By Jane P. Messina, Senior Postdoctoral Epidemiologist, Department of Zoology, University of Oxford, United Kingdom.
Crimean-Congo hemorrhagic fever (CCHF) is one of the most widely distributed tick-borne diseases in the world, ranging from southern Russia and the Black Sea region to the southern tip of Africa. It is caused by a virus which is transmitted to humans by ticks, and is considered as “emerging” across the globe, with countries such as Albania, Turkey, and Georgia reporting new infections in humans in recent decades. Human CCHF infection has also been recently reported after long periods of absence in some locations, for example in south-western Russia and Central Africa. The main genus of ticks that transmit CCHF to humans (Hyalomma ticks) are adapted to warmer and dry or semiarid environments, and are found in many parts of Africa, Asia, and Europe. In addition to humans these ticks feed on wild and domesticated animals, which can also become infected with CCHF virus but which do not show any disease symptoms. However, infection of these animals leads to further CCHF transmission to humans, as new ticks feed upon these animals and become infected.
Although human cases of CCHF are relatively rare, there is no widely available safe and effective vaccine against the virus, and severe complications often lead to death. Those living or working near livestock are particularly at risk of infectious tick bites, and those working in animal slaughterhouses are at risk for exposure through infected animal blood. Human-to human transmission is also possible via exposure to infected blood and other bodily fluids, and typically occurs amongst healthcare workers or relatives who have had close contact with CCHF patients. While it is important to take precautions such as wearing protective clothing and using repellents, the need for these measures is often poorly understood in places where disease risk is less certain (particularly in Africa). A good understanding of the geographic distribution of CCHF is crucial when it comes to raising awareness in slaughterhouse and healthcare workers, as well as in the general community. This information is further essential for accurate diagnosis of the cause of a returning traveller’s haemorrhagic fever.
Ebola, Marburg and Lassa fever viruses also occur in Africa and like CCHF, pose the risk of transmission from animal reservoirs and through secondary infections in the community. Creation of detailed databases of occurrence locations is a key first step to appropriately focus resources for hemorrhagic fever disease risk awareness. To improve available maps of global risk for CCHF, we looked at the environmental conditions in areas where CCHF is known to have occurred. We then used statistical models to infer where else transmission is likely to occur but has thus far gone unreported; possibly due to limited public health resources or misdiagnosis with other diseases that cause similar symptoms. The most important (and labour-intensive) step in this process was to compile an exhaustive database of known CCHF occurrence locations since its first reporting. We did this by searching through hundreds of published articles, case reports, and informal online reports of human CCHF infection since the 1950s. The compiled database can now be used alongside high-resolution environmental data (e.g., for land cover or average yearly temperatures) to identify characteristics that put certain locations at greater risk for CCHF transmission. This work is part of a wider initiative to improve understanding of the geographic distribution of important viral hemorrhagic fevers in Africa.
A global compendium of human Crimean-Congo haemorrhagic fever virus occurrence, by Messina et al., is available online at Scientific Data.