The mosquito salivary glands have to transmit the virus from the blood stream. Here's what they say in the science paper abstract:
One important step in the transmission of most vector-borne diseases is the entry of the disease agent into the salivary glands of its arthropod vector. The salivary glands of blood-feeding arthropods produce a complex mixture of molecules that facilitate blood feeding by inhibition of the host haemostasis, inflammation and immune reactions. Pathogen entry into salivary glands is a receptor-mediated process, which requires molecules on the surface of the pathogen and salivary gland. In most cases, the nature of these molecules remains unknownhttp://www.ncbi.nlm.nih.gov/pubmed/20621627
Short answer: nobody knows.