PPT Slide
Tick-borne flaviruses are pathogenic for humans and some animals. Some strains are more virulent than others but even the most virulent viruses are unlikely to produce high fatality rates. These viruses can infect via the alimentary tract and also when inoculated intranasally into experimental animals. Presumably, concentrated aerosols or high virus concentrations delivered as a powder contaminating food would be infectious.
TBEV are excreted in the urine and faeces of experimentally infected animals but it is unlikely that this form of virus would provide an efficient route of infection for humans. Perhaps their greatest weakness as biological weapons is the fact that they are normally transmitted to vertebrate hosts via the bite of an infected tick, and the natural habitat of ticks is the forest or moist thick grassy vegetation as found on uplands.
This means that humans and even most animals would be a dead-end for virus transmission because few humans are exposed to the bite of a tick.
Another important factor is that these viruses are all antigenically closely related. Therefore, immunity against one strain is likely to produce cross-immunity against the others. Moreover, in endemic regions there is a reasonably high level of immunity among the indigenous viruses.
Gritsun et al., Antiviral Res. 57, 129-146 (2003)