About sleeping sickness

Figure 1: Sleeping sickness is transmitted by tsetse flies (the genus Glossina) which are only found in Africa. The adult is distinguished from other flies by a cell at the centre of each wing that resembles the hatchet or a butcher’s cleaver.

Sleeping sickness, also known as human African trypanosomiasis (HAT), is one of the major neglected infectious diseases today. It is a vector-borne parasitic disease caused by protozoan parasites belonging to the genus Trypanosoma. The small, single-celled organisms called trypanosomes are transmitted through the bites of tsetse flies (Figure 1).

Tsetse flies are only found in sub-Saharan Africa. Different species have diverse habitats, but are mainly found in vegetation by rivers and lakes, in gallery-forests and in vast stretches of wooded savannah.

For reasons that so far remain unexplained, there are many regions in Africa where tsetse flies prevail but where there are no cases of sleeping sickness. Sleeping sickness spreads more easily in poor settings, generally occurring in remote rural areas where health systems are weak or non-existent. The rural populations that depend on agriculture, fishing, animal husbandry or hunting, are the most exposed to the tsetse bite and therefore to the disease. Displacement of populations, war and poverty are other important factors that lead to increased transmission.

The disease can develop in areas as small as a village or as large as an entire region encompassing several countries. Within a given area, the concentration of the disease can vary from one village to the next.

Growing momentum in the control of sleeping sickness

During the mid 1960s, sleeping sickness was successfully controlled through active case finding and treatment. However, from around 1970, the disease re-emerged as a major public health challenge in many parts of rural Africa, and received very little attention, both locally and internationally. Over the past decade, however, concerted efforts in surveillance and control of the disease by governments of endemic countries and the international community have succeeded in changing this trend, and the number of cases being reported has been falling progressively. Although reporting is inadequate, an estimated 50,000 to 70,000 people are thought to be carrying the infection at any one time.

Figure 2: Distribution of sleeping sickness foci in Africa. The chronic form of the disease (T.b. gambiense) which is more widespread occurs west of the red line, while the acute form (T.b. rhodesiense) is found to the east. Sleeping sickness is endemic in 36 African countries, including some of the least developed ones in the world.

The disease occurs in two forms, depending on the type of parasite involved:

Trypanosoma brucei gambiense causes the disease mainly in the lowland rain forests of West and Central Africa. It is spread primarily by peri-domestic tsetse flies, living in areas surrounding human habitats such as cultivated land, and near small rivers or pools of water, frequented by people. Thus, there is close contact between tsetse flies and people as they go about their daily activities. Gambiense sleeping sickness is a chronic disease with a long latency period and people can be infective for many years without knowing. This form represents more than 90% of the cases of sleeping sickness reported, and causes chronic infection. A person can be infected for months or even years without major signs or symptoms. However, once symptoms emerge, the disease is often in an advanced stage, meaning the patient’s central nervous system is affected. Studies have indicated that a small number of tsetse flies can maintain endemic transmission cycles at relatively high levels. In light of the above, it is not surprising that it is very difficult to stop transmission of gambiense sleeping sickness completely in a given locality, and in many villages, sleeping sickness recurs periodically.

Trypanosoma brucei rhodesiense is found primarily in eastern, central and southern Africa, where tsetse flies that carry the disease live primarily in the savannah woodlands of these regions. People are affected when they go into the savannah for activities such as gathering wood or honey, hunting, fishing, keeping cattle or cultivating land. This form represents less than 10% of reported cases and causes an acute infection. The first signs and symptoms are observed a few weeks or months after infection. The disease develops rapidly and parasites quickly invade the central nervous system. This form of the disease is much more virulent than gambiense and infected people usually die within a matter of months.

An important feature of African trypanosomiasis is its focal nature. It tends to occur in circumscribed zones. Observed prevalence rates vary greatly from one geographical area to another, and even between one village and another within the same area. Thus, it is important to understand the ecology and resulting transmission patterns in each locality.

Human African trypanosomiasis is different from American trypanosomiasis or Chagas disease. The latter occurs in 15 Central and South American countries. Although its pathogenic agent, Trypanosoma cruzi is a member of the same genus as the trypanosomes causing sleeping sickness, the clinical manifestations  of the disease, life cycle, geographic distribution and insect vectors are different.

Some facts about sleeping sickness

• More than 60 million people in Africa are at risk of being infected.
• The disease mainly occurs in the rural impoverished areas, affecting poor communities that cannot afford any diagnostic test.
• Around the year 2000, epidemics were causing more than 45,000 new infections a year, with estimates of between 300,000 to 500,000 people carrying the disease at any one time.
• Indications are that with the intensified interventions that have been undertaken recently, the number of cases is falling; WHO now estimates between 50,000 and 70,000 new infections per year.
• HAT occurs in foci which can be as small as a village. As a result, it does not attract national attention until it has flared into an epidemic.
• Trypanosoma rhodesiense also infects livestock without causing disease, making them an important source of infection for humans. In the case of T. gambiense, the role played by animal reservoirs in disease transmission has not been conclusively established.
• There are no clinical signs that are specific enough for sleeping sickness. At present, there is no screening test that is sensitive enough to guide treatment. The one in current use has to be followed by a second method to confirm disease.
• A positive diagnosis of sleeping sickness is always followed by a test to determine whether parasites have invaded the brain (a process referred to as staging). Once brain invasion has occurred, treatment is different and more risky than when the parasites are confined to other body organs.