A quantitative test of the relationship between parasite dose and infection probability across different host–parasite combinations

Abstract

Epidemiological models generally assume that the number of susceptible individuals that become infected within a unit of time depends on the density of the hosts and the concentration of parasites (i.e. mass-action principle). However, empirical studies have found significant deviations from this assumption due to biotic and abiotic factors, such as seasonality, the spatial structure of the host population and host heterogeneity with respect to immunity and susceptibility. In this paper, we examine the effect of the dose level of the bacterial endoparasite Pasteuria ramosa on the infection rate of its host, the water flea Daphnia magna. Using seven host clones and two parasite isolates, we measure the fraction of infected hosts after exposure to eight different parasite doses to determine whether there is variation in the infection process across different host clone-parasite isolate combinations. In five combinations, a pronounced dose-dependent infection pattern was found. Using a likelihood approach, we compare the infection data of these five combinations to the fit of three mathematical models: a mass-action model, a parasite antagonism model (i.e. an increase in the parasite dose leads to an under-proportionate increase in the infection rate per host) and a heterogeneous host model. We found that the host heterogeneity model, in which we assumed the existence of non-inherited phenotypic differences in host susceptibilities to the parasite, provides the best fit. Our analysis suggests that among 5 out of the 14 host clone-parasite isolate combinations that resulted in appreciable infections, non-genetic host heterogeneity plays an important role.

Effects of Potyvirus Effective Population Size in Inoculated Leaves on Viral Accumulation and the Onset of Symptoms

“…Both trends have been observed before in this experimental system (39), as well as for other plant viruses (11,20). Moreover, these trends appear to be quite general, since they also appear in other experimental systems (4,28,36,40), where it must be noted that a higher variance in the distribution of founders is equivalent to an increase in mixed-genotype infection frequency (36). Thus, although deviations from the model were not in themselves suffi- FIG 2 Dose-symptomatology relationship at 6 dpi.…”

“…Heterogeneity in host susceptibility has been implicated in departures from the IAH model; this model can reasonably account for dose-response and mixed-genotype infection data if the probability of infection is allowed to vary over hosts (4,36).…”

Disentangling the influence of parasite genotype, host genotype and maternal environment on different stages of bacterial infection in Daphnia magna

“…Although the influence of the environmental [23,24], genotype-by-environment [25,26] and genotype-by-genotype [3,27] interactions has been studied extensively in this system, insights from the stepwise infection process have only recently been incorporated. The recent development of P. ramosa clones, for example, has revealed that parasite susceptibility is highly specific [28] and only certain host-parasite genotype combinations are compatible [29]. Underlying this specificity is the attachment of the spores to the oesophagus of compatible host genotypes, a mechanism that is surprisingly unaffected by environmental variation [19].…”

Resistance to a bacterial parasite in the crustacean Daphnia magna shows Mendelian segregation with dominance

“…First, we employ a clone of P. ramosa (single genotype), not field isolates. Field isolates may contain more than one parasite clone (Jensen et al, 2006;Mouton et al, 2007;Ben-Ami et al, 2008;Luijckx et al, 2011). The use of P. ramosa clones negates the complicating factors intrinsic to mixed infections and allows for a more definitive interpretation of experimental results .…”