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9139
Rev. sci. tech. Off. int. Epiz., 1997,16 (2), 331-336
The role of epidemiology in public health
P.C. Bartlett & L.J. Judge
College of Veterinary Medicine, Michigan State University, Veterinary Medical Center, East Lansing,
Michigan 48824-1314, United States of America
Summary
Epidemiology is the study of disease in populations. Veterinarians and others
involved in the preventive medicine and public health professions use
epidemiological methods for disease surveillance, outbreak investigation, and
observational studies to identify risk factors of zoonotic disease in both human
and animal populations. Knowledge of these risk factors is used to direct further
research investigation and to implement disease control measures. The use of
hazard analysis critical control point (HACCP) systems depends greatly on
information produced by epidemiological studies. Epidemiological methods are
used for disease surveillance to identify which hazards are the most important.
Epidemiological studies are also used to identify risk factors which may represent
critical control points in the food production system.
Keywords
Epidemiology - Food hygiene - Public health - Risk factors - Zoonoses.
What is epidemiology?
The word 'epidemiology' comes from the Greek epi meaning
'upon' and demos meaning 'people or populace'. As such, it is
the study of 'that which is upon the people'. It was infectious
disease which was upon the people when the science of
epidemiology began, and if one thinks that the importance of
infectious disease is long past, one should read The coming
plague by L. Garrett (3).
Although many different definitions of epidemiology are
available in epidemiology texts, perhaps the simplest
description is that it regards the 'mass phenomena of disease'
(4, 5, 6 ) . Epidemiologists examine disease in populations
rather than in individual organisms (as the clinician would
do), or in individual cells (the histologist or microbiologist),
or in individual molecules (the geneticist or molecular
biologist). Many veterinary specialities are reductionistic in
that they learn about the world by focusing their attention on
the details of restricted areas of interest. When viewing an
unknown object through a microscope, some people will
switch to a higher power to see the object in more detail. Most
epidemiologists would probably switch to a lower power to
see more of the object in relation to its surroundings.
Epidemiologists are 'big picture people' (holistic) by nature, in
that they seek to understand disease by moving back from the
individual molecule, cell, or host organism to see the disease
in the population.
Veterinary epidemiology
For many years, epidemiologists at most veterinary schools
were also the public health specialists, and thus the two
specialities were inseparably linked. Although veterinary
epidemiologists sometimes conducted studies on nonzoonotic animal diseases, most had their training firmly
rooted in veterinary public health. Recently, a new breed of
veterinary epidemiologist has been created. This breed has no
public health background and uses epidemiological
techniques to investigate food animal production. After
experimenting with different labels such as herd health,
production medicine, preventive medicine and population
medicine, it has become widely recognised that it is the
statistical, population-oriented approach which defines the
discipline, not the species of host animal being studied. As
such, veterinary epidemiology is one discipline, regardless of
which host species or disease is being investigated.
Every few years, a debate is launched in the veterinary world
regarding the use of the words 'epidemiology, epidemic and
332
endemic' as applied to diseases in animal 'populations.
Although the historical origins of these words clearly relate to
disease in human populations (demos meaning people), most
veterinary epidemiologists believe that it is pointless to use
different words (epizootiology, epizootic, enzootic) when
referring to a disease in an animal population. The word
'blink' is used when a person, giraffe, snake, or any other
animal briefly closes both eyes. Similarly, people and animals
all have legs, livers, diabetes, and pneumonia: few would
suggest that we need to separate words for such things in
animals. Similarly, the creation of a separate set of words to
describe disease in populations of animals is unnecessarily
complex and serves no useful purpose. The words
'epidemiology, epidemic and endemic' should be used to
describe disease occurrence in all host species.
Health experts of the production
environment
R.K. Anderson has written a classic and comprehensive work
on the role of veterinarians in public health (1). This book
advances the argument that virtually all veterinarians of all
specialities are involved in some aspect of public health
service to the community. The food animal private
practitioner, in particular, is regarded as the health expert of
the food animal production environment, in which all
diagnoses are seen as screening for zoonotic disease, and
therapy, disease control, and disease prevention are actions
taken to reduce exposure of humans to zoonotic disease. The
veterinary practitioner is in a position to provide advanced
warning of potential zoonotic or environmental human
disease problems. Public practice veterinarians are often
extremely dependent on the observations of private practice
colleagues in the food production environment to give
warning when something with potential human health impact
is discovered. From this perspective, all food animal
veterinarians are involved in public health and are the first
line of defence in food safety. Trained in epidemiological
principles, the practitioner uses these and other tools to
monitor disease in the food animal population, diagnose
population disease problems, reduce exposure of humans to
zoonotic disease and provide for safe use of antibiotics and
other substances (pesticides, hormones) with potential
human health impact.
