A model for the development and growth of the parasitic stages of Parascaris spp. in the horse

•A model is described for the dynamics of Parascaris spp. in the young horse.•Worm survival in the intestine is the most important variable affecting dynamics.•Much is unknown about the role of host immunity in Parascaris biology.•Model output is consistent with published data.•Only treating foals after patency may slow the development of drench resistance. Literature documenting the growth and development of Parascaris spp. infections was used to develop a model describing worm dynamics in the young horse. The model incorporates four main variables; the rate at which larvae migrate through host tissues to return to the small intestine, the proportion of migrating larvae which succeed in returning to the small intestine, the rate of growth in size of maturing and adult worms and the survival rate of maturing and adult worms. In addition, the number of eggs laid each day by adult female worms is calculated as a function of worm size (length) and is used to calculate faecal egg output of the foal. Published data describing the rate of migration through host tissues, and the growth of worms following their return to the small intestine, was used to derive relationships describing these processes. However, only limited data exists relating the survival of migrating larvae and mature worms in the intestine to host age and experience of infection. Therefore, relationships and coefficients describing these variables were modified so that output aligned with published experimental results. As a consequence, the model has not yet been evaluated against an independent data set, and so remains as the best ‘current hypothesis’ for the dynamics of this parasite. Hopefully, future experiments designed to test specific assumptions and outputs of the model will lead to a better understanding of the biology of this important parasite. For example, the most influential variable in determining model output is the survival rate of worms in the small intestine. In the model, worm survival declines in response to both the increasing age of the horse and the increasing cumulative length of worms in the intestine (used as a proxy for crowding). Given the importance of this variable to model behaviour and the paucity of experimental data on this topic this would seem a priority for future study. Initial experiments using the model suggest that a single anthelmintic treatment, administered soon after patency of initial infection, may effectively control environmental contamination