The effectiveness of permethrin-impregnated bed nets against malaria for migrant workers in eastern Thailand.

A randomized, double-blind, field trial was carried out to compare the effectiveness of permethrin-treated bed nets with that of untreated nets as a method of malaria control for migrant workers in eastern Thailand. The study was conducted using 261 subjects in eastern rural areas that are known to be highly endemic for multidrug-resistant Plasmodium falciparum infection. One hundred twenty-six subjects used treated nets, while 135 used untreated nets. During the 35 weeks of observation, 23 subjects using treated nets and 33 workers using untreated nets developed 28 and 51 episodes of malaria, respectively (P = 0.029). The reduction in risk per subject due to treated nets was 0.06. The residual effects of permethrin were tested using a World Health Organization standard bioassay. Anti-mosquito activity was found to be present in the nets for more than 16 months. We conclude that because of the failure of the development of safe, effective, long-lasting prophylactic agents, integrating the use of impregnated nets with large-scale primary health care programs may be a partially effective method for controlling malaria in eastern Thailand.

Seroepidemiologic studies of humoral immune response to the Plasmodium falciparum antigens in Thailand.

We have investigated seroreactivity against Plasmodium falciparum crude parasite antigens, the P. falciparum ring-infected erythrocyte surface antigen (Pf155/RESA), as well as against two synthetic peptides (EENV)6 and (EENVEHDA)3 that represent important epitopes of Pf155/RESA. The study population consisted of 421 children and adult Thais living in an area with moderate malaria transmission. We related these serologic findings to some important epidemiologic baseline data collected in the study area. The parasite rate in study subjects was 18.76%. Sixty-two percent were seropositive to crude P. falciparum antigens, 30.3% to the Pf155/RESA antigen, 23.05% to (EENV)6, and 20.17% to (EENVEHDA)3. Antibody responses to crude P. falciparum antigens and to Pf155/RESA were age dependent and increased with exposure. There was evidence that Pf155/RESA antibodies might play a role in protective immunity in this population. Since Pf155/RESA is a potential vaccine candidate antigen, the information obtained from these field studies will provide some seroepidemiologic baseline data for subsequent vaccine trials.

Modeling factors influencing malaria incidence in Myanmar.

This is a documentary study to determine factors influencing malaria incidence in Myanmar. The period of study covered was from 1989 to 1998 using time series data. Multiple regression analysis was performed on the dependent variable, yearly incidence of malaria in Myanmar, with hypothesized independent variables including variables related to epidemiology, demography, service and socioeconomic status. Malaria incidence was inversely associated with the government budget for malaria control at the 5% level and with the case fatality rate of malaria at the 10% level. Other variables: yearly gross domestic product, yearly proportion of Plasmodium falciparum cases and yearly DDT use of spraying displayed expected signs but were not statistically significant.

Ex post and ex ante willingness to pay (WTP) for the ICT Malaria Pf/Pv test kit in Myanmar.

Willingness to pay (WTP) for the ICT Malaria Pf/Pv test kit was assessed by the contingent valuation method using a bidding game approach in two villages in Myanmar. Kankone (KK) village has a rural health center (RHC) and Yae-Aye-Sann (YAS) is serviced by community health worker (CHW). The objectives were to assess WTP for the ICT Malaria Pf/Pv test kit and to determine factors affecting the WTP. In both villages WTP was assessed in two different conditions, ex post and ex ante. The ex post WTP was assessed at an RHC in the KK village and at the residence of a CHW in the YAS village on patients immediately following diagnosis of malaria. The ex ante WTP was assessed by household interviews in both villages on people with a prior history of malaria. Ordinary least squares (OLS) multiple regression analysis was used to analyze factors affecting WTP. The WTP was higher in ex post conditions than ex ante in both villages. WTP was significantly positively associated with the average monthly income of the respondents and severity of illness in both ex post and ex ante conditions (p < 0.001). Distance between the residence of the respondents and the health center was significantly positively associated (p < 0.05) in the ex ante condition in a household survey of YAS village. Traveling time to RHC had a negative relationship with WTP (p < 0.05) in the ex post condition in the RHC survey in KK village.

A longitudinal study of seroreactivities to a major blood stage antigen (Pf155/RESA) of the malaria parasite Plasmodium falciparum in an endemic area of Thailand.

We have performed a longitudinal study of the formation of antibodies to Plasmodium falciparum in an area of Thailand where malaria transmission is moderate and seasonal. The study population comprised 118 subjects living in two villages 230 km southeast of Bangkok. All subjects included in this study were seropositive for antibodies to the blood stages of P. falciparum but only approximately 80% had antibodies to the blood stage antigen Pf155/RESA when assayed by erythrocyte membrane immunofluorescence (EMIF) or peptide ELISA during the period of maximal transmission. The reduced capacity to form these antibodies in a significant fraction of subjects living under comparable environmental and socio-economic conditions may reflect a genetic but antigen specific non-responsiveness. Both seropositivity and mean antibody titers to Pf155/RESA and its B-cell epitopes tended to be slightly higher during the rainy than during the dry season but the seasonal variations were slight and statistically not significant. Parasite rates were significantly higher in the rainy than in the dry season in both the EMIF positive and the EMIF negative groups. However, during the rainy season, the parasite rates in subjects with no or low titered antibodies to Pf155/RESA were significantly higher than those in subjects having such antibodies. The results suggest that antibodies to Pf155/RESA and some of its defined epitopes may be of importance for controlling parasitemias.