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Seasonal Abundance of Larval Stage of Culex Species Mosquitoes (Diptera: Culicidae) in an Endemic Area of Japanese Encephalitis in Mysore, India



Fakoorziba , Mohammad Reza and Vijayan V. Achuthan
 
ABSTRACT

Seasonal abundance of larval stage of Culex species including Culex vishnui subgroup, the major vectors of JE virus, was studied in Mysore and Mandya, Karnataka state, India during 2002-2004. The larvae of Culex sp. have been collected from temporary and semi permanent ground water pools. Simultaneously they have also been collected from plain rice field area from Mandya district an endemic area of Japanese encephalitis. Culex species including Culex bitaeniorhynchus, Cx. nigropunctatus, Cx. barraudi and Cx. hutchinsoni were seen only in paddy field habitat, whereas Cx. quinquefasciatus and Cx. gelidus were found only in ground pool habitats. Culex tritaeniorhynchus, Cx. vishnui, Cx. pseudovishnui, Cx. fuscocephala, Cx. murreli and Cx. fuscanus were recorded from both the breeding places. Cx. tritaeniorhynchus from Mysore city in ground pools breeds throughout the year except February and April, with a maximum density of 50.00 larvae per dip in May and the lowest being 0.25 larva per dip in September (2002-04). However Culex tritaeniorhynchus from Mandya district in paddy field was found to breed throughout the year, with the maximum density of 11.50 larvae per dip in March and the minimum being 0.06 per dip in October (2002-04). Two peaks of larval density of Cx. tritaeniorhynchus was observed in October and May in Mysore city during 2002-04 which coincides with the JE transmission season in Karnataka state. The seasonal abundance of Culex sp. from two habitats especially vectors of Japanese encephalitis are discussed.

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  How to cite this article:

Fakoorziba , Mohammad Reza and Vijayan V. Achuthan , 2006. Seasonal Abundance of Larval Stage of Culex Species Mosquitoes (Diptera: Culicidae) in an Endemic Area of Japanese Encephalitis in Mysore, India. Pakistan Journal of Biological Sciences, 9: 2468-2472.

DOI: 10.3923/pjbs.2006.2468.2472

URL: https://scialert.net/abstract/?doi=pjbs.2006.2468.2472

INTRODUCTION

Diseases transmitted by arthropods are among the major causes of illness and death in many tropical and subtropical countries. As such malaria, Japanese encephalitis (JE), filariasis, leishmaniasis, dengue and trypanosomiasis represent a significant impediment to economic development. Among these communicable diseases, Japanese encephalitis is endemic in at least 21 countries. It is the leading cause of viral encephalitis in Asia, with 30,000-50,000 cases annually (Gingrich et al., 2001). In India an estimated 378 million people are living at the risk of JE in 12 States including Karnataka State (Nagabhushana, 2000). Outbreaks of JE of varying intensity have been occurring in Kolar and Mandya districts of Karnataka state since 1977 (Mishra, 1984). In this area the JE virus has been isolated from more than 10 species of mosquitoes, of these five species viz., Culex tritaeniorhynchus, Cx. vishnui, Cx. pseudovishnui, Cx. gelides and Cx. fuscocephala are considered as important vectors responsible for the transmission of JE Another report shows that JE virus has been isolated from 16 species of mosquitoes in India, the Culex vishnui subgroup being the major vectors consisting of Cx. tritaeniorhynchus, Giles. Cx. vishnui, Theobald and Cx. pseudovishnui, Colless (Samuel et al., 2000). Cx. tritaeniorhynchus is considered as the most important vector of JE virus in most part of the South central Asia (Gingrich et al., 2001). Earlier observation made on Cx. tritaeniorhynchus has revealed two morphological variants from the two breeding sources of Mysore. The variety from Mysore city (ground pools) as type A and the variety from outside Mysore (paddy field) as type B (Fakoorziba and Vijayan, 2003).

Seasonal abundance study is useful when there is a need to know the density of the species and its potential in transmitting any particular disease (Rahman et al., 1977). In the other hands, study of the seasonal abundance of vector populations it is absolutely essential to develop suitable vector control strategies (Murty et al., 2002). So, in this investigation seasonal abundance of Culex species in different habitats in an endemic area of JE was studied.

