The Cook Islands Ministry of Health is concerned dengue fever may have spread to the northern island of Aitutaki.
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A spokesperson told the Cook Island News three probable cases of dengue type 1 virus have been identified on Aitutaki but have not yet been confirmed.
Two health protection officers and a mosquito control team have been sent to the island to check for the insects' breeding sites and try to prevent more people being infected with the mosquito-borne virus.
Last week the ministry reported 18 confirmed cases and 12 people classified as probably having the virus, all thought to be on Rarotonga.
The ministry said if the clinical presentation a patient is suspected having dengue a rapid test is performed in the Cook Islands and if the result is positive that case is classified 'probable dengue'.
Blood samples are then sent to New Zealand where the strain may be classified as 'confirmed dengue'.
Health officials are asking people to keep environments clear of water containers, to use bed nets and screens to avoid mosquito bites, and see a doctor if suffering from fever, headache, pain behind the eyes or muscle aches.