Human African Trypanosomiasis

African trypanosomiasis is an insect-borne parasitic infection of humans and other animals.

Human African trypanosomiasis (HAT), also known as African sleeping sickness or simply sleeping sickness, is caused by the species ''Trypanosoma brucei''. Humans are infected by two types, ''Trypanosoma brucei gambiense'' (TbG) and ''Trypanosoma brucei rhodesiense'' (TbR). TbG causes over 92% of reported cases.
Both are usually transmitted by the bite of an infected tsetse fly and are most common in rural areas.

Initially, the first stage of the disease is characterized by fevers, headaches, itchiness, and joint pains, beginning one to three weeks after the bite. Weeks to months later, the second stage begins with confusion, poor coordination, numbness, and trouble sleeping. Diagnosis involves detecting the parasite in a blood smear or lymph node fluid. A lumbar puncture is often needed to tell the difference between first- and second-stage disease.

Prevention of severe disease involves screening the at-risk population with blood tests for TbG. Treatment is easier when the disease is detected early and before neurological symptoms occur. The use of pentamidine or suramin treats the hemolymphatic stage of ''T. Brucei'' infection but if the disease progresses to the neurological stage dosages of eflornithine or a combination of nifurtimox and eflornithine can serve as a treatment for late-stage African Sleeping Disease. Fexinidazole is a more recent treatment that can be taken by mouth, for either stage of TbG. While melarsoprol works for both types, it is typically only used for TbR, due to serious side effects. Without treatment, sleeping sickness typically results in death.

The disease occurs regularly in some regions of sub-Saharan Africa with the population at risk being about 70 million in 36 countries. An estimated 11,000 people are currently infected with 2,800 new infections in 2015. In 2018 there were 977 new cases. In 2015 it caused around 3,500 deaths, down from 34,000 in 1990. More than 80% of these cases are in the Democratic Republic of the Congo. Three major outbreaks have occurred in recent history: one from 1896 to 1906 primarily in Uganda and the Congo Basin, and two in 1920 and 1970, in several African countries. It is classified as a neglected tropical disease. Other animals, such as cows, may carry the disease and become infected in which case it is known as nagana or animal trypanosomiasis.

Signs and symptoms

African trypanosomiasis symptoms occur in two stages: 1) the hemolymphatic stage and 2) the neurological stage. The hemolymphatic stage refers to the period when parasites are present in the blood and lymphatic system, prior to central nervous system involvement. The neurological stage, also called the meningoencephalitic phase, begins when Trypanosoma parasites cross the blood–brain barrier and invade the central nervous system. In addition to the hemolymphatic stage neurological symptoms can still present themselves, resulting in a difficulty in distinguishing the two stages based on clinical features alone.

The disease has been reported to present with atypical symptoms in infected individuals who originate from non-endemic areas (e.g., travelers). The reasons for this are unclear and may be genetic. Delayed or missed diagnosis in infected individuals who originate from non-endemic areas (travelers) have reported symptoms including higher susceptibility and quicker progression of advanced stages of the disease. The reasons for this are unclear but certain symptoms such as high fever could be due to genetic factors or a lack of previous exposure to non-human-pathogenic forms of trypanosomes. The low number of such cases may also have skewed findings. In such persons, the infection is said to present mainly as fever with gastrointestinal symptoms (e.g., diarrhea and jaundice) and with lymphadenopathy rarely developing.

Trypanosomal Ulcer

Systemic disease is sometimes presaged by a trypanosomal ulcer developing at the site of the infectious fly bite within 2 days of infection. The ulcer is most commonly observed in T. b. rhodesiense infection and rarely in T. b. gambiense infection, where ulcers are more common in persons from non-endemic areas.

Hemolymphatic Phase

The incubation period is 1–3 weeks for ''T. b. rhodesiense,'' and longer (but less precisely characterised) in ''T. b. gambiense'' infection. The first/initial stage, known as the hemolymphatic phase, is characterized by non-specific, generalised symptoms like: fever (intermittent), headaches (severe), joint pains, itching, weakness, malaise, fatigue, weight loss, lymphadenopathy, and hepatosplenomegaly.

