DISEASE E&E (Measuring Disease 1) Flashcards
Iceberg of infectious disease: clinical spectrum
Below: subclinical
-exposure without infection
-infection without disease
Above:
-mild
-severe disease
-death
Johne’s disease:
-paratuberculosis
-caused by bacteria called MAP
Johne’s disease iceberg:
Below:
-susceptible heifer calves
-non-clinical, non-shedding
-non-clinical, shedding
Above
-clinical cases
Quantifying the course of a disease:
-1.Incubation period
-2. Illness
-1.Latent period
-2. Infectious
*expressed as averages, but significant variability between individuals
Incubation period:
-individual has no symptoms
Ex. 3-5years for Johne’s
Illness:
-individual has symptoms
Latent period:
-from when they are infected to when they are infectious
-sometimes shorter than the incubation period
Johne’s latent period:
-2-4 years
-shorter than the incubation period=great way to spread the disease
Infectious;
-host can transmit the pathogen
Quantifying the course of rabies:
-incubation period: 3-12weeks, depends where they were bitten
-illness: 7 days (then death)
-latent period: infectious after the virus reaches the brain
>similar to the incubation period
Characteristics of agent in relation to communicable disease (5):
-infectivity
-pathogenicity
-virulence
-immunogenicity
-physical stability
Infectivity:
-proportion of individuals that are exposed and who become infected
Pathogenicity:
-proportion of infected individuals who develop clinically apparent disease
Virulence:
-proportion of clinically apparent cases that are severe or fatal
Immunogenicity:
-capacity to produce specific and lasting immunity in the host
Physical stability:
-ability to survive independently in environment
Outbreak definition:
-an increase in observed number of cases of a disease or health problem compared with expected number for
>a given place
>a specific group of animals/people over a particular period of time
Epidemic definition:
-occurrence in a community or region of cases of an illness, specific health-related behaviour, or other health-related events, clearly in EXCESS of NORMAL expectancy
Outbreak vs. epidemic:
-often used for more localized epidemic
>within a village, town or specific institution
Pandemic:
-outbreak on a global scale
Endemic disease:
-disease which is “normally” present or “regularly found” in a population
Ex. Johne’s disease in cows in North America
Case definition:
-standard set of criteria that investigators in an epidemiological investigation use to decide whether an individual should be classified as having the disease of interest
-don’t all need to have a lab diagnostic test
*different from a clinical diagnosis
Importance of case definition:
-helps count cases CONSISTENTLY over time and from place to place
-helps with comparing expected and observed numbers to decide if there is a real outbreak
3 categories for case definitions:
-confirmed
-probable
-possible
Confirmed:
-typical clinical features AND a lab test or epidemiologic link to a lab confirmed case
Probable:
-typical clinical features but no lab confirmation or epi link
Possible:
-fewer typical clinical features and no lab confirmation or epi link
Identifying and counting cases:
-critical to success of an outbreak investigation
-not all may seek/get care
-not all cases get tested
*need to case find to get max info and better understand the epi of the disease
Reported cases may not:
-representative of all cases
>mild disease may not be reported
Sometimes an ‘outbreak’ occurs because:
-we do a better job looking for or detecting cases
>new improved testing
>availability of diagnostics
>how hard we look
Counting cases/number of animals infected: includes
-infected
-exposed
-diseased (cases)
-dead
*need a case definition
*help understand the timing (latent period, incubation period, illness duration)
*need a concept of size of the population at risk (DENOMINATOR)
Clusters of cases:
-geographical or temporal collection of cases that seem GREATER than the EXPECTED number for that given place and/or time
*may help identify risk factors
How should we orient cases?
