Stochastic extinction and the selection of the transmission mode in microparasites

Abstract

Stochastic fluctuations in the transmission process of microparasites generate a risk of parasite extinction that cannot be assessed by deterministic models, especially in host populations of small size. While this risk of extinction represents a strong selection pressure for microparasites, it is usually not clearly separated from the deterministic ones. We suggest here that this stochastic selection pressure can affect the selection of the transmission mode of microparasites. To avoid extinction, parasites should maximize their inter-population transmission to ensure frequent reintroductions. Since the types of contacts may differ if congeners belong to the same or distinct populations, strains that are mainly transmitted through inter-population contacts might be selected. To examine this assumption, we analyse the issue of the competition between two strains differing in their transmission mode using a stochastic metapopulation model in which hosts may display different behaviours inside and outside their populations. We show that stochastic selection pressures may drive parasite evolution towards a transmission mode that maximizes the persistence of the parasite. We study the conditions under which stochastic selection pressures may surpass the deterministic ones. Our results are illustrated by the cases of feline immunodeficiency virus in cats and of sexually transmitted diseases in mammals.

Natural simian immunodeficiency virus transmission in mandrills: a family affair?

“…Yet, transmission mode is a key parameter to understanding the population dynamics of a pathogen and the selective pressure imposed by the social and spatial structure of the host population on its persistence [16]. SIV transmission is thought to be predominantly horizontal (i.e.…”

Modes of transmission of Simian T-lymphotropic Virus Type 1 in semi-captive mandrills (Mandrillus sphinx)

“…For example sexual transmission is preponderant in vervets [Cercopithecus aethiops, (Dracopoli et al 1986)] while the aggressive mode of virus transmission is predominant in captive baboons (d' Offay et al 2007) and mandrills (Nerrienet et al 1998;this study). These different patterns of transmission modes can be interpreted with regard to the social organization of populations (Bahi-Jaber et al 2008). For example, baboons and mandrills exhibit a similar multilevel social structure defined by dominance hierarchies in both males and females, and share the same aggressive mode of transmission for STLV-1 (d' Offay et al 2007, this study).…”

When domestic cat (Felis silvestris catus) population structures interact with their viruses

“…These males are important targets for viruses that must spread between sub-populations in order to survive. This may explain why FIV is primarily transmitted through bites [39], out of all the possible modes of transmission (e.g., sexual, or vertical in-utero or in-lacto) reported experimentally [54,55]. These results may clarify the apparent paradox of FIV in that it exhibits an endemic pattern in rural cat populations despite its low prevalence, relying on a small number of at-risk individuals.…”

“…In host populations of small size, viruses face a high risk of extinction due to stochastic fluctuations in the transmission process [37,38]. According to recent theoretical studies, stochastic extinction risk exerts an important selection pressure [4,39] on virus evolution, in order to maintain virus circulation through time. Cats, whose populations are fragmented and of small size (between 60 and 300 cats, [13]), represent an interesting case study to illustrate this theory.…”