The 1999-2000 avian influenza (H7N1) epidemic in Italy: veterinary and human health implications

Generation of candidate human influenza vaccine strains in cell culture - rehearsing the European response to an H7N1 pandemic threat

Background  Although H5N1 avian influenza viruses pose the most obvious imminent pandemic threat, there have been several recent zoonotic incidents involving transmission of H7 viruses to humans. Vaccines are the primary public health defense against pandemics, but reliance on embryonated chickens eggs to propagate vaccine and logistic problems posed by the use of new technology may slow our ability to respond rapidly in a pandemic situation.

Objectives  We sought to generate an H7 candidate vaccine virus suitable for administration to humans whose generation and amplification avoided the use of eggs.

Methods  We generated a suitable H7 vaccine virus by reverse genetics. This virus, known as RD3, comprises the internal genes of A/Puerto Rico/8/34 with surface antigens of the highly pathogenic avian strain A/Chicken/Italy/13474/99 (H7N1). The multi‐basic amino acid site in the HA gene, associated with high pathogenicity in chickens, was removed.

Results  The HA modification did not alter the antigenicity of the virus and the resultant single basic motif was stably retained following several passages in Vero and PER.C6 cells. RD3 was attenuated for growth in embryonated eggs, chickens, and ferrets. RD3 induced an antibody response in infected animals reactive against both the homologous virus and other H7 influenza viruses associated with recent infection by H7 viruses in humans.

Conclusions  This is the first report of a candidate H7 vaccine virus for use in humans generated by reverse genetics and propagated entirely in mammalian tissue culture. The vaccine has potential use against a wide range of H7 strains.

Back-calculation method shows that within-flock transmission of highly pathogenic avian influenza (H7N7) virus in the Netherlands is not influenced by housing risk factors

To optimize control of an avian influenza outbreak knowledge of within-flock transmission is needed. This study used field data to estimate the transmission rate parameter (beta) and the influence of risk factors on within-flock transmission of highly pathogenic avian influenza (HPAI) H7N7 virus in the 2003 epidemic in The Netherlands. The estimation is based on back-calculation of daily mortality data to fit a susceptible-infectious-dead format, and these data were analysed with a generalized linear model. This back-calculation method took into account the uncertainty of the length of the latent period, the survival of an infection by some birds and the influence of farm characteristics. After analysing the fit of the different databases created by back-calculation, it could be concluded that an absence of the latency period provided the best fit. The transmission rate parameter (beta) from these field data was estimated at 4.50 per infectious chicken per day (95% CI: 2.68-7.57), which was lower than what was reported from experimental data. In contrast to general belief, none of the studied risk factors (housing system, flock size, species, age of the birds in weeks and date of depopulation) had significant influence on the estimated beta.

Industrial food animal production and global health risks: exploring the ecosystems and economics of avian influenza

Many emerging infectious diseases in human populations are associated with zoonotic origins. Attention has often focused on wild animal reservoirs, but most zoonotic pathogens of recent concern to human health either originate in, or are transferred to, human populations from domesticated animals raised for human consumption. Thus, the ecological context of emerging infectious disease comprises two overlapping ecosystems: the natural habitats and populations of wild animals, and the anthropogenically controlled habitats and populations of domesticated species. Intensive food animal production systems and their associated value chains dominate in developed countries and are increasingly important in developing countries. These systems are characterized by large numbers of animals being raised in confinement with high throughput and rapid turnover. Although not typically recognized as such, industrial food animal production generates unique ecosystems -- environments that may facilitate the evolution of zoonotic pathogens and their transmission to human populations. It is often assumed that confined food animal production reduces risks of emerging zoonotic diseases. This article provides evidence suggesting that these industrial systems may increase animal and public health risks unless there is recognition of the specific biosecurity and biocontainment challenges of the industrial model. Moreover, the economic drivers and constraints faced by the industry and its participants must be fully understood in order to inform preventative policy. In order to more effectively reduce zoonotic disease risk from industrial food animal production, private incentives for the implementation of biosecurity must align with public health interests.

