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156 Cards in this Set

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define epidemiology
the study of patterns of health and disease in populations and the factors that influence these patterns.
epidemiology is concerned with what?
who in the pop is getting the dz

who is not

why are they getting the dz

why are some sick and some not sick

*concerned with the forest and the trees*
epidemiology involves a logical approach to _______ regarding _____, ______, & ______
A logical approach to solving disease problems regarding cause, predisposing factors & intervention
epidemiology involves describing patterns of dz in relation to ______, _______, & _____.


uses of epidemiology (6)
Investigation and control of a disease of unknown etiology

Identification of the etiology & risk factors of a disease

Determination of the origin of a disease of known etiology

To study the natural history and prognosis of a disease

Planning, monitoring & assessment of d’se control programs

Assessment of the magnitude and impact of a disease
epidemiology involves a systematic approach for ______
A systematic approach for the critical evaluation of scientific literature.
4 types of epidemiology
descriptive epidemiology
- Describes patterns & formulates hypotheses
- Questions answered: What, Who, Where, When?

analytical epidemiology
- Involves analysis of data to test hypotheses
- Questions answered: Why, How?

experimental epidemiology
- Involves pop. experiments to test hypotheses
- Example: Vaccine and drug trials

theoretical epidemology
- Uses mathematical models to study disease
7 epidemiological sub-disciplines
- Uses epidemiological principles in the care of indivs
- Is an important component of evidence-based medicine

- Uses computer science in the study of epidemiology
- Is used to formulate disease control strategies

- Is the study of epidemiology in related individuals

- Used to investigate disease outbreaks

- Uses local knowledge for improving anl. health

- Uses molecular techniques in epi investigations

- Concerned with spatial patterns of disease & health
what is the unit of concern in clinical medicine?


Clinical Medicine
- Sick Animal

- Dead Animal

- Healthy, sick and dead
What is the diagnostic procedure for clinical medicine?


Clinical Medicine
- Identify d’se based on signs & symptoms

- Identify d’se based on host responses

- Determine patterns of d’se distribution
What is the primary objective of clinical medicine?


Clin. Medicine
- Treatment of sick animals

- Treatment of future animals

- Control d’se & prevent future occurrence
What questions are asked in clinical medicine?


Clinical Medicine:
- What is it?
- How do I treat it?

- What is it?
- What is its mechanism?
- What caused it?

- What is it?
- Which animals have it?
- Where/ When is it occurring?
- What caused it?
- Why did it occur?
What is a determinant of dz/ health?
any factor that when altered produces a change in the frequency or characteristics of disease

Physical factors
Biological factors
Behavioral factors
primary vs secondary determinants
Primary determinants
- Have a major effect in inducing disease.
- Example: rabies virus

Secondary Determinants
- Are predisposing or enabling factors to disease.
- Example: poor hygiene
intrinsic vs extrinsic determinants
Intrinsic (Endogenous) Determinants
- Are Internal to the host
- Example: breed, species, sex, etc

Extrinsic (Exogenous) Determinants
- Are external to the host
- Examples: poisons, trauma, radiation, etc
epidemiological triad


epidemiological triad


The major component of epidemiology that differentiates it from other medical specialties is its focus on

a. Population
b. Animals
c. Diseased animals
d. Healthy animals
e. The environment
a. Populations
The epidemiologic triad consists of

a. Individual, place, time
b. Epidemiology, clinical medicine, pathology
c. Agent, host, environment
d. Intrinsic, extrinsic, & primary cause of disease
e. None of the above
c. Agent, host, environment
if we change a determinant, it will _______
change the nature or frequency of the dz
host range
the spectrum of hosts the agent can survive in

The broader the host range, the better the survival

The narrower the host range, the easier to control
Agent factors (determinants)
host range


infectious dose





antigenic stability
Ability to enter, multiply & produce change (may or may not be disease) in a host

Infectivity is variable from agent to agent

Infectivity may vary within one spp. of agent.
infection vs contamination vs pollution
Infection (not always disease)
- The entry and multiplication of an infectious agent in the body
- Infection may not necessarily lead to d’se (inapparent infections)

- Infectious agents on exterior surface of the body/object
- Can be internal (e.g. Pasteurella multocida – in air sacs)

- Presence of offensive matter in the environment
- The offensive matter does not have to be infectious
infectious dose
Quantity of agent necessary for transmission & infection.

Varies by agent
Ability of an agent to withstand environmental stress.

