Open Access Research article

Pertussis resurgence in Toronto, Canada: a population-based study including test-incidence feedback modeling

David N Fisman123*, Patrick Tang4, Tanya Hauck1, Susan Richardson56, Steven J Drews7, Donald E Low458 and Frances Jamieson45

Author Affiliations

1 Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, M5T 3M7, Canada

2 Department of Health Policy, Evaluation and Management, University of Toronto, 155 College Street, Toronto, M5T 3M7, Canada

3 Department of Medicine, University of Toronto, 1 Kings College Circle, Toronto, M5S 1A8, Canada

4 Public Health Laboratory--Toronto, Ontario Agency for Health Protection and Promotion, 81 Resources Road, Toronto, M9P 3V6, Canada

5 Department of Laboratory Medicine and Pathobiology, University of Toronto, 1 Kings College Circle, Toronto, M5S 1A8, Canada

6 Department of Microbiology, Hospital for Sick Children, 555 University Avenue, Toronto M5G 1X5, Canada

7 Alberta Provincial Public Health Laboratory, 3030 Hospital Drive Northwest, Calgary, T2N 4W4, Canada

8 Department of Microbiology, Mount Sinai Hospital, 600 University Avenue, Toronto, M5G 1X5, Canada

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BMC Public Health 2011, 11:694  doi:10.1186/1471-2458-11-694

Published: 7 September 2011

Abstract

Background

Pertussis continues to challenge medical professionals; recently described increases in incidence may be due to age-cohort effects, vaccine effectiveness, or changes in testing patterns. Toronto, Canada has recently experienced increases in pertussis incidence, and provides an ideal jurisdiction for evaluating pertussis epidemiology due to centralized testing. We evaluated pertussis trends in Toronto using all available specimen data, which allowed us to control for changing testing patterns and practices.

Methods

Data included all pertussis culture and PCR test records for Greater Toronto from 1993 to 2007. We estimated incidence trends using Poisson regression models; complex relationships between disease incidence and test submission were explored with vector autoregressive models.

Results

From 1993 to 2007, 26988 specimens were submitted for testing; 2545 (9.4%) were positive. Pertussis incidence was 2 per 100,000 from 1993 to 2004 and increased to 10 per 100,000 from 2005-2007, with a concomitant 6-fold surge in test specimen submissions after the introduction of a new, more sensitive PCR assay. The relative change in incidence was less marked after adjustment for testing volumes. Bidirectional feedbacks between test positivity and test submissions were identified.

Conclusions

Toronto's recent surge in pertussis reflects a true increase in local disease activity; the apparent size of the outbreak has likely been magnified by increasing use of pertussis testing by clinicians, and by improved test sensitivity since 2005. These findings may be applicable to changes in pertussis epidemiology that have been noted elsewhere in North America.