Protecting poultry workers from exposure to avian influenza viruses

Emerging zoonotic diseases are of increasing regional and global importance. Preventing occupational exposure to zoonotic diseases protects workers as well as their families, communities, and the public health. Workers can be protected from zoonotic diseases most effectively by preventing and controlling diseases in animals, reducing workplace exposures, and educating workers. Certain avian influenza viruses are potential zoonotic disease agents that may be transmitted from infected birds to humans. Poultry workers are at risk of becoming infected with these viruses if they are exposed to infected birds or virus-contaminated materials or environments. Critical components of worker protection include educating employers and training poultry workers about occupational exposure to avian influenza viruses. Other recommendations for protecting poultry workers include the use of good hygiene and work practices, personal protective clothing and equipment, vaccination for seasonal influenza viruses, antiviral medication, and medical surveillance. Current recommendations for protecting poultry workers from exposure to avian influenza viruses are summarized in this article.

The history of avian influenza

The first description of avian influenza (AI) dates back to 1878 in northern Italy, when Perroncito [Perroncito E. Epizoozia tifoide nei gallinacei. Annali Accad Agri Torino 1878;21:87-126] described a contagious disease of poultry associated with high mortality. The disease, termed "fowl plague", was initially confused with the acute septicemic form of fowl cholera. However, in 1880, soon after its first description, Rivolta and Delprato [as reported by Stubs EL. Fowl pest, In: Biester HE, Devries L, editors. Diseases of poultry. 1st ed. Ames, IO: Iowa State College Press; 1943. p. 493-502] showed it to be different from fowl cholera, based on clinical and pathological properties, and called it Typhus exudatious gallinarum. In 1901, Centanni and Savunzzi [Centanni E, Savonuzzi E, La peste aviaria I & II, Communicazione fatta all'accademia delle scienze mediche e naturali de Ferrara, 1901] determined that fowl plague was caused by a filterable virus; however, it was not until 1955 that the classical fowl plague virus was shown to be a type A influenza virus based on the presence of type A influenza virus type-specific ribonucleoprotein [Schäfer W. Vergleichender sero-immunologische Untersuchungen über die Viren der Influenza und klassischen Geflügelpest. Z Naturf 1955;10b:81-91]. The term fowl plague was substituted by the more appropriate term highly pathogenic avian influenza (HPAI) at the First International Symposium on Avian Influenza [Proceedings of the First International Symposium on Avian Influenza. Beltsville, MD. 1981, Avian Dis 47 (Special Issue) 2003.] and will be used throughout this review when referring to any previously described fowl plague virus.

A combination in-ovo vaccine for avian influenza virus and Newcastle disease virus

The protection of poultry from H5N1 highly pathogenic avian influenza A (HPAI) and Newcastle disease virus (NDV) can be achieved through vaccination, as part of a broader disease control strategy. We have previously generated a recombinant influenza virus expressing, (i) an H5 hemagglutinin protein, modified by the removal of the polybasic cleavage peptide and (ii) the ectodomain of the NDV hemagglutinin-neuraminidase (HN) protein in the place of the ectodomain of influenza neuraminidase (Park MS, et al. Proc Natl Acad Sci USA 2006;103(21):8203-8). Here we show this virus is attenuated in primary normal human bronchial epithelial (NHBE) cell culture, and demonstrate protection of C57BL/6 mice from lethal challenge with an H5 HA-containing influenza virus through immunisation with the recombinant virus. In addition, in-ovo vaccination of 18-day-old embryonated chicken eggs provided 90% and 80% protection against highly stringent lethal challenge by NDV and H5N1 virus, respectively. We propose that this virus has potential as a safe in-ovo live, attenuated, bivalent avian influenza and Newcastle disease virus vaccine.

Development of an antigen-capture ELISA for detection of H7 subtype avian influenza from experimentally infected chickens

Emergence of highly pathogenic avian influenza H7N1 was due to mutation of low pathogenic avian influenza H7N1 strain, which caused outbreaks in Italy between 1999 and 2000, and resulted in complete mortality of infected poultry. This outbreak places increased importance on the early detection of H7N1 AIV. Here we describe the development of a detection method for H7N1 virus from infected chickens using a specific antigen-capture-ELISA (AC-ELISA). A panel of mAbs was developed against the surface antigen HA of H7N1 AIV strain A/chicken/Singapore/94. The mAbs were screened by immunofluorescence assays, ELISA and immunoblotting. Selected mAbs 5E5 and 8F10 were of isotypes IgM and IgG and were conformation- or linear epitope-specific, respectively. These mAbs were used as capture antibodies for AC-ELISA development. The detection limit was as little as 10(2)-10(3) TCID(50) units of virus derived from tissue culture supernatants. Virus from the tracheal swab samples of experimentally infected chickens was detected from days 3 to 7 post-infection using the AC-ELISA, with results being confirmed by RT-PCR. AIV subtypes H4N1, H5N3 H9N2 and H10N5 did not react in the AC-ELISA but were RT-PCR positive, indicating that this AC-ELISA is specific for H7N1 strains.

