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 (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 , 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 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: , , 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). The five survey strata varied and answers to anticipated questions. in their response rates (Table 1). Four percent (19/505) of the

page 5 Surveillance 35 (3) 2008 Figure 1: Frequency histogram of distance (km) of all car movements from Thames-Coromandel district properties selected from LINZ CRS data (low frequency travel between 300 and 1,236 km is not presented) respondents living outside of the Thames-Coromandel district had not travelled into this district during the last 12 months.

Table 1: Summary of the sampling frame used to survey Thames-Coromandel district households from properties near SSM-infested sites; number, percentage and response rate by stratum

% Surveys Response Ratepayer location Stratum Number surveyed returned rate

Thames-Coromandel 1. Livestock and 21 100 10 48 district ratepayer contact soil/plants Figure 2: The spatial distribution of surveyed properties in the Thames- address 2. Soil/plants 13 100 6 46 Coromandel district selected from LINZ CRS data and related to SSM- 3. Livestock 126 100 75 60 infested sites 4. No livestock, or 281 100 175 62 soil/plants

Other New Zealand 5. Other New 1,764 29 353 70 territorial authority Zealand territorial ratepayer contact address authority contact address Total 2,205 45 619 68

Destination frequency After adjusting for the response rate and sampling fraction, there were 234,462 car movements to 44 places in the Thames- Coromandel district and 210 places outside this district over a 12-month period (Table 2). The locations of surveyed households were not evenly distributed around the clusters of SSM-infested sites. The majority of properties were closest to Whangapoua (73%) followed by Kennedy Bay (13%), Colville (8%), Waikawau (5%) and Waiaro (1%).

Only 2.5% of car movements were to destinations other than the Waikato (90%) and Auckland (7.5%) regions. The spatial distribution and density of car travel to various New Zealand destinations are presented in Figures 2 and 3.

Figure 3: A kernel smoothed density plot of car movements from surveyed locations to other places in the Thames-Coromandel district with overlying symbols representing the frequency of movements of plant/soil material and livestock

page 6 Surveillance 35 (3) 2008

15 Table 2: The percentage of car travel between Thames-Coromandel district properties Table 3: Frequency of car travel (rounded to the nearest 100) to and from destinations within 5 km of a SSM-infested site and other destinations defined by region within the Thames-Coromandel district from properties within 5 km of a SSM-infested site and related to the nearest SSM cluster (Whangapoua, Kennedy Bay, Waiaro, Region name % of car movements Waikawau and Colville) North Island Origin of travel related to SSM-infested site cluster Auckland 7.5 Destination Kennedy Colville Waiaro Waikawau Whangapoua Total Bay of Plenty 1 town name* Bay Gisborne 0.1 5 29 1 557 592 Hawke’s Bay 0.1 Coromandel 71 111 4 23 169 378 Manawatu-Wanganui 0.1 Thames 35 36 4 8 176 259 Northland <0.1 Matarangi <1 1 <1 4 176 181 Taranaki 0.1 Whangapoua <1 18 1 84 103 Waikato 90.0 Colville 39 10 <1 25 4 79 Kuaotunu 2 <1 71 73 Wellington 0.1 Te Rerenga 18 30 48 South Island Port Charles 15 5 9 1 30 Canterbury <0.1 Kennedy Bay <1 17 <1 5 23 Marlborough <0.1 Tairua 1 <1 16 17 Nelson <0.1 Amodeo Bay 6 6 4 1 17 Otago <0.1 Opito <1 4 9 13 Southland <0.1 Waikawau 1 10 1 <1 13 Tasman 0.1 Kopu 12 12 West Coast <0.1 Pauanui <1 <1 <1 <1 11 12 Te Puru 10 10 After adjusting for the response rate and sampling fraction <1 0 9 9 there were 193,389 car movements (82% of all movements) to Tuateawa <1 8 <1 9 Little Bay 1 3 4 <1 8 destinations within the Thames-Coromandel district (Table 3 and Otama 7 7 Figure 4). <1 1 4 5 Whangamata <1 1 2 2 5 Papaaroha 3 1 1 4 Whangaahei 4 <1 4 Kaimarama 4 4 Hahei <1 <1 4 4 Port Jackson 1 <1 2 <1 1 3 Te Kouma 2 2 Mill Creek 2 2 Manaia 2 2 1 1 Tapu <1 1 1 Waiaro <1 1 1 Waiomu <1 <1 1 Hikutaia <1 <1 1 Opoutere <1 <1 <1 1 Hikuai <1 <1 <1 Purangi <1 <1 <1 Puriri <1 <1 Whenuakite <1 <1 Matatoki <1 <1 Figure 4: A network diagram of car travel from SSM-infested sites Kareta <1 <1 to other Thames-Coromandel locations. Car travel between locations Ruamahunga <1 <1 with fewer than 500 car movements per year has not been included. The thickness of lines between locations relates to the frequency in Total 188 277 11 87 1,371 1,934 movement * The towns in italics are those in close proximity (<2 km) to SSM susceptible habitat Shading indicates travel between infested sites Twenty-one percent of car travel was for leisure or holiday activities and 41% for work related activities. The destinations for car travel associated with these activities was closely correlated (P <0.01, R2 = 88%).

