Viruses must pass a number of barriers to be transmitted by a mosquito. First the virus particles must be present in the blood of the reservoir (human or other animal) in sufficient quantity to be acquired by a female mosquito. The blood is first pumped through the mouthparts and esophagus and enters the crop of the mosquito, an internal storage sack that is lined with cuticle. The cuticular lining is relatively impermeable to both nutrients and virus particles. Blood slowly is passed out of the crop into the midgut where most absorption occurs.
If virus survives the digestive enzymes and environment of the gut it faces further barriers to transmission. The first barrier is a peritrophic matrix made of protein plus chitin that surrounds the blood meal. Many viruses and microorganisms will adhere to the membrane and be excreted. An infective virus must pass through the peritrophic matrix and access the cell layer of the midgut that separates the lumen from the hemolymph of the insect. The virus must then find a path through or around the midgut cells. Often that path entails a complex series of interactions with proteins in the cell membranes of the mosquito cells. If successful, the virus move into the hemolymph.
In the hemolymph, the virus must travel to the salivary glands and cross more cell membranes to enter salivary gland cells. Many successful mosquito transmitted viruses are capable of infecting and reproducing in cells of the mosquito salivary gland. The virus particle must be able to leave the salivary gland cells and enter the mosquito saliva. The successful virus particle must survive a mix of digestive enzymes and be able to interact with cell receptors in way that allow the virus to move from midgut to salivary gland.
Additionally, the virus must be able to replicate in cells of the mosquito. A sufficient number of virus particles must enter the host with the mosquito saliva to have a high enough probability that some will escape the host’s defenses.
Despite the many barriers, sufficient viruses evolve the necessary structure to cause disease and human misery. The complex process of virus navigation through the mosquito is being explored by scientists hoping to find ways to improve the barriers to transmission.