This article examines the outbreak of an unknown illness (later thought to be equine influenza) among the horses of Toronto and its subsequent spread as a continent-wide panzootic. Known as the Great Epizootic, the illness infected horses in nearly every major urban center in Canada and the United States over a 50-week period beginning in late September 1872. The Great Epizootic not only illustrated the centrality of horses to the functioning of nineteenth-century North American cities, but it also demonstrated that these cities generated ecological conditions and a networked disease pool capable of supporting the rapid spread of animal disease on a continental scale in localities from widely divergent geographies. This article invites environmental historians to broaden their view of cities to consider the ways in which networked urbanization produced forms of historical biotic homogenization that could result in the rapid and widespread outbreak of disease
