A possible role of high impact A possible role of high impact weather events in weather events in

waterborne disease outbreaks waterborne disease outbreaks in Canada, 1975-2001in 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, Ontario3. HPRP Project, Climate change effects on waterborne disease risk in Canada, University of Guelph 4. Environment Canada, Toronto, Ontario

OutlineOutline

IntroductionIntroduction Study ObjectivesStudy Objectives DataData MethodologyMethodology ResultsResults DiscussionDiscussion

IntroductionIntroduction

Safe water is of great concern to everyoneSafe water is of great concern to everyone Global need to determine what factors Global need to determine what factors

play a role in waterborne disease play a role in waterborne disease outbreaksoutbreaks

Potential impact of weather on waterborne Potential impact of weather on waterborne disease outbreaksdisease outbreaks

Canada’s climate is changing with the Canada’s climate is changing with the potential for more variable weather and potential for more variable weather and weather extremesweather extremes

Study ObjectivesStudy Objectives

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

To test the association between high To test the association between high impact weather events and waterborne impact weather events and waterborne disease outbreaksdisease outbreaks

Data ~ OutbreaksData ~ Outbreaks

Waterborne disease outbreak:Waterborne disease outbreak: 2 or more cases of disease, occurring at the same 2 or more cases of disease, occurring at the same

place and the same time, linked to a drinking water place and the same time, linked to a drinking water supplysupply

1975 through 2001 in Canada1975 through 2001 in Canada

168 in total 168 in total 58 Definite58 Definite 34 Probable34 Probable 76 Possible76 Possible

9292OutbreaksOutbreaks

Data ~ Exposure variablesData ~ Exposure variables

High Impact Weather Event:High Impact Weather Event: Short term weather event that contributes high Short term weather event that contributes high

volumes of water and causes substantial volumes of water and causes substantial overland flowoverland flow

Data ~ Exposure variablesData ~ Exposure variables

Meteorological stationsMeteorological stations Daily rainfall, maximum and minimum temperatureDaily rainfall, maximum and minimum temperature As geographically close as possible by latitude and longitude co-As geographically close as possible by latitude and longitude co-

ordinatesordinates

Rainfall (mm)Rainfall (mm) Maximum of rolling 5-day cumulative average amountMaximum of rolling 5-day cumulative average amount Maximum percentileMaximum percentile TimingTiming

Temperature (Temperature (ooC days)C days) Total maximum degree days aboveTotal maximum degree days above 0 0 ooCC Maximum of rolling 5-day cumulative average amountMaximum of rolling 5-day cumulative average amount TimingTiming

Data ~ Exposure variablesData ~ Exposure variables Stream flow stationsStream flow stations

Daily stream flowDaily stream flow As geographically close as possible by latitude and longitude co-As geographically close as possible by latitude and longitude co-

ordinatesordinates

Stream flow (mStream flow (m33/s)/s) Maximum of rolling 5-day cumulative average amountMaximum of rolling 5-day cumulative average amount Maximum percentileMaximum percentile TimingTiming

Stream flow peakStream flow peak Outbreaks January – May Outbreaks January – May Amount of maximum peakAmount of maximum peak Percentile of maximum peakPercentile of maximum peak Timing of peakTiming of peak

Data ~ Exposure variablesData ~ Exposure variables

Ecozone mapEcozone map Ecozone map is overlaid with map of outbreaksEcozone map is overlaid with map of outbreaks Canada is divided into 15 terrestrial ecozones based Canada is divided into 15 terrestrial ecozones based

on soil type, vegetation, climate and landformson soil type, vegetation, climate and landforms Categorical variable to control interaction between soil Categorical variable to control interaction between soil

type and other explanatory variablestype and other explanatory variables

MethodologyMethodology

Case-Crossover study designCase-Crossover study design11

Designed for acute events with transient Designed for acute events with transient exposuresexposures

EventEvent = Waterborne Disease Outbreak = Waterborne Disease Outbreak ExposureExposure = High Impact Weather Event = High Impact Weather Event IndividualIndividual = Community / Well System = Community / Well System

experiencing outbreakexperiencing outbreak Hazard Time PeriodHazard Time Period = 6 weeks = 6 weeks

1. Maclure, Malcolm. The case-crossover design: A method for studying transient effects on the risk of acute events. Am J Epidemiol 1991;133:144-53

MethodologyMethodology Time-stratified case-crossoverTime-stratified case-crossover

197

5

198

0

198

5

199

0

199

5

200

1

200

0

197

5

197

6

197

7

197

8

197

9

198

0

C X CCC

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

ResultsResults

ResultsResults

0

2

4

6

8

10

12

14

16

Janu

ary

Februa

ry

Marc

hApril

May

June Ju

ly

August

Septem

ber

Octob

er

Novem

ber

Decem

ber

Month

Num

ber

of O

utbr

eaks

.

ResultsResults

Relative odds of an Outbreak =

exp [ 1(total maximum degree-days above 0 oC) +

2 (accumulated rainfall 93rd percentile) + jIj ]

Where Where Ij are indicator variables for year effects and the j are the

corresponding parameters

Final ModelFinal Model

ResultsResults Accumulated rainfall 93Accumulated rainfall 93rdrd percentile percentile

OR=2.28311 (95% CI = 1.21644 – 4.28512)OR=2.28311 (95% CI = 1.21644 – 4.28512)

Total maximum degree days aboveTotal maximum degree days above 0 0 ooCC OR=1.00672 (95% CI = 1.00166 -1.01181)OR=1.00672 (95% CI = 1.00166 -1.01181)

ResultsResults

Average max Average max temp (6-week)temp (6-week) 20 20 ooCC 25 25 ooCC

Total maximum Total maximum degree-day degree-day above 0 above 0 ooCC

840840 10501050

Relative odds of Relative odds of outbreakoutbreak 277.53277.53 1132.741132.74

Impact of 5 Impact of 5 ooC C increase in tempincrease in temp

Over a 4-fold increase in Over a 4-fold increase in

relative odds of outbreakrelative odds of outbreak

DiscussionDiscussion

Significant association between extreme Significant association between extreme rainfall and waterborne disease outbreaks rainfall and waterborne disease outbreaks

Cumulative impact of total maximum Cumulative impact of total maximum degree days abovedegree days above 0 0 ooC is substantialC is substantial

Adds to body of literature on weather and Adds to body of literature on weather and waterborne diseasewaterborne disease

Novel use of Case-Crossover study designNovel use of Case-Crossover study design Provides Canadian perspective Provides Canadian perspective

AcknowledgementsAcknowledgements Dominique CharronDominique Charron David Waltner-Toews David Waltner-Toews Abdel MaaroufAbdel Maarouf John Holt John Holt Corinne SchusterCorinne Schuster Health Policy Research ProgramHealth Policy Research Program Public Health Agency of Canada Public Health Agency of Canada

(formerly Health Canada)(formerly Health Canada) Environment CanadaEnvironment Canada

• Dave Harvey, Don MacIver, Heather Auld, Joan KlaassenDave Harvey, Don MacIver, Heather Auld, Joan Klaassen Department of Population Medicine, University of GuelphDepartment of Population Medicine, University of Guelph ECCHO team ECCHO team www.eccho.cawww.eccho.ca

Thank youThank you

Any Questions?Any Questions?