New Study Shows Early Activity in Malaria Mosquitoes

September 19, 2002
By Michelle Vachon
The Cambodia Daily

A study conducted by the National Malaria Center shows that some malaria-carrying mosquitoes get active in the early evening earlier than previously believed.

A study of O'Dorng village, located in the dense forest of Stung Treng province near the Laos border, showed that the Anopheles minimus mosquito attacked people between 6 pm and 7 pm during November and December, before people had taken cover under insecticide-treated bed nets. At other times of the year, the mosquitoes bit later in the evening.

In O'Dorng village, another type of mosquito also started biting as early as 6 pm during all months of the year.

"There have been similar reports of early biting Anopheles dirus from Sabah [in Malaysia] and parts of Thailand," National Malaria Center official Tho Sochantha, who holds a masters degree in medical entomology from the London School of Hygiene and Tropical Medicine, wrote in the study report.

The distance between mosquito breeding grounds, which were close by in the Stung Treng forest, could partially explain their early activity, he said.

Another factor could be that since bed nets have been used for four years in the village, mosquitoes may have developed a new biting pattern in order to attack people before they slide under their nets, Tho Sochantha said.

In the case of both Anopheles dirus and Anopheles minimus, the biting took place before 10 pm, when people had not yet slid under their bed nets.

The study, funded by the European Commission's Malaria Control Project in Cambodia, was conducted in 2000 to find out how the biting habits of malaria-carrying mosquitoes vary from one season to the next.

Research done prior to 1970 identified four types of mosquitoes transmitting malaria in Cambodia, Tho Sochantha wrote in the report. Studies in the 1980s and 1990s in Cambodia, Thailand and Vietnam brought the number down to two major malaria transmittersÑthe Anopheles dirus and the Anopheles minimus.

Detailed records were lost during the Khmer Rouge era, and little is known on the fluctuation of their patterns throughout the year, Tho Sochantha said.

The study he led consisted of monitoring mosquito habits in three villages one each in Stung Treng, Battambang and Kompong Cham provincesÑon four occasions between March and December 2000.

Further research into malaria-carrying mosquito patterns is now under way at the National Malaria Center.