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Sleeping sickness or human African trypanosomiasis is a parasitic disease of people and animals, caused by protozoa of species Trypanosoma brucei and transmitted by the tsetse fly. The disease is endemic in certain regions of Sub-Saharan Africa, covering about 36 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. Three major epidemics have occurred in recent history, one lasting from 1896–1906 and the other two in 1920 and 1970.
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Encyclopedia
Sleeping sickness or human African trypanosomiasis is a parasitic disease of people and animals, caused by protozoa of species Trypanosoma brucei and transmitted by the tsetse fly. The disease is endemic in certain regions of Sub-Saharan Africa, covering about 36 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. Three major epidemics have occurred in recent history, one lasting from 1896–1906 and the other two in 1920 and 1970. In 2008 there was an epidemic in Uganda.
Symptoms & Clinical features
Symptoms begin with fever, headaches, and joint pains. As the parasites enter through both the blood and lymph systems, lymph nodes often swell up to tremendous sizes. Winterbottom's sign, the telltale swollen lymph nodes along the back of the neck, may appear. If untreated, the disease slowly overcomes the defenses of the infected person, and symptoms spread to include anemia, endocrine, cardiac, and kidney diseases and disorders. The disease then enters a neurological phase when the parasite passes through the blood-brain barrier. The symptoms of the second phase give the disease its name; besides confusion and reduced coordination, the sleep cycle is disturbed with bouts of fatigue punctuated with manic periods progressing to daytime slumber and nighttime insomnia. Without treatment, the disease is invariably fatal, with progressive mental deterioration leading to coma and death. Damage caused in the neurological phase can be irreversible.
In addition to the bite of the tsetse fly, the disease is contractible in the following ways:
- Mother to child infection: the trypanosome can cross the placenta and infect the fetus, causing prenatal death.
- Laboratories: accidental infections, for example, through the handling of blood of an infected person and organ transplantation, although this is uncommon.
- Blood transfusion
History
The condition has been present in Africa since at least the 14th century, and probably for thousands of years before that. The causative agent and vector were not identified until 1902–1903 by Sir David Bruce, and the differentiation between protozoa was not made until 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.
There have been three severe epidemics in Africa in recent history: one between 1896 and 1906, mostly in Uganda and the Congo Basin, one in 1920 in several African countries, and one that began in 1970 and is still in progress. The 1920 epidemic was arrested due to mobile teams systematically screening millions of people at risk. The disease had practically disappeared between 1960 and 1965. After that success, screening and effective surveillance were relaxed and the disease has reappeared in endemic form in several foci over the last thirty years.
Geographic distribution and epidemiology
The disease is found in two forms, depending on the parasite, either Trypanosoma brucei gambiense or Trypanosoma brucei rhodesiense. T. b. gambiense is found in central and western Africa; it causes a chronic condition that can extend 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 range. It is found in southern and eastern Africa; its infection emerges in a few weeks and is more virulent and faster developing.
According to recent estimates, the disability adjusted life years (9 to 10 years) (DALYs) lost due to sleeping sickness are 2.0 million.
Recent estimates indicate that over 60 million people living in some 250 foci are at risk of contracting the disease, and there are about 300,000 new cases each year.
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 kills more than 40,000 Africans a year.
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.
Horse-flies (Tabanidae) and Stomoxydinae possibly could play a role by mechanical transmission (in special situations) not only of Nagana (the animal form of sleeping sickness) but also of the human disease form.
Life cycle
The tsetse fly is large, brown and stealthy. While taking blood from a mammalian host, an infected tsetse fly (genus Glossina) injects metacyclic trypomastigotes into skin tissue. The parasites enter the lymphatic system and pass into the bloodstream
- Inside the host, they transform into bloodstream trypomastigotes
- are carried to other sites throughout the body, reach other blood fluids (e.g., lymph, spinal fluid), and continue the replication 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 cycle in the fly takes approximately 3 weeks to progress.
