Arenaviruses cause the South American haemorrhagic fevers, which produce hundreds of cases annually, with a case-fatality ratio as high as 33%. Hantaviruses cause hundreds of thousands of cases of haemorrhagic fever with renal syndrome (HFRS) in Europe and Asia each year, but were thought to be rare or absent in the USA until the appearance of HFRS in 1993. Since 1993, at least 20 additional hantaviruses have been isolated from rodents throughout the Americas; about half are known human pathogens.
Rodent-borne hemorrhagic fevers, among the most dramatic of recently emerging infectious diseases, are caused by two distinct groups of negative-stranded RNA viruses: the arenaviruses (family Arenaviridae) and the hantaviruses (genus Hantavirus, family Bunyaviridae). With few exceptions, each virus in these two groups is primarily associated with a single species of rodent host of the family Muridae. In the specific host, the virus establishes a prolonged infection, which rarely causes disease in the animal. The infected host sheds virus into the environment (in urine, feces, and saliva) for extended periods. These characteristics are key to the transmission of the viruses to humans (by the inhalation of aerosolized virus) and to other rodents (by horizontal and sometimes vertical mechanisms).
Not all arenavirus- and hantavirus-host associations have been described, and some species, such as the white-footed mouse and the cotton rat, are known hosts for multiple agents. Fighting and biting among adult male rodents may be a common mechanism of virus transmission.
Although arenaviral diseases of humans (other than lymphocytic choriomeningitis associated with the introduced Old World rodent Mus musculus) have not been recognized in North America, Tamiami virus has been recognized in association with cotton rats Sigmodon hispidus since 1969, and Whitewater Arroyo virus was identified from wood rats Neotoma spp. in the southwestern United States in 1995. The potential of Whitewater Arroyo virus for causing human disease is under investigation.