Swiss doctors have asked thousands of colleagues worldwide to provide data for the first global registry of women exposed to Zika, the team's lead researcher said Tuesday.

Such a database is urgently needed to better understand the deadly virus and how it is transmitted, said David Baud, a physician in the obstetrics research unit of the University Hospital in Lausanne.

‘Potentially thousands of pregnancies are affected worldwide,’ he told AFP.

Zika can cause crippling birth defects, and is suspected to trigger other neurological disorders.

The disease has swept through Latin America, the Caribbean and beyond since 2015, prompting the World Health Organization (WHO) to declare an international public health emergency in February this year.

As of early August, 65 countries have reported mosquito-born transmission in the last 20 months.

Initially thought to be spread only via the blood-sucking insects, Zika is now thought to be conveyed through sex and blood transfusions as well.

But much remains unknown.

- 'Tip of the iceberg' -

‘Does sexual transmission to a pregnant woman also induce foetal abnormality? Why do some babies developed abnormalities while others don't? Who is at risk?’

‘The only way to answer these questions is with 'big data',’ which can reveal otherwise hidden patterns, Baud said by phone.

What the world has seen so far could be ‘the tip of the iceberg,’ he added.

Estimates vary widely, for example, as to what percentage of foetuses of women infected with Zika during the first trimester are at risk of microcephaly, characterised by brain damage and small heads.

Research on French Polynesia, hit by Zika in 2013, put the odds at one in a hundred, while another study from Brazil -- the country hit hardest by the epidemic so far -- concluded that the risk was twice that high.

Yet another team of scientists reported foetal problems in 29 percent of women exposed to Zika during their pregnancies.

Some of the more than 4,000 gynaecologists and obstetricians to whom the appeal was made have already provided data on patients -- who remain anonymous -- using standardised online forms.

To encourage contributions, Baud has promised that doctors who participate will be listed as co-authors in future journal articles.

National and professional associations have also pledged to pass on the request to their members.

Baud said he would publish preliminary findings after his team had information for 100 women, but that the statistically significant threshold for new findings is 1,000.

At present, details on only 160 pregnancies exposed to Zika can be found in the scientific literature, and gaps often make comparisons difficult or impossible.

The new online registry gathers data on the general health of the women, blood profiles, medications taken, exposure to different viruses, and other relevant factors.

Last week, US health authorities declared a public health emergency in Puerto Rico due to the outbreak of Zika, which has now infected more than 10,000 people.

The United States registered its first locally transmitted cases of Zika in Florida in July.

Since February 2016, 11 countries have reported evidence of person-to-person infections, probably via sex.

In four out of five cases, the virus causes no symptoms. Those who do feel sick have reported fever, rash, body aches and conjunctivitis, or pink eye.

Baud and three colleagues published a comment explaining their initiative in the peer-reviewed journal The Lancet Infectious Diseases.