Diagnosis may be delayed due to the vagueness of initial symptoms. The disease may also be mistaken for malaria (which may occur as a co-infection).

Intermittent Fever

Fever is intermittent, with attacks lasting from a day to a week, separated by intervals of a few days to a month or longer. Episodes of fever become less frequent throughout the disease.

Lymphadenopathy

Invasion of the circulatory and lymphatic systems by the parasite is associated with severe swelling of lymph nodes, often to tremendous sizes. Posterior cervical lymph nodes are most commonly affected, however, axillary, inguinal, and epitrochlear lymph node involvement may also occur. Winterbottom's sign, is a clinical finding involving swollen lymph nodes at the base of the skull or along the back of the neck, particularly characteristic of T. b. gambiense infections.

Other Features

Those affected may additionally present with: skin rash, haemolytic anaemia, hepatomegaly and abnormal liver function, splenomegaly, endocrine disturbance, cardiac involvement (e.g. pericarditis, and congestive heart failure), and ophthalmic involvement.

File:PMC5373517 pntd.0005324.g001.png, Ulcer of human African trypanosomiasis
File:AcuteSleepingSickness.jpg, Typical fine-spotted pink rash of acute African trypanosomiasis on the skin of the abdomen ("trypanid rash")
File:SSHemorragicRash.jpg, Numerous spots of bleeding into the skin of the leg in a person infected with ''T. b. rhodesiense''

Neurological Phase

The second phase of the disease, the neurological phase (also called the ''meningoencephalic stage''), begins when the parasite invades the central nervous system by passing through the blood–brain barrier. Progression to the neurological phase occurs after an estimated 21–60 days in case of ''T. b. rhodesiense'' infection, and 300–500 days in case of ''T. b. gambiense'' infection.

In actuality, the two phases of African trypanosomiasis—the hemolymphatic stage and the neurological stage—often overlap, and their clinical features can be nonspecific or evolve gradually, making it difficult to distinguish them based on symptoms alone. While signs such as enlarged lymph nodes and intermittent fever are more characteristic of the early stage, and neuropsychiatric symptoms such as sleep disturbances, confusion, or motor abnormalities suggest progression to the later stage, these indicators are not definitive. Consequently, to accurately determine the stage of the disease, specifically to determine central nervous system involvement, a lumbar puncture is performed to analyze the cerebrospinal fluid (CSF). The detection of trypanosome parasites in the CSF confirms that the infection has progressed to the neurological phase. This assessment is crucial because treatment protocols differ depending on whether or not the central nervous system has been affected. In the later stage, more intensive drugs that can cross the blood-brain barrier are necessary to eliminate the parasites from the brain and spinal cord.

Sleep Disorders

Sleep-wake disturbances are a leading feature of the neurological stage and give the disease its common name of "sleeping sickness". Infected individuals experience a disorganized and fragmented sleep-wake cycle. Those affected experience sleep inversion resulting in daytime sleep and somnolence, and nighttime periods of wakefulness and insomnia. Additionally, those affected also experience episodes of sudden sleepiness.

This sleeping impairment is also related to disruptions of circadian rhythm, the body's internal clock which regulates rhythmic behavior including metabolic patterns in cells. Studies indicate '' T. brucei'' alters the oscillatory expression of clock genes in the suprachiasmatic nuclei (SCN), among other brain regions, charged with circadian regulation. This alteration of expression may be moderated by the host's immune responses, such as parasitic activity and inflammation resulting from elevated TNF-α levels.

Neurological/Neurocognitive Symptoms

Neurological symptoms include: tremor, general muscle weakness, hemiparesis, paralysis of a limb, abnormal muscle tone, gait disturbance, ataxia, speech disturbances, paraesthesia, hyperaesthesia, anaesthesia, visual disturbance, abnormal reflexes, seizures, and coma. Parkinson-like movements might arise due to non-specific movement disorders and speech disorders.

