Vector-borne diseases in urban environments are a major public health issue in the developing world. The outbreak of West Nile virus (WNV) in New York City (NYC) in 1999 revealed that even affluent societies are not invincible to emerging diseases. Global climate change is predicted to exacerbate this problem. Although there is a great body of knowledge about the biology of vector-borne diseases (including WNV) and transmission dynamics in rural and semi-rural landscapes, the urban ecology of such diseases is poorly understood. Particularly lacking is a comprehensive understanding of how the built environment directly and indirectly affects transmission, spread, and evolution and how urban environments under anthropogenic influence affect disease dynamics. This study will explore three hypotheses:
1. Spatial variation in the built environment leads to percolation-like spread rather than wave-like spread of classical epidemic models.
2. The pattern of annual outbreaks is explained by the interaction of seasonal temperature fluctuations and random variation in precipitation interacting with a dynamical infection process.
3. Evolution toward avirulence to humans is a byproduct of evolution toward avirulence in birds and/or adaptation in mosquitoes.
Each hypothesis will be addressed by theoretical models fit to data on WNV outbreaks in NYC between 1999 and 2006.
Results will identify links between the ecology of the built environment and the public health of nearly ten million New Yorkers. Theoretical disease ecology will be advanced by focusing on seasonal forcing, transmission in urban environments, and circulation of multi-vector multi-host pathogens. New knowledge of emerging urban vector-borne diseases will be produced by developing an understanding of this disease system as a model. The researchers will work closely with public health officers in the New York Department of Health and Mental Hygiene to develop recommendations and ecologically-based risk maps that will be useful for controlling WNV in NYC. The research also will include training of two undergraduate students, one graduate student, and one postdoctoral researcher.
The findings of our research have lead to greater understanding of the dynamics of West Nile virus in New York city in particular, and may also be generalizable to outbreaks of other vector-borne diseases in urban environments. We developed a mathematical model showing a link between land cover pattern and the spatial spread of West Nile virus in New York City. While standard models predict that a disease will travel at a steady or increasing rate of speed as it spreads, in the case of WNV in New York City, observation of actual outbreaks told a different story. Rather than remaining constant or accelerating, real outbreaks slowed down before reaching the edge of the city as the result of highly developed landscapes that are inhospitable to the mosquito vectors of the virus acting as barriers that the disease must travel around. The findings show a pattern of deceleration that has not been described before. Another major finding of this project was the identification of mosquito species that act as the primary vectors of West Nile virus. By combining information about mosquito traits from published literature with data from mosquitoes collected in New York by the Department of Health and Mental Hygiene, we were able to determine that species in the Culex genus were most likely to be responsible for the spread of West Nile virus, while also confirming earlier results that the same species of mosquitoes spread the virus between bird and from birds to humans. Looking beyond New York City, we demonstrated the connection between the life history of regionally different major mosquito vectors and the per capita incidence rate of total West Nile and neuroinvasive cases regionally and nationally at the county and state scale, through their association with different land-cover types which are distributed differently throughout the US. Taken together, the results of our work can be used to better understand West Nile virus and other mosquito-borne pathogens and potentially lead to better prediction of future outbreaks. One goal for future work will be the development of real-time "early warning systems." Our results could also help public health officials more efficiently target disease control efforts. Training and learning opportunities were made available during the course of this project for both current and future researchers. The project involved three post-doctoral research associates, one graduate student, five undergraduate students, and two high school math teachers. In addition, one of the high school teachers used his experience on the project to develop a teaching module that was later used in his high school calculus class as an example of application of mathematical approaches to solving biological problems.
