Risk factors for malaria infection among ethnic minorities in Binh Phuoc, Vietnam.

A cross-sectional study was undertaken to identify the prevalence of malaria infection, behavioral patterns of inhabitants and risk factors for malaria infection in a minority village in an area endemic for malaria in Vietnam. The target population were all the inhabitants of a village in Binh Phuoc Province, Vietnam. Interviews using a structured questionnaire and blood examination for malaria infection were conducted in house-to-house visits. Relationships between malaria infection and variables were examined using uni- and multivariate adjusted analysis. A total of 682 individuals from 159 households participated in both the interview and blood examinations. All households earned income through farming without forest activities at night, and the socio-economic status was generally not very low. The total prevalence of malaria infection was 6.2%, with a peak among 3- to 5-year-old children. Univariate analysis identified 3 - 5 year olds, a family size of > or =5 people, sleeping with >3 people in a bed, and living in a wooden/bamboo house as factors associated with malaria infection. Multivariate adjusted analysis after variable selection identified age 3 - 5 years old, a family size of > or =5 people and living in a wooden/bamboo house were significantly related to malaria infection. Malaria in this area can be controlled by basic activities, such as early diagnosis and treatment and prevention using bednets, since risk factors for malaria infection did not include forest activities, but were young age, living in a wooden/bamboo house and belonging to a large family. Continuous and intensive expansion of existing malaria control activities are required.

The specific status of Anopheles minimus s.l. collected from Taiwan.

At least three members (species A, C, and E) of the Anopheles minimus complex have been described in the Orient. This study investigated the specific status of An. minimus collected in the southern part of Taiwan by crossing experiments with species A from Thailand and species E from Japan. Crosses between Taiwan An. minimus and species A revealed genetic compatibilities. Post-zygotic isolation was observed in crosses between Taiwan An. minimus and species E. Hybrid progeny were only obtained from Taiwan female X species E male. F2 hybrid progeny were not obtained, since the hybrid males were sterile or almost sterile, with atrophied testes or abnormal spermatozoa. The hybrid females backcrossed with either Taiwan F1 progeny and species E males, and laid eggs with lower fertility and viability. This study supports previous published data regarding the analysis of the D3 region of the 28S gene of ribosomal DNA that An. minimus species A is indigenous to Taiwan. Whether other members of the An. minimus complex exist in Taiwan is not conclusive and needs more study.

Changes in malaria vector densities over a twenty-three year period in Mae Hong Son Province, northern Thailand.

Mae Hong Son Province in northwestern Thailand has a long history of malaria. During the last two decades the province has had one of the highest malaria incidences of all provinces in Thailand. Data were analyzed to determine whether the vector populations were stable or increasing during the last two decades and to determine the seasonal prevalence of the main vectors, and whether or not they were related to the malaria transmission peak, in the wet season. We compiled and analyzed accumulated entomological records from 1977 to 1999. The aim was to investigate long-term changes in mean densities of malaria vectors between two periods (1977-1989 and 1990-1999), and the differences in vector densities between two seasons (wet and dry). A total of 141,144 adult anophelines of 29 species were collected on indoor and outdoor human baits and animal baits during the study period. Of the main malaria vectors, the densities of Anopheles minimus s.l. and Anopheles maculatus complex increased significantly. Anopheles dirus s.l., however, was stable between the two periods. These vector populations were associated with consistently high malaria incidence in the province during the last two decades. An. minimus s.l. density was not significantly different between seasons. However, in the second period, both An. dirus s.l. and An. maculatus complex showed a tendency for higher wet season densities. This can explain the high malaria incidence in the rainy season in Mae Hong Son. Environmental and climatic factors seem to have been favorable for supporting a consistently high vector population in the province, and consequently a high malaria transmission rate during the period of study.

Molecular variation and phylogeny of the Anopheles minimus complex (Diptera: Culicidae) inhabiting Southeast Asian countries, based on ribosomal DNA internal transcribed spacers, ITS1 and 2, and the 28S D3 sequences.

Anopheles minimus (Theobald) is one of the most important vectors of human malaria in Southeast Asia. Morphological studies now have revealed five sibling species as its complex, designated as species A to E. The present study investigated the genetic divergence among An. minimus populations from four countries (Japan, China, Thailand and Indonesia), based on the DNA sequences data of the D3 (the third domain of the 28S ribosomal gene) and ITS2 (the second internal transcribed spacer of the ribosomal gene) is reported. The D3 and ITS2 phylogenetic trees, and the electrophoretic profile of ITS1 (the first internal transcribed spacer of the ribosomal gene) indicated that our An. minimus populations are comprised of three groups: the Japanese population as group I, the population from Guangxi Province of China (GX population) as group II, and others, as group III. The results showed the morphological similarity of group III and GX with the species complex A and B, respectively. It is interesting that both two species A (YN population) and species B (GX) occur in China, and that both species, An. minimus species A (LB-95 population) and the closer population An. flavirostris (Ludlow) (LB-00 population) appeared to be present on the Lombok Island of Indonesia, although in far separated localities. Moreover, this molecular evidence confirms the previous suggestion that the population from the Ishigaki Island of Japan should be classified as a new genetic status species E.

Distribution of two species of malaria, Plasmodium falciparum and Plasmodium vivax, on Lombok Island, Indonesia.

Medical and entomological surveys were conducted to determine the risk factors of Plasmodium falciparum and P. vivax infections on Lombok Island, Indonesia, to find the risk factors and the main mosquito vectors for each malaria. Multivariate longitudinal analysis demonstrated two significant risk factors for infection with P. falciparum: disappearance of P. vivax parasitemia (p<0.001) and a specific study site (p<0.001). In contrast, younger age (p=0.024) and the interpolated virtual density of An. subpictus (p=0.041) were significantly associated with increased risk of infection with P. vivax. Thus, it seems that the distribution of P. vivax was determined largely by the presence of An. subpictus, whilst that of P. falciparum was influenced by antagonism with P. vivax. This result shows the importance of following-up treated P. vivax patients to identify recrudescence of P. falciparum in this area.