Physiological changes during the evolution of diarrhea in preweaning calves prior to the onset of clinical signs.

Neonatal calf diarrhea remains a major issue in terms of mortality, economic, and animal welfare losses. In addition to different infectious and pathogenic causes, nonpathogenic factors, particularly stress and nutritional and management practices, also contribute significantly to the incidence of diarrhea in calves. This study aimed to investigate changes in blood metabolites and fecal parameters (pH value, short-chain fatty acids, and triglyceride content) preceding the onset of diarrhea signs in neonatal calves. Twenty-two calves were monitored up to d 13 of life, and blood and fecal samples were collected daily. Eleven calves developed diarrhea (DRH; fecal score of 3 on a scale from 0 to 3) at approximately 9.3 ± 2.7 d of age and were matched with healthy controls based on birth date, age, sex, and birth weight. Calves with diarrhea exhibited reduced milk intake and increased fecal scores from the first day of diarrhea (DOD 1). Notably, several metabolic indicators, including serum IgG, triglycerides, insulin, cholesterol, and plasma osmolality, were significantly lower in DRH calves up to 3 d before clinical manifestation. Fecal pH increased and fecal short-chain fatty acid concentrations declined in DRH calves from DOD 1 onward, whereas fecal triglycerides increased from DOD -1, indicating altered digestion and gut fermentation. Plasma glucose decreased, and plasma urea increased postdiarrhea onset. No differences were observed in nonesterified-fatty-acids, total protein, acute-phase proteins, or activities in hepatic enzymes. The early alterations in blood and fecal parameters before clinical symptoms indicate adaptation mechanisms in calves facing subclinical digestive stress. Further research is needed to elucidate the physiological thresholds and mechanisms triggering the progression from subclinical dysfunction to overt diarrhea. Optimizing feeding strategies and calf management may help mitigate such digestive disorders in neonatal calves.