Clinical manifestations

Presentation and course

HAT evolves in two distinct stages. In the first stage, the illness principally affects the hemolymphatic system with the parasite confined to the blood and lymphatic system. The second or late stage affects the nervous system, causing meningoencephalitis. There are, however, reports of nervous system involvement in the early stages as well. Neurologic symptoms and CSF inflammatory markers can appear in the first stage of the Trypanosoma brucei rhodesiense variant. Depending on the parasite strain, the clinical picture varies as do virulence and degree and rate of progression. Concurrent infections are common. The two major groupings are geographical. Trypanosoma brucei rhodesiense characteristically produces an acute course that is fatal in a few months from damage to the heart or viscera; Trypanosoma brucei gambiense (gHAT) frequently results in a chronic infection of several years, leading to the extensive nervous system involvement of classic HAT. Rarely, even the initial symptoms manifest after a long incubation period lasting up to 7 years (66).

Non-neurologic manifestations

In the virulent Trypanosoma brucei rhodesiense (rHAT) variant, a chancre develops at the site of the infective bite of Glossina. In both variants, local lymph gland enlargement occurs near the bite. Fever develops when the parasite invades the blood several hours or weeks after the bite. In the Gambian disease, there may be enlarged cervical lymph nodes (Winterbottom sign). In the Rhodesian form, the adenopathy principally involves the axillary and epitrochlear nodes. Irregular febrile episodes are accompanied by endocrine disorders (sterility, impotence or amenorrhea, and abortion) and moderate splenomegaly and hepatomegaly. Cardiac abnormalities progress from early tachycardia to other arrythmias to pericarditis and, in more advanced disease, peri-myocarditis. Anemia is also a consistent symptom, probably caused by changes on the red blood cell membranes that lead to erythrophagocytosis during acute stages of the disease (58). A peculiar deep hyperesthesia (Kerandel key, described in 1907 by Dr Jean-François Kérandel) can occur in a small number of patients. Human African trypanosomiasis can also be considered in cases with anterior uveitis in patients coming from endemic areas (50).

Neurologic manifestations

Neurologic manifestations, including headache, altered EEG, mood changes, may occur during the initial febrile period before trypanosomes can be detected in the cerebrospinal fluid and are associated with meningeal inflammation. Nevertheless, neurologic signs such as gait ataxia, somnolence, tremor, and abnormal Glasgow Coma Scale score may persist into the late stages (57). In the Rhodesian variety, the neurologic changes do not always become marked, with only drowsiness, tremors, and unsteadiness, preceding the terminal coma. In the Gambian form, meningitis progresses to encephalitis with serious motor impairment (dyskinesia and choreoathetosis); dysregulation of the circadian clock including reversal of sleep patterns with daytime somnolence interspersed with bouts of delirium, mania, and schizoid and aggressive behavior; and finally permanent sleep, coma, and death. Meningitis and encephalitis are marked by extensive infiltrations of monocytes, lymphocytes, and plasma cells. With gHAT, the interval between the start of the infection and the encephalitic stage is about 2 years, but an interval of as long as 8 years may elapse. With rHAT, the interval is usually a few months. The prominence of one symptom over the other, the rate of progression, and prognosis vary with geographic location as well (30). Symptoms may also be different among natives of endemic areas and those spending short periods of time in those areas primarily because of a difference in the pathogen. gHAT is extremely rare among tourists and visitors from nonendemic areas who tend mostly to become infected by rHAT. They, therefore, present with the more acute symptomatology (11).

Fever among Caucasian visitors is common with both species (Trypanosoma brucei rhodesiense [rHAT] 97.8% and Trypanosoma brucei gambiense [gHAT] 93.3%), as is headache (50% each). Chancres were more common with rHAT (84.4%) than with gHAT (46.7%). Surprisingly, although sleep symptoms are prevalent in endemic HAT, they are rarely reported in travelers. Insomnia was more common (rHAT 7.1%, gHAT 21.4%) than daytime sleepiness (rHAT 4.8%, gHAT none). Jaundice, a symptom never described in endemic HAT, was reported in 28% of travelers infected with rHAT (11). Neurologic and psychiatric symptoms predominate in the presentation of HAT among Africans from nonendemic areas because the early symptoms are often missed. The common symptoms were daytime sleepiness (46.2%), weakness (64.3%), hyperreflexia (14.3%), hallucinations (21.4%), and depression (21.4%).

A 2022 study tried to investigate the long-term effects of gHAT that may have been overlooked (40). A significant burden of depression and anxiety was identified in patients who were treated for stage 2 gHAT more than 15 years before. Furthermore, there was reduced physical and mental quality of life in former gHAT patients. Hence, there is a need for further research to investigate the long-term burden of this disease.

