State of Knowledge Regarding Transmission, Spread, and Management of Chronic Wasting Disease in U.S. Captive and Free-Ranging Cervid Populations (2025)
Chapter: 3 Mechanisms of Transmission and Potential Host Range
3
Mechanisms of Transmission and Potential Host Range
Of the naturally occurring prion diseases identified to date, CWD is singular in its transmissibility and spread among captive and wild cervid populations. The routes of transmission have not been determined definitively but are accepted generally to involve either oral or oral-nasal routes of exposure. As with all prion diseases, the incubation period has two separate stages, an extensive (months to years) preclinical phase in which the animal appears healthy, followed by a short (weeks to months) clinical phase prior to death. Shedding of infectious prions begins during the preclinical stage and correlates with the accumulation of misfolded prions in various tissues.
Vertical transmission of prions from female to fetus before (in utero) or shortly after birth has been described, but this contribution to CWD epidemiology is uncertain. Once shed in bodily wastes or through the decomposition of infected carcasses, prions effectively contaminate the environment, including soil, water, and plants, and persist for extended periods of time.
This chapter focuses on the mechanisms of CWD transmission. The persistence and movement of infectious prions across different elements of the environment and how susceptible cervids may become exposed are described. Finally, the potential of these infectious particles to induce prion diseases in non-cervid species, including humans, is discussed.
ROUTES OF EXPOSURE
The wide geographic dissemination of CWD can be explained, in part, by the efficient transmission of infectious prions between infectious and susceptible cervids. It is generally accepted that the primary route of exposure in natural settings is oral-nasal based on effective transmission reported through abrasions in the mouths of captive research cervids (Denkers, Telling, and Hoover, 2011), aerosolization of CWD prions (Denkers et al., 2013), and oral inoculation with infectious materials in the laboratory setting (e.g., Denkers et al., 2020).
The transmission events leading to CWD have been studied in experimental conditions. White-tailed deer, one of the best studied animal species of CWD prion transmission, are susceptible to the prion infectious agent by multiple routes of exposure, including intracerebral, intraperitoneal, intravenous, oral, and oronasal (Johnson et al., 2011a; Hamir et al., 2011; Denkers et al., 2020; Kincheloe et al., 2021). Although some of these routes (e.g., intravenous, intracerebral) are not practical in terms of natural transmission, they are useful to define the transmission potential of CWD prions, the temporal dissemination of infectious prions in different tissues and excreta, and the minimum quantities of prions needed to induce disease. Experimental infections have demonstrated that as little as 300 nanograms (ng) of infectious prions may result in clinical infection of white-tailed deer via the oral route (Denkers et al., 2020) (see Box 2.4). The oral-nasal route, as stated earlier, is the most likely route for CWD transmission in cervids as CWD prions are available to naive cervids directly via animal-to-animal contact or indirectly through contaminated environmental fomites (Moreno and Telling, 2018; Zabel and Ortega, 2017).
Another potential route of exposure involves mother-to-offspring transmission (Nalls et al., 2013; Selariu et al., 2015). CWD prions have been detected in fetuses and gestational tissues of naturally and experimentally infected white-tailed deer (Bravo-Risi et al., 2021; Nalls et al., 2021). Fawns born from experimentally infected Reeves’ muntjac deer (Muntiacus reevesi) dams presented increased risks of prion infection (Nalls et al., 2013). Mechanisms associated with CWD vertical transmission are yet to be defined.