Anthrax spores can be viable in the soil for over 20 yrs
Ability of an agent to withstand environmental stress.

Anthrax spores can be viable in the soil for over 20 yrs
Is the power of an agent to produce clinical disease.

Also used for toxic chemicals, etc.

Mathematically, it is the ratio of number animals developing d’se to the number exposed to infection
what is the difference b/w virulence and pathogenicity?
Virulence is the degree of pathogenicity (the severity of disease)

Pathogenicity is the ability to produce clinical disease (no association with severity of disease)

Both are dependent on host and agent factors
does virulence depend on host or agent factors
does pathogenicity depend on host or agent factors
Is the degree of pathogenicity of an agent

Indicated by the severity of disease

Mathematically, it is the ratio of the number of clinical cases to the number of infected animals. E.g. CFR
which animals are the most dangerous epidemiologically speaking

a. those with subclinical infections
b. those with mild disease
c. those with moderate disease
d. those with severe disease
e. susceptible individuals
a. subclinical dz
Ability of an agent to stimulate an immune response.

The likelihood of repeated infections is reduced if the agent is highly immunogenic.

Canine distemper– highly immunogenic

Herpes virus– poorly immunogenic
antigenic stability
The probability that the genome governing antigenic structure of an agent will undergo antigenic change.

Canine distemper virus is stable; influenza virus is not.
host factors (determinants)




species & breed

immunological status


size & conformation

social & ethological

coat color
Genotype is the genetic makeup of an organism.

Genetic d’ses are inherited e.g. hemophilia A&B in dogs

Genetic diseases belong to one of the three categories:

Chromosomal disorders

Mendelian (inherited) disorders
Multifactorial disorders – variable genetic component
- Interplay b/w environment and genetic factor (need both)
how does age affect disease
Most diseases have an association with age.

Lower immunity in the young & very old

Salmonellosis is deadly in chicks but mild in adult chickens

Knowledge is useful in disease control

Brucellosis vaccine - given to calves
how does sex affect dz (4)
- Males don’t have the stress of pregnancy, parturition & lactation
- Sex hormones may increase risk of diabetes mellitus in bitches

- Heartworm infection risk is higher in male hunting dogs

- Higher risk of bite wound abscesses in male than female cats

Genetic differences in susceptibility between sexes
- Sex-linked: Canine hemophilia A & B - more common in males
- Sex-limited: Orchitis (inflammation of testicles) and metritis (inflammation of the uterus)
- Sex-influenced: Lower d’se threshold in one sex than the other
how do species & breed affect dz
- Some d’ses affect almost all spp. e.g. rabies, tetanus
- Other d’ses are host-specific e.g. canine distemper
- Vultures & Jackals in Africa are resistant to anthrax (spread?)

Rottweillers and Dobermann Pinschers
- more susceptible to canine parvovirus infection than other breeds.
N’dama cattle:
- Very susceptible to rinderpest.
- Resistant to trypanosomiasis
Zebu cattle
- more resistant to tick-borne d’ses than European breeds.
differentiate b/w immunological status and immunogenicity
Immunological status is the status of the host (is it resistant, immunocompromised)

Immunogenicity is the ability of the agent to produce clinical disease
how does immunological status affect dz
Immunological status impacts d’se patterns

Immunity could result from:
- Dam to offspring
- Vaccination
- Natural infection
how does occupation affect dz?
Race horses - higher risk of leg injuries

Veterinarians - higher risk of zoonotic infections
how does size and conformation affect dz?
Size: Is a disease determinant (regardless of breed)
- Large dog breeds have higher risk of hip dysplasia

Conformation: is also a determinant of disease
- Cows with a small pelvic outlet relative to their size
- (e.g. Belgian Blue) are predisposed to dystocia
host factors: social & ethological
Tail biting in pigs

Gastric torsion in horses – rolling behavior

Lung cancer – higher risk in smokers

Lantana camara poisoning – some breeds learn to avoid the plant
how does coat color affect dz
Cutaneous squamous cell carcinoma in white cats
- Due to lack of pigmentation which protects from uv radiation

Photosensitization seen in animals with:
- Lack of pigmentation in parts exposed to direct uv radiation
- Liver problems
environmental factors (determinants)

location (geography)

climate (macro & microclimate)

husbandry (housing/ nutrition/ mgmt)
how does location affect dz
Geological formations, vegetation and climate

Non-specific chronic canine pulmonary d’se in old dogs
- Associated with urban residence. Why?

