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      Evaluating the control of HPAIV H5N1 in Vietnam: virus transmission within infected flocks reported before and after vaccination

      research-article
      Author(s): 1 , 2 , , 1 , 3
      Publication date (Electronic):
      Journal: BMC Veterinary Research
      Publisher: BioMed Central

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          Abstract

          Background

          Currently, the highly pathogenic avian influenza virus (HPAIV) of the subtype H5N1 is believed to have reached an endemic cycle in Vietnam. We used routine surveillance data on HPAIV H5N1 poultry outbreaks in Vietnam to estimate and compare the within-flock reproductive number of infection ( R 0 ) for periods before (second epidemic wave, 2004-5; depopulation-based disease control) and during (fourth epidemic wave, beginning 2007; vaccination-based disease control) vaccination.

          Results

          Our results show that infected premises (IPs) in the initial (exponential) phases of outbreak periods have the highest R 0 estimates. The IPs reported during the outbreak period when depopulation-based disease control was implemented had higher R 0 estimates than IPs reported during the outbreak period when vaccination-based disease control was used. In the latter period, in some flocks of a defined size and species composition, within-flock transmission estimates were not significantly below the threshold for transmission ( R 0 < 1).

          Conclusions

          Our results indicate that the current control policy based on depopulation plus vaccination has protected the majority of poultry flocks against infection. However, in some flocks the determinants associated with suboptimal protection need to be further investigated as these may explain the current pattern of infection in animal and human populations.

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          Most cited references17

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          Population biology of infectious diseases: Part I.

          R Anderson, R M May (1979)
          If the host population is taken to be a dynamic variable (rather than constant, as conventionally assumed), a wider understanding of the population biology of infectious diseases emerges. In this first part of a two-part article, mathematical models are developed, shown to fit data from laboratory experiments, and used to explore the evolutionary relations among transmission parameters. In the second part of the article, to be published in next week's issue, the models are extended to include indirectly transmitted infections, and the general implications for infectious diseases are considered.
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            Effect of vaccine use in the evolution of Mexican lineage H5N2 avian influenza virus.

            Jang Won Lee, Dennis Senne, D Suarez (2004)
            An outbreak of avian influenza (AI) caused by a low-pathogenic H5N2 type A influenza virus began in Mexico in 1993 and several highly pathogenic strains of the virus emerged in 1994-1995. The highly pathogenic virus has not been reported since 1996, but the low-pathogenic virus remains endemic in Mexico and has spread to two adjacent countries, Guatemala and El Salvador. Measures implemented to control the outbreak and eradicate the virus in Mexico have included a widespread vaccination program in effect since 1995. Because this is the first case of long-term use of AI vaccines in poultry, the Mexican lineage virus presented us with a unique opportunity to examine the evolution of type A influenza virus circulating in poultry populations where there was elevated herd immunity due to maternal and active immunity. We analyzed the coding sequence of the HA1 subunit and the NS gene of 52 Mexican lineage viruses that were isolated between 1993 and 2002. Phylogenetic analysis indicated the presence of multiple sublineages of Mexican lineage isolates at the time vaccine was introduced. Further, most of the viruses isolated after the introduction of vaccine belonged to sublineages separate from the vaccine's sublineage. Serologic analysis using hemagglutination inhibition and virus neutralization tests showed major antigenic differences among isolates belonging to the different sublineages. Vaccine protection studies further confirmed the in vitro serologic results indicating that commercial vaccine was not able to prevent virus shedding when chickens were challenged with antigenically different isolates. These findings indicate that multilineage antigenic drift, which has not been observed in AI virus, is occurring in the Mexican lineage AI viruses and the persistence of the virus in the field is likely aided by its large antigenic difference from the vaccine strain.
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              An analysis of the spatial and temporal patterns of highly pathogenic avian influenza occurrence in Vietnam using national surveillance data.

              Phan Q Minh, Dirk U. Pfeiffer, Vincent Faure … (2007)
              The objectives of this study were to describe the spatio-temporal pattern of an epidemic of highly pathogenic avian influenza (HPAI) in Vietnam and to identify potential risk factors for the introduction and maintenance of infection within the poultry population. The results indicate that during the time period 2004-early 2006 a sequence of three epidemic waves occurred in Vietnam as distinct spatial and temporal clusters. The risk of outbreak occurrence increased with a greater percentage of rice paddy fields, increasing domestic water bird and chicken density. It increased with reducing distance to higher population density aggregations, and in the third epidemic wave with increasing percentage of aquaculture. The findings indicate that agri-livestock farming systems involving domestic water birds and rice production in river delta areas are important for the maintenance and spread of infection. While the government's control measures appear to have been effective in the South and Central parts of Vietnam, it is likely that in the North of Vietnam the vaccination campaign led to transmission of infection which was subsequently brought under control.
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                Author and article information

                Journal
                Journal ID (nlm-ta): BMC Vet Res
                Title: BMC Veterinary Research
                Publisher: BioMed Central
                ISSN (Electronic): 1746-6148
                Publication date Collection: 2010
                Publication date (Electronic): 5 June 2010
                Volume: 6
                Page: 31
                Affiliations
                [1 ]The Royal Veterinary College, Department of Veterinary Clinical Sciences, Veterinary Epidemiology and Public Health Group, London, UK, Hawkshead Lane AL9 7TA, UK
                [2 ]School of Population Health, University of Queensland, Edith Cavell Building, Herston Road, Herston QLD 4006, Australia
                [3 ]Food and Agriculture Organization of the United Nation, Pro-Poor Livestock Initiative, Vialle Del Terme de Caracalla, 00153 Rome, Italy
                Article
                Publisher ID: 1746-6148-6-31
                DOI: 10.1186/1746-6148-6-31
                PMC ID: 2898779
                PubMed ID: 20525380
                SO-VID: 50586de5-f1d4-40a1-876f-b22a50c1bdf1
                Copyright © Copyright ©2010 Magalhães et al; licensee BioMed Central Ltd.
                License:

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                Date received : 14 July 2009
                Date accepted : 5 June 2010
                Categories
                Subject: Research article

                ScienceOpen disciplines: Veterinary medicine
                Data availability:
                ScienceOpen disciplines: Veterinary medicine

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