-space
-time
-animal
*very important for diagnostic process at the population level
Space:
-location
>province
>country
>farm
>kennel
>pen or pasture
Animals:
-species
-class of animal
-age
-immune status
Temporal variations of disease:
-seasonal patterns
-cyclical fluctuations (ex. measles, rabies)
-secular trends (long term movement)
Secular trends can be a result of:
-environmental changes
-diagnostic changes
-medical changes
Seasonal patterns:
-arthropod borne outbreaks
-related to environment and climate (foot rot in feedlot cattle)
-influenza in people (fall/winter)
-related to demographics in seasonal industry (calf scours in beef herds)
Arthropod borne outbreaks (seasonal pattern)
-WNV
-Western equine encephalitis
-summer
*gradual build up or ‘amplification’
Using a calendar as a tool:
-events in populations all take place in a continuum of time
-time=easily quantifiable
Prevalence:
-sometimes referred to as prevalence rate or prevalence proportion
-proportion of cases or infections in a population at ONE PARTICULAR TIME
>no distinction between old and new cases (INCLUDES ALL CASES)
-snapshot in time
-a proportion (0-1 or 0-100%)
Prevalence represents:
-the probability of an individual animal being infected AT a given POINT IN TIME
*important for diagnostic decision making
Prevalence example:
-you walk through a herd of 200 dairy cows and 20 of the cows are lame
=20/200=0.1 or 10%
Point prevalence:
-strictly considering the prevalence at a POINT in time
-can use an event as a ‘point of time’ (Ex. prevalence of hypocalcaemia at calving in dairy cows)
Period prevalence:
-calculated the same way, but the population is assessed over a PERIOD of time
Ex. Johne’s disease prevalence: September 2014 to January 2015
Johne’s disease prevalence:
-overall: 1.55%
-herd with at least one positive: 23.6%
-herd with 2 positives: 5.4%
Other reasons for using prevalence:
-can be used to assess frequency of behaviours that might be risk or protective factors for disease
Ex. prevalence of smoking or vaccine use
Incidence:
-measure of the occurrence of NEW CASES of disease during a span of time
-all measures should have a time component
2 methods of calculating incidence:
-incidence risk
-incidence rate
Incidence risk:
-probability that an INDIVIDUAL ANIMAL will contract or develop a disease during a defined period of time
-measure of RISK of becoming a case
-no units: ranges from 0-1
Incidence risk equation:
-# of new cases in a period of time/total population at risk
Incidence risk and length of observation period:
-longer the period=greater the risk
When is using incidence risk good?
-best for static populations
-good when risk period is related to a specific event (ex. calving)
Specific examples of incidence risk:
-attack rate
-case fatality rates
Attack rates:
-describes the frequency of disease in outbreak situations
-measures of risk
Attack rates equation:
=number of cases / size of population exposed
Attack rates used where:
-risk period is limited
-all cases are likely to occur within the that risk period
Ex. going to a wedding and getting food poisoning
Secondary attack rates:
-describes infectiousness of an agent
-ease of spread
-spread withing a herd or family and not ALL cases have resulted form a common source exposure
Secondary attack rates equation:
-(number of cases – initial cases) / population at risk
Case fatality rates:
-proportion of animals with a specific disease that die from it (within a specified time period)
-risk measure (proportion) instead of rate
Case fatality rates describe the:
-impact of epidemic-type disease or severity of acute diseases
Incidence rate:
-number of NEW cases of disease in a POPULATION per unit of ANIMAL-TIME during a given time period
-measures the rapidity with which new cases develop over time
Incidence rate equation:
=number of cases of disease in a defined period / (number of animals-time units at risk during that period)
Animal-time unit:
-one animal for a defined period of time
Ex. cow-mounth
Incidence rates are usually calculated using:
-only the first occurrence of the disease for each animal
>no longer part of the population of risk
*make assumption that on AVERAGE they enter or leave halfway through the time period
Incidence rate (approximate method):
=number of new cases of a disease that occurred in the population during a particular period of time / (# at risk at the start + # at risk at end)/2
Incidence rate (approximate method): change in population at risk size
Animals that leave:
-withdrawals (culls, deaths)
-cases of the disease (no longer at risk)
Animals that enter:
-purchases
-births
Incidence rate (exact method):
-sometimes called incidence density
-less commonly used
-denominator takes into account the EXACT amount of time each individual was at risk
-calculate per animal-time
Prevalence vs. incidence:
-prevalence is AT
>P=I x D
-incidence is DURING
Changes in prevalence can be due to:
Change in:
-incidence rate
-the average duration of the disease
-BOTH incidence and duration
