Persistency of transovarian dengue virus
in Aedes aegypt i
I
NTRODUCTIONDengue virus (DENV) infect ion cont inuous present s a serious healt h
problem in many t ropical areas of t he world. The disease is hyperendemic in
Sout heast Asia, where a more severe form, dengue haemorrhagic fever (DHF)
and dengue shock syndrome (DSS) is a maj or public healt h concern because of
t he severe and oft en f at al disease it causes in children (Rohani et al . , 1997).
Annually, 100 million cases of dengue fever (DF) and half a million DHF occur
worldwide. Ninet y percent of DHF subj ect s are children less t han 15 years of
age (Malange et al . , 2004).
In many t ropical and subt ropical areas, DENV infect ion is endemic and
occurs periodically or annual out breaks of disease (Gubler and Trent , 1994).
DENVs are t ransmit t ed t o human t hrough t he bit e of inf ect ed
mosquit oes. For many years, members of t he subgenus St egomyia, especially
Aedes aegypt i (Linn) and Aedes al bopict us (Skuse) have been recognized as
t he primary vect or of dengue (Boromisa et al ., 1987; Gubler, 1987; Rohani et
al ., 1997).
In t he absence of a saf e and ef f ect ive vaccine, t he prevent ion and
cont rol of DF/ DHF are dependent upon t he cont rol of t he vect or mosquit oes.
The Aedes cont rol st rat egy has focused mainly on surveillance for and
eliminat ion of Aedes larval breeding habit at s and emergency cont rol of adult s
(Chow et al . , 1998).
The cyclic nat ure of dengue epidemics and how t he virus is
maint ained during int er-epidemic periods in areas, where out breaks have
occurred previously pose quest ions, which have led t o st udies t o evaluat e t he
import ance of t ransovarian t ransmission in DENV maint enance (Lee et al . ,
1997).
D
ENGUE VIRUSDENV (serot ype I, II, III, IV) are members of t he family Flaviviridae.
They are plus sense, ss RNA viruses t hat cause dengue in human. The nat ural
hist ory of t he viruses suggest t hat t he viruses are biologically high adapt ed t o
t he mosquit o host s and t hey were most likely mosquit o viruses prior t o
becoming adapt ed t o lower primat es and humans(Gubler, 1997). The viruses
are maint ained in a ‘ human-mosquit o-human cycle’ . In addit ion t o t he
maint enance of dengue virus at t he low level of t ransmission wit hin human
populat ion, t ransovarian t ransmission is considered t o be an import ant aspect
in it s maint enance during int er-epidemic periods in species ot her t han it s
primary vect ors (Rodhain and Rosen, 1997).
Aedes aegypt i, PRINCIPAL VECTOR
Dengue inf ect ion f ollows t he bit e of a compet ent mosquit o vect or,
principally Aedes aegypt i, infect ed wit h one of t he four DENV serot ypes
(Plat t et al . , 1997). It breeds in st agnant wat er in all f orms of recept acles in
urban areas, especially following int ermit t ent rainfall in t ropical region
(Gubler, 1997).
Aedes aegypt i is considered t o be t he primary vect or of dengue
t hroughout t he world because of it s high degree of suscept ibilit y t o virus
inf ect ion and it s close associat ion wit h human habit at ion (Gubler, 1997).
Furt hermore, t he habit s of t he mosquit o of t aking more t han one blood meal
during a gonot hropic cycle can dramat ically increase t he vect orial capacit y
(Chow et al ., 1998). It has been demonst rat ed recent ly t hat most arbovirus
deposit ions occur ext ravascularly during probing. Ext ended period of probing
f or inf ect ed Aedes aegypt i would be expect ed t o enhance DENV t ransmission
(Plat t et al , 1997).
It has also been shown t hat suscept ibilit y of Aedes aegypt i t o DENV is
associat ed wit h vert ical t ransmission rat es, since higher t ransovarian
t ransmission in a st rain could be due t o t he presence of a higher number of
suscept ible, inf ect ed f emales (Joshi et al . ,2002).
The refract oriness of Aedes aegypt i mosquit oes t o bot h Flaviviruses
and Alphavirus is a dominant genet ic t rait . It is known t hat suscept ibilit y of
Aedes aegypt i t o Flaviviruses and Alphavirus is polygenic and a quant it at ive
t rait (Joshi et al ., 2002).
T
RANSOVARIAN TRANSMISSIONDespit e ext ensive research in vaccine development , t here is at
present no known met hod of cont rolling dengue, except by t he mosquit o
vect ors. Virologic surveillance, which involves t he cont rolled of DENV
infect ion in human, has been used as an early warning syst em t o predict
out break (Lam, 1993; Seah et al . , 1995). Such surveillance is based on
isolat ion of DENV from human serum by cell cult ure or mosquit o inoculat ion
and t ype specific ident ificat ion by immunof luorescent . This approach is less
ef f ect ive since t he virus is in t he human populat ion. A more ef f ect ive
approach is t o det ect t he virus in t he mosquit oes before it is int roduced int o
t he human populat ion (Rohani et al ., 2005).
One of t he great myst eries about t he epidemiology of dengue is how
t he viruses persist during int er-epidemic periods. Recent report s of
t ransovarian t ransmission of dengue were document ed (Rosen et al .,1998).
The demonst rat ion of t ransovarian t ransmission of dengue virus in t he st rain
of Aedes aegypt i, t he principal vect or, may play a significant role in t he
maint enance of DENV in nat ure (Lee et al .,1997). Furt her st udy by Maurya et
al . (2001) have shown t hat t ransovarially infect ed mosquit oes can orally
t ransmit t he virus. The st udy also suggest ed t hat when eggs obt ained from
inf ect ed f emales af t er several weeks of incubat ion at room t emperat ure, t he
vert ical t ransmission increases. Indicat ing t hat prior t o summer, if
mosquit oes get an opport unit y t o feed on viremic host and females lay eggs
in microniches, where some proport ions of eggs survive t hrough t he int
er-epidemic season, t he emerging adult s may init iat e t he ‘ human-mosquit
o-human cycle’ . It also envisages t he observat ion in underst anding t he possible
role of Aedes aegypt i in t he maint enance of virus in t ransovarian passage
over different generat ion (Maurya et al ., 2001).
Progeny of Aedes aegypt i mosquit oes infect ed wit h DENV was reared
t o subsequent generat ion. For each generat ion, blood-fed females were
confined individually and t he eggs obt ained f rom t he t ransovarian inf ect ed
females were pooled. The sevent h generat ion obt ained from t he infect ed
parent al mosquit oes showed t hat virus could persist in mosquit oes int o
successive generat ions t hrough t ransovarian passage (Joshi et al ., 2002).
Recent st udy suggest s t he occurrence of t ransovarian t ransmission of
DENV by Aedes aegypt i and Aedes al bopict us. Inf ect ed larvae were recovered
from 16 locat ions (10 in Trengganu, 5 in Kuala Lumpur, and 1 in Pahang).
Virus infect ion rat es (VIR) were higher in Aedes aegypt i (13.7%) compared t o
Aedes al bopict us (4.2%) (Rohani et al ., 2005).
Informat ion on t he abilit y of t he local vect ors t o t ransovarially
t ransmit DENV will be useful in assist ing t he public healt h personnel and t he
general public in implement ing a more effect ive campaign against dengue
and t he vect ors; for if t he t ransovarian t ransmission of DENV occurs, it is
obvious t hat t he cont rol of t he immat ure st ages of Aedes mosquit o and t he
eliminat ion of breeding sources must be furt her emphasized and priorit ized
(Rohani et al ., 1997).
R
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