Influenza virus infections in swine were first noticed in the US in 1918, during the human pandemic of the Spanish flu. In Europe, seroprevalences for the three most common swine influenza strains at the moment, H1N1, H3N2 and H1N2, range from 20-80% in finishing pigs at the end of the finishing period and in sows. However, little is known about the population dynamics of influenza virus within herds, or between herds. By investigating 53 outbreaks of acute respiratory disease over a period of four years, it was established that influenza virus infections are responsible for more than 50% of the acute respiratory disease outbreaks in swine. Virus circulation in weaned piglets was found to be very common, suggesting that weaned piglets may act as a reservoir for influenza virus in a herd. Virus infections at that time interfere with the presence of maternal antibodies, resulting in no noticeable clinical symptoms, but also in a lesser immunity than infections in fully susceptible pigs. The occurrence of influenza virus infections in finishing pigs was quite different in farrow-to finish herds when compared to finishing herds. For all subtypes, influenza virus infections in farrow-to-finish herds occurred mainly at a young age, up to and including the first half of the finishing period. In finishing herds on the other hand, the influenza virus infections mainly occurred in the second half of the finishing period. These differences may have implications for the choice of intervention measures, whether this is intervention through vaccination, or intervention through zoosanitary measures. Vaccines, while potentially highly efficacious, have the disadvantage that they are very specific for one infectious agent, or even one subtype of an infectious agent. The big advantage of zoosanitary measures compared to vaccination is that multiple infectious agents can be addressed at the same time with the same or similar measures. Measures with respect to air filtration and increased hygiene were therefore evaluated. These measures significantly reduced the number of influenza virus infections during the finishing period and therefore most likely also the magnitude of acute respiratory disease. These measures are unlikely to be cost-beneficial when they are only applied for influenza virus infections, but the application of such measures in a broader context may offer some prospect
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