Movement Patterns of Potential Conveyers of Aedes
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Movement patterns of potential conveyers of Aedes camptorhynchus in the The southern saltmarsh mosquito (SSM) habitat associated with a number of towns was identified as being important Thames-Coromandel district for surveillance based on analysis of conveyer movements. Alternative methods of estimating the relative travel to towns Aedes camptorhynchus, also known as southern saltmarsh mosquito and regions were identified for future SSM incursions. (SSM), is a mosquito of public and animal health importance. It is a vector of the zoonotic Ross River virus(1) (not present in New Zealand(2)) and a significant cause of nuisance biting(3), and is The sampling frame was amended by removing duplicate ratepayers involved in the transmission of diseases of importance to animal with more than one Thames-Coromandel property and ratepayers health(4) (Bull and Mules 1944(5)). It was first detected in New with an overseas contact address. Others were removed from the Zealand at Napier in late 1998 and since then in nine other locations, sampling frame if no contact name was provided for the resident or most recently in the Thames-Coromandel district in May 2006. if the contact name was a company or a government organisation. SSM-infested sites within the Thames-Coromandel district were Stratification unusual in that steep topography separating infested sites was likely to have prevented natural spread through wind dispersal. Approximately 56% of ratepayer contact addresses for Thames- MAF Biosecurity New Zealand (MAFBNZ) initiated the present Coromandel properties in the sampling frame were in Auckland, study to understand better the pathways of spread of SSM both 22% were in locations other than Auckland but outside of the within and between infested locations. The aim was to determine Thames-Coromandel district, 20% were within the Thames- the opportunities for spread of SSM from properties near infested Coromandel district and 2% were in another country. habitat in the Thames-Coromandel district. The study had the For sampling ratepayers, contact addresses for Thames-Coromandel secondary purpose of defining model parameters for local spread of properties were classified into two strata: those with a Thames- SSM in a spatial simulation model of spread in New Zealand. Coromandel contact address (441) and those with a contact address outside this district but not overseas (1,764). The former were sub- Materials and methods categorised into four additional strata based on whether they were within 0.5 km, or between 0.5 and 5 km, of an infested site, and Survey design and sampling frame whether or not they had livestock present on the property according A postal survey was the main data collection method for the study. to the Agribase livestock database (AsureQuality New Zealand, MAFBNZ conducted a postal survey of Thames-Coromandel Hamilton). properties located within 5 km of SSM-infested sites during November 2006. A questionnaire sent to selected households asked We assumed that SSM eggs associated with plant material or soil them to provide information on the movement frequency and had the potential to be moved if properties were within 0.5 km of destination of potential SSM conveyers for the previous 12 months. SSM-infested sites. Properties with livestock present were considered The conveyer movements were those associated with cars, livestock to have the potential to transfer adult mosquitoes when stock and plant material or soil. These were selected as the most likely were being moved. Questionnaires for the relevant strata were for human-mediated spread of SSM through movement of the eggs therefore customised to include additional questions relating to the amongst plant material or soil, and through the adult mosquitoes movement of plant material or soil, and livestock. being attracted to human hosts in cars and livestock hosts. Secondly, we assumed that properties with a contact address in a New Zealand location outside of the Thames-Coromandel district The Waikato District Council provided ratepayer data including the were likely to constitute holiday accommodation. Movements of Land Information New Zealand (LINZ) Core Record System (CRS) plant material, soil and livestock from these properties were not land parcels and Waikato district valuation data. The sampling considered to be significant, and so their questionnaires did not frame was selected from the LINZ CRS data according to its spatial include questions relating to movement of plant material, soil and proximity to the 12 SSM-infested sites identified by NZ Biosecure livestock. It was also assumed that the number of car trips was (B Gradwell, personal communication). Only parcels and associated low given the relatively short period of holidays and therefore car ratepayers within 5 km of infested sites were selected. For the usage at the property. The questionnaire for this stratum therefore purposes of data analysis, the 12 SSM-infested sites were grouped