The transmissibility of highly pathogenic avian influenza in commercial poultry in industrialised countries

Background

With the increased occurrence of outbreaks of H5N1 worldwide there is concern that the virus could enter commercial poultry farms with severe economic consequences.

Methodology/Principal Findings

We analyse data from four recent outbreaks of highly pathogenic avian influenza (HPAI) in commercial poultry to estimate the farm-to-farm reproductive number for HPAI. The reproductive number is a key measure of the transmissibility of HPAI at the farm level because it can be used to evaluate the effectiveness of the control measures. In these outbreaks the mean farm-to-farm reproductive number prior to controls ranged from 1.1 to 2.4, with the maximum farm-based reproductive number in the range 2.2 to 3.2. Enhanced bio-security, movement restrictions and prompt isolation of the infected farms in all four outbreaks substantially reduced the reproductive number, but it remained close to the threshold value 1 necessary to ensure the disease will be eradicated.

Conclusions/Significance

Our results show that depending on the particular situation in which an outbreak of avian influenza occurs, current controls might not be enough to eradicate the disease, and therefore a close monitoring of the outbreak is required. The method we used for estimating the reproductive number is straightforward to implement and can be used in real-time. It therefore can be a useful tool to inform policy decisions.

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.

Establishment of A Competitive ELISA (cELISA) System for the Detection of Influenza A Virus Nucleoprotein Antibodies and its Application to Field Sera from Different Species

A recombinant baculovirus (RBV) encoding the nucleoprotein (NP) of avian influenza virus (AIV) was generated and the appropriate protein was expressed in Sf9 cells. Purified recombinant NP and the NP-specific monoclonal antibody HB65 were used to establish a competitive ELISA (cELISA) system for the detection of NP-specific antibodies in sera of ducks, geese and wild birds. Tests to evaluate this method were carried out using sera of ducks experimentally infected with AIV, pre-immune duck and chicken sera, and poultry field sera, which tested negative in the haemagglutination inhibition (HI) assay, and field sera of several poultry species experimentally infected with other viruses. The evaluation of the test demonstrated a high sensitivity and specificity of this method. Tests carried out using field sera of duck and goose flocks revealed widely corresponding results obtained by HI assay and cELISA indicating that this test is applicable for flock diagnosis. Differing results were obtained for individual samples. It can be assumed that for the most part this was because of a better recognition of the conserved NP antigen by serum antibodies, although some results remained unclear.

Assessing genetic diversity in indigenous Veneto chicken breeds using AFLP markers

Genetic variation in four indigenous chicken breeds from the Veneto region of Italy was assessed using amplified fragment length polymorphism (AFLP) markers. A total of 99 individuals were analysed using three AFLP primer combinations that produced 70 polymorphisms. Four indigenous Veneto chicken breeds (Ermellinata, Padovana, Pépoi and Robusta) and a reference broiler line were included in the analysis. Breed-specific markers were identified in each breed. The expected heterozygosity did not differ significantly among the indigenous Veneto chicken breeds and the broiler line. The coefficient of gene variation (Gst) value across loci indicated that almost half of the total variability was observed among breeds. Nei's standard genetic distance between pairs of breeds showed that the distance between the broiler line and the Pépoi breed was greater than the distances between the broiler line and the other three chicken breeds. Cluster analysis based on standard genetic distances between breeds indicated that the Padovana and Pépoi breeds were closely related. Factorial analysis based on a binary matrix of the AFLP data showed a clear distinction of all breeds.

