Parasite transmission: reconciling theory and reality

Abstract

Summary 1.Arguably the most important and elusive component of host-parasite models is the transmission function. Considerable empirical and theoretical work has focused on determining the correct formulation of this function although, to date, there has been little attempt to combine these studies to develop general insights into how observed transmission rates affect host-parasite dynamics. 2. Here, estimates of transmission rates from a range of host-parasite systems in the literature are described using a phenomenological function which takes into account how transmission varies with host and parasite densities. This function is placed in the appropriate model framework to determine the consequences of the observed transmission rates for each system. 3. All of the parasites had decreasing per capita transmission rates with increasing parasite densities suggesting that parasites tend to saturate at high densities, either as hosts become limiting or due to heterogeneities amongst the host population. In terms of the responses to host density, the parasites fell into two groups: those with increasing or decreasing transmission rates. This dichotomy was due to the biology of the organisms; the former group infect through cannibalism, which increased at high densities as the individuals became stressed, whereas the latter group infected through free-living stages, resulting in a form of spatial structuring reducing the number of hosts available for infection. 4. A metapopulation model was developed where hosts and parasites interacted in discrete patches according to the appropriate transmission function, with neighbouring patches linked by dispersal. The model suggested that small-scale, localized transmission events can drive large-scale epizootics at the metapopulation level. This emphasizes the importance of correctly describing and quantifying the transmission function at the individual level. 5. Traditionally, the formulation of the transmission function has depended on the scale of observation. This work shows that transmission should be considered from the viewpoint of the organisms concerned. Observed transmission rates are a consequence of the biology of the individuals meaning it should be possible to develop a priori hypotheses concerning the nature of the transmission function from a basic understanding of the life history of the organisms concerned.

Combined Effects of Virus, Pesticide, and Predator Cue on the Larval Tiger Salamander (Ambystoma tigrinum)

“…While one might expect this increase in larval mortality to impact population dynamics (Sih et al 2004b), disease may further increase population impacts. Theoretically, an increase in disease susceptibility can also increase transmission rates and dramatically increase overall prevalence (Fenton et al 2002). This study did not specifically examine transmission, but understanding how these dynamics play out at a larger spatial scale with interacting individuals is an important next step.…”

Frequency–dependent incidence in models of sexually transmitted diseases: portrayal of pair–based transmission and effects of illness on contact behaviour

“…This approach hinges on considering the distinct contributions of each individual in forming a pair, and leads to nonlinear dependence of the transmission rate on the proportions of S and I individuals (beyond the usual bilinear SI form). A recent study found such nonlinearity to be a common feature of incidence data (Fenton et al 2002); disease effects on contact behaviour are an unexplored mechanism for this trend. Simulations in which individual pairing rates were reduced (or increased) by infection demonstrated that these behavioural effects could have striking impacts on epidemic progression ( figure 3b,c).…”

Transmission of Plant-Pathogenic Bacteria by Nonhost Seeds without Induction of an Associated Defense Reaction at Emergence

“…This contrasts strikingly with the situation for seed-borne plant-pathogenic bacteria and their hosts, for which little is known at this step, with efforts having been devoted mainly to interactions with fully developed plants. However, pathogen transmission is one of the most important parameters for fitness (16,32). It combines the ability to survive outside the host prior to infection, multiply on the host, and disperse and transmit to new ecological niches, including host seeds.…”