Journal of the American Mosquito Control Association, 18(4):290-293,20O2Copyright A 2002 by the American Mosquito Control Association, Inc.

SEASONAL PREVALENCE OF CULEX VISHNUI SUBGROUP THEMAJOR VECTORS OF JAPANESE ENCEPHALITIS VIRUS IN AN

ENDEMIC DISTRICT OF ANDHRA PRADESH, INDIAU. SURYANARAYANA MURTY,I D. V. R. SATYAKUMAR,' K. SRIRAM,I K. MADHUSUDHAN RAO,I

T. GOPAL SINGH,, N. ARUNACHALAMT eNo P PHILIP SAMUEL]

ABSTRACT. Seasonal prevalence of members of the Culex vishnui stbgroup, the major vectors of Japaneseencephalitis virus (JE) in an endemic district of Andhra Pradesh, was investigated. Approximately 15,500 mos-quitoes belonging to 2 genera and 5 species were collected by indoor-resting collections. The predominant specieswere found to be from the Cx. vishnui subgroup, which comprised 42.6Vo of the total collection, followed byAnopheles subpictus (4O.4Ea), An. hyrcanus (12.8E ), Cx. gelidus Q.sqd, and An. barbirostris (O.87o). Mosquitoesof the C"x. vishnui srtbgroup were collected throughout the year, and densities of females ranged from 2.3lman-hour in January 1999 to 26.0/man-hour in December 1999. Ninety{hree cases of Japanese encephalitis (27confirmed serologically) were reported, mainly during the monsoon months of November and December. Asignificant positive correlation between densities of mosquitoes of the C-r. vishnui subgroup and occurrence ofJapanese encephalitis cases (r = 0.765, P < 0.01) was observed. The factors responsible for transmission ofJEin the endemic district are discussed.

KEY WORDS Japanese encephalitis, Culex vishnui subgroup, Andhra pradesh, India

INTRODUCTION

Japanese encephalitis was lst recognized in Ja-pan and is prevalent in many Southeast Asian coun-tries. In the last 3 decades, Japanese encephalitisvirus (JE) has been responsible for major outbreaksof the disease in India. In India, JE was 1st rec-ognized in 1955 from cases of encephalitis admit-ted to the Christian Medical College and Hospital,Vellore (Tamil Nadu). The virus was isolated fromwild-caught mosquitoes in the same area and iso-lation from brain tissue of human cases followed in1958 (Reuben and Gajanana 1997).

Serological surveys carried out between 1955and 1972 showed that JE infections occurred inscattered areas in Gujarat, Maharashtra, Orissa, As-sam and Arunachal Pradesh, with the highest prev-alence in the 3 southern states of Tamil Nadu, An-dhra Pradesh, and Karnataka. Small numbers ofhuman cases were reported only in southern India.The lst major outbreak of Japanese encephalitis,involving more than 700 cases and 300 deaths, oc-curred in Burdwan and Bankura districts of WestBengal in 1973 followed by aZnd outbreak in 1976(Banerjee 1996). Between 1977 and 1979, exten-sive outbreaks involved new areas, including Ti-runelveli District in Tamil Nadu, Kolar District inKamataka, several districts in West Bengal and An-dhra Pradesh, Dhanbad District in Bihar, DibrugarhDistrict in Assam, and Gorakhpur District in UttarPradesh (Reuben and Gajanana 1997).

Since then, these areas have become prone tooutbreaks of Japanese encephalitis, with large num-

I Biology Division, Indian Institute of Chemical Tech-nology, Hyderabad 500 007, India.

2 Directorate of Medical and Health Services, Govern-ment of Andhra Pradesh, Hyderabad 5O0 O0l, lndia.

r Centre for Research in Medical Entomology (ICMR),No. 4, Chinna Chokkikulam, Madurai 625 OO2, lndia.

bers of cases occurring each year. Outbreaks fre-quently coincide with years having heavy or un-usual rainfall patterns. New areas recently havebeen involved in transmission of JE, including Cud-dalore (formerly South Arcot District) and Tiru-chirapalli districts in Tamil Nadu, the Union Terri-tories of Pondicherry, Goa, and most recently inHaryana and Kerala (Samuel et al. 2OOO).

