West Nile Surveillance: Classifying trap sites and surveillance study locations.

There are numerous reasons why traps are set to monitor or engage in surveillance activities focused on mosquito populations. In recent years, the use of mosquito trapping to survey mosquito ecology has been engaged in primarily for the purposes of west nile surveillance. The purpose for setting these traps includes an attempt to monitor and circumvent any possibility for west nile coming to a populated region, and secondly to better understand the distribution and envrionmental habits of its primary local vector species Culex pipiens-restuans, a hybrid of C. pipiens and Cx. restuans, and other mosquito population behaviors.

During the earliest years of west nile surveillance in lower New York state, another reason related to mosquito trapping was to hopefully uncover one or more sources for the west nile, prevent the spread of this disease, and based on our better understanding of its ecology, hopefully eliminate it from the local list of zoonotic diseases. The latter has not been the case, however, at least for now. Due to mosquito species behaviors and activities, even though west nile originated in another part of the world with a totally different set of mosquito populations, the west nile brought to America was able to find its niche of natural habitiat in very short time. It may even be that the predisposing factors for west nile become endemic to this region may have already been present along with predecessors to the west nile virus and insect carriers years ealier, only undetected at the time due to low priorities set for the local mosquito surveillance activities and the limited likelihood that the vector carrying this disease would chose to bit humans, much less cause west nile.

One of the first theories popular for why west nile came to New York was the introduction of a new species to the local ecological setting. Ochlerotatus japonicus was felt to be the culprit of this disease diffusion process, with a considerable amount of spatial evidence uncovered during the first several months of west nile infected species (mainly crows) pointing to some animal source, such as a zoo, research animals station, or international exotic animals shipping company responsible for bringing in an infected animal for sale as a pet. In spite of several years of trapping however, the ability to find japonicus species infected by west nile in this New York area have been unfruitful. Nearly all evidence points to the Culex species as its chief carrier and vector to other animal species and human.

Another popular reason given to west nile introduction into New York is the typical avian migration behaviors found with some game birds, water fowl, and even some raptors for the region. In particular, with game birds and waterfowl, it seemed possible that the birds could have been infected further south where mosquitoes, especially Ochlerotatus japonicus , are more abundant and likely to thrive throughout the year. It was possible that through an in-migration of these aves that several or more would either carry the mosquito with them imbedded in their feathers, or already be infected by a positive testing carrier much further south. In either case, the bites received by these mosquitoes in the previously uninfected New York settings could ultimately lead to the further spread of west nile virus into any of its subsequent victims given the proper ecological and environmental conditions.

Whatever the case, the natural events were responsible for west nile coming into New York, in association with the region’s long history of poor mosquito management, in association with thriving mosquito-breeding habitats, poorly managed wetlands and poorly monitored undeveloped lands, the west nile virus was able to establish a permanent ecological and home-setting by end of the first complete summer of its stay in 2000.

In spite of the fact that west nile was more than likely brought in by the Oc. japonicus species, evidence for infected japonicus pretty much remains lacking. Significant evidence found, however, for several ecological allies of Oc. japonicus were found to reside in similar ecological settings–3 Culex species.

What makes the Culex species stand out as a probable source for west nile is their ability to reside in fairly unnatural water settings. They are often found where few other mosquitoes will lay their eggs and thrive. They can be found in extremely foul water settings, such as the garbage-ridden areas located near old industrial settings, often in places close to human living and work settings. The best settings for both Oc. jap. and the Culex spp. are small containers such as tires, illegally disposed of gutters, old soda cans left in the woods, and even old leather shoes. This meant that some of the best human ecological settings for west nile carriers were areas where both raptor birds frequented like crows, blue jays, and hawks, and where people resided and lived–in fairly “dirty” living environment settings next to well populated areas rich in potential disease hosts.

Trap Sites History, by Year of First Use for Each Site

Trap Sites History, by Year of First Use for Each Site

To review the Dutchess County region, a total of 183 trap sites were identified and used. During the first season of trapping, much time was spent larva-dipping and setting traps in these ideal ecological settings for mosquitoes, like the edge of a large lake, a wetlands region covering a former farmer’s field, or a small marsh located in the middle of a neighborhood development. Some of these sites were selected due to their significant differences in ecologic and landuse conditions (i.e. a highest elevation residential area uphill from a low elevation creekbed). A number of sites had trap sites set at high elevations, and of considerable distance from the nearest stream or wetland, just to determine how far mosquitoes would be willing to fly at night in search of their victims.

