DengueDengueVirus, Vector and Virus, Vector and

EpidemiologyEpidemiology

I. VirusI. Virus

Dengue VirusDengue Virus

Causes dengue and dengue hemorrhagic Causes dengue and dengue hemorrhagic feverfever

Is an arbovirusIs an arbovirus Transmitted by mosquitoesTransmitted by mosquitoes Composed of single-stranded RNAComposed of single-stranded RNA Has 4 serotypes (DEN-1, 2, 3, 4)Has 4 serotypes (DEN-1, 2, 3, 4)

Dengue VirusesDengue Viruses

Each serotype provides specific lifetime Each serotype provides specific lifetime immunity, and short-term cross-immunityimmunity, and short-term cross-immunity

All serotypes can cause severe and fatal All serotypes can cause severe and fatal diseasedisease

Genetic variation within serotypesGenetic variation within serotypes Some genetic variants within each serotype Some genetic variants within each serotype

appear to be more virulent or have greater appear to be more virulent or have greater epidemic potentialepidemic potential

Transmission of Dengue VirusTransmission of Dengue Virusby by Aedes aegyptiAedes aegypti

Viremia Viremia

Extrinsic incubation

period

Days0 5 8 12 16 20 24 28

Human #1 Human #2

Illness

Mosquito feeds / acquires virus

Mosquito refeeds / transmits virus

Intrinsicincubation

period

Illness

Replication and TransmissionReplication and Transmissionof Dengue Virus (Part 1)of Dengue Virus (Part 1)

1. Virus transmitted to human in mosquito saliva

2. Virus replicates in target organs

3. Virus infects white blood cells and lymphatic tissues

4. Virus released and circulates in blood

3

4

1

2

Replication and TransmissionReplication and Transmissionof Dengue Virus (Part 2)of Dengue Virus (Part 2)

5. Second mosquito ingests virus with blood

6. Virus replicates in mosquito midgut and other organs, infects salivary glands

7. Virus replicates in salivary glands

6

7

5

II. VectorII. Vector

Aedes aegyptiAedes aegypti Mosquito Mosquito

Aedes aegyptiAedes aegypti

Dengue transmitted by infected female Dengue transmitted by infected female mosquitomosquito

Primarily a daytime feederPrimarily a daytime feeder Lives in and around human habitationLives in and around human habitation Lays eggs and produces larvae Lays eggs and produces larvae

preferentially in artificial containerspreferentially in artificial containers

Aedes aegyptiAedes aegypti Breeding Sites Breeding Sites

III. EpidemiologyIII. Epidemiology

World Distribution of DengueWorld Distribution of Dengue19991999

Areas infested with Areas infested with Aedes aegyptiAedes aegypti

Areas with Areas with Aedes aegypti Aedes aegypti and recent epidemic dengue and recent epidemic dengue

Reported Cases of DengueReported Cases of Denguein the Americas, 1980 - 1998in the Americas, 1980 - 1998

*Provisional data for 1998

0

100

200

300

400

500

600

700

800

Rep

orte

d C

ases

(T

hou

san

ds)

80 82 84 86 88 90 92 94 96 98*

Mean Annual Number of DHF CasesMean Annual Number of DHF CasesThailand, Indonesia and Vietnam, by DecadeThailand, Indonesia and Vietnam, by Decade

* Provisional data through 1998

Reported Cases of DHF inReported Cases of DHF inthe Americas, 1970 - 1998the Americas, 1970 - 1998

0

10000

20000

30000

40000

50000

60000

Rep

orte

d C

ases

1970s 1980s 1990s*

* Provisional data through 1998

Distribution of DEN-3Distribution of DEN-3in the Americas, 1998in the Americas, 1998

19941995

1998

Pan American Health Organization, 1999

1997

Recent Dengue in the U.S.A. Recent Dengue in the U.S.A. (Texas)(Texas)

Dengue epidemics occurred in the USA in the Dengue epidemics occurred in the USA in the 1800s and the first half of the 1900s1800s and the first half of the 1900s

Recent indigenous transmission:Recent indigenous transmission:• 1980: first locally acquired cases since 19451980: first locally acquired cases since 1945• 1986: 9 cases1986: 9 cases• 1995: 7 cases1995: 7 cases• 1997: 3 cases1997: 3 cases

Lack of recent epidemic transmission likely due Lack of recent epidemic transmission likely due to changes in life styleto changes in life style

Reasons for Dengue Expansion Reasons for Dengue Expansion in the Americasin the Americas

Extensive vector infestation, with declining Extensive vector infestation, with declining vector controlvector control

Unreliable water supply systemsUnreliable water supply systems Increasing non-biodegradable containers and Increasing non-biodegradable containers and

poor solid waste disposalpoor solid waste disposal Increased air travelIncreased air travel Increasing human population density in urban Increasing human population density in urban

areasareas

Challenges for the FutureChallenges for the Future

2.5 billion people at risk world-wide2.5 billion people at risk world-wide In the Americas, 50-fold increase in In the Americas, 50-fold increase in

reported cases of DHF (1989-1993 reported cases of DHF (1989-1993 compared to 1984-1988)*compared to 1984-1988)*

Widespread abundance of Widespread abundance of Aedes aegyptiAedes aegypti in in at-risk areasat-risk areas

* Organization of American States, Human Health in the Americas, 1996