Chikungunya virus transmission potential by local Aedes mosquitoes in the Americas and Europe

Abstract

Background: Chikungunya virus (CHIKV), mainly transmitted in urban areas by the mosquitoes Aedes aegypti and Aedes albopictus, constitutes a major public health problem. In late 2013, CHIKV emerged on Saint-Martin Island in the Caribbean and spread throughout the region reaching more than 40 countries. Thus far, Ae. aegypti mosquitoes have been implicated as the sole vector in the outbreaks, leading to the hypothesis that CHIKV spread could be limited only to regions where this mosquito species is dominant.

Methodology/principal findings: We determined the ability of local populations of Ae. aegypti and Ae. albopictus from the Americas and Europe to transmit the CHIKV strain of the Asian genotype isolated from Saint-Martin Island (CHIKV_SM) during the recent epidemic, and an East-Central-South African (ECSA) genotype CHIKV strain isolated from La Réunion Island (CHIKV_LR) as a well-characterized control virus. We also evaluated the effect of temperature on transmission of CHIKV_SM by European Ae. albopictus. We found that (i) Aedes aegypti from Saint-Martin Island transmit CHIKV_SM and CHIKV_LR with similar efficiency, (ii) Ae. aegypti from the Americas display similar transmission efficiency for CHIKV_SM, (iii) American and European populations of the alternative vector species Ae. albopictus were as competent as Ae. aegypti populations with respect to transmission of CHIKV_SM and (iv) exposure of European Ae. albopictus to low temperatures (20°C) significantly reduced the transmission potential for CHIKV_SM.

Conclusions/significance: CHIKV strains belonging to the ECSA genotype could also have initiated local transmission in the new world. Additionally, the ongoing CHIKV outbreak in the Americas could potentially spread throughout Ae. aegypti- and Ae. albopictus-infested regions of the Americas with possible imported cases of CHIKV to Ae. albopictus-infested regions in Europe. Colder temperatures may decrease the local transmission of CHIKV_SM by European Ae. albopictus, potentially explaining the lack of autochthonous transmission of CHIKV_SM in Europe despite the hundreds of imported CHIKV cases returning from the Caribbean.

Fig 1. Transmission efficiencies (A) and viral loads (B) in saliva of Aedes aegypti from Saint-Martin at different days after infection with CHIKV_LR and CHIKV_SM provided at a titer of 10^6.5 PFU/mL.

Mosquitoes incubated at 28°C were examined at days 2, 3, 5 and 7 to determine the transmission efficiency and the number of viral particles in saliva. Transmission efficiency corresponds to the proportion of mosquitoes with infectious saliva among the tested ones. An asterisk refers to a significant difference (P-value < 0.05). Error bars represent the confidence interval (95%) for transmission efficiencies, and the standard deviation for viral loads.

Fig 2. Transmission efficiencies (A) and viral loads (B) of CHIKV_SM in saliva of Ae. aegypti collected in the Caribbean (A) and the continental America (B).

At days 3, 5 and 7 after an infectious blood-meal, 20 mosquitoes per condition were sacrificed for saliva collection and saliva was titrated on C6/36 Ae. albopictus cells. Transmission efficiency corresponds to the proportion of mosquitoes with infectious saliva among the tested ones. In parenthesis, the number of analyzed mosquitoes. Error bars represent the confidence interval (95%) for transmission efficiencies, and the standard deviation for viral loads.

Fig 3. Susceptibility of Aedes aegypti compared to Aedes albopictus from the same locality when infected with CHIKV_SM and CHIKV_LR and examined at different days after infection.

Twenty mosquitoes per condition were processed to determine the transmission efficiency (A and B) and the viral load in saliva at days 3, 5 and 7 after an infectious blood-meal (C and D). Transmission efficiency corresponds to the proportion (%) of mosquitoes with infectious saliva among the tested ones. In parenthesis, the number of analyzed mosquitoes. An asterisk refers to a significant difference (P-value < 0.05). Error bars represent the confidence interval (95%) for transmission efficiencies, and the standard deviation for viral loads.

Fig 4. Effect of temperature on CHIKV transmission.

Ae. albopictus from Bar-sur-Loup (France) were infected with CHIKV_LR and CHIKV_SM and incubated at 28°C (A). Mosquitoes infected with CHIKV_SM were also exposed at a constant temperature of 20°C or at variable temperatures mimicking daily fluctuations around an average of 20°C (B). Transmission efficiencies and viral loads in saliva were determined as previously described using 20 mosquitoes per condition. In parenthesis, the number of analyzed mosquitoes. An asterisk refers to a significant difference (P-value < 0.05). Error bars represent the confidence interval (95%) for transmission efficiencies, and the standard deviation for viral loads.