The effects of host-vector relationships and density dependence on the epidemiology of visceral leishmaniasis

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Abstract

In Latin America, visceral leishmaniasis (VL) is caused by infection with
Leishmania infantum, an endemic but lethal parasite transmitted by Lutzomyia
longipalpis sandflies. Multiple hosts are implicated in VL transmission; therefore
sandfly biting preferences may be pivotal in determining transmission dynamics.
Host preferences are poorly understood with simple preference-host density
relationships being conventionally assumed.
Combined modelling and fieldwork approaches were used to investigate the
preference of sandflies for key host types (dogs, humans and chickens) and force of
infection (FOI) over a range of vector and host densities. In Brazil, variable vector
densities were (i) observed over a period of seasonal variation, and (ii)
experimentally manipulated via “trapping out” (sustained CDCLT capture to reduce
local vector density). Host density was also manipulated by (iii) the incremental
introduction of chickens to experimental sheds.
Results suggest that there is a significant link between alternative host density and
the absolute and relative preference of sandflies for humans and dogs. Investigations
also indicate that host choice has a vector density dependent element, which varies
significantly and nonlinearly depending upon vector density. Meta-analysis and
mathematical modelling of human and canine prevalences across Brazil also point
toward variable transmission rates to these hosts attributable to density-dependent
biting preferences observed in the field. These host choice dynamics ultimately
combine to demonstrate the influence of host and vector densities on FOI on dogs
and humans, but there are significant interactions between host and vector densities
resulting in complex FOI relationships.
Nonlinearities are likely explained by density dependent sandfly aggregation
behaviour upon outdoor living hosts, such as chickens, as vector density rises. This
preference behaviour may have far reaching implications for our understanding of
transmission and control, and potentially indicate host density manipulation as an
intervention measure.

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