The role of density-dependent arrested larval stages on parasite dynamics and stability: Lessons from nematodes and donkeys

The regulation of parasite populations is a very interesting field of study, relevant for both farm and wild host species. Traditionally, the mortality of parasites due to parasite-induced host mortality has been considered the main feature giving rise to the regulation and stability of host-parasite interaction, enhanced by the negative binomial distribution of parasites in host populations. Nevertheless, different possible density-dependent mechanisms of parasite regulation are likely to occur.For small strongyles (Subfamily: Cyathostominae), the most widespread intestinal parasites of equids, one of the mechanisms possibly regulating parasite population is the negative feed-back of luminal adult parasites on mucosal larval stage development.This paper proposes a deterministic model consistent with empirical data collected on donkey small strongyles. Its most innovative aspect is the inclusion of mucosal hypobiotic larvae regulation induced by adult parasite abundance. The explicit calculation of the expected value of the larval development rate, given the negative binomial distribution of adult parasites, was provided. Parasite-induced host mortality was irrelevant in the experimental donkey population studied and not included in the model, while density-dependent mortality of endogenous parasite stages was included.The main aim was to evaluate the possible stabilizing effect of the regulation of larval development. For this purpose, the comparison of the full model with a simplified model in which development rate is a fixed parameter was performed. The behaviour of the models after additional cyclical mortality was also studied. Cyclical parasite mortality can be viewed as the result of regular anthelmintic control measures used in the field.The main result obtained is that parasite extinction due to additional cyclical mortality is more difficult to be obtained if mucosal larvae development is regulated, notwithstanding the small transmission rate of the full model. Ecological regulation of parasite dynamics appears as a significant obstacle to parasite control; in particular the regulatory effect of adult parasites on mucosal larvae development enhances parasite viability allowing parasites to reach high population abundance with lower transmission rate and represents a clear advantage in terms of parasite survival to perturbations.