A community-based study of subclinical flavivirus infections in children in an area of Tamil Nadu, India, where Japanese encephalitis is endemic*

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1 A community-bsed study of subclinicl flvivirus infections in children in n re of Tmil Ndu, Indi, where Jpnese encephlitis is endemic* A. Gjnn,1 V. Thenmozhi,2 P. Philip Smuel,3 & R. Reuben4 A chrcteristic feture of the epidemiology of Jpnese encephlitis (JE) is the occurrence of lrge number of subclinicl infections. The reporting of only overt cses underestimtes the totl level of virus trnsmission, knowledge of which is essentil for the evolution of control strtegies. We crried out 3-yer prospective serologicl study between 1989 nd 1991 in primry helth centre in Tmil Ndu where JE is endemic. Ech yer pired specimens, tken before nd fter the trnsmission seson from cohort of schoolchildren ged 5-9 yers, were tested for hemgglutintion inhibition (HI) ntibody titres in order to study seroconversion. The seroconversion rtes in the successive yers were 37.5, 42.1 nd 25 percentge points, nd in third of such seroconversions it ws possible to estblish specific dignosis. Seroconversion ws ttributble predominntly to JE virus nd minimlly to West Nile virus. Reltively high dengue virus ctivity occurred only in There were sttisticlly significnt differences in seroconversion rtes between villges nd this ws relted to vritions in the rtio cttle:humns:pigs. Very high seroconversion rtes occurred mong children who were negtive for HI ntibodies before the trnsmission seson. HI ntibodies declined to undetectble levels 6-8 months lter in hlf the children who hd seroconverted. The verge net nnul increse of 16.2 percentge points in seropositivity ws nevertheless much higher thn vlues reported from other res of endemicity. The overll incidence of JE cses ws 15 per children ged 5-9 yers, nd the estimted rtio overt:inpprent infection ws 1:270. Introduction Jpnese encephlitis (JE) is n importnt public helth problem in south-est Asi, nd its trnsmission ppers to be incresing in severl countries (1). In Indi the disese ws first reported in 1955 (2), nd subsequently mny epidemics hve occurred in different prts of the country. JE virus infects lrge number of susceptible individuls but only few develop overt mnifesttions of the disese. Differences in the virulence of virus strins, host susceptibility, bckground immunity, nd severl other fctors my cuse vritions in the incidence of the * From: Centre for Reserch in Medicl Entomology, Post Box No. 11, No. 4 Srojini Street, Chinn Chokkikulm, Mduri , Indi. 1 Deputy Director. 2 Reserch Assistnt. 3Reserch Scientist. 4Director. Requests for reprints should be sent to this uthor. Reprint No disese without ffecting the totl infection rtes. Therefore the current system of reporting only cses of encephlitis does not reflect the totl level of trnsmission, mesurement of which is n essentil prerequisite for plnning control strtegies. Inpprent infection, resulting in the development of mesurble ntibodies to JE virus, cn be used to quntify seroconversion rtes mong susceptible groups during the trnsmission seson. In Indi, serologicl surveys hve provided vluble dt on the point prevlence of ntibodies ginst JE virus nd other flviviruses, but informtion on inpprent infection rtes nd the rtio of inpprent to pprent infections is indequte. We therefore crried out 3-yer prospective serologicl study of cohort of primry-school children in n re in Tmil Ndu where JE is endemic. At the sme time we monitored the JE virus infection sttus of sentinel pigs in the study re. Jpnese encephlitis in Indi. Pune, Ntionl Institute of Virology, 1980 (informtion document). Bulletin of the World Helth Orgniztion, 1995, 73 (2): World Helth Orgniztion

