The genus Hantavirus, of the viral family Bunyaviridae, currently encompasses more than 20 genotypic variants; it is thought that other, as yet unidentified strains also exist. Hantaviruses are transmitted by rodents in the family Muridae. Hantavirus strains can be categorized into those that cause hemorrhagic fever with renal syndrome (HFRS) and those that are the etiological agents of hantavirus pulmonary syndrome (HPS).
HFRS and HPS differ primarily in terms of target organ and disease severity. Occurrences of HFRS have been localized to Europe and Asia, where hundreds of thousands of cases each year yield a case fatality rate of up to 15%. HPS is localized to the Americas, where hundreds of cases are reported annually with a 50–60% case fatality rate. The dominant feature of both HFRS and HPS is vascular dysfunction, thought to be causally associated with the immune response rather than with the direct cytopathic effect of the virus.
Much remains to be learned regarding hantavirus pathogenesis in humans. Largely because of this lack of knowledge, no vaccines are currently available in the United States for HPS-producing viruses. A vaccine for one type of HFRScausing hantavirus has been offered to laboratory workers at the U.S. Army Medical Research Institute of Infectious Diseases. In addition, an inactivated (noninfectious) derivative of an HFRS-causing virus has been used to make a vaccine that is widely used in Asia; however, no vaccines for other HFRS-producing agents are currently available in the United States.
As causative agents of hemorrhagic fever, viral strains that produce HFRS are classified as Category A potential bioweapons; genotypes that produce HPS are categorized as Category C emerging pathogens. Although nonpathogenic hantaviruses exist, they are not the prime focus of this discussion.