Arboviruses differ from other viruses in their need to replicate in both 
vertebrate and invertebrate hosts. The invertebrate is a blood-sucking arthropod 
that is competent to transmit the virus between susceptible animals. Arboviruses 
transmitted by ticks must adapt to the peculiar physiological and behavioral 
characteristics of ticks, particularly with regard to blood feeding, bloodmeal 
digestion, and molting. Virus imbibed with the blood meal first infects cells of 
the midgut wall. During this phase the virus must contend with the heterophagic 
bloodmeal digestion of ticks (an intracellular process occurring within midgut 
cells) and overcome the as yet undefined "gut barrier" to infection. Genetic and 
molecular data for a number of tick-borne viruses indicate ways in which such 
viruses may have adapted to infecting ticks, but far more information is needed. 
After infection of midgut cells, tick-borne viruses pass to the salivary glands 
for transmission during the next blood-feeding episode. To do this, the virus 
must survive molting by establishing an infection in at least one cell type that 
does not undergo histolysis. Different tick-borne viruses have different 
strategies for surviving the molting period, targeting a variety of tick 
tissues. The infection can then persist for the life span of the tick with 
little evidence of any detrimental effects on the tick. Transmission to a 
vertebrate host during feeding most probably occurs via saliva that contains 
virus secreted from infected salivary gland cells. The virus then enters the 
skin site of feeding, which has been profoundly modified by the pharmacological 
effects of tick saliva. At least three tick-borne viruses exploit such 
tick-induced host changes. This phenomenon (saliva-activated transmission) is 
believed to underlie "nonviremic transmission," whereby a virus is transmitted 
from an infected to an uninfected cofeeding tick through a host that has an 
undetectable or very low viremia. Thus tick-borne viruses that have adapted to 
the feeding characteristics of their tick vectors may not need to induce a 
virulent infection (with high viremia) in their natural vertebrate hosts. 
Efficient transmission of tick-borne viruses between cofeeding ticks may be a 
means of amplifying virus infection prevalence in F1 generations infected by 
transovarial transmission.