|MicrobiologyBytes: Microbiology Notes: Trypanosomiasis||Updated: January 28, 2007||Search|
World Health Organization
Division of Control of Tropical Diseases
The parasite that causes sleeping sickness is called the trypanosome. It is transmitted to humans through the bite of a tsetse fly of the genus Glossina :
Human trypanosomiasis is therefore a vector-borne parasitic disease. The vector is found only in Africa, between the fifteenth parallels north and south. Its favoured habitat is the vegetation along watercourses and lakes, forest edges and gallery forests, extending to vast areas of scrub savanna.
The tsetse fly feeds on the blood of animals and humans. Once inoculated by an infected fly, the trypanosomes proliferate and gradually invade all the organs of the host. Most of the parasites are effectively destroyed by the host's natural defences, but some trypanosomes manage to evade the immune system by modifying their surface membrane, a process known as antigenic variation. The trypanosome can express thousands of variants, multiplying with each new surface change.
At first, the main clinical signs of human trypanosomiasis are high fever, weakness and headache, joint pains and pruritus (itching). Gradually, the immune defence mechanisms and the patient's resistance are exhausted. As the parasite develops in the lymph and blood of the patient, the initial symptoms become more pronounced and other manifestations such as anaemia, cardiovascular and endocrine disorders, abortion, oedema and kidney disorders appear.
In advanced stages of disease, the parasite invades the central nervous system. The patient's behaviour changes; they can no longer concentrate and become indifferent to their environment. Sudden and unpredictable mood changes become increasingly frequent, giving rise to lethargy with bouts of aggressiveness. Patients are overcome by such extreme torpor that eating, speaking, walking or even opening the eyes call for an unsurmountable effort. At night they suffer insomnia and during the day are exhausted by periods of sleep-like unconsciousness. Finally, patients fall into a deep coma and die.
Sleeping sickness was first described in the fourteenth century in what is now the country of Mali. For a long time, though they did not know its origin, caravanners recognized the signs of sleeping sickness that had often been observed in travels to southern kingdoms. It was only at the beginning of this century that the extent of destruction caused by sleeping sickness was recognized, and that millions were affected by an epidemic that left half a million dead. During the same period, the parasite and its tsetse fly vector were discovered and initial treatment methods were found. At that time the disease ravaged all of east, west, and south Africa, irrespective of the colonial administration. The human reservoir of the parasite was vast and transmission rapid. As the spread of the disease had to be stopped, control campaigns were organized, health services mobilized and administrative structures formed. Prevention campaigns employing the drug pentamidine covered entire populations, each person receiving one injection. At the time it was thought (wrongly) that a low dose of this drug protected each individual from sleeping sickness for six months. Additionally, agronol prevention, consisting of methodically clearing undergrowth from villages, water sources, paths, and bridges, dispersed the tsetse fly by destroying its natural habitat.
By the start of African independence, the disease was reduced to a few sporadic cases. It had taken half a century to bring down the incidence of sleeping sickness. But in many newly independent countries the human and financial resources were not available to keep up the effort to control and monitor the disease. This explains new outbreaks which have been reported in the last 30 years in old foci and in other areas previously unaffected.
In recent years the results of scientific and technical research produced new tools and improved field control strategies:
Social and economic changes coupled with political disorder occurring in many African countries are allowing a resurgence of trypanosomiasis. This happens when:
Millions of square kilometres of Africa are home to the tsetse fly, vector of trypanosomiasis:
It is estimated that today over 66 million people living in rural areas are at risk from the bite of the fly. If the entire population exposed to sleeping sickness could be under medical surveillance, the number of cases detected would undoubtedly reach the order of 250,000 to 300,000. Since this is a disease of rural areas, many sufferers go undiagnosed and untreated and die in their villages. Today, only one-tenth of the population at risk is under medical surveillance, and each year an average of 25,000 new cases are identified. Two-thirds of such cases are at the advance stage of the disease, when the nervous system is involved. Treatment with melarsoprol - still the only drug available - exposes almost 10% of them to serious risk, especially of arsenic encephalopathy, of which almost a thousand people die each year. For untreated cases of trypanosomiasis, death is certain. It is estimated that every year, some 250,000 to 300,000 women, people die for lack of diagnosis and treatment.
Of the 36 countries in which trypanosomiasis is endemic, 22 are actively involved in the WHO programme. The most effective approach for controlling sleeping sickness has three parts:
© MicrobiologyBytes 2007.