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MICROBIOLOGICAL METHODS
Hierarchy of Susceptibility of Viruses to Environmental Surface
Disinfectants: A Predictor of Activity Against New and Emerging
Viral Pathogens
SYED A. SATTA R
University of Ottawa, Centre for Research on Environmental Microbiology, Faculty of Medicine, 451 Smyth Rd, Ottawa,
ON, Canada K1H 8M5
Proper disinfection is crucial to interrupt the
environmental spread of many viruses. In the case
of new and emerging viruses still awaiting culture
and full characterization, it is proposed that any
official recommendations for disinfectant use be
based on the well-established hierarchy of
susceptibility to such chemicals as related to virus
particle size and structure.
Viruses represent nearly 15% of all infectious agents
catalogued as human pathogens (1). Viruses also
predominate in the list of nearly 40 human pathogens
discovered in the past 4 decades (2). Successful control of
many bacterial diseases, together with the discoveries of links
between viruses and chronic conditions, further enhance the
relative significance of viruses (3).
Although vaccination, insect vector control, screening
of blood and tissues, and barriers such as condoms can
effectively interrupt the spread of many viruses, others
require environmental control instead (4, 5); hand hygiene
and chemical disinfection of semicritical medical devices
and environmental surfaces constitute the backbone of such
control in healthcare settings. Hand hygiene and
disinfection of environmental surfaces are also crucial
where foods are handled.
In the United States, antiseptics are regulated by the U.S.
Food and Drug Administration (FDA), but that agency
currently has no system to review and register label claims of
such formulations against viruses (6). The FDA, which also
regulates chemi-sterilants and high-level disinfectants for use
on medical devices (7), requires data for efficacy against
viruses but simply refers to testing for virucidal activity as
specified by the U.S. Environmental Protection Agency
(EPA). The EPA deals mainly with environmental surface
disinfectants through specific rules on testing and approval of
label claims against human pathogens (see http://www.
epa.gov/opp00001/factsheets/antimic.htm). It also accepts the
use of surrogate organisms for claims of activity against
vegetative and spore-forming bacteria, mycobacteria, and
fungi (8). Apart from certain recently granted exceptions, this
does not apply to viruses, and testing is required against each
virus type to be listed on the product label. However, a
disinfectant can call itself a ‘virucide’ even with activity
against one or more enveloped, thus easier to kill, virus. This
is also in sharp contrast to regulations elsewhere. In Canada,
an environmental surface disinfectant can call itself a ‘general
virucide’ only upon demonstrated activity against the Sabin
vaccine strain of poliovirus type 1 (9), and in Europe, in
addition to the poliovirus, human adenovirus type 5 must be
used to make a similar claim (10). A general virucidal claim in
Australia requires testing against a poliovirus, an adenovirus,
and a herpesvirus; a parvovirus can be substituted for the
poliovirus (11). This paper highlights the implications of the
current EPA policy (http://www. google.ca/search?hl=en&q=
EPA+%2B+virus+surrogates&btnG) with particular
reference to new and emerging viral pathogens and proposes
an alternative.
VirusParticleSizeandStructureandActivityof
Disinfectants
The differences in the lipophilicity of enveloped
(hydrophobic) and nonenveloped (hydrophilic) viruses
were clearly delineated in the late 1950s (12, 13). Klein and
Deforest (14) subsequently showed that hydrophobic
viruses were considerably more susceptible to microbicidal
chemicals due to the presence of essential lipids in their
envelope. Among the hydrophilic ones, those with a
smaller particle diameter (25–35 nm) proved to be
comparatively less susceptible than those of a larger size
(40–75 nm). Those initial observations have been
repeatedly confirmed and extended over the years. The
Spaulding classification (15) is also frequently referred to
in this context; however, it relates exclusively to chemical
disinfectants to be used on medical and surgical devices,
which come under the purview of the FDA.
Table 1 is based on the hierarchy of susceptibility in
relation to the particle size of human pathogenic viruses when
tested under similar experimental conditions; other infectious
agents are included for contrast only. It should, however, be
noted here that nonenveloped viruses in general are often
considered more susceptible to chemical disinfectants than are
SATT A R :JOURNAL OF AOAC INTERNATIONAL VOL. 90, NO. 6, 2007 1655
Received June 3, 2007. Accepted by AH July 11, 2007.
