IMPACT OF CLIMATE VARIABILITY ON AEDES AEGYPTI INDICES AND DENGUE CASES IN THE CARIBBEAN REGION: A

PROSPECTIVE STUDY

DAVE D.CHADEE, INSECT VECTOR CONTROL

DIVISION, MINISTRY OF HEALTH, TRINIDAD, WEST INDIES

Dengue situation Globally and in the Caribbean Region

Background

• Over 2.5 billion people live in areas where dengue is endemic (latitude 45oN-35oS)

• Dengue causes more illness and death than any other arbovirus disease in humans

• Each year over 100 million cases of dengue and several thousands cases of DHF occur

• DHF is the leading cause of hospitalization and death among children in South East Asia

Dengue transmission within the Caribbean Region

• Dengue is caused by any of 4 different serotypes of the arbovirus (Den 1,2,3,& 4)

• An incubation period of 2-8 days after an infective bite by the Aedes aegypti mosquito.

• The disease usually begins with onset of fever, headaches followed by chills, retro-orbicular pain, photophobia, backache, severe muscle ache and joint pain

• Maculopapular rash, lymph node enlargement, petechiae and haemorrhagic manifestations like epistaxis and gastrointestinal bleeding (WHO 1997)

Emergence of dengue and DHF in the Caribbean Region

Attributed to:

1. anthropological factors including demographic (urbanization) and societal changes

2. post World War II increases in land and sea transportation

3. failure of Aedes aegypti programs due to a lack of political will (Breteau index of 5)

Emergence of dengue and DHF in the Caribbean Region

4. biological factors

a. insecticide resistance

b. infected vectors require longer feeding time

c. change in size and vector potential

d. introduction of new and different strains or serotypes

Emergence of dengue and DHF in the Caribbean Region

5. Climatic factors

a. smaller size mosquitoes (temperature)

b. quicker development time of life cycle

c. faster virus replication time (temp.)

d. increased risk of dengue transmission (temperature and humidity)

INTRODUCTION

Two of the most significant aspects of climate change relates to mosquitoes:

• Monitoring changes in vector abundance and disease transmission

• Indicating where and when intervention for control is appropriate

OBJECTIVESRetrospective and Prospective study

• To determine by a retrospective study whether there is any relationship between climate change, mosquito production and dengue transmission.

• Conduct a prospective study throughout the Caribbean region to examine changes in mosquito densities, climate and dengue transmission.

Map of Caribbean Region

Dengue prevalence and Aedes aegypti Breteau indices in Trinidad

and Tobago (1981-2001)

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1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Year

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Study sites

• We have started work in 5 countries

• Jamaica

• Barbados

• Trinidad and Tobago

• St. Kitts/Nevis

• St. Vincent and the Grenadines

Dengue Data from the Caribbean region (2)

Country 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

AnguillaAntigua & BarbudaArubaBahamasBarbadosBelizeBermudaBr.Virgin IslandsCayman IslandsCuracao & N. AntillesDominicaGrenadaGuyanaJamaicaMontserratSaint LuciaSt. Vincent & GrenadinesSt. Christopher & NevisSurinameTrinidad & TobagoTurks and Caicos IsTotal

120001830300030110280305260

616

2000000500200504004031089

0000000100000

2960370776420

1026

00008900-00180300211

1713060

-3354

03005480000011080367

1031504-

1626

02401

214910701100

29390

118475389271292820

4383

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132002500321346265564

6773588

-4629

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1855274000004121601461

3722081

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3365511

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15742984

07811

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69630000537-

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122660301038270259536

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674205001120290

3013897602594

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Qualitative model of the first prospective year (2002): distribution of dengue cases, vector density and climate

(Trinidad)

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J F M A M J J A S O N D

Months

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Rainfall

Breteau index

Epidemiological triad

Man

• Climate Dengue virus

• Environment

• Mosquito

What is a prospective study

• It measures the burden of disease in a population

• It shows the distribution of exposure to the disease- dengue

Prospective study

a. is distinguished by the fact that it starts with a group of subjects who lack a positive history of the outcome of interest yet are still at risk for it

b. There are two observational points

1. to determine exposure status and eligibility

2. to determine the number of incident cases that develop during the follow-up.

Type of Prospective study

• Population based representing a heterogeneous sample in terms of exposure (cohort)

• Unit of observation and unit of analysis in a cross-sectional study is the individual

Research strategy

• A prospective study is characterized by determining exposure levels at baseline (the present) and follow-up for occurrence of disease in two or more years

• DESIGN PAST PRESENT FUTUREProspective E D

Retrospective E D

Historical Prosp. E E D

E=Exposure D=Disease

Data Collection (March 2003-February 2004)

• Mosquito densities using the Breteau index (no.of positive containers in 100 houses)

• Dengue data to be collected from the Ministry of Health in each country

• Climate Data to be collected from our network of meteorologist or from weather stations nearby

Statistical analysis of Data

• Assessment of risk- 1. Relative risk- a measure of the extent to which those

exposed to a risk factor are likely to get a disease compared with the non-diseased general population.

2. Absolute risk- the incidence rate for a group exposed to

a risk factor

Data analysis

• 3. Attributable risk-

the difference in the incidence of a disease between the exposed (diseased) and non exposed (non-diseased) groups.

. 4. Analysis of Variance (Anova)

the association between climate, mosquito densities, dengue cases, seasonal changes etc.

5. Regression analysis

to determine changes in time and space etc.

Evaluating Epidemiologic Association

• could the association have been observed by chance?

• could the association be due to bias?

• could other confounding variables have accounted for observed relationship?

• to whom does this association apply?

• Does the association represents a cause and effect relationship?

Can we prove causation?

• Is there a logical time relationship• Is there a large relative risk• Can we demonstrate a dose-response

relationship• Is it reversible• Is it consistently found to be present in different

study sites• Is it consistent for various study designs• Is it biologically plausible

Collaborators

• Dr S.C. Rawlins, Caribbean Epidemiology Centre, Trinidad

• Dr A. Chen, University of the West Indies,

Mona, Jamaica