Vaccination and immigration rates influence raccoon rabies elimination and recolonization in simulated urban-suburban landscapes

Emily M. Beasley , Timothée Poisot 

The raccoon variant of the rabies virus (RRV) is primarily managed in the eastern United States and Canada via distribution of oral rabies vaccine (ORV) baits. The goal of ORV distribution is to reach seroprevalence rates (an index of population immunity) of 60%, the threshold thought to eliminate RRV. Seroprevalence rates in urban areas rarely reach target levels, predictably leading to rabies outbreaks. However, many managed urban areas have spent several years rabies-free, suggesting RRV can be eliminated from urban areas at below-target seroprevalence rates. Using an agent-based model to simulate raccoon populations in urban landscapes, we examined 1) effects of varying vaccination and immigration rates on rabies elimination probability and time to elimination, and 2) what vaccination threshold, if any, prevents subsequent rabies recolonization events. Rabies elimination rates ranged from 52–98% with a median of 82.6%. Vaccination rates increased elimination probability (1.6% per 10% increase in vaccination) and time to elimination (11 weeks per 10% increase in vaccination), whereas the weekly immigration rate and immigrant prevalence rate reduced elimination probability (3.7% per additional expected weekly immigrant and 1.1% per 1% increase in rabies prevalence, respectively). Recolonization was common and highly stochastic, with a recolonization probability ranging from 12.5–100% with a median of 89.5%. Immigration rates and immigrant prevalence increased the weekly probability of recolonization after rabies was eliminated (0.6% per additional expected weekly immigrant and 1.6% per 1% increase in rabies prevalence, respectively). Vaccination decreased weekly recolonization probability (1.6% per 10% increase in vaccination rate) and reduced the number of rabies cases following a recolonization event. Based on these results, we recommend continuous vaccination within the urban area and along raccoon movement corridors to prevent infected individuals from entering rabies-free areas, as both vaccination and immigration variables affected the likelihood of recolonization. Understanding long-distance movements of host individuals is crucial for managing diseases such as rabies which likely persist at the regional scale.