Noise is associated with:
- Decreased milk production in dairy cows
- Increased incidence of abortion in rats.

Temporal disease patterns affected by geography
- Temperate regions – seasonal variations in disease prevalence
- Tropics – variations during dry & wet months of the year
how does climate affect dz
Macroclimate: includes the weather conditions, etc
- Hurricanes
- Cold stress predisposes animals to disease
- Wind may carry infectious agents over long distances(FMD virus/ Arthropod vectors)
- Risk of respiratory diseases increases in cool damp weather
- Drought may lead to decreased milk yield and weight gain

Microclimate: Is climate in a small defined space.

Terrestrial microclimate – over the surface of leaves
affect the development of arthropods and helminths
- Right humidity and temp will help arthropods thrive

Biological microclimate – over surface of a host’s body
change during the course of a disease & enhance d’se spread
- Malaria during certain times (pathogenic state) there is a fever which leads to sweating which attracts more mosquitos to bite and spread disease

Larger microclimates –
Examples: calf house, a pen, etc
- Poor ventilation increases risk of chronic equine resp. d’se
how does husbandry affect dz
- Ventilation, bedding, and floor surfaces
- Housing protects from a harsh macroclimate.

- Well-nourished animals may show prolonged survival
- Feeding regimes are associated with gastric torsion

- Stocking densities
- Replacement policy
- Occupation - ‘Hump-sore’ (‘yoke gall’)
interaction & association of factors (determinants)
Association of Factors
- Association of a factor & d’se does NOT necessarily imply cause-effect relationship.
- Carrying matches is associated with lung cancer but doesn’t cause the disease.

Interaction of Factors
- Determinants do not act in isolation – they interact
- Porcine stress syndrome -Interaction between the host (gene) & Environment (stressors).
What will happen with a not very pathogenic agent in a non-immune population
Many will contract the agent and develop immunity w/o causing clinical disease????
What will happen with a very pathogenic agent in an immune population
It won’t get passed b/c the pop is immune
causation web
Is a method of conceptualizing how multiple factors combine to cause disease.

The causal web consists of direct and indirect causes of disease

These causes are interconnected through a series of interconnected causal chains or web structures
survival of an infectious agent depends on
its successful transmission to a susceptible host

its multiplication to maintain the life-cycle (aka life history)
1. _______ is the portal of exit

2. ______ is the portal of entry
1. reservoir

2. new host
define a reservoir
Living or non-living matter

Where infectious agent normally lives and multiplies

Maintains & perpetuates itself

From which it can be transmitted
differentiate reservoir from source of infection
Source of infection is where agent jumps from where it is to a susceptible host
- This is where the agent leaves
- Can be a subset of reservoir

Reservoir requires that the agent can survive there, multiply there, leave there.
is center where infection settles & from where it spreads

Example: abandoned chicken coops may be a nidus for Histoplasma capsulatum
source of infection
is an animal or substance from which an infectious agent passes directly to a susceptible host

SOI may or may not be a reservoir
- Reservoir and SOI are the same for most viruses.

SOI = Reservoir: Canine distemper
SOI ≠ Reservoir: Equine infectious anemia (horse = reservoir; biting fly = SOI)
what is a carrier
an infected person or animal
that sheds pathogenic/ potentially pathogenic organisms in the absence of discernible clinical disease & serves as a potential SOI

Carriers are very dangerous epidemiologically b/c they have the disease & spread it but you don’t know

ex: typhoid mary
types of carriers
- Shed the agent without developing clinical disease
- This state is called inapparent infection
- Is a very important type of carrier for disease control
- Example: Salmonella interrogans serotype typhimurium in swine.

- Shed agent prior to appearance of clinical signs
- Example: Canine distemper and foot-and-mouth disease

- Shed agent for short periods after signs have abated
- Examples: - Salmonellosis in cattle / Feline panleukopenia

- Intermittently shed agent for a moderate periods of time after recovery from disease
- Example: Toxoplasmosis in cats

- Shed agent for extended periods of time after recovery
- Example: Tuberculosis
healthy (asymptomatic) carriers vs. incubatory carriers
Healthy (asymptomatic) carriers
- Shed agent but never show clinical signs
- Hemophilia carriers
- Typhoid Mary
- Salmonella in snakes

