Modelling wildlife rabies: Transmission, economics, and conservation

Abstract

Rabies is a fatal zoonotic disease of mammals; it exacerbates the uncertainty of conserving populations of some threatened mammals (TM). Modelling affords an inexpensive, a priori way of studying key parameters of wildlife rabies transmission, rabies management economics, and TM conservation. Numerous models of rabies transmission have been published. Linear density dependent models predicted that a threshold density (K T 6 1.0), possibly attained by culling or contraception, would eliminate an epizootic through reduced contacts among host animals. Density independent models predicted less advantage of culling and contraception in rabies control due to limited contacts among territorial host animals. Recent stochastic, mixed models offer novel predictions about the role of culling, fertility control, and oral rabies vaccination (ORV) in disease management. Use of a ''threshold successful contact'' rate (C T ) as a parameter in these models predicts that density reduction of host animals will enhance ORV campaigns in non-TM contexts via more efficient bait delivery and vaccination. Economic analyses of medical, public health, and veterinary costs have shown post-exposure prophylaxis (PEP) and increased pet vaccinations (PV) to be major rabies-caused expenses during and after epizootics in North America.No modelling efforts have examined either the benefits-costs of rabies management strategies to conserve TM or the use of ORV, per se, to conserve TM -an omission due in part to the lack of methodologies for properly valuing TM (potential savings) and the expense or lower priority of using ORV for TM protection. This paper: (1) describes key aspects of rabies-transmission models in wildlife, (2) posits the use of C T to predict disease persistence, (3) reviews selected ORV strategies, economic studies, and benefit-cost models associated with the use of ORV as a means of rabies control in non-TM situations, (4) discusses implications of these models to the conservation of TM, and (5) recommends five steps to improve modelling of rabies transmission (wildlife disease in general), rabies-control economics, and TM conservation.

Demonstrating frequency-dependent transmission of sarcoptic mange in red foxes

“…The minimal effect of density on mange is supported by the observation that mange persists at low fox densities [20], because frequency-dependent diseases can be sustained at lower host densities than density-dependent diseases [26]. The contrast between our results and those of assessments of mange transmission in chamois, a species that does not show complex group structuring, and for which density-dependent transmission of mange was well supported [12], emphasizes the role of sociality in mediating disease dynamics within a population [1,2,5,19]. Further work is needed to examine how transmission mechanisms vary across different species affected by the same disease.…”

“…Previous mange models have only considered direct, density-dependent transmission [12,17] but off-host mite survival [9] and low inter-group contact in foxes [18] suggest that indirect transmission is likely. Moreover, the social nature of foxes suggests that the traditional assumptions of density-and frequency-dependent disease transmission might be complicated [19]. We developed a model of mange spread and fitted it to a long-term dataset.…”

Carnivore population trends in Spanish agrosystems after the reduction in food availability due to rabbit decline by rabbit haemorrhagic disease and improved waste management

“…Estimates of carnivore abundance are also important in assessing disease risks and the consequences of disease outbreaks (Sterner and Smith 2006). Diseases such as sarcoptic mange (Gortázar et al 1998) and rabies (Roboly 1985) can cause population declines which need to be quantified for management or for conservation purposes.…”

Assessing and predicting the spread of non-native raccoons in Germany using hunting bag data and dispersal weighted models

“…monitoring programs help to determine the distribution of non-native species, which is necessary in order to 55 assess the impact of non-native species in terms of disease risks, economic damage and negative effects on 56 native species and the environment, and plan management actions to reduce these impacts (Engeman et al 2006; 57 Sterner and Smith 2006; Yokomizo et al 2009). Monitoring programs for terrestrial mammals are usually based 58 on the collation of ad-hoc records (Roy et al 2014a), systematic surveys of abundance (such as road-kill surveys,…”