Prognosis and complications

With the advent of PCR, the presence of asymptomatic human carriers is now well documented. Treating these seropositive individuals may significantly cut down the risk of transmission, especially in areas of low disease prevalence. Reports of pregnancy miscarriage due to Trypanosoma brucei rhodesiense (rHAT) are rare in the literature and are poorly validated because they don’t include precise diagnostic methods and lack long-term follow-up. Such cases are probably due to atypical or anomalous parasite strains. Early detection reduces mortality significantly, even in the second stage. Without chemotherapy, the outcome is inevitably fatal once there is neurologic involvement. A study estimated that survival in stage 1 and stage 2 Trypanosoma brucei gambiense (gHAT) infections was 3 years in the absence of treatment. In both forms of the disease, prognosis is excellent, provided chemotherapy is given early. After the parasite is established in the central nervous system, chemotherapy with toxic drugs is required, and cure rates of around 90% may be obtained. Single or multiple relapses may occur, but the melarsoprol sometimes used in the resistant late stage may itself induce a fatal reactive encephalopathy, which in some studies has been put at 10%. In some patients, the parasites may be resistant to even melarsoprol. Surveillance should continue for up to 4 years after treatment of gHAT. If treatment commences when meningoencephalitis is advanced and the parasitic infection is successfully cleared, sequelae may include insomnia or sometimes attacks of violent behavior. Biological markers were developed to assess treatment outcome in late stage gHAT. Six months after treatment, CSF WBC counts of less than 5 cells/mL were a marker of low risk of relapse. Twelve to 24 months after treatment, the combination of high CSF white blood cell counts and high IgM levels were accurate markers of subsequent treatment failure. Infection by multiple different strains of gHAT results in a more severe neurologic course and higher mortality. Barriers to early treatment include difficulty of detection, logistics of treatment, regional socio-political instability, and cultural stigma (11).

A novel protein marker in the CSF, neopterin, is the most accurate marker to date of response to treatment as well as a predictor of treatment failure. High levels of neopterin in the CSF of those infected with gHAT were associated with a higher rate of treatment failure. In addition, 6 months after treatment, neopterin discriminated between cured and relapsed stage 2 patients with 87% specificity and 92% sensitivity, showing a higher accuracy than white blood cell numbers (60). CXCL13 is currently being studied for disease staging. Using both neopterin and CXCL13 in conjunction could be a sturdier predictor of disease staging than each alone. In addition, those markers could be detected in body fluids other than CSF, thus, avoiding additional lumbar punctures (08).

Clinical vignette

A 58-year-old man presented to an emergency room with lethargy and fever in Düsseldorf, Germany. He had just returned home from a 15-day vacation at a Lusaka game reserve in Zambia. Three days earlier he had noticed painful swelling on his right leg. During the examination he was drowsy, had slurred speech, and was febrile. He was mildly tachypneic and tachycardic with a Glasgow coma scale of 12/15. He was also noted to have facial swelling, a macular rash, two indurated skin chancres on his leg, and hepatosplenomegaly. Blood smears detected Trypanosoma brucei rhodesiense. Treatment was initiated first with pentamidine, but the patient failed to respond, and his condition deteriorated until he ended up in the intensive care unit, intubated. He then received intravenous suramin with six weekly doses of 20 mg/kg. After the second dose, the patient improved clinically, and both CSF and blood analyses showed no evidence of Trypanosoma. The patient recovered completely and was discharged after the last treatment. He was followed for 18 months after discharge and had no residual sequelae.

Biological basis

Etiology and pathogenesis

Sleeping sickness is caused by two strains of the flagellated protozoa Trypanosoma brucei (order Kinetoplastida) transmitted to humans by the bite of the tsetse fly, Glossina. The acute Trypanosoma brucei rhodesiense (rHAT) is transmitted by flies of the G mortisans group, and the chronic Trypanosoma brucei gambiense (gHAT) is transmitted by the G palpalis group, but all 23 species of Glossina appear capable of acting as vectors. Although the terminology Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense are adhered to, application of isoenzyme electrophoresis and DNA analysis is leading to more accurate understanding of the strains, which correlate with clinical features and geographical distribution. Human blood, unlike the blood of other mammals, has efficient trypanolytic activity due to apolipoprotein L1, an ionic-pore-forming apolipoprotein.

Tsetse flies acquire parasites by feeding on infected animal reservoirs, including domestic animals and cattle, game, and sometimes, humans. For instance, according to a study from Chad, 12% of domestic animals (pigs, dogs, sheep, and goats) studied in endemic areas harbored gHAT (65). A complex developmental cycle in the fly leads to infective metacyclic trypanosomes that are extruded during the probing and feeding process. The parasites spread to the hemolymphatic system and also occupy perivascular and connective tissue spaces. There is a general increase in inflammatory perivascular cellularity (lymphocytes and plasma cells). Waves of parasitemia are controlled by specific antibodies, but some parasites escape immune lysis by the mechanism of antigenic variation, which is an innate genetic switching process that enables the parasite to produce an antigenically different surface coat (03). A new multiplication cycle is then initiated. The result is a fluctuating parasitemia with progressive immunosuppression and multisystem pathology, which vary in intensity according to parasite strain, population, and host characteristics.