PRIONS IN THE ENVIRONMENT
CWD prions are introduced into the environment to different extents and through different animal products (e.g., urine, feces, or tissues released from decomposing carcasses or placenta) containing variable quantities of prion infectivity. Contaminated environments can remain a source of infection for decades or more (Georgsson, Sigurdarson, and Brown, 2006) and can contribute to the dissemination of prions. Infected carcasses may be consumed by mammalian and avian predators or scavengers (e.g., coyotes, vultures, feral pigs), as well as parasitic and non-parasitic invertebrates (e.g., insects, annelids) that may spread prions beyond where the host died (Soto et al., 2024; Pritzkow et al., 2021; Baune et al., 2021; Inzalaco et al., 2024; Nichols et al., 2015). CWD prions are found in soils (Kuznetsova et al., 2024), predator and scavenger fecal material, internal and external parasites including nasal bots1 and ticks (Haley et al., 2021b; Inzalaco et al., 2023), as well as plant material and water (Carlson et al., 2023; Pritzkow et al., 2015; Nichols et al., 2009; Plummer et al., 2018), although infectivity levels are unknown. Plummer and others (2018) detected the presence of CWD in samples collected at or near salt licks. Analysis of soils in and around deer carcass burial pits also resulted in the detection of infectious prions (Soto
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1 Nasal bots are the larva of bot flies that are found on the skin of the nose and mouth of white-tailed deer (e.g., https://myfwc.com/research/wildlife/health/white-tail-deer/nasal-bots; accessed April 30, 2024).
et al., 2023a; Schwabenlander et al., 2024). Infectious prions were detected in feeders in CWD-infected captive facilities (Soto et al., 2023a).
Environmental samples can be at least partially decontaminated in laboratory settings (Kuznetsova et al., 2018), but there are no practical approaches to eliminate prions efficiently and effectively in a contaminated natural environment. Environmental decontamination procedures being studied are discussed in Chapter 6.
Soils
Cervids consume significant amounts of soil both directly and inadvertently. In the lab, prions bind to soil and soil minerals, but binding capacity differs with the composition of the soil or mineral type (Johnson et al., 2006b). Laboratory experimentation has shown that prions bind more avidly to soils and clay minerals over time, making recovery increasingly more difficult, although infectivity remains unaffected (Kuznetsova et al., 2020). Prions bind readily to the clay mineral montmorillonite (Mte) and far less to sandy soils such as quartz sand. Areas rich in clay soils display higher CWD incidence compared to areas of other soil types (Walter et al., 2011). Furthermore, when prions are bound to Mte and then assessed for infectivity using bioassay (e.g., exposing lab cervids to a sample suspected to contain infectious prions), the incubation periods are shorter than when using prions not bound to Mte, suggesting that, in some cases, binding to soil or soil minerals enhances infectivity (Johnson et al., 2007).
Migration of prions through soil columns packed with different soil types is also soil-type dependent. When soil columns contain Mte or Mte-rich soils, prions do not migrate and remain near the top of the column. In quartz or sandy soils, prions move through the column, ultimately being detected in materials leached from the column (Kuznetsova et al., 2023). These results suggest that the availability of prions for subsequent transmission via the environment is dependent on soil type. The binding and lack of migration of CWD through clay soils underpins the recommendation that clay liners be used for carcass disposal pits (Jacobson et al., 2010). Differential binding of prions to different soil types—and the variable effect of soil components on prion detection methods—complicates the detection and quantification of CWD prions in the environment (see Chapter 4 for a discussion on detection methods).
Plants
Plants are the main component of cervid diets. Infectious CWD prions can mechanically bind to plants (Pritzkow et al., 2015), although it is important to note that current evidence related to prion-plant interactions comes primarily from research laboratories conducting controlled and proof-of-concept experiments (Pritzkow et al., 2015; Carlson et al., 2023). Grass plants exposed to infectious prions from different sources (including from brain extracts and excreta from experimentally infected cervids) have been found via prion replication analyses PMCA (protein misfolding cyclic amplification) and RT-QuIC (real-time quaking-induced conversion) to retain prions bound to their surfaces even after being extensively rinsed (Pritzkow et al., 2015; Carlson et al., 2023). Plants also can take up infectious prions from experimentally contaminated soil. Infectious prions were detected in the aerial portions of plants (i.e., stems and leaves) by PMCA (Pritzkow et al., 2015) and by fluorescently labelled PrPSc (Carlson et al., 2023). Experimental exposure of plants to CWD-contaminated soil suggests that edible plants can uptake prions from the soil and transport them to their aerial parts. For example, carrots grown in CWD-contaminated soils contained prions in both the roots and leaves, as detected by PMCA and confirmed by animal bioassay (Soto et al., 2023b). When root and leaf homogenates were individually inoculated intracerebrally into transgenic mice expressing deer prion protein, both roots and leaves resulted in clinical CWD infections (Soto et al., 2023b).2 However, there are no data demonstrating the presence of transmission-relevant concentrations of CWD prions in natural settings, nor is there evidence of natural infection transmitted to a cervid via plant tissues.
