Effects of transmission pathways, immunological thresholds, and long-distance dispersal on infectious spread: chronic wasting disease case study

A broad class of wildlife diseases is transmitted directly between individuals and indirectly through the environment. When environmental transmission requires threshold exposure, as will often be the case due to pathogen attenuation, there will be Allee-like behavior in the transmission pathway. We study this phenomenon in an integrodifference equation model of chronic wasting disease (CWD) in white-tailed deer. Our model considers direct and indirect transmission, as well as the impact of long-distance dispersal by juveniles. Direct transmission is identified with pulled fronts of invading CWD, while indirect transmission is primarily responsible for pushed fronts driven by dispersal. Environmental transmission allows for disease to spread even when the basic reproduction number for direct transmission is below one. We also find that even small proportions of long-distance dispersers can significantly impact CWD propagation. These findings have implications for understanding and managing wildlife diseases with environmental transmission pathways susceptible to threshold effects.