Immunology. Another mechanism by which the trypanosomes evade the immune response and, in addition, divert the host reactions to their benefit is the release of a specific substance termed trypanosome lymphocyte-triggering factor (TLTF). This substance binds to CD8+ T-cells and activates them to produce interferon-gamma, which in turn triggers proliferation of the parasite. This contributes to the Th-1-type immunological response triggered by stimulation of Toll-like receptor 9 on macrophages by parasite CpG DNA (38). The causes of the pathology are multifactorial and thought to result from direct parasite damage, toxic parasite products, and host-altered immune function and cytokine production. Progression from stage 1 to stage 2 in human African trypanosomiasis is associated with high CSF levels of IL-6, IL-10, and INF-gamma (57). Rather than clearing the parasites, tumor necrosis factor-alpha may act as an agent of host pathology. TNF-alpha-mediated inflammatory pathology is associated with rapid disease progression. The different severities observed in different populations may result from either differing trypanosome virulence phenotypes or from host immune response polymorphisms. CSF levels of CD19+ B cell increase with disease severity, whereas in the serum, CD19+ B cells are consistently high. Activated CD8+ T cells, on the other hand, are not elevated, and memory CD8 T-cell levels are suppressed (33).

High levels of endotoxins and antibodies to endotoxins from gram-negative bacteria occur in the blood of the chronically infected patient. There is good evidence from studies on animal models and research realized in Uganda that rHAT is associated with low-grade parasite-independent endotoxemia. Plasma endotoxin concentration is correlated to the occurrence of splenomegaly and lymphadenopathy. The source of endotoxin in this disease comes from increased intestinal leakage initiated by the parasite; elevated endotoxins in turn correlate with the alterations in nitric oxide and other proinflammatory cytokines such as IFN-γ and TGF-β (37).

CNS involvement. Many studies have focused on the mechanisms underlying the changes in the CNS, especially those associated with alterations in sleep that accompany the late disease stages. Most information is obtained from animal models. The neuropathology has many components; these include a meningeal and generalized perivascular inflammation, with the infiltrations consisting chiefly of lymphocytes and plasma cells. There is breakdown of the choroid plexus and compromise of the blood-brain barrier. Throughout the brain, there is activation of astrocytes and microglia, with alterations in cytokine and mediator expression and production. Studies in mouse models suggest that parasites gain entry to the brain parenchyma by capitalizing on the translocation of T cells across the blood-brain barrier. Release of TNF-alpha is necessary for their penetration of the endothelial tight junctions, and IFN-gamma activation of the chemokine CXCL10 is required for crossing the astrocyte basement membrane (38). Elevated levels of glial fibrillary acidic protein (GFAP) and light subunit neurofilament protein; autoantibodies against cerebrosides, gangliosides, and myelin basic proteins; as well as immune complexes and bacterial endotoxins can be detected in the CSF.

Pathophysiology. The oscillatory activities and circadian timing capacities of the suprachiasmatic nuclei of the hypothalamus become dysregulated. CSF hypocretin-1 levels were significantly higher in HAT (423.2 +/- 119.7 pg/mL) than in narcoleptic patients (40.16 +/- 60.18 pg/mL) but lower than in neurologic controls (517.32 +/- 194.5 pg/mL). No differences were found in CSF hypocretin levels between patients with stage 1 or stage 2 HAT; however, the presence of major sleep-wake cycle disruptions was significantly associated with lower CSF hypocretin-1 level. CSF tryptophan and kynurenine concentrations were elevated in early and late stages of the disease. Kynurenine pathway activation was associated with increased neuroinflammatory markers; however, a relationship with neurologic symptoms was not established (57). Studies suggest that trypanosomes shorten the period of the circadian clock in infected mice, most likely secondary to a secreted substance, as the effect precedes the detection of parasites in the brain of the sacrificed mice (54). This period shortening accumulates over time and leads to significant disruption of the mouse activity cycle. Such a disruption would be consistent with the observation that patients with HAT sleep the same amount during a 24-hour period as controls, but that sleep occurs at irregular intervals.

Epidemiology

Human African trypanosomiasis (HAT) is only endemic in areas where Glossina species are found. This is in turn determined by climate through its effect on vegetation. The ecological range of Glossina is bound approximately by a line commencing 14 degrees north from Senegal in the West to 10 degrees north in Somalia in the East, and 20 degrees south corresponding to the northern fringes of the Kalahari and Namibian deserts. The area is about 11 million square km or one third of the African continent. HAT of either strain is endemic in 36 countries, an area of approximately 10 million square kilometers. The annual incidence used to be approximately 300,000 cases; however, thanks to new chemotherapeutic agents, awareness campaigns, and the World Health Organization’s efforts of controlling transmission, the number of reported cases over the first decade of the 21st century dropped to only 10,000 in 2009 and 3796 in 2014 (31). That number further dropped to 1420 cases in 2017 (17). Most statistics show that there were 953 globally reported cases of Trypanosoma brucei gambiense (gHAT) and 864 of Trypanosoma brucei rhodesiense (rHAT) in 2019, which is a substantial decrease compared to more than 10,000 cases of both strains per year prior to 2009 (25).

