A sandwich enzyme-linked immunosorbent assay to study the antibody response against Psoroptes ovis in cattle

A sandwich enzyme-linked immunosorbent assay (ELISA) was developed using polyclonal antibodies from Psoroptes cuniculi infested rabbit and a crude P. cuniculi antigen. Sera from 133 Psoroptes ovis infested and 137 uninfested cattle were serially diluted from 1:100 to 1:102,400 and analyzed. Curves obtained by plotting log dilution versus optical densities (OD) were used and a working dilution of 1:400 was selected. Positive/negative cut-off values were calculated using the mean OD percentage (6%) of all negative sera plus 3 standard deviations. Sensitivity and specificity according to the resulting cut-off (25%) were 98.5% and 97.8% respectively. Expressed as the dilution giving the cut-off value, results in ELISA revealed a wide range of antibody titres (from 1:19 up to 1:2,240,000) with a geometric mean value of 1:35,480, and this is probably related to the well-described hyperglobulinemia in P. ovis infestations. In two trials performed at a 1 year interval, 19 Belgian White and Blue and six Fresian-Holstein calves were experimentally infested, and sensitivity and specificity were calculated throughout the experimental period. Sensitivity reached 78% on day 70 in trial 1 and 100% on day 49 in trial 2 (sensitivity was calculated using data from three control animals, three infected Fresian-Holstein animals and seven (trial 1) or six (trial 2) infected Belgian White and Blue animals). Specificity on day 0 was 100% in trial 1 and 92% in trial 2. At the end of both trials, specificity decreased in 80%. The development of the serum antibody activity to Psoroptes antigens was correlated with the development of lesions: in trial 1, the coefficient of correlation between the percentage of body surface with lesions and antibody titre was 0.82 (probability of no relationship P < 0.005) on day 56 post-infection (PI) and in trial 2, 0.83 (P < 0.025) and 0.87 (P < 0.025) on days 35 and 63 PI respectively.