Scientific Information Concerning the Issue of Whether Prions Are a “Pest” 

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

(12/17/08 Draft)

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 that proposes to establish a prion as a pest under FIFRA. 

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 again examined the relevant, available 

scientific evidence 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 more depth 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), research to characterize the precise nature of prions and 

resolve how they cause the diseases known as TSEs 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 (Prusiner et al. 1982). While a number of scientists have not ruled 

out an unknown virus as the agent of the TSEs (Manuelidis, 2007; Manuelidis et al. 2007), no 

virus producing a TSE disease has been isolated after 40 years of searching. Instead, the proteinonly hypothesis for the origin of the TSEs (Alper et al. 1966; 1967; Griffith, 1967; Brown et al. 

1990a) is now generally accepted by overwhelming 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 purified prion protein 

synthesized or propagated in cell free systems (Legname et al. 2004; Castilla et al. 2005) is 

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

accepted as the agent responsible for the TSEs, the precise structural properties of the prion 

protein responsible for infectivity and the chain of events that produce neural degeneration and 

fatal TSEs remain unknown (Barron et al. 2007). 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-helix rich conformation is 

converted to a misfolded, β-sheet rich conformation, the resultant protein by-passes the normal 

proteosomal degradation pathway, produces a TSE whose clinical features vary somewhat 

depending on the species (eg., cow, sheep, human, etc), and acquires the property of infectivity. 

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, polymorphism at PRNP gene codon 129 (129M/V) is common; 

2
however, homozygous genotypes at 129 predispose PrP transcripts to misfolding and 

heterozygous transcripts resist misfolding (Mead et al. 2003). Genetic polymorphism in the 

PRNP promoter may also influence susceptibility to TSEs, as in the case of BSE (Haase et al. 

2007). What distinguishes prion diseases from other neurodegenerative diseases featuring 

neurotoxic accumulations of protein plaques (such as Alzheimer’s, Parkinson’s, and ALS) is that 

misfolded PrP transcripts recruit and convert normal PrP transcripts to adopt the misfolded 

conformation; however, a low level of recruitment and conversion may be common to other 

amyloid plaque diseases. 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 

(known as plaques in histology), which are linked to cell death in neurons, and produce a number 

of fatal spongiform encephalopathies in humans, livestock and game animals (review Brown, 

2005). 

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

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

accounts for the resistance of prion protein particles to proteosomal degradation and, 

consequently, for the slow accumulation of brain plaques. 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 (kuru, 

scrapie, mad cow, etc), in fact, feature a novel disease mechanism. In the prion hypothesis of 

disease, it is thought that prion protein polypeptides in the misfolded, laque-forming 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 refers to an unknown, slow virus, but rather to this novel process of 

recruitment and conversion. Furthermore, as will be detailed below, Legname et al. (2004) show 

evidence that normal PrP can spontaneously convert to the misfolded isoform. 

While the outbreak of mad cow diseases in the UK appears to have effectively ended in 

cows (review Brown, 2001), the long, unresolved dormancy of prion diseases is a matter of great 

public concern (Prusiner et al. 1982; Carrell, 2004; Enserink, 2005), with the risk that prion 

disease might reappear in human populations long after the consumption of food contaminated 

with diseased CNS tissues. Also, surgical tools contaminated with prions are extraordinarily 

difficult to decontaminate (Brown et al. 1990; 2000; 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 

3
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

, 

the infectious isoform in scrapie is PrPP

Sc. Because a variety of isoforms produce different 

transmissible spongiform encephalopathies, PrPd

 has been introduced recently to connote any 

disease isoform. Also, PrPres has been used to connote a purified protein or recombinant 

polypeptide fragment that features the resistance to proteinase K digestion that is characteristic 

of TSEs. 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 a misfolded isoform of any 

prion protein; “prion protein” can refer to both the normal and 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. Between 380 and 660 days after inoculation of normal mice 

with these transgenic fibrils, all mice developed neuropathology, and their brain tissue, in turn, 

was infectious to other mice. The amino acid sequence of the human prion protein has been 

extensively probed and two cationic domains (PrP19-30 and PrP100-111) have been shown to be 

capable of homologous binding between PrPC and PrPSc, which would account for the 

recruitment of PrPC

 and 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). 

 

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. 

 3. Normal Prion Protein 

It is now known that 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 ( review Sakaguchi, 2005) located on chromosome 

20 in humans and on 2 in mice (Oesch et al. 1985; Basler et al. 1986). The Prnp gene appears to 

be 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 

homology 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 

4
(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). 