In nature, JE is maintained by a complex cycle thatinvolves pigs as amplifying hosts, ardeid birds as res-ervoirs, and mosquitoes as vectors. Members of theCulex vishnui subgroup of mosquitoes have been rec-ognized for many years as major vectors and theyplay an important role in the epidemiology of JE inIndia (Samuel et al. 2fi)0). Tlte Cx. vishnui subgroupincludes Culex tritaeniorhlmchus Giles, C-r. vishnuiTheobald, and Cx. pseudovishnui Colless. These spe-cies are extremely comnon, widespread, and breedmainly in paddy fields. Their abundance often is re-lated to rice cultivation. Blood meal analysis showedthat these mosquitoes were principally cattle-feeders,although human- and pig-feeding was also recordedin villages near Madurai and in Nallur Primary HealthCenter of Cuddalore District (Reuben et al. 1992\.

Japanese encephalitis control programs face se-rious problems because of the nonavailability ofsuitable cost-effective control strategies. Vaccina-tion and vector control are the only 2 availabletools for the containment of the disease. Becauseof the limited availability of JE vaccine, it is notpossible to vaccinate all the persons who are at riskof JE infection in endemic states. Hence, vectorcontrol is the only option available for disease con-trol. Study of the seasonal abundance of vectorpopulations it is absolutely essential to developsuitable vector control stratesies.

MATERIALS AND METHODS

In Andhra Pradesh, incidence of Japanese en-cephalitis has been recorded almost every year in

290

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ben 1971). A similar increase in the abundance ofmembers of the Cx. vishnui subgroup was noticedduring the monsoon season in Sarawak (Hill l97O).Because rainfall dominates all agriculture practices,

120

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Fig. 2. (A) Rainfall, (B) abundance of female mosquitoes of t|lle Culex vishnui stbgroup, and (C) occurrence ofJapanese encephalitis virus (JE) cases in Cuddapah District during 1999.

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therefore, rainfall also was thought to influencemosquitoes of the Cx. vishnui subgroup, whichbreed mainly in rice fields. In Cuddalore District, asimilar trend in abundance of mosquitoes of the Cx.

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vishnui subgroup was observed coinciding with thebeginning of rains and rice cultivation (Gajanana etal. 1997). Population densities of members of theCx. vishnui subgroup in Andhra Pradesh follow therainfall pattern, being least abundant in hot dry sea-son and most abundant during and after the North-east monsoon.

Sixty-eight villages were affected by encephalitisin 1999 and 93 cases were reported from these vil-lages. These data were collected from the Directorateof Health Services, Hyderabad. Out of these 93 cas-es, 27 cases were associated serologically with JEand these cases were reported mainly during Octoberto December (Fig. 2C). The correlation coefficientof abundance of members of the C.r. vishnui sub-group and occurrence of Japanese encephalitis caseswas calculated and a significant positive correlationwas noticed between densities of mosquitoes of theCx. vishnui subgroup and occurrence ofJapanese en-cephalitis cases (r : O.765, P < 0.01). In southernIndia, studies on seroconversion in school childrenshowed that seroconversion occurred between Julyand September and that more than 7O7o of the cohorthad seroconverted by October (Gajanana et al.1995). The same also may be true for Andhra Pra-desh because of the similar ecological conditions, thepresence of large numbers of pigs, and the higherabundance of mosquitoes of the Cx. vishnui sttb-group.

ACKNOWLEDGMENTS

We are grateful to K. V. Raghavan, Director, In-dian Institute of Chemical Technology, Hyderabad,and K. Satyanarayana, Directo! Centre for Re-search in Medical Entomology (CRME), Madurai,

for the encouragement and support. The assistancerendered by A. Veerapathiran and A. Venkatesh,

CRME, in the preparation of the manuscript is ac-knowledged.

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