LONDON, Dec 8 (Reuters) - The number of people killed by malaria dropped below half a million in the past year, reflecting vast progress against the mosquito-borne disease in some of the previously hardest-hit areas of sub-Saharan Africa.
The World Health Organization's annual malaria report showed deaths falling to 438,000 in 2015 - down dramatically from 839,000 in 2000 - and found a significant increase in the number of countries moving towards the elimination of malaria.
Malaria prevention measures - such as bednets and indoor and outdoor spraying - have averted millions of deaths and saved millions of dollars in healthcare costs over the past 14 years in many African countries, the report said.
And while Africa continues to bear the highest malaria burden of all regions of the world, death rates from the disease have fallen since 2000 by 66 percent across all age groups, and by 71 percent among children under five.
Margaret Chan, the WHO's director general, said the progress had been made possible "through the massive rollout" of effective prevention and treatment tools.
"In sub-Saharan Africa, more than half of the population is now sleeping under insecticide-treated mosquito nets, compared to just 2 percent in 2000," she wrote in the report published on Tuesday.
"A rapid expansion in diagnostic testing, and in the availability of antimalarial medicines, has allowed many more people to access timely and appropriate treatment."
Two countries, Nigeria and Democratic Republic of Congo, accounted for more than 35 percent of global malaria deaths in 2015.
Yet, the WHO said, an estimated 663 million cases of malaria have been averted in sub-Saharan Africa since 2001 as a direct result of the scale-up of three key malaria control interventions: bednets, indoor spraying and the use of malaria drugs known as artemisinin-based combination therapy, or ACTs.
Mosquito nets have had the greatest impact, it said.
Pedro Alonso, director of the WHO's global malaria programme, cautioned, however, that as the global burden of malaria declining, new challenges are emerging.
"In many countries, progress is threatened by the rapid development and spread of mosquito resistance to insecticides. Drug resistance could also jeopardize recent gains in malaria control," he said.
In the past five years, 60 of the 78 countries that monitor insecticide resistance have reported mosquito resistance to at least one insecticide used in nets and indoor spraying, the report found. And of these, 49 reported resistance to two or more classes of insecticide.
(Reporting by Kate Kelland; Editing by Mark Heinrich)