Determining whether plants in naturally infected environments can serve as reservoirs of infection is a critical question in CWD research. Concerns over the role of foraged crops in the geographic spread of CWD has led some governments (i.e., Norway) to ban the import of forage materials from any jurisdiction where CWD is present.3
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2 Transgenic studies are those in which genetic material from one species is artificially introduced to that of another species.
3 See https://lovdata.no/dokument/LTI/forskrift/2018-10-22-1599 (accessed August 21, 2024).
Parasites
The role of parasites in the transmission of CWD is largely unexplored. Whether parasites can act as vectors of CWD transmission is relevant, considering the wide variety of parasites (e.g., ticks, keds [a type of biting fly], mites, nasal bots, and several species of worms in lungs, stomach, muscles, liver, and arteries) that can occur on or in cervids. Parasites potentially could serve as vectors of CWD as they are transmitted between individuals, and CWD prions can be present in infectious amounts in cervid tissues during the long preclinical phase.
Ticks and nasal bots have been investigated for their potential roles in CWD transmission. Results related to transmission through ticks have been contradictory. Shikiya and others (2020) reported that ticks (Dermacentor andersoni) experimentally exposed to prion-infected hamsters could not infect new hamster hosts. These ticks also did not contain any prions, as measured by PMCA. It is important to note, however, that midgut contents present in tick homogenates (most likely blood) may affect detection using prion amplification methods. In contrast, prion seeding activity was demonstrated in winter ticks (Dermacentor albipictus) harvested from naturally infected North American elk by RT-QuIC assays (Haley et al., 2021a) (see Chapter 4 for a description of diagnostic tests). More recently, CWD prions were detected by PMCA and RT-QuIC in lab-fed black-legged ticks (Ixodes scapularis) and identified in black-legged ticks collected from naturally infected white-tailed deer (Inzalaco et al., 2023). The specific infectivity titers in these parasites, ergo their relevance for disease transmission, are unknown. The potential for ticks to transmit CWD needs to be carefully evaluated considering the low infectivity titers (i.e., the amount of infectivity in a sample) in blood of the tick’s host, the tick species’ natural life cycle in vertebrate hosts, the amount of infectivity in the tick, and the potential routes of infection (e.g., through transmission when the tick is consuming a blood meal versus inadvertent consumption of the tick during vertebrate grooming).
Some fly species, including bot flies (Cephenemyia phobifer), deposit their eggs in the nostrils of cervids where larvae mature within the nasal cavity and pharyngeal pouches (Soto et al., 2024). These anatomical structures are relevant for CWD as cervids can be readily infected via nasal exposure to CWD prions and prions are shed in nasal secretions. Moreover, cervids can host hundreds of nasal bots at a time (Texas Parks and Wildlife field personnel, personal communication, January 2021). CWD prions can be detected, via PMCA, in nasal bots collected from naturally infected, preclinical white-tailed deer (Soto et al., 2024). As the last stages of nasal bot larvae maturation occur in soil, there is also the potential for further contamination of the environment. As parasites do not express the cellular prion protein (PrPC) required for prion replication, they likely bind prions on their surface (Soto et al., 2024). The potential for parasites to serve as disease vectors depends on both their potential to bind prions and their interactions with the contaminated host or other potentially infected components (e.g., blood or other tissues). Animal bioassay studies have suggested that ingestion of a single bot would be sufficient to infect another deer (Soto et al., 2024); however, overall, there are more questions than answers regarding the role of fly larvae in potentially transmitting CWD.