High-risk foci (foyers) of Trypanosoma brucei remain, however, in Chad, Central African Republic, Democratic Republic of Congo, Equatorial Guinea, and Gabon with incidences of 0.1% to 1% per year. rHAT is endemic in woodland savanna, where the game animal reservoir and Glossina are prevalent. HAT foci seem to have moved from the North to the South of West Africa. Endemic HAT is presently limited to areas where annual rainfall exceeds 1200 mm. Currently, the most severely affected countries are the Democratic Republic of the Congo, Guinea, and Côte D’Ivoire whereas the northern countries seem less affected. There are 21 million people who live in areas classified as moderate to very high risk, where more than one HAT case per 10,000 inhabitants per annum is reported. The prevalence in Democratic Republic of the Congo for example, was estimated in 2011 to be 30 out of 100,000 almost twice the 2007 reported prevalence. This increase was due to better reporting that led to discovery of cases that were previously missed. According to the World Health Organization, Democratic Republic of Congo is the only country that currently reports more than 1000 cases per year and accounts for 84% of the cases reported in 2015. Overall number of fatalities in 2010 (although greatly underdocumented) was 9100, with an incidence of 0.1 out of 100,000 per year.

With increased awareness of the illness and increased travel to endemic areas, there is an increase in the number of HAT reported among European and other nonendemic expatriates. A comprehensive study reported 244 cases of HAT in people from nonendemic countries over a period of 110 years. The two peaks of incidence were from 1902 to 1920 (110 cases) and 2000 to 2012 (125 cases). Of the former group, 74.3% were missionaries and soldiers, and gHAT accounted for 78% of cases. Of the latter group, 73% were short-term tourists to game reserves, and rHAT was the more prevalent infection, accounting for 76.4% of all HAT cases in this cohort (47).

The Food and Agriculture Organization of the United Nations is developing a geographic database to track the distribution of the illness for future monitoring, control, and research activities. In 2012 a targeted door-to-door survey that focused on former HAT patients was performed in Bonon (Côte d’Ivoire), but no new cases of HAT were detected; however, when testing 1058 individuals from the immediate neighborhood of 72 former HAT patients, four new HAT cases were detected with 0.4% prevalence (32). In 2016, Zimbabwe launched a national atlas initiative for the gathering and storage of tsetse and animal African trypanosomiasis (AAT) (55). This initiative is an important factor in developing strategies to control and fight animal African trypanosomiasis. The atlas gathered data collected from 2000 records, which were updated through monthly reports. The atlas enables Zimbabwe to map areas that are epidemic with animal African trypanosomiasis and tsetse and to track changes in the distribution of tsetse, hence, enabling the authorities to make informed decisions on where and when to intervene. Further efforts to map tsetse distribution by a study aimed to predict the potential distribution of three tsetse species in Tanzania based on climate change (49). The information extrapolated from the study could serve as a predictor for hotspots and future endemic areas for HAT, which may guide prevention and treatment.

Prevention

Intensive campaigns are effective in eliminating trypanosomiasis in certain areas. These include insecticide use to kill tsetse flies and reducing their bites by wearing protective clothing. Trypanosoma brucei rhodesiense (rHAT) transmission has a 10-fold higher prevalence at geographic locations near riverine vegetation, crossings, villages, sacred sites, and water collection and washing sites. rHAT is considered to place at greatest risk those whose occupations bring them into more frequent contact with savanna Glossina: honey gatherers, fishermen, game wardens, poachers, and firewood collectors. Some campaigns to control tsetse flies have been counterproductive. Although some lead to a decrease in tsetse fly population densities, they allow for the emergence of insecticide-resistant flies (14). Because cattle can serve as a reservoir as well, particularly for the rHAT disease variant, livestock markets play a role in the transmission of the illness; transmission can be prevented if the animals are treated at the point of sale as required by the government of Uganda (29). This is not as common for the Gambiense variant as a study failed to find any Trypanosoma brucei gambiense (gHAT) in blood samples from cattle and pigs, making them unlikely reservoirs of the disease (14). In addition, there is evidence that infected humans can remain asymptomatic for long period of times, thus becoming cryptic reservoirs of the disease themselves. This issue needs to be further studied with proper diagnostic tools developed to aid in eliminating HAT (14). Large-scale screening of affected individuals is also difficult because of lack of privacy and associated stigma and misconceptions about treatment and the toxicity of the drugs used to treat HAT. A 2022 cross-sectional survey showed that the main reasons for not accepting HAT screening are fear of lumbar puncture and stigma from the community (19). Thus, there is a need for educational materials for this group. Additionally, diagnostic tools need to be improved to be more reliable and less invasive to increase acceptance and, hence, effectivity and utility.

