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The Science of Public Health Surveillance
THE TOOLS OF PUBLIC HEALTH SURVEILLANCE
Public health surveillance is the ongoing, systematic collection, analysis,
interpretation, and dissemination of health outcome-specific data for use by the
public health sector to reduce morbidity and mortality and to improve health
(Thacker and Berkelman, 1988~. Surveillance of many infections and intoxica-
tions, including those that are foodborne, has been a fundamental public health
activity for many years. Human foodborne disease surveillance is conducted for
three principal reasons: (1) to identify, control, and prevent outbreaks of food-
borne disease, (2) to monitor trends and determine the targets and efficacy of
control measures, and (3) to determine the burden of specific diseases on public
health (Potter et al., 2000).
By detecting outbreaks and their sources quickly, surveillance can lead to
control of an acute health hazard, for example, by removing a contaminated
product from the market or by temporarily closing a hazardous kitchen. Outbreak
investigations can also identify critical gaps in knowledge, leading to applied
research and ultimately to better long-term prevention as unsafe food handling
processes are corrected or new food hazards are identified and controlled.
The information gathered through surveillance and subsequent investiga-
tions of outbreaks and of sporadic cases can reveal the magnitude and trends of
foodborne diseases, which helps policy makers target prevention strategies. This
information is also critical to the design and evaluation of risk assessments.
Improved understanding of foodborne diseases, in turn, can help researchers
recognize new problems, such as entirely new hazards (e.g., microbes or toxins)
28
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29
Prevention
Measures
\
THE SCIENCE OF PUBLIC HEALTH SURVEILLANCE
Surveillance
Applied
Targeted
Research
FIGURE 2.1 The cycle of public health prevention.
-
Epidemiologic
Investigation
/
or known hazards that may appear in foods not previously associated with them.
Most foodborne pathogens were discovered during outbreak investigations, and
much of the knowledge we have about specific hazards and how they enter the
food supply also was gained during the course of investigations. As new
foodborne disease sources and agents emerge, the efforts to control them through
application of the Hazard Analysis and Critical Control Point (HACCP) system
and other control strategies must constantly evolve. Surveillance is a keystone in
the effort to define, control, and prevent foodborne diseases (Figure 2.1~.
In the United States, foodborne disease surveillance is primarily conducted
by local and state public health agencies. In fact, local surveillance for diseases of
public health concern has been conducted for centuries. In the nineteenth century,
fear of cholera led to the establishment of permanent municipal health depart-
ments and disease surveillance, even before the microbe that caused it was iden-
tified (Rosenberg, 1987~. Reporting of typhoid fever cases and deaths drove
many improvements in water and food safety at the beginning of the twentieth
century. Increased concern following the large Escherichia cold 0157:H7 out-
break in 1993 associated with consumption of undercooked ground beef (Bell et
al., 1994) stimulated enhancements in surveillance for foodborne infections (FSIS,
1998c).
Strategies in Public Health Surveillance
There are specific strategies to collect information that may serve as a basis
for making food safety policy decisions. The surveillance strategies for outbreaks
and sporadic cases of diseases that are often foodborne are:
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SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
1. Monitoring case reports of specific, notifiable infections
2. Investigating and reporting outbreaks of illnesses associated with events
or establishments
3. Investigating and reporting unusual clusters of cases of specific infections
4. Vigilantly surveilling (termed sentinel site surveillance) for specific con-
ditions that may or may not be notifiable
5. Laboratory subtyping of pathogens isolated from human infections
6. Surveying the population to measure trends in diarrhea! illness, consumer
behavior, and food consumption
One surveillance method may be more appropriate than another, and these
methods may also be used alone or in combination, depending on the purpose.
For example, subtyping of pathogens may be performed to confirm the source of
an outbreak.