Psychiatric/Behavioural symptoms

Individuals may exhibit psychiatric symptoms, which can sometimes dominate the clinical presentation. These symptoms include aggressiveness, apathy, irritability, psychotic reactions and hallucinations, anxiety, emotional lability, confusion, mania, attention deficit, and delirium.

Advanced/Late Disease and Outcomes

Without treatment, the disease is invariably fatal, with progressive mental deterioration leading to coma, systemic organ failure, and death. An untreated infection with '' T. b. rhodesiense'' will cause death within months whereas an untreated infection with '' T. b. gambiense'' will cause death after several years. Damage caused in the neurological phase is irreversible.

Circadian Rhythm Interactions

Circadian rhythm, an intrinsic clock that mediates rhythm of biological function, is affected by African trypanosomiasis. '' T. brucei'' alters the rhythmic activity of clock genes in basal forebrain, hypothalamus, thalamus, locus coeruleus,brainstem, etc
. Both parasitic activity and inflammation induced by elevated TNF-α levels impairs oscillatory expression within the host. These disruptions to regulate circadian gene expression are evidenced to contribute to key symptoms of African trypanosomiasis such as fragmented sleep, temperature changes, and abnormal hormone release.

African Sleeping Sickness Effect on Circadian Rhythm

Most organisms implement internal timing mechanisms to regulate the homeostasis of the body with the environment. These mechanisms, called circadian clocks, regulate pathways in core processes, where in mammals the primary circadian clock is the suprachiasmatic Nucleus (SCN). The SCN’s ability to serve as the organism's primary internal clock and send signals to adjacent clocks in order to collectively synchronize it can be affected by a variety of factors. Certain factors that induce inflammation such as viruses, bacteria, or parasites can disrupt the interactions between a cell’s circadian clock and the central pacemaker. Parasite aims to modify certain aspects of their host’s behavior in a way that favors their own survival and probability of transmission.To counteract this the internal clock on hosts' immune cells anticipate the time of infection by the parasite and thus optimize its cellular defenses or susceptibility to getting infected. In the case of Trypanosoma brucei, the parasite takes advantage of the host immune cells' dependence on rhythmic regulation; it attacks the internal clock of the cells in order to improve its survival and multiplication.

African sleeping sickness mainly disrupts the sleep/wake cycle alongside body temperature and hormonal regulation. After treatment, the sleep-wake cycle is able to revert back to normal, indicating that the parasites are responsible for circadian rhythm alteration rather than neuronal damage.

Sleeping sickness disrupts both sleep timing and architecture. The underlying causes were investigated in a mouse model where ''T. brucei'' infected mice had a reduced ability to maintain REM sleep and an inability for a homeostatic response to sleep deprivation. There were also reduced electrophysiological responses, electrical activity produced by the nervous system and heart, and behavioral changes. This presented a likelihood to ''T. brucei'' altering homeostatic adenosine signaling in addition to the inflammatory responses generated from the infection.

Effect of Inflammation on Circadian Rhythm Regulation

Inflammation modifies circadian physiology through altered homeostatic regulation. This is promoted by the response of the SCN to proinflammatory cytokines that most notably causes phase shifts in locomotor rhythms, seen in mice.

In a studied mouse model, the response to a ''T. brucei'' infection was analyzed where inflammatory molecules such as the proinflammatory cytokine interferon, IFN-γ, was released in positive correlation with a greater severity of sleeping sickness. The influence of IFN-γ on the circadian-timing system and the altered SCN function was observed.

Pro-inflammatory cytokines are enacted during an inflammatory response, generating reactions that alter a circadian clock. Cytokines such as TNF-alpha and IL-1 are associated with sleep sickness related symptoms such as fever, fatigue and sleep disturbances. The role of these cytokines is currently being explored, however, the sites of ''T. brucei'' infection generally involve an influx of inflammatory cells which introduce its potential role in the disruption of sleeping rhythms.

