A comparative study of serum and milk ELISAs for the diagnosis of ovine paratuberculosis (Ovine Johne's disease - OJD)

To evaluate the possibility of replacing serum with milk sample in the identification of Mycobacterium avium subsp. paratuberculosis (MAP) infected ovine flocks, a comparative study was carried out in Central Italy on 6 dairy farms with history of Paratuberculosis. Fresh milk and serum samples were collected from 192 randomly chosen sheep and tested by indirect ELISA (Pourquier®, France). The sheep were of the Comisana, Massese, and Sarda breeds, above two years of age, at early or peak lactation period. MAP infection was diagnosed in 28 (14.6%) and in 14 sheep (7.3%; 2= 5.43, P=0.02) by serum and milk ELISA, respectively. Higher mean OD values were observed in milk (1.96 ± SD 0.82) than in serum positive samples (1.77 ± SD 0.53; t=-0.90, P=0.37). The concordance between tests was moderate (k=0.5), higher at early (k=0.6) than in peak (k=0.4) lactation, in agreement with the Immunoglobulin G (IgG) rising in the milk in the beginning and at the end of lactation. The serum ELISA was able to identify more MAP positive sheep; nevertheless milk ELISA could be an available, cheaper and more feasible test to detect MAP infection in dairy sheep flocks by bulk tank milk analysis. Key words: Ovine Paratuberculosis, diagnosis, ELISA, milk, serum.