We will examine the incidence of outbreaks across two sets of year transitions: from 1999 to 2000 and from 2000 to 2001. For each year transition, we wish to determine whether new "outbreaks" geographically separated from contagion areas are more likely to emerge along flyways. The incidence of outbreaks along major flyways will be compared with those outside of flyways for mosquitoes, birds, veterinary cases, and humans.
The WNV is an Old World flavivirus whose natural cycle involves an avian host and mosquito vector. Since 1996 there has been an increase in the cases and severity of disease in humans and horses, both of which are "dead end" hosts for the virus. In recent history the first outbreak of WNV in the US was reported in 1999 in New Jersey. Subsequent testing and diagnosis over the next three years showed a dramatic increase in the instances and geographic spread of the disease. As of September 2002, the disease was reported in birds from 36 states and in humans from 31 states. The National Wildlife Health Center (February 2003) reported the disease from 193 species of wild and domestic birds. Because of the incidence of WNV in so many species of wild birds, epidemiologists have speculated that migrating birds may be playing a significant role in the spread of the disease.
The sampling, testing and reporting of WNV results has problems. Funding and manpower constraints have possibly skewed the acquisition of samples to areas near other positive areas. For the purposes of this study, the assumption is that where there were no tests, there was no WNV.
1999: First documentation of the disease occurring in the US in a small number of counties in New Jersey and New York state.
2000: The surveillance area includes Alabama, Connecticut, Delaware, the District of Columbia, Florida, Georgia, Louisiana, Maine, Maryland, Massachusetts, Mississippi, New Hampshire, New Jersey, New York, New York City, North Carolina, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, and Virginia.
2001: The surveillance area includes Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin and Wyoming.