Jakarta, Indonesia: An Indonesian research institute said Sunday it had found one positive Zika case on Sumatra Island, adding that the virus has been circulating in the country ‘for a while’.

Indonesia’s health ministry could not immediately comment on the report by the Eijkman Institute for Molecular Biology.

The mosquito-borne virus has sparked widespread alarm in parts of the Americas. It is suspected of causing grave brain damage in new-borns and has similar symptoms to dengue fever.

The institute said a 27-year-old man living in Jambi province on Sumatra Island who had never travelled overseas had been found to be infected.

It said it stumbled on the case while studying a dengue outbreak in the province.

Researchers set aside specimens which produced dengue symptoms such as rashes and fever but which tested negative for dengue, and researched them further.

‘Out of the 103 (dengue-negative) specimens that we checked, we found one positive for Zika,’ the institute’s deputy director, Herawati Sudoyo, told AFP.

Zika is transmitted by the Aedes aegypti mosquito, which also spreads dengue fever and the chikungunya virus. It produces flu-like symptoms including a low-grade fever, headaches, joint pain and rashes.

Sudoyo said the specimens were taken during a dengue outbreak in Jambi between December 2014 and April 2015. It was not known how and when the man, who never travelled overseas, contracted the virus.

‘We concluded that the virus has been circulating in Indonesia for a while,’ Sudoyo said.

The World Health Organization warned in the past week the virus is ‘spreading explosively’ in the Americas, with three million to four million cases expected this year.

Hardest-hit so far has been Brazil, with more than 1.5 million cases since April.

Health authorities there are investigating the possible linkage between Zika and more than 3,400 suspected cases of microcephaly—abnormally small skulls and brains—in babies born to infected mothers.