Analysis of the 1999–2000 highly pathogenic avian influenza (H7N1) epidemic in the main poultry-production area in northern Italy

We evaluated the effects of risk factors and control policies following the highly pathogenic avian influenza (HPAI) epidemic that struck northern Italy's poultry industry in the winter of 1999-2000. The epidemic was caused by a type-A influenza virus of the H7N1 subtype, that originated from a low-pathogenic AI virus which spread among poultry farms in northeastern Italy in 1999 and eventually became virulent by mutation. Most infected premises (IP) were located in the regions of Lombardy and Veneto (382 out of 413, 92.5%), and the eradication measures provided for in the European legislation were enforced. In Veneto, where flock density was highest, infection-control was also accomplished by means of depopulation of susceptible flocks through a ban on restocking and pre-emptive slaughter of flocks that were in the vicinities of or that had dangerous contacts with IPs. In Lombardy, such control measures were applied to a lesser extent. Infection incidence rate (IR) was 2.6 cases per 1000 flocks per day in Lombardy and 1.1 in Veneto. After the implementation of infection-control measures, the at-risk population, the percentage of flocks < or =1.5 km from IPs, and the HPAI-IR underwent a greater reduction in Veneto than in Lombardy. Although the proximity (< or =1.5 km) to IPs in the temporal risk window (TRW) was a major risk factor for HPAI at the individual flock level, its effect at the population level (population-attributable fraction) did not exceed 31.3%. Viral transmission therefore also occurred among relatively distant flocks. Turkey flocks were characterised by greater IR of HPAI compared with other bird species such as layer hens, broilers, gamebirds, and waterfowl, even when located at distances >1.5 km from IPs. In Lombardy, IR for species other than turkeys was also relatively high.

Highly pathogenic avian influenza H5N1, Thailand, 2004

Early detection and control curtail outbreaks.

In January 2004, highly pathogenic avian influenza (HPAI) virus of the H5N1 subtype was first confirmed in poultry and humans in Thailand. Control measures, e.g., culling poultry flocks, restricting poultry movement, and improving hygiene, were implemented. Poultry populations in 1,417 villages in 60 of 76 provinces were affected in 2004. A total of 83% of infected flocks confirmed by laboratories were backyard chickens (56%) or ducks (27%). Outbreaks were concentrated in the Central, the southern part of the Northern, and Eastern Regions of Thailand, which are wetlands, water reservoirs, and dense poultry areas. More than 62 million birds were either killed by HPAI viruses or culled. H5N1 virus from poultry caused 17 human cases and 12 deaths in Thailand; a number of domestic cats, captive tigers, and leopards also died of the H5N1 virus. In 2005, the epidemic is ongoing in Thailand.

Comparison of in vitro replication features of H7N3 influenza viruses from wild ducks and turkeys: potential implications for interspecies transmission

In previous work, it was shown that turkey H7N3 influenza viruses, presumably derived ‘in toto’ from interspecies transmission of duck viruses in Northern Italy, had only 2 aa differences in haemagglutinin and a few amino acid differences as well as a 23 aa deletion in neuraminidase compared with duck viruses. Here, the replication of these duck and turkey viruses in Madin–Darby canine kidney cells was investigated with respect to virus–cell fusion and viral elution from red blood cells. Duck viruses showed similar receptor-binding properties to turkey viruses but possessed a higher pH of fusion activation than the turkey viruses. Conversely, turkey viruses were not able to elute from red blood cells. These data confirm that neuraminidase-stalk deletion impairs the release of virions from cells and also confirm existence of naturally occurring viruses with different pH fusion activities, raising the possibility that these features may play a role in the evolution of influenza viruses in different hosts.

Measurement of antibodies to avian influenza virus A(H7N7) in humans by hemagglutination inhibition test

During the epizootic of highly pathogenic avian influenza A(H7N7) in 2003 in The Netherlands, RT-PCR and culture confirmed infection was detected in 89 persons who were ill. A modified hemagglutination inhibition (HI) test using horse erythrocytes and 2 hemagglutinating units of virus was applied to assess retrospectively the extent of human (subclinical) infection. Validation of the HI-test with sera from 34 RT-PCR and culture confirmed A(H7) infected persons and sera from 100 persons from a human influenza vaccine trial in autumn 2002 showed that this HI-test had a sensitivity of 85% and a specificity of 100% when using a cut-off titer of > or =10. Using this cut-off value, A(H7) specific antibodies were detected in 49% of 508 persons exposed to poultry and in 64% of 63 persons exposed to A(H7) infected persons. Correlation of seropositivity with the occurrence of eye symptoms in exposed persons who had not received antiviral prophylaxis and of reduced seropositivity with taking antiviral prophylaxis provided further evidence that the A(H7) HI antibody titers were real. In conclusion, by applying an HI-test using horse erythrocytes human antibodies against the avian A(H7N7) virus were detected with high sensitivity and specificity in an unexpectedly high proportion of exposed persons.

