The assessment of transfer of passive immunity in dairy calves affected by neonatal diarrhea

The assessment of immune status has practical implications in calves’ management and health. However, the presence of hemodynamic alterations like dehydration caused by Neonatal Calf Diarrhea (NCD), makes the diagnosis of failure of transfer of passive immunity (FTPI) challenging. This study aimed to evaluate the diagnostic performances of serum total protein refractometry (STP) and gamma-glutamyl-transferase activity (GGT) for assessing FTPI in dairy calves affected by NCD. 91 Friesian calves aged 1 to 10 days were enrolled for this study. Among them, 72 were admitted to the Veterinary Teaching Hospital of the University of Milan as affected by NCD, while 19 were healthy. Each calf underwent a complete clinical examination and dehydration assessment. Blood sampling was performed and serum was tested for STP concentration, GGT activity and IgG concentration. The effect of dehydration status and age on the correlation between the two methods under study and the gold standard (IgG) was investigated with Spearman's correlation index R for ranks. Receiver operating characteristic (ROC) curve analysis was performed to identify the optimal cut-off point to distinguish between diarrheic calves with or without FTPI (IgG < 10 g/L). Significant differences in IgG concentration (p= 0.009), STP concentration (p= 0.040), and age (p= 0.042) were found between healthy and NCD calves, while no statistical difference was found in GGT activity (p=0.081). GGT was affected by age (p= 0.003), while STP was influenced by dehydration (p= 0.013). The cut-offs for identifying FTPI were 52 g/L of STP in normohydrated calves, 58 g/L of STP in dehydrated calves, and 126 UI/L of GGT in calves with more than 3 days of age. In normohydrated diarrheic calves STP refractometry showed better diagnostic accuracy, whereas in dehydrated calves its accuracy dropped, and it resulted advisable to use GGT activity. This study confirms the hypothesized better performance of GGT activity in diagnosing FTPI in dehydrated calves.