Epidemiology Exam 1

What is epidemiology?

The scientific, systematic, data-driven study of the distribution (frequency and patterns) and determinants (causes and risk factors) of health-related states and events in specified populations, and the application of this information to control health problems.

Why does epidemiology matter when treating an individual animal?

Every individual exists within a larger population that influences disease risk, including its household, dog park, boarding facility, shows, and veterinary clinic. Epidemiologic information helps veterinarians make decisions for individual patients.

What is the difference between infection and disease?

• Infection: Presence of an agent that is not supposed to be there.

• Disease: Derangement of normal function.

What are the different stages or presentations along the disease continuum?

• Healthy: No apparent disease.

• Infected: An agent is present.

• Subclinical disease: Signs are not apparent without special testing or examination.

• Clinical disease: Functional derangement is apparent to a reasonably observant person.

• Mild, moderate, or severe disease: Increasing levels of disease impact.

• Death: The most severe outcome.

What is the difference between contagious and infectious diseases?

• Contagious: An infectious disease can spread within a susceptible population through direct or indirect contact with infected individuals; vector transmission is excluded from this definition.

• Infectious: Disease can be induced by transmission or inoculation of an agent or organism.

What is the incubation period?

The interval between exposure or acquisition of an agent and the development of clinical disease. Agent shedding and detection can vary during this period.

What is the infectious period?

The period during which an infected animal or person sheds an agent and is capable of transmitting the disease. Depending on the agent, it may begin before clinical signs appear and continue after clinical recovery.

What factors interact to cause disease?

The epidemiologic triad consists of:

• Agent: The infectious or toxic cause.

• Host: The animal or individual affected.

• Environment: External conditions that influence disease occurrence.

Disease development results from interactions among these factors and is not random.

What agent characteristics influence disease occurrence?

• Dose

• Environmental hardiness

• Infectivity

• Pathogenicity

• Virulence

• Toxicity

• Chemical or physical characteristics

Define infectivity, pathogenicity, and virulence.

• Infectivity: Proportion of exposed individuals who become infected. Formula: Number infected ÷ Number exposed.

• Pathogenicity: Proportion of infected individuals who develop clinically apparent disease. Formula: Number with clinical disease ÷ Number infected.

• Virulence: Proportion of clinically apparent cases that are severe or fatal. Formula: Number of severe or fatal cases ÷ Number with clinical disease.

What host immune factors influence disease risk?

• Innate immunity: For example, skin.

• Passive acquired immunity: For example, maternal immunity in neonates.

• Active acquired immunity: Previous exposure, vaccination status, and vaccine response.

What non-immune host factors influence disease risk?

• Age and sex

• Behavior (mutual grooming, dominance, pica)

• Production status (lactating vs. non-lactating)

• Reproductive status (pregnant vs. non-pregnant; intact vs. sterile)

• Anatomy

What is the difference between intrinsic and extrinsic host factors?

• Intrinsic factors: Non-changeable in the individual, such as age and genetics.

• Extrinsic factors: Changeable in the individual, such as vaccination status and intact versus neutered status.

What environmental factors influence disease occurrence?

• Animal stocking density

• Animal movement between groups

• Housing, including ventilation and sanitation

• Environmental conditions: temperature, humidity, wind velocity, and precipitation

• Nutrition, including protein, energy, macromineral, and micromineral status

Why should veterinarians consider the entire epidemiologic triad rather than just the pathogen?

Host and environmental factors may be more important than eliminating the agent alone. If host and environmental conditions continue to favor disease, eliminating one pathogen may allow another pathogen to fill the same niche.

What are the patterns of disease occurrence in populations?

• Sporadic: Occurs infrequently and irregularly.

• Endemic/enzootic: Habitual presence or predictable recurrence of disease in a population within a geographic area.

• Epidemic/epizootic: Disease occurs clearly in excess of what is expected in a population within an area. There is no universal numerical cutoff.

• Pandemic/panzootic: A worldwide or nearly worldwide epidemic; the classification can be subjective.

What are the patterns of disease occurrence over time?

• Seasonal: Associated with particular seasons.

• Cyclical: Occurs in recurring cycles.

• Secular: Long-term trends over time.

What demographic or case characteristics can reveal disease patterns?

Age patterns:

• Linear: Disease frequency increases or decreases with age.

