Avian influenza (AI) is a constant threat to the poultry industry in Europe. Up to date assessments of the factors influencing the probability of introduction of infection into poultry holdings are required for the implementation of informed prevention and control measures. We carried out a systematic literature review to identify and synthesise the evidence on risk factors of AI introduction into European poultry holdings. A review protocol was built, which considered the electronic search strategy, criteria for relevance screening, quality assessment and data collection. A total of 941 abstracts were evaluated and 145 relevant manuscripts were selected for evaluation of the full text. This assessment resulted in the selection of 29 manuscripts for data collection and analyses. These manuscripts provided information on 54 AI introductions (outbreaks) in poultry, with 25 of those being caused by highly pathogenic AI viruses and 29 caused by low pathogenic AI viruses. The identified risk factors influencing the probability of introduction of AI in the affected poultry holdings were: the poultry species and production system, contact with wild birds or material (fomites) contaminated by wild birds, abattoirs processing and commercialising contaminated fresh meat, holdings sharing personnel or equipment, undetected circulation of avian influenza in poultry and flaws in biosecurity of the affected holdings. The main source of introduction of AI virus was considered to be contact with wild birds or contaminated material, with 42 out of the 54 studied outbreaks providing evidence to support this source of introduction. This was further supported by the higher risk of introductions reported in free range poultry holdings. In conclusion, evidence on the different risk factors for AI introduction are summarised in this SR to support decision making.
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