Evidence of Experimental Vertical Transmission of Emerging Novel ECSA Genotype of Chikungunya Virus in Aedes aegypti

Background Chikungunya virus (CHIKV) has emerged as one of the most important arboviruses of public health significance in the past decade. The virus is mainly maintained through human-mosquito-human cycle. Other routes of transmission and the mechanism of maintenance of the virus in nature are not clearly known. Vertical transmission may be a mechanism of sustaining the virus during inter-epidemic periods. Laboratory experiments were conducted to determine whether Aedes aegypti, a principal vector, is capable of vertically transmitting CHIKV or not. Methodology/Principal Findings Female Ae. aegypti were orally infected with a novel ECSA genotype of CHIKV in the 2nd gonotrophic cycle. On day 10 post infection, a non-infectious blood meal was provided to obtain another cycle of eggs. Larvae and adults developed from the eggs obtained following both infectious and non-infectious blood meal were tested for the presence of CHIKV specific RNA through real time RT-PCR. The results revealed that the larvae and adults developed from eggs derived from the infectious blood meal (2nd gonotrophic cycle) were negative for CHIKV RNA. However, the larvae and adults developed after subsequent non-infectious blood meal (3rd gonotrophic cycle) were positive with minimum filial infection rates of 28.2 (1∶35.5) and 20.2 (1∶49.5) respectively. Conclusion/Significance This study is the first to confirm experimental vertical transmission of emerging novel ECSA genotype of CHIKV in Ae. aegypti from India, indicating the possibilities of occurrence of this phenomenon in nature. This evidence may have important consequence for survival of CHIKV during adverse climatic conditions and inter-epidemic periods.
Author Summary Although vertical transmission of arboviruses has been recognized for nearly a century, rates of transmission in laboratory experiments are low and their significance in terms of survival of virus during periods of low transmission appears debatable. Recently, major urban outbreaks of chikungunya have been recorded in many parts of Asia, Africa, and Europe. The occurrence of random sporadic cases of the disease in years following a major outbreak prompted us to investigate whether these might be attributable to survival of the virus by vertical transmission. Our experiments were designed to test two hypotheses: (1) The development of an egg-batch derived from an infectious blood meal is too rapid for the infection to reach ovaries; (2) The enormous distension of the membrane enveloping ovaries and ovarioles following oviposition, might facilitate virus penetration. We conclude that after the infected blood meal, oogenesis and oviposition were complete before virus had disseminated to infect the ovaries. Because similar experiments with infection in first gonotrophic cycle did not lead to infected progenies, it is presumed that expanded parous ovaries might support efficient infection. Therefore, it may be concluded that vertical transmission is a more common phenomena in mosquitoes during subsequent gonotrophic cycles following arboviral infection.