Exploratory Evaluation of the P22 ELISA for Antibody Detection in Fecal Samples from Tuberculosis-Infected Goat Herds

Background: Caprine tuberculosis (cTB), caused by members of the Mycobacterium tuberculosis complex (MTBC) represents a significant concern for animal and public health. Antibody-based diagnostic tests have emerged as promising tools to complement ante-mortem official diagnostic techniques in ruminants, which mainly use serum, although alternative, easier-to-collect samples, such as milk, oral fluid, or feces, could also be used. The objective of this exploratory study was to evaluate the reactivity of an in-house P22 enzyme-linked immunosorbent assay (ELISA) in fecal samples from goats belonging to cTB-infected herds, using serum P22 ELISA and official diagnostic tests as comparators. Methods: Serum and fecal samples were collected from 51 and 42 adult female goats from a high-prevalence herd (n = 149) and a low-prevalence herd (n = 220), respectively, which defecated during sampling. These animals were subjected to official diagnostic techniques: single and comparative intradermal tuberculin tests and the interferon-gamma release assay. As no TB-free herd was available, a preliminary fecal cut-off was established by receiver operating characteristic analysis using serum P22 ELISA as the reference classification. Results: The percentage of positive animals in fecal samples from the high-prevalence herd was 68.6%, while no positive results were observed in the low-prevalence herd. Serum samples showed a higher number of positives in both herds compared to fecal samples (p < 0.05). The agreement between official diagnostic techniques and P22 ELISA (using serum or feces) was slight in general terms. When a combined classification based on all official tests was used as the reference, fecal P22 ELISA detected 58.0% of test-positive goats, none of them in the low-prevalence herd. Conclusions: Under the conditions of this field study (no TB-free controls or post-mortem confirmation of infection were available), fecal P22 ELISA showed limited reactivity compared with serum, particularly in the low-prevalence herd, suggesting limited usefulness for antibody detection in TB infected goats.