Abstract
BackgroundBecause domestic dogs are reservoir hosts for visceral leishmaniasis (VL) in Brazil, one of the approaches used to reduce human disease incidence is to cull infected dogs. However, the results of controlled intervention trials based on serological screening of dogs and killing of seropositive animals are equivocal. A prophylactic vaccine to protect dogs from being infectious to the sand fly vector could be an effective strategy to provide sustained control. Here, we investigated whether a currently licensed commercial subunit rA2 protein–saponin vaccine (Leish-tec®) had an additional effect to dog culling on reducing the canine infectious populations.Methodology/Principal findingsThis prospective study was conducted in an L. infantum highly endemic area of southeast Brazil. At the onset of the intervention, all of the eligible dogs received through subcutaneous route a three-dose vaccine course at 21-day intervals and a booster on month 12. For the purpose of comparison, newly recruited healthy dogs were included as the exposed control group. To ascertain vaccine-induced protection, dogs were screened on clinical and serological criteria every 6 months for a 2-year follow-up period. Antibody-based tests and histopathological examination of post-mortem tissue specimens from euthanized animals were used as a marker of infection. The standardized vaccine regime, apart from being safe, was immunogenic as immunized animals responded with a pronounced production of anti-A2-specific IgG antibodies. It should be noted the mean seroconversion time for infection obtained among immunized exposed dogs (~ 18 months), which was twice as high as that for unvaccinated ones (~ 9 months). After two transmission cycles completed, the cumulative incidence of infection did differ significantly (P = 0.016) between the vaccinated (27%) and unvaccinated (42%) dogs. However, the expected efficacy for the vaccine in inducing clinical protection was not evident since 43% of vaccine recipients developed disease over time. Our estimates also indicated that immunoprophylaxis by Leish-tec® vaccine in addition to dog culling might not have an impact on bringing down the incidence of canine infection with L. infantum in areas of high transmission rates.Conclusions/SignificanceLeish-tec® as a prophylactic vaccine showed promise but needs to be further optimized to be effective in dogs under field conditions, and thereby positively impacts human incidence.
Highlights
Visceral leishmaniasis (VL; known as kala-azar) is a severe vector-borne disease, which if left untreated is almost always fatal
Anthroponotic VL occurs in regions where Leishmania donovani infections are endemic (South Asia, East Africa, and parts of the Middle East), whereas zoonotic VL is found in areas of Leishmania infantum
In the Neotropics, L. infantum is usually transmitted by Lutzomyia longipalpis, and the population density of these insects in the peridomestic setting can reach very high levels [4]
Summary
Visceral leishmaniasis (VL; known as kala-azar) is a severe vector-borne disease, which if left untreated is almost always fatal. Anthroponotic VL occurs in regions where Leishmania donovani infections are endemic (South Asia, East Africa, and parts of the Middle East), whereas zoonotic VL is found in areas of Leishmania infantum In VL-endemic regions of the Mediterranean and Latin America, the domestic dog (Canis familiaris) is the main parasitic reservoir host of human infection [9, 10]. In areas of high transmission, a high prevalence of canine infection is associated with high-risk of human disease [11]. Because domestic dogs are reservoir hosts for visceral leishmaniasis (VL) in Brazil, one of the approaches used to reduce human disease incidence is to cull infected dogs. We investigated whether a currently licensed commercial subunit rA2 protein–saponin vaccine (Leish-tec®) had an additional effect to dog culling on reducing the canine infectious populations
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