Corresponding author's e-mail: ssattar@uottawa.ca
mycobacteria (16). Although this may be true for the
larger-sized nonenveloped viruses, many types of
smaller-sized nonenveloped viruses show themselves to be
more difficult to inactivate than mycobacteria when tested
simultaneously (17). Prions are not included in the Table
because they are not only substantially different from viruses
in size and structure, but their communicability is also much
lower. Further, recent studies show them to be much less
resistant to chemicals than previously thought (18).
Viruses and Environmental Control
As stated earlier, many viruses have been discovered since
1968 (2, 19) and others have re-emerged (20), together
causing substantial numbers of human cases and
fatalities (21). Table 2 gives examples of the nonenveloped
viruses among those. Several of the listed viruses have the
potential to remain viable on nonporous surfaces and thus
their spread via such vehicles is potentially interruptible with
proper disinfection of the environment. Rotaviruses are a
suitable example, as they can survive well on environmental
surfaces (22), and their spread in experimental (23) as well as
field (24) settings can be prevented through the use of
chemical disinfection.
The discovery of a new pathogen elicits an immediate
demand for guidance on the selection and use of disinfectants
for its environmental control. In the United States, the primary
source for any such guidance is the Centers for Disease
Control and Prevention (CDC), which normally bases its
recommendations on EPA-registered label claims (25). This
may not readily apply to a newly discovered virus, particularly
a nonenveloped virus in which a greater degree of confidence
is essential for its environmental control.
The following additional factors may hinder the ready
availability of the information needed to base such
recommendations: (1) The agent may be refractory to growth
in lab animals and cell cultures, thus limiting experimentation
with it; human noroviruses illustrate this point well. (2)Even
if a newly discovered virus is cultivable in the laboratory, its
classification at biosafety level 3 or 4 would severely restrict
the number of laboratories that could handle it and thus
1656 SATT A R :JOURNAL OF AOAC INTERNATIONAL VOL. 90, NO. 6, 2007
Table 1. Hierarchy of susceptibility of human pathogens to chemical disinfectants
Level of
susceptibility Microbial class Virus family Examples of human pathogenic viruses
Lowest Bacterial spores
Small nonenveloped viruses (25–35 nm) Astroviridae
Caliciviridae
Circoviridae
Parvoviridae
Picornaviridae
Astro
Noro
Anellovirus
B19, bocavirus
Entero, hepatitis A, rhino
Mycobacteria
¯Large nonenveloped viruses (40–70 nm) Adenoviridae
Papillomaviridae
Polyomaviridae
Reoviridae
Adeno
Papilloma
SV40,Rota
Fungal conidia
Vegetative bacteria, yeast
Highest Enveloped viruses Arenaviridae Lymphocytic choriomeningitis
Bornaviridae Borna
Bunyaviridae Hanta, Rift Valley Fever
Coronaviridae SARS
Filoviridae Marburg, Ebola
Flaviviridae Yellow fever, hepatitis C
Hepadnaviridae Hepatitis B
Herpesviridae Cytomegalo, varicella
Orthomyxoviridae Influenza
Paramyxoviridae Mumps, measles
Poxviridae Smallpox, vaccinia
Rhabdoviridae Rabies
Retroviridae Human immunodeficiency
Togaviridae Rubella
impede the development of information on its susceptibility to
chemicals potentially applicable for its environmental control.
(3) Recently enhanced restrictions on the transportation of
infectious agents in general would also limit the availability of
the virus to researchers otherwise capable of testing
environmental surface disinfectants against it. (4) Lack of
vaccination and therapy against the virus may put staff at
testing labs at undue risk of laboratory-associated infections,
as shown by the severe acute respiratory syndrome (SARS)
incident in Singapore (26).
An Alternative Approach
An alternative approach in dealing with newly discovered
nonenveloped viruses is to apply the hierarchy of disinfectant
susceptibility as it relates to their size and structure (Table 1).
One plausible scenario is that a newly discovered virus is
assigned a tentative grouping based on either direct electron
microscopy only or by molecular and immunological means
with or without culture in the laboratory. Even this
preliminary information has a reasonable predictive value
based on the already available data on the relationship
between virus particle size and structure and susceptibility to
environmental surface disinfectants.
It is recommended that the data to be submitted for
consideration by the EPA be based on an approved carrier test
using at least one large-sized nonenveloped virus and one
small-sized nonenveloped virus. The actual choice of the
viruses may be determined depending on the predominant
clinical picture of the new or emerging virus under
consideration and the testing initiated with the following
criteria and conditions in mind: (1) The virus in question must
first be identified as to its tentative grouping, and the CDC
must make a formal announcement to that effect.
