How Virulent Should a Parasite Be to Its Vector?

Abstract

Abstract. Vector-borne parasites are commonly predicted to be less virulent to the vector than to the definitive host as the parasite gains little by harming its main route of transmission. Here we assess the empirical evidence from systems in which insects are vectors for vertebrate, plant, and invertebrate parasites. The body of evidence supports lower (but nonzero) parasite virulence to vectors than to plant or invertebrate hosts, but not to vertebrate hosts. We consider why this might be by assessing evolutionarily stable strategies for an insect parasite that can infect both predator and prey (or vector and definitive host) and can have distinct virulences in these two potential hosts. In a homogeneous environment, the parasite is predicted to be equally virulent to predator and prey. However, in a patchy environment it is predicted to become benign toward the more mobile of the two potential hosts, provided interpatch movement of free parasites is low and competitive displacement among strains in a patch is weak. This prediction meets reality in that the vector is usually more mobile between patches than is the definitive host in plant and invertebrate systems, but not necessarily in vertebrate hosts.

Transmission of Insect-vectored Pathogens: Effects of Vector Fitness as a Function of Infectivity Status

“…A primary assumption of research in this area is that evolution should favor effects on vector Þtness which increase transmission (Hurd 2003). Consequently, a commonly held generalization is that evolution should favor pathogens with low virulence toward their insect vectors as the pathogen is dependent on the vector for transmission (Elliot et al 2003). The results presented here support this contention.…”

“…Because studies in this area typically compare the Þtness of vectors reared on infected versus susceptible plants, it is often unclear if Þtness effects are caused by direct effects of the pathogen on the vector or indirect effects caused by changes in host plant quality (Elliot et al 2003). Effects of plant pathogens that do not propagate in vectors are expected to be indirect, whereas effects of plant pathogens that propagate in vectors may be direct or indirect.…”

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

“…With varying levels of virulence observed among different vector-pathogen systems, the body of evidence indicates that there is no consistent universal pattern, as, for example, among mosquito-Plasmodium systems (Ferguson & Read 2002). Although it has been suggested that virulence may depend on the relative mobility of the vertebrate host and arthropod vector (Elliot et al 2003), the degree of virulence to a vector and the advantage that it confers to the pathogen remain unclear.…”

Complex interactions between a plant pathogen and insect parasitoid via the shared vector-host: consequences for host plant infection

“…Effects on the performance of the vector mediated via virus-induced changes in the plant can subsequently have consequences for the success of the pathogen itself. It can be envisaged there will be evolutionary pressures on pathogens to manipulate plant biology so that vector performance is not significantly degraded, and, at best, somehow improved (Castle and Berger 1993;Kluth et al 2002;Eliot et al 2003;Belliure et al 2005). Many investigations into these ''tripartite' ' (or ''trilateral'') plant-insect-virus interactions have demonstrated that aphid growth rate and morphology can be modified when aphids feed on infected plants (e.g.…”