Ecology of Tick-Borne Disease and the Role of Climate

Crimean-Congo hemorrhagic Fever (CCHF) is unusual among vector-borne zoonoses in that, as well as the virus being transmitted to humans if they accidentally intrude on the natural transmission cycle by being bitten by ticks, the virus is also commonly transmitted directly to humans from its natural wildlife and livestock hosts (and even human patients) via contact or contamination with infected tissue or blood. The epidemiology of human disease therefore follows a pattern that is greatly influenced by employment practices that bring humans into close contact with livestock, whether alive or, even more riskily, dead. In tanneries, for example, ticks that detach from hides may reattach to humans, potentially transferring the infection much more rapidly than via the conventional transmission route that involves a long delay between the engorged infected tick of one stage and the feeding tick of the next stage. Nevertheless, like all other vector-borne diseases, the presence and persistence of zoonotic foci of infection depend on biological and ecological relationships between three very different kinds of organisms: virus, ticks, and vertebrates. These three must interact not only physically and biologically to permit each complete act of transmission, but also ecologically to permit continuing cycles of transmission. Although the focus of epidemiological interest is on the pattern of human cases, explanations for the described distribution and abundance of infections must come from understanding the underlying ecological processes.