The specific surveillance strategies are conducted either nationwide or in
several sentinel sites that represent the whole population. Surveillance conducted
to detect outbreaks and protect the public should cover the whole population,
should include conditions most likely to appear in outbreak form, and in some
instances, should focus on settings where outbreaks are likely to occur. Some
outbreaks are not tightly clustered in time and space, and thus are not evident in
surveillance conducted in one location. To detect dispersed outbreaks, it can be
critical to compare specific markers of the infecting organisms, such as genetic
"fingerprints," across many jurisdictions (Swaminathan et al., 2001~. Such com-
parison of subtypes may reveal an unusual clustering of infections with a single
strain of a pathogen that can then be further investigated. Public health laborato-
ries use subtyping methods and are linked in a national network to permit rapid
comparison of results and to provide warning of dispersed outbreaks. For example,
the network of state public health laboratories detected a multistate cluster of
Salmonella Newport infections that had the same pulsed-field gel electrophoresis
profile. As a result of the investigation of genetic profiles, 78 infections in
13 states were linked to consumption of imported mangoes (Sivapalasingam et
al., 2000~.
If the purpose of surveillance is to measure the public health burden of
disease or track long-term trends in the nation as a whole, more detailed data
collected from a representative sample of sites around the country is likely to
provide more accurate information (Angulo and Swerdlow, 1999~. This sentinel-
site approach can provide data on important illnesses, such as Campylobacter or
Vibrio infections, that are not well represented in national surveillance systems
because they rarely appear in outbreak form and are not reportable in many
jurisdictions.
For the purpose of determining the food source of infections, surveillance
based on outbreak investigations provides answers for those illnesses that fre-
quently appear in outbreak form. For illnesses that rarely appear as outbreaks,
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THE SCIENCE OF PUBLIC HEALTH SURVEILLANCE
31
case studies can give a general answer as to the source of illnesses that are
strongly tied to specific sources, and case-control studies can provide information
if the sources are complex. As described later, the committee feels that to con-
struct a detailed quantification of the contribution of specific animal or food
sources to foodborne diseases, systematic monitoring of pathogens in food and
animal reservoirs using molecular subtyping and comparison of strains with iso-
lates from human infections are urgently needed.
The following sections describe how these strategies are utilized in both
nationwide and sentinel site surveillance by public health agencies in the United
States. Specialized surveys that relate the contribution of consumer behavior to
the level of specific foodborne illness risk are described as well. Finally, several
factors that limit the value of surveillance systems are discussed.
Nationwide Surveillance of Notifiable Diseases
Many counties and states have collected notifiable disease reports for more
than a century, covering an ever-expanding list of illnesses. Since 1961, these
reports have been voluntarily submitted to the Centers for Disease Control and
Prevention (CDC), which publishes them as weekly and annual summaries
(Thacker, 1994~. At its annual meetings, the Council of State and Territorial
Epidemiologists decides which specific illnesses should be nationally notifiable.
This general umbrella of reporting covers all areas of the United States; provides
information useful to local, state, and national authorities; and is relatively inex-
pensive. Most disease reporting is passive from the standpoint of the public
health system, which means that clinicians and laboratories are asked to report
cases on their own initiative. Basic case surveillance has been enhanced for some
infections by further characterization of the infecting pathogen in public health
laboratories. This voluntary case surveillance was first begun for Salmonella.
Following large, multistate outbreaks of salmonellosis early in the 1960s, health
department laboratories in states and large cities began to serotype strains of
Salmonella isolated from humans; the results of this subtyping were shared with
CDC as well in order to detect outbreaks affecting more than one state. Since
1962, national Salmonella surveillance has depended on this serotype-based
reporting (Olsen et al., 2001~. These data have been critical to the detection of
many outbreaks of salmonellosis each year. Since 1990, these data have been
relayed electronically from states to CDC via the Public Health Laboratory Infor-
mation System (Bean et al., 1992~. In addition, since 1995 these data have been
routinely examined using an automated statistical outbreak detection algorithm
that compares current reports with the preceding 5-year mean number of cases for
the same geographic area and week of the year to look for unusual clusters of
infection (Hutwagner et al., 1997~. The usefulness of the outbreak algorithm is
limited by the timeliness of reporting and the high background rates of reporting
for common serotypes such as S. Typhimurium and S. Enteritidis. The greatest
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SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
sensitivity for Salmonella serotyping to detect meaningful clusters is for the rare
serotypes, whereas further differentiation is necessary for the most common sero-
types.
