African sleeping sickness, formally called human African trypanosomiasis (HAT), is a parasitic disease caused by single-celled organisms called trypanosomes, transmitted almost exclusively through the bite of the tsetse fly. It earned its common name from a hallmark late-stage symptom: a catastrophic disruption of the sleep-wake cycle that, left untreated, progresses to coma and death. The disease has shaped the ecology, demographics, and even the genetics of sub-Saharan Africa for centuries, and while case numbers have plummeted in recent decades, the story of sleeping sickness is far from over.
Two Diseases Under One Name
What most people call “sleeping sickness” is actually two distinct illnesses caused by two subspecies of the parasite Trypanosoma brucei. The West and Central African form, caused by T. b. gambiense, accounts for the vast majority of cases and tends to progress slowly over months or even years. The East African form, caused by T. b. rhodesiense, is rarer but far more aggressive, sometimes killing within weeks if untreated. The two subspecies occupy geographically separate zones, produce different clinical pictures, and even respond to different drugs.1PubMed Central. Comparative analysis of cerebrospinal fluid from the meningo-encephalitic stage of T. b. gambiense and rhodesiense sleeping sickness patients using TMT quantitative proteomics This matters because surveillance and treatment strategies that work for one form may not suit the other.
The gambiense form is the bigger public health concern simply because of numbers. It circulates in a slow, smoldering cycle: people can carry the parasite for months before feeling seriously ill, which gives the infection ample time to spread via tsetse bites. The rhodesiense form, by contrast, tends to flare up in zoonotic outbreaks, jumping from animal reservoirs to humans more sporadically. Both forms are fatal without treatment, but the timelines and the opportunities for intervention are very different.
How a Tsetse Bite Leads to Brain Invasion
When a tsetse fly bites a person, it deposits trypanosomes into the skin, where they multiply locally before entering the bloodstream. This first stage of the disease, sometimes called the hemolymphatic stage, produces relatively nonspecific symptoms: intermittent fever, headache, joint pain, swollen lymph nodes. Many people and even clinicians mistake it for malaria or another tropical infection.
The parasite’s ability to survive in the blood is remarkable. Each trypanosome wears a dense coat of a molecule called variant surface glycoprotein, or VSG. The immune system learns to recognize that coat and mount an attack, but before it can finish the job, a fraction of parasites switch to a completely different VSG. The immune system has to start over. This molecular shell game can repeat hundreds of times, producing the characteristic waves of fever as the parasite population surges, gets partially knocked down, and surges again.2PubMed Central. African trypanosomes expressing multiple VSGs are rapidly eliminated by the host immune system Each trypanosome expresses only one VSG at a time, and the switching is tightly regulated, making it an extraordinarily effective evasion strategy.
Eventually, the parasites cross into the central nervous system. Research on mouse models shows this happens in stages: the parasites first infiltrate the choroid plexus, a structure in the brain that produces cerebrospinal fluid and has relatively leaky blood vessels. From there they penetrate an epithelial cell layer to reach the ventricular system and eventually the brain tissue itself.3PubMed Central. Late stage infection in sleeping sickness White blood cells appear to pave the way, with immune signaling molecules like the cytokine IFN-γ and the chemokine CXCL10 playing key roles in amplifying the inflammatory response that lets the parasites breach the blood-brain barrier.4PubMed. Human African trypanosomiasis: How do the parasites enter and cause dysfunctions of the nervous system in murine models? Once inside the brain, the disease enters its second, meningo-encephalitic stage, and the signature neurological symptoms begin.