Laboratory diagnosis
The diagnosis rests upon demonstrating trypanosomes by microscopic examination of chancre fluid, lymph node aspirates, blood, bone marrow, or, in the late stages of infection, cerebrospinal fluid. A wet preparation should be examined for the motile trypanosomes, and in addition a smear should be fixed, stained with Giemsa (or Field), and examined. Concentration techniques 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. Isolation of the parasite by inoculation of rats or mice is a sensitive method, but its use is limited to T. b. rhodesiense. Antibody detection has sensitivity and specificity that are too variable for clinical decisions. In addition, in infections with T. b. rhodesiense, seroconversion occurs after the onset of clinical symptoms and thus is of limited use.
Three similar serological tests are 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. PMID 12481210
Treatment
First line, first stage
The current standard treatment for first stage disease is:
- Intravenous pentamidine (for T.b. gambiense); or
- Intravenous suramin (for T.b. rhodesiense)
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. . 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.
All patients should be followed up for two years with lumbar punctures every six months to look for relapse.
First line, second stage
The current standard treatment for second stage (later stage) disease is:
- Intravenous melarsoprol 2.2 mg/kg daily for 10 consecutive days.
Alternative first line therapies include:
- Intravenous melarsoprol 0.6 mg/kg on day 1, 1.2 mg/kg iv melarsoprol on day 2, and 1.2 mg/kg/day iv melarsoprol combined with oral 7.5 mg/kg nifurtimox twice a day on days 3 to 10; or
- Intravenous eflornithine 50 mg/kg every six hours for 14 days.
Resistant disease
In areas with melarsoprol resistance or in patients who have relapsed after melarsoprol monotherapy, the treatment should be:
- melarsoprol and nifurtimox, or
- eflornithine
Outdated protocols
The following traditional regimens should no longer be used:
- (old "standard" 26-day melarsoprol therapy) Intravenous melarsoprol therapy (3 series of 3.6 mg/kg/day intravenously for 3 days, with 7-day breaks between the series) (this regimen is less convenient and patients are less likely to complete therapy);
- (incremental melarsoprol therapy) 10-day incremental-dose melarsoprol therapy (0.6 mg/kg iv on day 1, 1.2 mg/kg iv on day 2, and 1.8 mg/kg iv on days 3–10) (previously thought to reduce the risk of treatment-induced encephalopathy, but now known to be associated with an increased risk of relapse and a higher incidence of encephalopathy);
History and research
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.
An international research team working in the Democratic Republic of the Congo, New Sudan and Angola involving Immtech International and University of North Carolina at Chapel Hill have completed a Phase IIb clinical trial and commenced a Phase III trial in 2005 testing the efficacy of the first oral treatment for Sleeping Sickness, known at this point as "DB289".
Trypanosomiasis vaccines are undergoing research.
Drug targets and drug discovery
The genome of the parasite has been decoded and several proteins have been identified as potential targets for drug treatment. The decoded DNA also revealed the reason why generating a vaccine for this disease has been so difficult. T. brucei has over 800 genes that manufacture proteins that the disease mixes and matches to evade immune system detection.
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.
A new treatment based on a truncated version of the apolipoprotein L-1 of high density lipoprotein and a nanobody has recently been found to work in mice, but has not been tested in humans.
The cover story of the August 25, 2006 issue of Cell journal describes an advance; Dr. Lee Soo Hee and colleagues, working at Johns Hopkins, have investigated the pathway by which the organism makes 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 avoidance of immunological capture. Dr. Lee and colleagues discovered trypanosomes use a novel fatty acid synthesis pathway involving fatty acid elongases to make myristate and other fatty acids.
Prevention and control
For in depth information on prevention of the disease via tsetse fly control see Tsetse fly control.
Prevention and control focus on, where it is possible, the eradication of the parasitic host, the tsetse fly. Two alternative strategies have been used in the attempts to reduce the African trypanosomiases. One tactic is primarily medical or veterinary and targets the disease directly using monitoring, prophylaxis, treatment, and surveillance to reduce the number of organisms which carry the disease. The second strategy is generally entomological and intends to disrupt the cycle of transmission by reducing the number of flies.
Instances of sleeping sickness are being reduced by the use of the sterile insect technique.