Circadian Cycle in Trypanosoma

Trypanosoma brucei is an extracellular parasite discovered to disrupt the circadian clock in the host with its own intrinsic clock. ''T. brucei'' is able to regulate its metabolism at two different stages in vitro. The genes that function with a circadian rhythm in ''T. brucei'' exhibit maximum expression at two different phases in a day.

Instead of a traditional transcription and translation feedback loop model, ''T. brucei'' has its genome primarily organized into polycistronic units (PCUs) that are already transcribed mRNA molecules and, has most of its gene expression regulated post-transcription, including cycling genes.

''T. brucei'' has a circadian oscillating transcriptome that is most likely entrained to the tsetse daily biting pattern for the most effective parasitic transmission and trigger metabolic parasitic changes.

Cause

''Trypanosoma brucei gambiense'' accounts for the majority of African trypanosomiasis cases, with humans as the primary reservoir for transmission. In contrast, ''Trypanosoma brucei rhodesiense'' is primarily zoonotic, with accidental human infections. The epidemiology of African trypanosomiasis is dependent on the interactions between the parasite (trypanosome), the vector (tsetse fly), and the host.

''Trypanosoma brucei (T. brucei)''

There are two subspecies of the parasite that are responsible for starting the disease in humans. ''Trypanosoma brucei gambiense'' causes the diseases in west and central Africa, whereas ''Trypanosoma brucei rhodesiense'' has a limited geographical range and is responsible for causing the disease in east and southern Africa. In addition, a third subspecies of the parasite known as '' Trypanosoma brucei brucei'' is responsible for affecting animals but not humans.

Humans are the main reservoir for ''T. b. 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''. These parasites primarily infect individuals in sub-Saharan Africa because that is where the vector (tsetse fly) is located. The two human forms of the disease also vary greatly in intensity. ''T. b. gambiense'' causes a chronic condition that can remain in a passive phase for months or years before symptoms emerge and the infection can last about three years before death occurs.

''T. b. rhodesiense'' is the acute form of the disease, and death can occur within months since the symptoms emerge within weeks and it is more virulent and faster developing than ''T. b. gambiense''. Furthermore, trypanosomes are surrounded by a coat that is composed of variant surface glycoproteins (VSG). These proteins act to protect the parasite from any lytic factors that are present in human plasma. The host's immune system recognizes the glycoproteins present on the coat of the parasite leading to the production of different antibodies (IgM and IgG).

These antibodies will then act to destroy the parasites that circulate in the blood. However, from the several parasites present in the plasma, a small number of them will experience changes in their surface coats resulting in the formation of new VSGs. Thus, the antibodies produced by the immune system will no longer recognize the parasite leading to proliferation until new antibodies are created to combat the novel VSGs. Eventually, the immune system will no longer be able to fight off the parasite due to the constant changes in VSGs and infection will arise.

Vector

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. Metacyclic trypomastigotes are the infectious form of the parasite that develops in the salivary glands of the vector and is transmitted through the bite. 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 body 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. The parasites then transform into procyclic trypomastigotes, specifically in the fly's midgut, 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 about three weeks. In addition to the bite of the tsetse fly, the disease can be transmitted by:
* Mother-to-child infection: the trypanosome can sometimes cross the placenta and infect the fetus.
* Laboratories: accidental infections, for example, through the handling of blood of an infected person and organ transplantation, although this is uncommon.
* Blood transfusion
* Sexual contact
Horse-flies (Tabanidae) and stable flies (Muscidae) possibly play a role in the transmission of nagana (the animal form of sleeping sickness) and the human disease form. Studies have noted a strain of Tetste fly Glossina palpalis that has proved to pose a public health challenge in animal livestock because of a high carrier rate of DNA of trypanosome parasites. further studies can observe the carrier rate across a range of domestic animals in addition to determining the prevalence and risk factors of nagana in different seasons and establish seasonal variation in animal trypanosomiasis transmission.

