Effects of Zika Virus Strain and Aedes Mosquito Species on Vector Competence

Abstract

In the Western Hemisphere, Zika virus is thought to be transmitted primarily by Aedes aegypti mosquitoes. To determine the extent to which Ae. albopictus mosquitoes from the United States are capable of transmitting Zika virus and the influence of virus dose, virus strain, and mosquito species on vector competence, we evaluated multiple doses of representative Zika virus strains in Ae. aegypti and Ae. albopictus mosquitoes. Virus preparation (fresh vs. frozen) significantly affected virus infectivity in mosquitoes. We calculated 50% infectious doses to be 6.1-7.5 log ₁₀ PFU/mL; minimum infective dose was 4.2 log ₁₀ PFU/mL. Ae. albopictus mosquitoes were more susceptible to infection than Ae. aegypti mosquitoes, but transmission efficiency was higher for Ae. aegypti mosquitoes, indicating a transmission barrier in Ae. albopictus mosquitoes. Results suggest that, although Zika virus transmission is relatively inefficient overall and dependent on virus strain and mosquito species, Ae. albopictus mosquitoes could become major vectors in the Americas.

Keywords: Aedes aegypti; Aedes albopictus; Zika virus; mosquitoes; vector competence; vector-borne infections; viruses.

Figure 1

Growth kinetics of Zika virus in A) mosquito (C6/36) and B) mammalian (Vero) cells. Cells were infected in duplicate with Zika virus strain CAM, PR, or HND, at a multiplicity of infection of 0.1. Concentration of Zika virus in supernatant was determined by plaque titration for 4 (Vero) or 7 (C6/36) days after infection. Values represent geometric means ± SD, and different superscript letters represent statistically different growth kinetics (repeated measures analysis of variance; p<0.05 by Tukey post hoc test).

Figure 2

Relationship between dose, infectivity, and preparation of Zika virus for Aedes aegypti mosquitoes. Quantitative reverse transcription PCR was used to test 12–25 processed Ae. aegypti mosquitoes for Zika virus 14 days after exposure to infectious blood meals containing various doses of Zika virus PR. Frozen stocks had been stored at −80°C and thawed before blood meal preparation, and fresh stocks were used directly after propagation without freezing. The difference in proportion infected when fresh and frozen stock at equivalent titers were compared was highly significant. *p<0.0001 by Fisher exact test.

Figure 3

Viral load of Zika virus in Aedes mosquito bodies at day 21 after infection. Zika viral load (PFU equivalents) was determined in whole mosquitoes by using Zika virus–specific quantitative reverse transcription PCR and strain-specific standards. The graph shows titers in individual mosquitoes after feeding on the highest dose (8.6–8.9 log₁₀ PFU/mL). Significant differences (t-test, p<0.05) were identified between mosquito species (*) and virus strains (†). Horizontal lines indicate means ± SD.

Figure 4

Relationship between dose and competence of Aedes aegypti and Ae. albopictus mosquitoes for Zika virus HND and CAM. Graphs show proportion of blood-engorged mosquitoes infected, with disseminated infections, and transmitting. Lines depict the best-fit linear relationships as determined by linear regression analyses. All relationships are linear and correlative (r² = 0.82–0.97). Doses at which 50% of mosquitoes are infected, have disseminated infections, and are transmitting (ID₅₀, DD_50, and TD₅₀, respectively) were calculated by using best-fit lines.