Life-history responses, transmission dynamics and epidemiology in a terrestrial gastropod infected with parasitic mites

Parasites are known to influence the life-history of their hosts. A parasite-induced reduction in host fecundity and survival may lead to a reduced rate of increase in the host population. Genetic variation in parasite resistance and parasite-induced host mortality is a precondition for parasite-induced selection on host populations and enables parasites to influence their host’s population genetics. The occurrence and the distribution of parasites in host populations are determined by parasite transmission. Transmission among host individuals is one of the most important characteristics in the life cycle of a parasite species, and is fundamentally important for the parasite’s fitness. Parasite-induced life-history responses and the dynamics of parasite transmission are key elements of the epidemiology. Mathematical models of parasite transmission and the host life-history may help to understand the dynamics of host- and parasite populations and are essential to predict the course of infectious diseases, which are especially important in the context of veterinary and human health care.
In the present thesis we investigated the impact of Riccardoella limacum, a parasitic mite of terrestrial gastropods, on life-history of Arianta arbustorum, a common host of R. limacum. Furthermore, we examined the transmission dynamics of R. limacum. Data on parasite-induced life-history responses in A. arbustorum and on parasite transmission were then combined in mathematical models, in order to predict the epidemiology and to assess disease relevant parameters, which may help to predict the course of R. limacum infections in A. arbustorum populations.
In order to assess the impact of R. limacum on the life-history of A. arbustorum, we quantified the reproductive output (number of eggs), the hatching success of the eggs and the activity of naturally infected snails. The reproductive output was reduced in infected snails, whereas the hatching success of the eggs was slightly increased in infected snails compared to parasite-free snails. Snail behavior differed between infected and parasite-free snails by a reduced activity of parasite infected snails. Furthermore, in 2 out of 3 A. arbustorum populations and in experimentally infected snails winter survival was reduced in infected snails compared to uninfected snails.
In order to quantify the additive genetic variation in parasite load and winter mortality, every second snail of 15 lab bred snail families was experimentally infected with R. limacum. Parasite load of infected snails showed high heritable variation among snail families, suggesting additive genetic variation in parasite resistance in A. arbustorum. Furthermore, parasite load increased with increasing snail size within snail family, which suggests that the proliferation of R. limacum is limited by resources provided by A. arbustorum.
Furthermore, the transmission route of R. limacum was investigated. Parasite transmission also occurred without physical contact from mite-infected to parasite-free snails. The investigation of the off-host locomotion of R. limacum revealed that mites prefer to move on fresh mucus. Apart from the transmission during long-lasting courtship and mating of the host, R. limacum is also transmitted via soil and may use fresh snail mucus a cue to locate new host.
Further transmission experiments revealed that transmission of R. limacum occurs for the most part during host contacts. Using experimentally assessed transmission probabilities per host contact and contact frequencies of A. arbustorum, we developed mathematical models based on contact frequencies and transmission probabilities of A. arbustorum. Data on parasite-induced life-history responses were used to assess the basic reproductive ratio (R0; expected number of secondary infections originated by an infected individual introduced into a susceptible population) and host threshold density for parasite persistence in three A. arbustorum populations. The models revealed that the population with the highest density showed larger R0-values (16.7–22.95) compared to populations with intermediate (2.72–7.45) or low population density (0.75–4.10). Host threshold population density for parasite persistence ranged from 0.35 to 2.72 snails per m2. The thesis shows that the incorporation of the disease-relevant biology of organisms may improve models of host-parasite dynamics. This approach may help to predict the epidemiology of host-parasite systems.