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Defining and detecting malaria epidemics in south-east Iran.

McKelvie WR, Haghdoost AA, Raeisi A - Malar. J. (2012)

Bottom Line: The best epidemic definition used a minimum duration of four weeks and week-specific and overall smoothed geometric means plus 1.0 standard deviation.It defined 13 epidemics.Untransformed C-SUM alerts also had the highest area under the ROC curve.

View Article: PubMed Central - HTML - PubMed

Affiliation: Diocese of Hyderabad, Ashraf Goth, Rattanabad, PO Box 21, Mirpur Khas 69000, Sindh, Pakistan.

ABSTRACT

Background: A lack of consensus on how to define malaria epidemics has impeded the evaluation of early detection systems. This study aimed to develop local definitions of malaria epidemics in a known malarious area of Iran, and to use that definition to evaluate the validity of several epidemic alert thresholds.

Methods: Epidemic definition variables generated from surveillance data were plotted against weekly malaria counts to assess which most accurately labelled aberrations. Various alert thresholds were then generated from weekly counts or log counts. Finally, the best epidemic definition was used to calculate and compare sensitivities, specificities, detection delays, and areas under ROC curves of the alert thresholds.

Results: The best epidemic definition used a minimum duration of four weeks and week-specific and overall smoothed geometric means plus 1.0 standard deviation. It defined 13 epidemics. A modified C-SUM alert of untransformed weekly counts using a threshold of mean+0.25 SD had the highest combined sensitivity and specificity. Untransformed C-SUM alerts also had the highest area under the ROC curve.

Conclusions: Defining local malaria epidemics using objective criteria facilitated the evaluation of alert thresholds. This approach needs further study to refine epidemic definitions and prospectively evaluate epidemic alerts.

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Related in: MedlinePlus

Weekly indigenous malaria counts in Chabahar City (21 Mar 2003-19 Mar 2008). When weekly case counts (black line) exceed both the mean of baseline counts in the surrounding 3 weeks plus 0.25 standard deviations (solid grey line) and also exceed the mean over all weeks in the baseline plus 0.25 standard deviations (dashed grey line), an alert is triggered (grey dots at top of graph). The epidemic weeks according to the primary definition (weekly smoothed log counts exceeded both weekly and annual means plus 1 standard deviation for at least 4 weeks) are marked with black dots at the top of the graph.
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Figure 1: Weekly indigenous malaria counts in Chabahar City (21 Mar 2003-19 Mar 2008). When weekly case counts (black line) exceed both the mean of baseline counts in the surrounding 3 weeks plus 0.25 standard deviations (solid grey line) and also exceed the mean over all weeks in the baseline plus 0.25 standard deviations (dashed grey line), an alert is triggered (grey dots at top of graph). The epidemic weeks according to the primary definition (weekly smoothed log counts exceeded both weekly and annual means plus 1 standard deviation for at least 4 weeks) are marked with black dots at the top of the graph.

Mentions: Six different alert types were tested, each consisting of both a week-specific and an annual threshold. Positive alerts required that weekly counts exceeded both weekly and annual thresholds. For all six alert types, annual thresholds were calculated using all weekly counts in the baseline. Weekly thresholds were calculated in one of two ways: 1) for "one week span" alerts, weekly thresholds were calculated from baseline counts corresponding to the current week alone; 2) for "three-week span" alerts, weekly thresholds were calculated from baseline counts corresponding to the previous, current, and following weeks. Figure 1 shows how this worked at one centre for one alert type using a three-week span.


Defining and detecting malaria epidemics in south-east Iran.

McKelvie WR, Haghdoost AA, Raeisi A - Malar. J. (2012)

Weekly indigenous malaria counts in Chabahar City (21 Mar 2003-19 Mar 2008). When weekly case counts (black line) exceed both the mean of baseline counts in the surrounding 3 weeks plus 0.25 standard deviations (solid grey line) and also exceed the mean over all weeks in the baseline plus 0.25 standard deviations (dashed grey line), an alert is triggered (grey dots at top of graph). The epidemic weeks according to the primary definition (weekly smoothed log counts exceeded both weekly and annual means plus 1 standard deviation for at least 4 weeks) are marked with black dots at the top of the graph.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3376027&req=5

Figure 1: Weekly indigenous malaria counts in Chabahar City (21 Mar 2003-19 Mar 2008). When weekly case counts (black line) exceed both the mean of baseline counts in the surrounding 3 weeks plus 0.25 standard deviations (solid grey line) and also exceed the mean over all weeks in the baseline plus 0.25 standard deviations (dashed grey line), an alert is triggered (grey dots at top of graph). The epidemic weeks according to the primary definition (weekly smoothed log counts exceeded both weekly and annual means plus 1 standard deviation for at least 4 weeks) are marked with black dots at the top of the graph.
Mentions: Six different alert types were tested, each consisting of both a week-specific and an annual threshold. Positive alerts required that weekly counts exceeded both weekly and annual thresholds. For all six alert types, annual thresholds were calculated using all weekly counts in the baseline. Weekly thresholds were calculated in one of two ways: 1) for "one week span" alerts, weekly thresholds were calculated from baseline counts corresponding to the current week alone; 2) for "three-week span" alerts, weekly thresholds were calculated from baseline counts corresponding to the previous, current, and following weeks. Figure 1 shows how this worked at one centre for one alert type using a three-week span.

Bottom Line: The best epidemic definition used a minimum duration of four weeks and week-specific and overall smoothed geometric means plus 1.0 standard deviation.It defined 13 epidemics.Untransformed C-SUM alerts also had the highest area under the ROC curve.

View Article: PubMed Central - HTML - PubMed

Affiliation: Diocese of Hyderabad, Ashraf Goth, Rattanabad, PO Box 21, Mirpur Khas 69000, Sindh, Pakistan.

ABSTRACT

Background: A lack of consensus on how to define malaria epidemics has impeded the evaluation of early detection systems. This study aimed to develop local definitions of malaria epidemics in a known malarious area of Iran, and to use that definition to evaluate the validity of several epidemic alert thresholds.

Methods: Epidemic definition variables generated from surveillance data were plotted against weekly malaria counts to assess which most accurately labelled aberrations. Various alert thresholds were then generated from weekly counts or log counts. Finally, the best epidemic definition was used to calculate and compare sensitivities, specificities, detection delays, and areas under ROC curves of the alert thresholds.

Results: The best epidemic definition used a minimum duration of four weeks and week-specific and overall smoothed geometric means plus 1.0 standard deviation. It defined 13 epidemics. A modified C-SUM alert of untransformed weekly counts using a threshold of mean+0.25 SD had the highest combined sensitivity and specificity. Untransformed C-SUM alerts also had the highest area under the ROC curve.

Conclusions: Defining local malaria epidemics using objective criteria facilitated the evaluation of alert thresholds. This approach needs further study to refine epidemic definitions and prospectively evaluate epidemic alerts.

Show MeSH
Related in: MedlinePlus