(2) Manufacturers of environmental surface disinfectants
must submit data to the EPA supporting claims of activity of
the proposed product against a virus expected to be equally or
less susceptible than the newly discovered virus. (3) Claims of
hierarchy-based virucidal activity must be limited only to
those products already classified as broad-spectrum and
hospital-grade disinfectants. (4) Claims for anticipated
activity against the target virus will not be allowed on product
label or in mass media, but may possibly be allowed on
company Web sites and tech bulletins. (5) Such a claim based
on the hierarchy of susceptibility to disinfectants will be
allowed only until actual information on the agent’s
susceptibility to environmental surface disinfectants becomes
available from actual testing which has been accepted by the
EPA. (6) Limited disinfectants and sanitizers, as defined by
the EPA, will be excluded from consideration in this context.
Conclusions
Since the work of Klein and Deforest (14) and
Spaulding (15), we know of many more human pathogenic
viruses and also better understand their susceptibility to a wider
variety of chemical disinfectants using more sophisticated test
methods (27). This information, while confirming the original
tenets, should allow for greater confidence in the scheme
proposed here. It should also be noted that the Spaulding
scheme was developed for the decontamination of medical
devices and is limited in its relevance to the chemical
disinfection of environmental surfaces.
New human pathogenic viruses continue to come to
light (28) and it is quite likely that such discoveries will
continue well into the future, while those already known may
also emerge or re-emerge as a result of ongoing societal
changes (2). In view of this, and in the United States in
particular, the existing regulations with regard to awarding
label claims for virucidal activity need urgent review. This
issue is especially pertinent when it comes to new and
emerging viral pathogens of humans. The proposed system,
based on the already well-recognized hierarchy of disinfectant
susceptibility, is not only scientifically valid, but it will not
require a major overhaul of the existing regulations.
The concept of using indicator strains for classes of
microbial pathogens has been in place for some time now.
More recently, the EPA has begun to allow surrogate viruses to
be used to substantiate virucidal activity for
SATT A R :JOURNAL OF AOAC INTERNATIONAL VOL. 90, NO. 6, 2007 1657
Table 2. Examples of nonenveloped viruses discovered as new and/or emerging since 1968
Virus
Year of
discovery
Virus family (approximate
particle size in nm) Associated disease(s)
Enterovirus 70 1968 Picornaviridae (30) Acute hemorrhagic conjunctivitis; rare cases of paralysis
Coxsackievirus A24 (variant) 1970 Picornaviridae (30) Acute hemorrhagic conjunctivitis
Enterovirus 71 1969 Picornaviridae (30) Aseptic meningitis; hand-foot-mouth disease
Norovirus (Norwalk agent) 1972 Caliciviridae (30) Acute gastroenteritis
Rotavirus 1973 Reoviridae (70) Acute gastroenteritis
Parvovirus Bl9 1975 Parvoviridae (25) Aplastic anemia
Hepatitis E 1988 Unclassified (30) Hepatitis
Anellovirus 1997 Circoviridae (17) Hepatitis
Bocavirus 2005 Parvoviridae (25) Respiratory infections
difficult-to-culture human pathogenic viruses such as
hepatitis B (http://www.epa.gov/oppad001/hbv.htm) and
hepatitis C viruses (http://www.epa.gov/oppad001/pdf_
files/hepcbvdvpcol.pdf), and norovirus (http://www.epa.gov/
oppad001/pdf_files/initial_virucidal_test.pdf). Further, the
concept of allowing the use of data based on testing with
surrogate viruses has been accepted by the EPA for new and
emerging enveloped viruses. As a general rule, nonenveloped
viruses are more stable outside hosts and have a greater
potential to spread by environmental means (29, 30).
Therefore, in the interest of human health and proper infection
control, the public and professionals alike have an immediate
need for recommendations to counter new and emerging viral
pathogens based on the best available science. It is
scientifically justifiable to substantiate those public health
recommendations and determine product efficacy based on
prior testing of environmental surface disinfectants with
appropriately related surrogate viruses using valid test
methods. Such an arrangement would add value to, and place
greater confidence in, any guidance issued by agencies such
as the CDC and EPA.
Acknowledgments
Comments on the manuscript were sought and received
from the Innovation and Reform Group (IRG) representing
The Clorox Co. (Pleasanton, CA), The Procter & Gamble
Co. (Cincinnati, OH), and Reckitt Benckiser, Inc.
(Montvale, NJ). The interaction with IRG was coordinated
by Pat Quinn of the Accord Group (Washington, DC). IRG
also financed travel for a meeting with the Antimicrobials
Division of the EPA.
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