The study, led by Deborah Buba of the University of Jos and Ahmadu Bello University in Nigeria, together with colleagues including Peter Mac Asaga of the University Medical Centre Freiburg, compared infections caused by two major African trypanosome species, Trypanosoma congolense and Trypanosoma brucei, in BALB/c mice. These two parasites are responsible for a large share of the animal trypanosomosis burden transmitted by tsetse flies across the continent. T. congolense is notorious among veterinarians for the severe anaemia it produces in cattle, while T. brucei, a close relative of the parasites that cause human sleeping sickness, is known for its ability to invade a wide range of host tissues. Despite their clinical importance, the combined influence of parasite species, inoculation route and host immune response on disease severity had rarely been examined within a single controlled experiment.

The researchers divided the animals into four infected groups of fifteen mice each, with one group receiving T. congolense and another T. brucei by intraperitoneal inoculation, and the remaining two groups receiving the respective parasites intramuscularly. A fifth group of five uninfected mice served as controls. Over the course of the experiment, the team tracked parasitaemia, body weight, rectal temperature and survival, and at defined endpoints they measured packed cell volume, a full panel of haematological indices, serum concentrations of the cytokines interferon-gamma and interleukin-10, and semi-quantitative pathology scores for the liver, spleen and kidney. The design, approved by the Ahmadu Bello University Committee on Animal Use and Care and reported according to the ARRIVE 2.0 guidelines, applied predefined humane endpoints so that no animal was permitted to reach a moribund state.

The survival data were unambiguous. Differences between the groups were highly significant, with a log-rank test yielding a p value below 0.0001. Mice infected intraperitoneally with T. congolense had the shortest median survival of any group, at just nine days. At the other extreme, mice infected intramuscularly with T. brucei survived longest among the infected groups. The two intermediate groups, intramuscular T. congolense and intraperitoneal T. brucei, fell between these poles. What makes this pattern remarkable is what the parasitaemia data did not show: the time until parasites became detectable in the blood and the peak levels they reached were broadly comparable across the infected groups. In other words, the animals that died fastest were not carrying the heaviest parasite burdens, a discordance that strikes at the heart of how virulence is conventionally assessed in laboratory models of trypanosomosis.

Haematological findings reinforced the clinical picture of acute disease. Infected animals developed anaemia, reflected in reduced packed cell volume, along with thrombocytopenia, a depletion of blood platelets. Both changes are hallmarks of African trypanosomosis in livestock and have been linked in previous work to the destruction or removal of red blood cells driven by inflammatory activation of the mononuclear phagocyte system. Earlier studies have implicated interferon-gamma produced by natural killer cells, natural killer T cells and CD8-positive T cells in driving myeloid cell activation and erythrophagocytosis, the engulfment of red blood cells by macrophages, which culminates in the acute anaemia characteristic of these infections. Other mechanisms, including the desialylation of red blood cells by parasite enzymes and cytokine-mediated haemodilution, have been proposed to contribute to the anaemia seen with different trypanosome species.

The cytokine measurements added a layer of nuance. Interferon-gamma, a pro-inflammatory signalling molecule central to the activation of macrophages during protozoal infections, was elevated in all infected animals relative to uninfected controls. However, its concentrations did not distinguish among the four infected groups, meaning this canonical inflammatory signal was similarly engaged regardless of parasite species or inoculation route. Interleukin-10, an anti-inflammatory cytokine that restrains excessive immune activation, told a different story. Its concentrations were higher in mice infected with T. congolense than in those infected with T. brucei, a species-associated pattern that suggests the two parasites elicit qualitatively different immunoregulatory environments within the same inbred host strain.

The histopathological analysis was equally revealing, both for what it showed and for what it did not. Semi-quantitative scoring of the liver, spleen and kidney confirmed that infection inflicted significant damage on all three organs: pathology scores were significantly higher in infected animals than in uninfected controls. Yet when the infected groups were compared with one another, the scores did not differ significantly. The severity of organ-level lesions, like the parasite burden, failed to track with survival. Mice that succumbed rapidly to intraperitoneal T. congolense infection did not carry measurably worse tissue damage than mice that survived longer with T. brucei. This second discordance, between lesion severity and outcome, implies that the functional consequences of infection, rather than the visible structural damage, may be what determines how quickly an animal deteriorates.

Taken together, the results paint a picture in which disease outcome in African trypanosomosis is governed by something more subtle than parasite load or tissue destruction. The authors suggest that species-associated host responses may contribute to disease outcome, although they are careful to note that further mechanistic investigation is required before the causal pathways can be pinned down. One plausible interpretation is that the balance between pro-inflammatory and anti-inflammatory signalling, exemplified here by the species-linked differences in interleukin-10, shapes the physiological toll of infection independently of how many parasites are present or how damaged the organs appear. Excessive or poorly regulated inflammation could accelerate clinical decline even when parasite numbers and histological lesions are comparable, a concept consistent with the broader literature on immunopathology in trypanosome infections.