Regular active surveillance, involving case detection and treatment, in addition to tsetse fly control, is the backbone of the strategy for control of sleeping sickness. Systematic screening of communities in identified foci is the best approach as case-by-case screening is not practically possible in highly endemic regions. Systematic screening may be in the form of mobile clinics or fixed screening centres where teams travel daily to the foci. The nature of gambiense disease is such that patients do not seek treatment early enough because the symptoms at that stage are not evident or serious enough to warrant seeking medical attention, considering the remoteness of some affected areas. Also, diagnosis of the disease is difficult and most health workers may not be able to detect it. Systematic screening allows early-stage disease to be detected and treated before the disease progresses, and removes the potential human reservoir.
Introduction
Human African trypanosomiasis, commonly known as sleeping sickness, is caused by the protist Trypanosoma brucei. There are two strains of T. brucei, T. brucei rhodensiense and T. brucei gambiense, which are separated by the African Great Rift Valley that divides Eastern Africa from Western Africa. African trypanosomiasis is transmitted among vertebrate hosts by several different species of Hematophagous glossina, which are commonly known as tsetse flies. The life cycle of T. Brucei can be divided into three main stages, two of which are spent in the tsetse fly and the other in a mammalian host like humans (Martinez, Wortmann, Kiminyo, & Lucey, 2007).
The name “sleeping sickness” gets its name from the gambiense strain of the protozoa, which crosses the blood-brain barrier only several years after invading the bloodstream, resulting in extreme fatigue in humans during those years. This is in contrast to the rhodensiense strain, which develops much more rapidly. In both forms of the disease, though, T. brucei initially enters the bloodstream of the host, staving off The protozoa eventually move to the central nervous system by crossing the blood-brain barrier and infect the central nervous system. Epileptic attacks, coma, and ultimately death are some typical late stage symptoms of African trypanosomiasis (Martinez, 2007).
Major outbreaks from 1920-1950 led to the awareness and subsequent extensive attempts to control the disease. Once thought to be almost completely under control, waves of infection are occurring again as it appears that T. brucei populations are gaining immunity to treatments (Martinez, 2007). It is estimated that between 50,000 and 70,000 around the world are currently infected with sleeping sickness, with the majority of cases concentrated in 37 sub-Saharan African countries (Martinez, 2007).
Life Cycle of T. brucei
A convenient place to begin looking at the life of a T. brucei trypanosome is when it first enters a tsetse fly, usually through the ingestion of blood from an infected mammal. At this point, the protist is short and thick, giving the phase its name: stumpy. (CDC) Using flagella for propulsion, they travel along the bloodstream to the midgut. Here, the protozoa enter the slender phase, transforming from the stumpy form (also known as bloodstream trypomastigotes) into procyclic trypomastigotes (Hendriks et. al., 2000). This differentiation involves a series of smaller changes: early procyclic, established procyclic, and proventricular mesocyclic. The primary reason for these changes is to facilitate proliferation. Because mitosis requires a great deal of energy, the parasite must be well-equipped to metabolize the nutrients in its environment. In the blood, glucose is primary energy source, but inside the midgut, proline is. The procyclic trypomastigotes are more efficient at metabolizing proline. Here, they proliferate through asexual reproduction (binary fission). (Katelyn & Matthews,
The next phase for the trypanosome is the epimastigote phase, which occurs in the bloodstream as the parasites leave the midgut. In the process of differentiating from procyclic to epimastigotic, asymmetric cell division occurs. This means that when undergoing mitosis, the cell does not split up evenly, resulting in an equal number of short and long epimastigotic forms. Although it is unknown what role the long form plays, the former continues on to complete in the infection cycle. There are two main purposes for this stage: sexual exchange (similar to subjunction) and further differentiation. These events occur inside the tsetse flies’ salivary gland. The trypanosome develops flagellipodium, which are similar to cilia. (Katelyn & Matthews, 2007) These attach to the microvilli on the epithelial cells inside the gland. The sexual exchange is believed to happen here. An experiment showed that placing florescent markers in one strain of T. brucei and mixing it with another, unmarked, strain resulted in hybrids in the next stage. Although the exact mechanism is unknown, surface protein, brucei alanine rich protein (BARP) is believed to play a role. According to the same study, a BARP coat is only present during the epimastigotic stage. Because epimastigotes cannot be studied in vitro, their exact purpose has not been ascertained. However, it is reasonable to infer that BARP surface receptors are used in sexual exchange of DNA (Urwyler, Erwin, Renggli, & Roditi, 2007).