Why It Is Called Sleeping Sickness
The name is a bit misleading. Patients do not simply sleep too much. Instead, their internal body clock falls apart. Sleep intrudes into the daytime, wakefulness intrudes into the night, and the normal 24-hour rhythm of sleep, activity, and body temperature becomes fragmented and chaotic. A 2018 study published in Nature Communications demonstrated that this is fundamentally a circadian disorder. In infected mice, activity during the daytime rest period increased dramatically: roughly 15 percent of their activity occurred during the light phase, compared to about 1 percent in healthy controls. Some infected mice ran more than a quarter of their daily activity during what should have been rest time.5PubMed Central. Sleeping sickness is a circadian disorder
The same study found that later in the infection, mice showed disrupted feeding patterns and spent more time in a sleep-like state during their active phase, even though they did not sleep more overall across a full day. Their circadian temperature rhythms shifted in step with the changes in sleep and activity. These patterns closely mirror what clinicians observe in human patients: fragmented sleep bouts scattered throughout the 24-hour day, rather than a simple excess of sleep.6PubMed. African trypanosome infections of the nervous system: parasite entry and effects on sleep and synaptic functions
Alongside the sleep disruption, the brain invasion triggers a neuroinflammatory cascade. High levels of pro-inflammatory signaling molecules have been documented both in the brains of infected animals and in the cerebrospinal fluid of human patients. The hypothalamus, which houses many of the cell groups that regulate sleep and wakefulness, appears especially vulnerable to early inflammatory damage, possibly because it sits near structures with more permeable blood vessels.7PLoS Neglected Tropical Diseases. Trypanosoma brucei Invasion and T-Cell Infiltration of the Brain Parenchyma in Experimental Sleeping Sickness: Timing and Correlation with Functional Changes This inflammation also contributes to psychiatric symptoms, including confusion, personality changes, and psychosis, that can precede the more obvious sleep disturbance.
The Parasite’s Hidden Reservoirs
For decades, sleeping sickness was understood as a blood-and-brain disease. But recent research has revealed that trypanosomes also colonize body fat. A landmark study using a mouse model found that adipose tissue constitutes a third major reservoir for the parasite. The trypanosomes found in fat can replicate and, when transferred to a healthy animal, are fully capable of starting a new infection.8Cell Host & Microbe. Adipose Tissue Is a Significant Reservoir for Trypanosoma brucei and Associates with Specific Metabolic Adaptations Follow-up work in naturally infected animals confirmed that trypanosome DNA can be detected across different fat deposits in real-world hosts, not just laboratory mice.9PubMed Central. Adipose and skin distribution of African trypanosomes in natural animal infections
This matters for treatment. A drug that clears parasites from the blood and cerebrospinal fluid might leave a pocket thriving in fat tissue, setting the stage for relapse. It also raises questions about diagnostic accuracy: standard tests look for parasites or their markers in blood and spinal fluid, so an infection quietly persisting in adipose tissue could be missed entirely.
Animal Hosts and the Spillover Problem
Sleeping sickness is not just a human disease. The same tsetse flies that bite people also bite cattle, pigs, and wildlife, transmitting closely related trypanosomes that cause a livestock disease called nagana.10PubMed Central. Through the dark continent: African trypanosome development in the tsetse fly Animals serve as a reservoir from which human-infective parasites can spill over. A study in Uganda found that about a third of trypanosome isolates from non-human hosts, mainly cattle, carried the gene that makes the rhodesiense subspecies dangerous to people.11PLOS Neglected Tropical Diseases. Genetic Diversity and Population Structure of Trypanosoma brucei in Uganda: Implications for the Epidemiology of Sleeping Sickness and Nagana
Pigs are an underappreciated part of the picture. In a study from Ghana’s forest zone, nearly half of sampled pigs were infected with trypanosomes, and blood-meal analysis of tsetse flies confirmed they were actively feeding on both domestic pigs and warthogs.12PubMed Central. The Transmission of Animal African Trypanosomiasis in Two Districts in the Forest Zone of Ghana These animal reservoirs complicate elimination efforts because even if every human case were found and treated, the parasite could persist in livestock and wildlife, waiting for a tsetse fly to bridge the gap again.
Asymptomatic Carriers and Surveillance Gaps
One of the most troubling recent findings is that some people carry human-infective trypanosomes without showing symptoms. A case report from southeastern Nigeria documented a pregnant woman with no signs of sleeping sickness whose blood harbored a trypanosome confirmed to be infective to humans through laboratory testing. She was mildly anemic with a history of recurrent miscarriage but was otherwise clinically silent.13PubMed Central. Silent transmission of human African trypanosomiasis: isolation of a human-infective trypanosome from an asymptomatic pregnant woman in south-eastern Nigeria The region where she was found had not previously been considered an active focus for sleeping sickness.
Asymptomatic carriers pose a double problem. They can sustain transmission without anyone realizing the disease is present, and they fall through the cracks of surveillance programs designed around symptomatic illness. How many such carriers exist, and for how long they can remain infectious, remain open questions. But their existence suggests that the true geographic range of sleeping sickness may be broader than current case maps indicate.
