Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
Sunpetch Angkititrakul, DVM, MSc, PhD
Department of Veterinary Public Health
Faculty of Veterinary Medicine, Khon Kaen University
Introduction of Epidemiology
Public health is primarily concerned with the prevention of disease in human
populations, and epidemiology is the branch of public health which attempts to discover
the causes of disease in order to make disease prevention possible. It thus differs from
clinical medicine both in its emphasis on prevention (rather than treatment) and in its
focus on populations (rather than individual patients). Thus, the epidemiological
approach to a particular disease is intended to identify high-risk subgroups within the
population, to determine the causes of such excess risks, and to determine the
effectiveness of subsequent preventive measures. Although the epidemiological approach
has been used for more than a century for the study of communicable diseases,
epidemiology has considerably grown in scope and sophistication in the last few decades
as it has been increasingly applied to the study of non-communicable diseases. At the
st
beginning of the 21 century, the field of epidemiology is changing rapidly, not only with
regards to its basic methods, but also with regards to the hypotheses which these methods
are used to investigate. In particular, in recent years there has been a revival in public
health applications of epidemiology, not only at the national level, but also at the
international level, as epidemiologists tackle global problems such as climate change.
This text does not attempt to review the more complex methods used to study such issues.
However, it does provide a coherent and systematic summary of the basic methods in the
field, which can be used as a logical base for the teaching and development of research
into these more complex issues.
What is epidemiology?
The word epidemiology comes from the Greek words epi, meaning “on or upon,”
demos, meaning “people,” and logos, meaning “the study of.”
Epidemiology is the study of the distribution and determinants of health-related
states or events in specified populations, and the application of this study to control of
health problems.
This definition of epidemiology includes several terms which reflect some of the
important principles of the discipline. As you study this definition, refer to the description
of these terms below.
Study: Epidemiology is a scientific discipline, sometimes called “the basic
science of public health.” It has, at its foundation, sound methods of scientific inquiry.
Distribution: Epidemiology is concerned with the frequency and pattern of health
events in a population. Frequency includes not only the number of such events in a
population, but also the rate or risk of disease in the population. The rate (number of
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
events divided by size of the population) is critical to epidemiologists because it allows
valid comparisons across different populations.
Pattern refers to the occurrence of health-related events by time, place, and
personal characteristics.
 Time characteristics include annual occurrence, seasonal occurrence, and daily or
even hourly occurrence during an epidemic.
 Place characteristics include geographic variation, urban-rural differences, and
location of worksites or schools.
 Personal characteristics include demographic factors such as age, race, sex,
marital status, and socioeconomic status, as well as behaviors and environmental
exposures.
This characterization of the distribution of health-related states or events is one
broad aspect of epidemiology called descriptive epidemiology. Descriptive
epidemiology provides the What, Who, When, and Where of health-related events.
Determinants: Epidemiology is also used to search for causes and other factors
that influence the occurrence of health-related events. Analytic epidemiology attempts to
provide the Why and How of such events by comparing groups with different rates of
disease occurrence and with differences in demographic characteristics, genetic or
immunologic make-up, behaviors, environmental exposures, and other so-called potential
risk factors. Under ideal circumstances, epidemiologic findings provide sufficient
evidence to direct swift and effective public health control and prevention measures.
It is useful to distinguish epidemiological from clinical approaches to disease
management. The clinical approach to disease management is focused on individual
animals and is aimed at diagnosing a disease and treating it. It involves physical
examination and generation of a list of differential diagnoses. Further examinations,
laboratory tests and possibly response to treatment are then used to narrow the list of
differential diagnoses to a single diagnosis. In an ideal world this will always be the
correct diagnosis. The success of this approach depends on two conditions:
• That the true diagnosis is on the list of differential diagnoses; and
• Clinical signs arise from a single (disease process in the individual.
The epidemiological approach
The epidemiological approach to disease management is conceptually different in
that there is no dependency on being able to precisely define the etiological agent.
It is based on observing differences and similarities between diseased and non-diseased
animals in order to try and understand what factors may be increasing or reducing the risk
of disease.
In practice, clinicians unwittingly use a combination of clinical and
epidemiological approaches in their day-to-day work. If the problem is relatively clearcut then an epidemiological approach plays a very minor role. If the condition is new or
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
more complex then the epidemiological approach is preferred since it will provide a
better understanding of what makes individuals susceptible to disease and once these
factors are known the measures required to control the disease become better defined.
Field epidemiology
Field epidemiology is really a type of applied field research since we are trying to
uncover what exists in an uncontrolled situation. The field epidemiologist attempts to
gather and organize data to bring order and meaning to it when there is an urgent need for
it. Field epidemiology can be applied to disease outbreaks, situation assessments and
policy evaluation. Field epidemiology relies on a systematic approach to gather and
organize data in a way that will support a better understanding of a disease situation.
