7 - Indian Council of Medical Research

Indian J Med Res 117, March 2003, pp 104-110
A long-term study on vector abundance & seasonal prevalence
in relation to the occurrence of Japanese encephalitis
in Gorakhpur district, Uttar Pradesh
P.C. Kanojia, P.S. Shetty & G. Geevarghese
National Institute of Virology (ICMR), Pune, India
Received April 26, 2002
Background & objectives: Japanese encephalitis (JE) virus is an important cause of viral encephalitis in
Gorakhpur district, UP. The area has been experiencing outbreaks of JE since 1978. No in-depth
longitudinal studies have been carried out on the mosquito species, particularly JE vectors prevailing
in the area. Entomological studies were carried out in the district in order to determine the species
composition, relative abundance and seasonal prevalence of mosquitoes in relation to the incidence of
JE.
Methods: Three JE affected villages were selected as study sites. Weekly visits were made to each
village and mosquito collections were made in and around the pig enclosures and cattle sheds with the
help of mouth aspirators, aided by flash lights at dusk.
Results: The overall mosquito population showed a bimodal pattern with short and tall peaks during
March and September respectively. Based on the elevated density and infection with JE virus, Culex
tritaeniorhynchus has been considered responsible for causing epidemics in the area. Cx. pseudovishnui,
Cx. whitmorei, Cx. gelidus, Cx. epidesmus, Anopheles subpictus, An. peditaeniatus and Mansonia uniformis
are suspected to have played some role in the epidemiology of JE in the region. JE cases were reported
between August and November with the peak in October when the vectors population, particularly
Cx. tritaeniorhynchus was on the decline.
Interpretation & conclusion: The results of the study indicate that Cx. tritaeniorhynchus, the most likely
vector of JE together with other known vector species remained more active during the period of
paddy cultivation. Integrated antilarval measures before the beginning of paddy irrigation may check
the breeding of JE vectors in the paddy fields. It may prove beneficial in reducing the vector population
during the JE transmission season.
Key words Culex tritaeniorhynchus - Cx. pseudovishnui - Gorakhpur district - Japanese encephalitis - JE transmission period -
vector abundance - vectors of JE
Japanese encephalitis (JE) virus is the most
important and widespread flavivirus in terms of
human morbidity and mortality. Approximately
50,000 sporadic and epidemic cases of JE are
reported annually in Asia, including India 1 . In
104
Gorakhpur, JE virus has been the leading cause of
encephalitis in humans. This district has experienced
two major outbreaks of JE, first in 1978 and another
in 1988 which claimed hundreds of human lives 2,3 .
For the last 40 years, Gorakhpur district has

KANOJIA et al : JE & ITS VECTORS IN GORAKHPUR DISTRICT 105
undergone a lot of ecological changes in the form of
construction of irrigation canals and small dams.
The habitats have been modified for development of
agriculture in the region which has resulted in the
vast expansion of water bodies which support
mosquito breeding. These environmental changes
probably have triggered JE in the region. Information
available on the mosquito species, particularly JE
vectors prevailing in Gorakhpur district is scanty.
Sixteen culicine species were recorded in 1961 4 , but
not in relation to the occurrence of JE. Nine species,
including 5 JE vectors were documented while
conducting entomological studies during the
declining phase of the JE outbreak reported in 1985,
however, this study provides little information on
the endophilic behaviour of JE vectors 5 . Barring this,
no detail information is available on the
entomological aspect of JE in the district, which is
known for its JE endemicity. In view of this,
entomological studies were carried out in order to
determine species composition, relative abundance
and seasonal prevalence of mosquitoes in relation to
the occurrence of JE in the district. The present
paper reports the results of the study carried out
from 1990 to 1996.
Material & Methods
Description of the area: Gorakhpur district has an
area of about 3483.1 km 2 with a population of about
30.66 lakhs with an average density of about 880
persons/km 2 . Paddy is cultivated during the monsoon
season. Out of 410721 hectares of cultivable land,
154601 (37.64%) were under paddy cultivation
(Gorakhpur district agriculture booklet 1990-91).