The second purpose of anchoring to the salivary gland wall is for further differentiation. The final stage of the trypanosome in the tsetse fly is the metacyclic phase. This is the form of T. brucei that will infect mammals, such as humans. There are three substages in becoming metacyclic: prometacyclic, nascent metacyclic, and mature metacyclic (Animated Trypanosome Lifecycle). In the first, the parasite is still attached to the microvilli of the salivary gland. The next substage, nascent, detaches itself from the wall and is free to move around. However, it still remains inside the salivary gland because it is missing something that would make the parasite very vulnerable in the bloodstream, i.e., an antigen coat. After the epimastigote phase, the BARP coat dissolved, leaving the trypanosome uncovered. In the final substage, mature metacyclic, the variable antigen coating is reproduced. This coating is essential to the infection of the mammalian host and to the evasion of the immune response (CDC).
Now that the trypanosome is ready to infect a mammal (in this case, humans), all it must do is wait for its tsetse host to bite one. The trypanosome is injected into the bloodstream via the bite, and once inside the bloodstream, the trypanosome transforms into a second slender form (LS), very similar to the one in the tsetse fly. This is also known as the human bloodstream trypomastigote (CDC). Again, the primary reason for this change is adaptation to the energy source (glucose now) and for proliferation (Katelyn & Matthews, 2007). In the bloodstream, trypomastigotes multiply very quickly. However, the sole purpose of the slender form is to multiply. It cannot, by itself, infect the brain. The next and final stage of the trypanosome is the stumpy form (SS). This is the form that crosses the blood-brain barrier and cause the final, fatal stage of sleeping sickness.
Although the mechanism that signals LS to begin differentiating into SS is unknown, several recent studies have determined a mathematical model that demonstrates an inverse relationship between the levels of SS in the bloodstream and the levels of LS (Seed & Black, 1997, Hamm, Schindler, Mecke, & Duszenko, 1990). This brings about the conclusion that once the LS concentration reaches a certain level, it recognizes that parasitemia is advanced enough for it to infect the brain. This is important, because if the signal is sent out too early, then the majority of the LS will differentiate. Since the stumpy forms cannot proliferate, the immune system will easily be able to destroy the infection before it can reach the safety of the brain (Reuner, Vassella, Yutzy, & Boshart, 1997). The conclusion was also reached by another study which attempted to fit mathematical models to this phenomenon (Tyler, Higgs, Matthews, & Gull, 2001). The results of the study showed that the immune response plays a strong role in limiting the number of slender trypanosomes. If differentiation into stumpy trypanosomes starts before LS concentration has reached a certain level, the immune system of the host would target and destroy both LS and SS cells before the parasite can infect the brain. Once SS has breached the blood-brain barrier however, there is no purpose left for the LS. Because blood cannot cross the barrier, there are no macrophages or lymphocytes to prevent the trypanosomes from wreaking havoc. Therefore, regardless of whether or not the immune system lowers the LS concentration to sub-infection levels, the disease will still progress (Grab et.al., 2004). Because SS cannot de-differentiate, the cycle of procyclic trypomastigotes, epimastigotes, and so on, would not be able to continue. Thus, when another tsetse fly comes along to bite the human, there must be a certain concentration of LS in the bloodstream for it to be infected with the trypanosome.
Symptoms
In most cases of African Trypanosomiasis, infection occurs following the injection of trypanosomes that accompanies a tsetse fly bite (the disease can also be transmitted by blood transfusions or from mother to fetus in rare cases) (Kennedy, 2004). The clinical symptoms of sleeping sickness are rather unreliable because of the variability and non-specificity of symptoms, and the fact that the length of time until the onset of symptoms depends heavily on host characteristics (Stich, Abel, & Krishna, 2002). However, common symptoms of the bite include the formation of a chancre, which is a hard, painless ulcer-type sore that generally forms at the site 5-15 days after infection (Martinez, 2007). Chancres are more frequently observed in the East African variant. Other superficial symptoms that may be observed include facial edema, which is a buildup of fluid beneath the skin, and itchy sensations (pruritus) at the site of infection (Martinez et. al., 2007).
There are two recognized stages to African Trypanosomiasis; the infection of the hemolymphatic system characterizes the early stage, while the late stage refers to the crossing of the blood-brain barrier by the trypanosomes and subsequent infection of the central nervous system (Kennedy, 2004).
