Travellers have been warned to reconsider visiting 22 countries in the Americas, the Caribbean, Africa and the Pacific Islands under threat from mosquito-borne viruses.

The Zika virus has already been contracted abroad by Australian travellers, according to a Sydney virologist.

Pregnant mothers are at risk from new symptoms of the virus, suspected of deforming the brains and skulls of Brazilian babies – a side effect not observed before.

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In Brazil, almost 4000 children have been born with smaller heads, and possibly brain damage, since the virus outbreak was reported in May 2015, prompting concerns for athletes, staff and tourists who will visit the Olympics in Rio De Janeiro.

A few Australian travellers have been infected abroad, but it is unclear if local mosquitos have contracted it, Westmead Hospital’s Professor Dominic Dwyer said.

“But we’re not entirely sure yet whether some of the Australian mosquitoes could carry Zika virus,” Professor Dwyer told the ABC.

The World Health Organisation (WHO) predicted this week the virus will spread rapidly beyond Brazil to all countries in the Americas except Canada and Chile.

Zika is already in 22 countries, but it is feared the spread of the virus will intensify.

Scroll down to see the countries travellers are being told to avoid

Many Brazilian cities have cancelled annual Carnivale celebrations, diverting funds to fighting three mosquito-borne illnesses: Zika, dengue fever and chikungunya.

There is no vaccine or treatment for these three viruses, although modern medicine can control symptoms (except for birth defects).

The Aedes aegypti species of mosquito, found in far north Queensland, can act as vectors for the Zika virus. Photo: US Dept of Health and Human Services

Officials from Rio said they would “intensify” efforts to eradicate mosquitos from sites where hundreds of thousands of tourists and athletes will visit for the Olympics.

“The Mayor’s office will be intensifying inspections,” Rio City Hall said in a statement.

“About a month before the opening of the Games a team will visit all competition sites to eliminate possible concentrations.”

The statement came after Brazilian Health Minister Marcelo Castro said last week the mosquito carrying Zika was gaining momentum.

“For nearly 30 years the mosquito has been transmitting these illnesses to our population and since then we’ve been fighting, but we are losing the war,” he was quoted as saying by Brazil’s G1 news site.

Rio officials said getting rid of stagnant water, a fertile breeding ground for mosquitos, was the focus for health workers.

WHO spokesman Christian Lindmeier said there were 3893 suspected microcephaly cases in Brazil, which included 49 deaths.

What is the Zika virus?

Zika virus is spread through mosquito bites. It is not contagious.

The most common symptoms are flu-like, including fever, rash, joint pain and conjunctivitis.

These symptoms are mild and can last for a few days to a week. Hospitalisation from the symptoms is rare.

A health worker and soldier check for mosquito larvae in Brazil. Photo: Getty

About one in five people infected by mosquitos develop the Zika virus, according to the United States Centre for Disease Control and Prevention (CDC).

The Olympic Games are being held in Rio’s winter, meaning mosquitos will be less active, however fears are still high.

It was first discovered in the Zika forest in Ghana in the late 1940s.

There is no vaccine or medicine for the virus.

The only way to prevent it is to avoid mosquitos by wearing insect repellent, long clothing, not keeping “standing” water lying around and to use air conditioning and fly screens.

It is rarely fatal, but it is believed to cause microcephaly, which deforms babies born to infected mothers.

The virus is also connected to Guillain-Barré syndrome causing paralysis. Scientists believe the virus moved to Brazil during the 2014 World Cup.

Brazil and many other of the nations potentially affected by Zika are also battling dengue fever.

What should Australians do?

Australia’s Department of Foreign Affairs and Trade (DFAT) issued a “high degree of caution” notice for Australians travelling to Brazil.

“Pregnant women should exercise particular caution as infection may cause babies to be born with cognitive impairment,” DFAT said in a statement.

Pregnant Australians should “exercise particular caution” in Brazil said DFAT. Photo: AFP

“The level of this advice has not changed. We continue to advise Australians to exercise a high degree of caution in Brazil.”

On Friday, the CDC extended the number of territories on its no-go list for pregnant women.

It added Barbados, Bolivia, Ecuador, Guadeloupe, Saint Martin, Guyana, Cape Verde and Samoa.

Last week, the agency urged pregnant women and women considering becoming pregnant to postpone visits to Puerto Rico, Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay and Suriname.

In a statement to The New Daily, a DFAT spokesperson said there were “references to the Zika virus in … advisories for Brazil, Paraguay, Solomon Islands, New Caledonia and Cook Islands”.

“DFAT is undertaking consultations to determine the most appropriate and practical advice for Australian travellers with regards to the Zika virus. Smartraveller will be updating its advice on the Zika virus once these consultations have been finalised.

“This will include a new bulletin for travelling Australians on particular countries of concern.”

The New Daily asked the Australian Olympic Committee if it had warned pregnant family members or staff to not travel to Brazil. It did not respond before publication.

– with ABC/AAP