Canine ehrlichiosis in Northern Australia: A sensitivity and elasticity analysis of R₀

Canine ehrlichiosis is a potentially fatal tick-borne bacterial disease of domestic dogs caused by Ehrlichia canis and transmitted in Australia by the brown dog tick, Rhipicephalus linnaei (Rhipicephalus sanguineus sensu lato "tropical lineage"). E. canis occurs throughout the United States, South America, Asia, and Africa, and recently emerged among domestic dogs in the Kimberly region of West Australia in May 2020. The pathogen has spread across the northern regions of Australia, prompting the development of awareness campaigns and bringing new attention to human-mediated dog movement. We constructed a next generation matrix to estimate the basic reproduction number, R₀, for E. canis. We find that there is considerable uncertainty in R₀ reflected in a lack of field studies on the abundance and life history of the immature stages of the primary vector. We calculated an average R₀ of 1.24 with 95% of the values falling in the interval (0.41,2.67). We also calculated the control effort required to prevent an outbreak when control is targeted at preventing infections in dogs. Elasticity analysis is performed to assess the relative contributions of ticks infected at different life stages to R₀; it reveals that ticks infected as larvae contribute the most to R₀. Global sensitivity indices indicate that the most direct way to reduce uncertainty in R₀ for E. canis is better information on the parameters governing the incidence and survival of the immature life stages of the brown dog tick.