2 A. Gjnn et l. Mterils nd methods Study site In 1981 n extensive epidemic of encephlitis in children ws reported in the South Arcot District of Tmil Ndu, in which serologicl dignosis of JE ws mde for 61% of the ptients exmined (3). Subsequently, mny cses of encephlitis hve been reported ech yer, minly in October nd November, coinciding with the period of the north-est monsoon. Nllur Primry Helth Centre, which covers bout people, ws chosen for the study becuse it is locted in one of the worst-ffected res. Between 50 nd 200 pigs re rered in ech villge, in ddition to other domestic nimls. Detils of cses nd deths due to encephlitis were obtined from the Tmil Ndu Public Helth Service. Mosquito blood mel identifiction A reltively high proportion of recognized vectors of JE virus in the re (Culex triteniorhynchus, C. vishnui, nd C. pseudovishnui) feed on humns nd pigs in ddition to cttle. Methods of identifying mosquito blood mels hve been described previously (4). Seroconversion in sentinel pigs Ten loclly procured piglets born during the nontrnsmission seson (Jnury-June) nd ged 3-6 months were plced in ech of four villges. Blood specimens were collected from them in heprinized vils t the time of plcement nd every fortnight therefter from June to December. The nimls were included in the study only when their mternl ntibodies, if present, disppered. Humn infections We crried out n explortory investigtion in eight villges in August nd November 1988 in order to pln subsequent seroconversion study. Smples of fingerprick blood ( pl) were collected in heprinized vils from nursery nd primry-school children ged 3-14 yers, following the informed consent of their prents. Fifteen villges were selected t rndom from those in which t lest one cse of encephlitis hd occurred since In ll the schools in these villges, primry-school children, minly ged 5-7 yers but with smll number ged 8-9 yers, who were present on the dy of the first visit, were recruited for the study, following informed consent. Only these children were followed up prospectively. None hd received JE vccine but they were ll covered by the Expnded Progrmme on Immuniztion. From ech child ,pl of fingerprick blood ws collected s described bove. Pired surveys 238 were conducted before nd fter the JE trnsmission seson, s follows: in August nd December 1989, August 1990 nd Jnury 1991, nd August 1991 nd Februry Blood specimens were trnsported on ice to the field lbortory for seprtion of plsm, which ws then shipped on ice to Mduri nd stored t -20 C pending exmintion. Serologicl tests The hemgglutintion inhibition (HI) test (5) ws performed on microtitrtion pltes on cetone-extrcted, goose-erythrocyte-bsorbed plsm. Ech specimen ws tested ginst JE, West Nile (WN), nd dengue (DEN-2) virus ntigens. Pired specimens were tested simultneously nd known positive nd negtive controls were included in ech dy's tests. The dignostic criteri used were s follows: - seronegtive, <1:10 HI titre for ll three viruses; - seropositive,.1:10 HI titre for t lest one virus; - seroconversion, pretrnsmission seson specimen negtive nd post-trnsmission seson specimen positive, or both specimens positive with fourfold or greter rise in titre in the post-trnsmission seson specimen; - seroreversion, post-trnsmission seson specimen positive nd next pretrnsmission seson specimen negtive; - specific virus infection, monospecific response or brodly recting, with HI titres to one virus tht were t lest four times greter thn those to others; nd - unclssified, less thn fourfold differences in HI titres for more thn one virus. Virus-specific IgM ntibodies were detected by IgM-cpture enzyme-linked immunosorbent ssy (ELISA) (6) using kits provided by the Ntionl Institute of Virology, Pune. The JE, WN, nd DEN-2 virus ntigens supplied in the kits were used to test ech smple. Sttisticl nlysis The precision level for the seroconversion rtes ws determined using the Epi Info softwre pckge (Centers for Disese Control, Atlnt, GA, USA, nd WHO, Genev, Switzerlnd). Results Seroconversion in sentinel pigs Of 124 nimls exmined, 95 (76.6%) seroconverted during the trnsmission seson. In individul villges WHO Bulletin OMS. Vol