HOST RANGE AND SPILLOVER TO OTHER SPECIES
As the geographic range and disease prevalence of CWD continue to increase, the frequency of exposure of other species to CWD also increases (Otero et al., 2021). The potential for spillover into a new species is dependent on the species barrier effect (the likelihood of infection of a different species) (Hill and Collinge, 2004). Multiple factors contribute to the species barrier and thus impact the likelihood of CWD spillover. These include (1) the amino acid sequences of the prion protein in both source and recipient cervids; (2) titer of the infectious agent; (3) the CWD strain; (4) the route of infection; and (5) possibly, the immune status of the newly infected cervids.4
Although other host factors may be involved, the prion protein is critical for prion infection as deletion of the gene encoding the prion protein abolishes the ability of an animal to be infected (Sailer et al., 1994; Richt et al., 2006). Transmission of prion diseases is more likely to occur when there is sequence homology or similarity between the amino acid sequences of the host PrPC and the pathogenic PrPSc (Prusiner et al., 1990; Hill et al., 1997). For example, although mice present a strong species barrier (i.e., are not readily infected) to several naturally existing prion strains, this barrier can often be overcome by removing the endogenous PRNP gene and replacing
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4 This sentence was revised after release of the report to specify the multiple factors that can contribute to the species barrier and CWD spillover.
it with the equivalent gene of the PrPSc-donor animal species (Browning et al., 2004; Herbst et al., 2022; Angers et al., 2010). Genetic modifications, particularly in rodent models of prion disease, have allowed identification of the specific amino acids contributing to the species barrier; however, susceptibility/resistance to infection by a specific prion strain can only be determined empirically (Cullingham et al., 2020).
Multiple strains of CWD prions have been identified (see Chapter 2). Different conformations of the misfolded prion protein result in CWD strains with different properties. Although most of the characterization of CWD strains is laboratory-based (i.e., analysis is based on transmission and biochemical properties), strains have been identified in naturally infected cervids. CWD strains can preferentially infect cervids with different PrPC sequences; for example, transgenic mice expressing 96S-PrP can be infected with some but not all CWD strains (Duque Velásquez et al., 2015; Hannaoui et al., 2021). Wisc-1, one of the best studied prion strains, can cause clinical disease in mice but not hamsters, while another CWD strain (H95+) can successfully infect hamsters but not cause clinical disease in transgenic cervidized mice (Herbst et al., 2017). These suggest that different strains may have different host ranges (i.e., the breadth of species able to be infected), impacting potential for transmission to non-cervid species, including humans.
The immune status of the newly infected cervids may impact intra- and inter-species transmission of prions. Several avenues of research suggest that the innate immune system (i.e., the immunity that an organism is born with) provides an early barrier to establishment of a CWD prion infection. These early defense mechanisms reduce the amount of infectivity in tissues important in the early stages of infection (see Pathogenesis of Disease section). Cells of the immune system also express considerable levels of PrPC and act, in non-cervid species, as early sites of prion replication as well as contribute to prion dissemination to different tissues (Blättler et al., 1997; Montrasio et al., 2000; Heikenwalder et al., 2005). Different levels of inflammation in various tissues have been shown to alter the distribution of prions in a positive manner (Heikenwalder et al., 2005; Ligios et al., 2005). Considering this, modifying the immune responses potentially can tip the balance into further preventing infection or allowing infection to occur (Carroll and Chesebro, 2019; Makarava et al., 2024). Although most of the experimental data, to date, are from laboratory animals, similar responses are anticipated to occur in cervid species. As many other pathogens will also activate the innate immune system, the status of the immune system in an individual animal may be predictive of whether an infection can be established.
