11/5 thursday incidence, prevalence, sensitivity, specificity, positive predictive value, negative predictive value

vm 709 csu

what is a diagnostic?

a diagnostic test is any device or process designed to detect or quantify a sign, substance, tissue change, or body response in an animal

examples of diagnostics for virology:

• virus isolation (VI)

• hemagglutination inhibition (HI)

• fluorescent antibody (FA)

• PCR

• agar gel immunodiffusion (AGID)

• enzyme linked immusorbent assay (ELISA)

• serum neutralization (SN)

infection does NOT equal…

…dz

• what is the cut off point in dz dx between healthy vs dz animals?

course of infection consists of 2 stages, _ and _

prepathogenesis & pathogenesis

pathogenesis includes:

• multiplication

• tissue and physiologic changes

• signs/symptoms

• illness

• disability

• defect

• chronic state → death

prevalence is

the proportion of a population that has a particular condition at a given point in time

“how common” something is - if 10 of 100 people have a condition, prevalence = 10%

what does prevalence help us understand?

the burden of a condition in a population and can inform healthcare planning, resource allocation, and research

incidence is

the number (or proportion of new cases of a conditioner that occur in a population over a specific period of time)

• very dynamic → worry about the incidence RATE

• fatalities NOT COUNTED

ex: if 5/10 sick, and 2 die, then 3/8 = incidence rate %. if 2 more get sick, then 5 total are sick, and incidence rate = 5/8%

incidence helps us understand…

…the risk of developing a condition and the speed at which it is occurring in a population over time

isues in animal disease diagnosis:

1. sensitivity vs specificity

2. presence of false +s / -s

3. some experts claim there is not perfect diagnostic test

test categories for a veterinarian

• screening tests (for clinically healthy animals)

• confirmatory/diagnostic tests (for clinically disease animals)

sometimes use both at a time, consider cost!

screening test characteristics

• cheap

• detects numerous possible cases of disease, less specific for IDing real cases

• fast, reasonably accurate

• not meant to supply complete info abt patho

• need follow up w confirmatory/diagnostic test

confirmatory/diagnostic test characteristics

• tests for clin dzd animals

• to confirm diagnosis or classify dz status among animals suspected to have dz

a rabies test is (how specific)?

highly specific (basically no false positives)

qualities of a test are

1. precision and (reproducibility, reliability, repeatability)

2. validity (accuracy)

goals in using diagnostics:

• establish dx in symptomatic Patients

• provide prognosis

• guide treatment/control strats

• monitor treatment effects

what can cause variation in results?

• variation w/in ani subjects

• variation in reading of test results by same reader (measuring tool)

• variation betw those reading test results

data layout for test evaluation

true disease status (+ or -) along horizontal

diagnostic test (+ or -) along vertical

validity is expressed in what four dimensions?

1. sensitivity

2. specificity

3. PPV

4. NPV

the “gold standard” for diagnostic tests is

a diagnostic test or benchmark that is best, available under reasonable conditions

• the criterion standard

sensitivity of a test is

the probability of the test to generate positive results among animals that actually possess the disease

• (true positive) / (true pos + false neg)

• a / (a+c)

specificity of a test is

the probability of a test to generate negative results among animals that are genuinely free of the disease

• true negative / (false pos + true neg)

• d / (b+d)

true positive means

positive result in diseased animal

• actual infection

false negative means

negative result in diseased animal

• natural or induced tolerance

• improper timing

• improper test

• non-specific inhibitors

• toxic substances

• antibiotic induced

• immunoglobulin suppression

• incomplete or blocking antibody

• insensitive tests

false positive means

positive result in non-diseased animal

• group cross rxn

• non-specific inhibitors

• non-specific agglutinins

true negative means

negative results in an animal with no disease

• absence of infection

what happens when you increase the sensitivity of the test (too far)?

end up getting false positives

when should you select a test with high sensitivity?

• when missing a dzd ani would be costly or dangerous (ex: foot and mouth)

• when screening for a dz or patho in imported animal

• demonstration of absence of a dz in a dz-free zone

what happens when you increase the specificity of the test?

increased false negatives

when to select a test with high specificity?

• when confirming a diagnosis

• when inc false positives would cause physical, emotional, or financial damages

  • ex: don’t want to say “dog has osteosarcoma” and then “oops nvm he doesn’t” → want a specific test for this

positive predictive value answers the question

“if the test result if positive, what is the prob that the ani actually has the dz?”

• horizontal row: a / (a+b) = PPV

negative predictive value answers the question

“if the test result if negative, what is the probability that the animal does NOT have the dz?”

• horizontal row: d / (c+d) = NPV

if you use a test in a population where the dz is more prevalent,

the PPV is increased, and the NPV is decreased

• basically, more likely that a positive result = actually dzd ani

predictive value is dictated by…

…the population, NOT the test

• when dz is rare, false + make up a larger share of the + tests

nidality of dz is

• tendency for a dz to remain localized w/in specific ecological “nest”

“certain zoonotic dz persist in specific natural landscapes, bc pathos, vectors, and ani reservoirs all live together in that ecosystem”

factors impacting ecology of parasitic infectious dz

• role in ecosystems/food webs

• host specificity

• habitate preference/niche/anatomic location

• parasite communities and competition

• adaptations

ecology is

the study of relationships betw orgs and their enviros

• factors regulating #s and distributions

• sometimes enviro is another org…

a host is a…

…rich, nutrient-dense environment

• most animals have body fluids containing, prots/AAs, carbs, NA precursors, etc

• parasites exploit their living enviro

why do adaptations come about in parasites?

so the lifestage can better survive abiotic conditions existing between hosts

does hosts adapt?

hosts adapt (move, and defend themselves)

• ex: immune rxns

a nidus is

a specific ecological niche, hab, or geographic focus

nidus is derived from Latin word for

“nest” or “breeding place”

• refers to where microbes are located and multiply

a parasite’s ecological niche provides resources from…

…the body of the host OR

…the abiotic conditions encountered by the transmission stages (like eggs, cysts, oocysts, spores, or juveniles)

one medicine

human med + vet med

• collab

• common physiological and pathological foundations

• zoonoses are a focus

one health

humans + anis + plants + ecosystems/environment

key facts of one health

1. hum/ani/eco all interlinked

2. links demand COLLABORATION, comm, coord

3. OH optimizes health of all involved

4. > 60% of emerging infectious dzs reported globally come from anis

5. human activities and stressed ecosystems have created new opportunities for dz spread

6. stressors include: animal trade, ag, livestock farming, urbanization, extractive industries, climate change, hab fragmentation, and encroachment into wild areas

encounter and compatibility filters