page 7 Surveillance 35 (3) 2008 Forty-three percent of respondents (32/75) with livestock on the Sixty-nine percent (11/16) of those that were within 0.5 km of a property (according to Agribase) indicated that livestock had been SSM-infested site moved soil or plants from their property. Table moved on or off the property. There were 34 livestock movements on 5 shows there were 35 movements of plant material or soil to to surveyed properties and 96 movements from surveyed properties to six different locations, most less than 5 km (86%, 30/35). One other locations. Table 4 shows there was a high percentage of livestock respondent with a Port Charles property, not within 0.5 km of an movements to: Thames 31% (30/96), Paeroa 15% (14/96), Colville SSM-infested site, indicated that 7,000 plants had been transferred 10% (10/96), Puriri 9% (9/96) and Whitianga 7% (7/96). The only to the property from various locations around Whangarei and the livestock movements on to surveyed properties from locations with Thames-Coromandel district. The spatial patterns of plant material/ SSM-infested habitat were two from Kennedy Bay to Colville. soil and livestock movements are presented in Figure 3:

Table 5: Frequency of plant material or soil movements (not corrected for sampling Table 4: Frequency of livestock movements (not corrected for sampling fraction) from fraction) from properties within 5 km of a SSM-infested site, and related to the nearest properties within 5 km of a SSM-infested site, and related to the nearest SSM cluster SSM cluster (Whangapoua, Kennedy Bay, Waiaro, Waikawau and Colville), to other (Whangapoua, Kennedy Bay, Waiaro, Waikawau and Colville), to other locations locations

Origin of stock movement related to SSM-infested cluster Origin of plant or soil movement related to SSM-infested cluster Destination Colville Kennedy Bay Waiaro Total Destination Colville Kennedy Bay Waiaro Waikawau Whangapoua Total Amodeo Bay 2 2 Thames 3 28 31 Colville 23 23 Paeroa 6 1 7 14 Coromandel 2 2 Puriri 9 9 Kennedy Bay 6 6 Port Charles 1 1 Colville 8 2 10 Waiaro 1 1 Whitianga 4 3 7 Total 26 8 1 35 Coroglen 4 2 6 Te Puke 3 3 Distance and trip frequency Waitoa 3 3 The median distance travelled for all car movements was 12 km

Auckland 2 2 (mean 35 km, STD 57 km). The median distance travelled by Thames-Coromandel district residents was 12 km (mean 27, STD 51) Patetonga 2 2 compared with 19 km (mean 52, STD 67) for Thames-Coromandel Tokoroa 2 2 property owners residing outside this district (Table 6 and Figure 1). Cambridge 1 1 The median number of car movements made per year by Thames-

Coromandel 1 1 Coromandel district residents was 162 (mean 370, STD 579).