In addition, the diagnosis of HAT is more complex than a simple screening, whereby positive screening should be followed by a parasitological confirmation. This is difficult on the logistic and resources level (42). Although screening may be time-consuming and expensive, it is effective in limiting case numbers while providing treatment and surveillance. Hence, several efforts have been placed into determining the most cost-effective method of screening. A study using a stochastic model for different villages found that the most cost-effective screening is annual screening with cessation after 3 years of zero cases in moderate to high endemic regions, which is in line with the WHO guidelines (16). Furthermore, another study assessed an alternative approach to screening whereby dried blood samples on filters were collected in villages and then tested in a regional laboratory (24). This is in contrast to the more cost and time-consuming traditional method of sending expertise to the field with point-of-care confirmation of disease.

Using insecticide treated traps in the shape of humans and cattle, vector control is another method of prevention that is somewhat successful, especially when integrated with other screening strategies. Political turbulence and resulting armed conflict, however, has made this type of prevention increasingly difficult. A single intervention with trypanocidal chemoprophylaxis targeted at cattle in Eastern Uganda was highly effective at removing rHAT parasites from cattle reservoir and significantly decreased cases of the disease in that area (21). Alternatively, for gHAT, where wild animal reservoirs play a larger role in transmission, vector control has also been effective. New, more cost-effective methods include using smaller electrified nets and small, inexpensive, insecticide-impregnated targets (61). A study deployed tiny targets that are insecticide-treated and attract and kill tsetse in Côte d’Ivoire (25). After identifying areas where locals were bitten by tsetse, they placed the traps there in 2016. Following the deployment of the tiny targets the daily catch of tsetses fell from 6.4 tsetse/trap/day to less than 0.3 tsetse/trap/day, which is a significant reduction of 95% and was translated into no cases of gHAT in Bonon (a city in Côte d’Ivoire) between 2016 and 2018. Hence, tiny targets are an important way to eliminate gHAT and have in fact been used in Côte d’Ivoire’s national strategy (25).

Vaccination is theoretically the most viable and complete option for prevention. It has thus far been unsuccessful because of the difficulty of finding an effective antigen and dearth of research projects. Partially effective trials were conducted with the following antigens: actin, tubulin, sialidase microtubule-associated protein, surface glycoproteins, and other enzymes (01). Variant surface glycoprotein-derived peptides (VSG) hold some promise as the target for vaccines as they are relatively conserved among strains (04).

Lastly, recognizing that the tsetse fly often infects hosts while they are indoors may lead to better methods of “fly proofing” individual houses (62).

In the past 20 years, the control of gHAT greatly improved due to better screening and treatment methods. The goal of eliminating gHAT as a public health problem by 2020 has been achieved. With further advances in diagnostics, vector control, and treatment, the goal of interrupting transmission by 2030 is achievable. The current epidemiological data suggest that an animal or cryptic human reservoir is unlikely to play a significant role in transmission. To prevent a resurgence of gHAT, sustained efforts and adaptation of control strategies are necessary to make optimal use of available innovations (22).

Differential diagnosis

The prevalence of other tropical diseases in areas where HAT is endemic together with the high incidence of intercurrent infections in patients makes accurate diagnosis essential. The early chancre may be confused with infected bites from insects other than tsetse. Among the many causes of fever in a HAT area are malaria, relapsing fever, typhoid, and kala azar. Hodgkin disease, with characteristic temperature fluctuations and adenopathy, infectious mononucleosis, and brucellosis are other alternatives. The enlarged glands may be due to tuberculosis or in some patients from pediculosis, scabies, or other skin lesions. The varied nervous manifestations of the chronic Gambian disease may result from meningovascular syphilis, tuberculous or viral meningitis, parkinsonism, or HIV-1 infection. The wasting may be associated also with HIV-1 or hookworm infections. The likelihood of double infection, in particular with hookworm, malaria, or HIV-1, is high. Differential diagnosis also includes autoimmune disorders such as systemic lupus erythematosus. Differential diagnosis has posed particular problems for emigrants or visitors returning from Africa, where the insidious and often long latent period of the Gambian disease, which can be up to 4 years, has resulted in misdiagnosis and unnecessary death. Additionally, American physicians may lack familiarity with HAT, and the initial symptoms can be vague. HAT diagnosis should not be overlooked in patients coming from endemic areas, especially if they have a clinical presentation consistent with subacute encephalitis (18; 11).

Diagnostic workup

As mentioned previously, diagnosis of sleeping sickness requires screening with a rapid diagnostic test followed by parasitological confirmation (42). The traditional mainstay of diagnosis has been the positive identification of trypanosomes in fluids or tissue from patients. Blood films may be examined directly under the microscope, but the major problem with both forms of the disease is that parasites may only be present in small numbers for periods of weeks or more. Sensitivity is increased by concentrating the parasites using either thick blood films or microhematocrit triple centrifugation followed by examination under dark-ground/phase contrast microscopy. Quantitative buffy coat technique combines centrifugation with acridine orange fluorescent detection. Parasites may also be separated by passing through a miniature anion exchange column (mAECT) and then visualized under a microscope. Aspirates of lymph fluid or, occasionally, bone marrow are also valuable. The main limitation of direct visualization is the inability to differentiate between the two types of trypanosomes.

