title: Strong density-dependent competition and acquired immunity constrain parasite establishment: implications for parasite aggregation.
abstract: The vast majority of parasites exhibit an aggregated frequency distribution within their host population, such that most hosts have few or no parasites while only a minority of hosts are heavily infected. One exception to this rule is the trophically transmitted parasite Pterygodermatites peromysci of the white-footed mouse (Peromyscus leucopus), which is randomly distributed within its host population. Here, we ask: what are the factors generating the random distribution of parasites in this system when the majority of macroparasites exhibit non-random patterns? We hypothesise that tight density-dependent processes constrain parasite establishment and survival, preventing the build-up of parasites within individual hosts, and preclude aggregation within the host population. We first conducted primary infections in a laboratory experiment using white-footed mice to test for density-dependent parasite establishment and survival of adult worms. Secondary or challenge infection experiments were then conducted to investigate underlying mechanisms, including intra-specific competition and host-mediated restrictions (i.e. acquired immunity). The results of our experimental infections show a dose-dependent constraint on within-host-parasite establishment, such that the proportion of mice infected rose initially with exposure, and then dropped off at the highest dose. Additional evidence of density-dependent competition comes from the decrease in worm length with increasing levels of exposure. In the challenge infection experiment, previous exposure to parasites resulted in a lower prevalence and intensity of infection compared with primary infection of naïve mice; the magnitude of this effect was also density-dependent. Host immune response (IgG levels) increased with the level of exposure, but decreased with the number of worms established. Our results suggest that strong intra-specific competition and acquired host immunity operate in a density-dependent manner to constrain parasite establishment, driving down aggregation and ultimately accounting for the observed random distribution of parasites.
authors.affiliation: Center for Infectious Disease Dynamics, Department of Biology, 208 Mueller Laboratory, The Pennsylvania State University, University Park, PA 16802, USA. ltl1@psu.edu
authors.forename: Lien T
authors.initials: LT
authors.lastname: Luong
authors.forename: Beth A
authors.initials: BA
authors.lastname: Vigliotti
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Hudson
mesh_terms: D056704:Adaptive Immunity; D000818:Animals; D004195:Disease Models, Animal; D006135:Gryllidae / Q000469:parasitology; D006790:Host-Parasite Interactions; D006801:Humans; D007303:Insect Vectors / Q000469:parasitology; D008297:Male; D051379:Mice; D009348:Nematoda / Q000254:growth & development / Q000276:immunology / Q000502:physiology*; D009349:Nematode Infections / Q000276:immunology* / Q000469:parasitology