• U-shaped: Young and old individuals are affected most.

• Curvilinear: Disease is less common at the youngest and oldest ages.

• Multinodal: Multiple peaks or patterns of disease occur.

• Sex

• Life stage, including pregnancy and lactation

• Production group or management system, such as free-range or pasture-based animals

What are spatial patterns of disease, and why are they important?

Spatial patterns describe the geographic distribution of disease and can reveal locations or environmental exposures associated with increased risk.

Example: John Snow investigated cholera outbreaks in 19th-century London and identified an association with the Broad Street pump.

What are the epidemiologic 5 W’s, and how do they help identify disease causes?

• Who: At-risk populations.

• What: Case definition.

• Where: Geographic distribution and associations.

• When: Time period, seasonality, and incubation period.

• Why/how: Causes and mechanisms of disease.

For infectious diseases, investigate routes of transmission and vectors. For noninfectious diseases, investigate risk factors. The answers help determine how to prevent or control disease.

What are primary, secondary, and tertiary interventions?

• Primary: Prevent disease by reducing exposure.

• Secondary: Intervene after disease occurs to reduce its impact.

• Tertiary: Intervene after significant disease impacts have occurred; attempt to restore lost productivity or ability (salvage).

What methods are used to measure the frequency of disease or other events?

• Simple counts

• Ratios

• Proportions

• Rates

What is a case definition, and what can it include?

A case definition is a set of standard criteria used to classify animals as diseased or healthy.

It may include:

• Clinical signs

• Diagnostic laboratory results

• Exposure history

• Restrictions involving time, place, or animal characteristics

Case definitions ensure everyone counts the same types of cases. Misclassification can have dangerous consequences.

What information do simple counts provide?

Counts establish:

• Number of cases (morbidity).

• Number of deaths (mortality).

• Number of animals that are neither diseased nor dead.

• Total population size.

What are the limitations of simple counts?

Simple counts alone do not tell you:

• How large the problem is relative to the population.

• Whether disease occurrence is higher than expected.

• The risk of another animal becoming a case.

The number of cases must be interpreted in the context of the population at risk.

What determines the population at risk?

The population at risk depends on the disease and its transmission risks.

Examples include:

• A litter of puppies

• A herd of cattle

• Horses attending an event

• Deer in a county

• Cats seen at a veterinary clinic

What is a ratio, and what does it tell us?

A ratio shows the relationship between two mutually exclusive groups.

Examples:

• Cats compared with cows.

• Puppies compared with adult dogs.

A ratio compares group sizes but does not, by itself, tell you the probability of disease.

Why must the case definition and population at risk be established before interpreting disease counts?

The case definition determines which animals qualify as cases, while the population at risk determines which animals belong in the population being evaluated. Without these definitions, counts may be inconsistent or misleading and cannot adequately describe the magnitude of disease or the risk of additional cases.

What are the four ways to quantify the frequency of events in a population?

Simple counts, ratios, proportions, and rates.

What determines the population at risk (PAR)?

The disease and its transmission characteristics. It includes animals capable of developing the disease during the period being studied.

What is the difference between a fixed and dynamic population?

• Fixed: Animals can leave, but new animals cannot enter; the population decreases over time.

• Dynamic: Animals can enter or leave; the population may increase, decrease, or remain stable.

What is a proportion, and how is it calculated?

A measure in which the numerator is included in the denominator.

Formula: Proportion = Number of cases / Population at risk (PAR)

Ranges from 0 to 1 and can be expressed as a percentage.

What is a rate, and how is it calculated?

A measure that incorporates time into the denominator.

Formula: Rate = Number of cases / Animal-time at risk

Expressed as cases per animal-time unit.

Why use rates and proportions instead of simple counts?

They account for the population at risk, allowing comparisons of disease frequency across different locations, time periods, and groups.

What is morbidity?

The proportion of animals affected by a disease in a population. It measures disease occurrence and can be crude or specific, and reported as a proportion or rate

What is incidence?

The number of new cases in a defined population during a specified period. It measures the risk or rate of developing disease and helps detect outbreaks, monitor trends, and evaluate prevention/control efforts.

What are the four ways incidence can be described?

• Incidence times: Time until an event occurs, such as days after exposure.

• Incidence counts: Number of new cases during a period.

