Genetic relationship in southern African Crimean-Congo haemorrhagic fever virus isolates: evidence for occurrence of reassortment.

Crimean-Congo haemorrhagic fever (CCHF) is a tick-borne viral zoonosis widely distributed in Africa, Asia and eastern Europe. Reassortment of CCHF genome segments has been shown to occur in nature. We therefore investigated the genetic relationship of southern African isolates using partial sequence data for each RNA segment, S, M and L, and comparing the tree topologies constructed using a neighbour joining method. A total of 21 southern African isolates were studied. The incongruencies which were identified in S, M and L sequence datasets involved group switching implying reassortment for 15 isolates. A higher fatality rate occurred in patients infected with isolates which had apparently acquired M segments from a group in which predominantly Asian strains are usually found. This suggests that reassortment may affect the pathogenicity of the virus.