Quantifying the Contribution of Hosts with Different Parasite Concentrations to the Transmission of Visceral Leishmaniasis in Ethiopia

Abstract

BackgroundAn important factor influencing the transmission dynamics of vector-borne diseases is the contribution of hosts with different parasitemia (no. of parasites per ml of blood) to the infected vector population. Today, estimation of this contribution is often impractical since it relies exclusively on limited-scale xenodiagnostic or artificial feeding experiments (i.e., measuring the proportion of vectors that become infected after feeding on infected blood/host).MethodologyWe developed a novel mechanistic model that facilitates the quantification of the contribution of hosts with different parasitemias to the infection of the vectors from data on the distribution of these parasitemias within the host population. We applied the model to an ample data set of Leishmania donovani carriers, the causative agent of visceral leishmaniasis in Ethiopia.ResultsCalculations facilitated by the model quantified the host parasitemias that are mostly responsible for the infection of vector, the sand fly Phlebotomus orientalis. Our findings indicate that a 3.2% of the most infected people were responsible for the infection of between 53% and 79% (mean – 62%) of the infected sand fly vector population.SignificanceOur modeling framework can easily be extended to facilitate the calculation of the contribution of other host groups (such as different host species, hosts with different ages) to the infected vector population. Identifying the hosts that contribute most towards infection of the vectors is crucial for understanding the transmission dynamics, and planning targeted intervention policy of visceral leishmaniasis as well as other vector borne infectious diseases (e.g., West Nile Fever).

Progress in the Mathematical Modelling of Visceral Leishmaniasis

“…L. infantum (Pourabbas et al, 2013;Mary et al, 2006;Moral et al, 2002;Biglino et al, 2010), and L. donovani (Miller et al, 2014; have been detected in blood in up to 58% of asymptomatic infections by a cross-sectional study. 14% of 4,695 asymptomatic Ethiopians were qPCR-positive in blood of which 3.2% had high genome equivalent counts (Miller et al, 2014), suggesting that distribution of parasitaemias is skewed and that perhaps the small proportion of asymptomatic infections with higher parasitaemia are more likely to develop disease and have a disproportional role in onward transmission. However of 44 Brazilian children with positive qPCR results at baseline, only 10 remained qPCR-positive 12 months later, showing a significant decrease in genome counts (dos Santos Marques et al, 2012).…”

“…kDNA for preparing qPCR standards) varies between amastigotes and promastigotes, and even between strains . Studies suggest that there exists a parasite density threshold above which canine and human infections are disproportionately infectious (Courtenay et al, 2014;Miller et al, 2014). In contrast, natural wildlife hosts (e.g.…”

“…For example, infection rates of sandflies from xenodiagnosis experiments in dogs have been shown to correspond to a threshold density of Leishmania parasites in skin snips taken from ears (Courtenay et al, 2014); so whilst the numerical values of transmission probabilities are for dog-sandfly interactions, it is likely that same biological process occurs in human-sandfly interactions. It is possible to perform xenodiagnosis directly in people, and there are currently attempts to conduct such experiments (Miller et al, 2014). Similarly, it is possible that the natural history of disease is related to the size of the inoculating dose of infection .…”

Serological tests fail to discriminate dogs with visceral leishmaniasis that transmit Leishmania infantum to the vector Lutzomyia longipalpis

“…These studies suggest that diagnostic tests can distinguish more infectious dogs, which are the actual sources of infection under natural conditions, from less infectious or non-infectious dogs. An elegant study in Ethiopia showed that in fact only a few individuals contribute to transmission 30 . A control program that discriminates infectiousness would identify such animals.…”

Standardization of a SYBR Green Based Real-Time PCR System for Detection and Molecular Quantification of Babesia bovis and B. bigemina in Water Buffaloes (Bubalus bubalis)

“…Identifying the hosts that contribute most to the infection of vectors population is crucial for understanding the transmission dynamics of vector borne diseases, as well as for planning intervention strategies targeting the relevant infected host groups [16]. The parasitemia is an important factor of the host reservoir competence on vector borne pathogens, influencing their contribution (host infectiousness profile) to the infected vector population [17]. The Giemsa-stained blood smears is a conventional method used to determine parasitemia in clinically infected cattle, but this test has low analytical sensitivity and is ineffective in carrier animals [15,18,19].…”