The Population Ecological And Colonization Risk Of Biomphalaria Glabrata As Intermediate Host Snails Of Schistosomia Mansoni

Schistosomiasis mansoni is widely endemic in 45 countries in Africa and America.It is an important infectious disease that harms human health in endemic areas and hinders social and economic development.Control and elimination of intermediate hosts is an important measure to interrupt the transmission of Schistosomiasis mansoni.Biomphalaria glabrata,a hermaphroditic,specificity freshwater aquatic snail,is widely distributed in Africa and South America and is one of the most important intermediate host snails of Schistosomia mansoni.Part I Research of Biomphalaria glabrata population ecology---reproduction and viabilityObjective:To compared the difference of fertility of Biomphalaria glabrata snails between autologous reproduction and variants reproduction,to observe the circadian rhythm of laying eggs,the effect of light on laying eggs and the tolerance of the snail to water and food deficiency,so as to provide the basis for control and elimination to Biomphalaria glabrata snails in the field.Methods:Under laboratory conditions,a single Biomphalaria glabrata egg for autologous reproduction was be separated and hatched individually,young snails were raised in different plastic boxes individually.The eggs for variants reproduction were hatched and young snails were fed in the same plastic box.The ability of spawn,the development of the eggs,and the number of snails growing from young to adult snails were compared between autologous reproduction and variants reproduction.The snails were be in water under four environments,all day illumination（3000lux）,all day without illumination（0 lux）,daytime lighting（8:00-20:00）and night（20:00-8:00 the next day）without illumination and daytime without illumination but night lighting.The eggs were collected and counted daily at 8:00 and 20:00.The circadian rhythm of spawn and the effect of illumination on spawn were observed.The adult snails were divided into 6 groups and exposed to environments with relative humidity of 0%,65%,87%and 100%,respectively.The survival rates of the adult snails exposed to different environments after different times were observed.The adult snails were placed at 25℃in the oven to remove water content from the soft body of snails.When the dehydration rates of the soft bodies achieved 10%,20%,30%,40%,50%,52%,55%,57%,60%and 70%respectively the survival rate of the adult snails exposed to the oven were observed.Results:The diameters of the snails first laying eggs were 10-12.5mm at autologous reproduction snails.The diameters of the snails first laying egg were7.5-10.mm at variants reproduction snails.In 25℃water the laying egg number,laying egg mass number and egg mass containing egg numbers for 15 days per snail were 8.77±16.92 egg/snail,1.93±3.30 block/snail and 4.01±1.87 egg/block respectively in the autologous reproduction snail.The laying egg number,laying egg mass number and egg mass containing egg numbers for 15 days per snail were149.71±142.28 egg/snail,5.36±4.20 block/snail and 24.20±12.18 egg/block respectively in the variants reproduction snails.There were a significant difference between autologous reproduction snail with variants reproduction snail（P<0.01）.In the natural environment of daytime with illumination and nighttime with darkness,the amount of laying eggs of Biomphalaria glabrata snail was944.07±392.53 egg/day during a whole day,among them the amount of laying eggs during daytime was 94.93±37.99 egg/day（account for 10.1%）and the amount of laying eggs during nighttime was 849.13±366.04 egg/day（account for 89.9%）,and the laying egg is given priority to with the night.If the circadian rhythm of laying eggs of Biomphalaria glabrata is changed and the snail is exposed to the environment of daytime with darkness and nighttime with illumination,the amount of laying eggs of Biomphalaria glabrata snail during a whole day was 577.07±279.76 egg/day,among them the amount of laying eggs during daytime was 431.13±239.19 egg/day（account for 74.7%）and the amount of laying eggs during nighttime was 145.93±123.98 egg/day（account for 25.3%）.The above results suggest that the dark environment is conducive to Biomphalaria glabrata snails to lay eggs.When snails were exposed to the environment of daytime and nighttime both with illumination,the amount of laying eggs of Biomphalaria glabrata snail during a