Experimental Inter-species Transmission
The potential transmission of CWD prions to non-cervid animal species is of great concern as inter-species transmission of prions is known to favor the emergence of novel prion strains with new infectious potentials and host ranges (Bartz et al., 1998; Morales, Abid, and Soto, 2007; Morales, 2017; Herbst et al., 2017). There is no evidence of CWD spillover resulting in clinical disease in non-cervid species in nature. In the laboratory, however, transmission experiments often utilizing direct brain inoculation suggest that multiple different species could be susceptible (i.e., be capable of replicating the infectious prions) to CWD (see Table 3.1). In experimental studies, CWD prions have been transmitted to raccoons (Moore et al., 2019), ferrets (Bartz et al., 1998; Sigurdson et al., 2008), cattle (Hamir et al., 2001), sheep (Hamir et al., 2006b), pigs (Moore et al., 2017), squirrel monkeys (Saimiri sciureus) (Race et al., 2009b), and multiple North American rodents that share the environment with cervids including meadow voles (Heisey et al., 2010; Carlson et al., 2015), red-backed voles (Carlson et al., 2015; Heisey et al., 2010), white-footed mice (Peromyscus leucopus), and deer mice (Peromyscus maniculatus) (Heisey et al., 2010). It is possible that these species could serve as reservoirs for CWD, although no evidence of natural occurrences has been reported. Transgenic mouse studies have also demonstrated that beaver may be susceptible to CWD (Herbst et al., 2022).
Several different carnivore species have been infected with CWD in experimental paradigms (Mathiason et al., 2013). Ferrets (Mustela furo) are a valuable model for many prion diseases, including CWD. Their susceptibility to CWD could provide information about the crossover potential of CWD to other species (Sigurdson et al., 2008). Mink (Mustela vison), on the other hand, are susceptible to CWD only by the intracranial route (Harrington et al., 2008). Oral and intracranial challenges of domestic cats (Felis catus) have resulted in no clinical disease and low attack rates on first passage, but, on second passage, 100% of the cats presented with clinical disease following intracranial challenge and 50% presented with disease via the oral route (Mathiason et al., 2013). In contrast, extensive, long-term natural exposure of mountain lions (Puma concolor) to CWD-infected cervid carcasses failed to result in infection (Wolfe et al., 2022) despite this species’ susceptibility to bovine spongiform encephalopathy
TABLE 3.1 Experimental Transmission of CWD to Different Animal Species
| Species | Route of CWD Transmissiona | References |
|---|---|---|
| Cervids | ||
| Muntjac deer (Muntiacus reevesi) | IC, PO, SC | Nalls et al., 2013 |
| Fallow deer (Dama dama) | IC | Hamir et al., 2011 |
| North American caribou (Rangifer tarandus caribou) | PO, environmental | Mitchell et al., 2012; Moore et al., 2016 |
| Livestock | ||
| Cattle (Bos taurus) | IC | Hamir et al., 2011; Greenlee et al., 2012 |
| Sheep | IC | Hamir et al., 2006b; Mitchell et al., 2015 |
| Pigs | IC, PO | Moore et al., 2017 |
| Non-human primates | ||
| Squirrel monkeys (Saimiri sciureus) | IC, PO | Race et al., 2009b |
| Rodents | ||
| Present in areas where CWD occurs: | IC | Heisey et al., 2010 |
| Meadow voles (Microtus pennsylvanicus) | ||
| Red-backed voles (Myodes gapperi) | ||
| White-footed mice (Peromyscus leucopus) | ||
| Deer mice (Peromyscus maniculatus) | ||
| House mice | ||
| Not present in CWD areas: | IC | Raymond et al., 2007; Di Bari et al., 2013 |
| Syrian golden hamster (Mesocricetus auratus) | Herbst et al., 2017 | |
| European bank vole (Myodes glareolus) | ||
| Carnivores | ||
| Domestic ferrets (Mustela furo) | IC, PO, IP | Bartz et al., 1998; Sigurdson et al., 2008; Perrot et al., 2012 |
| Mink (Mustela vison) | IC | Harrington et al., 2008 |
| Domestic cats | IC, PO | Mathiason et al., 2013 |
| Raccoons (Procyon lotor) | IC | Moore et al., 2019 |
NOTE: susceptibility to intracerebral (IC) exposure may not reflect susceptibility to exposure via natural routes.
a IC: intracerebral; PO: oral; SC: subcutaneous; IP: intraperitoneal.
SOURCE: Otero et al., 2021.
