Which of the following diseases involves a significant risk for small mammals?

La Crosse encephalitis is a disease primarily transmitted by mosquitoes, particularly affecting small mammals, especially the eastern chipmunk and the white-footed mouse, which act as amplifying hosts for the virus. Unlike other viruses in the options provided, La Crosse encephalitis has a life cycle that heavily relies on small mammal populations. These mammals become infected when bitten by mosquitoes carrying the virus and play a crucial role in maintaining and amplifying the virus within the ecosystem. The resulting higher viral loads in these hosts increase the chances of transmission to mosquitoes, which can then infect humans.

The other diseases listed do not show the same significant connection to small mammals as amplifying hosts. For example, West Nile virus and St. Louis encephalitis primarily involve birds in their transmission cycles, while Eastern Equine Encephalitis is also more associated with avian hosts. Therefore, the unique association of La Crosse encephalitis with small mammals highlights its significant risk to this particular group.