Trypanosomes

Trypanosoma brucei gambiense causes the chronic, slowly progressive form of West and Central African sleeping sickness. It is responsible for the large majority of reported human African trypanosomiasis cases. Disease evolves over months to years.

The haemolymphatic stage is prolonged and can remain relatively mild and unrecognised for an extended period before CNS involvement develops. Humans are considered the main reservoir, with limited animal reservoir contribution. This has made human case detection and treatment central to control efforts against this form.

Trypanosoma brucei rhodesiense causes East and Southern African sleeping sickness, a substantially more acute and rapidly progressive illness. The haemolymphatic and CNS stages often evolve over weeks rather than months to years.

Wild and domestic animals, particularly cattle and other livestock as well as wild game, serve as an important reservoir. Human case treatment alone therefore does not interrupt transmission in the way it can for gambiense disease. This zoonotic reservoir is a key epidemiological distinction between the two forms.

Both forms are transmitted by the bite of tsetse flies (Glossina species), which act as biological vectors.

Trypomastigotes ingested during a blood meal from an infected host undergo development and multiplication within the tsetse fly gut. They transform through several morphological stages before migrating to the salivary glands. There they develop into the infective metacyclic trypomastigote form over a period of several weeks. The fly then remains infective for life and can transmit infection at any subsequent blood meal.

A localised, painful nodule (trypanosomal chancre) may develop at the tsetse bite site within days to weeks of inoculation, reflecting local parasite multiplication before dissemination.

Following inoculation, trypomastigotes multiply extracellularly, first at the bite site and then disseminating via blood and lymphatics. Unlike Leishmania and Trypanosoma cruzi, African trypanosomes remain fully extracellular throughout the human infection, never entering an intracellular amastigote phase.

This produces the haemolymphatic (first) stage of disease:

• intermittent fever, headache, malaise
• generalised lymphadenopathy (classically prominent posterior cervical lymphadenopathy, described as Winterbottom's sign, particularly associated with gambiense disease)
• hepatosplenomegaly

A distinctive biological feature underlies the chronic, relapsing fever pattern seen especially in gambiense disease: antigenic variation. The trypanosome periodically alters the structure of its variant surface glycoprotein (VSG) coat. This allows successive parasite populations to evade the antibody response raised against previous antigenic variants.

The result is recurring waves of parasitaemia and fever. The immune system repeatedly clears one variant, only for a new antigenically distinct population to emerge.

Progression from the haemolymphatic first stage to the meningoencephalitic second stage occurs as trypanosomes cross the blood-brain barrier and invade the central nervous system. This produces the classic and clinically defining features of sleeping sickness:

• disturbed sleep-wake cycle (daytime somnolence with nighttime insomnia, the origin of the disease's common name)
• progressive personality and behavioural change, confusion, and other neuropsychiatric disturbance
• tremor and other movement abnormalities
• without treatment, progressive neurological deterioration, coma, and death

Staging (distinguishing first-stage haemolymphatic from second-stage CNS disease) is clinically essential because it determines treatment choice. Drugs effective against haemolymphatic disease do not reliably cross the blood-brain barrier, and second-stage disease requires CNS-penetrant therapy.

Staging has historically required examination of cerebrospinal fluid for trypanocidal white cell count and, where available, parasite detection.

Trypanosoma cruzi is the causative organism of Chagas disease, endemic across much of Latin America, from the southern United States to Argentina and Chile, and is distinguished from the African trypanosomes by its capacity for intracellular replication within host cells.

Chagas disease is transmitted principally by triatomine bugs (reduviid or "kissing" bugs), which characteristically feed at night on the face and around the mouth of a sleeping host.

Unlike the tsetse fly's salivary transmission, the triatomine bug transmits Trypanosoma cruzi through its faeces rather than its bite. The infected bug defecates near or at the bite site during or shortly after feeding. The host subsequently inoculates infective trypomastigotes present in the faeces into the bite wound (or adjacent mucous membranes, particularly the conjunctiva) through scratching or rubbing. Transmission is therefore dependent on host behaviour after the bite rather than the bite itself.