3 Subclinicl flvivirus infections in Tmil Ndu, Indi the seroconversion rtes were s follows: % in 1989, % in 1990, nd 58-66% in Seroconversions occurred in ll the months when tests were crried out, i.e., June-December, the pek being in November (Fig. 1). Of the 95 nimls tht seroconverted, 24 (25%) hd HI ntibodies to JE virus lone; nd 42 (44%) hd HI ntibody titres ginst JE virus tht were t lest four times greter thn the titres ginst WN/DEN-2. JE virus infection ws therefore confirmed in 69% of the nimls tht seroconverted; the remining 29 nimls (31%) were unclssified. Humn cses of encephlitis Between 1981 nd 1990, totl of 229 ptients with encephlitis were reported t the primry helth centre, the numbers corresponding to the successive yers being s follows: 24, 5, 18, 11, 15, 49, 39, 9, 30, nd 29. Dt for 1991 were indequte nd were therefore not included in the nlysis. As shown in Fig. 2, bout 99% of ptients were under the ge of 15 yers nd there ws distinct pek in the incidence mong 4-5-yer-olds. The mle:femle rtio ws 1:0.8, nd 80% of cses occurred in October nd November. Humn seroepidemiology In n explortory study, blood specimens from 793 children ged 3-14 yers were exmined for HI ntibodies to flviviruses, nd 271 (34.2%) proved positive ginst one or more ntigens. Of these seropositive children, 221 (81.5%) were positive for JE, 131 (48.3%) for WN, nd 112 (41.3%) for DEN-2, lone or in combintion. The ge-specific prevlence of HI ntibodies showed significnt liner regression of percentge positivity on ge (y = x; r = Fig. 1. Seroconversions in sentinel pigs in the re covered by Nllur Primry Helth Centre, by month, Monthly *-* Cumultive C 80 0 ~ o Jun Jul Aug Sep Oct Months Nov Dec Fig. 2. Age distribution of humn cses of encephlitis recorded t Nllur Primry Helth Centre, <1 I Z 3J s b I/ 9 IUU Age (yers) WHO , P <0.005). The vlue of the regression coefficient suggested tht the nnul increment in seropositivity ws bout 5%, nd on this bsis it ws considered tht smple size of 450 children would be required for precision of ±2% in estimting this increment. We therefore recruited bout 1000 children for the prospective study, in the expecttion tht bout 50% would drop out during the 3-yer study period. Our ssumption tht the rte of increse in cohort positivity ws bout 5% per yer proved to be incorrect. The precision levels chieved, s clculted from ctul smple sizes nd rtes of increse, vried from 2.9% in the first yer to 4% in the third, when blood smples were obtined in only nine of the originl 15 villges becuse of lck of coopertion (Tble 1). Prospective cohort study Among children from whom pired blood specimens were tken in 1989, 15.8% possessed ntibodies to flviviruses before nd 53.3% fter the trnsmission seson, n increse of 37.5 percentge points (Tble 1). In 1990, 33.7% of children were positive before nd 75.8% fter the trnsmission seson, n increse of 42.1 percentge points. Before the 1991 trnsmission seson, 48.2% of children were positive; fter this seson, 73.2% were positive, n increse of 25 percentge points. Tble 1 shows tht in the nontrnsmission seson, i.e., the period between the post-trnsmission survey nd the following yer's pretrnsmission survey, there ws considerble decline in seropositivity (19.6 nd 27.6 percentge points, in 1990 nd 1991, respectively). Thus, the net nnul increses in cohort positivity to HI ntibodies, s determined by comprison of pretrnsmis- WHO Bulletin OMS. Vol

