Vaccination reduces macrophage infiltration in bronchus-associated lymphoid tissue in pigs infected with a highly virulent Mycoplasma hyopneumoniae strain

The immunostaining revealed a lower density of macrophages in the BALT of the vaccinated groups compared to the non-vaccinated groups. The highest number of M. hyopneumoniae organisms in the BAL fluid was measured at 4 weeks PI for the HV strain and at 8 weeks PI for the LV strain. Vaccination reduced the number of organisms non-significantly, though for the HV strain the reduction was clinically more relevant than for the LV strain. At the level of the individual pigs, a higher lung lesion score was associated with more M. hyopneumoniae organisms in the lungs and a higher density of the investigated immune cells in the BALT.In conclusion, the infiltration of macrophages after infection with M. hyopneumoniae is reduced by vaccination. The M. hyopneumoniae replication in the lungs is also reduced in vaccinated pigs, though the HV strain is inhibited more than the LV strain.Mycoplasma hyopneumoniae is the causative agent of enzootic pneumonia (EP) in pigs. This disease is characterized by chronic non-productive coughing and poor growth rate and feed conversion ratio [1]. The disease occurs worldwide and causes significant economic losses to the pig industry [2].The infection pattern and the severity of the disease in pig herds can be influenced by many factors such as management practices, housing conditions and the virulence of the M. hyopneumoniae strain [3,4]. Based on protein and genetic studies of field isolates, there appears to be considerable diversity among this species [5,6]. Recently, it has been shown thaQ1t within a herd and even within a pig, different strains of M. hyopneumoniae can be present [7,8]. In a recent study, it was shown that disease signs aggravated after infection with a highly virulent (HV) strain when pigs had been previously inoculated with a low virulent (LV) strain [9]. The differences in virulence between isolates may be partly due to a differential capacity to provoke an inflammatory response in the host [10].The diversity between M.