mammals, including humans. Seroepidemiologic studies that evaluate the frequency and risk of human infection with avian influenza viruses in populations with exposure to domestic or wild birds can provide a better understanding of the pandemic potential of avian influenza subtypes. Investigations conducted in Hong Kong following the first H5N1 outbreak in humans in 1997 determined that exposure to poultry in live bird markets was a key risk factor for human disease. Among poultry workers, butchering and exposure to sick poultry were risk factors for antibody to H5 virus, which provided evidence for infection. A second risk assessment tool, the ferret, can be used to evaluate the level of virulence and potential for host-to-host transmission of avian influenza viruses in this naturally susceptible host. Avian viruses isolated from humans exhibit a level of virulence and transmissibility in ferrets that generally reflects that seen in humans. The ferret model thus provides a means to monitor emerging avian influenza viruses for pandemic risk, as well as to evaluate laboratory-generated reassortants and mutants to better understand the molecular basis of influenza virus transmissibility. Taken together, such studies provide valuable information with which we can assess the public health risk of avian influenza viruses.

Tenacity of avian influenza viruses

The goal of this review is to provide an overview of existing research on the environmental tenacity of avian influenza (AI) viruses, to identify gaps in our current understanding, and discuss how this information relates to AI control, eradication, and prevention. We are just beginning to understand the environmental factors that affect infectivity and the extent of variation in environmental tenacity that is present among these viruses. Because the environment can provide a bridge for AI virus transmission between many diverse hosts, including wild and domestic animals and man, understanding the importance of environmental transmission and identifying important points of contact are critical steps in preventing the spread of infection especially related to the introduction of these viruses to new host species.

Human HA and polymerase subunit PB2 proteins confer transmission of an avian influenza virus through the air

The influenza virus genes that confer efficient transmission of epidemic and pandemic strains in humans have not been identified. The rapid spread and severe disease caused by the 1918 influenza pandemic virus makes it an ideal virus to study the transmissibility of potentially pandemic influenza strains. Here, we used a series of human 1918-avian H1N1 influenza reassortant viruses to identify the genetic determinants that govern airborne transmission of avian influenza viruses. We have demonstrated that the 1918 HA gene was necessary for efficient direct contact transmission, but did not allow respiratory droplet transmission between ferrets of an avian influenza virus possessing an avian polymerase subunit PB2. The 1918 PB2 protein was found to be both necessary and sufficient for airborne transmission of a virus expressing the 1918 HA and neuraminidase. Also, it was found that influenza viruses that were able to transmit efficiently in ferrets were able to replicate efficiently at the lower temperature (33 degrees C) found in the environment of mammalian airway. These findings demonstrate that the adaptation of the HA and PB2 proteins are critical for the development of pandemic influenza strains from avian influenza viruses.

Evolution analysis of the matrix (M) protein genes of 17 H9N2 chicken influenza viruses isolated in northern China during 1998–2008

Matrix (M) protein genes of 17 H9N2 avian influenza viruses (AIVs) isolated from chickens in northern China during the last 10 years were completely sequenced and phylogenetically analyzed. Homology of nucleotide sequences in the M gene of 17 isolates was 92.7-99.9%. Phylogenetic analysis showed that 11 of the tested M genes belong to the A/chicken/HongKong/Y280/97 (Y280)-like lineage, while the other six belong to the A/Quail/HongKong/G1/97 (G1)-like lineage. This is also the first time that a G1-like M gene of a H9N2 virus was detected in chicken flocks in northern China. These newly appearing changes in M genes may be due to reassortment events of AIVs, or they may have come from the H9N2 strains of southern China which surged in northern China after translocation. An analysis of the viral amino acid sequence of M2 protein has revealed substitution of S31N in two isolates, which is the molecular characterization of amantadine resistance in AIVs. Results of this study suggest that long-term monitoring should be continued to track the transmission and evolution of H9N2 AIVs in chickens in China.

Transmission of low-pathogenicity avian influenza virus of subtype H6N2 from chickens to Pekin ducks and Japanese quail (Coturnix coturnix japonica)

In this experiment we evaluated the transmission characteristics of a chicken-adapted low-pathogenicity avian influenza virus (LPAIV) of subtype H6N2, from infected chickens to Japanese quail and Pekin ducks, which are commonly sold in live bird markets located in Southern California. The layout of the cages and bird-handling practices were similar to those found in Southern California live bird markets. Five out of 20 chickens were inoculated with LPAIV H6N2, and placed in direct contact with five chickens and in indirect contact with 10 chickens, 10 Japanese quail and 10 Pekin ducks. Transmission of LPAIV was followed in each bird daily for 15 days post inoculation by real-time reverse transcriptase-polymerase chain reaction testing of oropharyngeal and cloacal swab samples. This strain of H6N2 LPAIV, isolated from commercial poultry in California, was transmitted to chickens, quail, and ducks from chickens. An antibody response was detected in ducks by haemagglutination inhibition tests, but avian influenza virus was only detected by reverse transcriptase-polymerase chain reaction in one duck. Avian influenza virus was detected in quail (5 and 7 days post inoculation) before chickens (8 and 9 days post inoculation), all of which were in indirect contact with infected chickens; however, this difference was not statistically significant.

