1

Scientific Information Concerning the Issue of Whether a Prion Is a “Pest” 

under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA)

(2/17/10)

I. Introduction and Background

This paper reviews currently available scientific information pertaining to the issue of 

whether a prion (“proteinaceous infectious particles”) should be considered to be a “pest” as 

defined by the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). The scientific 

information in this review has been considered by the EPA work group that developed the Notice 

of Proposed Rulemaking (NPRM) that proposes to establish a prion as a pest under FIFRA. At 

EPA’s request, the FIFRA Scientific Advisory Panel (SAP) also reviewed this document and 

provided comments (USEPA 2009), and EPA has incorporated many of the SAP’s suggested 

changes and additional information into this document.

FIFRA provides EPA the authority to decide what entities are considered to be a “pest” 

under FIFRA. The sections that are potentially relevant to prions include:

 FIFRA Section 2(t): A pest is “... (2) any other form of terrestrial or aquatic plant or 

animal life or virus, bacteria or other micro-organism...which the Administrator declares 

to be a pest under section 25(c)(1).” 

 FIFRA Section 25(c)(1): “The Administrator, after notice and opportunity for hearing, is 

authorized–(1) to declare a pest any form of plant or animal life ... which is injurious to 

health or the environment.”

In September 2003, the EPA’s Office of Prevention, Pesticides, and Toxic Substances 

(EPA/OPPTS) decided that a prion should be considered to be a “pest” under FIFRA and that 

products intended to inactivate prions (i.e., “prion products”) should be regulated under FIFRA

(USEPA 2004). The Agency’s rationale may be summarized as the following key points:

o Prions are unquestionably injurious to human and animal health.

o Prions share many characteristics of other deleterious microorganisms.

o Prions share enough characteristics of “other micro-organism” or “form of life” 

to fall within the scope of FIFRA section 2(t)(2). They originate in living 

organisms, replicate within host organisms, and can be transmitted to other hosts.

o If prion products were not regulated under FIFRA, EPA could not provide 

assurance that such products would be effective or safe.

o It appears that the intent of Congress in the 1972 FIFRA was that EPA should 

regulate products used against deleterious forms of life share the characteristics of 

viruses and other microorganisms.

In order to provide sound, scientific input into the issue of whether a prion may be 

considered to be a “pest” under FIFRA, EPA has examined the available scientific information

relevant to the question. This document principally addresses the scientific evidence. Legal and 

policy considerations that factor into the Agency’s decision to issue a NPRM will be described in 

the Preamble to the NPRM. 
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II. The Science of Prions

A. Description

The leading theory on the causative agent for transmissible spongiform encephalopathies

(TSE) is that the agent is composed of an abnormal form of a nucleic acid-free, replicating 

protein (Prusiner et al. 1982; Bolton et al. 1982; Brown et al. 1990a), now called a prion. The 

term “prion” (Prusiner 1982) makes a distinction between molecular properties of the novel, 

proteinaceous infectious particles that cause scrapie disease and molecular properties of particles, 

such as viruses, plasmids and viroids that feature a core of nucleic acid. Fueled by the 

catastrophic outbreak of mad cow disease, or bovine spongiform encephalitis (BSE), originating 

in Essex County, England (1986; Wells et al., 1987), research to characterize the precise nature 

of prions and resolve how they cause the diseases known as transmissible spongiform 

encephalopathies (TSE) led to Prusiner’s award of a Nobel Prize in 1997. In the absence of 

mechanistic evidence, the assumption prior to this body of work was that kuru in humans and 

scrapie in sheep and goats were caused by unconventional viruses, virus-like agents, or slow 

viruses because the time interval between exposure and clinical disease was years or decades 

(Gajdusek, 1977; Prusiner et al., 1982). While a number of scientists have suggested that an 

unknown virus or co-factor is the causative agent of TSEs (Manuelidis, 2007; Manuelidis et al. 

2007), no such virus or co-factor has been isolated after 40 years of searching. Instead, a novel 

infectious particle containing no nucleic acid was suggested by Alper et al. (1966; 1967) and 

subsequently a protein-only (i.e., free of non-protein moieties) infectious agent was suggested by 

Griffith (1967) and Brown et al. (1990a). This protein-only hypothesis is now generally

accepted by strong weight of evidence, especially Prusiner’s hypothesis of a novel proteinaceous 

infectious agent – the prion (Prusiner et al. 1982; Bolton et al. 1982). In particular, experimental 

transmission of TSE diseases by prion protein synthesized or propagated in cell free systems 

(Legname et al. 2004; Castilla et al. 2005; Deleault et al., 2007) is accepted by many as 

definitive proof of the prion hypothesis. Though prions are now generally accepted as the agent 

responsible for TSE diseases, the precise structural properties of the prion protein responsible for

infectivity and the chain of events that produce neural degeneration and fatal TSEs (e.g., Barron 

et al. 2007), and what – if any - cofactors might be featured in the mechanisms of disease, remain 

unknown. As a result of the novelty of the prion hypothesis and controversy surrounding it as 

the agent for infectious diseases, the prion protein and its isoforms have been studied with 

unusual intensity; yet the physiological function of the normal protein and the role of the protein 

in producing neurodegeneration remain obscure (review Aguzzi et al. 2008). Highlights of prion 

research are presented in section II of this paper.

Numerous breakthroughs have been made in the past several years. As detailed below 

(B. Key Characteristics), the normal isoform of the prion protein (PrP) is now known to be 

encoded by the PRNP gene and highly expressed in neurons on the cell membrane. The prion 

protein possess a unique property, so that when the normal α-helical cellular protein is converted 

to certain misfolded, β-sheet rich conformations (Soto, 2006) by unknown events, the resultant

protein aggregates bypasses the normal proteosomal degradation pathway. This process

typically produces the non-amyloid or amyloid deposits featured in the TSE diseases, and 
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acquires the property of TSE infectivity. The clinical features of the TSE disease produced vary

somewhat depending on the species (e.g., cow, sheep, human, etc.) and TSE strain. 

Experimental intervention to restore proteosomal degradation of the misfolded protein 

conformation “cured” prion infectivity in cultured cells (Webb et al. 2007). The property of 

infectivity appears to reside in the β-sheet-rich conformation (Müller et al. 2007), which is the 

focal point of numerous ongoing investigations to define precisely the structural determinants of 

infectivity. In humans, genetic variation is common at the PRNP gene codon for amino acid 

129, resulting in high frequencies of amino acid substitutions (129M, 129V) in the translated 

prion protein. However, inheritance of identical codons from both parents results in prion 

protein homozygosity (129 MM or 129 VV), which produces a cellular pool of identical prion

proteins that are predisposed to misfolding and aggregation through homologous interaction. In 

contrast, inheritance of two different codons results in prion protein heterozygosity (129 M/V), 

which produces a cellular pool of polymorphic proteins that resist misfolding (Mead et al. 2003). 

In addition to mutations in the PRNP protein coding region, genetic mutations in the PRNP 

promoter region are thought to modulate prion protein expression levels and susceptibility to 

TSEs, as in the case of BSE (Haase et al. 2007). What distinguishes prion diseases from other 

neurodegenerative diseases featuring accumulations of protein plaques (such as Alzheimer’s, 

Parkinson’s, and ALS) is that misfolded PrP polypeptides recruit and convert normal PrP 

polypeptides to adopt the misfolded infectious conformation; however, a low level of prion-like 

transmission of altered protein may occur in other amyloid plaque diseases (Aguzzi, 2009), and 

“prinoid” behavior may be a feature of other protein aggregating neuropathologies. For 

example, experimental inflammatory amyloidosis in mice is accelerated when the mice are also 

administered purified amyloid fibrils (Lundmark et al. 2002). Similar seeding was observed in 

mice administered extracts from human Alzhemier’s brain (Meyer-Luehmann et al. 2006). 

These misfolded β sheet proteins produce aggregate particles (commonly described as plaques), 

which are linked to cell death in neurons, and produce a number of fatal spongiform 

encephalopathies in humans, livestock and game animals (Brown, 2005).