• Cumulative incidence (incidence risk): Probability of developing disease during a defined period.

• Incidence density (incidence rate): New cases per unit of animal-time at risk.

Which animals belong in the population at risk when calculating incidence?

Animals that do not have the disease at the start of the period and are capable of developing it during that period. Incidence always concerns new cases.

What is cumulative incidence (incidence risk)?

The proportion of initially disease-free animals that develop new disease during a defined period.

Formula:

Cumulative incidence = New cases / PAR during the period

It ranges from 0 to 1 and assumes a fixed population with the same period of risk for everyone.

How do you adjust cumulative incidence for withdrawals?

Subtract half the number of withdrawals from the denominator, assuming animals withdraw halfway through the observation period.

Adjusted cumulative incidence = New cases / [PAR − (0.5 × withdrawals)]

What is incidence density (incidence rate)?

The number of new cases per unit of animal-time at risk.

Formula:

Incidence density = New cases / Total animal-time at risk

It ranges from 0 to infinity and accommodates dynamic populations and different periods of risk.

What is animal-time at risk?

The total time contributed by all animals while at risk of developing disease.

Examples:

• 1 cow × 1 month = 1 cow-month.

• 100 cows × 1 month = 100 cow-months.

• 10 cows × 50 months = 500 cow-months.

Time units can be days, weeks, months, years, or hours.

How is the exact incidence rate calculated in a dynamic population?

Calculate each animal's actual time at risk, add the animal-time contributions, and divide the number of new cases by the total animal-time.

Example: 2 new cases / 3.34 cow-months = approximately 0.6 cases per cow-month, or 6 cases per 10 cow-months.

How do you calculate approximate incidence density when more than one case per animal is possible?

Formula:

Incidence rate = New cases / [(Start − 0.5 × withdrawals + 0.5 × additions) × Time]

Do not subtract half the number of sick animals because animals may experience multiple cases.

What is the difference between cumulative incidence and incidence density?

• Cumulative incidence: A proportion measuring the probability of developing disease; best suited to fixed populations with a defined period of risk.

• Incidence density: A rate measuring new cases per animal-time; accommodates dynamic populations and unequal time at risk.

Example: 50 cows are observed for 3 months. Ten become sick, 2 are culled, and 3 are purchased. Calculate the approximate incidence rate, assuming only one case per animal.

Denominator:

\[ [50-(0.5\times10)-(0.5\times2)+(0.5\times3)]\times3 \]

\[ =45.5\times3=136.5\text{ cow-months} \]

Incidence rate:

\[ 10/136.5=0.073 \]

Answer: 0.073 cases per cow-month, or 7.3 cases per 100 cow-months.

What is prevalence?

The proportion of the population that has a disease at a specific point or during a specified period, including all existing cases.

Formula: Prevalence = All existing cases / Population at risk (PAR)

It ranges from 0 to 1 and provides a snapshot of disease burden.

Example: Five puppies in a litter of 10 have parvovirus on Tuesday. What is the prevalence?

Prevalence = 5 / 10 = 0.5 = 50%.

How do incidence and prevalence differ, and when is each useful?

• Incidence: Measures new cases; useful for identifying risk factors and evaluating disease prevention.

• Prevalence: Measures all existing cases; useful for understanding disease burden, controlling disease, and reducing severity.

Prevalence is particularly useful for chronic diseases or when the date of disease onset is difficult to determine.

What determines prevalence, and why doesn't high prevalence always mean high incidence?

Prevalence is shaped by both incidence and disease duration.

\[ \text{Prevalence}\approx\text{Incidence}\times\text{Duration} \]

Prevalence can be high even when incidence is low if animals live with the disease for a long time.

When investigating disease causes, the suspected risk factor must precede disease development.

What is mortality, and what type of measure is it?

The occurrence of animal deaths from any cause within a population during a specified period. Mortality is an incidence measure because it concerns new events (deaths).

What is the crude mortality rate, and how is it calculated?

Measures deaths from all causes in a population.

Formula:

Crude mortality rate = Total deaths / (Animals at risk × Time at risk)

What is the disease-specific mortality rate, and how is it calculated?

Measures deaths caused by a specific disease in the population.

Formula:

Disease-specific mortality rate = Deaths from the specific disease / (Animals at risk × Time at risk)

What is the case-fatality rate, and what does it indicate?

