Abstract
BackgroundThe relationships between heterogeneities in host infection and infectiousness (transmission to arthropod vectors) can provide important insights for disease management. Here, we quantify heterogeneities in Leishmania infantum parasite numbers in reservoir and non-reservoir host populations, and relate this to their infectiousness during natural infection. Tissue parasite number was evaluated as a potential surrogate marker of host transmission potential.MethodsParasite numbers were measured by qPCR in bone marrow and ear skin biopsies of 82 dogs and 34 crab-eating foxes collected during a longitudinal study in Amazon Brazil, for which previous data was available on infectiousness (by xenodiagnosis) and severity of infection.ResultsParasite numbers were highly aggregated both between samples and between individuals. In dogs, total parasite abundance and relative numbers in ear skin compared to bone marrow increased with the duration and severity of infection. Infectiousness to the sandfly vector was associated with high parasite numbers; parasite number in skin was the best predictor of being infectious. Crab-eating foxes, which typically present asymptomatic infection and are non-infectious, had parasite numbers comparable to those of non-infectious dogs.ConclusionsSkin parasite number provides an indirect marker of infectiousness, and could allow targeted control particularly of highly infectious dogs.
Highlights
Studies of microparasites usually consider hosts as homogeneous infection units, despite knowledge that infections progress through states of clinical severity, that clinical severity is often associated with the number of infecting microorganisms, and that individual transmission potential may be related to infection load
Infectiousness to the sandfly vector was associated with high parasite numbers, and parasite loads in the skin was the best predictor of being infectious
Leishmania loads of infected dogs Parasite loads were quantified by quantitative PCR (qPCR) in 265 post-infection bone marrow samples from 82 dogs, and 185 post-infection ear skin biopsies from 64 dogs (Table 1)
Summary
Studies of microparasites usually consider hosts as homogeneous infection units (infected or uninfected), despite knowledge that infections progress through states of clinical severity, that clinical severity is often associated with the number of infecting microorganisms (load), and that individual transmission potential may be related to infection load. Heterogeneity in transmission can increase the basic case reproduction number R0 of a pathogen compared to that under assumptions of homogeneous mixing or density-dependent contact networks [9,11], and affect the effort required, and choice of strategy (mass or targeted), to interrupt transmission [7,8,9,12]. Molecular techniques, such as real-time quantitative PCR (qPCR), have been used recently to differentiate between infected individuals and to help understand the spread and treatment of emerging infectious diseases e.g. Tissue parasite number was evaluated as a potential surrogate marker of host transmission potential
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