The utility of serotyping as an international designation for Salmonella sub-
types has led to its widespread adoption. In a recent survey, 61 countries reported
that they used Salmonella serotyping for public health surveillance (Herikstad et
al., 2002a). A new World Health Organization (WHO) website (WHO, 2002)
collects and presents the results of this serotyping. This website is a new mecha-
nism for the global surveillance of foodborne diseases.
Molecular subtyping is now expanding the power of surveillance to detect
outbreaks that appear as sporadic cases and is improving the ability of public
health authorities to investigate outbreaks by comparing the molecular "finger-
print" of bacterial strains associated with sporadic cases of a foodborne disease.
These new techniques can define subtypes within a single species or serotype and
provide useful strain differentiation for a growing number of pathogens
(Swaminathan et al.,2001~. State public health laboratories began using an assay
standardized at CDC for E. cold 0157:H7 after it proved useful in the 1993 West
Coast outbreak associated with the consumption of undercooked ground beef;
they have now expanded the use of this technique to common serotypes of
Salmonella such as Typhimurium and Enteritidis, and to Listeria monocytogenes
(Swaminathan et al., 2001~. Developing this capacity at the state level also en-
hanced rapid detection of multicounty clusters within the state (Bender et al.,
1997, 2001~. Standardized subtyping protocols have now been developed for
seven pathogens; next-generation, gene-based technologies are under develop-
ment.
Recently, PulseNet, a national network formed by linking all state public
health laboratories via the Internet, with a national database maintained by CDC,
made it possible to rapidly identify and investigate multistate clusters. Once a
cluster of infections caused by strains with the same fingerprint is identified,
rapid epidemiological investigation can determine whether the cluster is a true
outbreak with a common source. Laboratories at the Food and Drug Administra-
tion (FDA) and the U.S. Department of Agriculture (USDA) also participate in
this network so that isolates from foods and animals can be compared within the
system. It is noteworthy that Canada has already adopted a compatible system
and that the European network for laboratory-based surveillance of foodborne
infections, EnterNet, has similar plans. The participation of Canada, Europe,
Asia, and other regions could make it possible to detect multiregional clusters of
foodborne disease (Swaminathan et al., 2001~.
Monitoring levels of antimicrobial resistance in foodborne pathogens is an-
other form of subtype-based surveillance. Since 1996, the National Antimicrobial
Resistance Monitoring System (NARMS) for enteric bacteria, a collaborative
effort of CDC, USDA, and FDA, has been monitoring the prevalence of resis-
tance in Salmonella, Campylobacter, and other foodborne bacterial pathogens
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THE SCIENCE OF PUBLIC HEALTH SURVEILLANCE
33
isolated from humans, animals, and foods (Marano et al., 2000~. This type of
surveillance provides information about the trends in microbial resistance to
specific drugs, identifies the emergence of new resistance threats, and permits the
comparison of strains identified in various locations. This information is useful to
public health officials who are involved in controlling highly resistant strains, to
clinicians making treatment decisions, and to regulators who can better evaluate
the association between antibiotics used in animals or the environment and resis-
tance developed in human pathogens.
In summary, nationwide surveillance systems for cases of foodborne infec-
tion are valuable tools for defining trends, identifying outbreaks, and evaluating
food safety programs. In some situations, serotyping and subtyping of pathogens,
coupled with nationwide surveillance, provide an ideal system to link a cluster of
cases.
