Tsetse flies are considered a nuisance in The Lion Guard universe by many animals for swarming around animals. The flies take a particular dislike to zebras due to their stripes.
In the Real World
Tsetse flies are dangerous to both animals and humans, not because of their bite but because of the deadly parasites they carry from victim to victim in their saliva. (There are over twenty different species of tsetse fly.) They carry many different types of parasites, called trypanosomes. These are simple life forms and they live in the blood of other animals. One type causes sleeping sickness in humans. An adult tsetse fly develops from a pupa.