The proportion of animals with a specific disease that die from that disease. Despite its name, it is a proportion, not a rate, and indicates disease severity.

Formula:

Case-fatality rate = Deaths from the specific disease / Total cases of that disease

It can also help evaluate treatment efficacy.

What is the key difference between crude mortality, disease-specific mortality, and case-fatality?

• Crude mortality: Deaths from any cause relative to the population and animal-time at risk.

• Disease-specific mortality: Deaths from a particular disease relative to the population and animal-time at risk.

• Case-fatality: Deaths from a particular disease relative to the total cases of that disease; indicates severity.

What are Hill’s causal considerations?

Time sequence, strength of association, dose-response relationship, plausibility, consistency, specificity, analogy, and experimental evidence.

What is the difference between causation and correlation?

Causation means one factor produces an effect; correlation means factors are associated but are not necessarily causally related.

What must occur for a cause-and-effect relationship to be possible?

The cause must precede the effect (time sequence).

What are continuous/quantitative variables?

Variables measured numerically.

• Interval: Equally spaced units but no true zero (e.g., dates).

• Ratio: Has a true zero (e.g., height, duration of illness).

What are categorical/qualitative variables?

Variables that describe categories.

• Nominal: Unordered categories (e.g., red, white, blue; yes/no).

• Ordinal: Ordered categories (e.g., mild, moderate, severe).

What is important to know about variable types?

Continuous variables can be categorized, but categorical variables cannot be made continuous. Variable type affects statistical analysis. A number does not automatically mean a variable is continuous (e.g., body condition score).

What is the purpose of descriptive studies?

Describe the nature and distribution of events, identify potential risk factors, and generate hypotheses.

What are the limitations of descriptive studies?

They lack a control group, cannot test hypotheses, cannot prove disease causation, and cannot determine which treatment is better.

What is the difference between a case report and a case series?

• Case report: Describes one disease case, often rare or unusual.

• Case series: Describes multiple cases of a disease and can help estimate prognosis.

• Both can increase awareness and generate hypotheses.

What can surveys investigate, and what affects their validity?

Surveys can estimate disease prevalence, determine whether a disease is present or detectable, and investigate multiple risk factors. Validity depends on the study population and potential recall bias, selection bias, and responder bias. Surveys can be active or passive.

Define target, source, and study populations.

• Target population: The population you want results to apply to.

• Source population: The population from which study subjects are drawn.

• Study population: The subjects actually included in the study.

The source population should represent the target population, and the study population should represent both

What defines an analytic (explanatory) study?

Compares subgroups of study subjects, such as exposed versus unexposed animals, to evaluate outcomes of interest.

What is the purpose of experimental studies?

Explore causal relationships and test hypotheses by comparing outcomes between treatment and control groups.

How do laboratory studies differ from clinical trials/field studies?

• Laboratory studies: Strictly control subject selection, treatment allocation, environment, and disease exposure. They may not reflect real-world conditions.

• Clinical trials/field studies: Have less control, are more similar to real-world conditions, and usually involve naturally occurring disease.

What are positive and negative control groups?

• Positive control: Animals are exposed to a factor expected to affect the outcome.

• Negative control: Animals are not exposed or receive something expected not to affect the outcome, such as a placebo.

What defines an observational study?

Disease is observed in its natural setting without assigning subjects to groups. These studies reflect real-world complexity but have less control and greater potential for bias and confounding.

What is a cross-sectional study?

A prevalence study that measures exposure status, disease status, and other variables at the same time. It provides a snapshot of a population.

What are the advantages of cross-sectional studies?

• Explore multiple risk factors simultaneously.

• Faster, easier, and less expensive.

• Less bias than case-control studies.

• Useful for permanent factors, such as breed and sex.

What are the limitations of cross-sectional studies?

• Not ideal for rare outcomes or temporary conditions.

• Cannot reliably establish temporal relationships.

• May be affected by survivor bias and confounding.

• Not ideal for transient risk factors.

What is a cohort study?

Identifies groups with and without a particular exposure and follows them over time to determine who develops disease. Measures incidence and can be prospective or retrospective.

What are the key requirements of cohort studies?

• All participants must be disease-free at the start.

• Exposed and unexposed groups must be comparable.