Considering that state and local public health systems provide the only nation-
wide population-based surveillance for foodborne diseases, and that outbreak
investigations are critical to identify new pathogens and new food safety hazards,
the committee recommends that foodborne outbreak investigation and reporting
by state and local health departments be enhanced. Training and personnel and
laboratory support should be provided to enable rapid, thorough, and accurate
investigation and reporting of foodborne outbreaks by local and state health
departments, with performance evaluated through systematic review of outbreak
reports. In addition, timely analysis and dissemination of results to regulators,
industry, and the public is essential. Time series analysis (as discussed in Chapter
3) would also be a valuable analysis technique in this area.
Sentinel Site Surveillance
In contrast to the national umbrella of routine notifiable disease surveillance
supplemented with public health laboratory subtyping, a different strategy, senti-
nel site surveillance, can provide more detailed information about specific ill-
nesses that are likely to be foodborne. This strategy was first developed for
monitoring cases of hepatitis, for which detailed laboratory and epidemiological
data are crucial (Bell et al., 1998~.
A more recent example of this type of surveillance is the Foodborne Disease
Active Surveillance Network (FoodNet), a collaborative program of CDC, senti-
nel sites (currently nine sites), USDA, and FDA under the aegis of CDC's
Emerging Infections Program (Angulo and Swerdlow, 1999~. The establishment
of FoodNet was stimulated by a request from USDA's Food Safety and Inspec-
tion System (FSIS) for a system to ascertain the public health impact of USDA's
Pathogen Reduction; Hazard Analysis and Critical Control Point Final Rule
(PR/HACCP rule). FoodNet began with an initial five-site area in 1996 and
expanded to nine sites by 2001. The current surveillance area covers 37.8 million
persons, or approximately 13 percent of the U.S. population (CDC, 2002a).
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SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
FoodNet conducts active case ascertainment for foodborne diseases, accompa-
nied by epidemiological studies designed to help better understand the epidemi-
ology of foodborne diseases in the United States. Active case ascertainment
means that public health authorities regularly contact clinical laboratories to
obtain case reports of diagnosed illnesses; therefore, the results do not depend on
which infections are locally notifiable or on local resources available for surveil-
lance. Thus, because reporting is more uniform and complete, active case ascer-
tainment yields better data than passive reporting systems. However, it is also
more expensive and limited in geographic scope. In addition to case ascertain-
ment, FoodNet surveys laboratory, physician, and patient practices that cause an
individual case to be diagnosed. Also, FoodNet has been a platform for conduct-
ing case-control studies of sporadic infections in order to identify general risk
factors for infection that distinguish the persons who get ill from those who stay
healthy. This information has been used to better define the burden of foodborne
illness (Mead et al., 1999), to evaluate the risk factors for specific infections (e.g.,
in the Campylobacter case-control study [Friedman et al., 2000bj), and to track
the trends in major foodborne infections (CDC, 2002a).
To provide real-time tracking of human case surveillance, the committee
recommends that the capacity of the sentinel sites of FoodNet to rapidly inter-
view (i.e., as soon as possible after the case is diagnosed, as opposed to two to
three weeks later when active surveillance contacts with the laboratory detect the
case, a cluster is identified, or some other event shows the need for follow-up)
individual illness cases that are potentially foodborne, to track real-time inter-
views, and to collect and subtype Listeria, E. cold 0157:H7, and Salmonella
isolates from human infections, be enhanced as soon as feasible. (Although sev-
eral subtyping schemes exist for Campylobacter, none has yet been shown to be
useful and practical in the public health setting for routine testing of all isolates.)
All cases of infection from pathogens covered by FoodNet surveillance should be
interviewed. In addition, the committee believes that international collaboration
and the sharing of methods and microbiological and illness surveillance data
between the United States and other surveillance systems such as WHO's Global
SalmSurv (WHO, 2002) and Europe's EnterNet must be strongly supported.