Transmission of Influenza Virus in a Mammalian Host Is Increased by PB2 Amino Acids 627K or 627E/701N

Since 2003, more than 380 cases of H5N1 influenza virus infection of humans have been reported. Although the resultant disease in these cases was often severe or fatal, transmission of avian influenza viruses between humans is rare. The precise nature of the barrier blocking human-to-human spread is unknown. It is clear, however, that efficient human-to-human transmission of an antigenically novel influenza virus would result in a pandemic. Influenza viruses with changes at amino acids 627 or 701 of the PB2 protein have been isolated from human cases of highly pathogenic H5 and H7 avian influenza. Herein, we have used the guinea pig model to test the contributions of PB2 627 and 701 to mammalian transmission. To this end, viruses carrying mutations at these positions were generated in the A/Panama/2007/99 (H3N2) and A/Viet Nam/1203/04 (H5N1) backgrounds. In the context of either rPan99 or rVN1203, mutation of lysine 627 to the avian consensus residue glutamic acid was found to decrease transmission. Introduction of an asparagine at position 701, in conjunction with the K627E mutation, resulted in a phenotype more similar to that of the parental strains, suggesting that this residue can compensate for the lack of 627K in terms of increasing transmission in mammals. Thus, our data show that PB2 amino acids 627 and 701 are determinants of mammalian inter-host transmission in diverse virus backgrounds.

Analysis of a point mutation in H5N1 avian influenza virus hemagglutinin in relation to virus entry into live mammalian cells

Binding to and infection of human cells is essential for avian influenza virus transmission. Since virus binding is not always predictive for efficient infection of the cells, here we wished to investigate how hemagglutinin (HA) mutations of avian influenza virus H5N1 influence virus post-binding events in a single cycle of replication. One mutation observed in H5 HA of avian and natural human isolates from mainland China, Hong Kong, Vietnam and Thailand was identified and analyzed. The effects of the mutation on receptor binding, fusion and virus entry into cultured cells were investigated using hemadsorption, polykaryon formation and pseudotyped virus that express luciferase in the cytoplasm of transduced cell. Our results revealed that replacing aspartic acid at residue 94 with asparagine enhanced virus fusion activity and increased the binding of HA to sialic acid alpha2,6 galactose, while it decreased pseudotyped virus entry into cells expressing the avian type receptor, sialic acid alpha2,3 galactose. Our result may have implications for the understanding of the role of HA mutations in virus entry into live cells that exclusively display one type of receptor.

Cross-Reactive Immunity to Clade 2 Strains of Influenza Virus A Subtype H5N1 Induced in Adults and Elderly Patients by Fluval, a Prototype Pandemic Influenza Virus Vaccine Derived by Reverse Genetics, Formulated with a Phosphate Adjuvant, and Directed to Clade 1 Strains

High fatality rates and multiple cases of transmission of avian H5N1 influenza viruses to humans illustrate the urgent need for an efficacious, cross-protective vaccine against H5N1 strains. Extensive genetic characterization of H5N1 strains has elucidated the natural evolutionary relationship of these strains, linking groups known as clades to a common ancestor. Although the clades and subclades probably differ sufficiently in their antigenic structure to warrant the preparation of different vaccines, there is some evidence that cross-reactive immunity can be afforded. We aimed to assess the immunogenicity of a clade 1 H5N1 (NIBRG-14) whole-virus vaccine with an aluminum phosphate adjuvant and to determine whether it can induce cross-reactive immunity against antigenically drifted clade 2 H5N1 strains, both those derived by reverse genetics and wild-type isolates. A total of 88 (44 adult and 44 elderly) subjects, who received one dose (6 microg) of the vaccine, were studied. As judged by U.S. and European licensing criteria based on hemagglutination inhibition, the subjects developed cross-reactive immunity against all studied H5N1 strains belonging to a clade different from that of the strain utilized to produce the vaccine. Our findings highlight the importance of stockpiling, since cross-immune reactions induced by prepandemic vaccines will likely reduce morbidity and mortality in case of a pandemic.

Avian influenza virus in water: Infectivity is dependent on pH, salinity and temperature

Wild birds in the Orders Anseriformes and Charadriiformes are the natural reservoir for avian influenza (AI) viruses. Transmission within these aquatic bird populations occurs through an indirect fecal-oral route involving contaminated water on shared aquatic habitats. In order to better understand the influence that aquatic environments exert on AI transmission and maintenance in the wild-bird reservoir system, we determined the duration of persistence for 12 wild-bird origin AI viruses under natural ranges of pH, salinity, and temperature. Viral persistence was measured using a laboratory-based distilled water model system. The AI viruses varied in their response to each of the examined variables, but, generally, the viruses were most stable at a slightly basic pH (7.4–8.2), low temperatures (<17 °C), and fresh to brackish salinities (0–20,000 parts per million (ppm)). Alternatively, the AI viruses had a much shorter duration of persistence in acidic conditions (pH < 6.6), warmer temperatures (>32 °C), and high salinity (>25,000 ppm). The results of this research suggest that the pH, temperature, and salinity in natural aquatic habitats can influence the ability of AI viruses to remain infective within these environments. Furthermore, these results provide insight into chemical and physical properties of water that could enhance or restrict AI virus transmission on an aquatic bird habitat.

