Host density and habitat structure influence contact rates and transmission of Batrachochytrium salamandrivorans
Malagon, D. A., L. A. Melara, O. F. Prosper, S. Lenhart, E. D. Carter, D. L. Miller, and M. J. Gray.  2019.  Ecological Society of America Annual Conference, Louisville, Kentucky.

Batrachochytrium salamandrivorans (Bsal) is an invasive fungal pathogen that is emerging in Europe and is highly pathogenic to some salamander species, especially those in the family Salamandridae. Given its widespread distribution and high abundance, the eastern newt (Notophthalmus viridescens) has the potential to significantly contribute to Bsal’s epidemiology if the pathogen emerges in North America. For pathogens that transmit via direct contact between hosts, management strategies that reduce contact rates can limit invasion potential and outbreak size. Thus, we designed two studies to estimate: 1) contact rates given different host densities and levels of habitat structure, and 2) the probability of transmission from infected to susceptible individuals as Bsal chytridiomycosis progressed. Using parameter estimates from these experiments, we modeled infection and disease outcomes for a population of eastern newts using a system of ordinary differential equations. Bsal transmission was very efficient between newts even at early stages of infection. For example, one contact of one-second duration between an infected and susceptible newt resulted in 100% transmission only 12 days after the infected newt was initially exposed to Bsal zoospores. Contact rates between newts were density dependent – reducing host density 4-fold reduced contact rates by 5 – 15X. At higher newt densities, adding plants to mesocosms reduced contacts by 3X. Our simulations show rapid transmission of Bsal among newts, resulting in >95% infection prevalence in one month and >80% mortality in three months, illustrating the potential for severe population impacts. Our results demonstrate that if Bsal is introduced into North America, the eastern newt has the potential to play a major role in its epidemiology. Additionally, reducing newt density or increasing habitat structure at Bsal positive sites might reduce transmission and outbreak size.