In summary, normal cellular protein is synthesized and eventually degraded through 

normal metabolic processes, whereas misfolding of the protein produces a conformational shape 

change that is prone to formation of prion aggregates. These changes account for the resistance 

of prion particles to proteosomal degradation and, consequently, for the slow accumulation of 

amyloid deposits or brain plaques, which are often observed in some TSEs. Furthermore, this 

slow progression of plaque formation and the prion diseases accounts for early speculation that 

the causative agent might be an unknown slow-virus. It is now clear that the transmissible 

encephalopathies (e.g., kuru, scrapie, BSE) feature a novel disease mechanism. In the prion 

hypothesis of disease, it is thought that prion protein polypeptides in the misfolded, aggregationprone isoform convert polypeptides in the normal cellular isoform to adopt the same misfolded 

conformation--a process that is now often described as recruitment and conversion. While 

researchers continue to describe prions as infectious protein particles, as defined by Prusiner 

(1982), it is important to note that this usage no longer necessarily refers to an unknown, slow 

virus, but alternatively to describe this novel process of recruitment and conversion. 

Since reaching a peak in 1992, the sharp decline in new outbreaks of BSE indicates that 

the epidemic in the UK appears to have effectively ended in cows (Brown, 2001). However, the

potential for long, unresolved dormancy of prion diseases in humans is a matter of great public 
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concern (Prusiner et al. 1982; Carrell 2004; Enserink 2005), with the risk that prion disease in 

humans resultant from the BSE epidemic, such as the corresponding peak of vCJD in 2000, 

might persist in human populations long after the consumption of contaminated food ended. 

Also, surgical tools contaminated with prions are extraordinarily difficult to decontaminate 

(Brown et al. 1990, 2000 and 2005; WHO 1999).

B. Key Characteristics

1. The prion protein and terminology

In its original description, the purified prion particles were characterized as comprised of 

a single protein, roughly 27,000 to 30,000 daltons in molecular weight, and resistant to 

proteinase K digestion. In particular, enrichment of the protein correlated with the titer of the 

infectious agent (Bolton et al. 1982) and failed to exhibit the spectra indicative of the presence of 

nucleic acids (Prusiner et al. 1984). Through the work of more than two decades, it is now 

believed that the infectious isoform of the prion protein is a misfolded isoform of a normal cell 

membrane protein, and various terminologies have appeared as numerous researchers have 

joined the investigations. The normal cellular isoform of the protein is typically designated PrPC, 

and the infectious isoform in scrapie is PrPSc. Because a variety of isoforms produce different 

transmissible spongiform encephalopathies, PrPdhas been introduced recently to connote any 

disease isoform (Brown, 2005). Also, PrPres has been used to connote a PrPdpolypeptide 

fragment that features the resistance to proteinase K digestion (Brown, 2005). Other less 

common usages also appear in the literature. More recently, PrPTSE has been used generically for 

the infectious conformation of the transcripts of any prion strain, which lends simplicity to 

communicating about the homologous prion proteins of different human and animal species 

(WHO 2006). As used in this paper, “prion” refers to an abnormal, misfolded, infectious

isoform of the prion protein, whereas “prion protein” is an all inclusive phrase that can refer to 

any normal or misfolded isoforms.

2. Current Status of Dr. Stanley Prusiner’s (1982) protein only hypothesis

Amyloid polymerization of recombinant prion protein produced in bacteria has been 

shown to accompany the acquisition of prion infectivity (Legname et al. 2004; Colby et al. 2007)

that is characteristic of the TSEs. Fibrils consisting of recMoPrP(89-230) were inoculated 

intracerebrally into transgenic (Tg) mice expressing MoPrP(89-231). Between 380 and 660 days 

after inoculation of mice with these transgenic fibrils, all mice developed neuropathology, and 

their brain tissue, in turn, was infectious to other mice. Regarding the mechanisms of infectivity 

intrinsic to Prusiner’s prion hypothesis, the amino acid sequence of the human prion protein, for 

example, has been extensively probed and two cationic domains (PrP19-30 and PrP100-111) have 

been shown to be capable of homologous binding between PrPCand PrPSc, which would account 

for the principle of recruitment of PrPCand conversion to infectious PrPSc (Lau et al. 2007). By

using guanidine hydrochloride as a denaturant and copper binding, apparent reductions in prion 

infectivity have been shown to be partly reversible, which supports the concept that the property 

of infectivity resides with the fibrillar, protease resistant conformation (McKenzie et al. 1998). 
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Large quantities of infectious PrPres can be produced in vitro by seeding the normal PrPC

isoform with small amounts of PrPres, followed by protein misfolding cyclic amplification in a 

cell free system (Castilla et al. 2005; 2008). Hamsters inoculated with PrPres produced in vitro

developed scrapie disease and died after about 170 days.

Significant recent work by Lindquist (e.g., Tessier and Lindquist 2007; Alberti et al. 

2009) uses yeast prion protein and recombinant DNA models to resolve the sequence 

requirements and mechanisms of prion protein misfolding and – most importantly – the 

recognition elements required for “prionogenic” infectivity. The publication of this work has 

been widely acclaimed in a series of news features that accompanied publication of Tessier and 

Lindquist (2007), which is regarded by many as a breakthrough paper that proves the prion 

hypothesis (Surewicz 2007; Jonietz 2007; Scientific American 2007).

3. Normal Prion Protein

The infectious prion protein is any one of several isoforms of an otherwise normal cell 

membrane protein (Stahl et al. 1990), encoded by a highly conserved gene, variously designated 

as PRNP, prnp or Prnp (Sakaguchi 2005) located on chromosome 20 in humans and on

chromosome 2 in mice (Oesch et al. 1985; Basler et al. 1986). In one view (see following 

paragraph), a low level of sequence homology may be evidence that the Prnp gene has been 

conserved across species from yeast to primates (Westaway and Prusiner 1986) and may share an 

ancestral origin by divergence with a related SPRN-like gene that codes for another brain protein 

known as Shadoo (Premzl et al. 2003; 2004). The neuronal protein Shadoo may have 

neuroprotective functions similar to those of PrPC. Shadoo is downregulated in prion disease, 

possibly indicating a companion role in the mechanism of prion infections (Watts et al. 2007). 

The prion gene family also includes Prnd which encodes Doppel, a protein with striking 

sequence similarity to PrPC. Doppel does not possess the conversion and recruitment properties 

of the prion protein but is neurotoxic, causing cerebellar degeneration when overexpressed in the 

brain (Whyte et al. 2003; Moore et al. 2001). Across the vertebrates, amino acid sequence 

homology between the PrP proteins of frog, turtle, chicken and mammal is conserved at a rate of 

about 30%, which is sufficient to preserve the same molecular architecture of mammalian PrPC

(Harris et al. 1991; Calzolai et al. 2005).

Another view is that the similarity of DNA sequences from yeast to vertebrate prion 

protein genes is an evolutionary convergence in the yeast and an example of non-Mendelian 

inheritance of phenotypes benefiting yeast adaptations to diverse environments (Alberti et al. 

2009). Based on the limited understanding of prion protein genes across a comprehensive array

of living representative species, the interpretation of homologous descent with modification 

(Westaway and Prusiner, 1986) of a homologous gene from yeast to vertebrates, or the 

interpretation of convergent evolution in the yeast to conserve phenotypic variation (Alberti et al. 

2009), are both plausible – but opposite – interpretations.

While attention has been focused on the disease producing isoforms, it appears that the 

normal isoform of PrPCis a copper binding metalloprotein (Brown DR 1997), and may function 

as an antioxidant. As noted above (McKenzie et al. 1998), copper binding status may also be an 

important cofactor in conversion of PrPCand stability of the PrPSc infectious conformation. 
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Disruption of normal PrPCfunction in vivo by binding with monoclonal antibodies produces 

rapid cell death in neurons (Solforosi et al. 2004), which is consistent with indications that PrPC 

functions include neuroprotection and that disruption of these normal functions produces 

neuronal cell death. Doppel-induced disease can be rescued by coexpression of wild-type PrPC, 

attributing antioxidant properties to the cellular prion protein (Wong et al. 2001).

Similar to the aggregates of neurotoxic proteins found in other neurodegenerative 

diseases (review Taylor et al. 2002), the neurotoxic PrP isoforms are produced by misfolding and 

aggregation of -rich isomers and accumulation of plaques in neurons. Homozygous PRNP gene 

codons for prion protein 129 methionine (Met, M) or 129 valine (Val,V) predispose PrP to 

recruitment and misfolding, so that in kuru, for example, homozygotes have an earlier onset of 

disease following cannibalism of contaminated central nervous system (CNS) tissues (Mead et 

al. 2003). Heterozygosity at codon 129 confers some resistance to prion disease, probably by 

inhibiting homologous protein interactions (Palmer et al. 1991).

