The Norwegian pig population has been free from influenza viruses until 2009. The pandemic influenza outbreak during the autumn 2009 provided an opportunity to study the clinical impact of this infection in an entirely na？ve pig population. This paper describes the results of a case-control study on the clinical impact of pandemic influenza (H1N1) 2009 infection in the nucleus and multiplier herds in Norway. The infection spread readily and led to seroconversion of 42% of the Norwegian nucleus and multiplier herds within a year. Positive and negative herds were identified based on surveillance data from the Norwegian Veterinary Institute. Telephone interviews were conducted with pig herd owners or managers between November 2010 and January 2011. Pigs with clinical signs were reported from 40% of the case herds with varying morbidity and duration of respiratory disease and reduced reproductive performance. Clinical signs were reported in all age groups. 1. Introduction Pandemic influenza A (H1N1) 2009 virus was first recorded in Norwegian pig herds in October 2009 [1]. Before that, documentation on freedom from several specific viral diseases in the pig population was provided by the surveillance and control program, where swine influenza (subtypes H1N1 and H3N2) has been included since 1997 [2]. All the nucleus and multiplying herds are included in this program. The Norwegian pig population is also documented free from porcine reproductive and respiratory syndrome virus, Aujeszky’s disease, porcine respiratory corona virus, and transmissible gastroenteritis [2]. In 2009 the pig population in Norway was declared free from enzootic pneumonia (Mycoplasma hyopneumoniae) [3]. Porcine circovirus type 2 is, however, presumed to be present in all Norwegian swine herds, including nucleus and multiplier herds. In contrast to Norway, the pig populations in most other countries are endemically infected with different swine adapted subtypes of influenza A virus [4–6]. Typical clinical signs associated with influenza infection are characterized by an acute onset of fever of short duration, inappetence, lethargy, coughing, dyspnea, and nasal discharge. Morbidity within infected herds is high (approaching 100%), but mortality is typically low (less than 1%) [7]. In recent experimental studies with pandemic influenza (H1N1) 2009 virus, a similar clinical picture has been described [8, 9]. Influenza viruses can also act synergistically with other viral and bacterial pathogens to cause porcine respiratory disease complex [10–12]. The course and severity of an influenza
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