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Open Access Research article

Relationship between the population incidence of pertussis in children in New South Wales, Australia and emergency department visits with cough: a time series analysis

Aaron W Cashmore12, David J Muscatello23*, Alistair Merrifield3, Paula Spokes3, Kristine Macartney45 and Bin B Jalaludin26

Author Affiliations

1 New South Wales Public Health Officer Training Program, New South Wales Ministry of Health, North Sydney, New South Wales, Australia

2 Population and Public Health Division, New South Wales Ministry of Health, North Sydney, New South Wales, Australia

3 School of Public Health and Community Medicine, University of New South Wales, Kensington, New South Wales, Australia

4 National Centre for Immunisation Research and Surveillance, Westmead, New South Wales, Australia

5 Discipline of Paediatrics and Child Health, University of Sydney, School of Medicine, Sydney, New South Wales, Australia

6 Centre for Research, Evidence Management and Surveillance, Sydney and South Western Sydney Local Health Districts, Liverpool, NSW, Australia

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BMC Medical Informatics and Decision Making 2013, 13:40  doi:10.1186/1472-6947-13-40

Published: 28 March 2013

Abstract

Background

Little is known about the potential of syndromic surveillance to provide early warning of pertussis outbreaks. We conducted a time series analysis to assess whether an emergency department (ED) cough syndrome would respond to changes in the incidence of pertussis in children aged under 10 years in New South Wales (NSW), Australia, and to evaluate the timing of any association. A further aim was to assess the lag between the onset of pertussis symptoms and case notification in the infectious diseases surveillance system in NSW.

Methods

Using routinely collected data, we prepared a daily count time series of visits to NSW EDs assigned a provisional diagnosis of cough. Separate daily series were prepared for three independent variables: notifications of cases of pertussis and influenza and ED visits with bronchiolitis (a proxy measure of respiratory syncytial virus (RSV) infection). The study period was 1/1/2007-31/12/2010. A negative binomial multivariate model was used to assess associations between the outcome and independent variables. We also evaluated the median delay in days between the estimated onset of a case of pertussis and the date the local public health authority was notified of that case.

Results

When notified pertussis increased by 10 cases in one day, ED visits with cough increased by 5.2% (95% confidence interval (CI): 0.5%-10.0%) seven days later. Daily increases in the other independent variables had a smaller impact on cough visits. When notified influenza increased by 10 cases in one day, ED visits with cough increased by 0.8% (95% CI: 0%-1.7%) seven days later. When ED visits with bronchiolitis increased by 10 visits in one day, ED visits with cough increased by 4.8% (95% CI: 1.2%-8.6%) one day earlier. The median interval between estimated onset of pertussis and case notification was seven days.

Conclusions

Pertussis appears to be an important driver of ED visits with cough in children aged under 10 years. However, the median delay in notification of cases of pertussis was similar to the lag in the pertussis-associated short-term increases in ED visits with cough. Elevations in RSV and influenza activity may also explain increases in the ED cough syndrome. Real time monitoring of ED visits with cough in children is therefore unlikely to consistently detect a potential outbreak of pertussis before passive surveillance.

Keywords:
Pertussis; Syndromic surveillance; Time series analysis