During the second year, traps were set based on reports filed about mosquito pestilence activities and the discovery of a dead bird possibly infected by west nile, with the goal of trying to equally distribute the traps across the entire county in order to engage in surveillance in the majority of the townships for the county. This enables some geographic features like major creekways to be tested in several areas, but due to equal distribution across the county, used up considerable time due to the need to travel between sites.

By the third year of this study, it was decided to set multiple traps at the same trap site for places where data had to be gathered, due to a recent positive west nile test result or a documented case history. This process was already in part underway due to the occasional failure of traps that occured due to technical or human-related issues, such as failure to reconnect a battery, failure to adequately recharge a battery or bring a replacement, or placement of the trap in an area unexpectedly impacted by heavy amounts of rainwater. During the second year of this field work, the issue of battery failure led twin traps to be set at specific positive testing sites to ensure that once species were captured they remained in the trap (the batteries run a fan that serves to drive and then keep them in the net). This immediately led to the idea of setting multiple traps at a particular location, especially positive-testing sites, giving way to the development of plants to test single sites in numerous places in order to assure first, that we increased the chance of capturing the positive testing mosquito expected to be around due to a positive testing animal or human case, and secondly, because this offered the opportunity to study the spatial relationships between places and species within a given research area. In short time, this led to the recognition in the field of sites viable for possible multiple-testing, and the first sites developed for testing the ecology of mosquitoes in areas immediately adjacent to possibly-infected livestock and human cases.

In the latter years, a number of these sites were set to once again for a more detailed study the ecology of west nile in relation to the various mosquito species of the region. These sites are easily recognizable on the map because they form lines along water and creek edges, and were used to determine spatial ecologic differences between the two largest creeks of the county–the Fishkill Creek and the Wappingers Creek. Another study I engaged in was to determine local vector species ecology in relation to the shoreline features of the Hudson River, an estuary water feature quite different from the more inland water settings in the county. On still another series of field trips, a series of wooded lakeshore areas were identified for ecologic analysis throughout the county. On yet another occasion, attempts were made to document mosquitoes spatially across the various ecological settings they frequented, focusing mostly on riparian floodplain and periurban settings. In terms of human ecological features, periurban settings with and without major wetlands or water-related features were analyzed, including a central urban setting in which catch basins and roadside drainage structures were evaluated for larva-adult vector presence and activities, and a periurban setting where significant natural settings existed for other non-infective species should have been found as a form of competition with the documented west nile carriers.

In spite of these aggressive attempts to better document vector ecology, most of the trap sites selected during the years were chosen due to some west nile related finding such as the discovery of a positive-testing dead host species (crows, blue jays, hawks), the documented presence of a horse infected by west nile or any of several other viruses spread by mosquitoes, proximity to a high risk group such as a retirement home of children’s camp, or the combination of a set of ecological, reported dead bird and high risk family setting at a particular location reported to the public health department. The presence of human cases remained a rare event leading to the selection of a trapping site location at this time. In two cases, local imports of malaria cases intp an international summer camp led to special studies of Anopheles to be performed in a particular area. Two or three west nile human cases were also evaluated ecologically in relationship to the local mosquito populations.

The following kinds of sites were established, with sites clustered at times for the various ecological and transect studies performed using GIS. The most common sites are of course water-related, but some are urban-based, to determine if infected species are capable of residing in large numbers within what seems to be a fairly uninhabitable setting, as well as within the actual culverts and catchbasins of smaller urban-like locations.

Note the major risk features evaluated/survey for in the field with regard to west nile ecology. Aside from the urban-related ecological feature, most of the highly active sites had 5 or more of these primary ecological requirements for heavy trap production to result. This does not mean they are the most likely sites to see west nile erupt, rather, they serve as a starting point for better understanding west nile-mosquito-host-related ecological requirements.


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