The harvesting of paddy is completed by the middle
of October, and wheat and gram are grown during
the subsequent months. Sugarcane is an annual crop
in the area. The year can be broadly divided into
three seasons: summer (March to May), monsoon
(June to September) and winter (October to
February). The area receives a total annual rainfall
varying from 807 to 1564 mm. Floods, water
logging and formation of shallow ponds are annual
features in the region.
Mosquito collections: Three villages viz., Bhathat
under Bhathat Primary Health Centre (PHC) area,
Kusmi-takia under Pipraich PHC area and Naharpur
under Chargawan PHC area, were selected for
regular mosquito collection. Each PHC covered
more than one lakh human population. These study
sites were chosen on the basis of past records of JE,
presence of pigs and cultivation of paddy in the area.
Weekly visits were made to each village and
mosquito collections were carried out in and around
pig enclosures and cattle sheds at dusk i.e., from
1915 to 2015 and 1750 to 1850 h during summer
and winter respectively with the help of mouth
aspirators, aided by torch lights. For monitoring
mosquito population, this type of sampling method
is considered superior quantitively as well as
qualitatively 6-8 .
Results
A total of 271355 mosquitoes belonging to 48
species and seven genera were collected in 3032
man hours. Fourteen species formed 98 per cent of
the total catch. In order of preponderance these
were: Cx. quinquefasciatus (25.3%), Cx.
tritaeniorhynchus (23.9%), Cx. pseudovishnui
(10.9%), An. subpictus (10.7%), Mansonia uniformis
(7.0%), An. peditaeniatus (4.9%), Cx. gelidus
(3.2%), Cx. whitmorei (3.0%), Cx. epidesmus
(2.1%), An. annularis (2.0%), An. vagus (1.5%),
Aedes lineatopennis (1.4%), Cx. fuscocephala
(1.2%) and Armigeres subalbatus (1.0%).
Percentages of some of the predominant species are
shown in the Table. The remaining 34 species,
which accounted for two per cent of the total catch
were: An. aconitus, An. barbirostris, An. culicifacies,
An. nigerrimus, An. pallidus, An. stephensi, An.
varuna, An . tessellatus, Ae. albopictus, Ae.
assamensis, Ae. indicus, Ae. jamesi, Ae.
pallidostriatus, Ae. pampangensis, Ae. pipersalatus,
Ae. punctifemoris, Ae. scatophagoides, Ae.
taeniorhynchoides, Ae. yusafi, Aedomiya venustipes,
Cx. bitaeniorhynchus, Cx. cornutus, Cx. fuscanus,
Cx. infula, Cx. (lophoceraomyia) species, Cx.
malayi, Cx. sinensis, Cx. sitiens, Cx. vishnui, Ma.
annulifera, Ma. crassipes, Ma. indiana, Mimomyia
chamberlaini and Mi. luzonensis.
Seasonal population fluctuations: The seasonal
fluctuations in the general mosquito population

106 INDIAN J MED RES, MA RCH 2003
Table. Percentage of some predominant species of mosquitoes collected between 1990 and 1996 in Gorakhpur district, UP
Mosquito species 1990 1991 1992 1993 1994 1995 1996
Culex tritaeniorhynchus 22.9 19.3 24.9 25.8 24.3 27.2 22.1
Cx. pseudovishnui 13.0 10.0 13.8 10.4 11.7 9.8 7.4
Cx. gelidus 3.6 4.3 2.7 3.0 3.2 1.7 4.2
Cx. whitmorei 3.6 1.9 2.4 4.6 1.3 2.2 3.7
Cx. quinquefasciatus 26.2 25.5 16.8 18.1 32.5 27.6 33.4
Anopheles peditaeniatus 2.9 4.6 4.2 6.4 3.7 6.2 6.0
An. subpictus 8.4 14.8 15.6 9.9 9.5 10.2 7.6
Mansonia uniformis 5.8 7.6 8.4 9.0 4.2 7.0 6.0
Total 47660 34439 35337 50372 31234 36300 36013
recorded during the entire study period are shown
in Fig. 1. The combined mosquito population
followed a bimodal pattern. A short early summer
peak was observed in March mainly due to the
high prevalence
of Cx. quinquefasciatus. Later, a tall late monsoon
peak was noticed in September chiefly due to the
high density of Cx. tritaeniorhynchus. The same
seasonal pattern was observed in all the years
except
Fig. 1. Combined mosquito density in relation to the rainfall, temperature and incidence of JE.