As the disease progresses from local symptoms to the first stage of infection, the injected trypanosomes further mature and divide in the blood and lymphatic system. Fever is one of the most common symptoms during this stage, and is part of the body’s natural immune response to the invading trypanosomes. The pattern of fever usually fluctuates over many weeks, reflecting the progressive cyclical waves of trypanosomes multiplying in the blood (Stich et. al., 2002). Weakness and fatigue, both as a result of anemia and the body’s allocating of resources to fight the infection, is another common symptom, especially in those infected with the East African strain (Smith, Pepin, & Stich, 1998). In East African trypanosomiasis, symptoms at this stage can already be severe, as about 10% of patients without rapid access to treatment will die (Martinez et. al., 2007).
The swelling of the posterior cervical chain of lymph nodes (lymphadenopathy) along the back of the neck caused by the trypanosomes as they travel in the lymphatic fluid is a frequent characteristic of West African trypanosomiasis (Martinez et. al., 2007). Hepatosplenomegaly and a faint rash are other common signs. (Stich et. al, 2002).
In the second stage of the disease, usually beginning after weeks in East African trypanosomiasis and months to years in the West African form, trypanosomes cross the blood-brain barrier by relatively unexplored mechanisms and invade the central nervous system, though it is suspected the mechanism involves circumventricular organs, particularly the choroid plexus (Schultzberg, Ambatsis, Samuelsson, Kristensson, & Meirvenne, 2004). This second stage results in a chronic encephalopathy and is associated with headache, mental changes, epileptic attacks, and maniacal behavior (Martinez et. al., 2007). Patients generally have reduced mental functions, difficulties concentrating, and insomnia, and eventually enter a terminal somnolent state (coma) (Kennedy, 2004). As such, both treatment options and survival rates are drastically reduced once the trypanosomes migrate to the central nervous system.
Pathology
What makes T. brucei so deadly to humans is how effectively it evades the human immune system. It does so by an intricately genetically programmed system of antigenic variation (Hutchinson et. al., 2007). Essentially, the trypanosomes have evolved a survival strategy based on the generation and expansion of new antigenic variants at a rate fast enough to prevent the recognition of the whole population by the host immune response. The site of antigenic variation for the trypanosome is the protective protein coat that covers its entire surface. The coat is composed of a single protein, the variant surface glycoprotein (VSG), which protects the cell surface proteins from recognition by host immunoglobulins (Ziegelbauer & Overath, 1993). An infecting population of trypanosomes will express a random variety of VSGs from a large reservoir of VSG sequences in the genome (Berriman, et. al., 2005). Different VSGs are genetically distinct due to extreme variation in their genetic sequences; however, all of them have a similar base structure, which is necessary for their function as a protective barrier (Ziegelbauer & Overath, 1993). The VSGs are composed of a combination of one N-terminal domain of around 350 residues and one or two C-terminal domains of 30 to 50 residues each (Hutchinson et. al., 2003). There appears to be no restriction on N and C-domain combinations (Hutchinson et. al., 2003).
When T. brucei crosses the epithelial layer via the bite of an infected tsetse fly, local pain and inflammation result as the innate immune system responds to the invader (Naessens, Jeale, & Sileghem, 2002). However, since in most cases the non-specific immune response is not sufficient to kill the parasite, the parasite will grow and multiply in the bloodstream, necessitating the adaptive immune response from the host, which is evident by the appearance of redness and swelling at the site of infection 1-2 weeks after the tsetse fly bite (Naessens, et. al., 2002). VSGs on the surface of the trypanosomes are recognized as foreign antigens by specific B cell receptors and T cell receptors, from which a specific B-cell mediated antibody response is generated, one against each type of VSG present, and this initial adaptive immune response will eventually kill all of the originally invading trypanosomes (Hutchinson et. al., 2007).
