dengue fever, epidemiology

7/31/2019 Dengue Fever, Epidemiology

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P

Infectious Diseases


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# Arthropod borne Infections are caused by arboviruses.# Over 100 viruses currently classified as arboviruses

produce disease in humans.

# They include the following best known families and genera :

* Family Flaviviridae ,,,,,, Genus Flavivirus,

* Family Togaviridae ,,,,,, Genus Alphavirus,

* Family Bunyaviridae ,,,,,, Genus Bunyavirus,Phlebovirus,


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# Flaviviruses are :

- Either mosquito- or tick-borne,

- but some of them have no recognized vectors.

# Dengue viruses ( DEN-1, DEN-2, DEN-3 and DEN-4).

belong to the Family Flaviviridae , Genus Flavivirus,


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# Dengue virusesare transmitted mainly by the mosquitoes :

Aedes aegypti, and

Aedes albopictus.

#They are capable of infecting humansincidentally causing

the dengue syndrome.


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# The dengue syndrome is either;

$ Asymptomatic infections, or,

$ Symptomatic infections, of tow types;

* Classical dengue fever.

* Dengue haemorrhagic fever without or with shock.


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* All 4 types of dengue fever may have the following manifestations:

AsymptomaticUndifferentiated fever

without haemorrhageSymptomatic Dengue fever

with unusual haemorrhage

No shock

DHF

DSS


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1. Dengue fever is the most common of all the arthropod-

borne viral diseases.

2. Also it is one of the most important emerging diseases

affecting urban & periurban areas. (Tropics, Sub-tropics).

3. Dengue viruses of multiple types are now endemic in mostcountries in the tropics ( In Asia, Africa, Americas ).


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Distribution of dengue, Eastern Hemisphere,CDC, 2005SOURCE :


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Distribution of dengue, Western HemisphereSOURCE : CDC, 2005


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It is estimated that , 2.5 3 billion

people live in the above-mentioned

areas where dengue viruses can be

Transmitted ( At risk population ).


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# 20 30 million infections areexpected to occur in SEAR.


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# It is estimated by WHO that each

year 50 millioninfections occur, with

500.000 cases of DHF and at

least 12.000 deaths, mainly among

children.


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The case fatality rates are high in

major endemic countries about 3.5%


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1. The viruses are maintained in a human / Aedes aegypti

mosquito cycle in urban tropics.

2. Also a monkey / mosquito cycle may serve as a reservoir

in the forests of southeastern Asia and western Africa.


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* Transmission is by the bite of infective mosquitoes, principally

Aedes aegypti. The mosquito becomes infective 8 - 12

days after the viraemic blood meal and remain so for life.

(The Aedes mosquitoes require blood meal every 2-3 days)

( Transovarian transmission )

A i i f i


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Aedes aegypti mosquito feedingThis is a female Aedes aegyptimosquito engorged withblood while feeding. Dengue viruses are transmittedduring the feeding process.


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Aedes mosquitoes are most abundant duringrainy season

Cool humid areas are suitable for Aedesaegypti mosquitoes


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* Aedes mosquitoes breed in dark & cool corners and artificial

accumulations of water in and around houses ( discarded

tins, broken bottles, fire buckets, flower pots, earthen

pots, tree holes etc ). They bite chiefly during the day.

They fly usually for less than 100 metres.


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days.74days, commonly143isIncubation period :*

No direct transmission fromPeriod of communicability :*

person to person, patients are infective for mosquitoes

from shortly before to the end of the febrile

period, usually 3 5 days (viraemic stage).


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* susceptibility in humans is apparently universal, but children

usually have a milder disease than adults.

* Recovery from infection with one Serotype provides lifelong

homologous immunity but only short-term protection against

other serotypes and may exacerbate disease upon subsequent

infections.


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* A prevalence of Aedes aegyptiand Aedes

albopictustogether with,

* Circulation of dengue virus of more than one

type in any particular area ;

tends to be associated with outbreaksof DHF/DSS.


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The first reported epidemics of DF occurred in

1779-1780 in Asia, Africa, and North America. The

near simultaneous occurrence of outbreaks on

three continents indicates that these viruses and

their mosquito vector have had a worldwide

distribution in the tropics for more than 200 years


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# A huge pandemic in 1998 with 1.2 million cases

of DF & DHF reported from 56 countries

is similar to the situation in 2001.


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# The increase of DF & DHF cases is due to;

* Uncontrolled population growth ,

* Urbanization without appropriate water

management,

* Global travel and trade, and

* The erosion of vector control programme.


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References

1- Parks Text Book of Preventive and Social Medicine, 17th Edition, 2003.

2- Control of communicable disease in man, 18th Edition, 2005, an official

report of the American Public Health Association.

3- CDC & WHO websites.

dengue fever, epidemiology

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