Because of the apparent link between misfolded isoforms of PrP and the production of 

fatal TSEs, the PrP has largely been studied in the brain. While most abundantly expressed in 

the brain, PrPChas been detected in the liver, kidney, heart and skeletal muscle, among other 

tissues (Bosque et al. 2002). New evidence indicates the normal isoform of the protein has 

important functions in non-neural tissues. Zhang et al. (2006) show that the prion protein is a 

biomarker for hematopoietic stem cells when it is normally expressed on the cell surface. When 

bone marrow is depleted of prion protein expressing stem cells, the capacity to regenerate is also 

lost. This evidence indicates normal prion protein expression is required for bone marrow stem 

cells to regenerate.

4. Neurotoxic mechanism 

The critical question in the prion hypothesis is how misfolding of an otherwise normal 

cellular protein leads to neuronal cell death, transmission to other neuronal cells leading to fatal 

brain neurodegeneration, and infectivity when prion-laden material is transmitted to others. 

Ongoing research activity is focusing both on the biochemistry of the prion proteins and related 

polypeptide domains featured in the initiating, misfolding events and on the long cascade of 

intermediate cellular events leading to clinical neuronal degeneration and infectivity.

One recent study (Jackson et al. 2009) supports the core tenets of the prion hypothesis by

showing that a single amino acid change in PrP is sufficient to induce a distinct 

neurodegenerative disease and spontaneous generation of prion infectivity. The intermediate 

steps linking the amino acid change to neurodegeneration and infectivity will no doubt remain 

speculative until the neurotoxicity pathway is resolved in detail. However, numerous

breakthroughs have been made in the past several years regarding associations between the

accumulation of prion protein aggregates and neuronal cell death, and the weight of these 

cumulative studies hints at the major features of the neurotoxicity pathway and infectivity of 

prions.

Removal of misfolded and damaged proteins is a normal part of cellular metabolism; 

however, misfolded prion protein accumulates in cellular aggregates. These aggregates are 
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thought to result from failure of the ubiquitin-proteasome pathway to adequately degrade 

misfolded protein, which is a feature prion diseases share with other neurodegenerative diseases, 

such as ALS, Parkinson’s disease and Alzheimer’s disease (Goldberg 2007). In the case of the 

TSE diseases, the misfolded prion protein conformation does not appear to be intrinsically

neurotoxic, and the precise mechanisms and intermediate steps of neurotoxicity remain 

unresolved in detail. As evidenced in several prominent studies, it appears that the pathology of 

scrapie prion in neurons may feature disrupting the function of the normal cell surface protein, 

and development of prion disease may require the presence of normal PrP expression and the 

continued conversion of PrPCto PrPSc (Brandner et al. 1996; Mallucci et al 2003). In these 

studies, it is thought that uncoupling normal PrP from the cell surface also uncouples the 

presence of PrPSc from the progression of disease. Key steps in resolving the mechanisms of the 

prion diseases include:

 Grafting transgenic tissue overexpressing PrP into embryonic forebrain of PrP deficient 

mice (Prnp0/0 ) and inoculating with PrPSc shows that grafted tissues accumulate PrPSc

and exhibit scrapie histology, whereas Prnp0/0 tissue remain unaffected (Brandner et al. 

1996). In other words, the β-sheet rich prion isoform is not intrinsically neurotoxic. 

 Depletion of PrPCin neurons during established infection arrests disease progression, 

even though extraneuronal PrPSc continues to accumulate (Mallucci et al. 2003). 

Transgenic mice programmed to disable the PRNP gene in neurons at 12 weeks of age 

were inoculated with PrPSc several weeks before PRNP inactivation. Within days after 

the PRNP gene was inactivated, the mice became depleted of PrPCand remained alive 

and disease free a year later. Inoculated control mice lacking the transgenic 

downregulation of PRNP all died.

 PrPCprotein is normally anchored to the neuronal cell membrane by a 

glycosylphosphatidylinositol (GPI) linkage. Disabling the GPI linkage in transgenic 

mice does not prevent the accumulation of PrPSc plaques in inoculated mice, but does 

lower the prion titer and alters the development of scrapie disease (Chesebro et al. 2005).

That the relationships between prion aggregation, plaque accumulation, infectivity and 

spongiform degeneration remain incompletely understood is shown by seemingly inconsistent 

evidence that the formation of oligomers (or small PrP aggregates) may be an intermediate step 

in neuronal degeneration (Simoneau et al. 2007) although in some circumstances plaque 

accumulation can occur in the absence of infectivity and degeneration (Piccardo et al. 2007).

While the mechanisms of prion neurotoxicity and infectivity will likely remain intense areas of 

scientific research and debate for some time, the weight of evidence remains pointed to the prion 

as the agent initiating disease and infectivity.

C. TSE Diseases 

Prion diseases are neurodegenerative diseases also referred to as transmissible 

spongiform encephalopathies (TSEs) due to the large vacuoles (Figure 1) that are seen upon post 

mortem examination of the cortex and cerebellum from affected individuals (Collinge 2001). 

These diseases are seen in both humans and animals (Table 1). Scrapie is the most widely
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known TSE naturally occurring in sheep; it was first recognized some 200 years ago in Europe,

and now present worldwide (Collinge 2001). More recently, other types of TSE have been 

recognized such as chronic wasting disease (CWD) in deer, moose and elk (Williams and Young, 

1980), bovine spongiform encephalopathy (BSE--mad cow disease) (Wells et al. 1987), feline 

spongiform encephalopathy (FSE) (Wyatt et al. 1991), and transmissible mink encephalopathy

(TME) (Hartsough and Burger, 1965; Burger and Hartsough, 1965; Marsh 1992). 

In humans, prion based diseases are generally thought to include Creutzfeldt-Jakob 

disease (CJD), Gerstmann-Sträussler-Scheinker (GSS) disease, fatal familial insomnia (FFI) and 

kuru. They are further categorized into three etiological categories: sporadic, inherited or 

acquired (Collinge 2001, Wickner et al. 2004; Brown 2005). The incidence of CJD worldwide is 

about one per million (Collinge 2001), and most cases appear to be sporadic. Sporadic CJD is 

characterized by a rapid dementia occurring in individuals between 45 and 75 years of age. 

Death can result as early as 2-3 months from onset (Collinge 2001). Other symptoms include 

fatigue, insomnia, depression, weight loss, headaches, ataxia, blindness and random pain 

sensations. On the other hand, variant CJD (vCJD) appears to be acquired from exposure by

ingesting prion contaminated beef. No mutations have been found with sporadic or acquired 

prion disease in humans. In contrast, over 20 distinct mutations of the prion protein gene (Prnp) 

have been found in inherited cases. Fifteen percent of human prion diseases are inherited 

(Collinge 2001). 

Kuru represents the earliest known example of human acquired prion disease. Kuru is 

thought to have occurred in Fore natives of Papua New Guinea in the early 1900’s when a case 

of sporadic CJD became entrenched in the population following cannibalistic rituals within the 

population. Because of the different roles adult men and women and children played in Fore 

communities, kuru was prevalent among women and children until the practice of cannibalism 

ended in the 1950s (Fischer and Fischer 1961; Gajdusek (1977); Alpers 1987 as cited in Collinge 

2001).

Creutzfeldt-Jacob disease - human Kuru – human

Bovine spongiform encephalopathy -

(cow)

Scrapie - (sheep)
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Figure 1. Pathology of Prion Disease in Humans and Animals

Table 1. Prion Diseases in Humans and Animals

Human Animal

Spongiform

 Creutzfeldt-Jacob disease 

(CJD); variant CJD (vCJD)

 Kuru

 fatal familial insomnia

Spongiform

 Scrapie – sheep

 Bovine spongiform 

encephalopathy

 Transmissible mink 

encephalopathy

 Chronic wasting disease of 

mule deer, elk, and moose

 Feline spongiform 

encephalopathy

 Encephalopathies in zoo 

animals*

 Gerstmann-SträusslerScheinker disease (GSS)

 Variant CJD 

* ankole, Arabian orynx, bison, cheetah, eland, gemsbok, greater kudu, nyala, ocelot, puma, 

Scimitar horned oryx, and tiger (Jeffrey and Wells 1988).

