Evaluation of a Commercial Serum Competitive Enzyme-Linked Immunosorbent Assay for Detection of Neospora caninum-Specific Antibodies in Raw Milk of Ruminants

Bovine neosporosis is an infection caused by the protozoan parasite Neospora caninum and has substantial veterinary hazards. Neosporosis cannot be controlled by vaccination or chemotherapy. Thus, accurate diagnosis followed by isolation and culling of infected animals is regarded as the most efficient method of control. In vivo diagnosis often relies on serologic testing of the animals, and milk represents a non-invasive and easy-to-collect sample matrix. However, indirect enzyme-linked immunosorbent assay (ELISA) specifically designed for antibody detection in milk are sometimes not easily available and it is tempting to use ELISA kits that are originally designed for use in serum in milk samples instead. Herein, we evaluated a widely used commercial ELISA (ID Screen (R) Neospora caninum competition Multispecies ELISA (ID. Vet, Grabels, France)), developed for detection of N. caninum antibodies in serum samples, for its performance on milk samples. Milk samples from dairy ruminants (cows, buffaloes, sheep, and goats; n = 149) were tested in parallel with the serum ELISA and a commercial milk ELISA as a standard test (Neospora caninum Milk Competitive ELISA, ID. Vet, Grabels, France). The detected prevalence values were 28.2% (42/149), 17.4% (26/149), and 17.4% (26/149) using milk ELISA, serum ELISA, and both ELISAs, respectively. Sensitivity, specificity, positive predictive value, and negative predictive value for the serum ELISA used with milk samples were 61.9%, 100%, 100%, and 87%, respectively. The agreement and kappa value between the two ELISAs were 89.3% and 0.70, respectively, suggesting substantial agreement. High values of Pearson correlation coefficient (0.904, p >= 0.0001) and area under the receiver operating characteristic (ROC) curve (0.789, p >= 0.0001) demonstrated the high diagnostic performance of the serum ELISA in milk samples. Also, a Bland-Altman Plot and histogram describing the frequency of distribution of ELISA optical densities confirmed the high agreement of both serum and milk ELISAs. The current results revealed the high specificity but moderate sensitivity of the serum ELISA used for milk samples compared with the milk ELISA. However, the excellent positive predictive value of the serum ELISA makes it an alternative option in case of the unavailability of milk ELISAs. With this study, we provided additional evidence that a widely used serum ELISA test kit may also be used for the detection of N. caninum antibodies in milk samples.