The effect of host heterogeneity and parasite intragenomic interactions on parasite population structure

Patient variability in the blood-stage dynamics of Plasmodium falciparum captured by clustering historical data

BACKGROUND

Mathematical models provide an understanding of the dynamics of a Plasmodium falciparum blood-stage infection (within-host models), and can predict the impact of control strategies that affect the blood-stage of malaria. However, the dynamics of P. falciparum blood-stage infections are highly variable between individuals. Within-host models use different techniques to capture this inter-individual variation. This struggle may be unnecessary because patients can be clustered according to similar key within-host dynamics. This study aimed to identify clusters of patients with similar parasitaemia profiles so that future mathematical models can include an improved understanding of within-host variation.

METHODS

Patients' parasitaemia data were analyzed to identify (i) clusters of patients (from 35 patients) that have a similar overall parasitaemia profile and (ii) clusters of patients (from 100 patients) that have a similar first wave of parasitaemia. For each cluster analysis, patients were clustered based on key features which previous models used to summarize parasitaemia dynamics. The clustering analyses were performed using a finite mixture model. The centroid values of the clusters were used to parameterize two established within-host models to generate parasitaemia profiles. These profiles (that used the novel centroid parameterization) were compared with profiles that used individual-specific parameterization (as in the original models), as well as profiles that ignored individual variation (using overall means for parameterization).

RESULTS

To capture the variation of within-host dynamics, when studying the overall parasitaemia profile, two clusters efficiently grouped patients based on their infection length and the height of the first parasitaemia peak. When studying the first wave of parasitaemia, five clusters efficiently grouped patients based on the height of the peak and the speed of the clearance following the peak of parasitaemia. The clusters were based on features that summarize the strength of patient innate and adaptive immune responses. Parameterizing previous within host-models based on cluster centroid values accurately predict individual patient parasitaemia profiles.

CONCLUSION

This study confirms that patients have personalized immune responses, which explains the variation of parasitaemia dynamics. Clustering can guide the optimal inclusion of within-host variation in future studies, and inform the design and parameterization of population-based models.

The genotypic structure of a multi-host bumblebee parasite suggests a role for ecological niche overlap

The genotypic structure of parasite populations is an important determinant of ecological and evolutionary dynamics of host-parasite interactions with consequences for pest management and disease control. Genotypic structure is especially interesting where multiple hosts co-exist and share parasites. We here analyze the natural genotypic distribution of Crithidia bombi, a trypanosomatid parasite of bumblebees (Bombus spp.), in two ecologically different habitats over a time period of three years. Using an algorithm to reconstruct genotypes in cases of multiple infections, and combining these with directly identified genotypes from single infections, we find a striking diversity of infection for both data sets, with almost all multi-locus genotypes being unique, and are inferring that around half of the total infections are resulting from multiple strains. Our analyses further suggest a mixture of clonality and sexuality in natural populations of this parasite species. Finally, we ask whether parasite genotypes are associated with host species (the phylogenetic hypothesis) or whether ecological factors (niche overlap in flower choice) shape the distribution of parasite genotypes (the ecological hypothesis). Redundancy analysis demonstrates that in the region with relatively high parasite prevalence, both host species identity and niche overlap are equally important factors shaping the distribution of parasite strains, whereas in the region with lower parasite prevalence, niche overlap more strongly contributes to the distribution observed. Overall, our study underlines the importance of ecological factors in shaping the natural dynamics of host-parasite systems.

The evolution of virulence and host specialization in malaria parasites of primates

Parasite virulence, i.e. the damage done to the host, may be a by-product of the parasite's effort to maximize its fitness. Accordingly, several life-history trade-offs may explain interspecific differences in virulence, but such constraints remain little tested in an evolutionary context. In this phylogenetic study of primate malarias, I investigated the relationship between virulence and other parasite life-history traits. I used peak parasitaemia as a proxy for virulence, because it reflected parasite reproductive success and parasite-induced mortality. Peak parasitaemia was higher in specialist than in generalist species, even when confounding life-history traits were controlled. While there was a significant phylogenetic relationship between the number of competitors per host and host specialization, peak parasitaemia was unrelated to within-host competition. Therefore, the key evolutionary factor that favours virulence is host specialization, and the evolutionary success of virulent parasites, such as Plasmodium falciparum, may be better understood when the trade-off in virulence between different hosts is considered. Such phylogenetic results may help us design better protection programmes against malaria.