Zika virus Public Health Emergency

A public health emergency has been declared this week by The World Health Organisation around the outbreak of Zika virus in Brazil, where as many as a million people may have been infected since last year.

Zika was first described - and named after - the Zika forest in Uganda, Africa, in 1947 where it was found in monkeys. But with the exception of sporadic human cases in the 1950s, the agent maintained a low profile.

Infected individuals were rarely symptomatic, and those that were had a fleeting rash, fever, headache and sore eyes and usually an uneventful recovery.

Zika appears to have spread indolently across Asia in the 1970s, borne by the Aedes aegypti mosquito that acts as a vector for the disease.

The range of the Aedes mosquito has been expanding in parallel with urbanisation. These mosquitoes are active in the daytime and prefer to bite large animals, like humans.

They are also attracted to towns and cities owing to breeding opportunities presented by nearby rubbish tips and detritus, like old tyres, which often provide pools of stagnant water.

By 2007 Zika had made it as far as the Pacific island of Yap where it caused a relatively inconsequential outbreak, but since then levels of activity have been low. But last year it began to spread rapidly across South and Central America. Some have speculated that the World Cup in 2014 may account for the arrival of the agent in Brazil, and other possible culprits include a competitors in a rowing competition who hailed from a Pacific island.

But what is worrying health officials is the association between the sudden uptick in Zika cases and increased reports of cases of infants born with microcephaly, a condition characterised by an abnormally small brain.

Since October 2015, the country has recorded 4000 such cases. At least a proportion of these appear to tally with histories from affected mothers of their having succumbed to a rash-like illness early in their pregnancy.

The genetic material has also been recovered from the brains or the amniotic fluid of some affected cases. Although Aedes mosquitoes appear to be the main carriage vector for the virus, a further case report has been released this week suggesting that Zika may also be able to spread sexually.

The partner of a US man who had returned recently from Venezuela succumbed to the infection. Tests showed blood in the man's semen, suggesting that this was the likely route for viral transmission.

Unanswered questions currently include how long a person remains infectious in this way after they recover from the disease.

Also, some infectious disease specialists are warning that microcephaly in babies born to women infected during pregnancy might represent only the more severe end of a spectrum of problems that awaits these infants as they grow up.

"It might be like rubella, or CMV," says Oxford's Peter Horby, "where there can be problems in other parts of the body too, and we don't know about those yet."