Viral hemorrhagic fevers

The term viral hemorrhagic fever (VHF) designates a syndrome resulting from infection with any of at least 30 different RNA viruses from four different taxonomic families (Table 1). Although they differ in certain features, all types of VHF are characterized by fever and malaise, a fall in blood pressure that can lead to shock, the development of coagulation defects that can result in bleeding, and for many VHF agents, high mortality. With the exception of dengue virus, which is maintained among human populations by mosquito transmission, all of the VHF agents persist in nature through cycles of infection in animals. In the past, therefore, the geographic range of each disease reflected that of the reservoir species. Human illness is an accidental event resulting from contact with an infected animal or its excretions or the bite of an infected arthropod. Subsequent human-to-human transmission through contact with infectious blood or secretions occurs with multiple haemorrhagic fever (HF) viruses and can cause devastating nosocomial outbreaks. Pathogenesis in humans, in most instances, only indirectly reflects the mechanisms by which the causative agent replicates in its reservoir host, but high levels of viremia are typical. Treatment is supportive for most VHFs, but progress is being made gradually in developing specific therapeutics. Vaccines are widely available for yellow fever, and recent studies indicate that effective dengue and Ebola virus vaccines are possible.