Ana Beatriz is held by her father Alipio Martin during a medical appointment at the Altino Ventura Foundation in Recife, Brazil. Picture: EPA/ Percio Campos

A senior World Health Organisation doctor tackling the Zika outbreak has warned that the impact of the virus is not yet fully understood and there is a significant risk it will spread rapidly to other parts of the world that could include southern Europe.

Anthony Costello, a British paediatrician who is head of maternal and child health for the UN body, warned that a vaccine may take years to develop.

He added that experts dealing with the dramatic rise in Zika cases in Latin America were being hampered by the lack of a commercially available diagnostic test as they try to track and investigate the virus.

The Brazilian President, Dilma Rousseff, warned yesterday that her country was losing the battle against the mosquito responsible for spreading Zika, which is suspected of being the cause of a dramatic increase - up to 4 000 cases - in the number of babies born with abnormally small heads.

The condition, known as microcephaly, can cause developmental issues and lifelong disability.

With the Olympics in Rio de Janeiro just seven months away, Ms Rousseff demanded a national effort to eradicate the insect.

Brazil has said it will deploy 220 000 troops to help to eliminate the Aedes aegypti mosquito from its breeding grounds in puddles and standing water.

Dr Costello echoed the concerns of the WHO's director general, Dr Margaret Chan, who warned this week that Zika is spreading “explosively” across the Americas.

Since the first cases in Brazil early last year, the virus has spread to 23 countries and territories, prompting epidemiologists to warn of up to four million cases a year across the continent.

Dr Costello, renowned for his work on improving survival rates among newborns in developing countries, told The Independent that medics were concerned that the speed of Zika's spread since its last outbreak on a group of Pacific islands in 2007 means it will jump to other continents.

Speaking from the WHO's headquarters in Geneva, he said: “It seems possible that the virus could move back into other parts of the world with vulnerable populations, places within the tropical belt and beyond - sub-Saharan Africa, South-east Asia, southern parts of the United States and southern Europe. Some of these places will be better able to deal with that threat than others.”

Researchers warned last year that parts of the Italian, French and Spanish Mediterranean coasts are suitable for the survival of the Aedes aegypti mosquito.

The insect also carries the dengue and chikungunya viruses, both from the same family of microbes as Zika.

But while surveillance systems and advanced healthcare mean that any outbreak could be rapidly isolated in Europe or the US, experts fear that developing countries in Africa and Asia would be vulnerable. Unlike malaria-carrying mosquitoes, the two Zika-carrying varieties feed in daylight, so the arrival of the virus would require a sea change in prevention habits.

The WHO, which was blamed for failing to act quickly in the early stages of the Ebola outbreak, is to meet on Monday to decide whether the Zika outbreak should be declared an international health emergency.

Unlike Ebola, the new virus, first discovered in Uganda in 1947, causes relatively mild symptoms and is not readily transmitted by human-to-human contact. While some may experience a fever and a rash, 75 percent of sufferers have no symptoms at all.

However, public health experts are concerned by its catastrophic effects on pregnancy and its ability to spark a rare immune response that can leave some adults paralysed.

Scientists have yet to prove conclusively that Zika is the direct cause of the rise in microcephaly cases, saying only that there is strong circumstantial evidence of a link. Doctors want to investigate whether mothers were also infected with dengue or chikungunya to see if a cocktail of viruses is to blame.

Dr Costello said WHO medics were nonetheless operating on the assumption that Zika is the cause and are racing to plug gaps in their knowledge. It is not known what percentage of mothers infected with the virus give birth to children with abnormalities.

The Independent understands that WHO figures suggest there has been at least a 10-fold increase in expected rates of microcephaly in Brazil.

Dr Costello said: “A really big issue is establishing the scale of the microcephaly problem and the damage it is doing to babies' brains at the most vulnerable point in their development... At the moment, we don't know if a pregnant woman has a 90 percent or 5 percent risk of her child developing microcephaly if she is infected.”

Among the priorities for those dealing with the virus is a cheap, commercially available diagnostic test in place of time-consuming laboratory tests. A vast effort to eliminate the disease-carrying mosquitoes will be made while the search for a vaccine gets under way.

Dr Costello said the best solution would be a vaccine that could be made available to girls in much the same way as the rubella jab. But he warned that while a vaccine can take just 12 months to develop, the reality may be a wait of years.

“We have done years of research on a vaccine for dengue but we don't yet have one.”

He warned that countries affected by Zika face a hefty financial burden in caring for thousands of children born with abnormalities.