United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington, DC 20460
Research and Development
EPA/600/S6-88/003  May 1988
 Project  Summary
 Pathogen  Risk Assessment
 Feasibility  Study
  This  report  evaluates  the
practicality of formulating guidelines
to assess the risk associated  with
exposure  to  pathogens in  sludge.
Risk assessment  may  be used  to
determine the  likelihood that an
environmental  agent  may  cause
human disease  (that is, potential  to
cause human cancer or toxicity). On
the  assumption  that the  agent
causes a  particular  disease, given
current and projected exposure
levels,  a  quantitative evaluation can
be made  on  the magnitude  of the
likely impact of  the agent on public
health. In this report, the feasibility of
performing a microbiological  risk
assessment for  pathogens  in
municipal wastewater sludge by
various   disposal  options  was
evaluated.
  This Project  Summary  was
developed by EPA's Environmental
Criteria  and Assessment  Office,
Cincinnati,  OH, to  announce key
findings of the research project that
is fully documented in a separate
report of the  same title (see Project
Report ordering information at back).

Introduction
  Pathogen risk assessment involves  an
evaluation of the information available  on
representative species  of microbes and
their potential  health  effects,  and
modeling, which  includes  fate,
persistence, and transport. The  result of
risk assessment is a focused output that
addresses the potential for human health
impacts. It  permits a decision about the
appropriate level of  concern about
existing sludge treatment  and  disposal
options used  to  reduce the  flow  of
pathogens  from sludges  to the human
population,  and whether  other options
should be considered.
  Pathogenic organisms found in sludge
of human origin include certain  bacteria,
viruses, fungi,  protozoa, and helminths.
 After treatment by anaerobic, aerobic,
 composting, lime  stabilization or other
 methods, the sludge and  remaining
 pathogens are disposed of in one of five
 major  ways  disposal  of  sludge in
 dedicated  sanitary  landfills;  direct
 application  of sludge to agricultural,
 pasture land, silviculture and reclamation
 areas;  distribution and  marketing  by
 direct application  of sludge to gardens
 and municipal areas such as roadsides,
 cemeteries and golf courses; dumping
 sludge  into  the ocean from a barge or
 tanker;  and  combustion  of sludge in a
 multiple hearth  or  fluidized bed
 incinerator.  Assessment  of risk
 associated  with  incineration  is not
 discussed  because pathogens are
 destroyed by this method.

 Mechanism of  Pathogen
 Transmission
   A number of possible pathways  by
 which  sludge  pathogens  and  other
 constituents  can be transferred through
 the environment  to exert  potentially
 adverse effects on humans have been
 identified. The pathways begin  at the
 point where wastewater  enters  a
 municipal treatment plant. The first three
 stages  represent wastewater treatment
 and  sludge processing  procedures
 necessary to dispose of sludge, or in the
 case of distribution and marketing, where
 a treatment  plant,  a retailer,  or a broker
 can  distribute  and market  sludge
 products.

 Selection  of Representative
 Pathogens for  Risk Assessment
 Study
   The  number and types of microbes
 found in municipal wastewater sludges
 varies from community to community and
 depend on  several factors.  These
 include, but are not limited to, the  degree
 of urbanization, population chemistry,
 sanitary  habits, season of the year, and