• Participants are followed to determine disease development.

What are the advantages of cohort studies?

• Good for rare exposures.

• Useful when assigning an exposure would be unethical.

• Can examine multiple outcomes from one exposure.

• Establishes time sequence more strongly.

• Less prone to some biases.

What are the disadvantages of cohort studies?

• Require larger groups.

• Not ideal for rare diseases.

• Time-consuming and expensive.

• Logistically difficult.

• Risk of loss to follow-up.

What is a case-control study?

Animals are selected based on disease status, and their previous exposures are compared.

• Cases: Animals that have or had the disease.

• Controls: Similar animals that do not have the disease.

How should controls be selected in case-control studies?

• Come from the same source population as cases.

• Have comparable exposure experience.

• Must be eligible to become a case during the same period they qualify as a control.

• Must be comparable to the cases.

What are the advantages of case-control studies?

• Good for rare diseases, but not rare exposures.

• Can investigate multiple risk factors.

• Require fewer animals.

• Often quicker and less expensive than cohort studies.

What are the disadvantages of case-control studies?

• Selection bias.

• Recall bias.

• Confounding.

• Potential problems with matching.

• Difficulty establishing temporal relationships, particularly with incident versus prevalent cases.

How do experimental and observational studies differ in evidence for causation?

• Experimental studies: Provide stronger evidence for causation because treatment precedes the outcome, confounders can be controlled, and association strength and chance can be evaluated.

• Observational studies: Generally establish correlation, with uncertainty about temporal relationships and limited ability to account for confounders.

What is internal validity?

Whether the study methods and results are valid for the source population. Requires no major biases and appropriate study design, execution, and analysis.

What is external validity, and what must come first?

External validity is the generalizability of results to the target population. A study must first possess internal validity.

What are the main steps and principles of the scientific method?

Form a good hypothesis, design an appropriate study, collect quality data, analyze data appropriately, and correctly interpret results. A single experiment rarely establishes the complete truth.

What is internal validity?

The extent to which a study's results and conclusions are valid for the subjects studied, without being distorted by bias, confounding, or errors.

What is the difference between internal and external validity?

Internal validity concerns whether the study conclusions are valid for the subjects studied. External validity concerns whether the results can be generalized to other populations or settings.

What are dependent and independent variables?

• Dependent variable: The outcome being measured; its value depends on other variables.

• Independent variable: A risk factor, exposure, or treatment that may influence the outcome.

What are the target, source, and study populations?

• Target population: The population you want the results to apply to.

• Source population: The population from which study subjects are drawn.

• Study population: The actual subjects included in the study.

How should the source and study populations relate to one another?

The source population should represent the target population, and the study population should represent both the source and target populations for results to generalize appropriately.

What is the difference between random sampling and convenience sampling?

Random sampling selects subjects using a random process, while convenience sampling selects subjects based on availability or ease of access. Convenience sampling may be more prone to selection bias

Why will two samples from the same population usually differ?

Random chance causes sampling variation, so samples are never perfect representations of their populations.

What is the difference between measures of central tendency and measures of spread?

• Central tendency: Mean, median, and mode.

• Spread: Range, standard deviation (SD), and interquartile range (IQR).

• Standard error (SE) measures the precision of an estimated statistic, such as the sample mean.

Why can using the mean be misleading for skewed data?

Extreme values can pull the mean away from the typical observation. The median is often more representative of the center of a skewed distribution.

What does it mean for samples or observations to be independent?

Each observation must not be related to or dependent on another observation. Independence is an assumption of most statistical tests.

What is clustering, and when does it occur?

Clustering occurs when observations are related rather than independent, such as animals housed on the same farm, animals in the same pen, or multiple samples collected from one animal.

How should statistical analyses account for clustering?

Use an appropriate method, such as stratified analysis or mixed regression analysis. Ignoring clustering can make statistical tests invalid.

What are the null hypothesis (H₀) and alternative hypothesis (Hₐ)?

• H₀: There is no statistical difference between groups (Treatment A = Treatment B).

• Hₐ: There is a statistical difference between groups (Treatment A ≠ Treatment B), typically using a two-sided hypothesis.

What is a p-value?

The probability, assuming the null hypothesis is true, of obtaining a result as extreme as or more extreme than the observed result.