| African trypanosomiasis | |
|---|---|
| Classification and external resources | |
Trypanosoma forms in a blood smear. |
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| ICD-10 | B56 |
| ICD-9 | 086.5 |
| DiseasesDB | 29277 13400 |
| MedlinePlus | 001362 |
| eMedicine | med/2140 |
Human African trypanosomiasis, sleeping sickness,[1] African lethargy,[1] or Congo trypanosomiasis[1] is a parasitic disease of people and animals, caused by protozoa of the species Trypanosoma brucei and transmitted by the tsetse fly.[2] The disease is endemic in some regions of sub-Saharan Africa, covering about 37 countries and 60 million people. It is estimated that 50,000 to 70,000 people are currently infected, the number having declined somewhat in recent years.[3] The number of reported cases was below 10,000 in 2009, the first time in 50 years.[4] It is believed that many cases go unreported. About 48,000 people died of it in 2008.[5] Four major epidemics have occurred in recent history: one from 1896–1906 primarily in Uganda and the Congo Basin, two epidemics in 1920 and 1970 in several African countries, and a recent 2008 epidemic in Uganda.[6]
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African trypanosomiasis symptoms occur in two stages. The first stage is known as the haemolymphatic phase and is characterized by fever, headaches, joint pains, and itching. Invasion of the circulatory and lymphatic system by the parasites is associated with severe swelling of lymph nodes, often to tremendous sizes. Winterbottom's sign, the tell-tale swollen lymph nodes along the back of the neck, may appear. If left untreated, the disease overcomes the host's defenses and can cause more extensive damage, broadening symptoms to include anemia, endocrine, cardiac, and kidney dysfunctions. The second stage, called the neurological phase, begins when the parasite invades the central nervous system by passing through the blood-brain barrier. The term 'sleeping sickness' comes from the symptoms of the neurological phase. The symptoms include confusion, reduced coordination, and disruption of the sleep cycle, with bouts of fatigue punctuated with manic periods leading to daytime slumber and night-time insomnia. Without treatment, the disease is invariably fatal, with progressive mental deterioration leading to coma and death. Damage caused in the neurological phase is irreversible.[6]
Tryptophol is a chemical compound that induces sleep in humans. It is produced by the trypanosomal parasite in sleeping sickness.[7]
The tsetse fly (genus Glossina) is a large, brown biting fly that serves as both a host and vector for the Trypanosome parasites. While taking blood from a mammalian host, an infected tsetse fly injects metacyclic trypomastigotes into skin tissue. From the bite, parasites first enter the lymphatic system and then pass into the bloodstream. Inside the mammalian host, they transform into bloodstream trypomastigotes, and are carried to other sites throughout the body, reach other blood fluids (e.g., lymph, spinal fluid), and continue to replicate by binary fission.
The entire life cycle of African trypanosomes is represented by extracellular stages. A tsetse fly becomes infected with bloodstream trypomastigotes when taking a blood meal on an infected mammalian host. In the fly's midgut, the parasites transform into procyclic trypomastigotes, multiply by binary fission, leave the midgut, and transform into epimastigotes. The epimastigotes reach the fly's salivary glands and continue multiplication by binary fission.
The entire life cycle of the fly takes approximately 3 weeks.
In addition to the bite of the tsetse fly, the disease can be transmitted in the following ways:
The gold standard for diagnosis is identification of trypanosomes in a patient sample by microscopic examination. Patient samples that can be used for diagnosis include chancre fluid, lymph node aspirates, blood, bone marrow, and, during the neurological stage, cerebrospinal fluid. Detection of trypanosome-specific antibodies can be used for diagnosis, but the sensitivity and specificity of these methods are too variable to be used alone for clinical diagnosis. Further, seroconversion occurs after the onset of clinical symptoms during a T. b. rhodesiense infection, and therefore is of limited diagnostic use.
Trypanosomes can be detected from patient samples using two different preparations. A wet preparation can be used to look for the motile trypanosomes. Alternatively, a fixed (dried) smear can be stained with Giemsa (or Field) and examined. Often the parasite is in relatively low abundance in the sample, so techniques to concentrate the parasites can be used prior to microscopic examination. For blood samples, these include centrifugation followed by examination of the buffy coat; mini anion-exchange/centrifugation; and the Quantitative Buffy Coat (QBC) technique. For other samples such as spinal fluid, concentration techniques include centrifugation followed by examination of the sediment.