MATERIALS AND METHODS

Breeding habitats surveyed: Mysore city and outskirts of Karnataka state, India were selected for the present study mainly because of the endemicity of JE and prevalence of Cx. tritaeniorhynchus, the major vector of JE. This city is situated at an altitude of 1085 MSL in a saucer shaped basin with Chamundi hill at the Southeast end. The city has an area of 40.05 km2 with the latitudinal and longitudinal coordinates of location, 12°18’ N and 76°42’ E. The city has a salubrious climate even though it is located in the tropics and in the interior of the peninsula. The average annual rainfall of about 86 mm and the irrigation facility in the rice fields are quite congenial for the proliferation of mosquitoes. This tropical city has no distinct spring, summer, autumn and winter seasons. But three seasons such as pre-monsoon, the hot season (March to May); monsoon, the rainy season (June to October) and post-monsoon, the cold season (November to February) could be differentiated.

There are quite a few tanks, ponds, puddles, ditches, ground pools etc., present throughout the city extensions which form the major sources of mosquito breeding (Vijayan and Ningegowda, 1993). Ground pools selected for this study are fields situated near the Mysore University Campus, which get flooded during rainy season and become grassy. Paddy fields situated around 26 km radius outside Mysore (Mandya district) are cultivated at least twice a year. These fields receive water from the famous Krishnarajasagar Dam.

Larval collection and rearing: The present investigation by the author was undertaken between January 2002 and January 2004. The larvae of Culex species have been collected from temporary and semi permanent ground water pools. Simultaneously they have also been collected from plain rice field area from a 26 km radius outside Mysore. An enamel dipper of 13 cm width and 6 cm depth with 350 mL capacity, attached to a long handle was employed for taking larval samples. In ground pools of about 10 to 12 m2 area, an average of 25 dipper samples were taken covering the whole water body. In paddy fields too varying number of dipper samples were taken covering all the sides of the field following the method of Service (1976). In all the habitats sampling was done twice every month. The larval density was calculated as the average number of immature per dip collected from each habitat. Larval counts were made carefully and the identification of species done following standard keys (Barraud, 1934; Sirivanakarn, 1976; Reuben, 1968). Third instar larvae were taken for categorization and reared until adult emergence for further confirmation of the species.

Data analysis: A total of 344 individual larvae were subjected to the investigation. Statistical description including mean, mode, standard deviation, standard error, range (min-max) and variance for all variables were processed with SPSS/pc ver 10.00. Data were also analyzed with this program following one-way ANOVA (analysis of variance).

RESULTS

Different Culex species found to breed in habitats surveyed. Culex bitaeniorhynchus, Cx. nigropunctatus, Cx. barraudi and Cx. hutchinsoni, were seen only in paddy field habitats, whereas Cx. quinquefasciatus and Cx. gelidus were found only in ground pool habitats.

Table 1: Larval density of different Cx. sp. in ground pools during 2002 and 2004
Each cell shows density in 2002-2003 (in parenthesis) and mean value

Table 2: Larval density of different Cx. sp. in paddy fields during 2002 and 2004
Each cell shows density in 2002-2003 (in parenthesis) and mean value

Culex tritaeniorhynchus, Cx. vishnui, Cx. pseudovishnui, Cx. fuscocephala, Cx. murreli and Cx. fuscanus were recorded from both the breeding places. The analysis of relative abundance of Culex species revealed that, in ground pools during 2002, prevalence of Cx. quinquefasciatus was maximum (51.91%), followed by Cx. tritaeniorhynchus (29.40%), Cx. vishnui (8.32%), Cx. fuscocephala (6.14%), Cx. fuscanus (2.14%), Cx. pseudovishnui (1.66%), Cx. murreli, (0.40%) and Cx. gelidus (0.02%) (Table 1). However in paddy field during 2002 the prevalence of Cx. tritaeniorhynchus was the highest (42.16%) followed by Cx. fuscocephala (22.49%), Cx. bitaeniorhynchus (15.92%), Cx. vishnui (10.70%), Cx. pseudovishnui (6.64%), Cx. fuscanus (1.97%), Cx. barraudi (0.11%) and Cx. nigropunctatus (0.01%) (Table 2).

Table 3: Influence of habitat on the larval density of Cx. tritaeniorhynchus during the years 2002 and 2004
Differences are statistically significant by one way (ANOVA) (p-value<0.01)

Likewise during 2003 at Mysore city ground pools, the prevalence of Cx. quinquefasciatus was the highest (52.13%) (Table 1) while in paddy fields Cx. fuscocephala (55.55%) showed the maximum density (Table 2).

