Modeling Routes of Chronic Wasting Disease Transmission: Environmental Prion Persistence Promotes Deer Population Decline and Extinction

Abstract

Chronic wasting disease (CWD) is a fatal disease of deer, elk, and moose transmitted through direct, animal-to-animal contact, and indirectly, via environmental contamination. Considerable attention has been paid to modeling direct transmission, but despite the fact that CWD prions can remain infectious in the environment for years, relatively little information exists about the potential effects of indirect transmission on CWD dynamics. In the present study, we use simulation models to demonstrate how indirect transmission and the duration of environmental prion persistence may affect epidemics of CWD and populations of North American deer. Existing data from Colorado, Wyoming, and Wisconsin's CWD epidemics were used to define plausible short-term outcomes and associated parameter spaces. Resulting long-term outcomes range from relatively low disease prevalence and limited host-population decline to host-population collapse and extinction. Our models suggest that disease prevalence and the severity of population decline is driven by the duration that prions remain infectious in the environment. Despite relatively low epidemic growth rates, the basic reproductive number, R 0, may be much larger than expected under the direct-transmission paradigm because the infectious period can vastly exceed the host's life span. High prion persistence is expected to lead to an increasing environmental pool of prions during the early phases (i.e. approximately during the first 50 years) of the epidemic. As a consequence, over this period of time, disease dynamics will become more heavily influenced by indirect transmission, which may explain some of the observed regional differences in age and sex-specific disease patterns. This suggests management interventions, such as culling or vaccination, will become increasingly less effective as CWD epidemics progress.

Deer density and disease prevalence influence transmission of chronic wasting disease in white‐tailed deer

“…Despite these advances, critical gaps are evident in our understanding of CWD, which can be transmitted from both direct contact and an environmental reservoir. Despite the importance of multiple transmission routes to future CWD infection patterns (Wasserberg et al 2009, Almberg et al 2011), we do not know the relative contributions of these factors in free-ranging populations. If infectious contacts are driven by contaminated environments, more information is required regarding the likely sources of concentrated prion contamination (e.g., mineral licks, scrapes, bait sites).…”

“…Sampling the environment for prions (Maddison et al 2010), especially across a range of densities of infected deer, could help determine the availability and importance of environmental sources of prions to CWD transmission. In general, we believe the role of environmental transmission in wild deer populations is a critical area of needed research to determine the future dynamics of CWD epidemics (Wasserberg et al 2009, Almberg et al 2011). Additionally, we know little about how CWD is transmitted between social groups of female deer or in adult male deer, despite their higher rates of prevalence (Miller and Conner 2005, Grear et al 2006).…”

Influence of the geographic distribution of prion protein gene sequence variation on patterns of chronic wasting disease spread in white-tailed deer (Odocoileus virginianus)

“…Once shed, prions have been shown to persist in the environment, potentially remaining infectious and furthering disease spread long after affected deer have dispersed [10–12]. If left unchecked CWD prevalence will increase slowly over time [13–16] and deer population densities will decline [17,18], possibly to the point of extirpation [19,20]. Negative economic impacts are also expected due to decreased hunter confidence, decline in wildlife viewing, and increased regulation of the captive cervid industry [21,22].…”

Challenges in managing the risks of chronic wasting disease

“…This suggests management interventions, such as culling or vaccination, will become increasingly less effective as CWD epidemics progress. (Almberg et al, 2011) The level of long-term risk represented by these factors is difficult to estimate at the moment, and they will need to be monitored on an ongoing basis.…”