The infection rate of Daphnia magna by Pasteuria ramosa conforms with the mass-action principle

Abstract

In simple epidemiological models that describe the interaction between hosts with their parasites, the infection process is commonly assumed to be governed by the law of mass action, i.e. it is assumed that the infection rate depends linearly on the densities of the host and the parasite. The mass-action assumption, however, can be problematic if certain aspects of the host-parasite interaction are very pronounced, such as spatial compartmentalization, host immunity which may protect from infection with low doses, or host heterogeneity with regard to susceptibility to infection. As deviations from a mass-action infection rate have consequences for the dynamics of the host-parasite system, it is important to test for the appropriateness of the mass-action assumption in a given host-parasite system. In this paper, we examine the relationship between the infection rate and the parasite inoculum for the water flee Daphnia magna and its bacterial parasite Pasteuria ramosa. We measured the fraction of infected hosts after exposure to 14 different doses of the parasite. We find that the observed relationship between the fraction of infected hosts and the parasite dose is largely consistent with an infection process governed by the mass-action principle. However, we have evidence for a subtle but significant deviation from a simple mass-action infection model, which can be explained either by some antagonistic effects of the parasite spores during the infection process, or by heterogeneity in the hosts' susceptibility with regard to infection.

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

“…The estimated infection probability is lower than the one we found previously (Regoes et al 2003), a difference we elaborate on below. We can calculate the ID 50 (i.e.…”

“…Although the purity of the parasite isolates is an important issue for a binary interpretation of the infection process, we would like to emphasize that it does not play a role in deviance from the mass-action model that we observed here. In a previous study, we showed that the observed deviations from the mass-action infection model cannot be explained by parasite heterogeneity or stochastic effects (Regoes et al 2003).…”

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

“…Juveniles were exposed to spores by placing them singly in 100-ml beakers filled with 20 ml of medium and adding spore suspensions containing 200 000 spores of P. ramosa clone C1. This dose is known to cause 100% infections without lethal effect in susceptible hosts (Regoes et al, 2003). Individual D. magna remained in 20 ml of medium for 4 days, at which point the beakers were filled up.…”

Responses of a Bacterial Pathogen to Phosphorus Limitation of Its Aquatic Invertebrate Host

“…Spore doses were created by homogenizing several infected Daphnia and counting the resulting spore concentration with a haemocytometer under a light microscope. We chose to infect Daphnia with a spore dose (16 000 spores) that was previously found to result in intermediate (60-80%) infection rates (Regoes et al 2003). Daphnia were exposed to spores for six days, after which they were transferred to fresh pathogen-free water.…”

“…Under conditions of high food quantity and quality, previous research has shown Pasteuria to be highly effective at infecting Daphnia (Regoes et al 2003). Pasteuria infections in Daphnia are generally nonreversible and greatly reduce daphnid fitness (Ebert 2005).…”