Foodborne Outbreak Reporting
A foodborne outbreak is a cluster of two or more similar infections that are
shown by investigation to result from ingestion of the same food (Olsen et al.,
2000~. Local and state health departments conduct most foodborne outbreak
investigations. Since 1967, CDC has collected reports of outbreaks of foodborne
illnesses investigated by local, state, and national public health authorities (Olsen
et al., 2000~. Reports of outbreaks include the nature of the pathogen or toxin, the
type of food that caused the outbreak, and some information about factors that
contributed to the outbreak. Before 1998, these reports were collected on paper
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THE SCIENCE OF PUBLIC HEALTH SURVEILLANCE
35
and slowly reviewed and compiled. The system is now being overhauled with an
improved form, the active solicitation of reports from states, the introduction of
Internet-based reporting (Electronic Foodborne Outbreak Reporting System), and
the more rapid analysis and dissemination of results (FDDB, 2002a).
The foodborne outbreak surveillance system has provided useful informa-
tion on long-term trends for many pathogens for which surveillance otherwise
does not exist, as well as summaries of the outbreaks caused by a particular
pathogen, hazard, or food (Bean and Griffin, 1990~. In the future, it may provide
more systematic detection of clusters of outbreaks, based on both laboratory
testing and epidemiological assessment of the outbreak presentation (Hall et al.,
2001~. The committee considers the systematic analysis of information on out-
breaks gathered through this system as an effective tool for allocating the burden
of many infections and other hazards across broad food categories.
Specialized Surveys of Behavior
FoodNet and other surveillance efforts also provide systematic data on be-
havior of the population and exposure to specific risks. Studies conducted through
the CDC Behavioral Risk Factor Surveillance System (BRFSS) documented the
high frequency of risky food behavior (Yang et al., 1998~. More recently, FoodNet
population surveys have provided population-based data on the incidence of
diarrhea! illness and the likelihood of seeking medical care for a diarrhea! illness;
this information was critical to develop a general estimate of the burden of
foodborne disease (Herikstad et al., 2002b; Mead et al., 1999~. The surveys also
provided general population-based data on the frequency of exposure to a wide
variety of foods and other potential sources of intestinal infection (Consumer
Studies Branch, 2002; FDDB, 2002b).
Another potential source of information is the complaint systems maintained
by local and state health departments to which individuals can report illnesses or
hazardous conditions they believe may be related to food (Samuel et al., 2001~.
While such systems are far less specific than systems built on diagnosed cases of
illness, they may provide an early warning of problems.
Limitations of Surveillance
One limitation inherent in all surveillance systems is that many cases go
unrecognized for a variety of reasons. For example, cases may not be detected
because people who are ill do not seek medical care, physicians and laboratories
may not make a specific diagnosis, diagnosed cases may not be reported to
authorities, and authorities with limited resources may not investigate or report
cases. This last factor becomes especially significant if the surveillance program
is voluntary, as is the case with outbreak reporting by local and state agencies.
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SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
Data collected in this voluntary manner do not correspond to a nationally
representative sample of the population because reporting depends on other vari-
ables, such as local resources or whether a particular disease is notifiable (CDC,
2001~. Even in active surveillance programs, such as FoodNet, the number of
cases is underestimated because people do not seek medical care or because cases
are reported only when they are confirmed by a laboratory. Therefore, the actual
number of cases that occurs is likely to be substantially greater than the number
of cases reported. For example, it has been estimated that 38 cases of salmonellosis
occur for every 1 that is reported (Voetsch et al., 1998~. Many outbreaks are also
likely to be unrecognized. A common-source outbreak in a restaurant may not be
recognized because patrons were exposed in small groups that were unknown to
each other. For some foodborne infections, the incubation period may be long
enough to obscure the relationship with the meal unless persons attending a large
gathering, such as a banquet or wedding reception, have some reason to compare
their experiences afterwards.
A second limitation is the difficulty in attributing a specific case to a specific
food. Many infections can be transmitted by a variety of foods and by routes
other than food. In the sporadic case of illness, the person may have consumed
many foods and may have had other potentially risky exposures in the days
preceding illness, making it difficult to determine the source of the illness. In an
outbreak setting, where careful comparison of food consumption patterns of a
group of ill persons with those of a group who remained well can identify the
immediate food vehicle, it is still difficult sometimes to determine which of the
various ingredients was the source of the illness. However, many outbreak inves-
tigations are definitive, and comparison of patterns observed among groups of
outbreaks can help define patterns.
