(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.
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.
While it is most common in tropical and sub-tropical climates such as the Philippines, India and Brazil, a recent study found that rising global temperatures caused by the climate crisis could see mosquitoes which carry dengue -- along with other diseases such as chikungunya, yellow fever and Zika -- move north, affecting the southern United States, inland Australia and coastal areas of China and Japan.
Dengue cases in the Philippines have historically surged every three to four years, and the sharp increase this year is in line with expectations after a spike in cases nationwide in 2016, the country's Health Secretary Franciso Duque said in a statement.
This surge in dengue cases comes as the country is grappling with a nationwide measles outbreak, which authorities say has killed more people so far this year than all of 2018 and puts 2.6 million children at risk.
The most effective ways to prevent dengue include searching for and destroying mosquito breeding places, liberal use of mosquito repellents, and seeking early consultation when the first signs and symptoms of disease strike, Philippines health officials said.