Transmission of H7N7 avian influenza A virus to human beings during a large outbreak in commercial poultry farms in the Netherlands

BACKGROUND

An outbreak of highly pathogenic avian influenza A virus subtype H7N7 started at the end of February, 2003, in commercial poultry farms in the Netherlands. Although the risk of transmission of these viruses to humans was initially thought to be low, an outbreak investigation was launched to assess the extent of transmission of influenza A virus subtype H7N7 from chickens to humans.

METHODS

All workers in poultry farms, poultry farmers, and their families were asked to report signs of conjunctivitis or influenza-like illness. People with complaints were tested for influenza virus type A subtype H7 (A/H7) infection and completed a health questionnaire about type of symptoms, duration of illness, and possible exposures to infected poultry.

FINDINGS

453 people had health complaints--349 reported conjunctivitis, 90 had influenza-like illness, and 67 had other complaints. We detected A/H7 in conjunctival samples from 78 (26.4%) people with conjunctivitis only, in five (9.4%) with influenza-like illness and conjunctivitis, in two (5.4%) with influenza-like illness only, and in four (6%) who reported other symptoms. Most positive samples had been collected within 5 days of symptom onset. A/H7 infection was confirmed in three contacts (of 83 tested), one of whom developed influenza-like illness. Six people had influenza A/H3N2 infection. After 19 people had been diagnosed with the infection, all workers received mandatory influenza virus vaccination and prophylactic treatment with oseltamivir. More than half (56%) of A/H7 infections reported here arose before the vaccination and treatment programme.

INTERPRETATION

We noted an unexpectedly high number of transmissions of avian influenza A virus subtype H7N7 to people directly involved in handling infected poultry, and we noted evidence for person-to-person transmission. Our data emphasise the importance of adequate surveillance, outbreak preparedness, and pandemic planning.

Nucleic acid sequence-based amplification methods to detect avian influenza virus

Infection of poultry with highly pathogenic avian influenza virus (AIV) can be devastating in terms of flock morbidity and mortality, economic loss, and social disruption. The causative agent is confined to certain isolates of influenza A virus subtypes H5 and H7. Due to the potential of direct transfer of avian influenza to humans, continued research into rapid diagnostic tests for influenza is therefore necessary. A nucleic acid sequence-based amplification (NASBA) method was developed to detect a portion of the haemagglutinin gene of avian influenza A virus subtypes H5 and H7 irrespective of lineage. A further NASBA assay, based on the matrix gene, was able to detect examples of all known subtypes (H1–H15) of avian influenza virus. The entire nucleic acid isolation, amplification, and detection procedure was completed within 6 h. The dynamic range of the three AIV assays was five to seven orders of magnitude. The assays were sensitive and highly specific, with no cross-reactivity to phylogenetically or clinically relevant viruses. The results of the three AIV NASBA assays correlated with those obtained by viral culture in embryonated fowl’s eggs.

Rapid and sensitive detection of avian influenza virus subtype H7 using NASBA

Nucleic acid sequence-based amplification with electrochemiluminescent detection (NASBA/ECL) is an isothermal technique allowing rapid amplification and detection of specific regions of nucleic acid from a diverse range of sources. It is especially suitable for amplifying RNA. A NASBA/ECL technique has been developed allowing the detection of RNA from avian influenza virus subtype H7 derived from allantoic fluid harvested from inoculated chick embryos and from cell cultures. Degenerate amplification primers and amplicon capture probes were designed enabling the detection of low and highly pathogenic avian influenza of the H7 subtype from the Eurasian and North American lineages and the Australian sub-lineage. The NASBA/ECL technique is specific for subtype H7 and does not cross-react with other influenza subtypes or with viruses containing haemagglutinin-like genes. The assay is 10- to 100-fold more sensitive than a commercially available antigen capture immunoassay system. The NASBA/ECL assay could be used in high throughput poultry screening programmes.