A new study underscores how local conditions can influence the spread of disease. Researchers at Mississippi State University did an in-depth statistical analysis of West Nile virus (WNV) encephalomyelitis incidence across the state in 2002, looking specifically at how rainfall and land cover—the amount and type of vegetation, water sources and areas developed by humans—correlated to reports of the disease at the county level.

A zoonotic disease first identified in this country in 1999, WNV is now considered endemic to the United States. Carried by birds—primarily crows and jays—WNV is transmitted by mosquitoes. Most horses exposed to the virus don’t become ill, but among the small percentage that do 30 percent die or are euthanatized. Clinical signs of infection include weakness and incoordination, muscle trembling and fever.

In analyzing data from the United States Department of Agriculture, the United States Geological Service and state climatologists, the researchers discovered that the risk of WNV was lower in counties with greater diversity in land cover and was inversely proportional to the amount of rainfall the previous year. In short, the counties with a greater variety of vegetation and landscape had fewer cases of WNV the following year.

“Local conditions are extremely important to the disease-infection rates,” says Guiming Wang, PhD. “Those conditions influence the abundance and composition of birds [a host for WNV] and mosquitoes [a vector] as well as WNV prevalence in birds and mosquitoes.”

The researcher notes that dry weather conditions probably reduced the number of predators that feed on mosquito larvae, such as fish, allowing the insects to flourish the following year.

Wang adds that land that has a mixture of environments, such as forests and plains, as opposed to a single type, had a greater variety of birds, which prevented the species that spread WNV from dominating the area.

Wang notes that the data was collected when WNV was first spreading across the country. At that point, horses had no natural immunity to the virus and vaccination was not yet widespread. The first WNV vaccine received a conditional Food and Drug Administration license in 2001; that product was fully approved in 2003 and four WNV vaccines are now available.

Even though WNV vaccination rates have greatly increased, Wang advocates continued monitoring of environmental influences on the disease’s spread at a local level to help authorities determine the best use of resources. “The county-level work is necessary, in my opinion, for forecasting equine WNV risks and allocating limited resources by state agencies to prevent or battle WNV outbreaks,” he says.

Reference: “Effects of weather and landscape on the equine West Nile virus infection risk in Mississippi, USA,” Geospatial Health, November 2015

This article first appeared in EQUUS issue #461