Why do leech bites itch?

Think about a leach as a mosquito. As you know, both leaches and mosquitoes suck blood but what you probably didn't know was that a mosquito injects a disease into your leg when it bites. Now, usually right when you feel it bite you shoo it away. If you would leave it alone it would then suck out the disease that makes you itch. Leaches are the exact same way however their process is much much, much slower!