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Introduction
Mycoplasma hyopneumoniae is the primary agent of enzootic pneumonia (EP) which affects mainly grow-finishing pigs and inflicts major economic losses on the pig industry ( Thacker and Minion 2012 ). Vaccination is a very common control measure which has been extensively proven to reduce performance losses, the severity of clinical signs and lung lesions ( Maes and others 1998 , 1999 , Jensen and others 2002 , Wilson and others 2012 , Del Pozo Sacristán and others 2014 ).
Currently, vaccination at weaning is commonly practised as the handling of the piglets is inevitable at that time ( Alarcon and others 2014 , Del Pozo Sacristán and others 2014 ). On the other hand, weaning is one of the most stressful events in the life of a piglet ( Pié and others 2004 , Campbell and others 2013 ). It is not recommended to vaccinate animals when they are severely stressed ( Chase and others 2012 ). For that reason, vaccinating the animals two to three days before weaning is also practised by some pig producers ( Gillespie and others 2010 ).
Consequently, one important question that remains unaddressed is whether vaccination at the day of weaning has an influence on the efficacy of M hyopneumoniae vaccines. A previous study investigated the efficacy of a single M hyopneumoniae vaccination three days before weaning or at weaning against experimental challenge infection with a virulent M hyopneumoniae field strain ( Arsenakis and others 2016 ). The results showed that the group that was vaccinated three days before weaning had the lowest macroscopic and histopathological lung lesions. A difference of three days between the two vaccination groups was chosen to allow the first and most critical steps of the immune response to develop before the stress of weaning ( Kick and others 2011 ). However, significant differences between the vaccinated groups were only obtained for the histopathological lung lesions. Thus, it was considered that a field study would provide further insight on the same topic, since it would permit to include more animals, to test the effect under practical conditions with concurrent infections with other respiratory pathogens and to investigate performance data until slaughter age.
The objective of this study was therefore to investigate the efficacy of...