They drink blood for about 20 minutes, consuming in that amount of time about 8 teaspoons of blood. The animal does not usually wake during the attack because these bats are so small. What is dangerous about the bites is rabies transmission. Most bats carry rabies, so vampire bats do transmit rabies through these bites. Farmers in these areas should be educated about the differences among the various kinds of bats so that other bats are not harmed in vampire control programs.
Bats are very interesting creatures, but they are often feared and misunderstood.
This is unfortunate since many bats help us and we do not even know it. In Tennessee, Brown bats help us by eating mosquitoes and other insects which can be pesky to humans. The more we learn about bats, the more we know how helpful many of them are to us and to the habitat in which we live.