4 A. Gjnn et l. Tble 1: Hemgglutintion inhibition ntibody ginst flviviruses mong the study children, Net nnul increse No. exmined No. positive Increse (percentge points) Precision level (%)8 in positivity (percentge points) 1989 Preseson (15.8)b Postseson (53.3) Preseson (33.7) 17.9 Postseson (75.8) Preseson (48.2) 14.5 Postseson (73.2) % confidence intervl. b Figures in prentheses re percentges. sion smples in successive yers, were 17.9 percentge points in nd 14.5 percentge points in The geometric men titres of JE virus ntibodies of ll the ser titrted in the post-trnsmission sesons did not revel differences between ge groups or between successive yers, vrying from 12.3 to 17.1 in December 1989, from 10.8 to 14.5 in Jnury 1991, nd from 10.6 to 12.2 in Februry 1992 Ṡeroconversion rtes were studied on pre- nd post-trnsmission seson smples. The rtes mong children who were negtive before the trnsmission seson were 46.2%, 76.6% nd 71.9%, respectively, in 1989, 1990, nd 1991 (Tble 2). The rtes mong children who were positive before the trnsmission seson were 32.7%, 27.8%, nd 7.4%, respectively. Seroreversion rtes were studied during the nontrnsmission period. Among positive children, 49.7% seroreverted in , nd 50.5% in The probbility of seroreversion ws inversely relted to the initil JE virus ntibody titres t the end of the trnsmission seson. Tble 3 shows tht 56.9% of children with n initil titre of <1:20 seroreverted, while 33.3% did so mong those with n initil titre of.1:40. The difference ws significnt (X2 test = 28.92, P <0.05). The postseson percentge point increses in seropositivity were similr for mles nd femles (36 nd 38 in 1989, 40 nd 42 in 1990, nd 23 nd 27 in 1991; X2 test, not significnt). However, sttisticlly significnt vritions were observed between villges, with the percentge point increses rnging from 23 to 50 in 1989, nd from 26 to 58 in 1990 (P<0.05). In 1991 only nine villges were followed up; in five of them seropositivity incresed (rnge: 4-69 percentge points); in one villge there ws mrginl increse (1 percentge point); nd in three villges there ws decrese (rnge: 43-4). For three villges, dt on the blood-feeding rtes of vectors were vilble. In Erppvur, where the cttle popu- Tble 2: Seroconversion nd seroreversion rtes ginst flviviruses in cohort of schoolchildren covered by the Nllur Primry Helth Centre, Seroconversion : Preseson Hlb ntibody sttus: Negtive Positive Seroreversion Yer n No. seroconverting n No. seroconverting n No. seroreverting (46.2)c (32.7) (49.7) (76.6) (27.8) (50.5) (71.9) (7.4) b c See text for definitions. Hemgglutintion inhibition. Figures in prentheses re percentges. 240 WHO Bulletin OMS. Vol

5 Subclinicl flvivirus Infections in Tmil Ndu, Indi Tble 3: Seroreversion rte in reltion to Jpnese encephlitis virus-hemgglutintion inhibition ntibody titre mong the study children Post-trnsmission JE-HI titre No. exmined No. seroreverting 1: (58.0)b 1: (53.3) 1: (38.3) 1: (28.0) 1: (31.3) Totl (48.6) <11: (56.9) >11: (33.3) JE-HI = Jpnese encephlitis virus-hemgglutintion inhibition. b Figures in prentheses re percentges. ltion ws high reltive to tht of humns, the percentge of JE vectors feeding on cttle ws greter nd the seroconversion rte ws lower thn in two other villges where the cttle:humns rtio nd the percentge of feeding on cttle were reltively low (Tble 4). The humns:pigs rtio nd the corresponding feeding rtes were pproximtely the sme in ll three villges. Among those children who seroconverted, virus specificity ws estblished in bout third, while the rest remined unclssified (Tble 5). JE-virus-specific seroconversions occurred in 29.6%, 