Prions are transmissible, proteinaceous agents that cause fatal neurodegenerative diseases. In deer elk moose prions cause chronic wasting disease (CWD). The incidence of CWD can be remarkably high both in captive and wild herds and epidemiological data suggest that efficient horizontal transmission drives epidemic dynamics. Although deer can be infected orally and seem to be able to contract CWD from contaminated environments, precisely how and when CWD prions are shed into the environment have not been described. Previous studies have identified CWD prions in saliva, blood, urine, antler velvet, and muscle, lymphoid and other tissues of symptomatic cervids with late-stage disease. These sources of CWD prions may contribute to the spread of CWD, but none explains natural CWD transmission both within and between species in the deer family. To fit observed patterns, a natural CWD transmission mechanism must be effected within biologically realistic limits of the carrier medium, cannot require cannibalism and should be indirect to explain both environmental persistence and spread among multiple host species. Because empirical data and modelling suggested faecal excretion of prions throughout much of the disease course as potentially important to CWD transmission, researchers investigated whether prions are shed in faeces from mule deer during the course of CWD infection.
Infectious prion diseases – scrapie of sheep and chronic wasting disease (CWD) of several species in the deer family – are transmitted naturally within affected host populations. Although several possible sources of contagion have been identified in excretions and secretions from symptomatic animals, the biological importance of these sources in sustaining epidemics remains unclear. Here we show that asymptomatic CWD-infected mule deer (Odocoileus hemionus) excrete CWD prions in their faeces long before they develop clinical signs of prion disease. Intracerebral inoculation of irradiated deer faeces into transgenic mice overexpressing cervid prion protein (PrP) revealed infectivity in 14 of 15 faecal samples collected from five deer at 7–11 months before the onset of neurological disease. Although prion concentrations in deer faeces were considerably lower than in brain tissue from the same deer collected at the end of the disease, the estimated total infectious dose excreted in faeces by an infected deer over the disease course may approximate the total contained in a brain. Prolonged faecal prion excretion by infected deer provides a plausible natural mechanism that might explain the high incidence and efficient horizontal transmission of CWD within deer herds, as well as prion transmission among other susceptible cervids.
Why should we care? Apart from the impact on wildlife, vCJD is a prion infection transmitted from infected cows to humans. People eat deer. Although there is no evidence that the CWN prion causes diseae in humans, this is definitely one to keep an eye on.