Sleeping sickness is a disease which is transmitted by the tsetse fly. It is caused by the protozoan parasitic species in the Trypanosoma genus and called trypanosomiasis. Human African trypanosomiasis (HAT) is caused by two sub-species of trypanosomes, Trypanosoma brucei rhodiense and T.b. gambiense which are prevalent throughout sub-Saharan Africa. These protozoan parasites exhibit markedly different types of immune evasion techniques and pathological disease than other intracellular parasites. The parasites have a complex life cycle which involves tsetse flies (Glossina spp.) and large mammals, such as humans Disease is caused by the entry of the parasites into the blood vessels when a human host (or other) is bitten by the tsetse fly. Because trypanosomiasis is a typical zoonosis, there is an ever-present wild animal reservoir which makes disease eradication basically unattainable. The number of people at risk is approximately 50 million, with monitoring highly underutilized and valid statistics shortcoming. The pathology and immunobiology has been heavily studied in the laboratory, as T. brucei readily infects laboratory rodents.

The parsites live in the blood stream and lymphatics, and in advanced cases, they enter the cerebral spinal fluid (CSF) and affect the central nervous system (CNS). It is this chronic stage of the infection which gives the common name of "sleeping sickness" to the disease. Late-stage infected patients exhibit increased lassitude and apathy. Early-infection symptoms of the disease include periodic fevers, which are accompanied by a plethora of other varying symptoms (including: headache, malaise, night sweats, weakness, and nausea or vomiting). Usually the periods between febrile attacks is asymptomatic and may last weeks to months. The initation of the infection results in the passage of metacyclic trypanosomes being passed to the host, where they take up residence in the lymphatic system. Often a large sore, called a chanchre, develops at the site of the infectious fly bite, but otherwise there is a asymptomatic incubation period. There is known to be parasite replication at the site of injection which causes an acute skin reaction taking the form of a chancre. The formation of this chancre in poorly understood in an immunological sense, as it does not form in rodent infections in the lab.

Over the next one to two weeks following parasite injection, the metacyclic parasites differentiate into long slender (LS) bloodstream forms. These forms migrate into the bloodstreram via the draining lymph fluid. They replicate by binary fission in the bloodstream at an exponential rate. They have evolved a means of checking this exponential growth to regulate the level of parasitemia in the host to prolong infection and increase chances of infection. The LS forms differentiate in to short stumpy (SS) forms which are in a state of G0 and are able to be taken up and infective to the tsetse fly. The cue for this differentiation is as yet unidentified, but has been speculated to be mediated by a parasite-produced paracrine factor as the process also occurs in culture.

Upon invasion of the lymphatics, the host displays a variety of early infection symptoms. Generally the initial symptoms vary in their diversity, magnitude and progression rate depending on the infected host; native Africans generally display less acute infections than non-Africans. The incubation period is variable in length, asymptomatic, and is accompanied by low levels of parasitemia. During this early stage trypanosomes can often be seen in blood and lymph fluid and lymph node enlargement often accompanies the fevers. The lymph glands of the posterior cervical region are most often involved and their enlargement is known as Winterbottom's sign (see image upper left. courtesy of CDC). The enlargement is due to the proliferation of the trypanosomes within the lymph nodes, and aspiration of the enlarged nodes will often reveal parasites, even if there are too few to visualize in blood smears.

Anemia is usually noted in HAT patients and there is also relatively high white cell count and thrombocytopenia (decrease in platelet count). There is a characteristically elevated IgM levels in patients (up to four times that of uninfected persons), which is thought to be a result of the variable surface antigens displayed by the trypanosomes. This is the parasites’ most highly studied and important forms of immune system evasion is through the use of the variable surface glycoprotein (VSG) they possess.

The primary component of the antibody (Ab) response to the VSG is T-cell independent IgM response. Also T-cell dependent B-cell responses are important in mediating VSG specific IgG responses. This is due to the fact that VSG epitopes on intact trypanosomes act as t-cell independent antigens, while buried nonvarient epitopes are only presented to t-cells after a dying or opsonized parasite has by phagosotyzed. This t-cell mediated response is important in establishing the cytokine response to trypanosomiasis. And although antigenic variation is important, there are times when it is circumvented by other host factors conferring resistance to the parasite. This is readily seen the N’Dama breed of cattle which are ‘typanotolerant’ and control parasitemia at very low levels. This has lead to the study of another facet of trypanosome immunobiology, which is the phenomenon of immuno-suppression.

