Microvascular congestion is prominent throughout the sections. These animals had rather characteristic clinical signs of swine influenza, but the severity was somewhat unusual. A variant of swine influenza which has recently been described causes proliferative and necrotizing pneumonia similar to the lesions seen in these cases. An acute fibrinohemorrhagic pneumonia caused by Streptococcus suis has also been recently described in which fibrinoid alveolar septal necrosis similar to that seen in these cases is a prominent feature. The extremely high morbidity and acute nature of the outbreak along with histologic features suggest swine influenza virus as the primary etiologic agent, but a dual infection with Streptococcus suis cannot be ruled out based upon the histologic lesions.
Swine influenza is an acute contagious disease caused by type A orthomyxoviruses.
Disease occurs in the form of explosive outbreaks with high morbidity but low mortality. The disease is often confined to young pigs and outbreaks occur mainly during the winter months. The classic disease is caused by the viral subtype H1N1, which contains distinct antigenic variants which may circulate concurrently in pig populations in different parts of the world. Subtype H1N1 is the primary agent of swine influenza in North America. Subtype H3N2 is prevalent in Europe.