| Objective To define the current situation of malaria and to evaluate its epidemic potential in the Yangtze River Three Gorges Reservoir area as well as analyze the influence from the construction of large-scale water conservation in the Three Gorges for proposing the appropriate preventive, control strategies and measures of this disease for this project.Methods①According to the diversity of environment as well as its recent malaria incidence, six counties were selected from the upper reaches and mid-streams as well as the downstream of the Three Gorges Reservoir area using a stratified design to provide a representative probability sample of the civilian population in the Three Gorges Reservoir area. Two townships of higher incidence from each county were selected and two villages from each township were selected, two surveys with at least150local residents from each village randomly selected for collecting filter paper blood spots for determination of IFA response were conducted in the same local population, at a six-month interval in pre-transmission period and post-transmission period (parasite diagnosis at the same time); in combination with the application of capture-recapture method for assessing the situation of incidence reporting in the Three Gorges Reservoir area.(2)A representative natural village from each county was selected separately for the quantitative survey of the ecological character of local Anopheles during the transmission period (5-10month) from2008to2009, the vectorial capacity calculated from the mathematical models (Macdonald).③The epidemic potential was assessed by the import in combination with the vectorial capacity; the stability as well as its critical man-biting rate were estimated at the same time by the formula of basic reproductive rate in the mathematical model of malaria.④The evaluation system composed of related index for assessing the malaria epidemic potential in the Three Gorges reservoir area was built through the Delphi Method, while the relationship between the epidemic potential and the natural environment or socioeconomic was analyzed with the application of Poisson regression analysis and Grey Systematic Theory.⑤The trends of the density of the malaria vectors as well as its relationship with meteorological factors was nailed down through the application of the linear theory and time series regression analysis.⑥Based on the professional theory, structural equation model with related ecological factors affecting the malaria epidemic potential in the Three Gorges Reservoir area was made and revised to confirm an optimum model; SAS9.1statistical software was applied for achieving the above analysis.Results①The incidence of malaria in the Three Gorges Reservoir area has been at a low level (0.08～0.64/10ten thousand) since the construction of The Great Dam, and its average incidence (0.26/10ten thousand) was lower than that of Hubei Province (4.79/10ten thousand) as well as that of the national (2.80/10ten thousand); while significant discrepancy of incidence existed among the different sections in the reservoir with particular missing-report of malaria cases in some areas; The overall percentage of positive response of the same population during post-transmission period was94.44%(1.40/0.72) higher than that in pre-transmission, an age-related increase in antibody prevalence was evident under15years of age and3.21times increase in antibody positive individuals, which the percentage of positive was significant statistically higher (P<0.05) than that of pre-transmission periods.②The malaria vectors in the Three Gorges Reservoir area was only Anopheles sinensis, while significant differentiation of its ecological character existed in different sections in this area, the result of this survey for man-biting rate, human blood index and the ratio of multifarious mosquitoes was0.29～8.92,0.03～0.21,0.51～0.66in turn, and the estimated range of vectorial capacity was0.01～0.64.③The ratio of imported malaria in different sections of this area was25%～100%, with relative high import in some area; while the receptivity index of malaria ranged from0.01～0.08, with relative high receptivity existing in some region. The stability index of malaria in the Three Gorges Reservoir area was0.06～0.28, indicating that malaria was unstable in this area; and the actual man-biting rate of Anopheles sinensis in some survey points (Wanzhou, Kaixian and Fuling) was5.72,3.37and2.87times that of the critical man-biting rate, confirming that the malaria transmission caused by the Anopheles sinensis was still continuing.