The Zika virus which causes devastating birth defects will rage throughout the continent, warns the World Health Organization with it saying that only two countries may be spared.

So far 21 countries and territories of the Americas have had cases since Brazil reported the first local transmission in May 2015, WHO's regional office for the Americas said in a statement. "Aedes mosquitoes — the main vector for Zika transmission — are present in all the region's countries except Canada and continental Chile."

The virus is expected to spread rapidly because there is no immunity due to a lack of exposure to it previously and so far there is no treatment or vaccination for the mosquito-borne disease. Zika is transmitted by both the Aedes aegypti mosquito and Aedes albopictus mosquitoes, which are found in the US and which also transmit dengue fever.

Zika has been linked to microcephaly, which causes abnormally small heads and brains and serious, sometime deadly, developmental problems in new-born babies whose mothers contract the illness.

"We've got no drugs and we've got no vaccines. It's a case of deja vu because that's exactly what we were saying with Ebola," Trudie Lang, a professor of global health at the University of Oxford told Reuters, adding: "It's really important to develop a vaccine as quickly as possible."

The incidents of micocephaly have skyrocketed in Brazil since the Zika outbreak there, with some 4,000 babies affected. The disease causes fever, rash, joint pain and red eyes but many who contract it have no symptoms even though Zika devastates the foetus of pregnant women with the disease.

Three cases have been identified in Britain in residents who travelled to Colombia, Surinam and Guyana. The virus has also been found in the US, including among pregnant women. A baby with microcephaly was recently born to a woman in Hawaii who had contracted the illness in Brazil during her pregnancy.

Health departments, including the US Centers for Disease Control, have issued guidelines warning pregnant women not to travel to countries with Zika outbreaks, a number which continues to climb. El Salvador has advised women not to get pregnant until 2018.

Concerns are mounting about visitors to the 2016 Olympic Games in Rio de Janeiro.

Reuters reported that the Sao Paulo-based Butantan Institute is currently leading the research charge on Zika although vaccine could take up to five years to develop.

Meanwhile British drug maker GlaxoSmithKline said on Monday (25 January) it was studying the feasibility of using its vaccine technology on Zika, while France's Sanofi (SASY.PA) said it was reviewing possibilities.