Incubatory carriers
- Shed prior to appearance of clinical signs
- Mononucleosis
- Canine distemper
convalescent vs. intermittent vs. chronic carriers
Convalescent carriers
- Shed after clinical signs have abated
- Salmonellosis in cattle

Intermittent shedders
- Intermittenetly shed agent for moderate periods of time after recovery from disease
- Toxoplasmosis in cats

Chronic carriers
- Able to shed for long periods of time after recovery
- TB
portal of exit
Is a place of exit of agents from the reservoir

It includes:
- Respiratory tract: Nose (secretions)
- Alimentary tract: Mouth (saliva, sputum); anus (feces)
- Urinogenital tract: Urine, semen, eggs
- Eyes: Tears (Pink eye – Haemophilus influenzae)
- Body surface: Skin & Hair/ lesions & crusts/ exudates
- Multiple exits: - Complicate control procedures(Q-fever & streptococci)
modes of transmission
- direct
- indirect

- Hereditary
- In Utero
- During birth
- Colostrum
- Suckled milk
how does direct transmission differ from indirect transmission

what is a primary vehicle
- The agent responsible for the disease
- Need HIV virus to get disease

- Risky behavior/ unprotected sex indirectly lead to disease

Primary vehicle
- Saliva
- Secretions
- Etc.
primary vehicle vs. secondary vehicle vs. vector
Primary vehicle:
- Secretions, excretions or other body fluids or tissues
- Examples: saliva, feces, blood, urine

Secondary vehicle
- Inanimate object contaminated with the primary vehicle
- Examples: water, food, grass, etc.

- Are invertebrates that transmit infectious agents
- Examples: fleas, ticks, flies, mosquitoes, snails
define direct transmission

what 2 ways may it occur through
Susceptible host gets infected either by contact with an infected host or host’s infected discharges

Direct transmission may occur through:
Contact (contagious diseases)
- ‘Fragile’ agents are generally transmitted directly
Coitus (venereal diseases)
- Occurs in both vertebrates and invertebrates
Indirect transmission may occur in the following ways:
Vehicle and fomites


Ingestion – food or water

airborne transmission can occur by
Dust, expiratory droplets, droplet nuclei, vapors & gases
define a vector

what are the 2 types
are living invertebrates responsible for the transmission of infectious agents

- The agent doesn’t change while in the vector
- No time lapse

Biological vectors
- There is some change in the agent (multiplication or maturation or reproduction)
- Time lapse
extrinsic differentiation period
Time from when vector picks up the agent to the time when it can infect another animal
incubation period vs. induction period
Incubation period
- Time between exposure and development of clinical signs

Induction period
- Time between exposure to chemical and development of signs of exposure
________ requires an extrinsic incubation period before transmission can occur.
biological vector
types of biological transmission
Developmental transmission
- An essential devpt phase of agent occurs in vector
- Example: Dirofilaria immitis in mosquitoes

Propagative transmission
- Multiplication of the agent occurs in the vector
- Example: arboviruses in mosquitoes

Cyclopropagative transmission
- Both development and multiplication occur in the vector
- Example: Babesia in ticks
True or False: In general it is easier to control if the host range is narrow
The severity of disease caused by an agent in a susceptible host is a measure of

a. Contagiousness
b. Infectivity
c. Virulence
d. pathogenicity
c. virulence
One of the following does not belong to the group. Which is it

a. Immunogenicity
b. Pathogenicity
c. Immunological status
d. Virulence
e. infectivity
c. immunological status
The extrinsic incubation period is the term used to refer to

a. Estimating the incubation period in a propogated curve

b. The time between infection and the 1st clinical signs (incubation period)

c. The time b/w exposure to a toxin and 1st clinical signs (induction period)

d. The time before an agent can be transmitted in biological vectors
d. The time before an agent can be transmitted in biological vectors
A localized reservoir is called a

a. Chronic carrier
b. Source of infection
c. Nidus
d. Convalescent carrier
c. nidus
True or False: Mechanical vectors cannot be reservoirs

The mechanical vector just transmits the agent but no change occurs (no propagation)
Reasons for special review:
Advancement or change in rating to pay grade E-7
Detachment for PCS
Detachment for intra-command reassignments if AO changes
Detachment of AO who directly supervises employee
Evaluee completes TAD, ADSW-RC, ADSW- AC for any length of time

Also for:
Convicted by Court Martial
with ________ infection can occur remotely from the vector
water inhabiting vectors

ex: Schistosomiasis
hereditary vs. congenital
- Carried in the genome of the parent e.g. retroviruses
- DNA copies of the virus carried in the hosts genome

- acquired in utero
contagiousness vs. communicability
Contagiousness refers to spread by direct contact b/w hosts

Communicability doesn’t differentiate how the agent is spread
At shore installations how is the CG ensign flown?
Starboard yardarm of the flag mast
_______ is the condition under which infection will occur
effective contact
seasonal vector-borne infections have a ______ effective contact.

