Nagana, also known as African animal trypanosomiasis, is a parasitic disease caused by protozoan parasites of the genus Trypanosoma. These parasites are transmitted by the bite of infected tsetse flies (Glossina species). The disease affects cattle, horses, pigs, goats, sheep, and other livestock, as well as wild animals.

Tsetse flies acquire the infection when they feed on the blood of an infected animal. The parasites undergo development within the fly, and after a period of time, the fly becomes infective. When an infected fly bites another animal, the parasites are transmitted through the fly's saliva and enter the bloodstream of the new host.

Within the new host, the parasites multiply and spread through the bloodstream and tissues, causing a variety of symptoms including fever, weakness, weight loss, anemia, and neurological problems. If left untreated, nagana can be fatal.

The transmission of nagana by tsetse flies is a complex process involving several factors, including the species of tsetse fly, the species of Trypanosoma parasite, and the immune response of the host animal. Different species of tsetse flies have varying degrees of susceptibility to infection with different species of Trypanosoma, and some species of animals are more susceptible to the disease than others.

The control of nagana is challenging due to the wide distribution of tsetse flies and the difficulty in treating and preventing the disease. Various methods are used to control the disease, including the use of insecticides to kill tsetse flies, the use of drugs to treat infected animals, and the use of traps and other methods to reduce contact between tsetse flies and livestock.