D. History of TSE Diseases

Though sporadic CJD (sCJD) occurs spontaneously in the human population, the disease 

is rare and occurs at the rate of about one per million people per year. Prion-related disease 

epidemics tend to result from acquired TSEs. It is now known that the first prion-related 

epidemic disease in humans was documented in 1957 in Papua New Guinea among native tribes 

known to practice ritualistic cannibalism (Zigas and Gadjusek 1957). The disease was called 

”kuru” and was closely related to Cruetzfeldt Jacob Disease (CJD) although the prion hypothesis 

was not known at that time. Kuru represents the earliest known example of human acquired 

prion disease. Kuru is thought to have occurred in Fore natives of Papua New Guinea in the 

early 1900’s when a case of sporadic CJD became entrenched in the population following 

cannibalistic rituals within the population. Because of the different roles adult men and women 

and children played in Fore communities, kuru was prevalent among women and children until 

the practice of cannibalism ended in the 1950s (Fischer and Fischer 1961; Gajdusek (1977); 

Alpers 1987 as cited in Collinge 2001).

The first major outbreak of transmissible encephalopathies having catastrophic public 

consequences linked to prions was not diagnosed until 1985 when mad cow disease (BSE) first 

appeared in Great Britain. By 1990, 14,324 cases of BSE were confirmed in British cattle, out of 

an estimated population of 10 million cattle. In 1992 and 1993, more than 1,000 cases of BSE 

were reported each week, and in 1995, at the height of the epidemic, there were approximately
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Distribution of CWD

(http://www.aphis.usda.gov/ 

animal_health/animal_diseases/cwd/downlo

ads/distribution_cwd.pdf)

146,000 accumulated cases of BSE. Other European countries also reported cases of BSE at that

time including Switzerland (200 cases), Ireland (120 cases), Portugal (30 cases) and France (13 

cases). As the practice of including animal parts in cattle feed was stopped (Feed ban 1988; 

Specified Bovine Offals ban 1990; Meat and Bone Meal ban 1996), the epidemic waned and feed 

security was assured 1996 (Brown et al. 2001; Univ. of Wisconsin 2004). However, since the 

introduction of monitoring programs to detect BSE in dead and slaughtered cattle, numerous 

other countries have also found their first cases of BSE: Austria, Canada, Czech Republic, 

Finland, Germany, Greece, Israel, Italy, Japan, Poland, Slovakia, Slovenia, Spain and the USA. 

Small numbers of cases have also been reported in the Falkland Islands (Islas Malvinas) and 

Oman, but solely in animals imported from Britain (WHO 2006).

In 1995 and 1996, a cluster of cases of vCJD was diagnosed in humans in Britain. The 

link between BSE and vCJD was announced publicly on March 20, 1996. By the end of 2003, 

145 cases of vCJD had appeared in Great Britain. Since then, the annual number of new vCJD

cases has declined, from a high of 28 in 2000 to 1 in 2008 (http://www.cjd.ed.ac.uk/cjdq56.pdf).

1The first case of BSE disease reported in the United States was on December 23, 2003 on a 

dairy farm in the state of Washington in a Holstein purchased from Canada. As a result, 4,000 

cattle on the ranch were destroyed. A second case of BSE was detected in Texas in November 

2004 in a cow of unknown origin and born before the U.S. ban on the use of mammalian protein 

in ruminant feed (diagnosis final in June 2005) (http://www.cfsan.fda.gov/~comm/bsefaq.html). 

The third and most recent case of BSE was discovered in Alabama 

(http://www.aphis.usda.gov/newsroom/hot_issues/bse/downloads/EPI_Final5-2-06.pdf; Heaton 

et al., 2008). 

To date, three cases of vCJD have been reported in the Unites States. The first and 

second patients (dates of onset 2001 and 2005, respectively) were both born in the United 

Kingdom and resided there during the defined period of risk for transmitted TSEs from cattle 

(1980-1996). A third US case of vCJD (onset and diagnosis, 2006) is likely to have originated in 

Saudi Arabia where the patient was born, raised and resided until 2005. None of the patients had 

medical histories to which transmission could be attributed (http://www.cdc.gov/ 

ncidod/dvrd/vcjd/factsheet_nvcjd.htm). However, there is no epidemiological evidence to 

suggest that any of these vCJD patients acquired the disease in the United States. They are 

presumed to have been infected in the United Kingdom or Saudi Arabia, respectively.

Chronic Wasting Disease (CWD) was first recognized as a clinical "wasting" syndrome 

in 1967 in captive mule deer in a wildlife research facility in Colorado. It was identified as a 

TSE in 1978. In 1981, CWD was detected in a wild elk and in 1985 it was detected in a wild 

mule deer in Colorado; shortly thereafter it was shown to 

be present as an endemic focus in wild deer, elk and moose 

in northeastern Colorado and southeastern Wyoming. 

Since nationwide wildlife surveillance was instituted in 

1997, CWD has been identified in wild deer and elk in 

several additional states: Illinois, Kansas, Nebraska, New 

Mexico, New York, South Dakota, Utah, West Virginia 

and Wisconsin (www.aphis.usda). However, numbers of 

positive animals remain low. For example, in 2002, only

302 samples were positive out of 91,636 samples tested (a 

prevalence of 0.3%) (www.aphis.usda). Within endemic 
11

areas, the prevalence of CWD has been estimated at <1% in elk and <1% to 15% in mule deer 

(Williams et al. 2002). Despite these low endemic levels, models suggest that epidemics of 

CWD could lead to local extinctions of infected deer populations although it is not known how 

the disease might become epidemic (Miller et al. 2000). There is no evidence of human cases of 

prion disease linked with CWD.

Scrapie, a neurological disease of sheep, has been present in Europe since the early 18th 

century. Affected sheep were observed rubbing or “scraping” their coat against a tree or a fence

post as if they itched. The disease is generally fatal after a long incubation period. It is 

estimated that scrapie costs the U.S. sheep industry $20 million per year in direct losses (USDA, 

2000). There are no known human cases associated with scrapie infected sheep. Scrapieinfected offal in cattle feed may have been the source of BSE.

Approximately 100 cases of feline spongiform encephalopathy (FSE) have been reported 

in Great Britain and Europe during the BSE epidemic in the early to mid 1990's (Cornell Feline 

Health Center 2004). British cats were believed to have gotten the disease by eating BSEcontaminated commercial cat food and butcher scraps. The number of new cases in domestic 

cats dropped off once measures were put into place to prevent materials potentially contaminated 

with BSE prions from entering the food chain. No new cases have been reported since 1999. No 

cases in U.S. cats have ever been reported (Cornell Feline Health Center 2006). Britain also 

reported cases in captive exotic cats including nine cases in cheetahs (three were diagnosed 

abroad but originated in Britain), three in pumas, three in ocelots, two in tigers and two in lions

(Cornell Feline Health Center, 2004). Outbreaks of transmissible mink encephalopathy (TME) 

are rare occurrences (Marsh and Bessen, 1993), possibly associated with a transmission of a rare 

TSE of cattle to mink (Liberski et al., 2009).

E. Transmission and Potential Routes of Exposure 

TSEs can be transmitted naturally and experimentally. The most efficient route of 

experimental transmission is intracerebral injection of infectious brain tissue (Weissman et al. 

2002a). Intraocular, intraspinal, intravenous, intramuscular, intraperitoneal, subcutaneous 

injections, scarification, intralingual, intranasal, intrasciated and oral administration have also 

been shown to be effective methods of transmission in the laboratory setting. Experimental 

transmission via the dental route has also been reported (Ingrosso et al. 1999). Consumption of 

contaminated food is generally thought to explain the natural pathogenesis of scrapie in sheep, 

the spread of kuru, and the human acquisition of vCJD from BSE-contaminated beef; however, 

the precise details about the fate of infectious prions in association with other food constituents 

as they pass through the alimentary canal and cross the gut mucosa to produce CNS disease 

remains elusive (Jeffrey et al. 2006; Rose et al. 2006; Austbø et al. 2007; Scherbet et al. 2007; 

Solomon et al. 2007).