(short or long)
effective contact depends on _______
Portal of exit

Portal of entry

Stability of the organism
If an agent that is lowly infective, highly pathogenic, and highly virulent was introduced into a totally susceptible population of animals, what would you expect to see clinically

a. A few severely affected animals

b. Many mildly affected animals

c. A few mildly affected animals

d. Many severely affected animals
a. A few severely affected animals
The extrinsic incubation period is the term used to refer to

a. Estimating the incubation period in a propagated curve

b. The time b/w infection and 1st clinical signs

c. The time b/w exposure to a toxin and the 1st clinical signs

d. The time before an agent can be transmitted in biological vectors
d. The time before an agent can be transmitted in biological vectors
True or False: Mechanical vectors can be reservoirs
For long-term survival, infectious agents require:

A: Hosts with low immunity
B: Hosts with moderate immunity
C: Hosts with high immunity
B: Hosts with moderate immunity
host immunity depends on
Nature of the agent

Challenge dose

Environmental factors
herd immunity
Resistant of the pop to infection or disease if infected
True or False: host immunity is absolute

it is relative
rate of infectious disease spread in a population depends on
Characteristics of the infectious agent

Host immunity of the animals in the pop

Pop. structure (proportion of susceptibles, resistant, etc)

Pop. dynamics (Movts., social interaction, behavior, etc)

Contact rate
herd immunity depends on
Population Structure
- proportion of immune & susceptible animals
- presence of alternative hosts & vectors
- More alternate hosts = more chance of infection

Population Dynamics
- Movements, social distance, behavior, etc.
- Contact Rate
herd immunity vs. host immunity
Herd immunity is affected by population structures and population dynamics whereas host immunity doesn’t get affected by those 2
what is contact rate

how does it affect herd immunity
Is the rate at which susceptibles interact with infected anls.

This interaction affects the degree of herd immunity.
- Reducing the prob. of contact (duration, interval, effectiveness) between infected and susceptibles may slow down d’se spread

Possible to control disease by changing herd immunity by manipulating herd structure without inducing host immunity
endemic occurrance
Is used to refer to:
- the usual frequency of occurrence of a disease in the population
- the constant presence of the disease in the population

Endemic occurrence can be used to describe both clinical and sub-clinical disease

It is applied to both infectious and non-infectious diseases

When describing endemic disease, the affected population and its location should be specified
hyperendemic vs hypoendemic vs. holoendemic
Hyperendemic occurrence - Disease is continuously present at a high level, affecting all age-groups equally.
- Rabies in Canada

Hypoendemic occurrence – Disease is continuously present at a low level, affecting all age groups
- TB in developed countries

Holoendemic occurrence – A high level of infection begins early & affects most of the young pop resulting in a state of equilibrium so that the adult pop. show evidence of disease much less commonly.
- Malaria in areas that it occurs (very young are more likely to be infected while the old have much lower incidence due to immunity)
define epidemic

when does it occur
the occurrence of disease at a level in excess of the expected (i.e. endemic) level.

Epidemic occurs when the pop is exposed to one or more factors that were not present previously
point source vs. propagating epidemic
Point source (common source epidemic)
- All individuals infected from same source at the same time
- Food borne illness

- 1 individual comes in and then infects others
- b/c not all infected from same source
_______ is when disease occurs irregularly and haphazardly.
sporatic occurrence
1. _________ phase is when # of cases is going up

2. __________ phase is when # of cases is going down

3. _________ affects the amplitude of an epidemic curve
1. progression

2. regression

3. infectivity
High infectivity = increased # of individuals infected (higher amplitude)
_______ makes you reach culmination point quicker (affects slope of epidemic curve)
Shorter incubation period
__________ has to be known before an epidemic can be recognized
The endemic level of the disease in a population
__________ is required to allow contact-transmitted epidemic to commence
A minimum (or threshold) density of susceptible animals
Factors affecting the shape of an epidemic curve
Incubation period of the disease
- shorter = steeper slope