The findings also carry practical implications for experimental design in trypanosomosis research. Laboratory models of animal African trypanosomosis have long been criticised for their reliance on syringe-passaged parasites and laboratory rodents, which can diverge from the natural tsetse-transmitted infections seen in clinically relevant livestock hosts. The demonstration that inoculation route measurably alters survival, with intramuscular delivery of T. brucei producing the most prolonged course, echoes earlier work showing that route of inoculation influences the virulence of both T. congolense and T. brucei brucei in mice. Researchers selecting a challenge model should therefore be aware that the route of infection is not a neutral variable but an active determinant of disease tempo, one that could confound comparisons between studies or mask genuine differences in parasite virulence or drug efficacy.

For the veterinary and medical communities, the study underscores a message that has been building across the trypanosomosis literature: virulence is a property of the interaction between parasite and host, not of the parasite alone. Recent work has revealed that African trypanosomes occupy unexpected tissue niches, including the skin and adipose tissue, where they may persist and adapt in ways that influence transmission and disease. The new data add a controlled, quantitative demonstration that two closely related parasite species, delivered at comparable infectious doses into the same host strain, can produce the same parasite burden and the same organ pathology while killing at different speeds and provoking different cytokine profiles. Untangling the mechanisms behind these discordances, particularly the role of interleukin-10 and other immunoregulatory pathways, may ultimately inform approaches that target the damaging host response as well as the parasite itself, an increasingly recognised strategy for diseases in which immunopathology drives the clinical picture.

Subject of Research: Comparative pathogenicity and multi-organ histopathology of Trypanosoma congolense and Trypanosoma brucei infection in mice

Article Title: Comparative Pathogenicity and Semi-quantitative Multi-organ Histopathology of Trypanosoma congolense and Trypanosoma brucei Infection in Mice

Article References: Buba, D., Adamu, S., Mohammed, B., Jatau, I. D., & Asaga, P. M. (2026). Comparative Pathogenicity and Semi-quantitative Multi-organ Histopathology of Trypanosoma congolense and Trypanosoma brucei Infection in Mice. Acta Parasitologica, 71(5), Article 232. https://doi.org/10.1007/s11686-026-01417-8

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01417-8

Keywords: Trypanosoma congolense, Trypanosoma brucei, animal African trypanosomosis, pathogenicity, inoculation route, anaemia, histopathology, cytokines, IFN-gamma, IL-10, murine model, parasitaemia

T. brucei leads a double life, cycling between the bloodstream of mammals and the gut of the tsetse fly. Within the mammalian host, the parasite exists mainly in two forms: the long slender form, which divides rapidly and drives parasitaemia upward, and the short stumpy form, a non-dividing stage produced through a quorum-sensing mechanism once parasite density climbs. The stumpy form is the parasite’s ticket to the next stage of its life cycle, primed to differentiate into procyclic forms once it reaches the fly. Yet stumpy cells are also fragile. Earlier work showed they survive only around four days in the bloodstream, and during the remission of the first parasitaemic peak in mice they disappear far faster than their natural turnover alone could explain. Something else, the researchers suspected, was accelerating their demise.

The team’s first clue came from body temperature. In C57BL/6J mice infected with pleomorphic T. brucei, parasitaemia first became detectable around three days post-infection, and by day four the slender forms had begun differentiating into stumpy cells just as fever set in. Both parasite load and body temperature peaked on days four and five, when stumpy forms made up nearly three-quarters of the population. Parasitaemia then collapsed to undetectable levels by day seven, coinciding with the return of body temperature to baseline. A Pearson correlation analysis confirmed a strong positive relationship between parasitaemia and temperature during the first week of infection, with a correlation coefficient of 0.678. A multiple linear regression model, which explained 67.54 percent of the variance in late-phase parasite levels, singled out the temperature window spanning days three to five as a significant predictor of subsequent parasite decline.

To test whether heat itself could kill stumpy cells, the researchers purified them from infected mice and cultured them at different temperatures. At 27 degrees Celsius, the temperature of the tsetse fly, the cells survived and even began differentiating into procyclic forms. At 37 degrees, normal mammalian body temperature, they underwent their expected natural turnover over roughly three to four days. But at 39 degrees, a fever temperature previously recorded in infected mice, mortality climbed steeply. Notably, long slender forms tolerated prolonged heat exposure far better; only the stumpy cells declined sharply within the first 48 hours. Fever, the results suggest, places a disproportionate burden on the very stage of the parasite that is already winding down.

The interplay with antibodies proved equally revealing. Plasma from mice at 21 days post-infection, rich in effective antibodies, suppressed stumpy cells in culture but could not eliminate them completely, unlike its effect on slender forms. Plasma from mice at day six, when the first peak collapses, showed no killing activity against either form. Yet when elevated temperature and day-21 plasma were combined, more than 99.5 percent of stumpy cells were eradicated within 24 hours, while either treatment alone left 15 to 18 percent alive. This synergy indicates that the well-documented IgM and complement-mediated clearance of the first parasitaemic peak is only part of the story. The parasite’s own heat sensitivity, amplified by fever, appears to be an underappreciated partner in that dramatic remission.