Natural transmission routes of prions vary among TSEs. Transmission of scrapie among 

sheep and goats is not completely understood. The scrapie agent is believed to originate from 

the placentas of infected sheep and goats (Pattison et al. 1972) although scrapie-contaminated 

feces have also been proposed (Weissmann et al. 2002b). Both scenarios represent direct 

horizontal transmission. Some studies suggest that indirect, horizontal transmission occurs via 

the oral route, such as from contaminated environments (Palsson 1979) or by prionuria (Shanked 
12

et al., 2001; Ligios et al. 2007). Vertical transmission (mother to offspring) of scrapie may be 

possible, but probably not significant. It is thought that a more likely route of transmission 

involves an early horizontal, post-natal event (Andreoletti et al. 2002). 

Transmission of CWD among deer and elk is not completely understood. Both direct and 

indirect horizontal transmissions are known to occur via contact with infected animals and 

environments contaminated with excreta or decomposed carcasses of infected animals (Miller et 

al. 2004). In this regard, prion-contaminated material in soil remains infectious for years (Brown 

and Gajdusek 1991; Seidel et al. 2007) and may serve as a reservoir of disease. Purified PrPSc

protein has been shown to adhere to soil minerals and remain infectious (Johnson et al. 2006; Ma 

et al. 2007), and this soil binding may actually enhance infectivity (Johnson et al. 2007), possibly

by a mechanism comparable to the recognized durability and seemingly enhanced infectivity of 

prions bound to surgical steel instrument surfaces (Zobeley et al. 1999; Flechig et al. 2001; 

Peretz et al. 2006). It is also perceived that, as in the case of BSE (above), if any maternal 

transmission of CWD occurs, it has not yet been documented and would be at very low levels 

(Miller and Williams, 2003; Wells and Wilesmith, 2004). A recent study confirms transmission 

of CWD in deer via blood and saliva, raising caution regarding contact with body fluids from 

infected animals (Mathiason et al. 2006). Currently, there is no evidence of a link between CWD 

and unusual cases of CJD in humans, though data are limited and more studies are necessary

(Belay et al. 2004). 

Several animal TSEs are associated with consumption of contaminated feed. BSE 

represents a large-scale common source epidemic transmitted by the feeding of BSE prioncontaminated meat and bone meal to cattle (Wilesmith et al. 1991). At that time, meat and bone 

meal were prepared from the offal of cattle, sheep, pigs and chicken and fed to cattle as a high 

protein nutritional supplement. [Note: In April 2009, the U.S. Food and Drug Administration 

imposed a feed ban rule which prevents this practice.] Given the weight of focus on 

contaminated meat and bone meal as the source of the BSE epidemic in the UK, the discovery in 

Alabama of a diseased animal with the BSE-linked 211K mutation (E211K) prompted Heaton et 

al. (2008) to survey over 6,000 US cattle, including five commercial beef processing plants 

Given the absence of any other observed instances of the gene allele for 211K in this survey and 

the size of the sample, they estimated that any additional atypical BSE cases in the U.S. with 

K211 would be vanishingly low. Similarly, in cats, feline spongiform encephalopathy (FSE) has 

been linked with consumption of commercial cat food and butcher scraps contaminated with 

BSE, and transmissible mink encephalopathy (TME) is associated with the feeding of tissue from 

scrapie-infected sheep or some form of TSE-infected cattle (Chesebro, 2003).

Human TSEs are associated with a variety of transmission routes. Most cases of CJD are 

classified as sporadic and are likely to result from a somatic mutation, the spontaneous 

conversion of PrPCto PrPSc, or reduced clearance of low levels of PrP

Sc that may normally be 

present (Peretz et al. 2001). However, CJD can be transmitted by exposure to contaminated 

surgical instruments or electroencephalogram electrodes, or exposure to infectious brain, 

pituitary, or ocular tissue or associated products via organ transplantation or other medical 

procedures (Blattler 2002). 
13

Both kuru and vCJD are caused by ingestion of contaminated food. As described earlier, 

Kuru, the TSE of New Guinea natives, is associated with consumption of infected human tissue 

during cannibalistic rituals. Exposure is also believed to have occurred through abrasions of 

oropharyngeal or conjunctival mucous membranes, as well as via open wounds on the hands 

(Gajdusek, 1977). vCJD is associated with consumption of BSE-contaminated beef products 

(Will et al. 1996, Collinge et al. 1996, Bruce et al. 1997, Will et al. 1999). Additionally, two 

cases of transmission of vCJD via blood transfusion have also been reported (one case 

manifested clinically and the second was identified at autopsy following death from an unrelated 

cause) (Llewelyn et al. 2004, Peden et al. 2004), and at the present time it is likely these numbers 

will continue to rise as surveillance continues (UK National Prion Clinic).

Prions have presented a major public health problem in the UK with the emergence of 

BSE. Through a major effort in the UK and other countries, this disease appears to be under 

control. However, because so little is understood about prions, they will remain an important 

public health issue for the foreseeable future, particularly since it is not clear if measures taken 

so far will adequately prevent future occurrences. In addition, scientists do not completely

understand several aspects of prion diseases, or whether effective treatments might be found 

through understanding of the misfolding and infectivity of the fibrillar structure (Karpuj et al. 

2007; Supattapone et al. 1999, 2001). Finally, the epidemic of CWD in deer and elk populations 

in the Western USA continues to spread and has certainly entered states east of the Mississippi, 

but the implications of this disease for humans remains unknown. 

Rapid advances in recombinant and cell free technologies are attacking the most puzzling 

question about the prion diseases (Dong et al. 2007; Müller et al. 2007; Tessier and Lindquist 

2007): Why are they transmissible whereas other neural protein misfolding diseases such as 

Alzheimer’s and similar amyloidoses are not? In addition to known hereditary prion diseases, it 

is an especially worrisome health problem that normal prion protein can also misfold under 

conditions that have not been defined to produce spontaneous TSE disease. For example, the 

UK commission report concluded that the catastrophic epidemic of BSE (1986 – 2000) began 

with the spontaneous emergence of an abnormal prion, followed by using contaminated bone 

meal to feed other cattle. The resultant intense surveillance now shows low level BSE in other 

countries – including three recent cases in the USA. 

In summary, prions are a persistent problem but the extent to which these diseases will 

pose a public health threat in the future is unclear.

F. Existing Occupational Safety Procedures 

Because TSEs are transmissible to humans by various routes (as described above), 

standard protective measures have been put in place to prevent or minimize occupational 

exposure to human or animal tissues or waste materials contaminated or potentially contaminated 

with prions. These measures are pertinent to health-care and veterinary care providers, mortuary

workers and laboratory workers, all of whom may handle or be exposed to contaminated blood, 

tissues or waste materials. Standard protective procedures include administrative and 

engineering controls, special work practices, personal hygiene measures, personal protective 

clothing and decontamination methods.
14

The Centers for Disease Control and Prevention (CDC) has published two documents that 

provide guidance for health-care workers pertaining to environmental infection control generally

and to “special pathogens” (including prions) specifically. Guidelines for Handwashing and 

Hospital Environmental Control (Garner and Favero, 1986; Bolyard et al. 1998: CDC/HICPAC

2003) encompasses general infection control, with emphasis on the handling of air, water and 

environmental surfaces; environmental sampling; and disposal of regulated and unregulated 

medical waste. Guidelines for Environmental Infection Control in Health-Care Facilities (CDC 

2003) provides extensive, detailed guidance on all aspects of infection control in health-care 

facilities and specifically addresses Creutzfeldt-Jakob Disease (CJD) in patient-care areas. This 

guidance notes that a limited number of cases result from direct exposure to prion-containing 

material (usually central nervous system tissue or pituitary hormones) acquired as a result of 

health care (i.e., iatrogenic cases). Six documented iatrogenic cases were associated with

neurosurgical instruments and devices that introduced residual contamination directly to the 

recipient’s brain. Because there is no evidence to suggest that either CJD or vCJD has been 

transmitted from environmental surfaces (e.g., during housekeeping activities), CDC states that 

routine procedures are adequate for cleaning and disinfection of a CJD patient’s room. However, 

the CDC recommends that hospitals identify patients with known or suspected CJD and 

implement prion-specific infection-control measures for the operating room and for instrument 

reprocessing. Such measures should include:

 Use of disposable, impermeable coverings during autopsies and neurosurgeries to 

minimize surface contamination

 Cleaning and decontamination of surfaces that have been contaminated with central 

nervous system tissue or cerebral spinal fluid (see decontamination methods described in 

next section).

