Managing infectious diseases without R0

The literature on infectious diseases in wild populations has focused primarily on a pathogen’s ability to expand within an existing host-pathogen system or invade new systems.  The basic reproductive ratio of the pathogen (R₀) is used to quantify this ability. Whenever R₀ < 1 the pathogen cannot invade or pathogen prevalence diminishes.  Using R₀ (or variations thereof), managers determine levels of harvesting, vaccination, or other efforts required to prevent a disease outbreak or to eradicate established pathogens.  R₀, however, has significant limitations as a management tool.  First, R₀ is calculated at a pathogen-free equilibrium.  Therefore, management based on R₀ is unresponsive to the current state of the world, and R₀ is an exogenous function of ecological parameters and unaffected by management choices.  Second, the threshold criterion R₀ = 1 only implies a minimum level of control, but not the most desirable level.  Moreover, this threshold is not uniquely determined in systems with multiple hosts or multiple management opportunities.  We therefore reject R₀ as a management tool and instead focus on host-density thresholds, which we show to be endogenous and time-dependent.  We further show how to quantify the degree to which one host is a reservoir for another.  Finally, we analyze management decisions using a multiple-host bioeconomic framework.  We illustrate that host-density thresholds are endogenously determined, as is the degree to which a population is a reservoir.  Improved disease control is achieved by jointly managing a population’s density and its threshold, which depends on managing the reservoir status of other populations.