Diagnosis in Remote Settings
Detecting sleeping sickness has always been difficult. The parasites circulate at low levels in the blood, making microscopy unreliable without concentration techniques, and distinguishing between the two disease stages requires a lumbar puncture to check cerebrospinal fluid. In rural sub-Saharan Africa, where most transmission happens, neither specialized labs nor trained personnel are readily available.
A rapid diagnostic test called the HAT Sero-K-SeT has been developed specifically for this context. It detects antibodies against the gambiense parasite and can be used without electricity or refrigeration. In a case-control study, the test achieved a sensitivity of about 99 percent and a specificity of about 99 percent, performing comparably to the older standard serological test (CATT) and the laboratory-based immune trypanolysis assay.14The Lancet Infectious Diseases. Accuracy of the HAT Sero-K-SeT rapid diagnostic test for gambiense human African trypanosomiasis: a case-control study A separate Phase III study in the Democratic Republic of the Congo confirmed high accuracy, though with a smaller sample of confirmed second-stage patients.15PubMed Central. A Phase III Diagnostic Accuracy Study of a Rapid Diagnostic Test for Diagnosis of Second-Stage Human African Trypanosomiasis in the Democratic Republic of the Congo Getting a reliable, portable test into health centers across the tsetse belt has been one of the practical victories of the past decade.
From Arsenic to Oral Pills
The history of sleeping sickness treatment is grim. For most of the twentieth century, the only drug available for the brain stage of the disease was melarsoprol, an arsenic-based compound administered intravenously. It was painful, toxic, and caused a severe encephalopathic reaction in a subset of patients that killed roughly half of those affected.16PubMed Central. Clinical Study on the Melarsoprol-Related Encephalopathic Syndrome: Risk Factors and HLA Association Patients sometimes called it “fire in the veins.” Yet without it, second-stage sleeping sickness was universally fatal, so clinicians had no choice but to use it and hope for the best.
The landscape has transformed. Fexinidazole, the first all-oral treatment for gambiense sleeping sickness, was approved in 2018. In a prospective trial enrolling patients with early-stage and early second-stage disease, treatment was effective at 12 months for about 99 percent of participants. The most common side effects were headache and vomiting, and none were severe enough to stop treatment.17The Lancet. Fexinidazole evaluating safety and efficacy in early-stage gambiense human African trypanosomiasis: a prospective, multicentre, open-label, single-arm cohort study A 10-day oral course replacing intravenous arsenic was a monumental step.
An even simpler option may be coming. Acoziborole, a single-dose oral drug, was tested in a Phase 2/3 trial enrolling patients with late-stage gambiense disease. The treatment success rate at 18 months was about 95 percent in the main analysis population and about 98 percent in the evaluable population, comparing favorably with the previous standard combination therapy.18PubMed Central. Efficacy and safety of acoziborole in patients with human African trypanosomiasis caused by Trypanosoma brucei gambiense: a multicentre, open-label, single-arm, phase 2/3 trial If approved and rolled out widely, a single pill that cures late-stage sleeping sickness would remove one of the biggest remaining logistical barriers to elimination: the need for patients to remain in a treatment facility for multiple days.
Targeting the Fly
Because tsetse flies are the nearly exclusive vector, reducing their numbers directly reduces disease transmission. A deceptively simple tool has proven effective: “Tiny Targets,” small panels of insecticide-treated blue and black cloth mounted on poles near riverbanks and vegetation where tsetse rest. Tsetse are attracted to the color and shape, land on the cloth, and pick up a lethal dose of insecticide.
In Cameroon’s Campo sleeping sickness focus, deploying Tiny Targets reduced tsetse catches by about 73 percent within twelve months.19PubMed Central. Impact of a small-scale tsetse fly control operation with deltamethrin impregnated “Tiny Targets” on tsetse density and trypanosomes’ circulation in the Campo sleeping sickness focus of South Cameroon In Uganda, a national-scale program covering about 4,000 square kilometers brought tsetse populations down to persistently low levels.20BMJ Global Health. Impact of a national tsetse control programme to eliminate Gambian sleeping sickness in Uganda: a spatiotemporal modelling study The targets are cheap to manufacture, easy to deploy, and do not require communities to change their behavior. They wear out and need replacing every few months, but the cost per unit is low enough to make sustained deployment feasible even in resource-limited settings.
