Vertical transmission of Japanese encephalitis virus in Mansonia species, in an epidemic-prone area of southern India

Japanese encephalitis ( JE) occurs in Japan, subpictus (Dhanda et al., 1997). However, when the role of Mansonia species in the North and South Korea, China and throughout South–east Asia (Reuben and Gajanana, transmission of JE virus in this region was studied in detail between 1998 and 2001, 1997). It remains a major public-health problem in India, where there are periodic the virus was also isolated from adult male mosquitoes that had been collected, during and cyclical epidemics, each generally involving 1000–3000 clinical cases and >100 routine mosquito collections, in and around cattle sheds and pigsties (see below). Densities deaths (Anon., 2000). The causative agent, JE virus (an arbovirus of the genus Flavivirus) of the major JE vector, Cx. tritaeniorhynchus, were very high (up to 397 females/manis maintained in nature by a complex cycle involving pigs as amplifying hosts, ardeid hour) in the paddy Ž elds, particularly during the main transmission season for JE virus in birds as reservoirs, and mosquitoes as vectors (Rosen, 1986). In southern India, members this area ( January–April). Mansonia indiana and Ma. uniformis, however, peaked in abundof the Culex vishnui subgroup of mosquitoes (Cx. tritaeniorhynchus, Cx. vishnui and Cx. ance (reaching 62 females/man-hour) during the monsoon months ( June–November), pseudovishnui ) are proven vectors of the virus (Reuben et al., 1988). Human beings and when the paddy Ž elds are  ooded and few Cx. tritaeniorhynchus could be found (collections cattle are considered to be ‘dead-end’ hosts, and epidemics only occur in human popufalling as low as 2.4 females/man-hour). The monsoon rains permit large populations of lations when there is substantial spill over of virus (Rodrigues, 1984). hydrophytes (such as Pistia, Salvinia and Eichhornia) to develop and these plants are Vertical transmission, in which the virus is transmitted from an infected female essential for the proliferation of Mansonia (Burton, 1960). mosquito to her eggs as they pass through the genital tract, is known to support the The male mosquitoes collected between June 1998 and February 2001 were tested, persistence of some arboviruses in nature (Rosen, 1988). There is Ž eld evidence of such in single-species pools of 25–50, for JE virus, using an antigen-capture ELISA for the vertical transmission of JE virus (Dhanda et al., 1989). In addition, infected male Cx. initial screening for  avivirus and inoculation of Toxorhynchites splendens combined with an bitaeniorhynchus can pass on the JE virus to females during mating, at least in the indirect immuno uorescence assay (Toxo– IFA) to conŽ rm infection with JE virus laboratory (Mourya and Soman, 1999). A recent outbreak of meningo-encephalitis, (Gajanana et al., 1995). Of the four pools of male Cx. gelidus ( N=116), 33 of Cx. beginning in early 1996 in the Kuttanadu area of Kerala, in south–western India, was tritaeniorhynchus ( N=1536), 25 of Ma. indiana (N=727) and 35 of Ma. uniformis identiŽ ed as JE. During the investigation of this outbreak, JE virus was successfully isolated ( N=1250) checked, three pools — two of male Ma. indiana (collected in October– from 12 wild-caught female mosquitoes: seven Cx. tritaeniorhynchus, three Mansonia November 1998) and one of male Ma. uniformis (collected in September 1998) — uniformis, one Ma. indiana, and one Anopheles