Salivary Gland

A salivary gland is an organ that secretes saliva into the mouth or onto the mouthparts. In humans, saliva moistens food and starts digestion, while in blood-feeding insects such as mosquitoes the glands produce a mixture of proteins that helps the insect to feed, and that disease agents can use to enter a host.

People have three pairs of major salivary glands, the parotid, submandibular and sublingual, along with many small ones, and together they produce roughly one to one and a half liters of saliva a day. Saliva contains water, mucus, antibacterial proteins and the enzyme amylase, which begins to break down starch. It also helps protect teeth and speech.

A female mosquito has a pair of glands, each with three lobes, in the front of the thorax. When she bites, her saliva carries compounds such as apyrase, which prevents platelets from clumping, and molecules that widen blood vessels and dampen local immune responses, so that blood flows freely. These proteins can also change how a host responds to the infection that follows the bite.

After a mosquito takes an infected blood meal, an arbovirus must cross the midgut, spread through the body and infect the salivary glands before it can be passed on. The time this takes is the extrinsic incubation period. The malaria parasite Plasmodium falciparum also gathers as sporozoites in the glands. Species such as Aedes aegypti and Culex transmit disease in this way, and salivary proteins are being tested as targets for vaccines.

• Extrinsic Incubation Period — the delay before a mosquito can transmit
• Arbovirus — viruses that use the gland to spread
• Aedes aegypti — a major disease-carrying mosquito