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Summary

Background Visceral leishmaniasis (VL) is a vector-borne disease induced by parasite Leishmania infantum and transmitted from animal reservoir to humans. It still represents a major challenge for control programs in the Americas. Domestic dogs are the proven reservoir, and along with sand flies, maintain L. infantum-endemic transmission. In some regions, recommended control measures to limit VL incidence in humans includes culling dogs shown to be seropositive to anti-Leishmania antibodies. Since not all seropositive dogs are infectious, this selection criteria include dogs that do not contribute to transmission. Current field-friendly serological diagnostic tests include kits based on rK39 and rK28 Leishmania antigens. However, there is no evidence to date that culling dogs results in reduced human infection rates. Low serological test specificity with respect to infectiousness often leads to poor dog-owner compliance with control programs. In longitudinal xenodiagnosis studies of naturally infected dogs, a large fraction of transmission events is due to a small fraction of the infected canine population, otherwise known as super-spreaders. Detection of these dogs is a research priority. Targeting super-spreaders to manage VL incidence more sustainably requires a different approach to the current control practises.

Aims The aim of the study is to discriminate super-spreaders in mixed infected canine population by testing current and novel anti-Leishmania antigens, and their combinations, as predictive immunological markers of inectiousness.

Methods To do so, ELISAs were performed on archived sera collected from a naturally infected cohort population of Brazilian dogs for which the infectiousness and transmission potential was measured by xenodiagnosis during a two years longitudinal study. The dog samples are well characterized in terms of exposure, infection, disease and infectiousness; and classified based on both point xenodiagnosis and infectious history.

Results and discussion Analyses demonstrated the potential of some of the novel candidate individual and fusion proteins to out-perform currently available antigens in their specificity to identify super-spreaders, and prior to their onset of infectiousness. Carefully selected thresholds improved test performance both to specific reduce canine population transmission potential, and to avoid selection of never-infectious dogs. The development of a commercial prototype is on-going in collaboration with the Infectious Disease Research Institute (USA). Based on these results, best strategies for field deployment of the novel diagnostic tool were explored to maximise impact on Leishmania transmission and infection under a range of demographic and epidemiological scenarios. The impact of the novel tool will be quantify under different population dynamic scenarios.

Specific objectives

The aim of the study are :

(1) to discriminate super-spreaders in mixed infected canine population by testing current and novel anti-Leishmania antigens, and their combinations, as predictive immunological markers of inectiousness.

(2) to develop a prototype in collaboration with the Infectious Disease Research Institute (USA).

(3) In conjunction with mathematical modelling, to analyse the performance of the diagnostic tool under various epidemiological scenarios to reduce transmission and explore the potential field efficacy.