Finally, surveillance can only count what is measurable and known. Because
diagnosis of Norwalk-like virus (recently designated "Noroviruses") infections is
not routinely performed in clinical laboratories, for example, this extremely com-
mon illness cannot be monitored with the same type of case-based surveillance
that is conducted for infections caused by Salmonella or Campylobacter, for
which routine diagnostic tests are available. The importance of Norwalk-like
virus infections can be defined from outbreaks where the typical combination of
signs, symptoms, incubation period, and duration of illness can be documented
and where specimens reach specialized laboratories that can make the diagnosis
(Bresee et al., 2002~. Similarly, enterotoxigenic E. colt, the cause of much travelers'
diarrhea, is increasingly recognized as a cause of outbreaks in the United States,
but may also be an unrecognized common cause of sporadic cases because the
specialized tests to detect it are rarely applied (Dalton et al., 1999; Guerrant et al.,
1990~. It is likely that there are many foodborne disease agents yet to be dis-
covered which, consequently, are not currently tested for in any laboratory (Tauxe,
1997~.
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THE SCIENCE OF PUBLIC HEALTH SURVEILLANCE
37
Similarly, behavioral risk-factor surveillance is subject to limitations. This
type of surveillance depends on what people can and will report. People may
overestimate how often they perform socially desirable behaviors such as hand
washing. Questions about risk exposures also depend on what the consumer
observes. People are not likely to know if the food they ate was cross-contaminated
in the kitchen, even if they prepared it themselves. The observations individuals
can make may be a less-than-perfect measure of risk. Although the FoodNet
population survey used consumption of pink ground beef as an assessment of
cooked meat doneness and safety, research has clearly demonstrated that cooked
meat color is not an acceptable indicator for these parameters (Berry et al.,1998;
FSIS, 1998a, 2000; Hunt et al., 1999~. Premature browning and a persistent pink
color are two conditions that can occur in ground beef patties, influencing inter-
nal beef patty color, whether or not a patty has been cooked to an internal
temperature of 160°F (Hunt et al., 1999; Killinger et al., 2000~. In a nationwide
evaluation, Berry and coworkers (1998) found 47.4 percent of hamburgers cooked
to 160°F retained some pink color, and 15.8 percent still retained some pink color
when cooked to 175°F. In addition, more than 25 percent of fresh-cooked ham-
burgers (meat was never frozen) were brown or nearly brown internally although
hamburgers were only cooked to 150°F.
RESULTS FROM PUBLIC HEALTH SURVEILLANCE
The Burden of Disease
An estimation of the burden of disease is very useful when regulatory agen-
cies make decisions about the focus and allocation of resources. The burden of
disease attributable to foods has only been estimated in a general way; if the
estimate of this burden was specific for particular foodborne diseases and food
groups, more informed decisions could be made by regulatory agencies.
Information from surveillance has recently been integrated into a general
estimate of the overall burden of foodborne disease in the United States (Mead et
al., 1999~. This estimate included the number of cases, hospitalizations, and
deaths that were attributed to specific pathogens and to the large number of
illnesses that remain unaccounted for. These pathogen-based point estimates can
provide a benchmark for assessing the economic impact of foodborne diseases,
such as the $6.9 billion estimated cost to society from the diseases caused by the
major foodborne bacterial pathogens (Buzby and Roberts, 1996~. Some foodborne
infections can also cause chronic complications in a small percentage of cases;
for example, kidney failure related to E. cold 0157:H7 has been reported in 4 to 8
percent of cases (Griffin et al., 2002), and Guillain Barre syndrome paralysis may
complicate 1 in 1,000 Campylobacter infections (Nachamkin et al., 2000~. There
may be other complications of and sequelae from foodborne diseases. For ex-
ample, it has recently been reported that people infected with multiresistant
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SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
salmonellae are more likely to die in the 6 months following the infection than
uninfected individuals (Helms et al., 2002~. The full impact of illnesses includes
acute morbidity and mortality, as well as the impact of subsequent complications
and of long-term effects, such as life-long impairments from congenital toxoplas-
mosis or early childhood diarrhea! illnesses in impoverished areas (Guerrant et
al., 2002~. With more information about the frequency, duration, and disability
caused by these complications, the burden of foodborne illness could be reesti-
mated on a basis such as Disability Adjusted Life Years, a measure used to
characterize the burden of many other public health problems (Murray and Lopez,
1997).