How Close Is Elimination?
Reported cases of gambiense sleeping sickness have dropped dramatically since the start of the century, thanks to systematic screening of at-risk populations, better diagnostics, improved treatments, and vector control.21CABI Reviews. Human African trypanosomiasis: current status and eradication efforts The World Health Organization’s current road map sets an ambitious target: zero reported cases of gambiense HAT by 2030, which would represent elimination of transmission. To be verified, a country would need to report zero cases of human infection with the gambiense subspecies for at least five consecutive years.22PubMed Central. The elimination of human African trypanosomiasis: Monitoring progress towards the 2021-2030 WHO road map targets
The progress is real but fragile. Between 2021 and 2022, about 4.5 million people were screened for gambiense HAT, the great majority through active screening campaigns that send mobile teams into endemic villages. Yet history provides a cautionary lesson: sleeping sickness has surged and ebbed multiple times over the past century, and every time political instability or funding cuts disrupted surveillance, cases bounced back. Animal reservoirs, asymptomatic carriers, and the parasite’s hidden tissue niches all complicate the push toward zero. The rhodesiense form, with its zoonotic reservoir in cattle and wildlife, is not included in the transmission-elimination target precisely because its animal cycle makes a human-only elimination strategy insufficient.
A Genetic Legacy in Human Populations
Sleeping sickness has left a mark in the human genome itself. A protein called APOL1, found in human blood serum, is one of the body’s natural defenses against trypanosomes. It kills most species of African trypanosome on contact, which is why those species cause disease only in animals, not humans. The two subspecies that cause sleeping sickness evolved ways to resist APOL1, which is precisely what makes them dangerous to us.23PubMed Central. Evolution of the primate trypanolytic factor APOL1
In populations of African ancestry, two APOL1 gene variants called G1 and G2 occur at unusually high frequency. Both are associated with a substantially increased risk of kidney disease. So why haven’t they been weeded out by natural selection? The answer appears to be sleeping sickness. The G2 variant confers roughly a fivefold dominant protective effect against infection with T. b. rhodesiense.24PubMed Central. APOL1 renal risk variants have contrasting resistance and susceptibility associations with African trypanosomiasis A separate study in Malawi confirmed the association, finding that carrying a G2 variant was linked to substantially reduced susceptibility to rhodesiense infection.25PLOS Neglected Tropical Diseases. Association of APOL1 renal disease risk alleles with Trypanosoma brucei rhodesiense infection outcomes in the northern part of Malawi
The picture gets more complicated with the gambiense form. The same G2 variant that protects against rhodesiense was associated with faster progression of gambiense disease, while G1 was linked to asymptomatic carriage and undetectable levels of parasites in the blood. In other words, the two variants push the two forms of sleeping sickness in opposite directions. This trade-off, protection against one threat at the cost of vulnerability to another plus a kidney disease risk, echoes the more famous example of sickle cell trait and malaria. It is powerful evidence that sleeping sickness has been a strong enough selective pressure over millennia to reshape human genetics across the continent.
Colonial-Era Campaigns and Their Shadow
Sleeping sickness control has a fraught history intertwined with colonialism. In the early twentieth century, devastating epidemics swept through colonial territories in Central and East Africa, and European administrators responded with aggressive, often coercive medical campaigns. In French-controlled Central Africa, a system devised around 1917 sent mobile teams of a French military doctor, trained African nurses, soldiers, and porters from village to village. Inhabitants were required, sometimes at gunpoint, to submit to examination: neck glands palpated for swelling, blood and lymph examined under a microscope.26PLOS Neglected Tropical Diseases. Sleeping Sickness Epidemics and Colonial Responses in East and Central Africa, 1900–1940 The explicit goal was not individual treatment but killing trypanosomes across the entire population to starve the transmission cycle.
These campaigns did reduce sleeping sickness incidence, and the mobile-team model persists in modernized form today. But the methods were embedded in systems of forced labor, restricted movement, and racial hierarchy. The legacy is complex: populations in some endemic regions remain wary of screening teams, a wariness that is historically justified and that public health programs still have to navigate. Understanding that history helps explain why community engagement and trust-building are not optional add-ons to elimination campaigns but core operational requirements.