1
2
3
4
5
6
The length of the plateau and descending branch will be influenced by factors such as the availability of susceptible animals , the time over which susceptible animals are exposed to the infection source and the minimum and maximum incubation periods of disease . Immunity across the group also affects the shape of the curve and will be influenced by previous exposure to the disease agent or interventions such as vaccination programs .
For outbreaks extending over longer periods , a seven‐day rolling average of the epidemic curve can be useful .
Secondary peaks are seen when there are new , susceptible introductions into the population at risk , movement of infected animals into a new area with susceptible animals or a change in mode of transmission .
HINT When producing the curve , try several different time intervals to best view the pattern and avoid missing secondary peaks .
A guide is to make the time interval ⅛ to ¼ of the estimated incubation period . E . g . EUS has an incubation period of 10 days , the number of new cases should be measured every two days .
7
8
9
Outbreaks can be referred to as being either :
• “ Common ” or “ point ” source : where a large number of cases result from exposure to a common source agent within a short time period . This pattern is generally associated with food or waterborne agents such as toxins or when a large proportion of the population is exposed to an infectious disease simultaneously .
• “ Propagating ” epidemic : where the disease agent spreads either directly or indirectly from an infected host to susceptible animals .
10
28