However, a small portion (less than 0.01 parasites per generation on average) of the initial trypanosome population randomly changes its VSG coating by altering which of the numerous VSG genes is expressed (Berriman, et. al., 2005). As mentioned above, each VSG gene encodes for structurally similar, but nonetheless unique VSGs, and because different VSGs are distinct enough from each other that each one is seen as a different pathogen by the immune system, a separate, primary adaptive immune response must be generated against the trypanosomes with new VSGs (Naessens, et. al., 2002). It is important to note, however, that most trypanosomes, as they do not switch VSGs, are eventually killed by the host immune system (Stockdale, Swiderski, Barry, & McCulloch, 2008). In the case of most infections, though, the rate of switching is high enough to maintain the infection for a prolonged period of time, provided that the reservoir of available VSGs is not exhausted within a given host, an extremely unlikely event considering the potentially thousands of VSG sequence donors in the genome (Berriman, et. al., 2005). For example, in some infections, more than 100 different VSGs are expressed during the course of the infection by the invading trypanosomes (Stockdale, et. al., 2008).
The time required to mount a sufficient primary adaptive immune response against each and every different VSG is what allows the trypanosome to divide, multiply, and occasionally change its VSG surface again (Stockdale, et. al., 2008). So although theoretically all the invading trypanosomes can eventually be killed by the human immune system, antigenic variation in VSGs is what allows the trypanosome to stay one step ahead of the adaptive immune system. This is why patients with African trypanosomiasis often suffer from chronic cycles of infection (Fig. 1). These cycles of fluctuating populations of trypanosomes in the body manifest themselves as the waves of chronic inflammation and fever that were previously discussed, while chronic fatigue and weakness is partially due to the body periodically funneling of resources to fight a never-ending infection (Naessens, et. al., 2002).
Your browser may not support display of this image.Fig. 1 The rise and fall of trypanosome populations with a given VSG over time. Note how the height of each cycle of the immune response corresponds to the rise of another trypanosome population with a different VSG, which in turn necessitates a new immune response.
Eventually, the immune response will be unable to contain the trypanosomes to the infection site, and they will proliferate and migrate via the bloodstream to other parts of the body such as the heart, spleen, liver, and most importantly the brain, where they cause more severe and irreversible damage (Stich et. al., 2002). Once the trypanosomes cross the blood-brain barrier, the body will have little to no protection against the neurological damage, and without treatment, coma and subsequent death is inevitable (Martinez et. al., 2007).
Diagnosis
As previously mentioned, the symptoms for African trypanosomiasis are very non-specific and often vary from case to case. Complicating matters even further is the need to differentiate between the East African and the West African strain; a failure to do so could lead to the administration of the inappropriate drug, which would either put the patient under unnecessary additional risks or just be ineffective.
There are some standard parameters that provide indirect diagnostic evidence for West African trypanosomiasis. For example, IgM concentrations in patients infected with West African trypanosomiasis can be up to 16 times the normal concentration as a result of non-specific B-cell activation (MacKenzie & Boreham, 1974). Studies have shown that during the accompanying specific immune response, a variety of non-trypanosome-specific antibodies and autoantibodies are produced, e.g., against fibrin, fibrinogen, DNA, and red blood cells, and central nervous system components such as myelin and neurofilament, for an unknown reason (MacKenzie & Boreham, 1974).
Highly raised IgM concentrations in the blood direct the diagnostic pathway towards more specific examinations. Despite the presence of other antibodies, West African trypanosomiasis-specific IgG and IgM antibodies are present in the highest concentrations and can be detected by immunofluorescence assays (Williams, Duxbury, Anderson, & Sadun, 1963). However, the test of choice when West African trypanosomiasis is suspected is the card agglutination test for trypanosomiasis (CATT) (Stich et. al., 2002). The CATT is a simple, 5-minute test that is based on the agglutination of whole, fixed, and stained trypanosomes in the presence of specific antibodies. (Inojosa et. al., 2006). The test is used by control programs for mass screening of endemic populations due to its ease of application and immediate results.
Although clinical suspicion might hint at African trypanosomiasis, the infection in practice is always confirmed by parasite detection before starting treatment because of the serious side effects of antitrypanosomal drugs. However, trypanosomes in West African trypanosomiasis can be difficult to detect as a consequence of the relatively low parasite population. For example, trypanosomes are generally easy to recognize in a wet blood preparation, but the technique generally has insufficient sensitivity for the detection of West African trypanosomiasis. Specialized techniques for trypanosome detection in the blood can provide a more straightforward solution. One technique that is widely employed is the microhematocrit centrifugation technique (Trc, Bailey, Doua, Laveissiere, & Godfrey, 1994). By centrifugation of blood in a hematocrit centrifuge, trypanosomes are concentrated at the level of the leukocytes, between the upper plasma layer and the lower erythrocyte layer, and can be seen under the microscope (Trc et. al., 1994). Experimental methods for trypanosome-specific DNA and RNA detection by polymerase chain reaction have been described for the diagnosis of trypanosomiasis, but PCR has not yet been fully validated, as problems with its reproducibility have been reported (Radwanska et. al., 2002).