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rate of disease  in  the  community.
Microorganisms found in wastewater and
sludges and their potential health effects
are identified in the report.
   Because of the limited  data base on
these and other species, and the lack of
appropriate  or simple  measurement
methods, specific representative species
are tracked through the waste handling
and  disposal  pathways.  Thus,  these
representative pathogens may be  used
to assess risk and are, in fact, pathogenic
surrogate organisms used  for  detection
of human health hazards. This latter fact
separates these organisms  from
"indicator organisms" that  are  used to
monitor the microbiological quality of the
environment, but may  not be pathogenic
or may only  pose a minimal  risk to
hurna.ns..	
   The criteria  used to select  these
representative  pathogenic  microbial
species include:
   • Known demonstrated occurrence in
     municipal sludge;
   • Known pathogen  in  the  general
     population;
   • More adequate information base for
     the  given  species than for other
     species of the principal  pathogen
     groups;
   • Known infectious doses; and
   • Relatively  hardy  species outside
     the host.
   Thus, in practice, species are selected
as examples from each of the principal
pathogen groups. Many  studies  have
used  the  following  representative
species: Salmonella  as an example of
enteric bacteria; enteroviruses  as an
example of human  enteric  viruses;
Entamoeba histolytica  (the  cause of
amoebiasis) or  Giardia  lamblia (the
cause  of  Giardiasis)  for  protozoans;
Ascaris lumbricoides  for helminths; and
Aspergillus fumigatus for fungi.
   Because a model is an approximation
of reality,  decisions have  to be  made
regarding which  components of reality
can  be relaxed and which cannot. It is
more feasible to model a few species as
opposed  to  hundreds  of  species.
Representative  species  are selected to
be  modeled and substitutes are  used
only when  necessary  The part of the
model  that must approximate  reality to
the  greatest  extent  possible  is the
tracking of these pathogens through the
treatment and disposal  pathway  to
human exposure sites. The available data
must describe  the  changes in viability
and concentration that occur in pathogen
populations  along  the pathways.  The
goal  of  the  model is  to  provide
reasonable  predictions, within  the
constraints of data uncertainties, of the
time-dependent  concentrations  and
locations  of   pathogens.   The
concentrations of pathogens can provide
a basis to  assess the  likelihood and
consequences of  infection, disease  or
fatality.

Uncertainties and Major Data
Gaps
  Some uncertainties  exist in the
methodologies  used  to  enumerate
pathogens in sludges, soils, groundwater
and  surface water.  The  quantitative
assumptions  used to   model  risk
exposure must  take this into account
Many of these uncertainties  can be
attributed to procedural  differences
among  laboratories,  even though the
same "standard procedure" is followed.
For  example, in reviewing  the literature
on the  efficiency  of pathogen  removal
from  wastewater during  treatment
processes,  one  may  conclude  that
quantitative information  should be
compared on  the basis  of  orders  of
magnitude.  This may be  true for
detection of pathogens  in general
because  of   the  laboratory-to-
laboratory variability in methods, and the
differences in pathogen recovery within a
single  laboratory  depending on  what
methods were  chosen.  Also,  as  new
methods  are  developed and  older
methods improved,  the  numbers  of
organisms  typically  isolated  from
wastewater  and sludges will probably
increase. Subsequent attempts  to
compare the new results with  older
results could be problematic.
  Campylobacter sp., for example, can
contaminate  drinking water  supplies and
cause enteritis. With increased attention
focused on Campylobacter  enteritis, new
methods resulting  in greater recovery of
the   organism   are  being evaluated
Recently,  several methods  were
evaluated for recovery of Campylobacter
from various specimens. Pretreatment,
growth medium type, incubation time and
temperature,   and  pre-enrichment
techniques  used,  all  affected the
quantitative results. Results of standard
tests, even  for  representative species,
are  subject to variability among different
laboratories.
  One  way  to  evaluate  the suitability of
quantitative data  among  different
laboratories  is  round-robin  testing that
involves  simultaneous  analyses of the
same  sample  by  several   different
laboratories.  The results reported support
the  conclusion that quantitative detection
of pathogens, especially viruses,  is not
highly precise.  For  modeling  purposf
the variabilities must  be reported,  i
order of magnitude,  plus or minus, m
be the only  reasonable starting point
lieu  of  rigorous  interlaborato
development of standard  methods  1
detecting pathogens in sludge.
   Major  data  gaps for  varioi
components  of a risk assessment exi
The following are specific examples th
must be dealt with:
   •  Microbes—population dynamii
     of important  pathogen species  a
     not  completely understoo-
     especially in regard to interactioi
     with other microbes or organisms
     their ecosystem;
   •  Treatment  and  storage—th
     survival  rate  of  sludge-bour
    pathogens needs  to be more ful
     clarified   along   with  th
     development   of   a   bette
     understanding of the importance
     such survival to human health;
   •  Disposal, transport  and  fate
     relationship of  key environment
     variables  to pathogen  survival  ar
     movement, especially as  related i
     pathogens being  bound to sludgi
     and
   •  Human  exposure-relationship  <
     infection to disease (case historic;
     needs to be more fully explored.
These  issues  are  among the mo:
significant ones that warrant  extensiv
research.
   The  dynamic  nature of som
pathogens  bound  to  sludge pose
questions of  reduced die-off  rate
during treatment and the development (
problems at the disposal  or  exposur
site. The rate  of  pathogen movemer
may  decrease and  allow for longe
retention  at or in a given site.  To dat«
these  processes are  not quantifiec
Finally, there is a  lack of conclusiv
evidence (case histories)  of  disease
resulting from pathogens in treate
sludge disposed by  any of the method
previously described.