In 2000, the World Health Organization issued its guidance document, WHO Infection 

Control Guidelines for Transmissible Spongiform Encephalopathies (WHO 2000), which 

focuses specifically on prion diseases. In this document, WHO designated central nervous 

system tissues and fluids as “high-infectivity,” which should be managed more conservatively 

than other diseases. Key recommendations include the following:

 Covering patient treatment surfaces with disposable non-permeable covers and uncovered 

surfaces are to be decontaminated. 

 Clinicians involved in patient handling are to wear single-use disposal coverings (mask or 

visor, goggles, cut-resistant gloves and liquid-repellent aprons over plastic gowns). 

 Treatments are to be conducted with equipment and tools that either are dedicated to a 

single patient, or are disposed after a single use. 

 Instruments that have contacted high-infectivity tissues are to be disposed by

incineration, as are all single-use coverings and tools. 

 Reusable instruments are to be kept moist until cleaning and decontamination; this 

measure is needed to prevent drying of tissues or fluids thereon. 

 Low- and high-infectivity instruments are to be stored separately between and during 

uses. 

 Bulk fluids are to be decontaminated or absorbed into sawdust, then incinerated. 
15

 Contaminated medical wastes are to be incinerated, as are the disposable materials.

 All incineration is to be conducted in hazardous-waste facilities. 

The WHO document provides similar specific guidelines for mortuary and laboratory

workers who may contact contaminated materials, and for decontamination of clinical or 

laboratory workers after occupational exposures. Safety recommendations are provided also for 

the transport of contaminated specimens by air. 

With respect to veterinary laboratories, the American Association of Veterinary

Laboratory Diagnosticians (AAVLD) has issued recommendations aimed at reducing potential 

risks to laboratory personnel posed by TSE agents in animal tissues and waste materials 

(AAVLD 2004). The AAVLD recommends:

 Using standard BSL2 safety precautions such as restricted access to laboratories, 

protective clothing and facial protection, special care with sharp instruments, and 

avoidance of aerosols.

 Using numerous specific protective measures for necropsy laboratories, histology

laboratories and laboratories handling fresh tissues from TSE suspect animals, such as 

extensive protective clothing, screened drains to trap tissue fragments, use of appropriate 

disinfectants to inactivate TSE agents on surfaces and non-disposable items, and disposal 

of contaminated tissues by certain methods.

G. Decontamination Methods

A number of decontamination methods have been developed and tested for their ability to 

inactivate prions on inanimate surfaces and on objects that may have contacted infected animal 

or human tissues. This development and testing has been especially spurred by the successful 

migration of BSE into the human population during the mad cow epidemic (UK 1986-2000) and 

by the compelling need for methods to decontaminate environmental surfaces and instruments in 

veterinary facilities, research laboratories, health care settings and other settings in which prioncontamination is a possibility . 

The infectious conformation of prion proteins is extraordinarily durable, and the 

measures necessary to destroy such prion proteins are extreme compared to decontamination 

methods for other biological pathogens, including bacterial spores. In particular, prions bind 

tightly to stainless steel, resist elution by exhaustive washing, and remain highly infectious after 

formaldehyde treatment (Zobeley et al. 1999), which accounts for historical reports of 

transmission of CJD by surgical instruments (Bernoulli et al. 1977).

Some of the currently recommended decontamination methods include treatment with

1-2 N NaOH or NaOCl (2%,or 20,000 ppm chlorine) for one hour followed by heat in a gravity 

displacement autoclave at 121o C for one hour, autoclaving at 134o C for 1 hr (or up to 5 hrs as 

in Peretz et al. 2006), or complete incineration (850o C or higher as in Brown et al. 2004). 

General descriptions of these procedures are found in several advisory reports, such as those of 

the CDC (1999), WHO (2000), AAVLD (2004), Canadian Food Inspection Agency (2005) and 

MSU (2005). However, these recommended decontamination methods are based on limited
16

academic research, and a number of issues bring into question whether these methods can 

demonstrate a disinfectant’s ability to totally inactivate prions on inanimate surfaces and objects.

1. The recommended prion-inactivation methods have not been tested using a validated 

efficacy test method, and such a method does not yet exist. Currently, EPA requires that 

disinfectants and sterilants be tested using specific, validated efficacy test methods before 

EPA will register such products, and such test methods are available (e.g., AOAC Use 

Dilution Test). However, no one has formally validated any of the prion-inactivation 

efficacy test methods used to date, so it is difficult to know whether such efficacy test 

methods provide consistent, reliable data.

2. The risk of transmissible infectivity persists at prion titers far below the limits of 

detection using analytical methods, such as gel electrophoresis and Western blot assay

(Yamamoto et al. 2001; Solassol et al. 2006). For example, Gregori et al. (2003) found 

that assaying for surviving prion infectivity in hamsters was 1000 fold more sensitive 

than visual detection by Western blot assay. Consequently, adequate confirmation in an 

animal model is required to confirm effective elimination of all prion infectivity.

3. What constitutes an adequate animal test model for determining the prion-inactivating 

efficacy of chemicals is uncertain. Although the Syrian hamster (Marsh and Kimberlin 

1975) and mouse (Chandler 1961) have a long history of utility in prion research, the life 

span of these laboratory animals is relatively short, whereas the time interval from prion 

exposure to the development of clinical disease can span decades of time, as in the human 

prion diseases, for example. In experimental models, it is known that as the titer of 

infective material decreases, the incubation time for the development of clinical disease 

in test animals increases (Jackson et al. 2005; McDonnell et al. 2005; Solassol et al. 

2006). Hence, the life span of the test animal may be too short to detect residual prion 

infectivity at very low titers, as might be expected with cleaning procedures that have 

been largely, but not entirely, effective. Ideally, efficacy studies should include positive 

controls to determine the relationships between prion titer, infectivity and the life span of 

the test animal. Otherwise, the absence of clinical signs during the short lifespan of the 

animal model might be mistaken as evidence of successful elimination of infectivity. 

Attempts to accelerate the progression of infectivity by using synthetic mammalian 

prions (Legname et al. 2004), as well as PrP over-expressing cell lines (Solassol et al. 

2004) and animals (for example, Peretz et al. 2006) potentially point to ways to solve the 

lifespan problem, but these approaches create new uncertainties by using animals already 

predisposed to overexpression of host or foreign prion strains.

4. In addition to the common use of two species of test animals--hamster and mouse--many 

different strains of prions are employed in decontamination research. These experimental 

variables might account for the discrepant results for Environ LpH obtained by Ernst and 

Race (1993) and Race and Raymond (2004), using Syrian hamsters as the test model, 

with those of Jackson et al. (2005), using mice. 

5. There are important barriers to the transmission of prion diseases between animal species 

and humans, and the degree of transmission of prion diseases from animals to humans 
17

varies greatly. For example, experimental studies using human and elk prion genes 

expressed in a transgenic mouse indicate that elk chronic wasting disease may not 

transmit readily to humans (Kong et al. 2005). On the other hand, the BSE epidemic in 

the UK (1986-2000) demonstrates that BSE can cross the species barrier and become

infectious to humans as vCJD (Brown 2001). Since there are no systematic studies of 

comparative infectivity of prion strains across different animal species and humans, it is 

not clear how precisely one can extrapolate human risk from efficacy studies performed 

in various animal species. Specifically, some of the strains used in efficacy studies may

pose little or no risk to humans. Conversely, it seems possible that some prion strains 

dangerous to humans may not be testable in lab animals commonly used in efficacy

studies. Finally, imperfect decontamination procedures may have consequences for risks 

to humans that vary from inconsequential to very significant, depending on the strain of 

prion being inactivated. For example, sheep scrapie is clinically unknown in humans and 

so the risks to humans resulting from imperfect decontamination of scrapie would be 

expected to be negligible compared to the risks from imperfect inactivation of BSE 

prions on surfaces to which humans are exposed.

6. Similarly, comparisons of the prion proteins of different species in a common animal 

model indicate there may be profound differences in the prions of different species with 

regard to their stability and resistance to inactivation during decontamination procedures. 