37.5%, nd 23.6% over successive yers; the corresponding vlues for WN virus were 3.8%, 0.3%, nd 0. DEN-2- Tble 4: Seroconversion rtes, niml nd humn popultion rtios, nd blood-feeding rtes of Jpnese encephlitis vectors in three villges covered by Nllur Primry Helth Centre Villge: Erppvur Sepkkm Kodikklm % Seroconversion in children Pigs:cttle:humns 1:12:25 1:4:20 1:4:29 Culex triteniorhynchus % Feeding on: Cttle Humns Pigs C. vishnui % Feeding on: Cttle Humns Pigs Men for 1989 nd Tble 5: Virus-specific seroconversions mong schoolchildren covered by Nllur Primry Helth Centre, Seroconversion to: Totl JE 142 (29.6)b 134 (37.5) 41 (23.6) 317 (31.4) WN 18 (3.8) 1 (0.28) 0 19 (1.9) DEN (15.5) 27 (2.7) Unclssified 320 (66.7) 222 (62.2)106 (60.9) 648 (64) Totl JE = Jpnese encephlitis virus; WN = West Nile virus; nd DEN = dengue virus. b Figures in prentheses re percentges. virus-specific seroconversions were seen only in 1991, when they ccounted for 15.5% of seroconverters. JE-virus-specific IgM ntibodies were demonstrted in 10 of 134 postseson smples from children who seroconverted in 1989, but in none of the 60 similr smples tested in No WN- or DEN- 2-virus-specific IgM ws detected in either yer. Apprent:inpprent infection rtio In the first 2 yers of the study, seroconversions were minly due to JE virus, nd it ws therefore ssumed for the purposes of clcultion tht the posttrnsmission rise in positivity ws due to JE virus infections. On this bsis we estimted the number of cses of inpprent infection mong the children ged 5-9 yers who were covered by the primry helth centre nd compred this with the number of cliniclly dignosed cses of encephlitis in this ge group, lso presumbly due minly to JE virus (Tble 6); the rtio ws 1:282 in 1989 nd 1:257 in 1990, with n verge of 14.9 cses per popultion. No clcultions were ttempted for 1991, since DEN-2 virus ws lso ctive nd dt on encephlitis cses were indequte. Discussion The present study followed the temporl chnges in trnsmission rtes over 3 yers in n re where JE ws endemic, unlike previous investigtions tht hve covered only one trnsmission seson (7-9). Schoolchildren ged 5-9 yers were selected so tht there would be n dequte number of susceptible individuls nd sufficient prticiption. Of the children ged 5-9 yers covered by the primry helth centre, the study smple size rnged from 10% in 1989 to 4.2% in The HI test hs severl dvntges mking it prticulrly pproprite WHO Bulletin OMS. Vol

6 A. Gjnn et l. Tble 6: Apprent:inpprent Infection rtios for children ged 5-9 yers in the re covered by Nllur Primry Helth Centre (popultion ), Estimted number of Observed number Apprent:inpprent Proportion inpprent of cses infection seroconverted infections of encephlitis rtio : :257 for following the sme cohort through more thn one trnsmission seson. It is sensitive nd ccurte indictor of subclinicl infection with JE virus in the erly postinfection phse (10), needing only smll quntity of serum or plsm, esily obtinble from fingerprick blood specimens. HI ntibodies develop fster thn neutrlizing ntibodies nd previously infected persons my show mesurble HI ntibody responses (7, 11). There ws intense flvivirus ctivity in ll 3 yers, nd the nnul sesonl increse in HI seropositivity rnged from 42.1 to 25 percentge points. This ws considered to be cused predominntly by JE virus ctivity in 1989 nd 1990, when the mjority of the seroconversions dignosed serologiclly were ttributble to this virus. The fct tht only JEvirus-specific IgM ntibodies were identified in smple of children provided supporting evidence; furthermore, 69% of the sentinel pigs in the re, ll primry responders, exhibited minly monospecific JE virus seroconversions during the seson. WN virus ctivity ws either bsent (in 1991) or present