Larval occurrence, oviposition behavior and biting activity of potential mosquito vectors of dengue on Samui Island, Thailand.

A 1995 outbreak of dengue haemorrhagic fever (DHF) occurred on Samui Island in Thailand with an incidence of almost 500 cases/100,000 population. To find and develop effective strategies to control this disease through cost-effective vector control programs, entomological studies were carried out on the island between 1996 and 1998. There were two species of DHF vectors, Aedes aegypti and Ae. albopictus prevailing on the island, and the population of Ae. aegypti remained relatively constant throughout the year while the abundance of Ae. albopictus increased substantially during the rainy season (May-December) and then declined drastically in the dry season (January-April). The ranges of the three Aedes larval indices, Breteau index (BI), house index (HI) and container index (CI) were 93-310, 43-89 and 16-50 respectively. The ceramic or earthen jars both inside and outside the dwellings and concrete water storage tanks (mostly in toilets and bathrooms) served as the main breeding places of Ae. aegypti whereas coconut husks and coconut floral spathes found outdoors were the major breeding sites of Ae. albopictus. The number of washing water jars, concrete tanks and natural sites infested with Aedes larvae increased significantly in rainy season, with 60% of ovitraps become positive for Ae. albopictus eggs with an average number of 26 eggs/trap in 3 days of setting. There was a complete lack of oviposition by Ae. aegypti in outdoor ovitraps (15 m away from the houses). The indoor biting rate ranged from 1.5 to 8.1 mosquitoes/man-hour, while the outdoor rate was between 5 and 78 mosquitoes/man-hour. Of the indoor biting mosquitoes, 75.4% were identified as Ae. aegypti and 99% of the outdoor ones were Ae. albopictus. The diel biting activity of Aedes during the period from 0800 h to 1700 h in the houses was higher in the morning than in the afternoon period, with a low prevalence between 1300 h and 1400 h.

Cockroach surveys in 14 provinces of Thailand.

Cockroach surveys using sticky traps were conducted in urban areas of 14 Thailand provinces. At least 30 houses in each province were randomly sampled for cockroaches. Each house was trapped in three areas: kitchen, bedroom and outside. A total of 2,648 cockroaches was caught by 550 out of 1,542 traps (35.7%), from 337 of the 514 houses (65.6%). Overall, relative density ranged from 2.6 to 9.1 with an average of 5.2 cockroaches/house. On the average, 47.7% of the cockroaches were caught in the kitchen, 24.4% and 27.9% were caught in the bedroom and outside of dwellings, respectively. There were 10 species of cockroaches caught from the 14 provinces: Periplaneta americana (60.9%), Periplaneta brunnea (15.4%), Neostylopyga rhombiofolia (9.6%), Periplaneta australasiae (9.2%), Pycnoscelis surinamensis (3.3%), Blatella germanica (0.6%), Periplaneta fuliginosa (0.5%), Supella longipalpa (0.3%), Blatella lituricollis (0.15%) and Nauphoeta cinerea (0.05%), belonging to six genera. According to the surveys in this study, Periplaneta americana and Periplaneta brunnea were the most abundant cockroach species in urban Thailand, whereas the kitchen was the major habitat.

Dengue vector mosquitos at a tourist attraction, Ko Samui, in 1995.

On Ko Samui, Thailand there were two epidemics of dengue hemorrhagic fever (DHF) in 1966 and 1967, followed by endemics up to 1994. Aedes aegypti and Aedes albopictus were the vectors. From January to July 1995, 51 cases of DHF were reported, out of these were many foreigners who still suffer from dengue fever and return home with negative impression. We carried out an entomological survey around the island and collected the mosquitos to detect dengue virus by digoxigenin-cDNA probe. The data revealed that Aedes aegypti and Aedes albopictus still were abundant and some were infected with dengue virus. Visual larval survey indices (HI, CI and BI) were 90.4, 61.3 and 301.3 respectively. Biting rate (BR) of Aedes mosquitos was high, the average indoor and outdoor BR were 9.7 and 100.8 mosquitos/man-hour. From 13 pools of mosquitos, 8 strains of dengue virus were detected (61.5%). The results may encourage the local authorities to improve vector surveillance and control before the famous island becomes an unpleasant island.