The tools of epidemiology are not restricted to veterinary
epidemiologists alone; they can be used by all veterinarians.
Unfortunately, many veterinarians forget that the tools of
epidemiology are available. Until recently, food hygiene
veterinarians used the tools of epidemiology to convince
colleagues and the public that the current meat inspection
system was not focusing on the appropriate health hazards,
and that the system would need to be changed (2).
Rev. sci. tech. Off. int. Epiz., 16 (2|
Types of epidemiological studies
'You can observe a lot just by watching', Yogi Bera once said.
Observational epidemiologists like to watch. They are
'biological accountants' in that they keep track of what they
see, and look for patterns and trends in the occurrence of
disease which might suggest the cause(s) of particular
diseases, and how these might be controlled.
Disease surveillance systems provide the data needed to
determine the. health status of populations. The population is
the patient for the epidemiologist and disease surveillance
resembles the physical examination. Each disease surveillance
system has advantages and disadvantages. For example, in the
United States of America (USA), human cases of zoonotic
foodborne diseases such as salmonellosis and Escherichia coli
0 1 5 7 : H 7 infection are reported by clinical laboratories and
physicians to local health departments, which then forward
the reports to State health departments, which in turn
centralise the data at the Centers for Disease Control and
Prevention. At each level of this pathway, epidemiologists
(including veterinary epidemiologists) are, or should be,
watching for patterns in the data. Such patterns might include
time clusters, geographical clusters, cycles and trends, which
could lead to disease control efforts and/or epidemiological
hypotheses which can then be tested with additional studies.
A disadvantage of this system is that fewer than 1 0 % of
human culture-confirmed cases of Salmonella, for example,
are reported through the system. Another disease surveillance
system is maintained by slaughter inspection systems,
although usually only rates of condemnation for very
non-specific reasons are available from this system.
Some disease surveillance systems measure incidence, and
others measure prevalence. Incidence is the occurrence of
new cases over a period of time, and prevalence is the number
of existing cases at one point in time. Epidemiologists have a
strong interest in the differences between incidence and
prevalence. In fact, many veterinarians probably think that,
along with calculating sensitivity and specificity, this is all that
epidemiology is about.
The case-control study is usually the first type of
epidemiological study conducted to determine which agent,
host, or environmental factors are associated with the
occurrence of a disease. Factors found to be associated with
disease are called risk factors. Identification of risk factors can
lead to increased understanding of the disease process and
possibly enactment of disease control measures long before
more detailed information about the cause of the disease can
be confirmed. For example, human cases of salmonellosis
found to be associated with a particular food item may lead to
the closure of a particular restaurant, or recall of the food item
from the market. An observed high prevalence of Listeria
contamination in hot dogs from plants using a particular
processing method may lead to identification of this
Rev. sci. tech. Off. int. Epiz., 16 (2)
333
Classic foodborne outbreak
investigation
Foodborne disease outbreaks have been investigated so many
times that a classic, recommended procedure has been
established. Cases (people or animals who meet the working
case definition of the disease) are summarised with regard to
symptoms and time of onset. An epidemic curve (Fig. 1) and a
tabulation of symptoms are made. If a hypothesised time of
exposure exists (such as a suspect meal), the mean and range
of incubation periods (time from exposure to disease onset)
can be calculated. The investigators should obtain food
histories (a list of food items eaten by each person) from all
people at the suspect meal, not just from those who were ill.
The most common mistake is to only obtain food histories
from the people at the meal who were ill. The resulting data
can then be tabulated as shown in Table I. An attack rate can
be calculated for those who ate each food item (number of ill
who ate a particular food divided by the total number who ate
that food), and an attack rate for those who did not eat each
food item (number of ill who did not eat a particular food
divided by the total number who did not eat that food item).
The most highly suspect food item will have a high attack rate
for those people who ate the particular food item, and a low
attack rate for those who did not eat the particular food item.
Calculation of relative risk or attributable risk for each kind of
food is a good way of finding the most highly suspected food.
A relative risk for each food item can be obtained by dividing
one attack rate by the other. An attributable risk for each food
item can be obtained by subtracting one attack rate from the
other. A chi-square test can be used to evaluate the possibility
that the difference in measured attack rates might be due to
chance alone. In the example given in Table I, turkey is the
most highly suspect food item, as this meat has both the
highest relative risk and the highest attributable risk.