④The result of positive coefficient, authority coefficients and coefficient of concordance in the evaluation system composed of six aspects including19related index for assessing the malaria epidemic potential in the Three Gorges Reservoir area built through the Delphi Method was0.92,0.76,0.39(X2=38.84, n=22, P<0.01) in turn, indicating that forecasting or assessing by application of this evaluation system should be confident. Univariate analysis showed that the relative risk of man-biting rate due to exposure to livestock, submerged land area, riparian zones and paddy was0.49(X2=6.38, P=0.01),1.04(X2=14.63, P=0.00),1.07(X2=23.49, P=0.00),11.84(X2=10.17, P=0.00) respectively; while multivariate analysis showed that the association between riparian zones and the man-biting rate was still statistically (X2=5.00, P=0.03); The grey relational grade between the human-blood index and the rate of sleep outdoors, the rate of mosquito nets use, the average livestock per man, the ratio of animal room screen use and the average distance between house and animal stall was0.87,0.67,0.62,0.56,0.5, respectively.⑤The trend regression coefficient for the density of malaria transmission vectors was statistically significant (b=-0.019, P<0.001), the correlation between the density of malaria vectors and the accumulative temperature of the recent three month was highest (r=-0.375, P=0.003); the total regression analysis showed that the density of malaria vector decreased as the accumulative temperature of the recent three month increased.⑥The fit indexes of the optimal structural equation model are as follows:Ration=5.227, GFI=0.9961, AGFI=0.9987, RMR=0.1094, RMSEA=0.1303, NFI=0.9952, NNFI=0.9929, CN=207, which showed that the model can reflect the relationship of variables; the direct effect of temperature factors, animal factors, humidity factors and breeding environment on the malaria vector density were0.0、-0.228、0.450、0.516in turn.Conclusion①The incidence of malaria in the Three Gorges Reservoir area had been a little decrease and stabilizing at a low level since the construction of The Great Dam, and the average incidence was lower than that of the surrounding areas in Hubei Province as well as that of the national; while significant discrepancy of incidence existed among the different sections in the reservoir area; The result of antibody levels in the whole population showed that the transmission-blocking has not yet reached during transmission period.②The malaria epidemic potential ranged from low to middle; There existed distinct levels of malaria epidemic potential in different sections of the reservoir area, while the malaria antibody levels in the local population as well as local incidence were commensurate with its epidemic potential; the stability index showed the malaria was unstable in the reservoir area and the actual man-biting rate much higher than critical biting-rate confirmed the malaria transmission continuing caused by the Anopheles sinensis. Therefore, the malaria was unstable with low endemic incidence in the reservoir area.③There existed significant discrepancy of ecological habitats of malaria vectors in the Three Gorges Reservoir area, therefore conspicuous divergence of epidemic potential among different sections was also presence; and its epidemic potential was closely related to such ecological environments as local crowd behavior (sleeping outdoors) and socio-economic (mosquito net use, livestock) as well as some environmental factors (paddy field area, riparian zones, submerged land area) and others.④The density of malaria vectors had been a downward trend and stabilizing at a low level since the construction of The Great Dam, the meteorological factors was closely related to the density of malaria vectors with big difference of the relation among different sections.⑤The result from the structural equation model revealed that the humidity condition was the driving force for the density of malaria vector. The result of this study indicate that the structural equation model is suitable for studying the relationship between the malaria epidemic potential and its related ecological factors which makes rewarding reference for similar studies.Suggestions①With different degree of malaria epidemic potential for different sections of the reservoir area, while relative high potential existing in some regions (Kaixian, Wanzhou, Fuling) and its malaria transmission caused by the Anopheles sinensis was still continuing, therefore strengthening the monitoring of malaria vector will provide an important reference for the assessment as well as prediction of the malaria situation in this area.②There existed relative high imported malaria in some region (Yubei, Wanzhou) of the reservoir area, strengthening the timely detection and its standard treatment of the malaria cases for shortening the course of disease and improving the recovery rate will have important significance for the control of endemic as well as the malaria elimination in this area.③The malaria situation of the reservoir area was unstable of low incidence, indicating that transmission trend may be varying with the change of local natural and social factor, the results also showed that the riparian zones was the risk factor for malaria transmission and the humidity factor was the driving force for malaria vectors in the reservoir area. Therefore, we should pay attention to the impact from the ecological environmental change on the potential transmission of malaria stability and carry out the risk prediction and early warning of malaria transmission for reducing the possible endanger. |
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