The Mosquito Population
Introduction. The Biogeography and Spatial Behavior of Disease.
There are four levels of understanding required for defining and predicting the spatial behavior of a vector-host-human-borne disease like west nile. First, we need to understand the disease vector itself–the mosquitoes–and the differences between vectors species and other non-vector species of mosquitoes residing alongside them. Second, we have to understand the behavior of each species of mosquitoes independently in relation to the various physiographic (physical-spatial) features that define their surrounding environment, including climate and weather patterns and local hydrographic, pedologic (soil), topographic and phytogeographic (spatial plant) features. Third, we have to understand the ecology of these environmental settings as a whole, detailing how and why the mosquito survives in this setting, how it acquires the disease-causing pathogen for west nile, and how and why an infected mosquito comes in contact with it potential host victims like the crow or the human. The goal of this spatial approach to understanding west nile behavior is to come to a better udnerstaedning of the disease-causing organism itself–the pathogen–and what it does to retain its place in the local ecological setting and infect human victims.
The fourth level for understanding diseases like west nile focuses on the human ecology of this event, which requires a description of the behaviors of the pathogen, vector and host populations in relation to human living settings. There are a number of ways in which interactions between people and the environment can help to promote the spread of a disease. In the anthropic (mankind-centered) sense, a disease is not a disease until it is able to affect or infect people. A “disease” may affect or impact people by killing off other organisms we have to come to rely upon, such as an orchard fruit-bearing plant or an ecologically important invertebrate species, or a disease can impact us directly by infecting us and impacting our chances for survival. When a disease takes place in a natural setting, regardless of human interaction, it is termed zoonotic or animal- based. The subsequent human-infecting form of this disease if it arises from a zoonotic form is termed anthropozoonotic. The West Nile fever in humans is an anthropozoonotic disease. It originated as part of a natural ecologic setting (presumably the West Nile area of Africa), from where it spread to other regions where over time it underwent the right genetic changes making it capable of infecting humans as well as other mammalian and non-mammalian species in other countries.
To study the spatial behavior of a disease like West Nile, we have to research it at all levels. In the past, the research of many anthropozoonotic diseases focused primarily on the human ecological level. The goal was to simply find the primary source for the contagion and them eliminate from our living experience. Scientists then try to better understand the cause for this disease by working backwards through the other non-human-based, natural forms of this disease. For the most part, the research of west nile has focused primarily upon two environmental features–climate and weather conditions. A considerable amount of time was also spent studying the long term migration of this disease, relating its spread pattern mostly to bird migrations. The ecological requirements for west nile in the United States have been minimally reviewed, with little to no evidence for reviews made using a natural ecological means for defining the sources for mosquitoes. For the most part, standard landuse-related methods of analysing west nile have been used, with several superficial attempts made to interject ecological information into such projects.
The documentation of vector type and behavior in the research area was the first task of this work on West Nile. The information on mosquito captures (for both adult and larva forms) was used to develop a better understanding on the species themselves, followed by an understanding of their spatial distribution in the research area in relation to both phytoecological and landuse types.
To better understand the impact on various spatial features on west nile, we first develop a database in which we can document the spatial pattern of the disease spread. We then try to relate the disease or disease-causing organisms (pathogen, vectors and hosts) to such things as weather patterns, host behaviors, and vector activities such as mating, swarming, breeding, egg laying and blood-feeding. Once we develop a better understanding of these natural behaviors of mosquitoes, we can then use this information to develop a better understanding of the disease they carry–the West Nile. We then follow this research with projects designed to provide us with a more complete understanding of the natural and human ecology of this mosquito-borne disease. The benefit of studying the ecology of a disease vector like the mosquito is that it provides us with an environmental understanding of the vector species involved with zoonotic (animal-borne) disease spread behaviors, an ecological understanding of the disease-causing organism borne by the vector, regardless of its infectious state to either man or animal, and a human ecological understanding of the disease as a human population phenomenon.