Once a disease agent(s) is/are identified a positive “case” is defined by establishing “case
definition”. Even if the agent is not yet identified the following basic disease methods of
disease control methods can be effectively to control the disease:
 Movement controls
 Stamping out
 Applying bio-exclusion and biosecurity principles
 Risk communication
 Vaccination may not likely be used for a new disease agent
Learning from history is important. While control measures can be taken in the
short term to control the disease, the field epidemiologist’s job is not only to help control
the disease but to understand the how the disease occurred in order to prevent it from
happening again in the future. This is a challenging task that uses both information and
data. Obtaining information from animal owners is the process that provides data. For
data to be useful, it must be collected, organized, summarized and reported in a
systematic way. What data needs to be collected?
Three initial questions need to be asked by the field epidemiologist so that the
appropriate data is collected:
1. How large is the disease problem? Where does it exist and where does it not exist?
First we seek and describe what we observe. Case finding and surveillance are key
activities.
2. How did the situation arise and what led to its presence? A thorough investigation is
required followed by preliminary analysis.
3. What can we do to better prevent and control the disease in the future? Further analysis
of the findings of investigations and studies is needed.
Health and Disease
Epidemiology is used to assess both the health and disease status of a population.
Epidemiology can be used to maximize the health of animals in order to increase milk,
meat or egg production which will benefit human health as well. Alternatively,
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
epidemiology is commonly used to prevention and control animal diseases affecting
either animals or humans requiring close collaboration between government and animal
producers.
At the population level disease can occur at different levels including sporadic, epidemic
or endemic patterns. These patterns may suggest some possible types of sources of the disease to
investigate more fully.
The Epidemiologic Triangle (Triad):
Disease is the result of complex interactions (some would say imbalance) between
the triad of the agent (toxic or infectious), the host and the environment. The components
of this interaction differ depending upon the specific circumstances of each group of
affected animals. Particularly for agricultural animals, this triad is strongly influenced by
husbandry and management factors, which are often the most important. For vector-borne
diseases, vector factors are also linked to the other factors.
Recognizing the different components of this triad is important because they are
the source of opportunities to reduce disease at multiple points in the transmission cycle.
A common mistake is to focus on only one aspect of the triad for disease control or
prevention and to overlook the others.
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
The host is the animal (or human) that may contract a disease. Age, genetic
makeup, level of exposure, and state of health all influence a host’s susceptibility to
developing disease. The agent is the factor that causes the disease (bacteria, virus,
parasite, fungus, chemical poison, nutritional deficiency etc) one or more agents may be
involved. The environment includes surroundings and conditions either within the host or
external to it that cause or allow disease transmission to occur. The environment may
weaken the host and increase its susceptibility to disease or provide conditions that flavor
the survival of the agent.
Agent Factor Examples:
 Dose
 Environmental hardiness
 Virulence (microbial)
 Infectivity (microbial)
 Toxicity (poisons)
Host Factor Examples:
 Innate resistance (e.g. gastric barrier, mucocilliary transport mechanism)
 Previous exposure
 Passive immune status (neonates)
 Vaccination status and response
 Age
 Gender
 Behavior (e.g. mutual grooming, dominance, pica)
 Production status (e.g., lactating vs. non-lactating)
 Reproductive status (e.g., pregnant vs. non-pregnant, sterile vs. intact)
 Genetics
Environmental Factor Examples:
 Animal stocking density
 Animal movement between groups
 Housing (e.g. ventilation, sanitation)
 Environmental conditions (e.g. temperature, humidity, wind velocity,
precipitation)
 Nutrition (protein, energy and macromineral and micromineral adequacy)
The Spectrum of Disease Severity
Because each group member is affected by one or more of the epidemiologic triad
factors differently, disease in a group is often manifested as a spectrum that can range
from inapparent to subclinical to clinical to fatal.
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Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
Subclinical means that signs of the disease cannot be detected without special
tests.
Clinical means that the signs of the disease can be detected during a normal
clinical exam.
Because the individual's body reacts to the disease, disease severity changes over
time. During disease progression in a herd, individuals exposed at different points in time
will be at different points in the natural history of the disease.
Status and Spectrum of Disease Severity
Exposure
Status
Unexposed
Infection
Status
Exposed
Uninfected
Infected
Subclinical
(Inapparent)
Disease
Status
Recovered
Clinical Disease (Apparent)
Morbidity
(Sickness)
Mild
Mortality
Severe
Fatal
Step in the disease process
1. Agent source
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Environment – land, water, air;
Live Animals – infected tissue (skin) and contaminated secretions (tears) and
discharges (oozing wounds, cuts);
Dead animals – carcasses;
Feed and Water;
Animal products – milk, meat, eggs, other;
Animal by-products – manure, feces, litter, offal (slaughter waste);
Introduced through human intervention (Iatrogenic);
Reservoir – includes wild animals, insects and other living sources of disease
agents for the population at risk prior to exposure;
Fomites – inanimate objects (clothing, equipment, vehicles) contaminated with
disease agents;
Vectors – insects and other living organisms can transmit disease agents either
mechanically or biologically where the agent replicates within the vector.