(BSE; Kirkwood and Cunningham, 1994). Transmission to raccoons was CWD strain-dependent, with both elk and white-tailed deer CWD, following intracerebral infection, showing low attack rates, while mule deer CWD did not transmit (Cassmann et al., 2022; Moore et al., 2019; Hamir et al., 2007).
It is important to recognize the limitations associated with experimentally testing the species barrier to CWD infection. Testing species barrier in experimental studies generally relies on intracranial inoculation of the infectious agent. This route circumvents many of the clearance mechanisms thought to reduce prion titers initially (Chang et al., 2024), and results are interpreted as proof-of-concept to assess susceptibilities. Thus, successful transmission via the intracerebral route of exposure does not necessarily mean that infections will occur via other routes (Harrington et al., 2008; Mathiason et al., 2013; Moore et al., 2017; Williams et al., 2018). Unfortunately, some of the most widely used mouse models of prion disease are not susceptible to peripheral routes of exposure (Bian et al., 2019). Oral infection can also be several orders of magnitude less efficient than intracranial infections, requiring exposure to higher amounts of infectivity (e.g., Moore et al., 2017).
Experimental Infection of Livestock
As sheep and cattle are both susceptible to non-CWD prion diseases (scrapie in sheep and BSE in cattle), there is concern that CWD could spill over to agriculturally important species. Although cattle and captive cervids can overlap geographically, there is no evidence that cattle can be infected with common strains of CWD prions via natural exposure routes (Williams et al., 2018; Gould et al., 2003). Intracranial inoculations of sheep with mule deer CWD prions did result in clinical disease, albeit with long incubation periods and low penetrance (Hamir et al., 2006b). Successful infection of sheep with CWD prions was dependent on the host PRNP genotype and the CWD strain (Hamir et al., 2006b). Infection of transgenic mice expressing multiple copies of a specific sheep PRNP allele did not result in clinical disease or accumulation of PrPSc in the brain; the CWD prions were, however, successfully replicated in the spleen. This suggests a subclinical infection of sheep. If these subclinically infected sheep can transmit the prion disease to another sheep (i.e., via shed secretions), it is possible the prions could adapt to the new host—generating a sheep-specific CWD prion with greater ability to infect sheep (Cassmann, Frese, and Greenlee, 2021).
Domestic pigs can also be infected, albeit poorly, with white-tailed deer CWD prions (Moore et al., 2017). The data, to date, suggest that the species barrier between cervid CWD prions and pigs is high. Even low levels of CWD transmission in pigs, however, are concerning due to the possibility that feral pigs, which share ranges with cervids, could become a reservoir of CWD prions. CWD prion detection in wild pigs collected from areas where CWD occurs was proportional to the prevalence of prion-infected deer in each area of collection (Soto et al., 2023c). Although there was no evidence of clinical CWD infections of wild pigs in that study, subclinical infections were reported in transgenic mice expressing deer prion protein when injected with tissue homogenates from these pigs. These data suggest that CWD prions were present in pig tissues but at very low levels. Mice expressing the pig prion protein (surrogates for pig susceptibility to CWD prions) did not present with prion disease either as clinical or subclinical prion infection (Soto et al., 2023c).
To date, there is no evidence of CWD spillover to the wide variety of species that share the landscape with free-ranging cervids. Comparison of the PRNP sequences of various nondomestic bovid species (bighorn sheep [Ovis canadensis] and mountain goats [Oreamnos americanus]) suggested that these species are potentially susceptible to CWD (Cullingham et al., 2020); however, natural cases have not occurred among bighorns despite opportunity for environmental exposure (Fox et al., 2021).
CWD ZOONOTIC POTENTIAL
The committee’s charge (see Box 1.2) does not include discussion of the state of knowledge regarding transmission of CWD to humans. Nonetheless, summarizing a few key points on this topic in the context of host range and potential for spillover to other species seemed appropriate in this report.