The following list describes the abbreviations to be used for the potential species expected to be found or trapped during the initial stages of this disease ecology project.
A general review of all captures made during the course of a season provides us with details as to when the mosquitoes in general swarm on a seasonal basis. This information is also of value in research performed on non-species specific behavior in relation to weather changes or non-species specific behaviors relative to place and habitat/landuse type.
The following are trends found reviewing all species captures on a per trapping day basis.
We can review overall productivity of a site. There are, of course, sites where productivity is exceptionally high. For these sites it is important to try to determine the reasons for thier productivity, especially when this success continued over a fairly long period of time (several weeks or more). The two most common reasons noted in this study for high productivity related to ecological setting (biodiversity and species richness) and a highly-impacted, periurban landuse setting.
The year prior to this part of the study, we had developed a fairly good method for identifying the highest risk areas and in particular the most productive landuse forms in relation to the known West Nile-carrying vector species. Much of this summary came about based on the 2002 data, which was reviewed over the following winter. The following chart depicts productivity in 2002; several of these sites can be correlated to the 2003 trap productivity.
To improve productivity, trap site performance was evaluated. The two measures for performance with traps are: 1) number of species relative to numbers of days a trap was set and became productive, and 2) the measure of trap failure (numbers of times you show up to pick up the trapped species and find the battery was dead.)
The following is an example of one of these productivity maps.
Aside from trapping, areas are assessed for larvae activity. These larva sampling activities required a great deal more time and aggressiveness that the trapping work in order to be successful. The following is an overveiw of the results for the 2003 larva gathering/identification activity.
Applications of GIS to Species Studies
The next 3 maps depict a chronological mapping of trap site use and results; this was based on the sum of the complete counts for the day, for all species captured.
2002 GIS project
During the Winter of 2002/3, a variety of assessments were done of the data to test out the applications of GIS to this project. Several fairly useful types of maps were produced by duplicating the methodology I employed in another GIS project (also at this blogsite), focused on the release of toxic and carcinogenic chemicals from EPA chemical release sites. A complex bargraph placed on the map provides information on sites in a relative fashions–the bars are sized based on the max and min values, which you help to define the final value (size) of the largest bars in the GIS program.
A number of species-specific maps were produced for 2002 depicting basic trap productivity and outcomes. These were done for 1-, 2- and 3-species groups. The first depicts Oc. japonicus average capture results (the primary suspected introductory vector, but not the carrier of west nile as is Cx. pipiens-restuans/PRE.) Positive testing dead crows are also depicted in the Aedes vexans map. The 3 Culex species were mapped relative to crow call density depicted using census block group-based graduated choropleths to depict crow density. For all of these maps, the counts are based on averages over the year and so often seem small due to significant numbers of 0 or non-capture days.
During the month of August, a cluster of positive testing crows was found resulting in the identification of the positive testing mosquito pool about a week later. The technique used to set up traps to find this pool is reviewed elsewhere. This technique required use of Aerial Photography and additional time spent GPS and mapping the wetlands and small creeks/rivulets of this region. Since this site was in the middle of the region where the highest density of Cx. pipiens-restuans was obtained in traps, and where a high density of crow calls were being recorded, this use of GIS became an example of small area analysis. The positive testing PRE could have been found without GIS in this case, and much of the work preparing for the capture required no use of GIS at all. In this case, GIS provided mostly a fast and easy overview of the region around the dead crow site, and through the use of extensions, we were able to define the centroid for the PRE species in the area. This was in close proximity to one particular trap site that could be developed fairly close to the centroid. (The centroid in fact was positions in a small temporary seasonal pool-like area on an open forest floor, covered by a shrub canopy, holding about 2 to 12 inches of water.)
2003 GIS project
2003 began with the successful retrapping of the positive testing PRE at the sites identified using a centroid-producing extension for the GIS.
The next series of maps were produced by the use of bars to depict the individual trap site productivity at a per species level.