Immunological and bacteriological quality of bovine colostrum, passive transfer of immunity, and associated maternal, colostral and management-related risk factors in dairy calves in northern Algeria.

Adequate colostrum management is critical for ensuring successful transfer of passive immunity (TPI) and reducing neonatal morbidity and mortality in dairy calves. However, comprehensive data on colostrum quality and its impact on calf health remain scarce in North African dairy production systems. Objectives: This prospective cohort study evaluated the immunological and bacteriological quality of first-milking colostrum and the prevalence of TPI, neonatal diarrhoea and mortality, and identified the independent risk factors associated with colostrum quality and failure of passive transfer (FPT) in commercial dairy herds in northern Algeria. A total of 188 cow-calf pairs (188 cows and 192 calves, with four twin pregnancies) from 16 commercial dairy farms in Tizi-Ouzou were enrolled between March 2025 and June 2026. Colostral immunological quality was assessed using the COL-IgG-Test® (BioX Diagnostics), while bacteriological quality was determined by standard culture and biochemical identification. Passive transfer was evaluated in calves aged 2-6 days using the Calf-IgG-Test®. Early-onset diarrhoea was recorded over the first 7 days of life, and mortality throughout the neonatal period. Associations were analysed using univariable and multivariable logistic regression, with model discrimination assessed by the area under the ROC curve (AUC). Overall, 58.0% (109/188) of colostrum samples were of good quality (IgG ≥ 50 g/L) and 28.7% (54/188) were bacterially contaminated, predominantly by S. aureus and E. coli (25.9% each). Successful passive transfer occurred in 60.4% (116/192) of calves, while 37.0% (71/192) developed neonatal diarrhoea and 24.0% (46/192) died. In multivariable analysis, dystocia (aOR = 29.89; 95% CI: 5.95-150.33) and positive/neutral energy balance (aOR = 0.015; 95% CI: 0.002-0.155) independently predicted poor colostrum quality. For FPT, the strongest predictors were poor colostrum quality (aOR = 11.48; 4.26-30.91), delayed first suckling > 2 h (aOR = 6.24; 2.32-16.79), colostrum intake < 4 L (aOR = 4.35; 1.72-11.02), positive bacterial culture (aOR = 3.63; 1.39-9.45), dystocia (aOR = 3.19; 1.18-8.61) and negative energy balance (aOR = 2.56; 1.01-6.46). The final model showed good discrimination (AUC = 0.82). Poor colostrum quality, bacterial contamination and inadequate management remain the main determinants of FPT and neonatal mortality in Algerian dairy herds. Routine Brix-based assessment, improved transition-cow nutrition, hygienic colostrum handling and administration of adequate colostrum volumes within 2 h after birth are recommended to improve calf health and productivity.