Vector competence of Aedes vittatus (Bigot) mosquitoes from India for Japanese encephalitis, West Nile, Chandipura and Chittoor viruses

BACKGROUND & OBJECTIVES

Aedes vittatus (Bigot), an anthropophilic mosquito, plays an important role in the maintenance and transmission of yellow fever (YF), dengue (DEN), chikungunya (CHIKV) and Zika (ZIK) viruses in Africa. In India, though natural isolation of none of these viruses was reported from the mosquito, experimental studies have shown vector competence to DEN and CHIK viruses. Despite wide prevalence in India, their potential in transmitting viruses of public health importance viz., Japanese encephalitis (JEV), West Nile (WNV), Chandipura (CHPV), Chittoor (CHITV) etc., has never been investigated. The objective of the present study is to determine the vector potential of the mosquito to these viruses.

METHODS

Mosquitoes were infected by intra-thoracic inoculation as well as by oral feeding, and growth kinetics was determined. Virus dissemination to organs was investigated by determining virus in the harvested organs on specified days' post infection (PI). Vector competence was determined by detecting the virus in saliva.

RESULTS

Intra thoracic inoculation has shown vector competence of the mosquito to JEV, WNV, CHIV and CHPV. However, using the oral route of infection, replication was observed with only WNV, JEV and CHITV. High degree of WNV replication (6.7log TCID₅₀/ml) with rapid dissemination to wings, legs and salivary glands was seen from 5^th day PI onwards. WNV was detected in saliva with a titer of 0.7log₁₀ TCID₅₀/ml on 5^th day PI. JEV and CHITV replicated in the mosquito yielding 3log and 4log₁₀ TCID₅₀/ml on 5^th and 10^th day PI respectively, but virus was not detected in saliva till 15^th day PI.

INTERPRETATION & CONCLUSION

From the results it is difficult to indict the mosquito as a vector of the viruses studied. However, presence of WNV in saliva of the mosquito shows its potential as a bridge vector and poses a concern especially when virulent WNV strains are circulating in the country.

Vector competence of certain Culex and Aedes mosquitoes for the Chittoor virus, the Indian variant of the Batai virus

Chittoor virus (CHITV), a mosquito-borne bunyavirus (Orthobunyavirus: Bunyaviridae) isolated in India, has been found to be antigenically close to the Batai virus (BATV), which has a wide distribution across Asia, Europe, and Africa. The latter virus causes influenza-like illness in humans and mild illness in sheep and goats. BATV has been involved in genetic reassortment with other bunyaviruses, generating novel genome combinations and causing severe clinical manifestations including hemorrhagic fever. Conversely, CHITV has never been associated with any major outbreaks in India, although neutralizing antibodies have been detected in humans and domestic animals. Repeated isolations and seroprevalence have prompted us to determine the vector competence of three important mosquito species, viz., Culex quinquefasciatus, Culex tritaeniorhynchus, and Aedes aegypti, for CHITV. The three mosquito species replicated CHITV to titers of 6.3, 5.0, and 5.2 log₁₀ TCID₅₀/mL, respectively, and maintained the virus for substantial periods. Both of the Culex species demonstrated vector competence, while A. aegypti did not. Horizontal transmission to infant mice was also demonstrated by both Culex species. Active circulation of the virus and the availability of both susceptible hosts and competent vector mosquitoes pose a serious threat to public health should there be a reassortment.