Likelihood of Exposure
   The  probability of  contact  betwee
sludge-originated  pathogens  am
humans  is  never  zero.  Rather, certaii
likelihoods of exposure can be  advancei
for pathogens  as  they  move  from  thi
various sludge  treatment and  dispose
options through the  exposure  pathways
Treatment, sludge management practice;
at the  treatment site,  sludge  dispose
methods,  and  pathogen survival  am
mobility in  soil,  water  and air greath
affect  pathogens and  limit  exposure t<
humans. For example, helminth eggs  ar<

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'arge, relative to viruses, and exhibit little
 ownward movement in  a soil  profile.
Thus, helminth  egg  infiltration  to
groundwater is unlikely unless the  water
table is near the  surface, the soil is very
porous, or a fissure exists that connects
the land surface with the saturated zone.
   The likely contact  between pathogens
and humans is discussed  in  the report.
Consideration  is  given  to  various
exposure routes as they relate to sludge
disposal  options.  This  scoring of the
various  exposure  routes  is meant to
guide and focus efforts for modeling and
data  collection that will allow consistent
risk   assessment  of exposure  to
pathogens in sludge.
   Most infections from pathogens follow
a  dose-response  relationship.
Therefore,  as  the  concentration  of
consumed   or  inhaled  pathogens
increases, there is a greater likelihood of
a  population becoming  infected. The
number of cases  of the  disease that
result is  eventually expressed as  an
incidence  rate. The measured response
in  humans to a microbial challenge  could
be in the  form of no infection,  infection
without illness (such as  subclinical, in
apparent  infection), or infection  with
illness (such as infectious  disease  in an
increasing proportion of test subjects).
Whether  or  not a response is  noted
depends on the dose of the pathogen the
human is  exposed to,  the susceptibility
of  the individual,  and the virulence of the
pathogenic  organisms.  Infection  is
detected  by  identifying progeny bacteria
in  body products, such as nasal or oral
secretions, blood, urine and feces,  or by
host  response  such  as  antibody
formation that results from infection.
   Host response to  an infectious  agent
has also  been  measured  in  terms of
disease production, that is, visible  signs
of  illness. However, this is a much less
objective measure of response and does
not include infections in which no clinical
disease is produced. In this "carrier"
state  the  agent  is still shed  in  body
products in a viable, communicable form.
   Viral  infectivity  can  be measured in
ways similar to  those described for
bacteria.  However,  viruses  are  also
measured in  cell cultures. Cell culture
methods  require  that the virus  replicate
and  kill  the infected  cell, and that
progeny virus, in turn,  replicate and kill
other cells in the culture.  The presence
of  the infectious  virus is detected by its
ability to  cause  destruction throughout
the cell monolayer (cytopathic effect) or
to  cause  cell  destruction in restricted
 egions  of  the  monolayer  (plaque
formation).
   Infectivity of protozoans is measured
by the detection of cysts in feces of the
host.  Depending on the strain,  1 to  10
cysts can produce an infection and many
of these infections  are asymptomatic.
Similarly, single eggs  (ova) of helminths
produce human infections, as measured
by the identification of the eggs in the
host's feces.
   The terms  "infective dose" (ID) and
"minimal  infectious dose" (MID)  are
actually a  discrete part  of the dose-
response. Generally, the MID is the dose
required to infect 50% of the population
(ID5o)  though  infectious doses such  as
ID-|  could  be  used  for  worst-case
scenarios.
   Minimum infectious doses for bacteria
are on the  order of  102  to  106.  Even
though these doses  are high,  such
concentrations can  be found  in some
sludges.  In  contrast, a single  viral unit
may initiate an infection. In this particular
case it was considered that about 1% of
the human  population  would become
infected from exposure to  one viral unit.
If 50% of the population were to respond
to an infection, the MID would be  5 to 30
viral  units.  Similarly, for  helminths and
protozoa, the  MIDs are lower than  for
bacteria. A single  egg  is  considered
infectious  to  man,  although   some
researchers  assume 10 cells or cysts to
be an infective dose. Much less is known
about infectious  doses  for  fungi
Individuals predisposed to lung problems
may be at high risk  from  inhalation of
Aspergillus spores  from  composting
sludge. The actual infective  dose  for
Aspergillus is not known,but exposure to
the fungus  seems to  be  much  less
important than levels of abnormal human
susceptibility.
   The information on  minimal infectious
dose  can  be  systematically  integrated
with  information  on  the number  of
pathogens that are likely to be present in
the various  exposure  pathways.  In this
matrix, consideration was  given  to the
survival and transport capabilities  of each
of the principal pathogen groups  as they
relate to the various exposure pathways.
For example, helminths move very little
in soil and  their  contamination  of
groundwater is  unlikely.   In contrast,
however, viruses can move through a soil
profile and contaminate  groundwater.
The  infectious dose for viruses  is also
very low. The integration of these facts
produces a high likelihood  of occurrence
relative to  the  previously described
example with a helminth.
   Relative  to  helminths  and protozoa,
bacteria and viruses are  more likely to
penetrate and move  along  exposure
pathways, and  finally come in contact
with  humans.  This  information,  when
coupled with infectious doses for viruses
and  bacteria directs risk  assessment
efforts  toward viruses because of the
large number of viruses in  sludge, their
relative  ease  of mobility, and  their low
infectious dose.