Three serological tests are also available for detection of the parasite: the micro-CATT, wb-CATT, and wb-LATEX. The first uses dried blood while the other two use whole blood samples. A 2002 study found the wb-CATT to be the most efficient for diagnosis, while the wb-LATEX is a better exam for situations where greater sensitivity is required.[10]
Two alternative strategies have been used in the attempts to reduce the African trypanosomiases. The primary method focuses on the eradication of the tsetse fly, which disrupts transmission rates by reducing the number of flies. Instances of sleeping sickness are being reduced by the use of the sterile insect technique. The second tactic is primarily medical or veterinary and tries to reduce spread of the parasite by monitoring, prophylaxis, treatment, and surveillance to reduce the number of people/animals that carry the disease.
Regular active surveillance, involving detection and prompt treatment of new infections, and tsetse fly control is the backbone of the strategy used to control sleeping sickness. Systematic screening of at-risk communities is the best approach, because case-by-case screening is not practical in endemic regions. Systematic screening may be in the form of mobile clinics or fixed screening centres where teams travel daily to areas of high infection rates. Such screening efforts are important because early symptoms are not evident or serious enough to warrant patients with gambiense disease to seek medical attention, particularly in very remote areas. Also, diagnosis of the disease is difficult and health workers may not associate such general symptoms with trypanosomiasis. Systematic screening allows early-stage disease to be detected and treated before the disease progresses, and removes the potential human reservoir.[11] There is a single case report of sexual transmission of West African sleeping sickness,[12] but this is not believed to be an important route of transmission.
The current standard treatment for first stage (haemolymphatic) disease is:
The drug Eflornithine — previously used only as an alternative treatment for sleeping sickness due to its labour-intensive administration — was found to be safe and effective as a first-line treatment for the disease in 2008, according to the Science and Development Network's Sub-Saharan Africa news updates. [1]. Researchers tracked over 1,000 adults and children at a centre in Ibba, Southern Sudan—the first use of eflornithine on a large scale— and it was highly effective in treating the issue.
according to a treatment study of Trypanosoma gambiense caused human African trypanosomiasis, use of eflornithine (DMFO) resulted in fewer adverse events than treatment with melarsoprol.[13]
The current standard treatment for second stage (neurological phase) disease is:
Alternative first line therapies include:
Combination therapy with eflornithine and nifurtimox is safer and easier than treatment with eflornithine alone, and appears to be equally or more effective. It has been recommended as first-line treatment for second stage T. b. gambiensis disease.[17]
In areas with melarsoprol resistance or in patients who have relapsed after melarsoprol monotherapy, the treatment should be:
The following traditional regimens should no longer be used:
After successful treatment, all patients should be followed up for two years with lumbar punctures every six months to look for relapse.
The disease is found in two forms, depending on the parasite, either Trypanosoma brucei gambiense or Trypanosoma brucei rhodesiense. Humans are the main reservoir for Trypanosoma brucei gambiense, but this species can also be found in pigs and other animals. Wild game animals and cattle are the main reservoir of T. b. rhodesiense. T. b. gambiense is found in central and western Africa; it causes a chronic condition that can remain in a passive phase for months or years before symptoms emerge. T. b. rhodesiense is the acute form of the disease, but has a much more limited geographic range. It is found in southern and eastern Africa and symptoms of infection emerges in a few weeks and is more virulent and faster developing than T. b. gambiense. According to recent estimates, the disability adjusted life years (9 to 10 years) (DALYs) lost due to sleeping sickness are 2.0 million.[20] Recent estimates indicate that over 60 million people living in some 250 locations are at risk of contracting the disease, and there were under 10,000 cases reported in 2009 according to WHO figures which represents a huge decrease from the estimated 300,000 new cases in 1998.[21] The disease has been recorded as occurring in 36 countries, all in sub-Saharan Africa. It is endemic in southeast Uganda and western Kenya, and killed more than 48,000 Africans in 2008.[6]
Horse-flies (Tabanidae) and stable flies (Muscidae) possibly play a role in transmission of nagana (the animal form of sleeping sickness) and the human disease form.[22]
The condition has been present in Africa since at least the 14th century, and probably for thousands of years before that. Because there was a lack of travel between indigenous people, sleeping sickness in humans had been limited to isolated pockets. This changed once Arab slave traders entered central Africa from the east, following the Congo River, bringing parasites along. Gambian sleeping sickness travelled up the Congo River, then further eastwards. In 1901 a devastating epidemic had erupted in Uganda, killing more than 250,000 people,[23] about two-thirds of the population in the affected lake-shore areas. According to The Cambridge history of Africa, "It has been estimated that up to half the people died of sleeping-sickness and smallpox in the lands on either bank of the lower river Congo."[24]
The causative agent and vector were identified in 1903 by David Bruce, and the differentiation between the subspecies of the protozoa made in 1910. The first effective treatment, atoxyl, an arsenic-based drug developed by Paul Ehrlich and Kiyoshi Shiga, was introduced in 1910, but blindness was a serious side effect. Numerous drugs designed to treat the disease have been introduced since then.
