PubMed Central and ISI Web of Science were searched, with the keywords: ‘trypanosomiasis’, ‘sleeping sickness’, ‘Chagas disease’, ‘T brucei’, and ‘T cruzi’. Preference was given to reviews that encompassed much of the primary published work on the findings that have led to the prevailing picture of these diseases. WHO websites for Chagas' disease (http://www.who.int/ctd/chagas/index.html; accessed Aug 20, 2003) and human African trypanosomiasis (http://www.who.int/health-topics/afrtryps.htm;
SeminarThe trypanosomiases
Section snippets
The parasites and their vectors
Superficially, there are many similarities between trypanosome species and the diseases they cause (table). Both are single-celled flagellates (figure 2) that are transmitted by insect vectors (figure 3). They share phases of local multiplication in their human host followed by dissemination and localisation in target organs, in which they cause potentially lethal damage. However, key differences between the organisms exist, which can account for why clinical manifestations and susceptibility
Evolutionary history
Comparison of 18S rRNA gene sequences from multiple trypanosome species obtained from diverse hosts, combined with other molecular approaches, suggests that the genus Trypanosoma is monophyletic.5 By superimposing estimates with the molecular clock on vicariance biogeography, it is suggested that T brucei and T cruzi shared a common ancestor around 100 million years ago.6
This prehistoric dating indicates that human beings were exposed to African trypanosomes concomitantly with their evolution.
Epidemiology and transmission
T brucei and T cruzi are transmitted by biting insects, but a fundamental difference between the means of transmission has been incorporated into the classification of these organisms. T brucei are known as salivaria because they are transmitted in tsetse saliva. T cruzi belongs to the stercoraria because transmission is via vector faeces.
Transmission of both species can also be via blood transfusion, contaminated needles, or the congenital route. Rarely, transmission of T cruzi by
Trypanosoma brucei
Trypanosoma brucei is divided into three subspecies. Only two cause human African trypanosomiasis.9 In west and central Africa, T brucei gambiense causes a chronic form of sleeping sickness. In east and southern Africa, T brucei rhodesiense causes an acute form. Natural tsetse-mediated transmission of both subspecies happens only in Uganda. Infection with either subspecies is uniformly fatal if untreated. T brucei brucei is not infectious to human beings.
Results of molecular studies are
Trypanosoma cruzi
In 1985, WHO estimated that about 100 million people in Latin America were at risk of acquiring Chagas' disease, with a prevalence of human T cruzi infection estimated at 18 million cases.8, 22 Since 15–30% of the infected population develops overt clinical manifestations, about 5 million people can be assumed to have clinical changes attributable to Chagas' disease today. Successful programmes in vector control have led to a decline in transmission in recent years (see later).
T cruzi is
The diseases
Human African trypanosomiasis and Chagas' disease are both chronic diseases that undergo distinct stages in their natural course. Both are potentially fatal. Sterilising acquired immunity does not exist after natural infection, and there are no vaccines.
Any pathogen must evade host-cell immunity to establish infection. In the mammalian host, by contrast with T cruzi and many other pathogenic protozoa that adopt an intracellular existence, which protects them from humoral immunity, African
Human African trypanosomiasis
This disorder is classed as stage 1 or 2 depending on whether parasites have become manifest in the cerebrospinal fluid. The pathology has been reviewed elsewhere.2, 36, 37, 38
Prospects for disease management through insect control
The epidemiological features of sleeping sickness and Chagas' disease are defined largely by the insect vectors that carry the parasites. Tsetse flies and triatominae differ ecologically. Tsetse flies are highly mobile winged dipterans needing special conditions of temperature, humidity, and vegetation. Triatominae usually crawl within peridomestic environments. Generally speaking, people are bitten by tsetse flies while active outside and by triatomines while asleep indoors. However, both
Outlook
Optimism that similarities in the biochemistry and ecological features of the trypanosome parasites may point to general control strategies has yet to bear fruit because of differences in relations between host, parasite, and the vector arthropods. In human African trypanosomiasis and Chagas' disease, control of transmission through efforts aimed at reducing the prevalence of insect vectors or mammalian reservoirs clearly works. It is essential to establish workable guidelines by which
Search strategy and selection criteria
References (125)
Leishmaniasis
Lancet
(1999)Clinical and epidemiological aspects of Chagas disease
Lancet Infect Dis
(2001)- et al.
The epidemiology and control of human African trypanosomiasis
Adv Parasitol
(2001) Epidemiology and diagnosis of African trypanosomiasis using DNA probes
Trans R Soc Trop Med Hyg
(2002)- et al.
Trypanosome lytic factors: novel mediators of human innate immunity
Curr Opin Microbiol
(2001) - et al.
Natural immunity to human African trypanosomiasis: trypanosome lytic factors and the blood incubation infectivity test
Trans R Soc Trop Med Hyg
(2002) - et al.
A gene expressed only in serum-resistant variants of Trypanosoma brucei rhodesiense
Mol Biochem Parasitol
(1989) - et al.
Identification of human-infective trypanosomes in animal reservoir of sleeping sickness in Uganda by means of serum-resistance-associated (SRA) gene
Lancet
(2001) - et al.
Strategic emphases for tropical diseases research: a TDR perspective
Trends Parasitol
(2002) Antigenic variation and allelic exclusion
Cell
(2002)