For example, Peretz et al. (2006) compared the resistance of hamster scrapie and human 

CJD prions in transgenic mice expressing either hamster PrP or a chimeric mouse-human 

PrP transgene and found that human sCJD prion is 100,000 fold more difficult to 

inactivate than hamster Sc237 prion. Preliminary additional studies indicate that the cow 

BSE prion may be even more resistant to inactivation than the human CJD prion (Giles et 

al. 2006; 2008 in press). This approach and the results obtained by the direct comparison 

of different prion strains in common test models indicate that decontamination 

procedures tested with rodent prions may not be effective when tested with more durable 

human and cow prions.

Accordingly, although a number of academic studies have demonstrated that certain 

chemical and/or physical decontamination methods provide some assurance of reduction of prion 

infectivity, current test methods cannot demonstrate that a disinfectant can totally inactivate

prions or TSE agents on inanimate surfaces and objects. 

III. The Science of Prions As It Relates to the Definitions of “Pest”

The Introduction to this paper described two sections of FIFRA that define or give the 

EPA Administrator the authority to define the term “pest.” This section addresses the extent to 

which the available scientific information on prions summarized in Section II relates to the issue 

of whether prions fit within the definition of “pest” specified in FIFRA Sections 2(t) or 25(c)(1).

A. Definitions of “Microorganism”

The definition of “pest” in FIFRA Section 2(t) has two parts. The first part is whether the 

pest occurs in “circumstances that make it deleterious to man or the environment.” Based on the 
18

available information described in Section II., prions clearly are deleterious to humans and 

animals. Exposure to prions may lead to infectious acquisition of irreversible and fatal TSE 

diseases. 

The second part of the definition of “pest” relevant to prions revolves around whether 

prions are “any other form of ... virus, bacteria or other microorganism....” Prions are neither 

viruses nor bacteria, as discussed in the preceding sections, so if prions are to be considered pests 

under FIFRA they would have to be “other micro-organisms.” Below, two approaches have 

been taken to the determination of prions as “other micro-organisms.” In the first case, prions 

are examined in the context of common dictionary definitions of what constitutes a 

microorganism. In the second case, prions are reviewed in the context of Koch’s postulates for 

microbial disease agents.

1. Dictionary definitions of microorganisms. Although there is no single, definitive 

definition of “microorganism,” the definition in the Oxford Dictionary of Biology (Fourth 

Edition, 2000) is representative.1The Oxford Dictionary of Biology defines “microorganism” as

“Any organism that can be observed only with the aid of a microscope. Microorganisms include 

bacteria, viruses, protists (including certain algae), and fungi.” “Organism” is further defined by

the Oxford Dictionary as “An individual living system, such as an animal, plant, or 

microorganism, that is capable of reproduction, growth, and maintenance.” These latter terms 

are defined by Dorland’s Medical Dictionary as follows:

 Reproduction—the sexual or asexual process by which organisms generate others of the 

same kind 

 Growth—the normal process of increase in size of an organism by accretion of tissue 

similar to that originally present 

 Maintenance—the process of holding a stable, steady state over a long period

Therefore, whether prions fall within the above definition of “microorganism” as it is 

generally used by biologists and the medical community arguably depends on whether they

exhibit reproduction, growth and maintenance. 

However, this interpretation of “organism” is at odds with the Oxford Dictionary of 

Biology’s definition for “microorganism,” inasmuch as that definition includes viruses as 

microorganisms, even though they do not have all of the listed characteristics of an “organism”,

as discussed in more detail below. 

2. Koch’s postulates. As indicated above, the properties that make prions infectious 

agents of disease overlap with common characteristics of microorganisms that cause disease (i.e., 

pathogens). In 1884, Robert Koch and Friederich Loeffler developed a list of general guidelines

that, if met, would indicate that a microorganism would likely cause disease (i.e., be a pathogen). 

 

1 This definition and those that follow were chosen from among several available to EPA. Those 

discussed herein were selected based on their inclusion in authoritative sources, their 

completeness, and their utility in addressing the scientific arguments considered. 
19

Koch later published these characteristics as four “postulates” (Koch 1893). While a number of 

infectious agents have been identified that do not meet Koch’s postulates, those postulates have 

historical importance in the field of microbiology (Jacomo et al. 2006). Following are Koch’s 

postulates:

(1) The microorganism must be found in abundance in all organisms suffering from the 

disease, but should not be found in healthy animals.

(2) The microorganism must be isolated from a diseased organism and grown in pure 

culture.

(3) The cultured microorganism should cause disease when introduced into a healthy

organism.

(4) The microorganism must be re-isolated from the inoculated, diseased experimental 

host and identified as being identical to the original specific causative agent.

Regarding the first of these postulates, it is clear that all clinical cases of TSEs feature 

measurable quantities of prions in the CNS tissues; however, it is equally clear that prions can be 

detected in CNS, blood and perhaps in other tissues in sub-clinical cases of disease (section II. 

E., for example). This is also the case for other diseases having asymptomatic manifestations or 

asymptomatic carriers, which Koch himself observed when he discovered asymptomatic carriers 

of cholera and typhoid fever. Regarding the 2nd postulate, prions can be isolated from diseased 

CNS (and other tissues). Prions can be grown in pure culture, in the sense that the disease 

producing conformation of normal cellular prion protein can be amplified by repetitive cycles of 

seeding existing, normal PrPC with prion isoforms, such as in the cell-free protein misfolding 

cyclic amplification systems described by Castilla et al. (2005; 2008; sections II.A. and II.B.2.

above). However, this is not the same as the genetic reproduction that is ordinarily used to 

satisfy Koch’s second postulate.

The 3

rd and 4th postulates are satisfied by the numerous demonstrations that prions 

enriched from diseased animals or synthesized de novo in cell-free systems, are both infectious 

and produce disease (see II.A.2, above). Thus, the mechanisms of prion disease and infectivity 

generally conform with Koch’s postulates for the microbial basis of disease. 

B. Definitions of “Life”

FIFRA Section 25(c)(1) authorizes the EPA to declare “a pest any form of plant or 

animal life...which is injurious to health or the environment.” Because prions are already known 

to be injurious to human and animal health, the remaining issue is whether prions are a “form of 

life.” 

Dorland’s Medical Dictionary (2004), defines “life” as:

“The aggregate of vital phenomena; a certain peculiar stimulated condition of organized 

matter; that obscure principle whereby organized beings are peculiarly endowed with 

certain powers and functions not associated with inorganic matter. Generally, living 

things share, in varying degrees, the following characteristics: organization, irritability, 

movement, growth, reproduction, and adaptation.” 
20

This definition adds four more characteristics to the characteristics beyond those included 

in the definition of “microorganism” and “organism” and addressed in the preceding section (i.e., 

reproduction, growth, metabolism). These additional characteristics that generally are shared, to 

varying degrees, by living things are:

 Organization—the act or process of being organized.

 Irritability—the property of living organisms that allows them to respond to stimuli.

 Movement—the act or process of changing of place or position.

 Adaptation—adjustment to environmental conditions by modification of an organism to 

make it more fit for existence.

C. Whether Prions Are Microorganisms or Alive

Given the unique attributes that make prions an infectious agent, and given the imprecise 

nature of what constitutes a form of “life” or a “microorganism,” there are at least two different 

ways to view the issue of whether prions may be classified as either alive or a microorganism. 

One can focus on the biochemical properties of prions as a type of protein molecule and argue 

that prions are neither alive nor a microorganism. Alternatively, one can focus on the attributes

that make prions infectious agents (such as in Koch’s postulates above) and argue that prions 

should be classified as microorganisms. These two approaches are outlined below.

1) Prions are not microorganisms or alive:

From the perspective of protein chemistry, the research studies summarized in Section II 

indicate that a prion protein (whether normal or misfolded) does not have all of the

characteristics of life or of a microorganism. A protein is a molecule and joins the lipids, 

carbohydrates and nucleic acids as the basic molecules from which microorganisms and 

the cells of multicellular organisms are made. Regarding key properties of 

microorganisms (reproduction, growth and maintenance), the prion protein is a normal 

cellular protein constituent in vertebrate species that is transcribed by a homologous gene 

conserved in the vertebrate species. The individual molecule of prion protein cannot 

reproduce, grow or perform self-maintenance in the same way that microorganisms do. 

