Molecular and Serological Evidence of Brucella Infection in Dogs from Selected Indian Cities: A Bayesian Latent-Class Evaluation of Diagnostic Tests

Canine brucellosis is an under-recognized zoonotic disease with important reproductive and public health implications. This study investigated molecular and serological evidence of Brucella infection in owned domestic dogs presented to veterinary clinics and referral facilities in Delhi, Bareilly (Uttar Pradesh), and Mannuthy (Kerala), and evaluated the performance of commonly used diagnostic tests. A total of 448 sampling units were included. Overall, 382 blood-derived DNA samples were screened by genus-specific PCR targeting the bcsp31 gene, 356 serum samples were tested by ELISA and agar gel immunodiffusion (AGID), and 22 vaginal swab-derived samples were screened by PCR. PCR detected Brucella DNA in 40/382 (10.47%) blood-derived samples, with all positive samples originating from Delhi (40/174; 22.99%), and in 1/22 (4.50%) vaginal swab-derived samples. ELISA detected antibodies in 34/356 (9.55%) samples, while AGID detected 13/356 (3.65%). Overall, 43/356 (12.08%) samples were positive by at least one serological test. Agreement between ELISA and AGID was 89.04%, with Cohen’s κ = 0.12; when ELISA was used as the comparator, AGID showed a relative sensitivity of 11.76% and relative specificity of 97.20%. Bayesian latent-class analysis estimated the model-derived latent Brucella-positive infection-related state at 17.84% (95% credible interval [CrI]: 11.99–25.13%). This estimate represents an underlying infection-related state inferred from the joint diagnostic-test results and should not be interpreted as confirmed active infection or clinical disease. Positivity was significantly higher among dogs older than two years (p < 0.05), while no significant association was observed with sex. Overall, the findings provide molecular and serological evidence of Brucella in the sampled owned domestic dogs and highlight the complementary information provided by molecular and serological testing. Further species-level characterization and structured surveillance are warranted to better define the epidemiology and public-health significance of Brucella infection detected in dogs in India.