There has been a limited number of epidemiological studies on the topic of spillover of CWD from cervids into humans, and these have found no causal links between CWD exposure and increased frequencies of human prion disease (Mawhinney et al., 2006; Olszowy et al., 2014; Abrams et al., 2011; Belay et al., 2001). A review of those studies is included as part of a larger effort by Waddell and others (2017). However, the increasing prevalence of CWD and broader geographic expansion of the disease in cervid herds can increase opportunities for human exposure, as there are more infected cervids that may be handled or consumed. Given that, it is important to recognize that populations with high frequency of cervid consumption (e.g., subsistence hunters) and that have unique traditional practices (e.g., brain-tanning of hides) may be at disproportionate risk of exposure to CWD prions in areas where CWD occurs (Tranulis and Tryland, 2023; Maraud and Roturier, 2021; Parlee et al., 2021). Monitoring and investigations of potential cases for evidence of CWD spillover in the United States are ongoing.
The risk of spillover to humans has also been examined indirectly in experiments using a variety of laboratory models including two non-human primate species, “transgenic” cervids expressing human prion protein, and in vitro amplification assays. Squirrel monkeys, considered universally susceptible to prion diseases, were susceptible to infection with CWD (Marsh et al., 2005; Table 3.1). In contrast, a study in rhesus macaques—a species considered more closely related genetically to humans than spider monkeys (Goodman et al., 1998)—suggested they were refractory to CWD infection (Race et al., 2009b; Table 3.1).
Most studies using transgenic mice expressing the human prion protein and developed to enhance their susceptibility potential have failed to identify any evidence of CWD transmission, thereby suggesting that the species barrier preventing CWD infection in humans is relatively high (Race et al., 2022; Wilson et al., 2012; Sandberg et al., 2010; Kong et al., 2005; Race, Williams, and Chesebro, 2019). One published study in transgenic mice that overexpress the human prion protein found no evidence of disease but did report PrPCWD amplification in spleen and other peripheral tissues suggestive of some potential for subclinical infections (Hannaoui et al., 2022). A study using transgenic fruit fly models expressing either human or non-human primate prion proteins reported susceptibility to CWD (Thackray et al., 2024). In considering the practical relevance of various transgenic model results, it is worth noting that these studies relied on genetically altered model systems and inoculation routes that bear little resemblance to the circumstances that might surround natural exposure of humans to CWD.
In vitro assays (e.g., cell-free conversion, RT-QuIC, or PMCA; see Chapter 4), developed to assess the ability of CWD prions to replicate using normal human prions as a conversion substrate, also suggest that the species barrier is high between CWD and humans (Raymond et al., 2000; Davenport et al., 2015). While an earlier study reported inefficient conversion of normal purified human prion protein by CWD prions (Raymond et al., 2000), a later study found that PrPCWD could convert recombinant human PrP (Davenport et al., 2015). The failure of CWD prions to infect human cerebral organoids, developed using human stem cells, further suggests a high species barrier to infection (Groveman et al., 2024). Taken together the collective results of research to date using a variety of molecular and animal models suggest the species barrier between humans and CWD prions is likely high, although perhaps not absolute.
The potential for animal prion agents to infect humans has been recognized (e.g., EFSA Panel on Biological Hazards, 2011, 2015, 2017) since clear evidence emerged that consumption of classical BSE-infected materials resulted in variant Creutzfeldt-Jakob Disease (CJD) (Will et al., 1996; Bruce et al., 1997; Hill et al., 1997). Determining the actual zoonotic risk of CWD in the United States is epidemiologically complex due to the number of CWD and other animal prion strains present currently, the long incubation of prion diseases in humans (perhaps decades) (Bartz et al., 2000; Hill and Collinge, 2003; Huillard d’Aignaux et al., 2002), and the potential for subclinical (silent) infection (Gill et al., 2013, 2020). The identification of spillover to humans would require a statistical increase in human prion disease in a given geographic location, large-scale screening of exposed populations for potential subclinical infections, or the presentation of a prion disease with different clinical symptoms/characteristics or biochemical signature than currently known forms of CJD (e.g., Will and others [1996] provides a historical perspective).
