The objective of this work was to evaluate the performance of piglets in the nursery phase submitted to activation of immune system with vaccine against Haemophilus parasuis and to different levels of metabolizable energy in the diets. It was used 160 barrows and females at 30 days of age, weaned at 23 days of age, with initial weight of 7.97 ± 0.54 kg, distributed in 2 × 4 factorial arrangement (with vaccination and without vaccination with four levels of metabolizable energy - 3,200; 3,300; 3,400 and 3,500 kcal/kg). Each combination was evaluated with five repetitions of four animals per experimental plot (two males and two females). Energy levels reduced linearly the intake, feed conversion and the total number of leukocytes in blood in the experimental period, regardless of the use of vaccine which in turn reduced daily average weight gain only in the first week after second application and it increased the number of limonocytes and spleen weight after three weeks. There was no difference in the C-reactive protein quantity in the blood in the periods when it was evaluated. The use of vaccine reduces performance immediatelly after its application but after it occurs recovery of performance with changes in the number of monocytes and spleen weight. Dietary energy levels do not smooth this reduction in the performance, but they influence intake, feed conversion and blood parameters in the total period of use.
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