Immunotherapy of Canine Leishmaniasis by Vaccination with Singlet Oxygen-Inactivated Leishmania infantum

Background: Canine leishmaniasis is notoriously difficult to manage by chemotherapy alone, necessitating the consideration of supplemental or alternative treatment. Evidence is presented to support the feasibility of immunotherapy of diseased dogs through vaccination. Methods: The vaccine format used consisted of cultured promastigotes of Leishmania infantum, which were rapidly and completely killed by intracellularly generated singlet oxygen. A total of 33 owned dogs of different breeds and ages diagnosed positive for leishmaniasis were enrolled and divided into three groups for treatments as follows: (1) immunotherapy alone (9 dogs); (2) immunotherapy after chemotherapy (14 dogs); and (3) chemotherapy alone (10 dogs). All dogs in Groups 1 and 2 received intradermally three identical dosages of the vaccine format mentioned at the same schedules. The outcomes were assessed for one year at a post-treatment interval of 2–4 months by determining lymph node parasite loads and clinical scores based on established methodologies. Results: Spaghetti plots of the values for parasite loads obtained revealed that they scattered widely over time with a significant decline by 8–12 months post-treatment in all three groups. Sankey plots of clinical scores in stacked bars also showed that they followed erratic patterns of flow over time, albeit toward lower levels in all cases. Ordinal logistic regression analysis of clinical scores indicated that, while the odds for the emergence of severe clinical symptoms declined in all three groups, the lowest risk was associated with Group 2 dogs treated with immunotherapy after chemotherapy. The evidence presented thus suggests that immunotherapy of the diseased dogs with the vaccine format diminished their parasite loads and improved their clinical scores, especially when applied after chemotherapy. Dogs in Groups 1 and 2 that received immunotherapy, on average, lived twice as long as those in Group 3 that received chemotherapy alone. The risk of death estimated by analysis of the clinical scores using the Cox proportional hazard model was also found to be lower for Groups 1–2 dogs receiving immunotherapy than those in Group 3 receiving chemotherapy alone. Conclusions: Post-therapeutic survival time thus may be an additional parameter suitable to assess treatment efficacy by vaccination. In vitro approaches to mitigate some limitations of this study were proposed for future investigation.