Additional recognised transmission routes:

• congenital (transplacental) transmission
• blood transfusion
• organ transplantation
• oral transmission via consumption of food or beverages (classically fruit juices in parts of Latin America) contaminated with triatomine faeces or crushed infected bugs

The oral route is associated with outbreaks of acute, sometimes severe disease.

After inoculation, trypomastigotes invade local host cells, notably macrophages and other nucleated cells including cardiac and smooth muscle. They transform into amastigotes and replicate intracellularly by binary fission. This directly parallels the Leishmania intracellular cycle, though it occurs in a much broader range of host cell types rather than being restricted to macrophages.

Infected cells eventually rupture, releasing amastigotes that transform back into trypomastigotes. The trypomastigotes enter the bloodstream, where they are the form typically detected on blood film during the acute phase. They can invade further cells locally or at distant sites, notably cardiac and gastrointestinal smooth muscle, establishing the tissue reservoirs responsible for chronic disease.

The acute phase is often mild or entirely asymptomatic, particularly in adults. It may produce fever, malaise, and localised inflammatory signs at the site of parasite entry:

• Romana's sign (unilateral painless periorbital oedema, when the conjunctiva is the portal of entry)
• a chagoma (localised indurated skin swelling at the bite site)

Trypomastigotes are detectable on blood film during this period.

Following acute infection, most individuals enter an indeterminate chronic phase. It is characterised by positive serology and persistent low-grade infection without clinically apparent organ disease. This can last for decades or lifelong in many infected individuals.

In a substantial minority (commonly cited as roughly 20 to 30 percent), progressive chronic Chagas disease eventually develops, principally affecting the heart and gastrointestinal tract.

Chronic Chagasic cardiomyopathy results from a combination of direct parasite-mediated myocyte damage and chronic immune-mediated inflammatory injury to cardiac tissue and its autonomic innervation. It comprises dilated cardiomyopathy, conduction system disease including right bundle branch block and complete heart block, ventricular arrhythmia, apical aneurysm formation, thromboembolism, and progressive heart failure.

Gastrointestinal disease (megaoesophagus and megacolon) results from progressive destruction of the myenteric (Auerbach's) plexus. This produces oesophageal and colonic dysmotility, dysphagia, and severe constipation.

The two trypanosomiases differ across every major axis despite shared genus membership.

Vector biology differs fundamentally. African trypanosomes are transmitted through the tsetse fly's saliva during the bite itself (inoculative transmission). Trypanosoma cruzi is transmitted through triatomine bug faeces deposited near the bite and subsequently rubbed into the wound or mucosa (contaminative transmission). This has direct implications for vector control strategy (bite prevention versus housing improvement and bug faeces exposure reduction).

Geographically, African trypanosomiasis is confined to the tsetse fly belt of sub-Saharan Africa. Chagas disease spans the Americas from the southern United States to southern South America. This reflects the very different ranges of their respective insect vectors.

Morphologically and cellularly, African trypanosomes remain extracellular throughout human infection, multiplying in blood and lymph and relying on antigenic variation for immune evasion. Trypanosoma cruzi is intracellular during tissue replication (amastigote form) and extracellular only during bloodstream transit (trypomastigote form). It lacks the extensive antigenic variation system of the African species.

Disease course also differs sharply. African trypanosomiasis is characterised by progressive haemolymphatic disease followed by CNS invasion. Without treatment, death from neurological disease occurs within a variable but generally more clearly staged timeframe (particularly compressed in rhodesiense disease).

Chagas disease typically follows a mild or silent acute phase, then a prolonged clinically silent indeterminate phase lasting years to decades. In a minority, chronic cardiac or gastrointestinal disease follows, driven by cumulative tissue damage. There is no equivalent CNS invasion phase, since Trypanosoma cruzi does not characteristically produce the meningoencephalitic syndrome that defines late-stage African sleeping sickness.