reflected a low number of car trips. into five clusters: Whangapoua, Kennedy Bay, Waikawau, Colville and Waiaro. The spatial location of each ratepayer land parcel was Consequently, there were five versions of the questionnaire relating recorded as the cluster to which it was closest. to the five strata (Table 1): page 4 Surveillance 35 (3) 2008 1. livestock present, the property is within 0.5 km of a SSM- Survey questions infested site, and the ratepayer contact address is in the Thames- Households were asked to list the places they drove to and the Coromandel district; frequency of those visits on a weekly, monthly or yearly basis, as 2. no livestock present, the property is within 0.5 km of a SSM- appropriate. They were also asked the type of activity (work, shops, infested site, and the ratepayer contact address is in the Thames- sport and recreation and other) associated with each car trip. Coromandel district; Households with properties within 0.5 km of a SSM-infested site 3. livestock present, the property is between 0.5 km and 5 km from were asked to provide the frequency and destination of movement a SSM-infested site, and the ratepayer contact address is in the of any plant material or soil. Those with livestock present on Thames-Coromandel district; the property were asked to provide information on livestock 4. no livestock present, the property is between 0.5 km and 5 km movements, including the month of travel, stock type, transport from a SSM-infested site, and the ratepayer contact address is in company, whether the movement was on or off the property and the the Thames-Coromandel district; destination of the livestock. 5. the ratepayer contact address is in another New Zealand Destination and distance frequency Territorial Authority district. The reciprocal of the response rate and sampling fraction were used The purpose of creating different versions of the questionnaire was to adjust the travel frequency of car movements for each of the to encourage householders to respond by ensuring the questions five strata (Table 1). Adjusted frequencies were used to determine were relevant to the group. the distribution of distances travelled. Distance travelled for each car movement was determined as the hypotenuse of the ‘x’ and ‘y’ Sample size distances travelled, calculated from GIS maps. Figure 1 shows the A random sample (29%, 505/1,764) of households from the stratum frequency of all car movements. with ratepayer contact details outside the Thames-Coromandel district was selected for the survey. For the other four strata all Other data sources households were selected. Data were also obtained from two additional sources. The Department of Conservation (DOC) provided data captured from Survey procedures its booking system for a six-month period between 13 October 2005 Before undertaking the survey, the clarity of the questionnaire was and 14 April 2006 on the place of origin of visitors staying at the tested through a pilot study of 10 households (80% response rate). DOC-managed Waikawau Bay campsite. The questionnaire format was finalised after addressing comments from the respondents. Tourism New Zealand provided data for the period 2002 to 2004 on visitor origin extrapolated from yearly surveys carried out on 15,000 The survey was carried out as follows: New Zealand households and 5,000 departing overseas tourists. 1. MAFBNZ sent households in the sampling frame an initial letter advising them of the SSM survey and asking them to participate. Statistical analysis and GIS All analyses were performed using SPSS version 14 (SPSS Inc, 2. Two days later, households were sent the questionnaire along Chicago IL, USA). Regression statistics for mesh-block area data with a covering letter, a postage paid envelope and a chocolate (Statistics New Zealand) and frequency of car travel to specific gift. destinations were determined from data normalised through the 3. In the same week the questionnaire was posted, a local logarithmic transformation. newspaper ran an article provided by MAFBNZ . The article was designed to lift the profile of the survey and encourage Kernel density maps were created using the spatial analyst function responses. of ARCMAP (ESRI, New York, USA) using a search radius of 7 km. A network map of car travel in the Thames-Coromandel region was 4. One week after the questionnaire was posted, 215 non- created using R version 2.4.1 for Microsoft Windows (Freeware). respondents were telephoned over five working days and encouraged to fill out the questionnaire if they had not done so. Results The 360 non-respondents not listed in the Telecom white pages were not telephoned. Telephonists made the phone calls between the Response rate hours of 3–7 pm to increase the likelihood that someone would be Of the 946 questionnaires sent to property owners, 37 were returned home. The telephonists had been briefed on the biology of the SSM with no known address. The response rate for correctly addressed and the study objectives, and were provided with a telephone script questionnaires was 68% (619/909).