The flies are pool feeders. Their saliva, which contains anticoagulants, a number of enzymes and histamine, is mixed with the blood, preventing clotting until it is ingested by the fly. These bites cause localized tissue damage, and if the number of feeding flies is sufficient, their feeding may produce a blood-loss anaemia.
The host’s reaction to fly attacks may include systemic illness, allergic reactions or even death, presumably mediated by histamine. In humans, this systemic reaction is known as “black fly fever” and is characterized by headaches, fever, nausea, adenitis, generalized dermatitis, and allergic asthma.
(From: Wikipedia, April 2017)