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Predicting the spread of disease
The temporal information displayed in the epidemic curve is useful for analysing the outbreak as it is happening .
However often we want to know what the situation will be tomorrow or next week for planning purposes . The Estimated Dissemination Ratio ( EDR ) is useful in assisting with this planning . In cattle outbreaks of FMD , the EDR at a farm level has been used to predict the trajectory of outbreaks and assist with planning control approaches . The EDR is calculated by the number of new cases in a defined time period ( e . g . seven days ) divided by the number of cases in the previous defined period .
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EDR = cases in x days
cases in previous x days
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Calculating the EDR ( at a tank level ) for “ Happy Abs ” is relatively simple . Let ’ s assume it is day 11 of the outbreak and all the data is in for the previous 10 days . We are interested in what has happened in 5‐day time periods .
The EDR will be 55 / 14 = 3.9 . This is calculated by summing the case tanks from day 6 – 10 of the outbreak divided by the case tanks from day 1 – 5 . An EDR > 1 indicates the epidemic is continuing to grow and an EDR < 1 indicates the epidemic is declining .
If we calculate the EDR for days 11 to 15 and divide by the previous 5 days we calculate the EDR as 45 / 55 = 0.8 . This figure indicates the outbreak is declining . Recall that given this event is only occurring on one farm it is an outbreak , not an epidemic .
Care is required in outbreaks with few cases as very small changes in case numbers will result in a large apparent change in the EDR . Five days was chosen in this instance , but a longer period ( such as a week ) could have been selected .
Photo . Tasmanian salmon farms . ( Photo courtesy of Huon Aquaculture )
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