KANOJIA et al : JE & ITS VECTORS IN GORAKHPUR DISTRICT 107
in 1994 where the second peak was recorded in
October instead of September. The other culicine
species such as Cx. pseudovishnui, Cx. whitmorei, Cx.
gelidus and Ma. uniformis had also shown peak
occurrence in September. The seasonal prevalence of
some of the Culex species is shown in Fig. 2. Among
the Anopheline species, An. subpictus and An.
peditaeniatus showed high prevalence during July and
September respectively.
During the JE transmission period, Cx.
tritaeniorhynchus was the predominant species,
constituting 32 per cent of the total catch. This was
followed by Cx. pseudovishnui (16.3%), Ma. uniformis
(11.0%), An. peditaeniatus, (8.6%), An. subpictus,
(5.7%), Cx. whitmorei (5.0%), Cx. gelidus, (5.0%), Cx.
epidesmus (3.1%), and Cx. fuscocephala (1.7%).
Epidemiological features: A total of 885 JE cases were
reported from the 20 PHCs of the district between
1990 and 1996 (JE cell, district hospital,
Fig. 2. Seasonal abundance of Culex species.
Gorakhpur). The maximum number of cases
(313) were registered in 1991. Nagar Mahapalika
PHC was the worst affected and 142 JE cases
were reported from this area followed by
Chargawan (116), Pipraich (90), Sardarnagar (79),
Bhathat (71), Jangal Kodia (37), Khorabar (36),
Gaugaha (32), Kauri Ram (29), Bansgaon (28),
Sahjanawa (28), Campierganj (27), Derwa (26),
Piprauli (26), Brahmpur (25), Gola (24), Khajani
(19), Uruwa (16), Belghat (13), Pali (12) and
untraced (9). Generally, cases were reported
between August and November with a peak in
October. But during 1995 and 1996 cases were
reported a little earlier i.e., in June and July
respectively. During 1990, the epidemic
prolonged up to December. Age-wise analysis
revealed that the maximum number of cases
occurred in children aged between 1 and 15 yr
with the highest occurrence in the age group 6-10
yr. It was also observed that males were more
affected than females. The epidemics were
usually preceded by substantial rainfall followed
by an elevation in mosquito population.

108 INDIAN J MED RES, MARCH 2003
Discussion
The combined mosquito population showed two
density peaks, one a short peak in March where
Cx. quinquefasciatus contributed in large measure
to the total density. The population of this species
increases during March due to the availability of
favourable breeding sites containing stagnant water
polluted with decayed organic matter. Another tall
peak was observed in September in which
Cx. tritaeniorhynchus was the main contributor.
This species is known to breed in paddy fields and
its abundance is related to rice cultivation. In
Gorakhpur, a single paddy crop is grown and as a
result the majority of JE vectors have shown one
peak in a year i.e., in September. The occurrence of
JE in the region was closely associated with this
peak. Similarly, a peak of JE vectors was observed
in the Kolar district of Karnataka where a single
paddy crop was raised (NIV, unpublished data). But
two peaks of JE vectors were recorded in the areas
where double paddy crops were cultivated in South
Arcot and Madurai districts of Tamil Nadu 9,10 .
