Efficacy of a candidate vaccine against Leishmania infantum on naturally exposed dogs to sand flies

The aim of this study was to determine the efficacy of a Leishmania candidate vaccine administered to dogs kept in an open-kennel under controlled conditions and naturally exposed to the sand fly vector, Phlebotomus spp. during three sand fly seasons. One hundred, male (n = 47) and female (n = 53) healthy and fully vaccinated with all mandatory vaccines Beagle dogs, aged 6 to 10 months old, were enrolled and randomly allocated to two treatment groups: (i) vaccinated (n = 50) and (ii) control (n = 50). Vaccination consisted of three primary vaccinations (days 0, 21 and 42), during which dogs remained at a Biosafety Level 2 (BL-2) facility protected from the sand flies, followed by two annual boosters during the exposure phase. General health of the animals was observed daily, while bone marrow aspirates, whole blood and serum samples were collected every 3 months. Molecular, serological testing for Leishmania infection detection, as well as complete blood counts were routinely performed. Levels of γ-IFN were measured prior to and after annual boosters. At the end of the study the dogs were euthanized, and necropsies performed. Six (12 %) vaccinated and 18 (36 %) control dogs were characterized as "diseased" (i.e., qPCR/parasitology positive + ≥1 clinical/laboratory sign of leishmaniosis) at the end of the study (Odds Ratio (OR): 0.240 [0.084-0.686]; p = 0.0084). Parasitic load was up to 825 copies in 1 mL of bone marrow of vaccinated dogs, while up to 4,407 copies in the controls. Antibodies against the parasite were detected in both groups, while also gross lesions were recorded during necropsies. Overall, the candidate vaccine was safe and demonstrated comparable potential in preventing the development of clinical leishmaniosis in dogs.