What happens inside a heat-stressed stumpy cell? The researchers found that apoptosis-like events, detected through phosphatidylserine exposure, rose from almost nothing at 27 degrees to 3.72 percent after 24 hours at 37 degrees and 30.63 percent at 39 degrees. Slender forms showed essentially no such events at any temperature. Gene expression analysis pointed to the mitochondrion as the driver: Metacasp-3, a mitochondrial-associated apoptosis gene, was upregulated roughly twofold at 37 degrees. Fluorescence imaging of mitochondria stained with MitoTracker revealed progressive damage, with punctate dye aggregation appearing in about 43 percent of cells after 24 hours at 37 degrees and over 95 percent after the same period at 39 degrees. Transmission electron microscopy confirmed the structural toll, showing fenestrated mitochondrial membranes and multivesicular structures suggestive of a stress-induced degradative pathway resembling autophagy.

Mitochondrial dysfunction went hand in hand with oxidative stress. Using MitoSOX staining, the team measured mitochondrial reactive oxygen species accumulating in stumpy cells exposed to heat: 15.30 percent of cells showed oxidative stress after 24 hours at 37 degrees, rising to 44.58 percent at 39 degrees, while cells kept at 27 degrees remained largely unaffected. The reactive oxygen species co-localized with the damaged mitochondria, reinforcing the link between heat, mitochondrial injury, and cell death. Mitochondrial membrane potential, a key indicator of mitochondrial health, collapsed in parallel, falling to 51.36 percent of baseline at 37 degrees and just 19.38 percent at 39 degrees. Slender forms, by contrast, showed neither mitochondrial oxidative stress nor membrane potential loss under the same conditions.

Untargeted metabolomics added a metabolic dimension to the picture. Comparing stumpy cells incubated for 12 hours at 37 versus 39 degrees, the researchers detected 306 significantly altered metabolites among more than 22,000 measured. The most striking change was a sixfold accumulation of succinate at fever temperature, alongside elevated glucose-6-phosphate and 3-phosphoglycerate, pointing to disrupted energy production through glycolysis and the TCA cycle. The team attributes the succinate surge to mitochondrial fumarate reductase, an enzyme known to generate reactive oxygen species and highly expressed in stumpy cells. Lipid metabolism was also disturbed: myristic acid, a key component of the parasite’s surface GPI anchors, accumulated at 39 degrees, while carnitine, needed for fatty acid degradation, dropped significantly, suggesting the cells could neither properly remodel nor dispose of their lipids under febrile stress.

The findings carry broader implications for how trypanosome infections unfold. Stumpy cells lack complexes III and IV of the electron transport chain and maintain their membrane potential through glycolysis-derived ATP and a reversed ATP synthase, a precarious arrangement the authors describe as dancing on a knife-edge. This metabolic mismatch between a partially activated TCA cycle and a limited antioxidant arsenal, including lower levels of iron superoxide dismutase than in slender forms, may be an evolutionary trade-off that pre-adapts the parasite for the insect gut at the cost of vulnerability to mammalian fever. It may also explain why trypanosomes take refuge in the cooler, peripheral tissues of the skin and adipose tissue, and why livestock species with naturally higher baseline body temperatures often show low parasitaemia yet remain reservoirs for transmission.

The authors caution that they were unable to create a fever-suppressed mouse model to prove causation directly, though earlier veterinary studies found that anti-inflammatory drugs raised parasitaemia in infected animals, indirectly supporting the fever-clearance hypothesis. They also note that natural infections initiated by tsetse bites produce lower first peaks than experimental injections, so fever’s role there may be more modest, though still relevant before specific antibodies appear. Because fever is among the most common clinical signs in human patients, and because stumpy cells resist antibody-mediated elimination better than slender forms do, the study proposes that temperature-driven clearance complements immunity throughout infection. Understanding the molecular basis of this thermo-sensitivity, the researchers conclude, could open new avenues for controlling a disease that still threatens millions across sub-Saharan Africa.

Subject of Research: Heat-induced degeneration of short stumpy Trypanosoma brucei forms and the role of fever in clearing the first parasitaemic peak

Article Title: Thermo-related degeneration of stumpy forms of Trypanosoma brucei, the pathogen of African sleeping sickness

Article References: Thermo-related degeneration of stumpy forms of Trypanosoma brucei, the pathogen of African sleeping sickness. (n.d.). https://doi.org/10.1007/s44307-025-00081-9

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00081-9

Keywords: Trypanosoma brucei, sleeping sickness, short stumpy form, fever, mitochondrial damage, oxidative stress, apoptosis-like cell death, metabolomics, glycolysis, TCA cycle, parasitaemia, tsetse fly