A Zika virus from America is more efficiently transmitted than an Asian virus by Aedes aegypti mosquitoes from Asia

Abstract

Zika is a mosquito-borne disease associated with neurological disorders that causes an on-going pandemic. The first outbreak was recorded in Micronesia in 2007, then in French Polynesia in 2014 from which it spread to South America in 2015 and ignited a widespread epidemic. Interestingly, Zika outbreaks in Asia remained of moderate intensity although the virus is circulating. To understand these epidemiological variations, we investigated the entomological determinants of ZIKV transmission in Asia. We used oral infection of mosquitoes collected in Singapore to identify the vector species, to quantify the blood infection threshold and to compare transmissibility between an Asian ZIKV strain (H/PF13) and an American strain collected in Brazil (BE H 815744). We have confirmed the vector status of Aedes aegypti and determined that 10³ pfu/ml of blood is sufficient to infect mosquitoes. We showed that only the American strain was present in the saliva 3 days post-infection, and that this strain had a 30-40% higher rate of saliva infection in Ae. aegypti from 3 to 14 days post-infection than the Asian strain. Our data suggests that American strains are more efficiently transmitted than Asian strains, which raises concerns about the introduction of American strains in Asia.

Figure 1

Comparison of susceptibility to Zika virus infection between Ae. aegypti, Ae. albopictus and Cx. quinquefasciatus. Mosquitoes were orally infected with PF13 at a titer of 10⁶ or 10⁵ pfu/ml. (A) At 7 days p.i., ZIKV genome copies were quantified in whole mosquitoes. (B) At 14 days p.i., ZIKV genome copies were quantified in salivary glands. Infection rate and gRNA copies per infected mosquitoes are shown. Each point represents one mosquito sample. Line shows average of gRNA copies. Twenty mosquitoes were tested per condition. Asterisks indicate significant difference with Ae. aegypti in the same condition following a Z-test and T-test for infection rate and gRNA copies, respectively.

Figure 2

Zika virus can infect mosquitoes at a titer of 10³ pfu/ml in the blood. Aedes aegypti mosquitoes were orally infected with 10⁵, 10⁴, 10³ or 10² pfu of PF13 per ml of blood. At 7 days p.i., ZIKV genome copies were quantified in whole mosquitoes. Each point represents one mosquito sample. Twenty mosquitoes were tested per condition. Line shows average of gRNA copies.

Figure 3

Aedes aegypti mosquitoes infected with a strain from Brazil have a higher infection rate of saliva. Aedes aegypti mosquitoes were orally infected with 10⁵ pfu/ml of PF13 or the Brazil virus. At 3, 7, 10 and 14 days p.i., Zika virus genome copies was quantified in (A) whole mosquitoes and (B) saliva. Percent of infection rate and average ± s.e.m. for genome copies per infected mosquitoes or saliva are presented. Thirty mosquitoes were analyzed per condition. Asterisks show differences between the two viruses within the same day following a Z-Test and T-test for infection rate and gRNA copies, respectively.