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Epidemiology of outbreaks

What are the kinds of skills that people who investigate outbreaks have to have? Dr David Muscatello explains.
DAVID MUSCATELLO: Hello. I’m Dr David Muscatello and I’m going to be talking to you a little bit about the epidemiology of outbreaks.
Put simply, an epidemiologist studies the occurrence of disease in populations. Epidemiologists who are involved in outbreak investigations have to have strong quantitative skills. To understand the severity, risk factors, and possible causes of an outbreak requires analysis of data collected about the people affected by the outbreak.
The first data analysis an epidemiologist would do would be to describe the outbreak in terms of the three fundamental epidemiological dimensions. They are time, person, and place. The time dimension could involve plotting what’s called an epidemic curve, which is simply a graph which has the date the person developed symptoms on the x-axis and the number of cases of people developing symptoms on each date on the y-axis.
On the presentation, you can see an example of an epidemic curve from an outbreak of salmonella infections. It shows the date on the x-axis and the number of cases on the y-axis. Epidemiologists can learn things from an epidemic curve, such as what the time interval is between successive cases in the outbreak. Importantly, if most cases develop their symptoms around the same time, that often indicates a point or common source outbreak. That might happen, for example, if a contaminated food product is sold to many people on the same day, from the same food outlet. The contamination usually occurs unintentionally. But deliberate contamination has been known to occur and should be considered as a possibility.
The person dimension might involve looking at the age distribution of cases. This might give clues about the susceptibility to the disease or exposures that may allow people to come into contact with the disease, for example.
The place dimension might give clues as to where people came into contact with the disease. So displaying a map of where cases live could be useful. One important indicator of severity of an outbreak is the case fatality ratio. This is simply the number of patients who die from the disease divided by the total number of cases of people with the disease. Epidemiologists may also conduct what is known as an analytical study, to provide statistical evidence on the cause of the outbreak. These studies need to collect data, not just on cases of the disease, but also on people who don’t have the disease. These people may be called controls, in what is known as a case control study.
Some outbreaks are suitable for what is called a retrospective cohort study. The cohort study involves comparing the exposures of people who have the disease with exposures among people who don’t have the disease. Different risk of illness or exposure between people with and without the disease provides evidence about the cause of the outbreak.
In this presentation, we discuss the kinds of skills that people who investigate outbreaks have to have. We provide examples of the way outbreak information is presented and analysed, to help the outbreak investigators understand the severity of the outbreak and the people who are most at risk. We also introduce the idea of conducting an analytical study to find the cause of the outbreak.
Recently an outbreak of Zika virus started in Brazil and is spreading across the world. Zika virus spreads by mosquitoes and usually causes a mild illness. If pregnant women are infected, Zika infection can cause their baby to be born with a small head circumference and under-developed brain. This birth anomaly is known as microcephaly. Microcephaly can cause major health and developmental problems for the baby. A situation report for the Zika virus outbreak is linked below.
After watching this video, in the comments section below discuss your answer to the following question:
What does this report tell you about person, time and place for this outbreak?
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