whole day was 999.07±444.00 egg/day,among them the amounts of laying eggs during daytime and during nighttime were 428.47±302.08 and 570.60±257.17pieces/day respectively.There was no significant difference（P>0.05）between daytime and nighttime.When snails were exposed to the environment of daytime and nighttime both with darkness,the amount of laying eggs of Biomphalaria glabrata snail during a whole day was 602.93±510.68 egg/day,among them the amount of laying eggs during daytime was 47.67±52.68 egg/day（account for 7.9%）and the amount of laying eggs during nighttime was 555.27±469.89 egg/day（account for92.1%）.The above results suggest that light can promote the increase of spawning of Biomphalaria glabrata.When Biomphalaria glabrata was exposed to the environments with relative humidity of 0%,65%,87%and 100%in 25℃respectively,the longest survival times of snails were 7d,70d,150d and 100d respectively.In 25℃water,the snails in the control group can survive for 50 days without food.The adult snails were placed at 25℃in the oven to remove water content from the soft body of snails.When the dehydration rates of the soft bodies achieved 10%,20%,30%,40%,50%,52%,55%,57%,60%and 70%respectively the survival rate of the adult snails exposed to the oven were 100%,100%,100%,100%,70%,30%,0,0,0 and 0 respectively.Conclusion:Biomphalaria glabrata can achieve the reproductive process by variants reproduction or autologous reproduction.There is a significant difference in reproductive ability between variants reproduction snail and autologous reproduction snail,variants reproduction is stronger than autologous reproduction,but the rate of reproduction in autologous reproduction is higher than variants reproduction which suggests that a single survived Biomphalaria glabrata snail has the possibility of forming a new population through the process of autologous reproduction in certain environmental conditions.Therefore,it is believed that the existing measures for elimination of Biomphalaria glabrata can reduce the amount of snail populations but cannot eliminate the snail breeding completely.Biomphalaria glabrata are mainly spawn at night,illumination interfere the spawn.The dark environment is conducive to spawning,and light can promote the increase of spawning.Biomphalaria glabrata have a strong ability to withstand harmful environments,such as lack of water and food.The laying egg of Biomphalaria glabrata is given priority to with the night.The dark environment is conducive to Biomphalaria glabrata snails to lay eggs.Light can promote the increase of spawning of Biomphalaria glabrata.Biomphalaria glabrata snail can survive 150 days in the environment with 87%relative humidity and without food in 25℃.It indicates that snails which still alive after the dry season in Africa play an important role in local schistosomiasis transmission and control.Part II The colonization risk and potential geographical distribution of Biomphalaria glabrata as the intermediate host snails of Schistosomia mansoni in the mainland ChinaObjective:To predict the colonization risk and potential geographical distribution of Biomphalaria glabrata in the mainland China.Methods:The survival extreme high temperatures and low temperatures of Biomphalaria glabrata egg,young and adult Biomphalaria glabrata snails and the average effective accumulated temperature of generation development had been determined in laboratory conditions.The temperature data in January and July from1955 to 2010 had been collected from the national meteorological monitor sites in the southern part of China,included Chongqing,Zhejiang,Yunnan,Sichuan,Jiangxi,Hunan,Hainan,Guizhou,Guangdong,Guangxi and Fujian,11 provinces.A database of ambient temperature related to Biomphalaria glabrata had been established based on the Geographic Information Systems（GIS）.The colonization risk and potential geographical distribution of Biomphalaria glabrata in the southern part of China had been analyzed and predicted by ArcGIS 10.1 software.Results:The half lethal low temperatures of Biomphalaria glabrata egg,young and adult Biomphalaria glabrata snails were 6.89℃,6.34℃and 6.60℃respectively;The half lethal high temperatures of Biomphalaria glabrata egg,young and adult Biomphalaria glabrata snails were 35.99℃,33.59℃and 32.27℃,respectively.The highest and lowest temperatures layed eggs were 31.75℃and 9.91℃,respectively;The average effective accumulated temperature of generation development