Several serological methods are available that are based on direct agglutination, hemagglutination, gel precipitation, immunofluorescence, and antigen or antibody capture (ELISA). Because antibody production against the surface glycoproteins of Trypanosoma brucei rhodesiense (rHAT) is relatively limited regardless of geographical location, their detection provides a fairly reliable marker. The card agglutination test (CATT) is the most practical and in widespread use. Unfortunately, it has many limitations, including poor sensitivity and specificity. The immune trypanolysis test improves the accuracy of the card agglutination test in endemic areas (15), as does the ELISA test and the latex agglutination. The use of these tests, however, applies primarily for the detection of Trypanosoma brucei gambiense (gHAT). CATT remains the best serological screen for rHAT. Since 2010, rapid diagnostic tests (RDT) have been used as a complement of CATT. SD BIOLINE® HAT is a RDT that has higher sensitivity and slightly lower specificity than CATT, which means that the need for a test with better performance persists (35). A test using similar technology, HAT Sero-K-SeT, has excellent sensitivity and specificity (100% and 97%, respectively) in a phase III diagnostic accuracy trial, although the study was limited by its small size (07). It also remains quite expensive due to use of native antigens. In contrast, SD BIOLINE® HAT 2.0, which is considerably cheaper due to use of recombinant antigens, was assayed against the CATT and the original SD BIOLINE® HAT in a much larger trial that looked at both active (healthcare providers actively seeking HAT cases in hospital databases) and passive (cases identified when the individuals with HAT came seeking medical attention) screening (44). In this trial, SD BIOLINE® HAT 2.0 had a sensitivity of 98.4% in passive screening and 83% in active screening. There was some evidence suggesting that a test incorporating multiple antigens may be more sensitive and specific to account for parasite variation (36).

The advent of real-time PCR for the detection of Trypanosoma brucei in human blood has increased the accuracy of diagnosis and may eventually replace microscopy. Moreover, PCR is helpful in differentiating between different Trypanosoma species. There is, however, a limitation to PCR use in the follow-up of treated patients as 20% of individuals, despite cure, continue to have PCR positivity for up to 2 years. Two other techniques that have been developed and require fewer resources than PCR are loop-mediated isothermal amplification (LAMP) and nucleic acid sequence-based amplification (NASBA).

A study done in Eastern Zambia confirmed that a technique combining microscopy and PCR is superior in sensitivity to microscopy technique alone. The use of PCR, however, may be limited and impractical in rural African areas where the disease is endemic. Thus, microscopy may remain the most practical diagnostic method in the field in rural areas. The study further showed that to better diagnose using microscopy, the staff should be well trained to diagnose using the microscope, the microscopes should be well maintained, there should be a better funding model, and an enhancement in quality control is needed (43).

A CRISPR-based HAT test using SHERLOCK (Specific High-sensitivity Enzymatic Reporter unLOCKing) can differentiate between gHAT and rHAT and has potential for HAT diagnosis and monitoring in the field, with more technical advancements (56).

Polysomnography can demonstrate normalization of sleep-wake patterns after treatment is instituted. Actigraphy, a simpler and less costly sleep diagnostic tool shows promise in evaluating sleep-wake disturbances in patients with HAT. Although actigraphic changes do not mirror white blood cell counts in the CSF, they do correspond to polysomnographic changes and may demonstrate clear sleep-wake improvement once treatment is instituted (48).

The late, or second-stage, disease with central nervous system involvement is diagnosed by identification of cerebrospinal fluid abnormalities. For identification of trypanosomes in CSF, fresh fluid must be centrifuged and quickly analyzed because the parasites are rendered more fragile than in blood. Multiple centrifugation procedures may be employed. Parasites may be absent from the cerebrospinal fluid sample even though the blood-cerebrospinal fluid barrier has been breached and meningoencephalitis is established.

The most suitable ways to determine CNS involvement in the field are miniature anion exchange and modified single centrifugation analysis.

The widely accepted criteria for CNS involvement following cerebrospinal fluid analysis are either the presence of trypanosomes or white blood cells greater than five cells per mm2, together with protein greater than 40 mg. If HAT has already been diagnosed, the lymphocyte pleocytosis and elevated protein is considered a strong indication of CNS involvement or relapse. New research has been directed toward the identification of biomarkers of disease in blood that will enable discrimination between stage 1 and 2 (both with CNS involvement), removing the need for lumbar puncture from HAT-staging algorithms (64).

