Evaluation of infrared thermography compared to rectal temperature to identify illness in early postpartum dairy cows

This study evaluated and compared infrared thermography (IRT) and rectal temperature (RT) as screening tests to identify sick transition dairy cows. Holstein cows (n = 72; 42 primiparous) had RT and IRT temperatures taken daily from 1 to 12 days in milk (DIM). Health examinations were performed daily to diagnose retained fetal membrane, milk fever and metritis, and blood was analyzed for β-hydroxybutyrate at 6 and 9 DIM to diagnose ketosis. Plasma concentrations of cortisol, interleukin-6, tumor necrosis factor α and serum amyloid A at 3, 6, 9 and 12 DIM were included as additional indicators of illness. Cows were categorized as true sick if clinically diagnosed with an illness, or if at least 2 blood parameters were above the normal range. Diagnostic test performances for RT and IRT variables were determined for each variable at a test referent value that provided the highest Youden's (J) index. The best performing screening test depended on the definition of true sickness. In general, the J index for RT was 0.15-0.17 whereas the highest J index for the IRT variables was 0.22 for the mean eye temperature and 0.19 for the mean cheek temperature. Infrared thermography was at least comparable to RT and some IRT variables performed better as a screening tests than RT. Future studies into the automation of IRT for surveillance of early postpartum diseases is warranted.