Host Restriction of Avian Influenza Viruses at the Level of the Ribonucleoproteins

Although transmission of avian influenza viruses to mammals, particularly humans, has been repeatedly documented, adaptation and sustained transmission in the new host is a rare event that in the case of humans may result in pandemics. Host restriction involves multiple genetic determinants. Among the known determinants of host range, key determinants have been identified on the genes coding for the nucleoprotein and polymerase proteins that, together with the viral RNA segments, form the ribonucleoproteins (RNPs). The RNP genes form host-specific lineages and harbor host-associated genetic signatures. The functional significance of these determinants has been studied by reassortment and reverse genetics experiments, underlining the influence of the global genetic context. In some instances the molecular mechanisms have been approached, pointing to the importance of the polymerase activity and interaction with cellular host factors. Better knowledge of determinants of host restriction will allow monitoring of the pandemic potential of avian influenza viruses.

Using Mean Infectious Dose of High- and Low-Pathogenicity Avian Influenza Viruses Originating from Wild Duck and Poultry as One Measure of Infectivity and Adaptation to Poultry

The mean infectious doses of selected avian influenza virus (AIV) isolates, determined in domestic poultry under experimental conditions, were shown to be both host-dependent and virus strain-dependent and could be considered one measure of the infectivity and adaptation to a specific host. As such, the mean infectious dose could serve as a quantitative predictor for which strains of AIV, given the right conditions, would be more likely transmitted to and maintained in a given species or subsequently cause an AI outbreak in the given species. The intranasal (IN) mean bird infectious doses (BID50) were determined for 11 high-pathogenicity AIV (HPAIV) isolates of turkey and chicken origin for white leghorn (WL) chickens, and for low-pathogenicity AIV (LPAIV) isolates of chicken (n = 1) and wild mallards (n = 2) for turkeys, and WL and white Plymouth rock (WPR) chickens, domestic ducks and geese, and Japanese quail. The BID50 for HPAIV isolates for WL chickens ranged from 10(1.2) to 10(4.7) mean embryo infectious dose (EID50) (median = 10(2.9)). For chicken-origin HPAIV isolates, the BID50 in WL chickens ranged from 10(1.2) to 10(3.0) EID50 (median = 10(2.6)), whereas for HPAIV isolates of turkey origin, the BID50 in WL chickens was higher, ranging from 10(2.8) to 10(4.7) EID50 (median = 10(3.9)). The BID50 of 10(4.7) was for a turkey-origin HPAIV virus that was not transmitted to chickens on the same farm, suggesting that, under the specific conditions present on that farm, there was insufficient infectivity, adaptation, or exposure to that virus population for sustained chicken transmission. Although the upper BID50 limit for predicting infectivity and sustainable transmissibility for a specific species is unknown, a BID50 < 10(4.7) was suggestive of such transmissibility. For the LPAIVs, there was a trend for domestic ducks and geese and Japanese quail to have the greatest susceptible and for WL chickens to be the most resistant, but turkeys were susceptible to two LPAIV tested when used at moderate challenge doses. This suggests domestic ducks and geese, turkeys, and Japanese quail could serve as bridging species for LPAIVs from wild waterfowl to chickens and other gallinaceous poultry. These data do provide support for the commonly held and intuitive belief that mixing of poultry species during rearing and in outdoor production systems is a major risk factor for interspecies transmission of AIVs and for the emergence of new AIV strains capable of causing AI outbreaks because these situations present a more diverse host population to circumvent the natural host dependency or host range of circulating viruses.

An Inhibitory Activity in Human Cells Restricts the Function of an Avian-like Influenza Virus Polymerase

Transmission of avian influenza virus into human populations has the potential to cause pandemic outbreaks. A major determinant of species tropism is the identity of amino acid 627 in the PB2 subunit of the heterotrimeric influenza polymerase; glutamic acid predominates in avian PB2, whereas lysine occupies this position in human isolates. We show that a dominant inhibitory activity in human cells potently and selectively restricts the function of polymerases containing an avian-like PB2 with glutamic acid at residue 627. Restricted polymerases fail to assemble into ribonucleoprotein complexes, resulting in decreased genome transcription, replication, and virus production without any significant effect on relative viral infectivity. Understanding the molecular basis of this species-specific restriction should provide strategies to prevent and treat avian influenza outbreaks in humans.

Avian influenza viruses in wild birds: A moving target

The long-standing evolutionary and ecological relationships between wild birds and influenza A viruses has created a broad pool of viral genetic diversity and a reservoir of potentially transmissible viruses. An understanding of these relationships can help us identify and modify critical control points to reduce transmission of avian influenza viruses into animal and human populations.

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.