t low level (in 1989 nd 1990), wheres in postepidemic survey in South Arcot District in 1982 the prevlence of WN neutrlizing ntibodies ws significntly higher thn tht of JE (12). This indictes shifting ptterns of virus ctivity in the re. Thus DEN-2 virus, which ws not ctive in 1989 nd 1990, exhibited reltively high ctivity in 1991, when 16% of seroconversions in children were due to it. Previous studies in southern Indi hve shown tht, mong cses of serologiclly confirmed JE, HI ntibody titres begn to fll bout 3-4 months fter onset (13). In the present study, for bout hlf the children who suffered inpprent infections during the trnsmission seson HI ntibody titres hd declined to undetectble levels 6-8 months lter, before the strt of the next trnsmission seson; lso, there ws n inverse reltionship between seroreversion rtes nd initil ntibody titres. Similrly, in Chingmi Vlley, Thilnd, 21% of people who hd monospecific JE virus titres of 1:20 s result of subclinicl infection hd reverted to <1:20 within 12 months, while none of those with titres of.1:40 reverted (11). The high seroreversion rte in the present study rose t lest prtly becuse only smll 242 children were investigted. It is interesting tht the only other record of high seroreversion (28%) ws mong Americn servicemen in Kore, ll of whom were nonimmune before the trnsmission seson, who lost detectble HI ntibodies in 7-9 months (10). Notwithstnding the high seroreversion rtes, the net nnul increses in HI positivity (on verge 16.2 percentge points, were much higher thn those reported for popultions in other res of endemicity, lthough the dt re not strictly comprble becuse of differences in the study designs nd the ge groups observed. Among Jpnese schoolchildren ged 6-12 yers, nnul rtes of increse of 5% (7) nd 9-10% (8) hve been reported. In the Chingmi Vlley, nnul increments were in the rnge % in individul villges, while mong urbn schoolchildren the increment ws 4.3% (11), nd for under-40-yer-olds in Srwk the overll estimte ws 6% (14). However, 50% of group of susceptible Americn servicemen t n irbse in Kore developed ntibodies in single seson (15). Very high seroconversion rtes were observed in children who did not possess HI ntibodies before the trnsmission seson (46.2%, 76.6% nd 71.9% in successive yers). Ech seroconversion ws presumbly the result of t lest one infective mosquito bite. The minimum probbility of child receiving n infective bite during the trnsmission seson ly in the rnge Studies re in progress to detect virus in wild-cught vectors in order to confirm whether such intense trnsmission cn tke plce. Presumbly the children who were lredy HI-positive t the beginning of the seson received the sme number of infective bites, but they did not ll exhibit detectble response. In successive yers, with incresing cquisition of immunity, the seroconversion rte in the group of positive children decresed. Sttisticlly significnt differences were observed between seroconversion rtes in individul villges. At lest, in prt, these differences were possibly cused by vritions in cttle popultions, since low seroconversion rte in children ws ssocited with high cttle:humns rtio. Vector bundnce ws pproximtely the sme in these villges (Centre for Reserch in Medicl Entomology, Mduri, unpublished dt). WHO Bulletin OMS. Vol