was（1970.07±455.10）days·degree.The GIS overlay analysis of the half lethal low and high temperatures of the Biomphalaria glabrata showed that the local temperature conditions in all Hainan and part regions in Yunnan,Guangxi,Guangdong and Fujian conformed to the survival temperature of Biomphalaria glabrata snails;The regions,where the average effective accumulated temperature was more than the average effective accumulated temperature of generation development of Biomphalaria glabrata,were Guangdong and Hainan,and parts regions of other 9 provinces.The overlay analysis of GIS maps of the survival extreme high temperatures and low temperatures of Biomphalaria glabrata with the GIS map of the average effective accumulated temperature of generation development in 2010 showed that whole region of Hainan and part regions of Guangdong,Guangxi,Yunnan and Fujian were potential geographical distribution regions of colonization risk of Biomphalaria glabrata.The overlay analysis of GIS maps of the survival extreme high temperatures and low temperatures of Biomphalaria glabrata with the GIS map of the average effective accumulated temperature of generation development from 1955 to 2010showed that the potential geographical distribution regions of Biomphalaria glabrata was expanding from whole region of Hainan and part regions of Guangdong in 1955to whole region of Hainan and part regions of Guangdong,Guangxi,Yunnan and Fujian in 2010.Conclusions:If Biomphalaria glabrata snails were introduced into the mainland China,the potential geographical distribution regions would be whole region of Hainan and part regions of Guangdong,Guangxi and Yunnan.The change of risk range and risk intensity presented the trends of expanding and increasing from the south to the north gradually.Part III Molluscicidal effect of niclosamide compound preparation against snails and eggs of Biomphalaria glabrataObjective:To evaluate the molluscicidal effects of niclosamide compound preparation against Biomphalaria glabrata,to seek the more suitable molluscicidal products for Biomphalaria glabrata control in the field.Methods:The three preparations of niclosamide compound preparations,just as26%suspension concentrate of metaldehyde and niclosamide（MNSC）,26%niclosamide compound preparation,31%niclosamide compound preparation,and the50%Wettable powder of niclosamide ethanolamine salt（WPN）were formulated into solutions with niclosamide concentrations of 1.000,0.500,0.250,0.125 mg/L and0.063 mg/L.The snails,10 Biomphalaria glabrata snails per group,were immersed in100 ml of the solution at 25°C for 24,48,and 72hr,respectively,then washed with dechlorinated water at 25°C.The distinguish between survivor and death,had been made by water detection mathed.The date was analyzed and LC₅₀ and LC₉₅5 were calculated by the linear regression of weighted mothod.The three preparations of compound preparations and WPN above were formulated into solutions with niclosamide concentrations of 0.500,0.250,0.125,0.063 mg/L and 0.032 mg/kg,respectively.The Biomphalaria glabrata eggs,50-100eggs/group,were immersed in solution liquid at 25℃for 24,48,72hr,respectively.The eggs were removed and placed in new dechlorinated water at 25℃for 15 days.The development and hatched of snail eggs were observed day by day and the number of newly hatched young snails were counted.The data was analyzed and LC₅₀ and LC₉₅ were calculated by the linear regression of weighted mothod.Results:When the adult snails were immersed in the solutions of MNSC,26%compound suspension concentrate of niclosamide,31%compound suspension concentrate of niclosamide and WPN,LC₅₀ for 24hr were 0.197,0.565,0.191 mg/L,and 0.216 mg/L,and LC₉₅ for 24hr were 0.253,0.664,0.271 mg/L,and 0.262 mg/L;LC₅₀ for 72hr were 0.170,0.536,0.173 mg/L,and 0.172 mg/L,and LC₉₅ for 72hr were 0.203,0.637,0.248 mg/L,and 0.221 mg/L,respectively.When the snail eggs were immersed in the solutions of the four drug solutions above.LC₅₀ for 24hr were 0.094,0.121,0.094 mg/L,and 0.155 mg/L,and LC₉₅ for24hr were 0.122,0.167,0.121 mg/L,and 0.241 mg/L;LC₅₀ for 72hr were 0.071,0.108,0.084 mg/L,and 0.102 mg/L,and LC₉₅5 for 72hr were 0.092,0.155,0.109 mg/L,and 0.151 mg/L,respectively.Conclusion:The molluscicidal effect of MNSC and WPN against the adult snails and snail eggs of Biomphalaria glabrata is much better than other compound preparations,and it is suitable molluscicides for controlling Biomphalaria glabrata in the field.