A possible role of high impact weather events in waterborne disease outbreaks in Canada, 19752001

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A possible role of high impact weather events in
waterborne disease outbreaks in Canada,
1975-2001
Presented by Kate Thomas
M. Kate Thomas1, Dominique Charron2, David
Waltner-Toews1, Corinne Schuster3 Abdel R.
Maarouf4 and John D. Holt1 1. University of
Guelph, Ontario 2. Public Health Agency of
Canada, Guelph, Ontario 3. HPRP Project, Climate
change effects on waterborne disease risk in
Canada, University of Guelph 4. Environment
Canada, Toronto, Ontario
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Outline

• Introduction
• Study Objectives
• Data
• Methodology
• Results
• Discussion

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Introduction

• Safe water is of great concern to everyone
• Global need to determine what factors play a role
  in waterborne disease outbreaks
• Potential impact of weather on waterborne disease
  outbreaks
• Canadas climate is changing with the potential
  for more variable weather and weather extremes

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Study Objectives

• To describe the incidence and distribution of
  waterborne disease outbreaks in Canada and high
  impact weather events occurring prior to an
  outbreak
• To test the association between high impact
  weather events and waterborne disease outbreaks

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Data Outbreaks

• Waterborne disease outbreak
• 2 or more cases of disease, occurring at the same
  place and the same time, linked to a drinking
  water supply
• 1975 through 2001 in Canada
• 168 in total
• 58 Definite
• 34 Probable
• 76 Possible

92 Outbreaks
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Data Exposure variables

• High Impact Weather Event
• Short term weather event that contributes high
  volumes of water and causes substantial overland
  flow

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Data Exposure variables

• Meteorological stations
• Daily rainfall, maximum and minimum temperature
• As geographically close as possible by latitude
  and longitude co-ordinates
• Rainfall (mm)
• Maximum of rolling 5-day cumulative average
  amount
• Maximum percentile
• Timing
• Temperature (oC days)
• Total maximum degree days above 0 oC
• Maximum of rolling 5-day cumulative average
  amount
• Timing

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Data Exposure variables

• Stream flow stations
• Daily stream flow
• As geographically close as possible by latitude
  and longitude co-ordinates
• Stream flow (m3/s)
• Maximum of rolling 5-day cumulative average
  amount
• Maximum percentile
• Timing
• Stream flow peak
• Outbreaks January May
• Amount of maximum peak
• Percentile of maximum peak
• Timing of peak

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Data Exposure variables

• Ecozone map
• Ecozone map is overlaid with map of outbreaks
• Canada is divided into 15 terrestrial ecozones
  based on soil type, vegetation, climate and
  landforms
• Categorical variable to control interaction
  between soil type and other explanatory variables

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Methodology

• Case-Crossover study design1
• Designed for acute events with transient
  exposures
• Event Waterborne Disease Outbreak
• Exposure High Impact Weather Event
• Individual Community / Well System
  experiencing outbreak
• Hazard Time Period 6 weeks

1. Maclure, Malcolm. The case-crossover design A
method for studying transient effects on the risk
of acute events. Am J Epidemiol 1991133144-53
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Methodology

• Time-stratified case-crossover

• Within each stratum one year is the case (X) the
  other 4 are controls (C)

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Results
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Results
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Results

• Final Model

• Relative odds of an Outbreak
• exp ?1(total maximum degree-days above 0 oC)
• ?2 (accumulated rainfall 93rd percentile) ??jIj
  

• Where Ij are indicator variables for year
  effects and the ?j are the corresponding
  parameters

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Results

• Accumulated rainfall 93rd percentile
• OR2.28311 (95 CI 1.21644 4.28512)
• Total maximum degree days above 0 oC
• OR1.00672 (95 CI 1.00166 -1.01181)

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Results
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Discussion

• Cumulative impact of total maximum degree days
  above 0 oC is substantial

• Significant association between extreme rainfall
  and waterborne disease outbreaks

• Adds to body of literature on weather and
  waterborne disease

• Novel use of Case-Crossover study design

• Provides Canadian perspective

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Acknowledgements

• Dominique Charron
• David Waltner-Toews
• Abdel Maarouf
• John Holt
• Corinne Schuster
• Health Policy Research Program
• Public Health Agency of Canada
• (formerly Health Canada)
• Environment Canada
• Dave Harvey, Don MacIver, Heather Auld, Joan
  Klaassen
• Department of Population Medicine, University of
  Guelph
• ECCHO team www.eccho.ca

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Thank you
Any Questions?
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