The 2003 outbreak of Highly pathogenic avian influenza (HPAI) A(H7N7) in the Netherlands, Belgium and Germany resulted in significant genetic diversification that proved informative for tracing transmission events. Building on previous investigations on the Dutch outbreak, we focused on the potential transnational transmissions between the Netherlands and Belgium. Although no clear epidemiological links could be identified from the tracing data, the transmission network based on concatenated HA-NA-PB2 sequences supports at least three independent introductions from the Netherlands to Belgium and suggests one possible introduction form Belgium back to the Netherlands. Two introductions in the Belgian province of Limburg occurred from nearby farms in the Dutch province of Limburg. One introduction resulted in three secondary infected farms, while a second introduction did not cause secondary infections. The third introduction into Belgium occurred in the north of the Antwerp province, very close to the national border, and originated from the North of the Dutch province Brabant (long distance transmission, >65 km). The virus spread to two additional Belgian farms, one of which may be the source of a secondarily infected farm in the Netherlands. One infected turkey farm in the province of Antwerp (Westmalle) was geographically close to the latter introduction, but genetically clustered with the first introduction event in the Limburg province. Epidemiological tracing data could neither confirm nor exclude whether this outbreak was a result from long distance contacts within Belgium or whether this farm presented a fourth independent transboundary introduction. These multiple transnational transmissions of HPAI in spite of reinforced biosecurity measures and trade restrictions illustrate the importance of international cooperation, legislation and standardization of tools to combat transboundary diseases.
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