Transmission of arboviruses without involvement of arthropod vectors

Transmission of arboviruses (arthropod-borne viruses belonging to various virus families) without involvement of arthropod vectors has been documented for years, but the reports have not been reviewed systematically. The recent report of West Nile (WN) virus isolation from a hawk in mid-winter in New York (Garmendia et al., J. Clin. Microbiol. 38, 3110-3111, 2000) generated a considerable interest in this mode of arbovirus transmission. In this article, the data available worldwide are analyzed according to the factors involved in such a transmission under natural conditions, mode of infection, virus entry mechanism, administration and efficacy evaluation of vaccines, and significance in agricultural trade and public health. Analysis of numerous reports compiled for this review revealed that peroral and intranasal/aerosol transmissions are very common among arboviruses. The mechanism of virus infections in animals was most extensively studied for intranasal/aerosol infection, confirming two routes of virus spread to central nervous system (CNS), olfactory and hematogenous. To rule out the possibility of asymptomatic, cryptic infection the efficacy evaluation of candidates for vaccines against neurotropic arboviruses should include virus isolation from tissues of not only symptomatic but also of asymptomatic animals that survive intranasal virus challenge. Human activities, such as feeding livestock animals with food containing virus-contaminated meat and assembling a large number of livestock from many geographically-separated locations, have been identified as a cause of spread of some arboviral diseases. Despite numerous laboratory reports, the significance of this mode of transmission of arboviruses under natural conditions was rarely investigated, except for a few viruses important for veterinary medicine.