Disease outbreak investigation is one of the most useful
methods of learning about disease in populations. Disease
outbreaks can be 'experiments of nature'. Due to ethical
considerations, the infection of people (or sometimes animals)
with a particular disease agent in order to learn about the
disease is not possible. Therefore, the examination of a
naturally-occurring 'experiment of nature' is often the only
way investigators can study the epidemiology of a given
disease.
The basic principles of the case control study can be applied
equally well to risk factors other than food histories. For
example, a food hygiene veterinarian may use the same
technique to identify particular carcass characteristics
associated with bacterial contamination, certain processing
Number of new cases
processing step as a 'critical control point' which could
become the basis for regulatory action.
-3
Date of onset of disease (January 1997)
Fig. 1
Onset of foodborne illness in people eating suspect meal
methods associated with contamination, or meat from
particular herds associated with increased rates of disease
among consumers. Veterinary clinicians - and others
interested in pre-harvest food safety - can also look for
associations using the methods described in Table I, except
that the suspected risk factors would not be particular food
items on the menu but might be factors such as gender, age,
exposure to a group of infected cattle, exposure to a particular
wildlife reservoir, etc. Delineation of important risk factors in
the production environment may help determine the
epidemiology of a particular disease agent on the farm.
However, unless the relative risk of an on-farm factor is
extremely high, control of this factor might reduce the level of
contamination in the human food source but would probably
never eliminate contamination to the extremely low safety
levels required for human consumption. Therefore,
pre-harvest control measures will need to be used in
conjunction with control measures at other levels of the
food-processing pathway.
'Cohort' or prospective studies are usually more expensive
and take longer to conduct than do case-control studies. An
example of a cohort study would be a comparison of the
contamination rate in samples of meat produced during the
warm day shifts to the contamination rate of meat produced
during the cool evening shifts at a processing plant. In a
cohort study, the two groups (cohorts) to be compared are
identified before the beginning of the study, and the cohorts
are then monitored for disease occurrence.
Controlling bias
Veterinarians also become involved in experimental studies,
in which risk factors are randomly and blindly assigned to
study participants before the monitoring period commences.
Such assignment tends to balance the study groups with
respect to all other factors which might otherwise affect (bias)
the measurement of the study outcome. These other factors
334
Rev. sci. tech. Off. int. Epiz., 16 (2)
Table I
Typical food history from a foodborne outbreak
For each food item, attack rates are calculated for those who ate the item, and for those who did not eat the item. The most highly suspect food item will be the
one with a high AR for those who ate the item, and a low AR for those who did not eat it. This is usually determined by dividing the two ARs to obtain a relative
risk, or subtracting them to obtain an attributable risk
Food item
Carrots
Turkey
Beans
Bread
Milk
Peas
Fish
Gravy
No. of people who ate a
food item
No. ill
Total
6
9
9
3
9
1
3
8
12
11
19
8
18
3
4
11
AR
50
82
47
37
50
33
75
73
No. of people who did
not eat a food item
No. ill
Total
AR
4
1
1
7
1
9
7
2
8
9
1
12
2
17
16
9
50
11
100
58
50
53
44
22
Relative risk
Attributable risk
1.0
7.45
0.47
0.64
1.0
0.62
1.7
3.3
0
71
-53
-21
0
-20
31
51
AR: attack rate (%|
(confounders) can cause large problems in observational
studies. In an observational study, selection bias occurs when
the manner in which study participants are selected for
membership in a study group (case versus control or risk
cohort versus control cohort) predetermines the study
outcome. Informational bias occurs when the way in which
the study outcome is measured systematically affects the study
results.
Confounding bias occurs when a third factor is associated
with both the study factor and the study outcome. For
example, suppose one is trying to study the relationship
between cow size and the rate of pneumonia. Housing type
(indoor versus outdoor) might be a confounding variable,
since larger cows tend to be dairy cows housed inside where
the rates of pneumonia are probably greater. A random
sample of large cows in a particular country may therefore
predominantly contain indoor cattle. A comparison group of
small cows may contain predominantly beef cattle living
outdoors. While investigators may think that they are
comparing large cattle to small cattle, they are also
simultaneously comparing indoor cattle to outdoor cattle,
since housing and cattle size are associated. The comparison
between cattle size and pneumonia is thus confounded by
housing type. Much of what epidemiologists do is to use
various techniques such as stratification, matching, restriction
and multivariable statistics to un-confound the results of
observational studies.
Clinical epidemiology
Clinical epidemiology is a subspeciality which emphasises
those epidemiological tools used in support of clinical
medicine (5). Much of clinical epidemiology relates to
interpretation of diagnostic test results and evaluation of
treatment efficacy, although considerable overlap exists with
subject areas long considered the core science of
epidemiology.
