Disease Outbreak Investigation Flashcards
What is an outbreak?
the occurrence of more cases than expected (even one case can count!)
- disease of health-related event occurring in excess than would be normally expected for a given time period, area, or among a specific group of animals
- series of disease events clustered in time, involving a part of a herd, whole herd, country, or the world
What is the difference between an epidemic and a pandemic?
EPIDEMIC - and outbreak in a larger geographic area
PANDEMIC - epidemic that has spread to severe countries or continents
What are some local, regional, and national/international sources of information used to detect an outbreak?
LOCAL - reports from field veterinaries and producers
REGIONAL - reports from laboratories and routine surveillance activities
NATIONAL/INTERNATIONAL - general public/media and online web-servers, like ProMed, HealthMap, WOAH, and WHO
What are the 5 overall goals of an outbreak investigation?
- identify source of illness
- put in place timely health control interventions to stop the outbreak (prevent new cases)
- increase knowledge of disease (transmission, risk factors, etc.)
- prevent new episodes and outbreaks
- evaluate or establish surveillance system
What 6 things are specifically identified during outbreak investigation?
- causal agent
- mode of transmission
- source of infection
- carriers (if any)
- population at risk
- risk factors associated with disease
What are the 8 steps of an outbreak investigation?
- verify the outbreak - sporadic, unrelated cases may look like an outbreak
- establish case definition
- enhance surveillance
- describe the outbreak
- formulate a hypothesis
- test hypothesis - analyze data
- implement control interventions
- communicate findings
What should be done before leaving for the investigation to verify an outbreak? How is an outbreak verified?
gather preliminary information, like area affected, species involved, animals health situation, and likely cause of disease
characterize the problem with the aim to formulate a working case definition and verify clinical signs, morbidity, and mortality
Do cases need to be lab-confirmed at the time of verifying an outbreak?
no - can still investigate without knowing the agent, sufficient clinical signs is enough
How can we be sure that current cases are in true excess during investigation?
- ensure the disease event is clustered in time, place, or animal/person —> do not rely on rumors, memories, or perceptions
- be aware of artefactual causes in increase (or decrease) in frequencies of reporting cases —> change in reporting practice, increased interest
What is a case definition? What is the point of establishing it?
operational criteria for person, place, time, and clinical features of a case
allows for simple and uniform way to identify cases, standardize the investigation, and ensure the outcome is consistently defined across space and time
What are 2 parts of a good case definition?
- characteristic of the population at risk
- what distinguishes cases from other members of the population (clinical signs, lab results)
What are the 3 categories of cases?
- CONFIRMED - clinical signs are characteristic of a particular agent, confirmed laboratory test results, epidemiological link (little room for doubt)
- PROBABLE - clinical signs confirmed, no laboratory confirmation or epidemiological link
- SUSPECT - reported, unconfirmed clinical signs and no laboratory or epidemiological link (hearsay, reports)
Jura Salmonella typhimurium outbreak, confirmed vs. probable cases:
CONFIRMED = lab detection
PROBABLE = no lab detection, but in contact with confirmed case
What is active surveillance? Why is intensive surveillance required?
field investigation with interviewing the public about contact with cases, animal movement, cases in the area, etc.
allows for the accumulation of enough cases to draw conclusions and document success/failure to recommend interventions
What is an effective tool to enhance surveillance at the national level?
media coverage - radio, TV, producer organizations/publications
Why are reported cases just the tip of the iceberg?
- active cases in the community
- mild cases or no clinical signs
- self-recovering
- treatment by owner without reporting
- deaths
all potential spreaders that may not be recognized
Why is it important to conduct surveillance while considering spectrum of disease?
there is a specific spectrum of observable disease —> mild, severe, fatal —> many areas where disease is present but not observed
How are outbreaks described?
- WHO (person/animal) - identify those infected, species, breed, sex, age, and diet, and compare the number of affected with non-affected
- WHERE (place) - locations of infected —> create maps with cases of distribution
- WHEN (time) - time of infection, create epidemic curve using various time intervals, establish type of outbreak
According to the data, who in the population is most at risk of becoming infected with Salmonella typhimurium in Jura?
children
What 3 things do epidemic curves allow us to determine?
- probable/actual date of exposure
- incubation period of disease
- mode of transmission
What are some point sources of exposure? What 3 epidemic curves could be generated from this information?
food, water, vectors
- point source without propagation - all cases appear to occur within one incubation period and do not spread
- continuing point source - group are exposed to a single noxious influence and exposure continues over a long time
- intermittent point source - common source that is not well controlled, so outbreaks recur (flood + anthrax)
What are propagative epidemic curves? What are the 2 types?
cases spread from person-to-person or animal-to-animal
- index case with limited propagation - single “index” case infects other people and cases arise after an incubation period
- index case with propagation - index case spreads and develops into a full-blown epidemic with secondary cases infecting new individuals
How can the probable time of exposure be calculated if only the time of onset of illness is known and the epidemic curve suggests a point source outbreak?
- determine the median time of onset of illness
- calculate the time between occurrence of first and last cases (width of epidemic curve)
- count back this amount of time from the median to obtain probable time of exposure
How can the probable time of exposure be calculated if the organism and the time of onset of illness are known and suggests a point-source outbreak? When can these methods not be used?
- find minimum and maximum incubation period for the disease
- identify the last case and count bac on the x-axis one maximum incubation period
- identify the first case and count back the minimum incubation period
- two dates will be similar and represent the probable period of exposure
if secondary spread is involved or exposure is prolonged
Jura Salmonella typhimurium outbreak, hypotheses:
What is the general rule of thumb for the number of individuals to include in the control group of case-control studies?
30 in a sample size shows increased power
Jura Salmonella typhimurium outbreak, case-control study:
- questionnaires were filled out by both groups
- <1 OR = protective!
- 3.89 is the most statistically significant - ingestion of Morbier cheese puts people at higher risk for infection
What measure of association is best calculated in case-control studies? Why?
odds ratio —> odds will always be the same (or extremely similar) when comparing odds of exposure in diseased and odds of disease in exposed
Jura Salmonella typhimurium outbreak, analysis:
Early implementation of control interventions:
CDC steps for outbreak investigation:
