Description of innate immunity and haematological changes in Holstein calves during the gradual weaning process

The weaning process negatively affects the haematological parameters and innate immune response of dairy calves, even when managed under an intensive milk program. Here we describe haematological and innate immunity changes in 47 Holstein calves aged 69-85 days subjected to a gradual weaning process. Blood samples were collected at six (D-6), four (D-4), and two (D-2) days before and on the weaning day (D0) for the phagocytosis assay and to measure the production of reactive oxygen species (ROS) after stimulation with Staphylococcus aureus, Escherichia coli, and Mannhemia haemolytica, in addition to total protein (TP), haptoglobin (Hp), and iron concentration. The highest mean neutrophil number was recorded at D-2. The absolute number of monocytes was initially high on D-6 and D-4 but declined as the calf progressed to weaning. The number of basophils decreased rapidly, reaching a low value on D-4, and remained low for the remainder of the study period. The TP, Hp, and Fe concentrations decreased. Overall, polymorphonuclear leukocyte phagocytosis activity induced by S. aureus and E. coli decreased from D-6 to D-2, indicating persistence of the low phagocytosis rate for S. aureus. ROS production was constant for all bacterial stimulations from D-6 to D-2, followed by an increase on D0. Phagocytosis and ROS production indicate that the weaning process dampens the innate immune response relative to exposure to these common pathogenic bacteria in dairy calves. Phagocytosis and the corresponding indicators of intracellular killing activities (ROS production and myeloperoxidase index) represent the most accepted core mechanisms for the early elimination of pathogenic microorganisms in calves. Despite a slow gradual weaning management system, the study concluded that intensive milk production programs contribute to innate immune response suppression during weaning.