Standardizing container classification for immature Aedes aegypti surveillance in Kamphaeng Phet, Thailand

For the development of community-based vector control programs for dengue prevention, one of the key components is to formulate an adequate classification scheme for the different containers in which immature Aedes mosquitoes develop. Such a standardized scheme would permit more efficient targeting of efforts and resources in the most productive way possible. Based on field data from Kamphaeng Phet, Thailand, we developed a classification method that consists of the shape (S), use (U), and material (M) of the container (SUM-method). We determined that by targeting the four container classes that held the most Ae. aegypti pupae, adult mosquito production could theoretically be reduced by 70%. The classification method may be equally suitable for similar studies elsewhere in the world. Main advantages of the classification scheme are that categorization of containers does not need to be done a priori, that there is no "miscellaneous" class, and that different immature control strategies can be easily and prospectively tested with a local database. We expect that the classification strategy will 1) facilitate comparison of results among different ecological and geographic settings and 2) simplify communication among vector control personnel and affected communities.

Molecular serotyping of dengue viruses in field-caught Aedes mosquitos by in-house RNA extraction/RT-PCR reagent kits

We developed in-house RNA extraction and RT-PCR reagent kits for the molecular serotyping of dengue viruses in field-caught Aedes mosquitos. Mosquitos that showed positive results by ELISA or IFA were selected for the identification of dengue viruses in order to predict the distribution of the four dengue serotypes. Total RNA was extracted from one whole mosquito as well as from one dissected mosquito by our in-house RNA extraction reagents using the modified method of guanidinium thiocyanate denaturation and isopropanol precipitation. The extracted RNA was amplified by our in-house RT-PCR reagents specific for each dengue serotype under optimized conditions. Dengue viral RNA extracted from a single mosquito as well as from the head and thorax of one dissected mosquito could be detected successfully; it could not be found in the abdomen, however. These results indicated that most of the dengue viruses were located in the head and thorax rather than in the abdomen. The results of dengue serotyping showed a pure specific PCR product for each dengue serotype at 490, 230, 320 and 398 bp for DEN-1, DEN-2, DEN-3, and DEN-4 respectively. In addition, the detection sensitivity was very high: an amount of RNA template equivalent to approximately 1/80 of a single mosquito could be detected by agarose gel electrophoresis and ethidium bromide staining. The coupling of RT-PCR-based surveillance of dengue viral infection with disease mapping data (Geograpical Information System, GIS) could serve as an alternative epidemiological means of providing an early warning of dengue fever/dengue hemorrhagic fever epidemics.

Soldier acceptability of a camouflage face paint combined with DEET insect repellent

Under operational conditions, current doctrine requires separate application of the standard military insect repellent and camouflage face paint. Working with an industrial partner, Amon Re, Inc., we developed a combined camouflage face paint and insect repellent that offers excellent protection from arthropod disease vectors. We undertook a study to determine whether the new product was acceptable to soldiers under field conditions. A new formulation of camouflage face paint containing the insect repellent N,N-diethyl-3-methyl-benzamide (DEET) was tested for user acceptability during a joint-service, multinational, military field training exercise (Operation Cobra Gold 1999) in Thailand. Soldiers testing the camouflage face paint were members of one of three companies (A, B, and C) of 1st Battalion, 21st Infantry Regiment, 25th Infantry Division. Soldiers in A company (N = 98) received standard military camouflage face paint and the U.S. military's Extended Duration Topical Insect and Arthropod Repellent containing 33% DEET, soldiers in B company (N = 75) received a new formulation of camouflage face paint without DEET, and soldiers in C company (N = 88) received the new formulation of camouflage face paint containing 30% DEET. Every soldier who volunteered to participate completed the study and submitted responses to the questionnaire. The different treatments were evaluated while soldiers spent 4 days conducting simulated combat exercises in a hot, tropical environment in central Thailand. Soldiers were provided the test materials, given a briefing on the study, and completed an initial questionnaire on May 19, 1999. Soldiers completed a final questionnaire after they returned from the training exercise on May 24, 1999. Results of the study indicated that soldiers found the new formulation of 30% DEET camouflage face paint easier to apply (88% of respondents) and remove (77%) than the current standard issue camouflage face paint. Soldiers liked the new 30% DEET camouflage face paint formulation better (61%) and were more likely to recommend it (67%) than the old camouflage face paint formulation. Although the addition of DEET to the camouflage face paint occasionally caused minor irritation after application for a short period (less than 15 minutes), the new camouflage face paint formulation with 30% DEET was evaluated most often (72%) as either good or excellent. The new 30% DEET camouflage face paint formulation offers significant operational advantages: protection from disease-transmitting arthropods, elimination of the need to carry separate insect repellent and camouflage face paint, and reduction in the time required to apply repellent and camouflage face paint separately.

New host and distribution record of Amblyomma javanense (Acari: Ixodidae) in Thailand

Amblyomma javanense (Supino) was collected from a Malayan pangolin (Manis javanica Desmarest) and a wild boar (Sus scrofa L.) from Tak province on the western boundary of Thailand along the Myanmar (Burma) border. To date, this tick species has not been recorded from this area and from a wild boar.