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

normal isoform of PrPC

 is a copper binding metalloprotein (Brown DR, 1997), and it may 

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

also be an important cofactor in conversion of PrPC

 and stability of the PrPP

Sc infectious 

conformation. Disruption of normal PrPC

 function 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 β-rich isomers and accumulation of plaques in neurons. Homozygous codons for 

methionine (Met, M) or valine (Val,V) at codon 129 in the human PRNP gene predispose PrP to 

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

disease following cannibalism of contaminated 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, spongiform encephalopathies, the PrP has largely been studied in the brain. While most 

abundantly expressed in the brain, PrPP

C

 has 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 hematopoetic stem cells where 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 

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

between the accumulation of prion protein aggregates and neuronal cell death. These aggregates 

are thought to result from failure of the ubiquitin-proteasome pathway to adequately degrade 

misfolded protein, which is a feature of prion disease 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. Rather, it appears that the scrapie prion kills neurons by disrupting the 

function of the normal cell surface protein, and development of prion disease requires the 

presence of normal PrP expression. Uncoupling normal PrP from the cell surface also uncouples 

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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 PrPP

Sc shows that grafted tissues accumulate PrPSc

P

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 PrPC in 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 PrPP

Sc several weeks before PRNP inactivation. Within days after 

the PRNP gene was inactivated, the mice became depleted of PrPC

 and remained alive 

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

downregulation of PRNP all died. 

• PrPC

 protein is normally anchored to the neuronal cell membrane by a 

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

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

prevent the development of scrapie disease (Chesebro et al. 2005). 

Relationships between prion aggregation, plaque accumulation, infectivity, and 

spongiform degeneration are incompletely understood, as 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). 

C. TSE Diseases 

 

 Prion diseases are neurodegenerative diseases also referred to as transmissible 

spongiform encephalopathies (TSE) 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 

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 in deer, moose, and elk (Williams and Young, 1980), 

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

(Wyatt et al. 1991), and transmissible mink encephalopathy (Marsh, 1992). 

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

disease (CJD), Gerstmann-Straussler syndrome (GSS), 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 sporadic CJD worldwide 

is about one per million (Collinge, 2001). 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, 

6
headaches, ataxia, blindness and random pain sensations. 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). 

 Most prion disease in humans is acquired. 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; Alpers, 

1987 as cited in Collinge, 2001). 

Creutzfeldt-Jacob disease - human Kuru – human 

Bovine spongiform encephalopathy - 

(cow) 

Scrapie - (sheep) 

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): sporadic, iatrogenic, 

familial 

 Kuru 

 Sporadic and familial 

insomnia 

 Acquired variant CJD 

Spongiform 

 Scrapie – sheep 

 Bovine spongiform 

encephalopathy 

 Transmissible mink 

encephalopathy 

 Chronic wasting disease of 

mule deer & elk 

7
(vCJD)  Feline spongiform 

encephalopathy 

 Encephalopathies in zoo 

animals* 

Dominantly Inherited 

 Gerstmann-StrausslerScheinker Syndrome (GSS) 

Acquired by ingestion 

 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 occurs spontaneously in the human population at the rate of one per 

million people per year, prion-related disease epidemics tend to result from acquired TSEs. The 

first prion-related epidemic disease 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). However, the first 

major outbreak of transmissible encephalopathies was not diagnosed until 1985 when mad cow 

disease (i.e., bovine spongiform encephalopathy; 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 146,000 cases of BSE. Other European countries 

also reported cases of BSE during this time including Switzerland (200 cases), Ireland (120 

cases), and Portugal (30 cases). As the practice of including animal parts in cattle feed was 

stopped, the epidemic waned (Brown et al., 2001; Univ. of Wisconsin, 2004). However, since 

the introduction of monitoring programs to detect BSE in dead and slaughtered cattle, 12 

countries have found their first cases of BSE: Austria, Czech Republic, Finland, Germany, 

Greece, Israel, Italy, Japan, Poland, Slovakia, Slovenia, and Spain. Small numbers of cases have 

also been reported in Canada, 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 variant CJD 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 2007 

(http://www.cjd.ed.ac.uk/cjdq56.pdf). 1The first case of mad cow 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 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 

8
(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). 

Distribution of CWD (www.aphis.usda.gov) 

 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 

Transmissible Spongiform Encephalopathy (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 and elk 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 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 or a 

fence post as if it 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. Scrapie 

infected 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 

9
abroad but originated in Britain), three in pumas, three in ocelots, two in tigers and two in lions 

(Cornell Feline Health Center, 2004). 

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, or subcutaneous 

injections, scarification, or 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 (Palson, 1979) or by prionuria (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 recognized that, as with BSE, maternal transmission of CWD likely 

occurs, but at 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). Meat and bone meal are 

10
prepared from the offal of cattle, sheep, pigs, and chicken and fed to cattle as a high protein 

nutritional supplement. For the U.S., a recent study by Heaton et al. (2008) surveyed over 6,000 

cattle, including five commercial beef processing plants, and determined that BSE cases with the 

K211 allele that occurred in Alabama would be unlikely. 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 BSE-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 PrPC to PrPSc, or reduced clearance of low levels of PrPSc 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). 

