Assessing The Risk Of Transmission Of Zika Virus

Background: Zika virus(ZIKV)is a single-stranded positive sence RNA virus,belongs to the genus Flavirus,family Flaviridae.The main mosquito vectors are Aedes aegypti(Ae.Aegypti)and Aedes albopictus(Ae.Albopictus).And nomosquito tranmission,for example sexual transmission and vertical transmission,plays a role in ZIKV transmission.What's more,mother-child transmission of ZIKV will cause congenital microcephaly.Most of ZIKV infected persons have no symptom,only 20% infected persons will show fever,popular rash,arthritis or other symptoms of Zika virus disease(Zika)after an incubation period of 2 to 12 days.Researches proved that people infected ZIKV is a risk factor of Guillain–Barrésyndrome.Zika cases first diagnosed in Nigeria in 1953,since then,sporadic cases were reported in Africa and Asia.Some small-scale outbreaks of Zika have been reported.In 2015,ZIKV struck Latin America and caused an unprecedentedly large outbreak,which caused the most widespread and most human cases reported in history.What's more,new cases are reported in America and the incidence in Vietnam,Thailand and other Southeast Asian countries was growing by the end of December,2016.With the global economic integrative development and frequent international trades,the cross-regional and cross-country population movement is increasing,so that all countries are facing a serious imported risk of Zika.Assessing the distribution of Zika and analyzing the risk environment factors of ZIKV transmission are very important for making targeted prevention and control measures to control global transmission of ZIKV.Objectives and Contents: Our study included three sections.(1)Predicting and Mapping the global distribution of Ae.aegypti and Ae.albopictus: building a database about global reported locations of Ae.aegypti and Ae.albopictus,and assessing the probable seasonal distribution of these two Aedes mosquitos based on ENM.The aim of this research was providing vector-distribution information for those Ae.Aegypti or Ae.Albopictusborne diseases.The research also offered data base for selecting key monitor-areas.(2)Assessing and mapping the global risk of transmission of ZIKV: building a database of this global epidemic of Zika aimed to describe the spatial distributionand temporal trend of Zika.By building boosted regression tree(BRT)models,we analyzed the risk factor of ZIKV autochthonous vector-borne transmission,predicted the seasonal distribution of ZIKV and evaluated the population lived in high risk areas of ZIKV transmission.So that our study could provide information supports for the regional prevention and control of this disease.(3)Risk assessment of Zika invasion and transmission in mainland China: we described the spatial distribution and epidemiological characteristics of imported Zika cases in mainland China.And we quantitatively evaluated the imported risk of ZIKV in major cities,in which there are international flights with epidemic focuses or which are near the border in southern China.Based on this result,we assessed the risk of vector-borne transmission of ZIKV in mainland China,the result offered scientific evidences for making Zika and vector monitoring ports,disposeing medical resource reasonably and making workable prevention and control strategies.Methods:(1)Predicting and Mapping the global distribution of Ae.aegypti and Ae.albopictus: the global observed locations of these two Aedes mosquitos were collected from published references then the locations were unified to the county level.19 bioclimatic variables were unified to 0.1°×0.1°(about 11 km × 11 km near the equator)grid level by ArcGIS10.2 software.We built ENMs separately based using a maximum entropy method(Maxent)to predicate the presence of this two Aedes mosquitos.Then the model-predicted probabilities of the two outcomes were mapped to visualize the global distribution of Aedes mosquitoes.Considering the mean temperature within 15°C to 36°C as the necessary condition of Aedes mosquitoes presence,we created the map of the seasonal risk probability of Aedes mosquitoes presence by restricting the predicted probabilities to this temperature range.(2)Assessing and mapping the global risk of transmission of Zika virus: we collected global reported Zika cases and neonates with congenital microcephaly in Brazil from March,2015 to December,2016 and linked them to the digital map of global administrative boundaries according to their locations at the municipality level using Geographic Information System(GIS)technologies,then created the thematic maps in ArcGIS10.2 software.The BRT models were created based on the reported locations of autochthonous ZIKV transmission in the Americas during October 2015–February 2016 and predicted aedes' distribution(models I–II: Ae.aegypti and the combination of Ae.aegypti and Ae.albopictus were respectively included in the models)during corresponding months,climatic variables(September 2015–February 2016,one-month lag effect considered),as well as eco-geographical variables.The process realized in R language environment(version3.1.1;R Core Team 2014).We predicted the globallyseasonal risk of autochthonous transmission of ZIKV based on the BRT models and we analyzed risk factors of ZIKV transmission.Then we calculated the cut-off value of the risk probability of human ZIKV infection presence based maximum Youden index.Areas for their risk probability above the cut-off value were considered as high risk areas of the human infection with ZIKV.Then we assessed the population in high risk areas according to the classification of the WHO.