Comparison On Effect Of Aedes Trap And Larval Survey On Mosquito Density Surveillance

BackgroundAedes albopictus and Aedes aegypti are important vectors for dengue virus (DENV), Zika virus (ZIKV) and Zika virus (ZIKV). Dengue is an acute mosquito-borne disease with symptoms including fever, headache, rash, myalgia and arthralgia. According to symptoms and severity of the disease, it can be divided into dengue fever, dengue hemorrhagic fever and dengue shock syndrome. Annually, 50,000,000 to 200,000,000 dengue infections occur, and approximately 20,000 die of dengue fever. Dengue has become one of the most serious mosquito-borne diseases. ZIKV was first isolated in Uganda in 1947. Since then, only sporadic human infections in Africa and Asia have been reported. The first Zika fever outbreak took place in Yap Island, Micronesia. A large epidemic was reported in French Polynesia in 2013 and 2014. In May 2015, patients presenting "dengue-like syndrome" emerged in northeast Brazil. Symptoms included fever, headache, rash, myalgia and arthralgia. Many were confirmed to be ZIKV infected. Up to February 2016, more than 400,000 ZIKV infections was reported. It spread rapidly to 22 other countries and territories in the America. An increase in the number of infants born with microcephaly has been recognized since the outbreak of ZIKV infection. Several mothers of infants with microcephaly were infected. Besides, brain tissue and cerebrospinal fluid from fetus were found to be positive for ZIKV. ZIKV infection in pregnancy may be associated with fetal microcephaly.In China, dengue epidemic was primary caused by imported infections. As international personnel exchanges and trade increase, dengue is reported every year. And epidemics take place every 4-6 years. Guangdong is a southern province in China. With perennial adequate rainfall and high temperature, Guangdong is always considered to be an epidemic areas, accounting for 70% dengue cases in China. An outbreak of dengue was identified in Guangdong in 2014. More than 40,000 cases were reported. Both ZIKV and DENV are arboviruses of the Flaviviridae family and genus Flavivirus. Both are mostly transmitted by Aedes mosquitoes. Guangdong is facing a severe test in the circumstances that ZIKV spread rapidly.In the absence of vaccine and effective drug, patients infected by DENV or ZIKV can only receive supportive treatment. Prevention and control of dengue and ZIKV fever mainly depend on vector control. DENV and ZIKV are transmitted by Aedes mosquitoes. Aedes albopictus plays the most important role in China. Vector surveillance can estimate risk of transmission and becomes a part of prevention measures. To date, labor hour method, larval survey, ovitrap, sticky trap, mosquito adult trap and mosquito-ovitrap are applied to vector control while labor hour method and larval survey are widely used in China. In labor hour method, field workers collect mosquitoes attracted by their bare legs with hands or aspirator. However, that makes field workers at risk of getting infected in epidemic areas. The fact that attractions and collecting skills often differ among workers leads to low quality of this method. Traditional larval survey has a number of shortcomings. It is labor intensive and time consuming and it does not take into account that the sizes of containers and houses vary. Entomological surveys in private premises require the owner’s permission. As Aedes albopictus prefers to breed outdoors, many breeding containers are not likely to be counted in. In 2003, a new trap called mosquito-ovitrap was developed by Lin. Mosquito and oviposition index (MOI) showed significant correlation with Breteau index (BI), House index (HI), and Container index (CI). Evidences indicated it could be an tool for mosquito monitoring. But some studies doubted its efficiency.Base on mosquito-ovitrap, we made an improved trap to meet the biologic habits of Aedes albopictus, for instant, its preference to breeding in dark containers, habit of oviposition. The new Aedes trap was confirmed to be more effective compared to the mosquito-ovitrap. Nevertheless, relationship between MOI and larval indices continues to be unknowed. That may limit its application for vector surveillance. Furthermore, more research are required to evaluate its application environments and cost-effectiveness and ease or difficulty as a new tool.Objective1. To explore the relationship between larval indices and Aedes trap indices and evaluate the effectiveness for reflecting mosquito density.2. To identify the important Aedes albopictus producing