 Both kuru and vCJD are caused by ingestion of contaminated material. 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). Variant CJD (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 was manifested clinically and the second was identified at autopsy following death 

from an unrelated cause) (Llewelyn et al. 2004, Peden et al. 2004). 

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

mad cow disease. 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. Certain prion diseases are infectious to 

humans, including BSE or mad cow disease, but it is not clear if the 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 chronic wasting disease in deer and elk 

populations in the Western USA continues to spread, 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 mad cow (1986 – 2000) 

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

11
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. 

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 (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 prionspecific 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 Spongifiorm Encephalopathies (WHO, 2000), which 

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

12
system tissues and fluids as “high-infectivity,” which should be managed more conservatively 

than others. 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. 

 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. 

 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 

13
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 and are most difficult to inactivate on this surface (Zobeley et al. 1999) 

 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 

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 

14
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 different 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 

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 mad cow epidemic 

in the UK (1986-2000) demonstrates that BSE readily crosses the species barrier and is 

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. 

15
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 FIFRA Definition 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. How Do Prions Relate to Common 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 

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

animals. Exposure 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”. 

Although there is no single, definitive definition of “microorganism”, the definition in the 

Oxford Dictionary of Biology (Fourth Edition, 2000) is representative.1

 The 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 Dictinonary 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 this definition of “microorganism” as it is generally 

used by biologists and the medical community would depend on whether they exhibit 

reproduction, growth and maintenance. The research studies summarized in Chapter II indicate 

 

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. 

16
that a prion protein (whether normal or misfolded) does not have these characteristics. The 

normal prion protein is a cellular protein that is transcribed by a gene ubiquitous in vertebrate 

species. It cannot reproduce, grow or perform self-maintenance as microorganisms do. When a 

prion protein becomes misfolded, it converts normal prion proteins to adopt the same misfolded 

conformation through recruitment and conversion. Thus, a misfolded prion protein cannot 

reproduce itself; it can only induce other prion proteins to take on a misfolded shape. All other 

infectious agents—algae, fungi, bacteria, viruses and viroids—produce infectious diseases by 

infecting and propagating their numbers in 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 to adopt the same misfolded conformation. There is neither synthesis of new prion 

protein nor of genetic material. Finally, recognition that prions are distinct from the entities that 

biologists and the medical community generally recognize as microorganisms lies at the heart of 

Nobel prize for the discovery that prion particles are (a) comprised solely of protein and (b) are 

the causative agent of a novel infectious process (Prusiner, Nobel Prize, 1997). 

Although prions do not fit the criteria biologists generally use to define microorganisms, 

the term “microorganism” is sometimes used more broadly. For example, the Oxford Dictionary 

of Biology’s definition for “microorganism” cited above includes viruses as microorganisms, 

even though they do not have all of the characteristics of an “organism” as listed above. Viruses 

cannot reproduce by themselves, but do replicate themselves by injecting their DNA into host 

cells and taking over the host cell’s reproductive machinery. Nevertheless, they 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 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, at least in casual speech. 

 B. Are Prions Alive? 

 As mentioned above, 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: 

 

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). 

17
“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.” 

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 of 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. 

Of the six properties of living things identified in the Dorland definition, the only 

property of living things that prions share is organization. Prions are made of protein, which has 

four levels of organization (Leninger, 1970): 

• Primary structure is the sequence of amino acids that comprise one polypeptide chain. 

• Secondary structure describes the polypeptide chain's repetitive domains of alpha 

(helical) and beta (pleated sheets) structures. 

• Tertiary structure describes the full, 3-dimensional folding conformation of the 

polypeptide, which is determined by the content of alpha and beta structures, as well as 

by disulfide cross-linkages. 

• Quaternary describes the organization of individual polypeptide chains to form a larger, 

more complex molecule comprised of individual subunits, such as first described in 

hemoglobin. [Note: prions are single polypeptide chains, so they do not have this level of 

organization.] 

Although prions feature the above kinds of “organization,” they do not differ in this 

respect from any other biological proteins. Inasmuch as other biological proteins are not 

generally considered to be forms of life, these kinds of “organization” do not appear indicative of 

forms of life. With respect to “irritability” and “movement,” prions are not capable of reacting to 

stimuli or initiating movement on their own. Finally, with respect to “adaptation,” prions have 

no known mechanism for either adapting to their environment or passing along their 

characteristics via DNA because they contain no nucleic acids. 

18
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