GENEVA/LONDON (REUTERS) - The World Health Organisation on Monday (Feb 1) declared the mosquito-borne Zika virus to be an international public health emergency as the disease linked to thousands of birth defects in Brazil spreads rapidly.

WHO director-general Margaret Chan told reporters an international coordinated response was needed, although restrictions on travel or trade were not necessary.

The emergency designation was recommended by a committee of independent experts to the United Nations agency following criticism of a hesitant response so far. The move should help fast-track international action and research priorities.

The WHO said last week the Zika virus was "spreading explosively" and could infect as many as four million people in the Americas.

The agency was criticised for reacting too slowly to the Ebola epidemic in West Africa which killed more than 10,000 people, and has promised to do better in future global health crises.

The WHO's International Health Regulations emergency committee brings together experts in epidemiology, public health and infectious diseases from the Americas, Europe, Asia and Africa.

Brazil has reported nearly 4,000 suspected cases of microcephaly, in which infants are born with smaller-than-usual brains. The health ministry has linked the condition to Zika, although the connection is not yet definitive.

Brazilian Health Minister Marcelo Castro told Reuters that the epidemic was worse than believed because in 80 per cent of the cases the infected people had no symptoms.

As the virus spreads from Brazil, other countries in the Americas are also likely to see cases of babies with Zika-linked birth defects, experts believe.

The Pan American Health Organisation says that Zika has now spread in 24 nations and territories in the Americas.

FAQs on Zika

Q What is Zika?

It is a disease caused by a virus transmitted by the Aedes mosquito. It was first detected in Zika Forest in Uganda in 1947 in a rhesus monkey, and in the Aedes africanus mosquito in 1948.

Very few cases of human infection were reported before 2007, when an outbreak occurred on Yap Island in Micronesia. Brazil has borne the biggest brunt of the disease so far, with an estimated one million people infected.

Q What are the symptoms?

The incubation period is likely to be a few days. The symptoms are similar to other mosquito-borne diseases such as dengue and chikungunya: fever, skin rashes, conjunctivitis, muscle and joint pain, malaise and headache. Symptoms are usually mild and last between two and seven days.

Q Why are there outbreaks now?

Deforestation, increasing urbanisation and rising temperatures have encouraged mosquitoes to breed. With the high volume of travel across countries, the virus has a greater chance of getting imported as well.

Zika is new to the Americas, so the virus can spread quickly there to a big population of susceptible hosts who have no immunity to it. Latin America is also known to have a large number of Aedes mosquitoes.

In South-east Asia, there have been a small number of cases detected in Cambodia, East Malaysia, Indonesia, the Philippines and Thailand in recent years.

Q How is it harmful?

When French Polynesia experienced an outbreak in 2013, instances of Guillain-Barre syndrome linked to the viral infection were reported, as well as a spike in microcephaly, a rare condition in which babies are born with abnormally small heads.

Evidence is mounting in Brazil linking the virus to microcephaly, while Colombia and Venezuela are both reporting a jump in cases of Guillain-Barre syndrome - a rare disorder that causes muscle weakness and temporary paralysis. Most people recover from it.

No Zika-related deaths have been reported so far, according to the World Health Organisation.

Q Is it likely to spread to Singapore?

The Health Ministry and National Environment Agency (NEA) say it is "inevitable" the virus will be imported into Singapore, given the high volume of travel by Singaporeans and tourists, and the presence of the virus in the region.

Aedes mosquitoes are also present in Singapore, making subsequent local transmission likely.

The NEA said it will boost vector control activities should a case be detected, while anyone found to have the disease will be admitted to a single room in a public hospital.

Q Is there a vaccine or a drug for Zika?

No, as the disease was rare and mild up until last year.

Those infected are usually advised by doctors to get plenty of rest and drink lots of fluids, while pain and fever are treated with available medicines.

Scientists are currently studying how to produce vaccines, but it could take five to seven years before one is commercially available.

Q How can you protect yourself?

The same way you would with dengue if you're travelling to countries affected by Zika - wear long, covered clothing and apply insect repellant.

At home, take precautions to prevent mosquitoes from breeding.

Tan Dawn Wei

Sources: World Health Organisation, Centres for Disease Control and Prevention