Statistician or epidemiologist?
Epidemiologists should be experts in study design, data
collection, and control of bias. Statistical analysis is only one
of the duties of an epidemiologist, and frequently is not the
most important duty. In obtaining the correct answer to the
study question, quality of data is usually more important than
the sophistication of the statistical analysis. Although
analytical epidemiologists occasionally like to impress each
other with new and complicated statistical approaches, the
more mundane matters regarding accurate and precise data
collection probably have a greater impact on whether or not
the study results accurately represent reality.
Conclusion
Epidemiological methods are frequently used by food hygiene
and public health veterinarians to determine relevant risk
factors associated with disease occurrence. Knowledge of
these risk factors is used to direct further research
investigation and to implement disease control measures. The
following papers in this issue give information about hazard
analysis critical control points. Bear in mind that
epidemiological tools are used for disease surveillance to
identify which hazards are the most important, and
epidemiological studies are used to identify risk factors which
may represent critical control points in food production
systems.
•
335
Rev. sci. tech. Off. int. Epiz., 16 (2)
Rôle de l'épidémiologie en santé publique
P.C. Bartlett & L.J. Judge
Résumé
L'épidémiologie est l'étude des maladies dans une population donnée. Les
méthodes épidémiologiques sont utilisées par les vétérinaires ainsi que par les
professionnels de la médecine préventive et de la santé publique lors des
programmes de surveillance des maladies, des enquêtes sur les foyers et des
études appliquées en vue d'identifier les facteurs de risque de zoonoses pour les
populations humaine et animales. La connaissance de ces facteurs de risque
permet d'orienter les enquêtes ultérieures et de mettre en œuvre des mesures de
prévention. L'utilisation de la méthode de « l'analyse des risques, points critiques
pour leur maîtrise » (hazard analysis and critical control point: HACCP) dépend
dans une large mesure des données épidémiologiques disponibles. Les méthodes
épidémiologiques permettent également de détecter les risques les plus
importants lors des programmes de surveillance des maladies. Enfin,
l'épidémiologie contribue à identifier les facteurs de risque pouvant représenter
des points de contrôle critiques dans le système de production agro-alimentaire.
Mots-clés
Épidémiologie - Facteurs de risque - Hygiène alimentaire - Santé publique - Zoonoses.
El papel de la epidemiología en la salud pública
P.C. Bartlett & L.J. Judge
Resumen
La epidemiología es el estudio de una enfermedad en el seno de una población
dada. Los veterinarios y otros profesionales de la medicina preventiva y la salud
pública se sirven de métodos epidemiológicos para la vigilancia sanitaria, la
investigación de brotes infecciosos y los estudios sobre los factores de riesgo de
enfermedades zoonóticas en poblaciones tanto humanas como animales. El
conocimiento de estos factores de riesgo sirve para orientar ulteriores
investigaciones y para aplicar medidas de control de enfermedades. La
utilización de sistemas de análisis de riesgos y control de puntos críticos (hazard
analysis and critical control point: HACCP) depende en buena medida de la
información que suministran los estudios epidemiológicos. También se emplean
métodos epidemiológicos para llevar a cabo acciones de vigilancia sanitaria
destinadas a identificar los riesgos más importantes y para determinar factores
de riesgo que puedan constituir puntos críticos de control de los sistemas de
producción agroalimentaria.
Palabras clave
Epidemiología - Factores de riesgo - Protección de alimentos - Salud pública - Zoonosis.
336
Rev. sci. tech. Off. int. Epiz, 16 (2)
References
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3.
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of veterinarians as they relate directly to human health.
University of Minnesota School of Public Health,
Minneapolis, Minnesota, 1-13.
Committee on the Scientific Basis for our Nation's Meat and
Poultry Inspection Program (1985). - Meat and poultry
inspection: The scientific basis of the nation's program.
National Academy Press, Washington, DC, 209 pp.
Garrett L. (1994). - The coming plague: newly emerging
diseases in a world out of balance. Virago Press, London,
750 pp.
4.
Martin S.W., Meek A.H. & Willeberg P. (1987). - Veterinary
epidemiology: principles and methods. Iowa State University
Press, Ames, Iowa, 343 pp.
5.
Smith R. (1995). - Veterinary clinical epidemiology: a
problem-oriented approach, 2nd Ed. CRC Press, Ann Arbor,
Michigan, 1.
6.
Thrusfield M. (1986). Veterinary
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epidemiology.
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