Immunosuppression in trypanosomiasis has been extensively studied, and occurs in all mammalian hosts of the disease. There are profound alterations is the macroscopic make up of the lymphoid system during infection, including splenomegaly, massive accumulation of b-cells and null cells and lymph node enlargement. In mice there is dramatic suppression of both b- and t-cell responses to specific antigens and mitogens in the spleen, lymph nodes, and peripheral blood. This suppression cannot be attributed to dilution of specific lymphatic factors due to non-specific proliferative responses. It affects t-cell independent responses and both primary and secondary t-cell dependent responses. These effects are not directly mediated by the parasite, but instead result from suppressor macrophages. Massive polyclonal b-cell activation is exhibited in both natural and experimental infections which manifests itself in elevated IgM levels, auto-antibody production, and elevated b-cell background proliferation.

In mice, phenotypic analysis shows an increase of MHC II expression, release of active oxygen intermediates, prostaglandin-E2 (PGE2) release, and nitric oxide (NO) production. These are characteristics of typical macrophage activation. Activated macrophages are needed to phagocytose opsonized trypanosomes, but they are also mediators of immunosuppression. When PGE2 was experimentally suppressed, a partially restored proliferative response was observed in both splenic and lymph node cell cultures from infected mice. There was also a suppression of IL-2 expression in the lymph nodes which was restored after PGE2 was suppressed. These tests demonstrating PGE2 as an effector product of suppressor macrophages has not been verified in vivo, nor is there data for human or bovine infections.
Inhibition of NO synthesis also led to partial resoration of immune cell proliferation in splenic cell cultures and directly correlates to suppressor macrophage activity. When trypanosome infected mice were treated with NO-synthase inhibitors there was a significant restoration of mitogen-driven T-cell proliferation and a reduction in the first parasitemic peak. Therefore, NO is a infection-exacerbating factor in murine trypanosomiasis. It has also been proposed that NO released by bone marrow macrophages may cause dyserythropoeisis associated with infection, as NO infected mice treated with NO-sythase supressors results in recovery from anemia. Interestingly, NO as a parasite suppressor in typanosomiasis is fairly useless. Although in vitro parasite cultures are effectively killed by either chemically generated or macrophage released NO; the addition of RBC’s in physiological concentrations negates this activity. This is because hemoglobin acts as a high affinity sink for NO in the form of met-haemoglobin. This means that T. brucei’s niche in the bloodstream is protective from NO damage.

After a variable amount of time, the trypanosomes will invade the central nervous system (CNS). This invasion marks the on-set of the late stage of infection. During the late stage parasites can be seen in aspirations from the cerebralspinal fluid (CSF) and in the brain parenchyma. It had previously been proposed that trypaonsomes enter the CNS through an early attack on the choroid plexus and then subsequent incorperation in to the CSF. In studies by Mulenga et. al., it was shown that the trypomastigotes invaded the brain tissue without disruption of the vascular endothelium in the capillaries of the brain. But the parasites were also found uniformly throughout the brain tissue, sugessting they were not filtering in through the CSF. Therefore Mulenga et. al. have proposed a process of transcytosis through the vascular endothelium as a means for trypanosomes to enter the CNS. This is an amazing finding and warrants further investigation, as if transcytosis were the means of invasion, the research for new chemotherapies would necessarily need to aim for a late-stage treatment that could cross the blood-brain barrier.

It is eventually the presence of the trypaonsomes in the CNS that leads to the death of the infected patient. They begin to exhibit increased apathy and lassitude at this point of infection and eventually slip into a coma, then death. There is currently only one drug in production to treat late-stage infection, Melarsoprol. The side-effects associated with this drug are profound, as it is arsenical in nature. Namely, it causes ensephalitis and can be so complicating that death occurs because of the treatment. Eflornithine is a late-stage treatment drug that was in production between 1991-1999, and it was better recieved with less side effects than melarsoprol.

There are currently two drugs in use for the treatment of early-stage infections, and they are specific for the two sub-species of T. brucei. These drugs are suramine, for the treatment of East African trypanosomiasis, and pentamidine, for treating West African trypanosomiasis.

Mulenga, C., J.D.M. Mhlanga, K. Kristensson, and B. Robertson. Trypanosoma brucei brucei crosses the blood-brain barrier while tight junction proteins are preserved in a rat chronic disease model. Neuropathology and Applied Neurobiology. 2001. 27, pp. 77-85

Sternberg, Jeremy M. Immunobiology of African Trypanosomiasis. Chemical Immunology. (1998). Vol. 70, pp186-199.