| Malaria is an infectious disease transmitted by mosquito bite and is threatening human health. Mosquito control is an important strategy to block malaria transmission and epidemiology. Chemical insecticides are being commonly used in mosquito control. However, it can pollute the environment and is harmful to human being and livestocks. Besides, the resistance of mosquitoes against insecticides was increased dramatically during the past years. Now, more and more attentions are focused on biolarvicides such as Bti and Bs, which will not pollute the environment and are safe to human and livestocks because of their specific targets. The related basic research for application is emergently needed to provide dosage standard and subsequent effects. Without the dosage standard, incorrect using often results in sublethal effect, which can accelerate emerging of mosquito resistance to larvicides. And, some mosquitoes can survive from the sublethal dose of larvicides. As a result, the vector capacity of the survived anopheles might be changed under the selection by biolarvicides or by exposure to biolarvicides, which can probably change the innate immunate and gut microflora of mosquitoes. As known, the vector capacity of anopheles mosquitoes is the decisive factor to malaria transmission and epidemiology. Bioassay was firstly conducted in this study to investigate the killing effect of Bti on Anopheles stephensi and reasonable dosage. Then, the impact of sublethal dose of Bs on plasmodium susceptibility to anopheles was studied. Finally, we investigated the mosquito innate immunity involved in the change of vector capacity by Bs. This study will provide a dosage standard and risk reference for application of biolarvicides. It will be helpful to block transmission of malaria by mosquito control.This study includes the following three parts.1.First part: Bioassay of killing effect of Bti on Anopheles stephensi.Firstly, linear regression analysis was carried out by Probit analysis method to investigate the relationship between doses of Bti and the accumulated mortality of Anopheles stephensi larvae. And, some key parameters such as LC50 were calculated based on the obtained formula. It’s helpful to determine the killing effect of Bti on Anopheles stephensi. Then, the influence of Anopheles larvae density on the killing effects of Bti was also detected by treatment with a sub-lethal dose of Bti on groups with different larvae densities. The results showed that Bti had a good killing effect on An. stephensis larvae with a rapid onset. At 12 h post treatment, deaths of larvae were observed obviously. The accumulated mortality of the 4th instar larvae of Anopheles stephensi increased with the raises of incubation time and concentration of Bti. There was an ideal linear relationship between the logarithm of concentration of Bti and the Probit value of the accumulated mortality. The LC50 s at 12 h, 24 h, 48 h and 72 h post treatment were(1242±60)IU/L,(1112±45)IU/L,(1010±73)IU/L and(855±32)IU/L, respectively. The LC95 s at 12 h, 24 h, 48 h and 72 h post treatment were(4170±138)IU/L,(3746±444)IU/L,(3462±409)IU/L and(3080±464)IU/L, respectively. And with the increase of larval density, and the cumulative mortality rate decreased significantly.2.Second part: Impact of Bs treatment on vector capacity of anopheles to malariaBoth susceptible animal model(Plasmodium yoelii and Anopheles stephensi) and resistant animal model(Plasmodium yoellii and Anopheles dirus) were used to investigate the impact of Bs treatment on vector capacity of anopheles. An. stephensi and An. dirus were divided into normal control and Bs treated groups, respectively. On day 9 post blood meal, the midguts of anopheles were dissected and checked under microscopy. The infection rates and densities were analyzed and compared between control group and Bs group. The results showed that the susceptibilities of plasmodium to anopheles were decreased significantly by Bs treatment. And, there was no difference of melanization rates and densities between control group and Bs group, which indicates that melanization mechanism was not involved in the impact of Bs on change of vetor capacity.3.Study of innate immunity involved in the change of vector capacity of anopheles to malariaReal-time PCR and semi-quantitative PCR were carried out to compare the transcriptional level of TEP1 of An. stephensi and An. dirus between control group and Bs group. The result showed that the transcriptional level of TEP1 was increased in multiple stages of An. stephensi, especially at 24 h post infection by plasmodium. And the TEP1 level was also increased in larvae, pupae and adults of An. dirus.Conclusion:1.Bti has a strong and rapid killing effect on the 4th instar larvae of Anopheles stephensi, which indicates that Bti is a kind of ideal bio-larvicide to control mosquitoes. And, the larval densities should be considered to decide the doses in the application of Bti. The dose of Bti should be increased accordingly with higher larval density.2.Treatment with sublethal dose of Bs can decrease susceptibilities of Plasmodium yoelii to both Anopheles stephensi and Anopheles dirus.3.Innate immunity was involved in the change of vector capacities of anopheles mechanism by regulating the transcriptional level of TEP1 but not melanization. |
PDF Full Text Request