Diagnosis of ovine caseous lymphadenitis by blood and milk gamma interferon assays

Diagnosis of Corynebacterium pseudotuberculosis (C. pseudotuberculosis) infection in sheep herds, caseous lymphadenitis (CLA), usually depends on individual animal testing by blood based immunoassays. This study was carried out to investigate the reliability of newly developed milk and heparinized whole blood based gamma interferon (IFN-γ) assays for the diagnosis of ovine CLA. It was conducted on 65 (16 lactating and 49 non-lactating) sheep at Semella village, Gharbia Governorate (in the central region of the Egyptian Delta). Ten (15.3%) animals, 5 lactating and 5 non-lactating, had superficial lymph node abscesses at the head region. C. pseudotuberculosis recombinant phospholipase D (rPLD) was used as a stimulating antigen for blood and milk IFN-γ assays. Blood IFN-γ assay had showed a sensitivity and specificity of 70% and 80% compared to 78.1% and 72.7% of milk IFN-γ assay. A total of 18 (27.6%) animals were positive by blood IFN-γ assay. In addition, 7 out of 10 (70%) of clinically affected animals were positive and 76.5% of apparently normal animals were negative by blood IFN-γ assay. On the other hand, a total of 7 (43.7%) lactating animals were positive by milk IFN-γ assay. Considering the relation between the existence of a superficial lesion and milk IFN-γ assay, 4 out of 5 (80%) affected lactating ewes were positive and 72.7% of apparently normal lactating ewes were negative. These findings declared that the existence of superficial lymph node abscesses in sheep is highly indicative of the true disease status of the tested animals. In addition, milk IFN-γ assay is a newly developed technique for the diagnosis of CLA in lactating ewes had a similar performance to blood IFN-γ assay. However, further wide scale studies are required for testing, evaluation and standardization of milk IFN-γ assay on tank milk to judge its validity to identify the disease status on the herd level, particularly in countries where there are dairy small ruminant herds.