The mean value for Culex tritaeniorhynchus (Type A) density in Mysore city, ground pool habitats, was 59.64 larvae per dip whereas for Type B that was 11.09 larvae per dip in Mandya district, rice field habitats (Table 2).

DISCUSSION

The relative abundance of Culex species in Mysore city during 2002 coincided with the data of the following year. However during the year 2002, abundance of Cx. tritaeniorhynchus type B (42.16%) was maximum among the Culex species followed by Cx. fuscocephala (22.49%), Cx. bitaeniorhynchus (15.92%), Cx. vishnui (10.71%) and Cx. pseudovishnui (6.64%) (Table 2). Further, during the year 2003, the relative abundance of Cx. fuscocephala (55.55%) in paddy field habitats was high among the Culex species followed by Cx. tritaeniorhynchus type B (25.16%), Cx. pseudovishnui (14.83%), Cx. bitaeniorhynchus (2.11%) and Cx. vishnui (1.64%) (Table 2). During the year 2003, due to shortage of irrigation water in the rice fields, paddy cultivation has declined and agricultural activities changed. So, the relative abundance of Culex species showed difference between 2002 and 2003. Similar to this, Takagi et al. (1997) have pointed out that modification of agricultural practices could possibly reduce the abundance of Cx. tritaeniorhynchus.

Gingrich et al. (2001) have reported that the JE transmission in Southern India is as follows:

May through October in Goa.
October through January in Tamil Nadu.
August through December in Karnataka with a second peak from April through June in the Mandya District.
September through December in Andhra Pradesh.

In the current investigation, two peaks of larval density of Cx. tritaeniorhynchus in ground pools (type A) was observed in October and May in Mysore city during 2002-03 which coincides with the JE transmission season in Karnataka state. The mean value for larval density of Cx. tritaeniorhynchus type A (59.64 larvae per dip) was found to be significantly more than that of type B (11.09) during the study period (p< 0.01) (Table 3). So, these results will be useful for integrated vector management in control of JE vectors in epidemics.

ACKNOWLEDGMENTS

Authors are thankful to Mr. George Jacob, National Institute of Virology (NIV) field station at Bangalore, India for confirmation of species identified, ICCR India for financial assistance and Chairman, Department of Studies in Zoology, University of Mysore, Mysore, India for the facilities provided.

REFERENCES
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Fakoorziba, M.R. and V.A. Vijayan, 2003. Variation in morphological characters of Culex tritaeniorhynchus diptera culicidae a Japanese encephalitis vector at Mysore India. J. Commun. Dis., 35: 206-213.
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Gingrich, J.B., H.J. Harlan, P.V. Perkins and J.H. Trosper, 2001. Regional Disease Vector Ecology Profile South Central Asia. Walter Reed Army Medical Center, London, pp: 49-50.

Mishra, A.C., 1984. Monitoring of the vector of Japanese encephalitis. Proceedings of the National Conferences on Japanese Encephalitis, (NCJE'84), Indian Council of Medical Research, New Delhi, pp: 62-62.

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Nagabhushana, R.P., 2000. Japanese encephalitis for doctors health workers and parents. http://openmed.nic.in/1012/.

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Reuben, R., 1968. A description of the species Cx. Vishnui theo with notes on Cx. Pseudovishnui colless and Cx. Tritaeniorhynchus giles from Southern India. Bull. Entomol. Res., 58: 643-652.

Samuel, P.P., S.J. Hiriyan and A. Gajanana, 2000. Japanese encephalitis virus infection in mosquitoes and its epidemiological implications. Ind. Med. Res. Bull., 30: 40-43.

Service, M.W., 1976. Mosquito Ecology Field Sampling Methods. Applied Science Publishers, London, pp: 583.

Sirivanakarn, S., 1976. Medical Entomology Studies. Contributions of the American Entomological Institute, America, pp: 129-134.

Takagi, M., W. Suwonkerd, Y. Tsuda, A. Sugiyama and Y. Wada, 1997. Effects of rice culture practices on the abundance of Culex mosquitoes diptera culicidae in Northern Thailand. J. Med. Entomol., 34: 272-276.
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Vijayan, V.A. and N. Ningegowda, 1993. Susceptibility difference in two populations of Culex quinquefasciatus Say diptera culicidae to three synthetic pyrethroids. Southeast Asian J. Trop. Med. Publ. Health, 3: 540-543.
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