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
2. Exposure
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Initial introduction into the population
Transmission within the population (individual to flock/herd to larger population)
Direct transmission between infected host and susceptible host within the same
population or among different populations
 Horizontal
 Vertical e.g. Brucellosis, tuberculosis
Indirect transmission through contaminated clothing, equipment and vehicles
Marketing systems
Exposure dose of disease agent
Route of exposure (oral-fecal versus venereal) within host population
Animal density increases chances for secondary transmission in the PAR
Survival time of the disease agent in the environment and host
Frequency of exposure
Critical periods for apparent infections when clinical signs are observed:
Incubation period related to introduction of new animals or other contacts
The shedding period is developed through experimental studies
The period of clinical signs and recovery period can be observed visually when
evident; sometimes animals die very suddenly without observing previous
clinical signs
Carrier state can be assessed through sampling at time intervals
3. Host susceptibility
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Species, breed, strain
Age
Sex
Genetics
Animal management and husbandry
4. Outcome (clinical / subclinical)
It is important to measure disease outcomes using practical, available data including
existing records, conducting surveys and surveillance to determine the number of either incident
or prevalent cases. The ability to define a case appropriately will affect whether disease control or
disease eradication is possible. Basic measures (indices) used to assess health and disease
outcomes include productivity, morbidity and mortality. Important data that can be used in
production systems include production records, treatment records, and mortality records.
Outcomes can only be assessed if we collect these data.
Modes of transmission
After an agent exits its natural reservoir, it may be transmitted to a susceptible
host in numerous ways. These modes of transmission are classified as:
 Direct
o Direct contact
o Droplet spread
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
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Indirect
o Air-borne
o Vehicle-borne
o Vector-borne
 Mechanical
 Biologic
In direct transmission, there is essentially immediate transfer of the agent from a
reservoir to a susceptible host by direct contact or droplet spread. Direct contact occurs
through kissing, skin-to-skin contact, and sexual intercourse. Direct contact refers also to
contact with soil or vegetation harboring infectious organisms. Thus, infectious
mononucleosis (kissing disease) and gonorrhea are spread from person-to-person by
direct contact. Hookworm is spread by direct contact with contaminated soil. Droplet
spread refers to spray with relatively large, short-range aerosols produced by sneezing,
coughing, or even talking. Droplet spread is classified as direct because transmission is
by direct spray over a few feet, before the droplets fall to the ground.
In indirect transmission, an agent is carried from a reservoir to a susceptible host
by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries.
Most vectors are arthropods such as mosquitoes, fleas, and ticks. These may carry the
agent through purely mechanical means. For example, flies carry Shigella on appendages;
fleas carry Yersinia pestis (agent that causes plague) in the gut and deposit the agent on
the skin of a new host. In mechanical transmission, the agent does not multiply or
undergo physiologic changes in the vector. This is in contrast to instances in which an
agent undergoes part of its life cycle inside a vector before being transmitted to a new
host. When the agent undergoes changes within the vector, the vector is serving as both
an intermediate host and a mode of transmission. This type of indirect transmission is a
biologic transmission.
Causation
The basis for most epidemiological investigations is the assumption that disease
does not normally occur in a random fashion something causes it. As a result we can use
epidemiological investigations to identify causal relationships and potential risk factors.
Most scientific investigations are aimed at identifying cause-effect relationships.
Webster’s dictionary defines a cause as ‘something that brings about an effect or a
result.’ A cause of a disease is an event, condition, or characteristic which plays an
essential role in producing an occurrence of the disease. Knowledge about cause-andeffect relationships underlies every therapeutic manoeuvre in clinical medicine. The
situation is complicated if multiple causes are involved. Koch (1884) provided a
framework for identifying causes of infectious disease. He specified that the following
criteria (known as Koch’s postulates) had to be met before an agent could be considered
as the cause of a disease:
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
• The agent has to be present in every case of the disease.
• The agent has to be isolated and grown in pure culture.
• The agent has to cause disease when inoculated into a susceptible animal and
the agent must then be able to be recovered from that animal and identified.
In the late nineteenth century Koch’s postulates brought a degree of order and
discipline to the study of infectious diseases, although the key assumption of ‘one-agentone disease’ was highly restrictive (since it failed to take account of diseases with
multiple etiologic factors, multiple effects of single causes, carrier states, and non-agent
factors such as age and breed).
