A number of times calls were received regarding specific mosquito or west nile issues that ended up requiring a small area analysis in order to better understand the problems at hand. From a public health perspective, not only are human west nile cases a cause for concern. Equally important to the area are livestock-related mosquto-borne cases, be they west nile or some of the form of disease transmitted by mosquitoes, and the problems associated simply with excessive mosquito swarms impacting an area where significant numbers of people reside.
Reviewing livestock-related cases, in particular equine (horse) related issues is important to the region due to the substantial role this industry plays in the local economy. This county is positioned at a fairly managable distance from downstate facilities (i.e. NYC and Westchester County) which bring to it ample amounts of financial support in the form of week-end getaways, indoor and outdoor recreation facilities, touring sites and scenery, examples of unique regional cuisines, local wineries, independent organically-focused family-run farming businesses, a regionally famous annual county fair, numerous historical landmarks and visitor centers, a traditional fox-hunt facility that was privately run, and numerous racing horse breeding and training facilities.
Before west nile made its way to New York, the most important mosquito-related health issue actually pertained to the local equine industry. Several horse deaths occur per year in the local setting due to an encephalopathy related to the equine viruses transmission by mosquitoes in this region. According to several local breeders I spoke with about this issue, this had the potential for creating a significant impact on the local economy due to the significant amounts of income generated by these breeding and training facilities and the relationship of these businesses to several fairly large race-horse operations, in Saratoga, Monticello, and several tracks in New Jersey.
West Nile Surveillance: identification of primary location for a positive testing vector species (Culex pipiens-restuans) infecting local Corvus (crow)animal host populations.
The simplest way to do this is to just focus on the suspected vectors, which in this case are Ochlerotatus japonicus and three Culex species. To do this however requires that all captured mosquitoes be identified and either included or ruled out from your study. For this reason, it is best to document all species captured. More importantly, as the results of a later study suggest, interactions between different vector species also play a role in determining whether a region is statistically more likely to infect a local human resident. For the most part, NIH/CDC specialists’ recommendations are that all species be recorded and monitored, so perhaps this is a moot point. However, it helps to note that by monitoring all competitive species, we also develop a much braoder understanding of mosquito species as a whole, so be it.
Small area analysis of properties surrounding a positive testing human case.
A positive human case was documented causing the public health department to request surveillance of the woodlands adjacent to the home of the infected victim (M., approx. 65 yo). Field inspections revealed several suspicious microecological settings where a vector carrying west nile may reside, mate and hatch. Traps were therefore set right adjacent to two water sources: a small temporary pond situated in a woodlands area with light penetration suggesting a prime setting (150-350) for vectors. Another site was a small brook emptying the temporary pond which flowed from the forest floor to a small field at the edge of the road. Hiking deep into the woodlands in several directions from the west nile victim’s home, no other water bodies or drainage routes could be located. Traps set around the home resulted in catches that suggest only species natural to this woodlands setting could be captured. Although this does not suggest that species responsible for west nile may not have resided inthe area several weeks earlier, the time of the year in which the west nile infection was documented (early June) and the numbers of other species abundant in the immediate setting suggested it was unlikely the local vector populations bore west nile infected mosquitoes. It is possible that temporary residence of these vectors existed, but were no longer alive in the immediate setting. More importantly, the predominant species in the area were primary frog-biters, not human-biters. This small area study based on local species ecology led to the elimination of suspicions that a local positive-testing mosquito pool existed that was responsible for west nile. This remained the finding for the rest of the year and into the following spring season following the initial field inspections of specimens that survived the winter.
Locating sources for an exceptional Ae. vexans population swarm late in the season using GIS grid mapping techniques in association with land use history and field surveillance information.
In late August we received a call that an exceptional swarm of mosquitoes had been effecting an area for about two weeks. No west nile cases were ever reported for this area, and upon the initial field inspection it appeared the predominant species was Aedes vexans, a species easily recognizable with field inspection and not associated with west nile locally. At first the cause for the swarm was uncertain. Possible sources included numerous century old drainage system remnants in the form of cisterns and catch basins. Inspection of these led to no discovery of a potential source for a swarm. Immediately adjacent to the location was the estuarine Hudson River, this too had already been proven an unlikely source for the positive testing vectors of the area based on previous studies done all along the 60 miles of river-edge (this is another study reviewed later on this page). At the river’s edge there was a sewage treatment facility which prepared the well-monitored runoff and sewage waters for their release into the river. The wooded areas revealed no unexpected water surfaces, such as run off rivulets, creeks, ponds, small glacial ice ponds, puddles, etc. The absence of an adequate source for approximately 1/4 to 1/32 mile around the apartment units complaining of these swarms suggested it was, as the locals suspected, a recent spillover and release of sewage water from a century old building into the adjacent grassy field in front of the building. To inspect the actual site around the building, a sanitarian officer had to be contacted. To obtain the evidence needed to obtain the support of the county sanitarian officer, traps were set all around the edges of the property and along the fence which surrounded this century old sewage treatment building. Traps were also set in increments walking away from the site, approximately every 1/20th to 1/10th of a mile in both a northward direction and a southward direction. (Fortunately, both of these properties were open space and well established recreational areas for fishing and for the use as a city park.) At several sites along the river edge, traps were also set linearly inland, up to about 1/3 mile from the river edge, along a path and into an open woodland/field setting. Traps were set in the appropriately lit settings; even a trapsite located in the centrer of the field to the south was placed beneath a tree situated in the middle of this area. The first night of trapping resulted in 1000+ samples of Ae. vexans. This enabled us to contact the sanitarian officer, who met with us at the entrance to the property and allowed us to check this location. The immediately adjacent grassy area (2′ height fescue and such) was covered with a light brown layer of dried sludge, extending about halfway up the grass blades. The source for this spill was obvious (overflow within the building, spilling out its lower windows) and the cause and source immediately recognizable upon opening a mancover located in the center of this property (access to the water surface required some scaling of its ladder). This area was treated, and the region monitored for the next few weeks. The Aedes swarms persisted for about a month. No other sources for Aedes swarms were uncovered.
Locating possible Anopheles sources for documented Malaria cases at an in-migrant summer camp community setting.
Locating possible sources for local equine mosquito-vectored disease hosts and vectors at four local equestrian settings.
- West Nile Surveillance