Evaluating the efficacy of serum haptoglobin concentration as an indicator of respiratory-tract disease in dairy calves

The serum concentration of haptoglobin (S-Hp) was measured in 833 group-housed dairy calves from nine herds in south-west Sweden once at 24–56 days of age to evaluate the potential of S-Hp as an indicator of clinical respiratory-tract disease (CRD). Presence of disease (treated and non-treated) was assessed clinically by farmers and by a project veterinarian visiting the farms every third week. The median S-Hp of healthy calves was 0.06 g/L (80% central range: 0.04–0.23), of calves with diarrhoea within the 10 days before sampling 0.07 g/L (80% central range: 0.04–0.63), and of calves with CRD within the 14 days before sampling 0.09 g/L (80% central range: 0.04–0.69). Eight different cut-off values were used to define a positive S-Hp analysis result: >0.05, >0.06, >0.07, >0.08, >0.09, >0.10, >0.15 and >0.20 g/L. A rectal temperature >39.5 °C was denoted as fever. A positive result of five different diagnostic tests for CRD was defined as: (1) a positive S-Hp with fever absent, (2) a positive S-Hp with fever present, (3) either a positive S-Hp or fever, (4) both a positive S-Hp and fever, and (5) fever (regardless of S-Hp). The sensitivity (Se) and specificity (Sp) of each test were calculated from regression coefficients of generalized linear mixed models of the binary test results, applying a logit link. Apart from CRD status (within the 14 days before sampling; no or yes), the models included sex (bull or heifer), and for the test based on S-Hp alone, also rectal temperature (fever, no or yes). Confidence intervals (CI) of Se and Sp were estimated by simulation. Based on Se, Sp, and areas under Receiver Operating Characteristics curves, test 3 was considered the best. At optimal performance, giving equal importance to type I and II errors, i.e. at a S-Hp cut-off of 0.15 g/L in heifer calves, Se was 0.64 (95% CI 0.50–0.77) and Sp 0.71 (95% CI 0.60–0.80), and at a S-Hp cut-off of 0.08 g/L in bulls, Se was 0.52 (95% CI 0.40–0.64) and Sp 0.80 (95% CI 0.74–0.85). The other tests were judged as unsatisfactory indicators of CRD. In heifers, the proportion of CRD-positive calves in the herd was strongly associated with the proportion of test positives (S-Hp or fever; S-HP and fever), suggesting potential as a herd-level indicator.