Surveillance data can subdivide the burden of a specific infection. For ex-
ample, the contribution of specific Salmonella serotypes to the overall burden of
salmonellosis can be derived from their frequency. More specifically, the three
most common serotypes of Salmonella, Typhimurium, Enteritidis, and Newport,
together accounted for nearly half of all reported cases of salmonellosis in 2001,
and thus of the burden of salmonellosis (Table 2.1~.
The burden of reported foodborne outbreaks can also be measured. National
foodborne outbreak reporting from 1998 through 2000 gave a combined annual
incidence of 4.8 outbreaks per 1 million persons in the population (FDDB, 2002a).
However, in addition to the limitations mentioned above, measuring the burden
of disease due to outbreaks presents special challenges. For example, small out-
breaks are particularly likely to go unrecognized and unreported, and it is likely
that outbreak surveillance undercounts the true frequency of events for the rea-
sons noted earlier. Moreover, a substantial fraction of outbreak investigations do
not determine either the causative agent (the etiology) or the specific food that
TABLE 2.1 The Top Ten Salmonella Serotypes Reported from Humans in 2001
Rank Serotype Number of Reported Cases Percentage of the Total
1 Typhimurium 6,999 22.1
2 Enteritidis 5,614 17.7
3 Newport 3,158 10.0
4 Heidelberg 1,884 5.9
5 Javiana 1,067 3.4
6 Montevideo 626 2.0
7 Oranienburg 595 1.9
8 Muenchen 5 8 3 1.8
9 Thompson 514 1.6
10 Saint Paul 469 1.5
Subtotal 21,509 67.9
Total 3 1,675
SOURCE: FDDB (2002c).
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SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
and the major seed distributors put these instructions on the seed packages. Since
then, outbreaks of salmonellosis have been linked to a sprout producer that reported
disinfecting the seeds following those guidelines (Proctor et al., 2000), as well as
to a sprout producer using less chlorine than recommended (Winthrop et al.,
2003~. Another recent outbreak involved a single lot of clover seed shipped to
two sprout producers in Colorado (Brooks et al.,2001~. The first did not disinfect
the seed before sprouting and caused 1.13 documented infections per 50 lb-bag of
seed sprouted, whereas the second did disinfect the seeds and caused only 0.29
infections per bag of seed. These outbreaks show that the disinfection strategy
works partially, but is by itself insufficient to completely protect the public. In
addition to disinfection, FDA also recommended lot-by-lot testing of the irriga-
tion water for Salmonella (FDA, l999~. One outbreak occurred that was linked to
sprouts that had passed such a test, suggesting that false negative tests may occur
(Winthrop et al., 2003~. Continued surveillance and investigation indicate that the
challenge of preventing outbreaks of salmonellosis from sprouts has been par-
tially met, but complete prevention has still not been achieved.