7 Subclinicl flvivirus infections in Tmil Ndu, Indi The overll incidence of JE cses in the Nllur Primry Helth Centre ws 6.0 per popultion in the ge group 1-19 yers, higher thn in Chingmi Vlley (Thilnd) where it ws 3.8 per for the sme group (16). However, in Nllur the incidence mong 1-9-yer-olds ws strikingly higher (9.4 per ) thn tht mong yerolds (2.3 per ), wheres in Chingmi Vlley the incidence for the younger ge group ws lower thn tht for the older (3.1 nd 4.6 per , respectively). This suggests tht immunity develops erly in Nllur s result of repeted exposure to intense trnsmission. The rtio of overt:inpprent JE infections (1:270) for children ged 5-9 yers in the present study ws similr to the rtios observed in Chingmi Vlley (1:300) (11), Srwk nd Chin (Province of Tiwn) (1: ) (11), Jpn (1:590) (8), nd West Bengl (1:400) (17). The present study hs reveled tht young children in n re of endemicity in southem Indi re grvely t risk of developing JE during the trnsmission seson becuse of the high probbility of receiving n infective mosquito bite. However, only bout 1 in 270 will develop the disese nd others will suffer ltent infection resulting in high HI seroconversion rtes, followed by high rtes of seroreversion during the nontrnsmission seson. Further studies re required to elucidte the implictions of these findings for the development of protective immunity in the popultion. Acknowledgements We re very grteful to ll the children who donted blood smples nd thnk their techers s well s the doctors nd helth workers of the Tmil Ndu Public Helth Deprtment for their invluble help. Dr P.S.S. Sundr Ro, Professor nd Hed, Deprtment of Biosttistics, Christin Medicl College, Vellore, Tmil Ndu, is thnked for his help in designing the study. The excellent technicl ssistnce by the stff of the Centre for Reserch in Medicl Entomology, Mduri, nd its field sttion t Vriddhchlm is grtefully cknowledged. The study would not hve been possible without the generous dontion of flvivirus ntigens, positive nd negtive ser, nd ELISA kits by the Ntionl Institute of Virology, Pune. Mr R. Ilnthiryn ws involved in prt of this study t the Centre for Reserch in Medicl Entomology. Resume Etude dns l communute des infections infrcliniques flvivirus chez l'enfnt dns une r6gion du Tmil Ndu (Inde) ou l'enc6phlite jponise est end6mique Les infections infrcliniques pr le virus de l'encephlite jponise (JE) d6pssent de tres loin les enc6phlites d6clr6es. L notitiction des seuls cs d6clr6s sous-estime donc le niveu gen6rl de trnsmission du virus, dont l d6termintion quntittive est indispensble pour Il'bortion de strtegies de lutte. Nous vons r6lis6 une enquete serologique prospective sur l p6riode , sur une cohorte de coliers g6s de 5 9 ns, s6lectionn6s dns 15 villges couverts pr le centre de snte primire de Nllur, u Tmil Ndu, ou 1'enc6phlite jponise est end6mique. Pour chque enfnt, on d6termin6 les titres d'nticorps pr inhibition de Ihemglutintion (HI), l'grd du virus de l'enc6phlite jponise (JE), du virus West Nile (WN), et du virus de l dengue type 2 (DEN-2), vnt et pres l sison de trnsmission. On 6glement etudie, prllelement, les s6roconversions chez des porcs sentinelles. Chez l'enfnt, I'ugmenttion post-sisonniere du pourcentge de positivit6 en HI 6tit, pour les trois nn6es successives, de 37,5, 42,1 et 25 points. Chez environ un tiers des enfnts ynt op6r6 une s6roconversion, un dignostic virl specifique 6t6 6tbli. L s6roconversion 6tit due u virus JE chez 29,6%, 37,5% et 23,6% des enfnts pour chcune des trois nnees, lors que pour le virus WN les vleurs correspondntes 6tient de 3,8%, 0,3% et 0. Une ctivit6 du virus DEN-2 n' ete observ6e qu'en 1991, u cours de lquelle 15,5% des enfnts etient tteints. L pr6dominnce du virus JE dns l r6gion 6t6 prouvee pr l mise en 6vidence d'lgm nti-virus JE chez 7,5% des enfnts ynt op6r6 une s6roconversion, lors qu'il n' ps 6te d6tect6 d'nticorps dirig6s contre les virus WN et DEN-2. Le tux de seroconversion chez les porcs 6tit de 76,7%, dont 69,9% du fit du virus JE. Chez pres de l moitie des enfnts ynt fit une s6roconversion, les titres d'nticorps vient biss6 jusqu' n'etre plus decelbles lors de nouvelles nlyses effectu6es 6 8 mois