Infectivity of the agent
- higher = increased amplitude

Proportion of susceptible hosts in the population
- less = lower amplitude and slower to reach culmination point
how do you construct an epidemic curve
Identify the date of onset

Set the time Interval - 1/3 or less of IP

Create X-axis lead and end periods
- Should be equal to 2 incubation periods (lead and end periods)

Draw tick marks & label time intervals

Assign area equal to one case

Plot the cases on the graph

Mark Critical events on graph & add labels
common source epidemic

Rapid Progression and Regression

Compressed in time

Approaches symmetry at average incubation period
common source epidemic

Rapid Progression and Regression

Compressed in time

Approaches symmetry at average incubation period
propagated epidemic

Initial (primary) cases infect susceptible individuals that become secondary cases

Index case – Is the first identified primary case

Gradual progression (ascending limb)

Tendency to plateau

A feature of vector & animal-to-animal spread
why can you get a second peak with a propagating epidemic?
Introduction of susceptible animals into the area

Movt of infected animals from the epidemic area & subsequent contact with susceptible animals.
temporal patterns
Short-term - Are exhibited in epidemics

Cyclical Trends
- Result of periodic fluctuations in disease occurrence
- Seasonal Trends. Are due to:
- changes in host density
- management practice or behavior of the host
- survival of the infectious agent
- vector dynamics

Secular trends - are long term (several years) increases or decreases in disease rates
Cyclical Trends
- Result of periodic fluctuations in disease occurrence

Seasonal Trends. Are due to:
- changes in host density
- management practice or behavior of the host
- survival of the infectious agent
- vector dynamics
Secular trends: are long term changes where, in addition to short term ups and downs, the curve either climbs or declines more or less steadily over an extended period of time, usually years.
Random Spatial occurrence
are exhibited by sporadic outbreaks
Clustering spatial pattern
Is the aggregation of disease or other events in amounts greater than would be expected by chance alone.
space time clustering
Is an interaction between the place and time of occurrence of a disease.

An epidemic is clustering of disease over both time and space.
Ability to detect disease

Se = (TP) / (TP +FN)
clinical epidemiology
The application of the concepts of epidemiology and biostatistics to the practice of medicine on the individual patient (or herd/flock).
evidence based medicine
Process of systematically finding, appraising, and using research findings as the basis for clinical decisions.
what is outbreak investigation
a systematic procedure to identify causes and sources of epidemics
what is an outbreak
an epidemic that is limited to a localized increase in the incidence of a disease
sensitivity vs. specificity
Sensitivity: Ability of a test to accurately classify a group of patients known to have a disease

Specificity: Ability of a test to accurately classify a group of patients known to be free a disease
10 steps of an outbreak investigation
Determine the existence of an outbreak

Confirm the diagnosis

Define a “case” and count cases

Describe the patterns & determine who’s at risk

Determine the magnitude of the problem

Develop & test hypothesis

Compare the hypothesis with established facts

Plan a more detailed & systematic study

Prepare a written report

Execute Control & prevention measures
attack rate

attack rate ratio
# sick/ total population

attack rate in exposed/ attack rate in unexposed
accuracy refers to
What proportion of all tests have given positive results?
search widely for food or provisions
positive vs. negative predictive value
Positive predictive value- the probability that a patient whose test result is positive has the disease.

Negative predictive value- the probability that a patient whose test result is negative does not have the disease
1. ________ & _________ Measure the accuracy of a diagnostic test when applied to a group of patients whose disease status is known. What does it depend on?

2. _______ & _______ Measures the ability of a diagnostic test to accurately classify a patient whose disease status is unknown.
What does it depend on?
1. sensitivity & specificity
test properties

2. predictive value
Depends on the sensitivity, specificity and pre-test probability (prevalence) of disease in the population from which the patient was selected.
prevalance vs. point prevalance
Prevalence: fraction or proportion of a group that possess a clinical outcome/disease during a given period (point) in time.

Point prevalence is measured by a single survey of the group.
what effect does serial testing have on sensitivity & specificity?
- Improve specificity
- Decrease sensitivity

- Improve sensitivity
- Decrease specificity
What effect does serial testing have on sensitivity & specificity?

