Why does a virus trigger issues in a single area however not in one other? A research presents insights
Why does a disease hit some countries or regions hard and largely spare others?
With COVID-19, for example, the virus has soared in North and South America. However, this was less of a problem in Africa and many parts of Asia.
Nobody knows the answer for COVID-19 yet. But for another emerging disease called Zika, scientists now have a better understanding of why the virus isn’t a major problem in Africa, despite its origins on the continent. It probably boils down to the animal that is spreading the disease – or, more specifically, the subspecies of the animal.
That comes from a new article published in Science magazine on Thursday. The study shows that a species of mosquito found around the world not only likes to bite people, but is also really good at making us sick.
In 2015, Zika was traveling Central and South America, infecting more than 100 million people and causing at least 3,500 cases of microcephaly in newborns. Microcephaly is a condition in which babies are born with very small heads and brain damage.
The mosquito-borne virus also broke out in the Pacific Islands and parts of Asia. Many health officials feared that this would ultimately lead to major problems in Africa.
But that did not happen, says virologist Louis Lambrechts from the Pasteur Institute, a biomedical research foundation in Paris. “Zika has never caused a major human outbreak in Africa, with the exception of [the islands of] Cape Verde, which is technically off the coast of Africa. ”
Zika is mainly spread by a species of mosquito called Aedes aegypti that thrives in tropical climates around the world. The mosquito ingests the virus when it bites a person. Then the virus replicates in the mosquito. And if the mosquito bites another person, it can transmit the virus.
Not all A. aegypti are created equal, says Lambrechts, who led the new study. The species occurs in two main types: the forest dweller form prevalent in Africa and the human-loving form that lives in Latin America and Asia.
The forest-dwelling form prefers to feed on animals such as rodents or antelopes. “It’s not about people,” says Rockefeller University neurobiologist Leslie Vosshall, who was not part of the new study. “It’s almost like actively avoiding people.”
In contrast, the human-loving form really likes to feed on human blood, says Vosshall, and spends its life around people. That difference in feeding preference is probably one of the reasons why Zika wasn’t such a big problem in Africa, Vosshall says.
Lambrechts and his team also wondered whether the human-loving mosquito in Latin America is more susceptible to infection with Zika than the forest-dwelling form and is better able to transmit the virus.
To find out, he and his colleagues carried out a simple experiment. They took mosquitoes from Africa, Asia and Latin America and fed them with blood that was infected with varying amounts of the Zika virus. Then they determined what percentage of the insects were infected at each virus concentration.
“The results were so clear. It was almost too good to be true,” says Lambrechts.
The mosquitoes from Africa – the form of forest dwellers – were more resistant to Zika infections than the human-loving mosquitoes from Latin America. The African mosquitoes only ingested the virus from their blood meal if it contained higher concentrations of Zika. In contrast, the human-loving mosquitoes were infected by the blood, even if it contained far fewer Zika particles. The human-loving mosquitoes from Asia fell between the other two populations.
The infection pattern fits remarkably well with the epidemiological pattern of Zika infections, says Lambrechts. Regions with mosquitoes very susceptible to Zika infections (i.e. Latin America) have had large, explosive outbreaks. Regions with less susceptible mosquitoes did not have large outbreaks (i.e. Africa). And the regions with moderately susceptible mosquitoes were somewhere between the two extremes (i.e. Asia).
The researchers then show that the African mosquitoes are less able to transmit Zika to mice in the laboratory than the Latin American mosquitoes. Those in Africa are less likely to ingest the virus from infected blood and are less likely to have Zika virus in saliva after feeding on an infected animal.
These new findings help explain why Zika suddenly became a problem in Latin America, says Vosshall, but hadn’t been a problem in Africa. It depends on what taste of mosquito is hanging around.
“We already knew that the human-loving form loves to bite people, and whatever is infected with the mosquito, it will infect the person very efficiently,” says Vosshall. “Now this paper shows that the human-loving form is really good at growing the Zika virus too [inside its body]. ”
“So it’s really our bad luck,” she adds, because the mosquitoes that love to be around and feed on our blood are also really good at replicating a virus that makes us sick.