Exploring specific biomarkers in blood for in vitro diagnosis of caprine tuberculosis

Caprine tuberculosis (cTB) is a zoonotic disease with significant implications for animal and public health and economic impact. Therefore, accurate ante-mortem diagnosis is essential. Diagnosis in goats rely on the single and comparative intradermal tuberculin test (SITT and CITT, respectively), both of which showing limited diagnostic performance. The aim of the study was to characterize the immune and physiological responses to cTB in naturally infected goats and to identify potential in vitro biomarkers that could improve the reliability of cTB diagnosis.

The study was conducted in a cTB infected herd and goats were classified as reactors (positive to SITT or IFN-γ release assay and considered as infected) and non-reactors. Basal production of fifteen key immune cytokines was measured in plasma samples (n = 19) and compared with levels observed after stimulation with bovine purified protein derivative (PPDb). Acute phase proteins (haptoglobin/Hp and serum amyloid-A/SAA) and oxidative stress markers (total antioxidant status/TAS and malondialdehyde/MDA) were determined in serum samples (n = 90) while complete blood count (CBC) was performed on whole-blood samples (n = 44).

Regarding cytokine expression patterns, plasma levels of IFN-γ and IFN-γ-inducible protein 10 (IP-10) in PPDb-stimulated samples were significantly higher (p < 0.005 and p < 0.001, respectively) in reactor goats than in non-reactor. When considering the difference between PPDb and PBS-stimulated samples (Δ-cytokine), levels of Δ-IFN-γ, Δ-IP-10 and Δ-IL-1α were significantly higher (p = 0.0006, p < 0.0001 and p = 0.022, respectively) in reactor goats compared to non-reactor. In addition, significantly higher (p = 0.014) levels of serum TAS were observed in reactor vs. non-reactor goats whereas no significant differences (p > 0.05) were found for Hp, SAA and MDA. Finally, among the CBC parameters, only the percentage of mid-range cells was significantly higher (p < 0.05) in reactor vs. non-reactor goats. This study demonstrates the differential expression of immunological (IFN-γ, IP-10 and IL-1α) and physiological (TAS) biomarkers in goats reacting to TB diagnostic tests and suggest their potential as biomarkers of cTB infection. These findings could lead to the development of novel diagnostic in vitro methodologies and contribute to more effective cTB eradication strategies.