Attempts to create algorithms for early detection and disease staging incorporate clinical findings, CATT, and other serological testing in blood, CSF, and lymph. Five regional algorithms exist with a sensitivity ranging from75% to 90% and a maximum positive predictive value of 75% at 1% prevalence. Approximately 50% of stage 1 cases were misclassified as stage 2 (13).

A specific marker of cure is spliced leader RNA. When compared to DNA detection in serum, its specificities were 98.4% to 100% as compared to only 77% for DNA detection (23).

A study using brain MRI imaging in HAT demonstrated unique high FLAIR signals in the midbrain, pons, medulla, corona radiata, caudate nuclei, hypothalamus, and cerebellar white matter (09). These changes were not reported before in humans. The uniqueness of these findings and the correlation with clinical findings may be an important area to explore in order to further enhance the ability to diagnose HAT.

In a study done by Camara and colleagues the skin acted as a potential reservoir for Trypanosomes and T brucei was detected in the extravascular dermis of all unconfirmed seropositive patients and all confirmed seropositive patients. Hence, the skin could be a target for the confirmation of the disease in unconfirmed seropositive patients in the future (12).

Management

Fexinidazole has replaced pentamidine as the first-line treatment in patients with first-stage Trypanosoma brucei gambiense (gHAT) infection (34). Fexinidazole is administered orally once a day for 10 days. If the patient is an adult weighing more than 35 kgs, the patient should receive a loading dose of three tablets (1800 mg) each day for 4 days and a maintenance dose of two tablets (1200 mg) each day for the remaining 6 days. If the patient is a child over 6 years of age and weighs more than 20 kg and less than 35 kg, or if the patient is an adult weighing 35 kg or less, they should receive two tablets (1200 mg) each day as a loading dose for 4 days and then 600 mg (one tablet) each day as a maintenance dose for the remaining 6 days (34; 46; 26; 27). Unfortunately, no literature exists addressing the treatment of children under 6 years of age. The approval for the usage of fexinidazole is a milestone because it is the first oral drug approved for the treatment of HAT. This eliminates the need for injectable treatment, which has been a hurdle in the elimination of HAT, because it is more accessible. Fexinidazole has a relatively safe side effect profile when compared to other drugs. The side effects are mainly gastrointestinal, such as nausea and vomiting; they may also include headache, insomnia, tremor, and dizziness. Fexinidazole should always be taken with a meal because of low bioavailability in the fasting state. In addition, fexinidazole has not been approved for use in children younger than 6 years of age or those who weigh less than 20 kg. There is an ongoing clinical trial (DNDiFEX09HAT) assessing the safety and efficacy of fexinidazole in vulnerable populations, such as pregnant women, breastfeeding women, and patients with a poor nutritional status (34; 46).

For late-stage infection with gHAT, the traditional first line of treatment has been a combination of eflornithine and nifurtimox or NECT. The dosing is usually eflornithine 200 mg/kg every 12 hours intravenously for 7 days and oral nifurtimox 15 mg/kg orally per day in three divided doses for a duration of 10 days (28). Compared to eflornithine alone at the same total daily amount divided into four doses but for 14 days, NECT was equally effective and was less toxic (28). Third-line treatment would be intravenous melarsoprol 2.2 mg/kg daily for 10 days.

Head-to-head comparison shows that eflornithine saves more lives than melarsoprol, but melarsoprol is slightly more cost effective. Switching from melarsoprol to eflornithine can be considered a cost-effective option according to the WHO choice criteria.

Clinical trials examining oral racemic eflornithine show that there isn’t enough systemic exposure due to poor bioavailability. A study investigated the anti-trypanosomal activities of racemic eflornithine, L-eflornithine, and D-eflornithine in vitro against three gHAT strains (06). The study showed that the potency of L-eflornithine is 9-fold higher than D-eflornithine. L-eflornithine needs to be further studied against more strains of gHAT in addition to determining the efficacy and safety in vivo. However, this study paves the way for the possibility of the future development of an effective oral eflornithine drug, which is more practical and feasible than intravenous administration in rural areas.

A substance that has shown in vitro trypanocidal activity is the antihelminthic drug, niclosamide. In a phase 3 clinical trial, this agent was noninferior for the treatment of stage 2 gHAT (39). Its oral delivery route is a distinct advantage over NECT. Acoziborole (NCT03087955) is another drug being studied for gHAT that may offer a new perspective because it is a single-dose oral treatment (20).

In phase 3 trials, the first state of gHAT responded to the experimental antihelminthic, pafuramidine. The cure rates 3 months posttreatment were 79% in the 5-day pafuramidine group, 100% in the 7-day pentamidine group, and 93% in the 10-day pafuramidine group (10). When directly compared to pentamidine, the overall cure rate at 12 months was 89% in the pafuramidine group and 95% in the pentamidine group (52).

Lately however, the recommendation has evolved to using fexinidazole instead of nifurtimox-elfornithine combination therapy in patients with second-stage gHAT and less than 100/μL cerebrospinal fluid white blood counts. The recommendation remains to use nifurtimox-elfornithine in patients with more than 100/μL cerebrospinal fluid white blood counts (34).