Matatoki 1 1 Table 6: Percentiles for distance (km) travelled for all car movements, Thames- Coromandel residents, and Thames-Coromandel property owners living in another Morrinsville 1 1 Territorial Authority district

Northland 1 1 Car movement category Percentiles for distance travelled (km) Ramarama 1 1 10% 25% 50% 75% 90%

Waitakaruru 1 1 All car movements 4 9 12 47 86

Total 27 6 13 2 48 96 Thames-Coromandel residents 4 8 12 25 59

Non Thames-Coromandel residents 4 12 19 82 121

Population demographics There was a statistically significant relationship between the destination frequency of car travel to towns within 12 km of infested sites (median distance of all car movements) and the mesh- block area associated with that town (P <0.01, R2 = 0.42). This relationship was not statistically significant when all towns in the Thames-Coromandel district were included in analysis (P = 0.07).

Waikawau camping ground data Of camping parties staying at the Waikawau camping ground, 88% were from the Auckland (47%, 499/1,061), Waikato (32%,

page 8 Surveillance 35 (3) 2008 344/1,061) and Bay of Plenty (9%, 92/1,061) regions. Camping Colville and Waikawau. While this does not necessarily mean that parties from the Thames-Coromandel district comprised 26% cars are responsible for transfer, it does show that a suitable network (91/344) of the Waikato region’s camping parties staying at the exists between SSM-infested sites within the Thames-Coromandel camping ground. Within those from the Thames-Coromandel district. district, 94% were from three locations: Coromandel (27%, 25/91), The highest frequency of car travel from surveyed locations to Thames (44%, 40/91) and Whitianga (23%, 21/91). places within the Thames-Coromandel district was to the towns of Tourism New Zealand data Whitianga, Coromandel, Thames, and . All these locations have associated SSM-susceptible habitat. Surveillance Approximately 2,455,499 visits are made to Coromandel annually. of habitat could be further prioritised based on the risk status of the Ninety-six percent of these visitors are from Auckland (43%), origin of travel. The SSM-infested sites of Waiaro and Kennedy Bay Waikato (31%), Bay of Plenty (8%), Wellington (2%) and other could both be considered to have had a high risk of SSM transfer as countries (12%). high numbers of adult SSM have been found at both these locations Discussion (B Gradwell, personal communication). A high proportion of travel from these locations has been to Coromandel and Thames. This study quantifies the movement frequency of three potential SSM conveyers from properties in close proximity to SSM-infested Coromandel, Thames and Whitianga were also important sites in the Thames-Coromandel district. Results provide broad destinations for car travel to and from the DOC-managed Waikawau patterns in conveyer movements reflecting movement to the nearest camping ground. The camping ground is close to infested habitat. town rather than to a specific site. Recall bias is likely to have had The risk of adult SSM transfer from this site is high as adult some influence on data quality, as it is unrealistic to assume that mosquitoes may become trapped in tents or camping equipment or respondents will have accurately recalled all conveyer movements cars when campers pack up to leave a campsite. for the previous 12 months. The response rate for the survey was We hypothesised that holiday sport and recreation activities high (68%) particularly considering the effort required to complete constitute a greater risk for spreading adult mosquitoes than work- the questionnaire. The high response rate probably reflects the related activities, as people are likely to come into closer proximity survey design and procedures adopted but is also likely to reflect to SSM habitat. However, patterns of car travel were similar for these community concern regarding SSM and its potential role as a activity types suggesting it is not necessary to take into account the disease vector. type of activity when studying car movement patterns. The results showed that the frequency of travel between the Thames was the main destination for livestock movements, Thames-Coromandel district and other Territorial Authority although the number reported was low. It is probable that livestock districts or regions is related to the distance that separates them. movements to Thames represent stock sent to slaughter, as the town A high proportion of travel to the Thames-Coromandel district has a livestock slaughter plant but no saleyards. Livestock saleyards was from the Auckland and Waikato regions. Assuming that travel with a Thames-Coromandel catchment are located at Colville, frequency is an important factor influencing SSM transfer, these Coroglen, Coromandel and Paeroa. From these locations, livestock regions are likely to be important as possible sources and potential were sent to Colville and Paeroa. The Colville saleyard has sales six locations for spread. For future SSM incursions, LINZ CRS data monthly whereas sales at Paeroa are weekly. No susceptible habitat (describing ratepayer location) and Tourism New Zealand data has been identified in Paeroa. (describing tourist numbers and place of origin) could be used as an alternative to the survey methods presented here to understand long A high proportion of respondents within 0.5 km of SSM-infested distance car travel between districts. sites carried out activities resulting in plants or soil being moved, most over short distances. Nevertheless, long distance movements Potentially, an estimate of local travel to surrounding towns could were identified. One property, not within 0.5 km, received a be predicted from population density statistics using mesh-block substantial number of plants from a variety of locations within area (Statistics New Zealand). The frequency of car travel to towns and outside the district, to establish new habitat for New Zealand within 12 km of infested sites was correlated to the population wildlife. While the property does not represent an immediate risk, density of the town. However, the distance from the site of it does show the importance of identifying similar properties. It is interest will have an influence on the level of correlation between likely that government agencies such as DOC and Regional Councils destination frequency and population density as there was no have information that could identify properties carrying out similar correlation when analysis was repeated including all towns in the activities. Thames-Coromandel district. Ninety percent of travel by those surveyed was less than 58 km from Within the Thames-Coromandel district there was a high frequency the respondent’s property location. The frequency distribution of of car travel between SSM-infested sites; particularly between distance travelled may be of use in prioritising surveillance of SSM Kennedy Bay and Waikawau/Whangapoua/Colville and between

