Abstract

Avian influenza (AI) is a disease of poultry* caused by Influenza A viruses belonging to the Orthomyxoviridae family. These viruses are classified into subtypes on the basis of the possession of surface antigens, the haemagglutinin (H) and the neuraminidase (N). There are 16 known types of H protein (1-16) and 9 N types (1-9), and viruses possessing virtually all possible H and N combinations have been isolated from birds. In poultry AI causes two distinct clinical forms – Highly Pathogenic Avian Influenza (HPAI) and Low Pathogenicity Avian Influenza (LPAI). HPAI is caused only by some viruses of the H5 and H7 subtypes which exhibit certain molecular characteristics that are at the basis of systemic infection. LPAI may be caused by viruses belonging to all H subtypes which lack these molecular traits, thus causing localised infection. LPAI viruses of the H5 and H7 subtype represent the progenitors of HPAI as they may mutate and acquire the molecular traits of HPAI following introduction into poultry. Current EU legislation is only directed at HPAI. Until recently, AI infections were of little or no relevance to the poultry industry and to the scientific community. Thus since its recognition in 1955, as a consequence of the paucity of natural outbreaks in most areas, the disease was not investigated thoroughly. In recent times however, the disease has escalated resulting in a global crisis with 50 million birds culled or dead in the EU alone between 2000 and 2003. The extensive circulation of the virus in Asia could represent the origin of a pandemic virus for humans and a great number of questions have been raised with a view to finding a way to combat the ongoing AI crisis. Due to the lack of field and experimental data certain questions are not yet answered although these gaps should be filled following the results of the ongoing research efforts of the scientific community. Nevertheless, on the basis of the scientific evidence currently available, it is possible to draw some conclusions on certain issues included in the mandate of this opinion although these do not include the human health aspects of AI infections. It is universally accepted that LPAI viruses of the H5 and H7 subtypes should be included in AI legislation as they represent the progenitors of HPAI. These viruses are perpetuated in nature in wild bird populations, particularly in waterfowl, which represent an ever-present reservoir of LPAI for poultry. Pigeons do not appear to have a relevant role in AI epidemiology. As a consequence of the HPAI H5N1 epidemic in South-East Asia this infection has spilled over to resident and migratory wild bird populations which could represent a means of the virus reaching the EU. The unavailability of a comprehensive set of data on the ecological, virological and patho-biological aspects of this event make any forecast imprudent. A multidisciplinary effort is necessary to clarify the likelihood and implications of such occurrence. Some poultry holdings located under migratory flyways or in close proximity with wild bird breeding or resting sites are considered to have a greater risk of exposure to AI. This risk is enhanced in case of establishments with outdoor access and areas that have a high poultry density. Specific additional methods to prevent exposure should be implemented in these establishments. In addition, there is a need to address the issue of minimum distances between poultry establishments. With reference to the presence of HPAI and LPAI in poultry commodities, it should be stated that at present safeguards only apply to HPAI. In addition, safeguards are currently only applied to legally imported commodities and rely on the detection of clinical signs. The detection of clinical signs may not be observable in certain avian species and by definition during the incubation period in all species. Illegal importation of avian commodities represents an additional risk as pre-export safeguard measures are avoided. In all cases, the systemic nature of HPAI, suggests that given infection, commodities have a greater risk of containing viable virus than with LPAI. Fomites and packaging material may also be contaminated with AI virus and may represent a source of viable virus. Insufficient specific data on the effectiveness of commodity processing is available and few indications can be deducted from limited studies. However, the exposure pathways are complex and a certain degree of dilution and degradation will occur both for LPAI and for HPAI. Biosecurity is the primary tool to combat AI, both for the prevention of the index case and for the avoidance of secondary spread. However, breaches in biosecurity systems occur and under certain circumstances they may result in massive spread. This has been observed in densely populated poultry areas worldwide, and has resulted in some cases in the implementation of vaccination programmes. Vaccination for AI viruses of the H5 and H7 subtypes was not considered until recent times, and has only had a relatively limited use in the field. Given the animal and public health threat, vaccination should be used as a tool to support eradication of AI. The evidence collected indicates that emergency vaccination will achieve the goal of eradication only if used in conjunction with enhanced biosecurity and other restriction measures. Similar measures should be implemented in case of prophylactic vaccination. The animal welfare aspect of AI infection is a significant issue, and includes the aspect of animal suffering following infection, the application of appropriate culling methods by trained staff, and the correct use of vaccination. Methods for killing birds which minimise poor welfare are listed in the recommendations. * According to EU legislation (Council Directive 1990/539/EEC) 'poultry' means fowl, turkeys, guinea fowl, ducks, geese, quails, pigeons, pheasants and partridges and ratites (ostriches, emus etc) reared or kept in captivity for breeding, the production of meat or eggs for consumption, or for re-stocking

Published in

EFSA Journal
2005, number: 266, pages: 1-21
Publisher: EFSA

SLU Authors

• Algers, Bo

  • Department of Applied Animal Science and Welfare, Swedish University of Agricultural Sciences
    

• Wierup, Martin

  • Department of Animal Biosciences, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Veterinary Science
Animal and Dairy Science


Permanent link to this page (URI)

https://res.slu.se/id/publ/7058