For stage 1 rHAT, suramin is given. For adults in a reasonable state of health, a test dose of intravenous suramin 100 to 200 mg is followed by 1 g intravenously on days 1, 3, 7, 14, and 21. Children should receive lower doses proportional to weight and health. Suramin is not readily available in non-endemic countries, and distribution is permitted by the WHO after a case is diagnosed. Hence, pentamidine treatment is usually initiated as a bridge treatment until suramin is available. Long-term pentamidine regimens have been associated with good results in treating rHAT. In a case report in the Netherlands, suramin was not available for a patient diagnosed with rHAT parasitemia in the initial hemolymphatic phase of rHAT. At the time of diagnosis, suramin was not available in the hospital; therefore, a patient was treated with one dose of 4 mg/kg intramuscular pentamidine. After 12 hours, suramin was delivered to the hospital; however, prior to administering suramin, another blood specimen was drawn to study the effect of a single-dose-pentamidine on the parasitemia and motility of the trypanosomes. The results showed a drop in 75% of trypanosomes and a significant effect on the motility, whereby the trypanosomes had a sluggish motility. Hence, a single dose of pentamidine could be of added value in the management of rHAT as a bridge treatment until suramin is available (63).

For the late stage of rHAT, melarsoprol is given by intravenous injection at 2.2 mg/kg/day for 10 days. If melarsoprol fails, then a pretreatment with a single 4 to 5 mg/kg of suramin is recommended followed by a repeat course of intravenous melarsoprol 2.2 mg/kg/day for 10 days. In fact, most experts recommend pretreatment with suramin to treat the parasitemia and avoid a Jarisch-Herxheimer reaction that is a severe inflammatory reaction associated with a sudden release of endotoxins.

Because melarsoprol is a highly toxic drug and fexinidazole has shown promise of killing Trypanosoma brucei rhodesiense in vivo and in vitro, a clinical trial assessing the efficacy of fexinidazole in the treatment of rHAT has just been completed (NCT03974178) (34). Results are pending.

One possible strategy for treating rHAT is targeting rhodesain, the main cysteine protease of Trypanosoma brucei rhodesiense, because it plays essential roles in the Trypanosoma life cycle. Many classes of rhodesain inhibitors have been developed, but most of them showed poor selectivity towards the target enzyme. The incorporation of unnatural amino acids and peptide backbone modifications has led to ligands with improved selectivity. In a study, small Michael acceptors (pseudopeptides) were evaluated for their biological activity against Trypanosoma brucei. The pseudopeptides showed good binding affinity and selectivity against the target enzyme rhodesain, and some of them also showed promising inhibition of Trypanosoma brucei growth, suggesting multiple targets of action (53).

In addition, a promising class of medications to treat both species is the selective tubulin inhibitor group, which holds great promise in animal studies because it inhibits the growth of trypanosome cells, sparing viability of mammalian cells (45; 05).

Finally, there may be a novel way to target protozoa through targeting the two key enzymes involved in folate metabolism: dihydrofolate reductase and pteridine reductase 1. A study investigated the ability of the drug pyrimethamine to target the two key folate metabolism enzymes that may open the doors for future research and design in dual enzyme inhibitors in the treatment of HAT of both species (59).

Despite all of these efforts, underdiagnosis, underdetection, and poor compliance with treatment and clinic follow-ups remain huge problems in the fight against trypanosomiasis. WHO states that the successful elimination of the illness requires the participation of existing health systems for surveillance and control sustainability; the development of new diagnostic tools and drugs is crucial to guarantee the effective participation of existing health structures, and the maintenance of a specialized central structure at national level is necessary to ensure the coordination and overall technical assistance needed. Training of nonspecialist healthcare workers improves the rate of syndromic referrals to hospitals for HAT diagnosis and management (51). Furthermore, despite the recent decline in reported cases, the high level of underreporting and increased socioeconomic instability in endemic regions create significant obstacles to the eradication of HAT (61; 02).

Special considerations

Pregnancy

Vertical transmission, although uncommon, is well documented for both forms of the disease. The overall risk is unknown, but screening women in HAT endemic areas and testing newborns to mothers infected with Trypanosoma is highly recommended. One study explored long-lasting sequalae among patients with congenital gHAT, contracted from their mothers, who were themselves infected during pregnancy (41). It demonstrated that five out of six patients were seriously disabled. Some of the other neurologic symptoms in these patients included behavioral changes, tremor, involuntary movements, convulsions, and pyramidal signs, among others. Although there is a risk of embryo toxicity with treatments during pregnancy, the benefit of treatment has to be weighed against the risk of congenital HAT by the treating physicians. With the registration of fexinidazole, there may be an opportunity to treat pregnant patients after conducting further safety studies. There should also be further research and follow up on newborns to infected pregnant patients in order to further understand the true impact of HAT congenital transmission and to develop better treatment and prevention strategies (41).