Portuguese health authorities on Thursday reported two cases of full-blown mosquito-borne dengue on the popular tourist archipelago of Madeira and said another 22 people were suspected of carrying the virus that causes the fever.
Authorities in Madeira "reported 24 cases, two of them confirmed by laboratory analyses", DGS health agency director Francisco George told AFP.
Those affected are Madeira residents and have not left the island lately.
"Steps have been taken to follow the situation ... The monitoring system is in place and we believe that there is no reason for alarm," said George.
He said the outbreak was "not unexpected as the mosquitoes' existence on the island has been known since 2005", but no one was infected so far.
Health officials called on residents to use protection against mosquito bites and asked airlines to step up precautions so the mosquitoes do not spread.
According to the UN's World Health Organisation, between 50 and 100 million dengue infections occur each year in more than 100 countries. In 1970, the disease was endemic in just nine countries.
Half a million people each year develop severe dengue, a feverish condition requiring hospitalisation.