Zika may have gone quiet during the winter, but as mosquito season approaches it looks like it could spread further and have longer term health effects than we thought
By Jessica Hamzelou
ZIKA virus is set to return to the fore once the mosquito season starts again in the coming months, and it looks like it could spread further and do more damage than we thought.
Cases of Zika virus – and the neurological disorders it causes in babies – have been declining across the Americas in recent months, in part because of a drop in mosquito numbers during winter. There is also evidence that people in affected countries are developing immunity to the virus – although this may be short-lived if the virus evolves, or as newly vulnerable people are born or move to affected areas.
Yet while Zika may have gone quiet, research into the virus has continued in earnest. It was assumed that only a few species of mosquito could spread Zika, including Aedes aegypti and Aedes albopictus, found in tropical regions across the globe. But now it seems many more species could carry the virus, including 26 not previously considered a threat (eLife, doi.org/b2ps). Some are found in more northerly reaches of the US, to which Zika hasn’t yet spread. The onset of warmer weather in the US could bring many more cases, as mosquitoes begin to breed and feed on blood.
Meanwhile, other means of transmission may further propagate the virus. Gabriela Paz-Bailey and her colleagues at the US Centers for Disease Control (CDC) have been investigating how long Zika can remain in human body fluids. It seems to linger in semen the longest; of the 55 infected men who have donated semen samples so far, half had cleared the virus within a month, but 5 per cent still had traces three months later. Bailey presented her findings at the Conference on Retroviruses and Opportunistic Infections in Seattle last month.
“Only a handful of sexually transmitted cases have been identified so far, but there are probably more”
Although only a handful of sexually transmitted cases have been identified, there are probably more out there, says Laurent Hébert-Dufresne at the Santa Fe Institute in New Mexico. What’s more, the discovery that Zika survives for so long in semen means that men who have sex with men are at particular risk of becoming infected and spreading the virus. They are also less likely to be tested for the virus, Hébert-Dufresne adds.
Testing is offered during pregnancy as babies born to infected women are at risk of microcephaly, which causes them to have small heads, along with nervous system disorders. Last week, the CDC reported a 20-fold increase in such birth defects in Massachusetts, North Carolina and Georgia. Even babies that appear healthy at birth can develop neurological problems down the line, and it may be a while before the full impact of the virus is known.
Those aren’t the only possible side effects of Zika infection in the womb. Problems with vision, limb development, hearing, digestion and breathing are beginning to emerge in babies exposed to Zika. Around 42 per cent of infected pregnant women have babies with at least one of these issues (JAMA Pediatrics, doi.org/b2qz).
“It does more than microcephaly,” says Catherine Spong, deputy director of the US National Institute of Child Health and Human Development.
Zika may have consequences for adults, too. A handful of cases of Guillain-Barré syndrome, in which a person’s immune system attacks their own nerves, have been reported. Research in mice also suggests the virus may affect the growth of neurons in adult brains, and potentially shrink the testicles.
Other question marks surround the presence of the virus in South East Asia; we don’t yet know if it is as dangerous, or how it might spread. “We see Zika there, but we’re not sure if it’s the same, or if there could be larger outbreaks,” says Alain Kohl at the University of Glasgow in the UK.
Globally, efforts are under way to control the virus by destroying mosquito habitats, killing the insects or using genetically modified insects to drive them into oblivion. Several groups are also working on vaccines for the virus, one of which appears to protect mice and monkeys.
For now, though, there are no treatments available. “Until we get to a situation where we can treat it effectively, we need to be worried about it,” says Kohl.
This article appeared in print under the headline “Dormant Zika is back for round two”