Suramin was introduced in 1920 to treat the first stage of the disease. By 1922, Suramin was generally combined with Tryparsamide (another pentavalent organo-arsenic drug) in the treatment of the second stage of the gambiense form. It was used during the grand epidemic in West and Central Africa in millions of people and was the mainstay of therapy until 1969.
Pentamidine, a highly effective drug for the first stage of the disease, has been used since 1939. During the fifties, it was widely used as a prophylactic agent in Western Africa, leading to a sharp decline in infection rates. At the time, it was thought that eradication of the disease was at hand.
The organo-arsenical melarsoprol (Arsobal) was developed in the 1940s, and is effective for patients with second stage sleeping sickness. However, 3-10% of those injected have reactive encephalopathy (convulsions, progressive coma, or psychotic reactions), and 10-70% of such cases result in death; it can cause brain damage in those who survive the encephalopathy. However, due to its effectiveness, melarsoprol is still used today. Resistance to melarsoprol is increasing, and combination therapy with nifurtimox is currently under research.
Eflornithine (difluoromethylornithine or DFMO), the most modern treatment, was developed in the 1970s by Albert Sjoerdsmanot and underwent clinical trials in the 1980s. The drug was approved by the United States Food and Drug Administration in 1990, but Aventis, the company responsible for its manufacture, halted production in 1999. In 2001, however, Aventis, in association with Médecins Sans Frontières and the World Health Organization, signed a long-term agreement to manufacture and donate the drug.
The genome of the parasite has been sequenced and several proteins have been identified as potential targets for drug treatment. Analysis of the genome also revealed the reason why generating a vaccine for this disease has been so difficult. T. brucei has over 800 genes that make proteins the parasite "mixes and matches" to evade immune system detection.[25]
Recent findings indicate that the parasite is unable to survive in the bloodstream without its flagellum. This insight gives researchers a new angle with which to attack the parasite.[26]
A new treatment based on a truncated version of the apolipoprotein L-1 of high density lipoprotein and a single-domain antibody has recently been found to work in mice, but has not been tested in humans.[27]
The cover story of the August 25, 2006 issue of the journal Cell describes an advance in understanding how Trypanosomes escape the immune system. Dr. Lee Soo Hee and colleagues, working at Johns Hopkins investigated the pathway by which Trypanosomes make myristate, a 14-carbon length fatty acid. Myristate is a component of the variant surface glycoprotein (VSG), the molecule that makes up the trypanosome's outer layer. This outer surface coat of VSG is vital to the trypanosome's ability to avoid destruction by the host's immune system. Dr. Lee and colleagues discovered trypanosomes use a novel fatty acid synthesis pathway involving fatty acid elongases to make myristate and other fatty acids.
An international research team working in the Democratic Republic of the Congo, Southern Sudan, and Angola involving Immtech International and University of North Carolina at Chapel Hill have completed a Phase IIb clinical trial and began a Phase III trial in 2005 testing the efficacy of the first oral treatment for sleeping sickness, pafuramidine (DB289).[28][29] Trypanosomiasis vaccines are undergoing research.
Two independent variants of the APOL1 gene found in African haplotypes carrying signatures of natural selection have been shown to confer protection against the acute version of sleeping sickness caused by Trypanosoma brucei rhodesiense while at the same time increasing risk of kidney disease when inherited from both parents.[30]
Synthetic and computer based approaches are used for the development of newer anti-trypanosomal analogues with improved efficacy and oral bioavailability.[31]
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