WUCHERERIA BANCROFTI FILARIASIS IN SOUTH INDIA

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TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE (1996) 90,643-644

Transovarial transmission of dengue 3 virus by Aedes aegypti

Vinod Joshi, Manju Singhi and R. C. Chaudhary Desert Medicine Research Centre (Indian Council of Medical Research), Post Box no. 122, New Pali Road, Jodhpur- 342005, India

Keywords: dengue 3 virus, Aedes aegypti, transovarial transmis- sion, India

Dengue fever, associated with dengue haemorrhagic fever (DHF) is an important public health problem in the countries of south-east Asia and the western pacific (WHO, 1975). Although dengue associated with DHF has emerged in almost all the parts of India (BANERJEE, 1994) persistence of the virus in nature and in suscepti- ble human hosts could not be substantiated in the In- dian subcontinent by ILKAL et al. (1991), who were un- able to demonstrate transovarial transmission of dengue virus in mosquitoes in nature. In Jalore district of the arid western Rajasthan region, India, epidemic dengue has appeared twice, the first epidemic being reported in 1985. The virus strain dengue 3 has been reported to be active in the region (CHOUHAN et al., 1991).

In the present paper we report investigations into the occurrence of transovarial transmission of dengue virus in Aedes aegypti. We have shown the presence of tran- sovarial transmission of the virus in nature as well as to the Fl progeny of experimentally virus-inoculated fe- male mosquitoes. This is the first report of transovarial transmission of dengue 3 virus in the Indian subconti- nent.

A. aegypti is the principal vector of dengue virus in In-

dia (RODRIGUES et al., 1972). Larvae of A. aegypti were collected from an endemic locality on the outskirts of Jalore town in India, each month from January to De- cember 1993 from domestic water containers. The larvae were reared in the laboratory and the emerged adults were kept for 3-4 d in Barraud cages with a supply of 4% glucose solution. Head squashes were prepared and ex- amined by the indirect fluorescent antibody test (IFAT) described by ROSEN & GUBLER (1974). High-titre im- mune rabbit serum raised against dengue virus was ob- tained from the National Institute of Virology, Pune, In- dia.

Dengue 3 virus was also inoculated intrathoracically to 15 fully fed female A. aegypti, and their progeny in the Fl generation were subjected to the IFAT to detect any virus transmitted transovarially. The experimental mos-

Table. Monthly rates of transovarial transmission of dengue virus in female Aedes aegypti

Prevalence of Mean Mean relative infectiona temperature humidity

Month No. No. (“Cl (%I (1993) examined infected Min. Max. Min. Max. January February March April May June July August September October November December

t :

:i 20

8

26.4 66 30.3 41 :: 32.6 40

4 (8.3%) 19.8 38.6 41 2:

i NDb

25.1 25.3 43.6 40.9 41 38 4: 22.9 34.5 63 80

1: 0 21.8 21.5 35.2 35.2 56 54 74

12 : 28.4 37.8 53 2;: 27 58 i 10.8 6.1 296 31.7 47 56 ;:

Total 388 22 (5.6%) - - - -

aRate of infection with dengue 3 virus in adult female A. aegypti reared in the laboratory from field-collected larvae; deter- mined by the indirect fluorescent antibody test.

bNot done.

644 VINOD JOSH1 ETAL.

quitoes’had been obtained from an area without any re- ported case of dengue fever and a few used were tested by the IFAT to ensure that they were uninfected before virus inoculation.

The Table shows the monthly prevalence of dengue virus in field-caught female mosquitoes from the study area. Of the total number of 599 larvae reared to adults, 22 females (3.6%) showed the presence of dengue anti- gen; no male mosquito was found to be infected. The highest rate of transovarial transmission occurred in March and April.

In the experimental studies, the 15 virus-inoculated female A. aeBvpti produced 17 female Fl progeny, 15 of which (88%) contained dengue antigen. No male prog- eny gave a positive IFAT result.

Earlier studies from India have failed to demonstrate the occurrence of transovarial transmission of dengue vi- rus (ILKAL et al., 1991). Our study of adult A. aegypti reared from field-collected larvae in a dengue endemic area has clearly shown dengue antigen, indicating the occurrence of transovarial transmission under natural conditions. It was interesting that the rate of tran- sovarial transmission was highest during March; the number of reported cases of dengue fever in the study area was also maximal during March and April (JOSHI et al., in press). In March the mean temperature ranged be- tween 13°C and 33”C, and the mean relative humidity between 40% and 56%. This range of temperature and humidity seems to be favourable for transovarial trans- mission, as similar observations have been reported by WATTS et al. (1987) and KOOPMAN et al. (1991). This may indicate the appropriate time for anti-larval measures to be undertaken.

Our experimental studies have also demonstrated transovarial transmission of dengue 3 virus. Tran- sovarial transmission of dengue 2 and dengue 3 virus in Myanmar and Trinidad has been reported by KHIN & KHIN (1983) and HULL et al. (1983), respectively; this is the first report from the Indian subcontinent. It was in- teresting that virus was transmitted only to female prog- eny and not males. Whether the virus could not multiply in male mosquitoes or whether it did not persist throughout development to the adult remains to be de- termined. Whether transovarial transmission through successive generations of mosquitoes leads to enhance-

ment or reduction of virus propagation also needs to be studied, since any such effect would be of considerable epidemiological importance.

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Received 9 May 1996; revised 31 July 1996; accepted for publication I August 1996