A researcher at Thailand’s Department of Medical Sciences’ National Institute of Health (NIH) has found that a single mosquito can carry two different strains of the dengue fever virus, with larvae inheriting the disease from their mothers.
The finding coincides with a warning by the NIH that it expects there to be a serious dengue fever outbreak across the country this year, due to the the recent extreme changes in the weather, and figures from Thailand’s Public Health Ministry (PHM) that in 2010 24,816 cases of malaria were reported nation-wide.
NIH researcher Usavadee Thavara studied 25 Thailand provinces with reported dengue fever outbreaks between 2006 and 2010 and found that the females of two species of mosquitoes, Aedes aegypti and A albopictus, were responsible for spreading the four strains of the disease.
The four-year research project also found that the two mosquito species had evolved, from previously living at altitudes below 500m (1,640ft), to now being found as high as 1,509m (4.950ft) at Doi Tung in Chiang Rai and 1,928m (6,325ft) at Doi Ang Khang in Chiang Mai.
Whereas previously both species of mosquito were only active during daylight hours, they now were also equally active at night.
According to Dr. Usavadee, larvae samples collected from dengue fever outbreak zones showed the dengue fever virus was passed from the mother to the next generation, with one mosquito capable of carrying two different strains of the disease.
The findings put an end to the theory that mosquitoes contract the virus from the blood of an infected host, before continuing on to spread the disease. In recognition of the value her study of the biology and infection rate of dengue fever carrying mosquitoes had provided, the NIH presented Dr. Usavadee with an award for outstanding research.
The research findings coincide with a report by NIH scientist Apiwat Thawatsin, that 2010 saw a doubling of dengue victims in 2009, with 113,017 reported cases, while deaths from the debilitating disease increased by 78 percent to 139 people.
Dr. Apiwat said the research finding make the removal of areas where mosquitoes could breed even more vital, while standing water should be treated with the larvicide temephos, an organophosphate insecticide commonly referred to by the trade name Abate, in Thailand.
Coinciding with the announcement by the NIH, the PHM were announcing figures for malaria throughout the kingdom, with the number of reported cases for the first three months of 2011 decreasing over the same period in 2010
Dr Wichai Satimai, director of Thailand’s Vector-Borne Disease Bureau, said there had been a total of 5,540 confirmed cases of malaria in Thailand between January and the end of March 2010, with 2,320 Thais infected, a drop of 44 percent of the same period in 2010, and 3,220 non-Thais, a drop of 19 per cent over over the same period last year.
While the number of malaria cases was generally decreeing, Dr. Satimai said malaria remains a threat in Thailand’s south, particularly Yala Province, where ongoing civil unrest was preventing medical staff and vector control teams from entering villages to distribute medical supplies and undertake vector control measures.
“As a result, the incidence of malaria in Yala Province has increased from 30 cases a year to 3,000 annually over the last several years”.
Also of concern are provinces along the Thailand-Burma border, with 15,181 of the 24,816 malaria cases confirmed in Thailand in 2010, or 63.47 per cent, being reported in these areas.
Dr. Charles Delacollette, manager of the World Health Organization’s (WHO)
Mekong Malaria Programme, said the number of malaria cases in Burma had increased to more than 400,000 in 2009.
“If there is no collaboration between Thailand and Burma, they are going to fail in disease prevention,” Dr. Delacollette said, while noting that there is declining interest from governments and donors in malaria control and prevention programs.
Dr. Wichai said that while current indications were good, poor community participation
and involvement had led to inadequate public awareness of risks of malaria, while reorganization of public health staff nationwide and an inflexible malaria surveillance system were hampering controlling the disease.
Sounding a similar warning as Dr. Apiwat, Dr. Wichai said the risks of increasing cases of malaria in Thailand were higher this year because of the erratic weather recently, while the approaching wet season of May through July was a period his agency monitored intensely.
“People cannot be complacent. They must search for and destroy anywhere mosquitoes can breed around their property. If people actively did this it would have a dramatic affect on the incidence of mosquito-transmitted diseases.”
Malaria and dengue fever often exhibit identical symptoms with the former the result of a parasite and the latter a virus. Though reliable and inexpensive tests exists to confirm malaria, a reliable test for dengue has not yet been found.
Symptoms for malaria and dengue fever appear about eight to 30 days after a bite from an infected mosquito, with no tested and approved vaccine for the dengue flavivirus.
For malaria doxycycline and a combination of atovaquone and proguanil hydrochloride (Malarone) are the best tolerated prophylactics, with mefloquine (Lariam) associated with higher rates of psychiatric and neurological symptoms
Typical dengue fever infections last between two to seven days, with a smaller peak of fever at the trailing end of the disease, while attacks of malaria can occur for years if it is not diagnosed and treated.
Symptoms for both diseases are very similar to influenza and can very in intensity, but include fever, bad headaches, muscle and joint pains, skin rashes, and overall weakness in the body.
Due to the risk of bleeding with dengue hemorrhagic fever (DHF), people in dengue fever areas who exhibit symptoms should not be given aspirin, though paracetamol is okay, and should seek immediate medical treatment.
According to the WHO there were more than 225 million cases of malaria in 2010, resulting in the deaths of about 781,000 people, while it estimates there may be 50 to 100 million dengue infections worldwide every year, including 500,000 DHF cases, resulting in about 22,000 deaths.
According to US Centers for Disease Control and Prevention (CDC) 40 per cent of the world’s population, or some 2.5 billion people now at risk from dengue fever, while 3.3 billion people (half the world’s population) in 109 countries live in areas at risk of malaria transmission.
© 2011 John Le Fevre
Thailand’s Vector-Borne Disease Bureau maintains the Journal of the Vector – Borne Diseases website with a large amount of information, documents and a discussion forum. Full reports are available in Thai language only, though precise are provided in English language
Thai researcher debunks dengue fever myth – warnings for 2011 wet season,
He is currently deputy editor and Thailand / GMS region editor for The Establishment Post
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