(3)Risk assessment of Zika invasion and transmission in mainland China: We collected the imported Zika cases information in China from 2015 to 2016 and linked them to the digital map then we created the thematic map to describe the distribution of these cases.And we created histogram and pie charts to describe the epidemiological features of Zika cases in mainland China.Based on monthly reported Zika cases in epidemic focuses and monthly traveller flight itinerary data from epidemic focuses to China cities,we assessed the import risk of ZIKV in these cities.We evaluated the import risk in cities near border in southern China based on the distance between these cities and land transportation ports.Then we assessed seasonal risk of autochthonous vector-borne transmission of ZIKV in mainland China combing imported risk,population density,probability of distribution of Ae.aegypti or Ae.albopictus and meteorological factors.We mapped the distribution of seasonal risk of ZIKV transmission in ArcGIS10.2 software.Results:(1)Predicting and Mapping the global distribution of Ae.aegypti and Ae.albopictus:(1)The distribution of Ae.aegypti was mainly driven by the ambient temperature,and the effect of those variables(min temperature of coldest month and mean temperature of coldest quarter),which are mirrors of low temperature,is greater than those high temperature inflected variables.(2)The probability of Ae.albopictus presence was dependent on both of precipitation and temperature,and distribution probability of Ae.albopictus was less dependent on temperature than Ae.aegypti.(3)The environmentally suitable regions of Ae.aegypti mainly lie between 35°N and 35°S.(4)The distribution range of Ae.albopictus is wider than Ae.aegypti.The environmentally suitable regions of Ae.albopictus mainly lie between 60°N and 40°S.There has distribution probability in the southern areas of Europe,the middle-east areas of the United States and major areas in the south of yellow river in China.(5)Starting from March,regions with high probabilities in the Northern Hemisphere slowly expanded northwards,whereas,those in the Southern Hemisphere expanded southwards since October.(6)The seasonal tendency of distribution of Ae.Albopictusis changed obviously in the Northern Hemisphere,but the tendency is inconspicuous in the Southern Hemisphere.The distribution range expanded northeastward during March to September and shrank toward equator from October to Feburaryin the Northern Hemisphere.(2)Assessing and mapping the global risk of transmission of ZIKV:(1)The BRTmodels showed decent predictive power with the mean AUC more than 93%(2)The risk of ZIKV transmission was low where the monthly average temperature was lower than 22°C.When the monthly average temperature was higher than 22°C,the risk of ZIKV transmission sharply increased with a fluctuation around 26°C.The effect of the monthly precipitation(mean relative contribution: 22–24%)showed a nonlinear pattern with a peak risk around 60 mm.(3)The distribution of ZIKV transmission by Ae.Aegypti was similar with the distribution of Ae.aegypti.But there are some differences,the probability of ZIKV transmission is lower and the probability of Ae.aegypti distribution is higher in the southeast areas in Brazil(3)Based on BRT model I,the seasonal trend of distribution of ZIKV transmission is similar with the seasonal tendency of Ae.aegypti distribution.The risk areas expanded northward and southward in spring and summer and the areas shrunk to equator in autumn and winter.(5)The areas with predicted high risk of ZIKV transmission found by BRT Model II were larger than those by BRT Model I,and the additional areas were mainly distributed in China,the United States,and southern Europe,Some of areas in Latin America,Africa,and India showed reduced risks,we considered the vector function of Ae.aegypti and Ae.albopictusis at the same time in BRT model II.(6)Our research found that 91% of the global population at high risk to of ZIKV transmission distributed in South-East Asia,Africa and the Western Pacific Region,and 71% of the population at high risk were under the highest risk during June–November.In particular,India was top one,have the largest population at high risk areas.Four South-East Asia countries were among the top 10 countries.China was ranked the top 5.None of the American countries were among the top 10 countries in terms of the size of population at high risk of ZIKV transmission,however,three countries in the Americas,Brazil,Colombia and Venezuela,were 1st,6th and 7th countries in terms of the largest areas under high risk of ZIKV transmission.(3)Risk assessment of Zika invasion and transmission in mainland China:(1)China reported 26 imported Zika cases in 2016,62% of cases came from Venezuela.The main symptoms were fever and erythema,individual patients showed headache,pruritus,arthralgia,pinkeye and diarrhea or other atypical symptoms.(2)There were high risk of importing ZIKV by international flight in Beijing,Shanghai and Guangzhou.And there were high risk of importing ZIKV through land transportation ports in Dehong,Fangchenggang and Beihai.(3)Zhanjiang,Haikou and Sanya had high risk of ZIKV transmission in Feburary,August and November.And Dehong and Lincang had high risk of ZIKV transmission during Feburary to November.(4)Considered vector function of Ae.albopictusis,there was high risk in Guangzhou in March,April and November.Conclusion:This study had effectively combined spatial information technology and mathematical modeling to predict global vector-borne transmission of ZIKV and analysis the risk factors,which may provide an advanced approach and a new direction in the future study about predicting and assessing the transmission of the vector-borne disease.The study proved:(1)Temperature and precipitation are main risk environmental factors of ZIKV transmission.Therefore,the distribution of risk areas of ZIKV transmission will change if there are extreme weathers,for example,the abnormal warm or cold and drought or flood caused by El Ni?o,which will influence the transmission risk of ZIKV.(2)The results of assessing and predicting the seasonal distribution of ZIKV transmission showed there were high risk of ZIKV transmission in some countries and regions in the Southeast Asia,West Africa,northeast of South America and the Garibbean Sea,and the population lived in high risk areas is largest in July.It reminded that some monitor measures of vectors and patients should been taken in these areas and in July.(3)There was high imported risk by international airline in Beijing,Shanghai and Guangzhou,so that Inspection and Quarantine Organization should strengthen the border quarantine and detect Zika cases promptly.And there was high risk for autochthonous vector-borne transmission of ZIKV in Zhanjiang in Guangdong,north of Yunnan and Hainan,so Zika cases and surveillance of Aedes mosquitos should be reinforced to detect outbreak and take prevention and control measures as soon as possible.