Based on John Stuart Mill’s rules of inductive reasoning from 1856, Evan
developed a unified concept of causation which is now the generally accepted means for
identifying cause-effect relationships in modern epidemiology. Evan’s unified concept of
causation includes the following criteria:
• The proportion of individuals with disease should be higher in those exposed to
the putative cause than in those not exposed.
• Exposure to the putative cause should be more common in cases than in those
without the disease.
• The number of new cases should be higher in those exposed to the putative
cause than in those not exposed, as shown in prospective studies.
• Temporally, the disease should follow exposure to the putative cause.
• There should be a measurable biologic spectrum of host responses.
• The disease should be reproducible experimentally.
• Preventing or modifying the host response should decrease or eliminate the
expression of disease.
• Elimination of the putative cause should result in lower incidence of disease.
The "Iceberg" phenomenon
In outbreaks of most disease in animal groups, both clinical cases (the tip of the
iceberg) and subclinical cases (unobserved beneath the ocean surface) are present in the
group. For many infectious agents, particularly those that are endemic, more of the
infections in a group are subclinical (silent) than are clinical. For some exceptions, such
as rabies, few if any subclinical infections occur and almost all if not all clinical
infections end in death. This iceberg concept of severity distribution also holds for most
induced, non-infectious diseases affecting a group, such as hypomagnesemia, ketosis and
hypocalcemia. Disease in an individual is often evidence of a group phenomenon because
the factors that caused the disease in that individual are usually affecting others adversely
as well.
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
Measure of health
One of the most fundamental tasks in epidemiological research is to quantify the
occurrence of disease. This can be done by counting the number of affected individuals
however, to compare levels of disease among groups of individuals, time frames and
locations; we need to consider counts of cases in context of the size of the population
from which those cases arose.
A ratio defines the relative size of two quantities expressed by dividing one
(numerator) by the other (denominator). Proportions, odds, and rates are ratios. Say we
have a herd of 100 cattle and 58 are found to be diseased. The odds of disease in this herd
are 58:42 or 1.4 to 1.
A proportion is a fraction in which the numerator is included in the denominator.
Say we have a herd of 100 cattle and 58 are found to be diseased. The proportion of
diseased animals in this herd is 58 ÷ 100 = 0.58 = 58%.
A rate is derived from three pieces of information: (1) a numerator: the number of
individuals diseased or dead, (2) a denominator: the total number of animals (or animal
time) in the study group and/or period; and (3) a specified time period. To continue the
above example, we might say that the rate of disease in our herd over a 12-month period
was 58 cases per 100 cattle.
The term morbidity is used to refer to the extent of disease or disease frequency
within a defined population. Two important measures of morbidity are prevalence and
incidence. As epidemiologists we must take care to use these terms correctly.
Prevalence
Strictly speaking, prevalence refers to the number of cases of a given disease or
attribute that exists in a population at a specified time. Prevalence risk is the proportion of
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
a population that has a specific disease or attribute at a specified point in time. Many
authors use the term ‘prevalence’ when they really mean prevalence risk, and these notes
will follow this convention.
Prevalence can be interpreted as the probability of an individual from a population
having a disease at a specified point in time.
Two types of prevalence are reported in the epidemiological literature: (1) point
prevalence equals the number of disease cases in a population at a single point in time (a
snapshot), (2) period prevalence equals the point prevalence at the beginning of a study
period plus the number of new cases that occurred during the remainder of the study
period.
Incidence
Incidence measures how frequently initially susceptible individuals become
disease cases as they are observed over time. An incident case occurs when an individual
changes from being susceptible to being diseased. The count of incident cases is the
number of such events that occur in a defined population during a specified time period.
There are two ways to express incidence: incidence risk and incidence rate.
1. Incidence rate
Incidence rate (also known as incidence density) is the number of new cases of
disease that occur per unit of individual time at risk, during a defined time period. The
denominator of incidence rate is measured in units of animal (or person) time.
2. Incidence risk
Incidence risk (also known as cumulative incidence) is the proportion of initially
susceptible individuals in a population who become new cases during a defined time
period.
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Introduction of Epidemiology
Veterinary Field Epidemiology in Action
1-25 September 2009 Khon Kaen University, Thailand
Study design
A study generally begins with a research question. Once the research question has
been specified the next step is to choose a study design. A study design is a plan for
selecting study subjects and for obtaining data about them. Figure 10 shows the major
types of epidemiological study designs. There are three main study types: (1) descriptive
studies, (2) analytical studies, and (3) experimental studies.
Descriptive studies are those undertaken without a specific hypothesis. They are
often the earliest studies done on a new disease in order to characterise it, quantify its
frequency, and determine how it varies in relation to individual, place and time.
Analytical studies are undertaken to identify and test hypotheses about the association
between an exposure of interest and a particular outcome. Experimental studies are also
designed to test hypotheses between specific exposures and outcomes — the major
difference is that in experimental studies the investigator has direct control over the study
conditions.
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Introduction of Epidemiology