After diagnosing the condition as African sleeping sickness, cerebrospinal fluid (CSF) is customarily examined for leukocyte count, protein concentration, and the presence of trypanosomes (Cattand, Miezan, & Raadt, 1988). These examinations are performed in order to distinguish between the first and second disease stage, which determines the choice of treatment. Second-stage trypanosomiasis is diagnosed in patients with a CSF leukocyte count of more than 5 cells/µL or with trypanosomes in their CSF. The latter can be detected by centrifugation of the CSF (Cattand et. al., 1988). In Africa, total protein determination in CSF is only rarely done, due to lack of reagents, variability of the results, and low (Lejon et. al., 2002). Due to the problems with the sensitivity and specificity of the current disease-stage parameters, alternative techniques for second-stage diagnosis have been proposed such as detection of IgM synthesis, IgM detection in CSF by a latex agglutination field test, and detection of autoantibodies against brain components (Lejon et. al., 2002).
Treatment
The treatment of African Trypanosomiasis is encumbered by many factors, including the limited number of available drugs, the toxicity of trypanocidal drugs (the drugs must be able to kill the eukaryotic trypanosomes, which necessarily causes damage to the body as well), difficulties associated with administering the drugs, and parasitic resistance, as evidenced by the increasing number of cases where treatment failed.
As mentioned above, the treatment is stage specific. The drugs used for the first stage—pentamidine and suramin—were both developed more than half a century ago. Pentamidine (C19H24N4O2) has been used since the 40s, primarily for West African trypanosomiasis. It is usually well tolerated by the human body, with hypotension reported as the most common side effect (Nok, 2003). The present recommended regimen is 7-10 doses of 4mg/kg per day given once daily or every other day, depending on the progression of the infection (Nok, 2003). Suramin (C51H40N6O23S6) has been used since the early 20s for the treatment of early East African trypanosomiasis (Nok, 2003). The protocol is 5 mg/kg at day 1, 10 mg/kg at day 3, and 20 mg/kg at days 5, 11, 23, and 30, given by slow intravenous injection (Nok, 2003). Severe side effects have often been reported, though, including anaphylactic shocks, neurotoxicity, and renal failure (Nok,
Three drugs are used to treat second-stage sleeping sickness: melarsoprol, eflornithine, and nifurtimox. All three necessarily share the ability to cross the blood-brain barrier, which is what allows them to kill the trypanosome at this stage. Melarsoprol (C12H15AsN6OS2) is an organo-arsenical compound that has been in use since the 50s for both strains, and has the curious ability to melt plastic, making administration only possible through special glass IV tubes and syringes (Nok, 2003). It is believed to work by disrupting ATP generation in the trypanosome due to the high affinity of melarsoprol for sulfhydryl groups, which form the active sites of many enzymes (Kennedy, 2004). Once inside the trypanosome, melarsoprol inactivates the enzyme pyruvate kinase, which inhibits glycolysis and subsequently the synthesis of ATP, causing the trypanosome to eventually die as a result of insufficient energy production (Kennedy, 2004). The most common treatment protocol for melarsoprol consists of three to four series of three or four injections (one intravenous injection per day) separated by rest periods of 4-5 days (Nok, 2003). However, an alternative protocol for the West African strain has recently been proposed. It consists of ten consecutive injections of 2.2 mg/kg per day (Blum & Burri, 2002). Preliminary results suggest a similar efficacy but a higher, non-significant proportion of adverse skin reactions when compared with the standard protocol (Blum & Burri, 2002). Being a toxic organic compound of arsenic, melarsoprol is a dangerous drug that causes many adverse conditions, including encephalopathy and neuropathy, the effects of which are chills, fever, headache, rigidity, confusion, convulsions, loss of consciousness, restlessness, slurring of speech, and tremors, leading to death in about half the patients who experience the conditions (Nok, 2003). Another problem with melarsoprol is the recent reports of high therapeutic failure rates of around 25-35%, which could be due to the appearance of strains of trypanosomes with reduced sensitivity to melarsoprol (Robays et. al., 2008). However, the mechanism of resistance is still unknown.