page 9 Surveillance 35 (3) 2008 susceptible habitat in the immediate area surrounding a newly located infested site.

This study has introduced the association of conveyer movements between infested and non-infested SSM sites. Factors other than those considered in this study are likely to influence whether a conveyer movement results in a successful transfer. While the study in some cases did show that there was a relationship between the frequency of car travel and infestation status, it did not explain why some locations have not become infested despite being a frequent destination for travel from properties close to SSM-infested habitat. Further studies investigating these factors and the resulting probability of transfer for each type of conveyer may be of benefit to understanding SSM spread.

Acknowledgements The authors thank Angela Grocott and Shandi Diamond-Reti from the Investigation and Diagnostic Centre for their help in managing the logistics of the postal survey. Others providing support to the project were Graeme Mackereth (Investigation and Diagnostic Centre), Bryn Gradwell (NZ Biosecure), Mark Stevenson (Massey University Epicentre), Robert Sanson (AsureQuality New Zealand) and Ruth Frampton (Critique Limited).

References (1) Ballard JWO, Marshall ID. An investigation of the potential of Aedes camptorhynchus (Thom.) as a vector of Ross River virus. Australian Journal of Experimental Biology and Medical Science 64,197-200, 1986.

(2) Mackereth GF, Hearnden MN. A survey for arbovirus infection in New Zealand livestock exposed to the exotic mosquito Aedes camptorhynchus. Surveillance 26(4), 10-1, 1999.

(3) Holder P, Brown G, Bullians M. The mosquitoes of New Zealand and their animal disease significance. Surveillance 26(4), 12-5, 1999.

(4) Russell RC. The relative importance of various mosquitoes for the transmission and control of dog heartworm in southeastern Australia. Australian Veterinary Journal 67, 191-2, 1990.

(5) Linley JR, Geary MJ, Russell RC. The eggs of Aedes australis and Aedes camptorhynchus (Diptera: Culicidae). Mosquito Systematics 24, 29-39, 1992.

Andrew McFadden Investigation and Diagnostic Centre (Wallaceville) PO Box 40742 Upper Hutt Email: [email protected] Mark Bullians Investigation and Diagnostic Centre (Tamaki) PO Box 2095 Auckland 1015

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