Is serology a realistic approach for monitoring red deer tuberculosis in the field?

Tuberculosis (TB) is a zoonotic mycobacterial infection with great importance in human health, animal production, and wildlife conservation. Although an ambitious eradication programme in cattle has been implemented for decades, TB-free status has not yet been achieved in most of Spain, where animal TB persists in a multi-host system of domestic and wild hosts, including the red deer (Cervus elaphus). However, information on long time series and trends of TB prevalence in wildlife is scarce. The diagnosis of TB in wild red deer is often based on gross pathology and bacteriological culture confirmation, although recently serological assays have been developed to detect anti- Mycobacterium tuberculosis Complex (MTC) antibodies. Particularly, protein complex P22 has demonstrated to yield good specificity and sensitivity in the serological diagnosis of MTC for red deer, as well as cattle, goats, sheep, pigs, wild boar, and European badger. Thus, the objective of the present study was to compare the performance of the P22-ELISA with TB-compatible lesion detection, as well as to assess the potential application of each technique for determining spatiotemporal trends and risk factors of MTC infection in wild red deer from low and high TB prevalence areas of Spain over the last two decades. We tested 5095 sera from 13 wild populations by indirect ELISA using P22 as antigen. Mean seroprevalence (13.22%, CI₉₅: 12.32-14.18) was compared with the prevalence of macroscopic TB-compatible lesions (6.94%, CI₉₅: 6.18-7.79). The results evidenced a poor agreement between both techniques (K < 0.3), although generalized TB-lesions and anti-P22 antibodies showed a positive association (χ² = 9.054, P = 0.004). Consequently, TB-lesion based prevalence and seroprevalence cannot be considered as equivalent for TB surveillance in red deer. Regarding the spatiotemporal trend of TB in red deer in Spain, we observed a North-South gradient of TB occurrence [North: 1.23% (CI₉₅: 0.77-1.97) of TB-lesions and 12.55% (CI₉₅: 10.91-14.41) of P22-ELISA; Centre: 7.10% (CI₉₅: 6.04-8.33) and 8.74% (CI₉₅: 7.57-10.08); South: 21.04% (CI₉₅:17.81-24.69) and 23.09% (CI₉₅: 19.73-26.84), respectively]. Overall, there was a stability over time, with higher prevalence in adults belonging to densely populated sites. We conclude that the P22-ELISA alone is not sufficiently reliable for TB surveillance in red deer at large spatiotemporal scales. Instead, we recommend combining gross pathology and P22-ELISA.