Distribution of dengue and Japanese encephalitis among children in rural and suburban Thai villages

In the rainy season of 1989, IgG and IgM antibodies against dengue and Japanese encephalitis viruses (measured by enzyme-linked immunoassay [ELISA]) in serum from all primary-school children in two areas of central Thailand were sampled in order to choose a study site for more detailed epidemiological and entomological analysis. Students in three schools in the largely non-agricultural, suburban community of Bang Bua Thong, Nontaburi Province were sampled in late June and July. Of 1,477 children, 33/1,000 had recent dengue infection and 7/1,000 had recent JE infection. The rate of dengue infection in each village influenced the rate in schools, in that the rate of the school could be predicted from the proportion of students coming from each village. This result suggested that most transmission occurred in the residential environment; otherwise, the rate in each village going to a single school would be identical. Serum samples were taken in late August in the agricultural community of Hua Samrong, Chachoengsao Province. Of 748 students in two schools, 95/1,000 had signs of recent dengue infection and 32/1,000 had signs of recent JE infection. Two of 12 villages had significantly less flavivirus infection than some other villages and three villages had significantly more flavivirus infection. The children from one village had a dengue infection rate of 256 per 1,000, which was higher than the national average for the worst year (1987) previously recorded in Thailand. Within Hua Samrong, there was evidence for significant dengue transmission in one of the schools and concentrated transmission in small areas of two of the villages. The younger age group (3-8 years old) had significantly higher risk of infection by either flavivirus than older children. Elevated homes with wooden floors had significantly higher risk of dengue in the largest village. The observations from 1989 describe the epidemiological situation in rapidly developing, rural villages. This stage of development is probably being repeated throughout Southeast Asia as formerly isolated, rural villages become connected by transportation and economy to urban centers. What appears to be a single dengue outbreak based on passive surveillance conducted on a regional basis may actually be a variety of epidemiological situations. The practical implication of this conclusion is that application of a combination of vaccination and vector control should be targeted to higher risk areas in order to increase the likelihood of regional dengue virus eradication.

Effect of size and geographic origin of Aedes aegypti on oral infection with dengue-2 virus

Differences in larval habitats cause variation in the size of Aedes aegypti (L.) adults. We suspected that such size variation was related to the ease with which the mosquitoes could be infected with dengue virus. Using a rearing procedure that produced three distinct size classes of mosquitoes, we determined the percentage of mosquitoes that developed disseminated dengue-2 infection following oral feeding with a suspension containing 3.3 x 10(7) plaque-forming units/ml. Mosquitoes were reared from eggs deposited by females captured in either of two villages in Chachoengsao Province or in Bangkok, Thailand. More of the larger mosquitoes (10.7%) were infected than the medium (5.6%) or small (5.7%) mosquitoes. Mosquitoes from Bangkok were less easily infected (5.0%) than mosquitoes from either of the two villages (8.5% and 10.7%). These results suggest that quantitative risk assessment of dengue transmission may be very difficult unless inoculation rate is measured directly. Also, control procedures that reduce density of larvae in individual containers may exacerbate dengue transmission by creating larger mosquitoes that are more easily infected.

Effects of sublethal dosages of methoprene on Anopheles dirus species A and B

Anopheles dirus species A and B individuals were exposed as 4th-stage larvae to sublethal concentrations of methoprene. Median lethal concentrations were 0.21 ppb for species A and 0.17 ppb for species B. When exposed to 0.10 ppb methoprene the sex ratio of species. A changed from fewer males to more males; no effect was observed in the sex ratio of species B. Exposure to methoprene had no effect on wing length and survival of either species. Results indicate that exposure to methoprene significantly affected fecundity of both species of An. dirus.

ELISA for detecting dengue and Japanese encephalitis viral antigen in mosquitoes

An indirect ELISA has been developed for detecting two viruses in triturated, experimentally infected mosquitoes: dengue (DEN) in Aedes aegypti (L.) and Ae. albopictus (Skuse) and Japanese encephalitis (JE) viral antigen in Culex tritaeniorhynchus Giles. DEN antigen from four strains of the virus, representing each of the four serotypes, and JE antigen were captured with a polyclonal anti-flavivirus IgG and detected with a monoclonal antibody (4G2) that reacts with all flaviviruses. Minimum viral titres detectable by the flavivirus antigen-capture ELISA, measured as multiples of the dose infecting 50% of mosquitoes (MID50), were 10(6.4), 10(4.5), 10(6.3), 10(6.9) and < or = 10(6.9) MID50/ml for DEN 1, DEN 2, DEN 3, DEN 4 and JE, respectively. Pools of up to 100 larvae or adults of Ae. aegypti and Ae. albopictus did not reduce the sensitivity of the ELISA to DEN 1, 2, 3 or 4. The results indicate that the antigen-capture ELISA could readily screen for DEN antigen in individual and pooled mosquitoes.