containers. That will guide our vector control practice.3. To investigate the perceived ease or difficulty and value and time effectiveness of Aedes trap and larval survey from field workers.Methods1. Larval survey:A village in Baiyun district, Guangzhou, was selected as our study area. All containers both indoors and outdoors were inspected to determine whether they were wet or dry and for the presence or absence of Aedes albopictus larvae and pupae by trained personnel. All aquatic containers were classified into three types:(1) flower pots; (2) water storage containers; (3) idle containers; (4) man-made pool and gutterways; (5) nature containers; (6) discarded containers. Larval indices including BI, CI and HI were calculated.2. Aedes trap monitoring:The prepared traps, which were filled with 60ml tap water after circular filter paper had been placed in, were deployed at ground level, leeward, humid and shaded locations for 4 days. Traps were placed every 5 premises or every 10m in the open space. The geographic location of each trap was located using a hand-held GPS unit. Traps were deployed in the same locations. Every trap was inspected to determine whether it was positive. Eggs and adult mosquitoes and larvae were counted. MOI, MDI and EDI were calculated. Vector surveillance was carried out once every half month.3. Qualitative research:We conducted semistructured interviews with 14 field staff from basic health departments in Baiyun District, Guangzhou (one from each department). The outline of questions were prepared before the interviews. Demographic characteristics of participants, perceived ease or difficulty in field monitoring, time and manpower needed, perceived effectiveness of Aedes trap and larval survey and defects of both methods were included. The interviews were audio-taped with participants’permission. Notes were recorded if the participants did not agree.4. Statistical analysis:Spearman correlation analysis was used for analysing relationship between Aedes trap indices and larval indices, x 2 test was used for comparing two constituent ratios. P values less than 0.05 were considered to be statistically significant. All tests were two-tailed. All statistical tests were analyzed using SPSS 20.0(?).5. Qualitative research analysis:Audio recordings and notes were transcribed verbatim in Microsoft Word. All textual data were entered into ATLAS.ti 6 and coded with a set of existing categories above. Emergent codes and themes were allowed.Result1.1 One-year mosquito monitoring:3983 Aedes traps were deployed during the one year period July 2014 to July 2015.3816 were recovered. The recovery rate was 95.81%.812 traps were identified adult mosquito and (or) egg positive, with a positive rate of 21.28%.24 larval surveys were conducted during July 2014 and June 2015.2123 premises and 2932 aquatic containers were inspected totally.1010 aquatic containers were larvae and (or) pupae positive, with a positive rate of 34.45%.1.2 Climate factors in the village:The village was warm all the year round. Temperature began to drop in October, and average temperature was above 10 ℃ in winter. The average minimum temperature was 9.42 ℃. Temperature began to rise in March. From July to September, average temperature were above 30 ℃. It had a rainy season from April to September and dry season from October to March. Rainfall throughout the whole year reached 1686.8mm.1.3 Aquatic containers types:The most abundant container types were idle containers (1974,67%), the least abundant were flower pots (63,2%) and nature containers (54,2%). Water storage containers (454,16%) and discarded containers (264,9%) were also abundant. Larvae and pupae were most often found in discarded containers (positive rate 39.4%), flower pots (38.1%), idle containers (35.6%) while man-made pool and gutterways (21.1%) were less common. The aquatic habitat positive rate in rainy season was significantly higher than that in dry season (43.6% vs 10.7%, x2=283.29, P<0.01). There was a correlation between average aquatic containers per premises and rainfall (r=0.75, P<0.01).1.4 Dynamics of Aedes albopictus density:BI was 93.14 in the first half month of July. And it did not stop rising until BI reached 106.67 in September. BI began to decline in October and soon got under 10 at the end of November. BI kept low level (<5) from December to the beginning of March. In the second half of March, BI began to rise. At the end of April, BI reached 38.89 and in May BI was high (104.21). It remained high level (>90) in June. CI, as well as HI, shared the same trend of BI. In July 