- Improve specificity
- Decrease sensitivity

- Improve sensitivity
- Decrease specificity
when should you use a sensitive test?
When there is a penalty for missing a disease (heartworm, brucellosis in cattle, equine infectious anemia [EIA])

Early in diagnostic process to “rule out” disease

Few false negative results (high NPV)

Sensitive test most helpful when result is negative

Useful to screen patients and eliminate those subjected to a more expensive and time consuming confirmatory test
when should you use a test with high specificity?
When there is a penalty for false positive result (Feline infectious peritonitis, Johne’s disease)

Useful to “rule in” disease (few false positives, high PPV)

Specific test most helpful when result is positive

Used as follow-up test to confirm disease status on subset of patients positive on screening test.
A. 100% sensitive

B. Optimal combination
of sensitivity and

C. 100% specific
the best spot is one near the top left corner
A serosurvey was conducted on a sample of dogs to determine the prevalence heartworm disease in Knox County, TN. Results 50 out of 500 dogs (10%) tested positive. This estimate is the (more than one answer may be correct):

a. Rate of heartworm infection among dogs

b. Ratio of heartworm infection among dogs

c. Proportion of heartworm infection among dogs

d. Prevalence of heartworm infection among dogs
c. Proportion of heartworm infection among dogs

d. Prevalence of heartworm infection among dogs
The mean white blood cell count in a sample of dogs with parvovirus is 3,000/mm3 with a standard deviation of 750. Which of the following ranges would include approximately 95% of the dogs in this sample?

a. 2,250 to 3,750
b. 750 to 5,250
c. 2,500 to 3,500
d. 1,500 to 4,500
e. 2,000 to 4,000
d. 1,500 to 4,500
In a study to determine the effect of an new experimental treatment (antibiotic) on reproductive performance among broodmares with uterine infections, the attending veterinarian gives the new “experimental” treatment to mares she thinks are more severely affected. This action would affect the:

a. Internal validity

b. External validity
a. Internal validity
internal vs external validity
Internal Validity: The extent to which the design and conduct of the trial eliminate the possibility of bias.
- Are the results valid for the patients studied?
- Could the results be due to bias or chance?

External Validity: The extent to which the results of a trial provide a correct basis for generalization to other circumstances (= generalizability or applicability)
- Are the patients in the study similar to the ones in my practice?
- Are they so different that the results do not apply?
- To which group of patients can I apply the results?
liklihood ratio pros and cons
- Probability of disease given a test result that falls within a limited range of values
- Degree of measurement abnormality considered

- Use of odds
- May calculate or use nomogram to convert to probabilities
- LR differs from sensitivity and specificity in that result of test is probability of disease.
evidence based medicine steps
Formulate the problem

Search for the best evidence available

Critical appraisal of the evidence

Implement results in your clinical practice

Evaluate our performance
types of bias
Selection bias‑ patients in one of the treatment groups differ from those in the comparison group with regard to one or more factors that can influence the outcome being measured.

Observation bias‑ measurement of the outcome is systematically different between treatment groups.

Confounding bias‑ An unknown factor (X) is associated with the suspected risk factor and the outcome (disease).
measurement of characteristics
objective vs. subjective data
Objective (hard data)
Laboratory, demographic e.g., age, sex, breed

Subjective (soft data)
Quality of life; improvement of clinical signs, severity of lameness, assessment of pain, extent of nursing care

Increasing quality of subjective (soft) data
accuracy vs. precision
Accuracy (validity): The extent to which observations represent the truth

Precision (Reliability- repeatability): The extent to which repeated measures of the same factor are close together
rate vs ratio vs proportion
affected/total (in a given time period)


affected/total (no time period)
how do we define abnormality
Abnormal as unusual (infrequent)
- Mean + 2 SD
- Extreme 2.5% at each end of distribution

Abnormal as associated with disease
- Cut point to determine successful passive transfer in calves.

Abnormal as associated with response to treatment
- Heart murmur w/o signs of congestive heart failure (CHF) dog < 30 lbs.
- considered “normal” (80% of old dogs)
- Older dog > 30 lbs. with a heart murmur and signs of CHF is treated
If this were the distribution of white blood cells in a group of dogs with parvovirus, and the mean was 3,000 with a standard deviation of 500 then 68% of dogs would have values between 2,500 and 3,500 and 95% would have values between 2,000 and 4,000.
If this were the distribution of white blood cells in a group of dogs with parvovirus, and the mean was 3,000 with a standard deviation of 500 then 68% of dogs would have values between 2,500 and 3,500 and 95% would have values between 2,000 and 4,000.