Conclusions
   Pathogens  in  sludge,  especially
pathogenic bacteria,  viruses,  protozoa,
helminths  and fungi,  have been  studied
for  many years.  Studies  range  from
enumeration of microorganisms before
and  after  various treatments  to
epidemiological documentation of the
role  of  aerosol pathogens in  human
infection and  disease. Priorities can  be
set for  what exposure situations should
be recognized and examined first.
   Data  available for  microbiological risk
assessment for  sludge pathogens varies
in quality and quantity for all parts of the
process for a limited number of pathogen
species. Uncertainties can  be  identified
and  rational  assumptions  justified  to
augment the evaluation. Risk assessment
of  pathogens  in sludge  is a reasonable
activity to  undertake  at  this  time.
Although the  models  can  be improved,
sufficient data are  available that should
approximate reality

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   This Project Summary was prepared by staff of the Environmental Criteria and
    Assessment Office-Cincinnati, Cincinnati, OH 45268.
   Larry Fradkin  is the EPA Project Officer (see below).
   The complete report,  entitled  "Pathogen  Risk  Assessment  Feasibility  Study,"
    (Order No.  PB 88-191 4401 AS;  Cost:  $25.95,  subject to change)  will  be
    available only from:
           National Technical Information Service
           5285 Port Royal Road
           Springfield,  VA22161
           Telephone:  703-487-4650
   The EPA Project Officer can be contacted at:
           Environmental Criteria and Assessment Office
           U.S. Environmental Protection Agency
           Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300

EPA/600/S6-88/003
         0600329    PS
         U  S
                                         40604
                                                                                 GOVERNMENT PRINTING OFFICE. 1988—548-013/87051

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