The Norwegian pig population was considered free from influenza A virus infections until the first case of porcine pandemic influenza A (H1N1) 2009 virus infection in October 2009. Human to pig transmission of virus was suspected. Unusual lung lesions were observed in fattening pigs, with red, lobular, multifocal to coalescing consolidation, most frequently in the cranial, middle, and accessory lobes. The main histopathological findings were epithelial degeneration and necrosis, lymphocyte infiltration in the epithelial lining and lamina propria of small bronchi and bronchioles, and peribronchial and peribronchiolar lymphocyte infiltrations. Infection with pandemic influenza A (H1N1) 2009 virus was confirmed by real-time RT-PCR and immunohistochemical detection of influenza A virus nucleoprotein in the lesions. This investigation shows that natural infection with the pandemic influenza A (H1N1) 2009 virus induces lung lesions similar to lesions described in experimental studies and natural infections with other swine-adapted subtypes of influenza A viruses. 1. Introduction Infections with swine-adapted subtypes of influenza A virus are enzootic in most pig-producing countries and cause a highly contagious respiratory disease in pigs [1, 2]. The pathological changes are restricted to the respiratory tract with necrotizing bronchitis and bronchiolitis and multifocal lobular purulent bronchopneumonia as the main lesions [3]. In April 2009, a novel strain of influenza A H1N1 virus, now known as pandemic influenza A (H1N1) 2009 (pandemic H1N1 2009 virus), affecting humans was detected in Mexico and USA [4]. The new strain was shortly afterwards reported to be transmitted from humans to pigs, and the following few weeks infection in pigs was reported from Argentina, Canada, Australia, Northern Ireland, Ireland, and the United States [5–7]. The first porcine case of pandemic H1N1 2009 virus in Norway was detected in the beginning of October 2009 [8]. A farm staff member had shown influenza-like illness and had tested positive for pandemic H1N1 2009 virus in the same period as a sow showed signs of respiratory disease, and human to pig transmission was suspected. In the following three months, infection with pandemic H1N1 2009 virus was widespread and detected in 91 Norwegian pig herds [9]. The Norwegian pig population is relatively small, totaling 2,500 herds and with annual production of 1,5 million fattening pigs. Up to October 2009 the population was free from influenza A virus infection (subtypes H1N1 and H3N2) as judged by a serological surveillance
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