During the JE transmission period, Cx.
tritaeniorhynchus was the most abundant species and
constituted 32 per cent of the total collection. The
major proportionate contribution of this species has
been found to be a typical feature in several JE
affected areas in India 7,8,11,12 Cx. tritaeniorhynchus
has also been incriminated as a major vector of JE
in India as well as in many countries of Southeast
Asia 13-17 . JE cases were first reported in August,
coincided with the high density of mosquitoes,
chiefly contributed by Cx. tritaeniorhynchus.
Generally, the epidemics reached their peak during
October, which coincided with a decline in the
general mosquito population, particularly Cx.
tritaeniorhynchus. Such a relationship between the
epidemic and declining vector population has also
been observed in Japan and this type of pattern was
attributed to the differences in total and infected
populations of vector species 18 . Predominance of the
vector species prior to the occurrence of the
epidemic might hasten the enzootic infection and
result in a large number of infected mosquitoes being
present in the population 8 . One isolate of JE virus
was obtained from Cx. tritaeniorhynchus during the
study period 19 . In view of its abundance and large
number of virus isolates that have been made in
India and elsewhere in Southeast Asia, this species
is likely to be a vector of JE virus in Gorakhpur
district.
Cx. pseudovishnui was the second most abundant
species encountered during the JE transmission
period. This species is considered to be an important
vector of JE in India and Sri Lanka 20,21 .
Cx. whitmorei and Cx. gelidus were the other
abundant species caught during the JE transmission
period. Cx. whitmorei is considered to be a
secondary vector of JE in India and Sri Lanka, where
few isolates of JE virus have been obtained 21,22 .
Cx. gelidus has been recognized as the most
important vector of JE in Sri Lanka, Thailand,
Malaysia, Vietnam and Sarawak 16,17,21,23,24 . But
relatively few isolations have been made from this
species in India 9,25 . Recently, JE virus has been
isolated from Cx. gelidus in Australia 26 .
Cx. fuscocephala was also collected in
appreciable density during the JE transmission
period. This species is an efficient vector of JE in
Thailand 27 . Experimental transmission studies
carried out in that country demonstrated its high
vector efficiency 28 . Isolations from field caught
specimen have been reported from India 9,25 . But
vector competence studies on Cx. fuscocephala
failed to prove its transmission capability though
mosquitoes got infected with JE virus (NIV
unpublished data). Hence, it seems that this species
may differ in its susceptibility in different
geographical areas. The other known vectors of JE
in India viz., Cx. epidesmus, An. subpictus, An.
peditaeniatus and Ma. uniformis 29 were also caught
in good numbers during the JE transmission season.
This suggests that all these species may be playing
some role in the epidemiology of JE in the area. Cx.
quinquefasciatus was found to be the predominant
species during the drier months of the year. A few
isolates of JE virus have been made from this species
in India and Vietnam 24,25 but the species is regarded
as a poor vector of JE 30 .
Surface water bodies like ponds and tanks are the
perennial breeding sources. Antilarval applications

KANOJIA et al : JE & ITS VECTORS IN GORAKHPUR DISTRICT 109
of chemical larvicides (organophosphate),
larvivorous fish (Gambusia affinis) and biological
larvicides (Bacillus thuringiensis) to all the
permanent water bodies before the commencement
of paddy irrigation may check proliferation of
breeding of Cx. tritaeniorhynchus as well as other
JE vectors in the rice fields and may contain the
vector population during JE transmission season.
Acknowledgment
Authors thank Dr D.A. Gadkari, former Director and
Dr V.S. Padbidri, former Officer-in-charge, NIV, Pune for
encouragement and advice. The assistance rendered by the field
staff of the NIV, Field Station, Gorakhpur is also gratefully
acknowledged. Authors also thank the Chief Medical Officer,
District Hospital, Gorakhpur for providing epidemiological data
on encephalitis cases and the Director, Meteorological Centre,
Lucknow and India Meterological Department, Pune for
providing the meteorological data of Gorakhpur district.
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Reprint requests : Dr P.C. Kanojia, Assistant Director, National Institute of Virology (ICMR), 20-A Dr Ambedkar Road,
Pune 411001, India