Sperm donors returning from Zika-infected areas have been told they will be banned from donating until there is no chance of the virus being in their systems. The mosquito-borne infection can cause birth defects in babies whose mothers had the virus in pregnancy, with thousands of cases of microcephaly, which causes an abnormally small head in newborns, confirmed in Brazil where Zika has spread rapidly.

Advice issued by the NHS in the UK has stated pregnant women should postpone non-essential travel to Zika-hit areas, with a large US sperm donor clinic now following suit.

On its website, Xytex stated that it would reject donors who had travelled to countries where Zika was present within the last month, with men having to wait if they wished to donate sperm after travelling to affected areas.

In a statement, the chief medical director for Xytex J Todd Spradlin said: "Donors who have travelled to a country or territory designated by the CDC as an area of active Zika virus transmission within the prior 28 days will not be allowed to donate. This procedure is in line with that being followed by the American Red Cross for blood donors."

As many people have no symptoms while carrying the Zika virus, blood and sperm donation centres are taking precautions so as not to risk infecting anyone further with the virus.

Zika is usually transmitted via a bite from the Aedes mosquito, but there has been a confirmed case of sexual transmission, while birth defects from Zika have been confirmed in their thousands.

The high rate of microcephaly in Brazil also prompted warnings against travel to infected areas for women considering becoming pregnant. In Brazil itselft, women were told by the government to consider postponing starting a family while the virus was spreading.