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78 Epidemiology of Infectious Disease 835

Point source epidemic curve Continuous common source
VetBooks.ir 20 10 epidemic curve
Number of animals (cases) 10 Number of animals (cases) 8

15

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0
0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
Date of onset Date of onset

Intermittent common source Propagating epidemic curve
epidemic curve 20 incubation period
Number of animals (cases) 5 4 3 2 1 Number of animals (cases) 10 5
Hypothesized
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15

0
7
5
9 11 13 15 17 19 21 23 25 27 29 31
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Date of onset 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
Date of onset
Figure 78.1 Epidemic curves.
Continuous common source epidemic curves also Separate epidemic curves can be created for subpopu-
indicate a common pathogen exposure in a group of ani- lations of animals based on breed, age, or sex. Differences
mals. Sharp onset and more gradual ending to the out- in the shapes of the curves may indicate potential risk
break are similar to point source epidemics with the factors for disease, helping to identify potential patho-
overall duration of the outbreak extending across a gens or intervention measures with a greater likelihood
longer period of time which overlaps into multiple incu- of success. Combining the temporal spread of a disease
bation periods. with the geographic spread can also provide further
Intermittent common source epidemic curves look insight into potential pathogens and mechanisms for
like a series of point source epidemics across the meas- spread of disease between susceptible populations.
ured time period. These epidemics may be misclassified For example, the CDC has published annual maps of the
as point source in nature when the studied time period is incidence of West Nile virus in people and animals from
too brief to detect the sequential pathogen exposures. 2003 to 2019. These maps provide a visual reference for
Periods between onset of case clusters can be used to the spread of the disease from New York to California
estimate the incubation period of a suspected infectious (wwwn.cdc.gov/arbonet/maps/ADB_Diseases_Map/
pathogen. index.html). Patterns of spread mapped against topo-
Propagating epidemic curves resemble intermittent graphic information may support theories of vector‐
common source epidemics, but subsequent waves of borne or water‐borne disease and again may help with
infection peak at progressively higher levels of infection. designing successful intervention strategies.
These waves of infection represent secondary and ter-
tiary cases of disease as a result of animal‐to‐animal Step 4: Formulate and Test Hypotheses
spread of the pathogen. As with intermittent common
source epidemic curves, periods of time between the Hypotheses related to disease introduction, spread,
onset of waves of infection can be used to estimate incu- cause, and intervention will be formulated based on data
bation period of a suspected infectious pathogen. If the gathered from animals meeting the case definition.
pathogen responsible for the outbreak is yet undeter- If considered in isolation, the data can be informative
mined, incubation period information can be used to but potentially deceptive. Without a nonaffected com-
narrow the list of potential pathogens. parison population, false assumptions about prevalence