Chikungunya, a debilitating mosquito-borne viral disease, has taken hold in French Polynesia, spreading rapidly and threatening neighboring Pacific nations, regional health authorities said on Wednesday.

The disease, typically found in low levels in Africa and Asia, this year has infected almost a million people in Latin America and the Caribbean, according to the World Health Organization.

French Polynesia, with a population of more than 268,000, said four people had died and more than 18,000 people had sought treatment for the disease since October, the first outbreak in the archipelago.

"We think that about half of the population will be affected in this cycle," said public health surveillance head Henri-Pierre Mallet, adding that the start of the rainy season would help the disease spread.

Samoa has reported more than 2,500 cases since July, while American Samoa and Tokelau had also suffered outbreaks, the Secretariat of the Pacific Community (SPC), based in New Caledonia, said.

"We are only half way through the chikungunya wave," said Dr Adam Roth, an epidemiologist at the SPC. "And there is certainly a big risk of infection for the (14) countries which have not had it yet," he said. SPC is a regional organization that helps the development of 22 countries.

Chikungunya is spread by two mosquito species and can cause severe symptoms including fever, headache and joint pain. There is no treatment nor vaccine.

Among mosquito-borne diseases, chikungunya has the longest period of illness with some people developing long-term debilitating joint pain that can keeping them away from work for months.

"It's not only a big burden for healthcare systems but also for the economy," Roth said.

French Polynesia's $5 billion economy was hit by the global financial crisis which caused a severe drop in tourism, the nation's lifeblood.