By all accounts, it’s saliva. They have a separate tube for drinking and another for injecting saliva that numbs and also prevents blood clotting. Because of this system, regurgitation is rare.

The saliva also transmits disease, in fact the saliva makes the infection more effective via suppressing the immune system response and has been shown to attract immune cells to the bite site making it easier for some viruses to ‘recruit’ them and spread faster.