Hundreds of children have been hospitalised with dengue fever in Phnom Penh and thousands of cases reported across Cambodia amid a major outbreak of the deadly disease. Several other south-east Asian nations are also battling a sharp spike in the number of dengue cases, according to the World Health Organisation, including Laos, Malaysia, the Philippines, Singapore and Vietnam. Cambodian Health Minister Mam Bunheng said on Thursday that 12,000 cases of the mosquito-borne disease had been reported nationwide and that 21 people had already died, according to local media. The Kantha Bopha foundation, which runs five hospitals, said on its Facebook page that as of Monday it had treated 4305 children for dengue as outpatients. A total of 2990 children had been diagnosed with a serious case of dengue and had been admitted for treatment and of that cohort, 577 children had been diagnosed with severe haemorrhagic dengue fever and 89 children had had surgery. Dengue fever is a virus carried and spread by mosquitoes and widespread throughout south-east Asia and in parts of far north Queensland. It causes flu-like symptoms, including high fever, muscle and joint pains, nausea and vomiting. In severe cases of dengue fever, it can cause internal bleeding, bleeding from the mouth and gums and can be fatal if the infection becomes severe and is not treated. Outside one of the Kantha Bopha hospitals in Phnom Penh on Thursday, more than 100 families were waiting for treatment for their children. Thirty-one year old Cambodian man Sophia, who only uses one name, from the provincial town of Pursat, cradled his 2½-year-old son Thearith in his arms. Sophia said his son had been sick with the virus for three days and he had travelled the 200 kilometres to Phnom Penh at the urging of his mother, who lives in the capital. "I got his blood tested, I was told that he got dengue fever his temperature had been up and down, between 37 and 38 degrees celsius," he said. "I got some medication to bring down his temperature. After the medicine had no more effect, he became hot again." It was when his son's temperature began to rise again that he took the decision to come to Phnom Penh and seek specialised care. "One of my siblings once had dengue haemorrhagic fever, and my mother brought him to this hospital. He recovered very quickly after getting medical treatment," he said. Phally, the mother of a four and a half month old baby girl, Noroyanash, had also travelled to Phnom Penh from Tbong Khmom province to seek medical care at the hospital after her daughter had had fever for four days. "Her body temperature was totally abnormal the doctor [in her home town] said that her condition is difficult, so he asked to refer her to the main hospital. He said that this hospital is specialised in treating children," she said. According to the WHO, dengue is a leading cause of serious illness and death in south-east Asia. There is no specific treatment for the virus but early detection and medical care, including maintaining the body's fluid levels and reducing fever with paracetamol, can reduce mortality rates. The WHO recently reported the number of cases in Cambodia was on the rise, compared to last year. In Malaysia, 52,941 cases and 81 deaths have ben reported in the first 22 weeks of 2019, twice the number that occurred in 2018, while in the Philippines 77,040 cases of dengue and 328 deaths have been reported - twice the number for the same period of time in 2018. In Laos, 4216 cases of dengue including 14 deaths - the highest figures for five years - had been reported. Vietnam has seen 59,959 cases and four deaths reported in the first 19 weeks of 2019 - three times the number for the same period in 2018, while Singapore has reported 3886 cases in the first 21 weeks of 2019 - four times the number of cases in 2018. Monash University professor Cameron Simmons, the director of the Oceania hub of the World Mosquito Program, said dengue was a seasonal disease throughout south-east Asia and that every few years, a sharp rise in cases of the virus occurred. "Some years are bigger than others. In the last two or three years, dengue incidents in southern Vietnam, Cambodia and Laos has been lower than average but what happens is it happens in cycles, there is a big epidemic every three to five years. It's quite possible that’s what we are facing at the moment. South Vietnam is also having a bigger year than the last couple of years," he said. "There is one particular mosquito, the Aedes Aegypti mosquito, that is the main domesticated mosquito that loves to feed on us, take blood from us, and it is an effective transmitter of dengue." "For travellers going to Cambodia and other parts of south-east Asia, there is always a risk. Australia typically has 500 to 1500 cases each year of Australian travellers returning from places like Thailand or Bali or Papua New Guinea Png with dengue, and being diagnosed in a hospital [when they return]." However, Professor Simmons said that a major program to stop the spread of dengue in far north Queensland had been largely successful. A report published in the journal Nature Microbiology on Monday warned that climate change could increase the spread of dengue around the world, including to the south-eastern United States, coastal areas of China and Japan and inland regions of Australia. There are 100 million cases of dengue infections reported around the world each year, and about 10,000 deaths. with The New York Times.