Study of role of zoonotic cycle of dengue virus in maintaining/amplifying endemic DF and causing DHF in different settings of Rajasthan, India Vinod Joshi and Manju Singhi
Date of Commencement: December, 2005
Duration: Two Years
Status: New
Objectives
1. To confirm occurrence of zoonotic cycle (ZC) of Dengue virus among tree hole breeding mosquitoes and inhabiting monkey populations. Study of its occurrence and epidemiological significance in desert settings of Rajasthan. 2. Study of role of Zoonotic cycle in maintaining and/or amplifying endemic cycle of dengue fever. 3. Study of possible role of Zoonotic cycle as factor causing Dengue Hemorrhagic Fever.
4. To undertake experimental studies to confirm monkey- mosquito cycle of dengue in
laboratory models of monkeys.
Rationale
Dengue fever (DF) associated with Dengue Hemorrhagic Fever (DHF) is an emerging public health problem in countries of South East Asia and Western Pacific Region. In Rajasthan the epidemics of DF have been reported for number of times in past. In the absence of any chemotherapy or vaccination against DF, prevention of infection is the only tool to save susceptible human population from the disease. With the increasing population and development of rapid means of transportation, exchange of vectors and virus of dengue across different ecotypes of the country has become more pronounced than before. Consequently, dengue is spreading fast and risk of its more severe form i.e. DHF & DSS has increased tremendously. In the current scenario of available knowledge and expertise it appears essential that reproducible research on developing entomological indices for transmission risk evaluation of an area is undertaken. In addition, studies on mosquito-virus interaction aspects to detect maintenance foci of virus in nature, may also play a key role in understanding aggravation of disease. The virus –mosquito –monkey cycle on trees can serve as one of the maintenance foci of virus. Or extrinsic virus stock being exchanged vertically across mosquito generations within tree holes may serve as such foci. These virus may also contribute to amplify viraemia among towns situated adjacent to these peri-urban niche. In addition, these may also cause DHF. _________________________________ 16 ______________________________
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Progress of the work
Site and Season selection for the study Present understanding of etiology of Dengue Hemorrhagic Fever (DHF) is based on cross reactivity of two dengue strains appearing in a common host with a gap of few months/years of infectivity from each other. Keeping this in view, we have presumed that endemic dengue has a uniform infectivity by a one DEN strain and that zoonotic or sylvatic foci of virus within tree hole breeding Aedes albopictus and monkeys or through vertically transmitted virus within generations of Aedes albopictus, may offer another DEN strain to either aggravate or cause DHF/endemic dengue. Thus we have selected following two areas: 1. Umaid Garden with monkeys in zoo 2. Mandore Garden with natural habitats of monkeys Collection of tree hole breeding Aedes albopictus has been made since December, 2005 at fortnightly interval. Mosquito rearing and Virus isolations Field work pertaining to collection of breeding stages of Aedes albopictus from tree holes has been started from December, 2005 through April, 2006. In all, 10 point collections with fortnightly intervals have been made. A total of 83 samples of adult Aedes albopictus mosquitoes have been reared in laboratory form eggs collected from the soil samples of tree holes. The laboratory reared adults have been subjected to Indirect Fluorescence Antibody Test (IFAT) for virus isolations from salivary glands. Mosquito mid-guts of all IFAT processed mosquitoes have been subjected for SDS PAGE for resolving their proteins as the possible molecular basis of vector competence. Details are provided in the Tables 1-2 below:
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Table 1. Details of monthly prevalence of tree hole breeding of Aedes albopictus and vertically transmitted virus isolations Period collection of No. mosquitoes subjected IFAT 7 12 1 17 46 of No. +ve for IFAT for 1 0 0 4 9 140 0 0 235 195 Positive / 1000 mosquitoes
December’05 January’06 February’ 06 March’ 06 April’ 06
IFAT= Indirect Fluorescence Antibody Test
Table 2. Poly Acrylamide Gel Electrophoresis (PAGE) of midguts of Aedes albopictus and association with TOT Period of study Virus positive/1000 % Absence of 200 kDA protein 86 0 0 55 100
December’05 January’06 February March April
140 0 0 235 195
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Important Leads:
Present study aims to establish epidemiological links between extrinsic virus foci and appearance of DHF in the area. The ultimate evidences will be generated through genotyping the viral strains circulating among endemic dengue and sylvatic dengue foci. Work done so far has highlighted an initial lead that a high percentage of mosquito positivity of vertically transmitted dengue virus has been observed among tree hole breeding Aedes albopictus species. It is expected that during forthcoming rainy season (July-September) when possible mixing of endemic dengue with sylvatic foci takes place, evidences of mixing of two strains emerging from two foci will be produced more to prove this novel research concept. As the present group is also pursuing the contention of absence of 200 kDA proteins of mid gut as possible marker of vector susceptibility, the same issue has been applied in the present project to study the association of virus presence with absence of 200 kDA protein.. Table 2 depicts a strong association of this type.
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