Pathophysiology

Tryptophol is a chemical compound produced by the trypanosomal parasite in sleeping sickness which induces sleep in humans.

Diagnosis

The gold standard for diagnosis is the identification of trypanosomes in a sample by microscopic examination. Samples that can be used for diagnosis include ulcer 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, so is of limited diagnostic use.

Trypanosomes can be detected from 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 using Giemsa's or Field's technique and examined under a microscope. Often, the parasite is in relatively low abundance in the sample, so techniques to concentrate the parasites can be used before microscopic examination. For blood samples, these include centrifugation followed by an 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 an examination of the sediment.

Three serological tests are also available for the detection of the parasite: the micro-CATT (card agglutination test for trypanosomiasis), 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.

Prevention

Currently, there are few medically related prevention options for African trypanosomiasis (i.e. no vaccine exists for immunity). Although the risk of infection from a tsetse fly bite is minor (estimated at less than 0.1%), the use of insect repellants, wearing long-sleeved clothing, avoiding tsetse-dense areas, implementing bush clearance methods and wild game culling are the best options to avoid infection available for residents of affected areas.

Regular active and passive surveillance, involving detection and prompt treatment of new infections, and tsetse fly control are 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 centers where teams travel daily to areas with high infection rates. Such screening efforts are important because early symptoms are not evident or serious enough to warrant people 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. A single case of sexual transmission of West African sleeping sickness has been reported.

In July 2000, a resolution was passed to form the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC). The campaign works to eradicate the tsetse vector population levels and subsequently the protozoan disease, by use of insecticide-impregnated targets, fly traps, insecticide-treated cattle, ultra-low dose aerial/ground spraying (SAT) of tsetse resting sites and the sterile insect technique (SIT). The use of SIT in Zanzibar proved effective in eliminating the entire population of tsetse flies but was expensive and is relatively impractical to use in many of the endemic countries afflicted with African trypanosomiasis.

A pilot program in Senegal has reduced the tsetse fly population by as much as 99% by introducing male flies that have been sterilized by exposure to gamma rays.

Treatment

The treatment is dependent on if the disease is discovered in the first or second stage of the disease. A requirement for treatment of the second stage is that the drug passes the blood–brain barrier.

First Stage

The treatment for first-stage disease is fexinidazole by mouth or pentamidine by injection for ''T. b. gambiense''. Suramin by injection was previously used for ''T. b. rhodesiense'', but fexinidazole has replaced this as the first-line treatment since 2024.

Second Stage

Fexinidazole may be used for the second stage of TbG, if the disease is not severe. Otherwise a regimen involving the combination of nifurtimox and eflornithine, nifurtimox-eflornithine combination treatment (NECT), developed by the Drugs for Neglected Diseases initiative and partners, or eflornithine alone appear to be more effective and result in fewer side effects. These treatments may replace melarsoprol when available. NECT has the benefit of requiring fewer injections of eflornithine.

Intravenous melarsoprol was previously the standard treatment for second-stage (neurological phase) disease and is effective for both types. Melarsoprol, which causes death in 5% of people who take it and carries a risk of resistance development , was previously the only treatment for second stage ''T.b. rhodesiense''. In 2024, it has been replaced by fexinidazole as the first-line treatment for sleeping sickness caused by ''T.b. rhodesiense''.

Drug Development Projects.
A major challenge has been to find drugs that readily pass the blood–brain barrier. The latest drug that has come into clinical use is fexinidazol, but promising results have also been obtained with the benzoxaborole drug acoziborole (SCYX-7158), developed by the Drugs for Neglected Diseases initiative. This drug is currently under evaluation as a single-dose oral treatment, which is a great advantage compared to currently used drugs. Another research field that has been extensively studied in ''Trypanosoma brucei'' is to target its nucleotide metabolism. The nucleotide metabolism studies have both led to the development of adenosine analogues looking promising in animal studies, and to the finding that downregulation of the P2 adenosine transporter is a common way to acquire partial drug resistance against the melaminophenyl arsenical and diamidine