Transmission of the Zika virus in Africa is low due to genomic variation in mosquitoes, according to a new study published in The Lancet Planetary Health on December 11, 2024.
Research by scientists at the High Meadows Environmental Institute (HMEI) at Princeton University, Institut Pasteur, and University of California, San Diego shed light on why the virus remains relatively scarce in Africa, the continent where it was originally discovered.
The mosquito species that spreads Zika has two forms, each with different feeding preferences and capacity to transmit the disease.
Aedes aegypti exists in two different forms, sometimes referred to as subspecies. These differ morphologically, genetically and ecologically.
The globally invasive, light-coloured form found primarily outside of Africa, Aedes aegypti aegypti (Aaa), is dependent on human habitats, primarily breeds in artificial water containers, and preferentially feeds on humans as a blood source.
The native, dark-coloured African form, Aedes aegypti formosus (Aaf), is a generalist found in a variety of habitats including forests, breeds in natural sources of standing water such as tree holes and rock pools, and feeds on humans and animals. The mixed diet lowers the chances of an infectious mosquito biting a human.
Both forms live in Africa according to the study. But the amount of variation within mosquito populations might account for the variation in Africa’s Zika burden.
The African native form is less effective at acquiring and transmitting Zika virus than its human specialist counterpart, leading to a natural barrier that reduces the spread of Zika virus in Africa.
The researchers developed models that considered genetic effects on mosquito-biting preferences and capacity to spread the virus, as well as the role of temperature in affecting mosquito development, survival, and transmission capacity.
The scientists selected 59 urban centres in Africa with current populations of more than one million.
Temperature could be responsible for the low disease burden, according to the study. While many parts of sub-Saharan Africa have the ideal climate for Zika virus transmission, areas with temperatures too hot or cold for the virus might restrict its spread.
The study found that mosquito population genetics influence Zika transmission in Africa more strongly than climate.
The proportion of the human specialist mosquitoes — Ae aegypti aegypti (Aaa) — in different populations across Africa closely resembled the Zika virus burden across the continent.
However, climate change and rapid urbanisation may make African cities more vulnerable to Zika virus outbreaks in the future.
Among 59 African cities, 23 cities, or 39 per cent, are already suitable for a Zika virus outbreak, the study estimated.