pres l sison de trnsmission. II y vit, pr cons6quent, une ugmenttion nnuelle nette moyenne du pourcentge de s6ropositivite de 16,2 points dns l cohorte. Les tux de s6roconversion vriient de f,on sttistiquement significtive d'un villge l'utre, et dns trois d'entre eux, ces vritions etient liees des differences u niveu du rpport bovin:homme:porc. Chez les enfnts n6gtifs pour les nticorps en HI vnt l sison de trnsmission, des tux de s6roconversion tres elev6s ont 6t6 observes (46,5% 76,6%), montrnt 1'existence d'une intense trnsmission du virus dns l region. L'incidence des cs de JE dignostiqu6s d'pres l'exmen clinique 6tit de 14,9 pour chez WHO Bulletin OMS. Vol

8 A. Gjnn et l. les enfnts de 5 9 ns. En supposnt que l plus grnde prtie de l s6roconversion observ6e u cours des deux premieres nnees de 1'etude 6tit due u virus JE, nous vons estim6 que le rpport entre l'infection declree et l'infection inpprente 6tit de 1:270. II est n6cessire de determiner plus vnt l significtion des forts tux de seroconversion, qui peuvent etre ttribues des tux 6lev6s d'inocultion pr les moustiques, en ce qui concerne I'cquisition d'une immunit6 protectrice. References 1. Umenl T et l. Jpnese encephlitis: current world sttus. Bulletin of the World Helth Orgniztion, 1985, 63: Work TH, Shh KV. Serologicl dignosis of Jpnese B type encephlitis in North Arcot District of Mdrs Stte, Indi, with epidemiologicl notes. Indin journl of medicl reserch, 1956, 10: Mohn Ro CVR et l. The 1981 epidemic of Tmil Ndu nd Pondicherry. Indin journl of medicl reserch, 1988, 87: Reuben R et l. Mosquito blood feeding ptterns s fctor in the epidemiology of Jpnese encephlitis in southern Indi. Americn journl of tropicl medicine nd hygiene, 1992, 46: Clrke DH, Csls J. Techniques for hemgglutintion nd hemgglutintion-inhibition with rthropod-borne viruses. Americn journl of tropicl medicine nd hygiene, 1958, 7: Burke DS, Nislk A. Antibody cpture immunossy detection of Jpnese encephlitis virus immunoglobulin M nd G ntibodies in cerebrospinl fluid. Journl of clinicl microbiology, 1982, 16: Scherer WF et l. Ecologic studies of Jpnese encephlitis virus in Jpn. Americn journl of tropicl medicine nd hygiene, 1959, 8: Southm CM. Serologicl studies of encephlitis in Jpn. II. Inpprent infections by Jpnese encephlitis virus. Journl of infectious diseses, 1956, 99: Soe T et l. Study of vector, mplifier nd humn infection with Jpnese encephlitis virus in Rngoon community. Americn journl of epidemiology, 1988, 128: Hlsted SB, Russ B. Subclinicl Jpnese encephlitis. II. Antibody responses of Americns to single exposure to JE virus. Americn journl of hygiene, 1962, 75: Grossmn RA et l. Study of Jpnese encephlitis in Chingmi Vlley, Thilnd. III. Humn seroepidemiology nd inpprent infection. Americn journl of epidemiology, 1973, 98: Rishbudh AR et l. Post-epidemic serologicl survey of Jpnese encephlitis virus ntibodies in South Arcot District, Tmil Ndu. Indin journl of medicl reserch, 1991, 93: Crey DE, Myers RM. Jpnese encephlitis studies in Vellore, South Indi. Prt IlIl. Neutrlizing ctivity of humn survey ser. Indin journl of medicl reserch, 1968, 56: Simpson DIH et l. Jpnese encephlitis in Srwk. Virus isoltion nd serology in Lnd Dyk villge. Trnsctions of the Royl Society of Tropicl Medicine nd Hygiene, 1970, 64: Hlsted SB, Grosz CR. Subclinicl Jpnese encephlitis. I. Infection of Americs with limited residence in Kore. Americn journl of hygiene, 1962, 75: Grossmn RA et l. Study of Jpnese encephlitis virus in Chingmi Vlley, Thilnd. VI. Summry nd conclusions. Americn journl of epidemiology, 1974, 100: Bhttchry KK. JE in West Bengl. In: Proceedings of the Workshop on Jpnese Encephlitis, Jnury Delhi, Ntionl Institute of Communicble Diseses, 1988: WHO Bulletin OMS. Vol