In the past few weeks, there have been a number of reports about the rise of Zika virus, another member of the family that includes yellow fever virus, dengue virus and West Nile virus. But what is Zika virus? How is it transmitted? We asked Dr Derek Gatherer from Lancaster University, who recently co-wrote a review on the virus for the Journal of General Virology.
How did the current outbreak start?
Zika virus was first isolated in Uganda in 1947, and the first human case was reported in Nigeria in 1954. It spread across Africa, and eventually made its way to South Asia, where it was detected in Malaysia in 1966. It seems to have remained here on and off, then in 2007 spread to the island of Yap in Micronesia. In 2013, an outbreak in French Polynesia caused 28,000 infections.
We think that the current outbreak in the Americas arrived from Polynesia. The virus may have been introduced to Brazil during the 2014 FIFA World Cup or a canoe race where Polynesian athletes competed. Since then, the virus has spread through several Latin American countries, including Colombia, Mexico, Paraguay and Venezuela.
Just this week there has been a confirmed case of Zika virus in the USA. The patient had recently returned from Latin America.
How is the virus transmitted?
The primary method of transmission is bites from mosquitos of the genus Aedes. Aedes aegypti mosquitos – known for spreading diseases like dengue in Africa – do transmit Zika virus, but so do many of the other Aedes species. We know a lot less about which species spread the disease in Asia and the Americas.
There are also some cases of male sexual transmission of the disease, although fortunately this seems to be very rare. Similar to Ebola, some male survivors can transmit the virus in their semen. This is due to reduced immune system activity in the testes, meaning the virus can remain there and be transmitted sexually.
Another possibility is that the virus could appear in blood stocks of infected areas, meaning it could be transmitted through transfusions. Care will need to be taken that this does not happen.
What are the symptoms of Zika virus?
The symptoms are similar to other viral diseases like dengue and chikungunya. Patients get a fever and a characteristic rash, with blotches appearing on the trunk, back, arms and neck. There is pain in muscles and joints, but this is less severe than in dengue fever. There have also been reports of nausea, vomiting and headaches in some people.
Does infection have any potential complications?
There is no evidence to suggest the virus is fatal to healthy adults, but there are some potentially concerning associations. In the Polynesian outbreak, some Zika patients developed Guillain-Barré syndrome, a post-viral autoimmune condition that casues muscle weakening. In the case of Zika virus, none of the Guillain-Barré patients died of this, but some developed temporary partial paralysis.
In South America, the concern is that Zika virus could be linked with birth defects – infants born with unusually small heads (microcephaly). In Brazil, this has been happening in the same parts of the country and at the same time that the Zika virus outbreak has been occurring.
Some affected pregnant mothers were tested and showed antibodies to the Zika virus, but this in itself does not prove a connection between the two. Many people may already have the virus, including some pregnant women. At present, we don’t have any real evidence that Zika virus is causing microcephaly; it could be that two separate epidemics have superimposed themselves. But the scale of the outbreak and the speed of its progression means lots of information hasn’t made it into the public domain.
Is it possible to treat the virus?
There is currently no cure or treatment for Zika virus, although patients normally recover by themselves within a week.
How can Zika virus be prevented?
No vaccine has been developed for Zika virus. Producing vaccines for other flaviviruses has been difficult in the past, so it’s unlikely that treatments will emerge any time soon.
The main prevention measure is avoiding mosquito bites. This can be done with mosquito repellant and covering up exposed skin. Nets aren’t effective as the mosquito species bite during the day. Removal of mosquito breeding sites, such as water butts and wells, will also reduce the spread. But for people living in poorer areas, who often can’t afford these measures, bites are much harder to avoid.
Gatherer, D. & Kohl, A. (2015) Zika virus: a previously slow pandemic spreads rapidly through the Americas. Journal of General Virology doi: