Serological progression and time to seroconversion in serodiscordant and seronegative dogs tested for visceral leishmaniasis in an endemic area of Brazil.

Visceral leishmaniasis is a disease of major public health importance in Brazil, where dogs act as the main reservoir in urban transmission cycles. The immunochromatographic rapid test (RT) is currently used as a screening tool for canine visceral leishmaniasis (CVL), with the enzyme-linked immunosorbent assay (ELISA) serving as a confirmatory test. Dogs with non-reactive RT results are classified as seronegative, whereas those with reactive RT and ELISA results are classified as seropositive. Dogs with discordant results are considered uninfected according to the current diagnostic protocol. However, these animals may have epidemiological relevance, as they represent a diagnostic uncertainty and may correspond to early infections or to animals with evolving antibody responses. This study analyzed the database of the Zoonosis Surveillance Unit in the Federal District, Brazil, to investigate the serological evolution of seronegative and serodiscordant dogs. Out of 21,268 tests performed during the study period, 1434 dogs met the inclusion criteria and were included in the longitudinal analysis. During follow-up, 28.8 % of serodiscordant dogs and 5.3 % of seronegative dogs converted to seropositive status. Survival analysis showed that serodiscordant dogs were 7.8 times more likely to become seropositive than seronegative dogs and that seroconversion occurred earlier in this group. Additionally, a proportion of animals remained serodiscordant over time, indicating heterogeneous serological evolution among dogs under surveillance. These findings show that serodiscordant dogs represent a subgroup at higher risk of subsequent seroconversion and may be epidemiologically relevant in the transmission dynamics of CVL.