When a prion protein becomes misfolded, it converts other normal prion protein 

molecules to adopt the same misfolded conformation through recruitment and 

conversion, which is analogous to the way crystals grow. Thus, a misfolded prion protein 

cannot reproduce itself; it can only induce a copy of its shape in other prion proteins. All 

other infectious agents—algae, fungi, bacteria, viruses and viroids—produce infectious 

diseases by infecting and propagating their numbers within the host species, which is 

accomplished through genetic reproduction and the propagation of new generations from 

genes comprised of DNA or RNA. Unlike these other infectious agents, the aggregated 

particles of a prion protein are devoid of any genetic element and have no mechanism for 

genetic reproduction. The property of infectivity is limited to that of misfolded prion 

protein converting other, existing, homologous protein molecules to adopt by copying the 

template of the misfolded conformation. There is neither synthesis of new prion protein 
21

nor of new genetic material. From this perspective, prions would not be considered 

microorganisms.

Examining the six properties of “life” identified in the Dorland definition cited above, 

one could argue that the only property of living things that prions share is organization. 

With respect to reproduction, growth and maintenance, prions cannot carry out these 

functions via conventional mechanisms due to their lack of genetic material. With 

respect to irritability and movement, prions are not capable of reacting to stimuli or 

initiating movement on their own as most living organisms are. Further, with respect to 

adaptation, prions cannot genetically adapt to their environment or pass along their 

characteristics because they contain no DNA. Finally, individual prion particles cannot 

modify themselves to make themselves “more fit for existence” although prions can adapt 

or evolve non-genetically, as described in the next section.

Applying the definitions of “microorganism” and “life” narrowly, therefore, one could 

argue that prions are neither microorganisms nor alive.

2) Prions are microorganisms and/or alive:

By focusing on the properties of prions that make them infectious to humans and animals,

and distinct from conventional protein toxins, one can argue that prions are

microorganisms and/or are alive. The terms “microorganism” and “life” have previously 

been expanded to accommodate newly identified infectious agents, and because the 

behavior, study and control of prions have more in common with the behavior, study and 

control of infectious microorganisms than of conventional chemical toxicants, it is 

practical and reasonable to characterize them as infectious microorganisms.

For example, the Oxford Dictionary of Biology’s definition for “microorganism” cited 

above includes viruses as microorganisms even though viruses are not considered to be 

alive, and do not have all of the characteristics of a microorganism as listed above. 

Viruses do not grow, have no metabolism, and cannot reproduce by themselves, although 

they do replicate themselves by injecting their DNA into host cells and taking over the 

host cell’s reproductive machinery. Nevertheless, viruses are often grouped with bacteria 

and other entities that meet the entire microorganism criteria described above.2 In the 

absence of a more general term that expressly includes viruses, viroids and other 

infectious agents that do not meet all the criteria of microorganisms, this broader use of 

the term “microorganism” is common among professionals in the biological, medical and 

public health sciences.

 

2 Microbiology textbooks also routinely devote significant space to viruses; see, for example: Textbook of 

Diagnostic Microbiology, Connie Mahon, George Manuselis, Donald Lehman (3d ed., Elsevier 2006); 

Microbiology: Principles and Explorations, Jacquelyn G. Black (6th ed. Wiley 2005); Bailey & Scott's Diagnostic 

Microbiology, Betty A. Forbes, Alice S. Weissfeld, Daniel F. Sahm (Elsevier Health Sciences 2007); Medical 

Microbiology, Patrick R. Murray, Ken S. Rosenthal, Michael A. Pfaller, Ken S. Rosenthal (Elsevier Health Sciences 

2005); Koneman’s Color Atlas and Textbook of Diagnostic Microbiology, Elmer W. Koneman, Stephen D. Allen, 

Gail L. Woods, William M. Janda, Paul C. Schreckenberger (Lippincott Williams & Wilkins 2005); Sherris Medical 

Microbiology: An Introduction to Infectious Diseases, Kenneth J. Ryan, C. George Ray, John C. Sherris (McGrawHill 2003).
22

Prions are distinguishable from viruses in their lack of DNA and RNA, but the 

International Committee on Taxonomy of Viruses nevertheless considers prions to be 

within its scope, and includes taxonomy of various prions in its Index of Viruses 

(http://www.ncbi.nlm.nih.gov/ICTVdb/Ictv/fs_prion.htm). However, possession of such 

genetic material is not among the characteristics of “microorganism” discussed above. 

Prions have been characterized as non-Mendelian elements of phenotypic inheritance 

(Alberti et al. 2009) because they are capable of transmitting structurally and functionally 

distinct states of the protein molecule. Recognizing that prions are the agents of TSEs, 

Miller (1999) classifies prions as “Subviral Agents,” and Büchen-Osmond (2003) lists

prions in the table of viruses infecting humans with the notation that prions have the 

unique property of a self-replicating infectious protein with no nucleic acid. 

Regarding whether prions are a form of life, prions show a number of characteristics of 

living organisms. First and foremost, prions are infectious agents that cause disease in 

humans in a manner very similar to pathogenic viruses and bacteria. Specifically, prions 

convert normal prion proteins in animal nervous system tissues to abnormal, 

dysfunctional prion proteins (additional prions) over a long period of time, leading to 

neurological disease and death. Unlike conventional chemical toxicants, prions 

successfully replicate themselves, increase in number, and spread from one host organism 

to another, much as pathogenic viruses and bacteria. Prions move within an organism, 

move to other organisms, and move through the environment, and these movements are 

closely analogous to the movements of viruses and bacteria. While movement of prions 

appears to be passive rather than active, such as by motile mechanisms, that is also true 

for many organisms universally regarded as living. Finally, prions can adapt or evolve

(non-genetically) in two ways: (a) when injected into a new host, the introduced prions 

(e.g., BSE prions introduced into humans) can recruit and convert the new host’s prion 

protein into new prions (e.g., variant CJD prions) and (b) over time, a prion strain can 

change (through a process similar to natural selection) its characteristics such as method 

of transmission and incubation period. Based on these points, prions could be considered 

to have some of the characteristics of “life.”

The FIFRA SAP has also weighed in on the issue of whether prions are microorganisms 

or a form of life. Specifically, the SAP stated: 

A majority of the Panel concluded that prions can be included in the category of 

“other microorganisms.” A minority of Panel members contended on scientific 

basis that prions lack the significant features typical of microorganisms. (USEPA 

2009)

Several reasons the SAP articulated in support of its majority position included:

 Research spanning the last 50 years leaves no doubt that the agents which cause 

prion disease can reproduce, adapt, grow and move within the host.
23

 Replication of prions has been amply demonstrated by the fact that they can be 

titered and that animals infected with a low level of prion infectivity eventually

accumulate exponentially higher levels of infectivity. This can only occur if 

prions can replicate themselves. 

 As demonstrated by the isolation and careful characterization of multiple mouse 

and hamster-adapted sheep scrapie strains, prions can adapt to new species. 

 Peripheral inoculation of TSE infectivity eventually leads to infectivity in the 

brain, something that can only occur if the infectious agent moves within the host. 

 Prions most definitely exhibit a stable steady state over a long period of time 

(maintenance). Established stocks of TSE infectivity can be maintained for 

decades with no change in either their titer or their biological characteristics.

 Prions are environmental pathogens that infect mammals, replicate virus-like, and 

cause disease according to Koch’s postulates.

The SAP also stated the following conclusion concerning whether prions are “alive.”

A majority of the Panel agreed that once revised to incorporate revisions 

suggested by the Panel the White paper will adequately identify and summarize 

the scientific literature concerning prions and that prions are “other 

microorganisms” exhibiting some features of living entities. (USEPA 2009)

Applying the definitions of “microorganism” and “life” broadly, therefore, one could 

argue that prions are microorganisms.

D. Summary/Conclusion

With respect to the issue of whether prions are “microorganisms” or are “alive,” EPA 

recognizes the possibility of at least two different points of view: (1) A narrower interpretation of 

current definitions that prions are not microorganisms or alive, and (2) a broader interpretation of 

current definitions that prions are a unique type of microorganism and/or are alive. In either 

case, prions are infectious and do produce disease, and in doing so, they generally adhere to 

Koch’s postulates for the causal relationships of microbial diseases. EPA also acknowledges the 

FIFRA SAP’s position that prions should be considered to be microorganisms and alive, and is 

taking this recommendation into account in formulating a regulatory position on whether prions 

should be considered to be a “pest” under FIFRA.
24

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