Multidrug-Resistant Salmonella Newport and Foods of Bovine Origin
One of the latest food hazards to emerge in the United States is a new and
highly resistant strain of S. Newport (Zansky et al., 2002~. This strain was first
identified through NARMS surveillance in 1998, and its detection increased
rapidly in 1999 and 2000. The strain is resistant to at least nine antibiotics because
it possesses a large plasmid bearing several resistance genes, including an unusual
gene, the AmpC cmy2 gene, which confers resistance to most cephalosporins. In
2001, a retrospective study of these strains in Massachusetts identified the same
strains in ill and dying dairy cattle, and showed that visiting or working on dairy
farms was a risk factor for illness (Gupta et al.,2001~. Later that year, an outbreak
in Connecticut was traced to traditional cheese made from insufficiently pasteur-
ized milk from Massachusetts dairy farms (McCarthy et al., 2002~. In 2002, an
investigation of a multistate cluster of cases in the Northeast linked the illness to
eating ground beef traced to meat from a single slaughter plant (Zansky et al.,
2002~. Surveillance of human infections indicates a sharp increase in S. Newport
infections, which in 2001 represented 10 percent of human salmonellosis (FDDB,
2002c). Many of the S. Newport strains are multidrug resistant (CDC, 2002b).
The same organism has been detected since 1998 among isolates from animals,
including bovines (Fedorka-Cray et al., 2002~. Among S. Newport isolated from
cattle in 2000,74 percent had the AmpC multidrug resistance profile (ARS, 2002~.
The evidence to date indicates that this strain has spread in epidemic fashion
among cattle herds and that it affects the animals themselves, persons in contact
with the animals, and consumers of bovine products (including meat, cheese, and
other foods). Once control measures begin, success can be measured by monitor-
ing animals and meat for this strain, by trends in human illness, and by outbreak
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THE SCIENCE OF PUBLIC HEALTH SURVEILLANCE
59
surveillance. Surveillance activities in animals, meat, and poultry can also pro-
vide early warning of the spread of this strain or its plasmid to other food-animal
populations.
ANTICIPATING THE FUTURE
In the future, it can be expected that new pathogens and new foodborne
modes for transmission of such pathogens will continue to be recognized. New
diagnostic strategies will identify some pathogens that currently are often or
completely missed. Globalization of the food supply and concentration of food
production, in turn, will create new challenges for detection, investigation, con-
trol, and prevention of microbial foodborne hazards.
The committee concludes that enhanced public health surveillance for human
foodborne illnesses will be vital to identify and investigate these new challenges.
In addition, it believes that a flexible monitoring system is needed that permits
comparison of information from multiple points in the food supply. Just as moni-
toring individual cattle at slaughter is an important strategy for documenting the
continuing absence of bovine spongiform encephalopathy, a system for docu-
menting the frequency of microbial or other foodborne hazards at the point of
slaughter or processing could be critical to assessing and controlling these haz-
ards in the future. Systematic surveys of potential hazards, such as the appearance
of antibiotic resistant microbial strains in live animals in production, already
provide information useful to industry, regulators, and the public health sector. In
the future, similar systematic surveys of microbial contamination in various cat-
egories of processing plants and at various points along processing lines could be
equally useful for risk assessors. Preventing or minimizing contamination early
in the chain, as well as identifying foods at higher risk of being contaminated so
that they can be diverted out of the raw product market and into safer processing,
may become the norm. For some foods, irradiation and other terminal microbial
decontamination steps hold great potential (Tauxe, 2001~. High-pressure process-
ing, for example, is already commercially available. Preventing foodborne dis-
ease means preventing contamination before food reaches the consumer. Risk-
management policies applied throughout the food system on farms, fisheries,
and orchards; in slaughter facilities and processing plants; during transportation
and storage; and in retail food stores, food service establishments, and homes-
are all key parts of food safety.
For certain products, it may be possible to define varying levels of process-
ing depending on microbiological and other markers of the risk that they are
contaminated. Already, eggs that are cracked or that come from farms contami-
nated with S. Enteritidis are routinely approved for marketing after pasteuriza-
tion; milk for manufacturing purposes meets standards that are different from
Grade A milk; and an occasional carcass is passed for cooking rather than being
OCR for page 60
60
SCIENTIFIC CRITERIA TO ENSURE SAFE FOOD
allowed to go through standard slaughter. In the future, such treatments of higher-
nsk food may be a useful tool for achieving pathogen reduction in other foods.
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Representative terms from entire chapter:
food safety