2014, MOI was above 40. In spite of decline in August, MOI was still above 35. Then it began to decrease. In October, MOI was 9.13. In the winter season from December to March, MOI was 0. MDI and EDI shared the same trend of MOI, but EDI was fluctuant.1.5 Relationship between larval indices and Aedes trap indices:MOI was significantly correlated with BI (r=0.92, P＜0.01). MOI was significantly correlated with HI (r=0.96, P<0.01). MOI was significantly correlated with CI (r=0.95, P＜0.01). MDI was significantly correlated with BI (r=0.89, P<0.01), CI (r=0.87, P＜0.01), HI (r=0.90, P<0.01). EDI was significantly correlated with BI (r=0.66, P＜0.01), CI (r=0.65,P<0.01), HI (r=0.66, P<0.01). Aedes trap indices had high correlations with larval indices other than EDI.2.1 Demographic characteristics of participants:14 field staff were recruited from 14 basic health departments in Baiyun District. There were 5 males and 9 females. Participants varied from 25 to 32 years old (28.21±2.19). They had been in their current position for 3.43±1.79 years. All had a bachelor degree.12 participants had their medical licenses. The interviews lasted for 36.57min approximately.2.2 Cost for labor:Larval surveys were said to be inconvenient because inspecting premises needed assistance of neighborhood committees or village committees. It took 5-10 staff 4 hour to finish inspection of 100 premises. However, it usually took 1-2 staff 2 hours to deploy the traps.2.3 Methodological characteristic of both methods:Larval survey was claimed to have many defects:(1) equal treatment to all positive containers; (2) primary breeding containers can’t be counted as Aedes albopictus prefers to breed outdoors. But staff can propagate knowledge about dengue fever and eliminate aquatic containers when inspecting premises. Aedes trap was perceived to effective with its cone shaped holes. Transparent cup wall was conducive to distinguish mosquitoes collected. But the sizes of the cup body and cone shaped holes were perceived to be too small for attracting mosquitos.4-day placement made it not suitable for prompt detection of vectors after case notifications. Participants suggested that there was no comparability between data from different environments for Aedes trap monitoring.2.4 Perceived ease or difficulty in the field:Staff mentioned several factors: (1) necessary assistance of neighborhood committees or village committees; (2) inaccessibility of urban premises; (3) irresponsible attitudes; (3) poor skills of field staff; (4) dislike and incompatibility of premises owners, which made larval survey hard to operate. Aedes trap was perceived to have a few problems:(1) it could not be placed in villages-in-city which were densely built and populated; (2) vector surveillance program needed a certain number of traps to make it work; (3) it was hard to found and identify the eggs when they were rare in the traps. Egg incubation period was long.2.5 Perceived effectiveness of Aedes trap and larval survey:2 participants considered larval survey to effective.4 suggested the data of larval survey were inferior because survey only covered a small area and inaccessibility of urban premises were common.8 considered BI was always too low for breeding sites were primary in public areas for Aedes albopictus.1 staff thought it’s not effect because all containers were equal treated. Aedes trap was said to be effective by 6 participants, but 5 thought the positive rate were always too low to reflect the true vector density.1 staff mentioned nature Aquatic containers could compete against the traps and the number of nature Aquatic containers may affect the Aedes trap indices.Conclusion:1. Aedes mosquitoes density begins to rise in March and keeps high from May to September. It begins to decline in October and remains low in the winter season from December to March.2. Idle containers, water storage and discarded containers are primary breeding sites for Aedes albopictus rather than flower pots.3. Aedes trap indices and larval survey indices have the same trend of density dynamics. BI, HI and CI are correlated significantly.4. Larval survey is labor-consuming. Inaccessibility of premises, equal treatment to all containers and preference to breeding sites outdoors can influence the quality of the data. But staff can propagate knowledge about dengue fever and eliminate aquatic containers when inspecting premises.5. Aedes trap method is labor-saving. It is not suitable for crowded areas. The cup body and cone shaped holes may be too small. Effectiveness of Aedes trap is controversial among field staff.