EPIDEMIC CURVES & DETERMINANTS OF DISEASE Flashcards
Latent period
Latent period = microbe is replicating but not yet enough for the host to become infectious.
Incubation period
Incubation period = microbe is replicating but not symptomatic yet. Does not always correlate with the latent period.
Infectious
Infectious- disease caused by the invasion and multiplication of a living agent in/on a host
Infestation-
Infestation- invasion, but not multiplication of an organism in/on a host (fleas/ticks, sometimes parasites)
Contagious
Contagious- disease transmissable from one human/animal to another via direct or airborne routes.
Isolation is a way that you could control contagious diseases
Communicable-
Communicable- disease caused by an agent capable of transmission by direct, airborne, or indirect routes from an infected person, animal, plant or a contaminated inanimate reservoir.
A disease can be communicable but not contagious. When a disease is communicable, you have to worry about vector control, enviornmental etc
Epidemic Curves
Represent the number of new cases of disease, over time
Are simple to make and interpret
Can tell you:
– Most probable source of the outbreak
– If the pathogen is contagious
– If the outbreak is ending – or will continue
– Incubation period of the pathogen (sometimes)
– About outliers
What separates each wave?
What type of a curve is this?
Incubation Period
Propagative Curve- as the days increase, the amount of new cases increases- it is contagious
What kind of curve is this?
Epidemic Curve:
Common Source Single Point Exposure- steep rise, comes to a peak and then falls off
only one incubation period
- All animals are exposed at once
- All are exposed to the same source of infection
- Not contagious
- Can determine the minimum, average, and maximum incubation time
What kind of curve is this?
Common Source w/ Intermittent Exposure
Animals are exposed at different times
Exposed to the same source
Incubation period is NOT clearly shown
What determines the shape of the epidemological curve?
Depends on several factors:
Host
– Immunity or other resistance to disease
– Direct transmission
Agent
– Infectiousness of agent
– Latent and incubation periods
– Duration of infectivity
Environment
Endemic Stability
– Endemic (enzootic) vs. epidemic (epizootic)
A situation in which all factors influencing disease are relatively stable, resulting in little fluctuation in disease incidence over time
– New cases occur at a regular, usually low, level
– Young individuals may enter the population
– Old individuals die or are removed
DETERMINANTS
Factors that help DETERMINE the probability, distribution, or severity of a disease in an animal or population of animals.
Host susceptibility is one kind of determinant – but not the only one!
these are important because:
- id’s animals at risk
- disease prevention
- aid to differential dx
Primary Determinants
Primary = a MAJOR contributing factor, usually a NECESSARY one
It has to be present for the disease to occur
Secondary Determinants
Secondary = factors that make the disease more or less LIKELY; predisposing or enabling factors
Intrinsic Determinats
Intrinsic = determinants that are internal to the animal (age, breed, sex, etc.)
Extrinsic Determinants
Extrinsic = determinants that are external to the animal (housing, medical treatment, etc.)
PRIMARY Determinants
Primary determinants MUST ALWAYS be there in order for disease to occur
– AKA “necessary causes” in Rothman’s model of diseases (later)
Primary determinants satisfy the “gotta have it” test!
– Everything else is “secondary”, just a “component cause”
i.e. for anthrax- you have to have the spores and you have to have the suceptible cattle
Mutations can include
Mutations can include
– Increased infectivity within typical hosts
– Ability to infect new species / populations of hosts – Acquisition of new toxins
– Immune system evasion
Antimicrobial resistance can be obtained
Antimicrobial resistance can be obtained either de novo, through mutation, or via lateral transfer from another organism
Genotype
Genotype = a term describing the DNA sequence, or “type”, of an individual
Genetic diseases are ENTIRELY determined by
Genetic diseases are ENTIRELY determined by genotype
What determines genetic susceptibilties?
Genetic susceptibilities are PARTIALLY determined by genotype and partially by other factors
Herd Immunity
The idea that infectious diseases can be contained if the population’s resistance to infection is high enough
– Does NOT protect individuals
– Some non-immune individuals will probably
become infected…
– Many others will be protected, indirectly, by the immunity of their herd-mates
üCan be applied to populations of people as well as populations of animals!
Is it easier if the latent and the incubation period? Which one is more important?
Latent period is more important and yes it is easier
What kind of determinate do you want to target for prevention?
Primary extrinsic factors
