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American Samoa's Department of Health has declared a dengue fever outbreak, with six confirmed cases.
This follows the discovery of the disease last week in travelers returning to the territory.
Territorial epidemiologist Adam Konrote told a press conference on Wednesday - streamed on the government-owned KVZK TV - that four of the confirmed cases were imported, but the two newest ones were locally acquired.
"The two most recent cases had no travel history, confirming that dengue virus transmission is now happening within the community," he said.
"This is a serious development and requires immediate coordinated action from both health authorities and the public."
Dengue is not spread from person to person but through the bite of infected mosquitoes.
Travellers returning from Fiji, Samoa or Tonga, which have previously declared outbreaks, are advised to monitor their health for up to 14 days for any symptoms of dengue.
ASCC entomologist Dr Mark Schmaedick said there are 12 species of mosquito in American Samoa, but only two of them are known to carry dengue fever.
"These are the black-and-white patterned mosquitoes that you see coming around to bite you in the day time.
"Generally these species don't feed at night."
Schmaedick said dengue-carrying mosquitoes tend to develop in habitats of water-holding containers, such as buckets, tyres and ice cream containers.
He said they are not strong flyers and tend to stay close to where they hatched.