What Causes Sleeping Sickness

Causes

Two subspecies of the Trypanosoma brucei parasite cause sleeping sickness or human African trypanosomiasis (HAT).

West African sleeping sickness is spread by the parasite T. b. gambiense. East African sleeping sickness is spread by the parasite T. b. rhodesiense.

These parasites are only found in sub-Saharan Africa and spread by the bite of the tsetse fly (Glossina species). The percentage of tsetse flies carrying these parasites is low.

East African sleeping sickness is found in Eastern and Southeastern Africa and caused around 5% of cases in 2022.

Western African sleeping sickness is found in central Africa and limited areas of West Africa. It is the most common type, making up 95% of cases in 2022.

Read more: Areas in sub-Saharan Africa where sleeping sickness spreads.

How it spreads

You can get sleeping sickness from the bite of an infective tsetse fly.

Tsetse flies live in:

• Rural areas
• Woodlands and thickets in the East African savannah
• Forests and vegetation along streams in central and West Africa

Tsetse flies bite during daylight hours. Both male and female flies can spread the parasite that causes the disease.

Other causes

Pregnant people can occasionally pass the T. b. gambiense parasite that causes West African sleeping sickness to their unborn baby.

Although rare, the condition may also spread through:

• Sexual contact
• Blood transfusion
• Organ transplantation
• Accidental laboratory exposure

However, such cases are poorly documented.

Risk factors

Sleeping sickness affects people in rural areas of African countries. Travelers to urban areas in those countries are at low risk.

People at higher risk include:

• Hunters
• Villagers with infected cattle herds
• Tourists and others working in or visiting game parks

Your risk of infection increases with the number of times you are bitten by the tsetse fly. This is because most flies are not infective.

Tsetse flies that spread sleeping sickness are found only in sub-Saharan Africa.