Study system

The investigated system consists of three interacting populations: the host population (raccoons, H), the parasite population (adult B. procyonis, P) and the free-living infective stage of the parasite (B. procyonis eggs, E). The simulated intervention strategies are: (i) raccoon depopulation, (ii) anthelmintic treatment and (iii) faeces removal.

Firstly, we performed a preliminary sensitivity analysis of the parameters representing the different intervention strategies, to compare their influence on the number of environmental eggs [32]. Then, we analysed the system by combining the analysis of equilibria (S1 Text) and simulations to evaluate both the efficacy and the efficiency of each intervention strategy. Multiple simulations were performed using the simple Euler forward integration method (function euler of”deSolve” package in R 3.6.3 software). In order to assess efficacy, we evaluated whether each specific intervention strategy was able to eliminate the egg population within 50 years. Efficacy was computed by using the equations for the analysis of the system equilibria reported in S1 Text. To assess the efficiency of each intervention strategy we evaluated the effective time needed to reach the new steady state. Since we focused our attention on intervention strategies eliminating the egg population and both adult parasite and egg populations consist of a discrete number of individuals, we considered that an intervention strategy reaches the steady state when the computed number of eggs and parasites is lower than one.

We chose to consider a 50-years time frame to clearly show and compare the effects of the intervention strategies on both the number of eggs and the time needed to reach the equilibrium. For this reason, interventions requiring more than 50 years to reach the equilibrium were not considered in the analysis and we only reported the number of eggs reached at the 50^th year.

To simplify the comparison between intervention strategies, we will focus hereafter on the proportion of subjects (raccoons/parasites/eggs, depending on the intervention strategy) treated per day, expressed as a percentage of the whole population size on that day and named hereafter “treatment coverage”. It must be noted that the proportion of subjects treated does not strictly represent a constant number of raccoons/parasites/eggs treated per day, because it will depend on the population size of that day.

Due to the recent introduction of raccoons and B. procyonis in areas outside their natural North and Central American distribution range [21], two different scenarios have been explored:

The ‘native population’ scenario: represented by a raccoon population in its native range, where the host population is close to its environmental carrying capacity (K) and the system is close to its steady state.

The ‘introduced population’ scenario: represented by a raccoon population recently introduced in a new area, where neither the environmental carrying capacity nor the system’s steady state have been reached yet. In this scenario, we considered, as initial sizes of H, P and L, the values reached from the system when the host population reaches 50% of its environmental carrying capacity.

We considered these two scenarios to take into account their epidemiological and demographic differences, as they differently affect the feasibility and efficacy of management and intervention strategies.