Host Defenses Interferon is an early defense mechanism. Viremia ceases with the appearance of humoral antibody, which does not alter established encephalitis, retinitis, or renal lesions. The distribution of each disease is determined by the distributions of the vector and vertebrate host. Except for hantaviruses, biologic transmission is by a tick, mosquito, midge, or sand fly vector. Arthropods are infected for life.
Transovarial transmission is common in arthropods.
Wild or domestic vertebrates usually are needed to maintain the cycle. Humans are usually dead-end hosts for all these viruses except phleboviruses. There is a secondary nosocomial spread of Crimean-Congo hemorrhagic fever. Hantaan virus cycles among rodents, probably by aerosol or fomite transmission from infected rodent urine. Human infection is incidental.