Assigning Risk

WEST NILE SURVEILLANCE

Assigning Risk to an Area or Location

You can assign risk to the possibility of a specific location bearing west nile or becoming a potential breeding spot in any of several ways. This risk may focus on the people, by defining risk areas based on the density of people residing in an area which has a high likelihood of resulting in humans being bitten, and by chance, infected with west nile. This method of assigning risk of course assumes that the high risk individuals are active and outside their home long enough to be bitten, by just the right species of mosquito, at just the right time for them to become infected. In theory, this method of defining risk works best as a method applied to case surveillance practices, animal or human. It provides with little detail on what we need to know about the ecology of the vector though.

The second way we can assign risk for an area regarding west nile infection is to base this risk assessment process on the infected host or west nile carrier populations uncovered in the field. This requires that many corpses (usually crows) be found, reported, collected, mailed, and then tested for west nile in the lab. In general, this is a retrospective study of west nile location at best due to the amount of time it takes to test animal blood for west nile virus infection (often up to 3 weeks). And even then. the animal has carried the west nile now for several days to a week, meaning that the location of its demise may not even relate to where it was initially infected. Typically, this review of dead animal host data serves more as an alarm for locating areas possible carrying infected mosquitoes a few weeks back. These areas then have to undergo more detailed trapping, field inspections and monitoring in order to be documented as actually being a west nile nidus (disease nest).

Interestingly, during my time of trapping and collecting, I found that relying upon the host-related data demonstrated some other major spatial flaws related to human activities and behaviors. The fact is that most dead animals that are potential west nile carriers are not reported, and with time, much less appropriately collected and prepared for testing and/or adequately tested by the lab due to their condition upon initial discovery and receipt by the lab. Another factor limiting this part of west nile research is the cost of performing the expensive laboratory tests required for this review of the west nile virus distribution. Whereas at first, all suspected avian species were tested in order to rule out other potential hosts for the most part, with time, this type of activity became limited by costs and the times and numbers of sample collected reduced in number. And as the lab costs increased, west nile became old-hat to some and the much-needed financial resources began to dissipate.

So, during the first two years of testing, the potential animal hosts to be submitted underwent an increasing amount of scrutiny before being shipped, fewer were being tested. The data related to these collection of dead crows served three main purposes: to document another aspect of west nile ecology locally by determining the human and natural ecologic, and physiographic requirements for their presences, to document the behavior of crows as a social species, and to document the spatial distribution of crows whenever they are found to occur in aggregates with positive testind results for the west nile immunologic tests performed on their tissues.

The limits to this infected host-carrier method of surveillance of west nile are obvious. Human geography and human activity play a key role in how the samples are found and then defined. People determine whether or not they will undergo the adequate testing in time for west nile history, regardless of where nature leaves her clues. For this reason in combination with the fact that the documentation of west nile infected crows is really a retrospective view of the virus history locally, other methods had to be developed to better undnerstand the spatial behaviors of this disease and its vectors, in order to predict west nile activity and behaviors in the years to come. Whereas traditional methods of prediction seem to focus mostly on meteorological features, my work focuses on ecology.

The third method of monitoring and testing west nile surveillance involves simply remaining active in the field in search for the actual vector species. The goal of this approach should be determine the west nile virus-vector-host ecologic requirements, and then determine whether or not it species of mosquitoes already present have a history of exposure to west nile virus. This is the primary method in which we can monitor and test for the west nile virus in all of its possible environmental settings. Although such settings are very much impacted by host-animal behaviors, and of course human behaviors and lifestyle, the best way to determine whether or not west nile is in a surrounding community is to regularly monitor the vector ecology as well as engage in secondary activities like monitoring potential host and animal/livestock populations, and on occasion monitoring any animal or human cases that erupt. The spatial (human ecological) distribution of each of these features has to be mapped and then statistically analyzed.

Trap Site Ecology

Dead Crows and larva Counts on provide a small part of the Ecological Picture for West Nile Ecology

Three Culex species were evaluated spatially in terms of their distribution; in theory we are overreacting by stating that any site with these species is high risk

Temporal behaviors of Vectors

Using GIS, a temporal model of vector activity and behavior can be produced as well. This kind of analysis can be correlated with local meteorological events, in particular recent rain and temperature-related changes, and then be used to identify what changes in the weather are required to get significant numbers of hatches and swarms to develop. Of course, this is really a standard in much of the current west nile research typically engaged in nationwide in various state and county settings. We researchers would all like to understand the meteorological requirements for west nile to blossom in our communities.

It is important to recognize however that other factors play an important role in potential west nile spreading vectors. The biodiversity of vector species has the ability to prevent the spread of west nile, more than incresing the likelihood it exists locally, in spite of increased positive testing species density. (See the Fishkill Creek Flood Plain Study a little further down this web page.) More impartantly, for some species, the most important determining factor in their vector-related behavior has to do with the phytoecological activities, findings supported by this use of the canopy light penetration study.

A chronology of trapping activities in Northern Dutchess County

Using census data on the most susceptible populations to define High Risk Areas

Most forms of surveillance used to evaluate population risk focus on census tract, block group and block data to identify regions with the highest risk populations for west nile and its potential for illness and death. The population of highest concern locally were residents >65 years of age or <18 years of age. For this reason, special attention was drawn to the retirement settings and communities and to blocks with evidence for larger numbers of children per household or family in order to find areas with large numbers of these potential victims.

One goal of this in part was to increase applications of standard preventive activities, including public health education programs on west nile and the promotion of mosquito-prevention and killing practices such as the elimination of stagnated water sources and the administration of bacillus thuringiensis to the most potentially productive sites in order to kill mosquitoes during their waterbound larval stage. Another goal of this monitoring process is to continue ongoing surveillance of specific locations for which we have reason to be concerned should a human-borne west nile case be reported (see later on this case).

To define those regions of highest risk, potential mosquito sites and the related trap locations were reviewed, resulting in the following maps.