(CNN) Health authorities in the Philippines have declared a "national dengue alert" after a spike in cases of the viral disease which has left more than 450 people dead since January.

Around 100,000 dengue cases were reported across the Philippines in the first six months of 2019, an increase of 85% on the same period last year.

Epidemics have been declared in four regions of the country -- Mimaropa, Western Visayas, Central Visayas, and Northern Mindanao -- which between them are home to more than 20 million people, or roughly 20% of the Philippines population.

Health officials are also monitoring the situation in nine other regions, including Davao, the home province of Philippines President Roderigo Duterte, but said dengue remains "localized" rather than a national epidemic, CNN Philippines reported

Filipinos infected with dengue rest on beds at the Quirino hospital in Manila.

A mosquito-borne viral infection, dengue causes flu-like symptoms, including piercing headaches, muscle and joint pains, fever and full body rashes. Of the millions of people infected with dengue every year worldwide, an estimated 500,000 develop severe symptoms requiring hospitalization, and of those some 12,500 people die, according to the World Health Organization

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