Eflornithine (C6H12F2N2O2) is the only new drug registered for the treatment of African trypanosomiasis over the past half-century. It has been used successfully in the treatment of second stage West African trypanosomiasis patients since the 80s, but seldom for the East African strain, which has been found to be more resistant to this drug (Chappuis, Udayraj, Stietenroth, Meussen, & Bovier, 2005). Eflornithine appears to kill trypanosomes by acting as an inhibitor of the enzyme ornithine decarboxylase, which regulates cell division by catalyzing the first step in the synthesis of proteins (Chappuis et. al., 2005). Theoretically, as the humans metabolize the drug quickly but not trypanosomes, the drug should only be able to harm trypanosomes. The administration of Eflornithine is tedious, requiring 400 mg/kg per day in four separate injections for seven or fourteen days, depending on the progression of the infection (Chappuis et. al., 2005). Because of its proposed mechanism of action, it is better tolerated by the body than melarsoprol, but reported side effects include diarrhea, convulsions, and hallucinations (Chappuis et. al., 2005).
Nifurtimox (C10H13N3O5S) is an orally administered drug conventionally used to treat American trypanosomiasis (Chagas disease) (Checchi et. al., 2007). In a series of small-scale studies, nifurtimox showed inconclusive results in the treatment of West African trypanosomiasis, most notably with relapse occurring in about half of the cases (Pepin et. al., 1992). The protocol most widely employed today is 15 mg/kg per day in three separate doses for two weeks (Pepin et. al., 1992). The type and frequency of adverse reactions are poorly documented, although the toxicity of the drug seems to increase with both dose and duration of treatment. (Checchi et. al., 2007). Anorexia and neurological side-effects are also common. It is thought that nifurtimox could represent an effective therapeutic alternative, especially in combination with other drugs, at least for the West African strain (Priotto et. al., 2007). However, no treatment schedule has been validated yet, as more clinical trials are needed.
It is widely accepted that for African Trypanosomiasis, treatments involving a combination of existing drugs delay the occurrence of resistance and lower the chances of relapse. It is also speculated that combination treatment might also improve the efficacy of treatment, possibly allowing for a reduction of the dosages and the toxicity of the drugs. However, only a few combinations have been used on a compassionate basis in people, and knowledge of the safety of these protocols is too limited to recommend their systematic use. For example, for late-stage West African trypanosomiasis, some patients were reportedly fully treated with a combination of eflornithine and melarsoprol in Equatorial Guinea in 1996 after several ineffective cures of melarsoprol (Mpia & Pepin, 2002). For late-stage East African trypanosomiasis, a combination of nifurtimox with eflornithine has been tested with some success (Priotto et. al., 2007).
In terms of novel treatments, DB 289 is a drug that has shown potential as an effective treatment for sleeping sickness. A diamidine derivative, DB 289 is currently being developed for use against Pneumocystis carinii. However, it has shown good activity against African trypanosomes in vitro as well as in different animal models in the first stage of the infection (“Immtech’s DB-289 Drug to Combat African Sleeping Sickness Proven Effective in Animal Studies,” 2000). Phase I clinical trials have been recently concluded and no significant adverse drug reactions were noted. However, even if all clinical trials are successful, DB 289 will not reach the market for at least 5 years (“Immtech…,” 2000). Currently, no other substance is near preclinical or clinical development.
Conclusion
Although science has a good understanding of the pathogenesis of Human African trypanosomiasis, its mechanism, and the life cycle of its associated protist, T. brucei, there is much work to be done on the diagnosis and treatment fronts, both of which have remained relatively stagnant for an extended amount of time now. It is best that these efforts be expended now before immunity spreads even more among the T. brucei populations.
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There is a single case report of sexual transmission of West African sleeping sickness. This is not believed to be an important route of transmission. A case of sexually transmitted sleeping sickness was the focus of an episode of the television program House.
See also
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