History and Metabolic Context

History and Metabolic Context

Molecular Basis of Disease State

Treatment and Disease Management

Conclusions and Proposals for Future Work

Annotated Bibliography

Definition and Symptoms

Human African trypanosomiasis (HAT), also known as African sleeping sickness, is an infectious disease caused by trypanosomes, a type of parasite. The disease is caused by two different trypanosomes: Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense. Most cases of HAT are caused by Trypanosoma brucei gambiense. It is a disease of the circulatory and central nervous system (Stich, Abel, & Krishna, 2002).

 

There are two stages of the disease, characterized by different symptoms. The first stage of the disease is called the hemolymphatic stage. This stage is characterized by symptoms such as intermittent fever (corresponding with cycles of parasite multiplication in the blood) , joint pain, rash, and swollen lymph nodes, particularly at the base of the neck (Stich, Abel, & Krishna, 2002). It can also present as anemia (Pays et al, 2014).

 

Trypnosomes in the blood stream. Image taken from Brun et al, 2010.
Trypnosomes in the blood stream. Image taken from Brun et al, 2010.

 

The second stage of disease begins once the parasites have infiltrated the central nervous system. It is characterized by a disruption of sleeping schedule (which is where the disease gets its name), psychosis, headache, and reduced mental function. If left untreated, the disease will kill its victim (Stich, Abel, & Krishna, 2002). The more common, chronic form (caused by T. b. gambiense) can take years to cause death, whereas the rarer, acute form (caused by T.b. rhodesiense) causes death in months (WHO).

 

History

 

 

The disease is thought to be thousands of years old, originally existing in isolated areas in Africa, then spreading along the Congo river. It was specifically referenced as early as the 14th century, when the Arabian historian Ibn Khaldun gave a report of trypanosomiasis. He discussed an emperor of Mali who had been struck by the disease:

“He told me that Jata [the empreror] had been smitten by the sleeping illness, a disease which frequently afflicts the inhabitants of that climate, especially the chieftains who are habitually affected by sleep. Those afflicted are virtually never awake or alert. The sickness harms the patient and continues until he perishes (Steverding, 2008).”

One of the main impetuses of the quest to determine the cause of HAT was the slave trade. Slavers noticed that the humans they were trafficking were dying of a mysterious sleeping sickness, and wanted to prevent this from happening (Steverding, 2008).

There was an epidemic in Uganda from 1896-1906, an epidemic  in Africa in 1920, and one that began in the 1970s, each killing thousands of people (WHO).

 

The tsetse fly, the vector for human african trypanosomiasis. Image from Google Images.
The tsetse fly, the vector for human african trypanosomiasis. Image from Google Images.

 

In 1903, a scientist named David Bruce identified the vector of the disease: the tsetse fly (Strong, 1944). An effective treatment was not developed until 1910- an arsenic-based drug called atoxyl that had severe side effects. Some older drugs are still used today, such as pentamidine (discovered in 1949) and melarsoprol (also discovered in 1949). Pentamidine is used during stage one of illness. Melarsoprol is used in stage two of the disease. It is arsenic-based like atoxyl, and can have severe side effects, including encephalitis and death (WHO).  It is highly preferred to diagnose the disease while it is still in its hemolymphatic stage.

 

Diagnosis

 

The disease is usually diagnosed by viewing parasites in the blood under a microscope. If one wishes to asses whether the disease has progressed to stage two, a lumbar puncture is performed in order to determine whether parasites can be found in cerebrospinal fluid (WHO). Lately, there is a new, promising technique to diagnose HAT in the field. The typical preparation of a sample involves chilling it at multiple step and much preparation, which can be difficult to perform without a proper lab in less-developed areas. Recently, there has been a promising new protocol applied in the field called LAMP (loop-mediated isothermal amplification of DNA). This protocol cuts out the many chilling steps required: it uses dried blood samples, eliminating the need for temperature control of liquid samples. Also, since the PCR for DNA amplification in LAMP mostly takes place at one temperature, expensive equipment is not needed (Hayashida et al, 2015). Prior to this paper, LAMP kits were available under a company called Eiken Chemical Co Ltd, but they were very expensive, and therefore did not make sense for an under-equipped laboratory to use. Hayashida et al attempted to make their own LAMP protocol without access to the tightly guarded secrets of the company, using their understanding of the process. It looks like they had success, so it is possible that LAMP will be used more often to diagnose HAT more quickly. Still, if one is to be completely confident in a diagnosis of trypanosomiasis, one must see the parasites in the blood.

 

Affected Biomolecules

If one is to understand the mechanism by which trypanosomes cause disease, one should first have an understanding of the non-disease state functions of the biomolecules that HAT affects. These biomolecules include tumor necrosis factor, hemoglobin, and haptoglobin.

 

Tumor necrosis factor is a cytokine (mostly produced by macrophages) that can cause apoptosis, and is also involved in fever and cachexia (“wasting away”). There are two types of tumor necrosis factor receptors in the body: TNFR1, which is found in all cells and about which we have the most information, and TNFR2, which is found in immune cells only. When TNF binds to its receptor, the receptors form trimers. This causes a conformational change that leads to the release of a molecule called SODD from the receptor’s “intracellular death domain”  (Wajant, Pfizenmaier, & Scheurich, 2003). This domain then binds to an adaptor protein called TRADD. In this form, the protein can initiate one of three pathways. The first of these three pathways is the activation of the NF-kB pathway, which ends with the increased production of transcription factors involved in cell survival and proliferation. The second is activation of the MAPK pathways, which end in the production of transcription factors that promote cell differentiation and apoptosis. The third pathway is for TRADD (attached to the TNF receptor) to interact with FADD. FADD is a cysteine protease, that, when activated, can lead to cell death (Chen & Goeddel, 2002). HAT inhibits the production of TNF through a mechanism discussed under the page “Molecular Basis of Disease State.”

A representation of the TNF signaling pathway from Chen & Goeddel, 2002.
A representation of the TNF signaling pathway from Chen & Goeddel, 2002.

 

Hemoglobin is the molecule that carries oxygen throughout the body via blood. Haptoglobin is a protein that binds hemoglobin with high affinity, and then removes it from circulation. This prevents loss of iron, and inhibits hemoglobin’s oxidative damage potential. Trypanosomes scavenge heme from the hemoglobin-haptoglobin complex (Pays et al, 2014).

7 Replies to “History and Metabolic Context”

  1. Hi Gabby! This was both interesting to read and very informative. I found it particularly interesting that treatments were able to be discovered as early as 1910 and even more so that drugs from the late 40’s are still part of our first line of defense against both stage 1 and stage 2 HAT. My question, however, deals with the diagnosis of HAT. You mentioned that a new protocol by the name of LAMP has been tested in the field as a way to amplify DNA without the use of traditional PCR equipment. How does this method cut out the numerous chilling steps and ensure proper primer annealing during DNA replication? (I understand this may not be relevant to your project, but I was curious) More on topic, why is diagnosis still dependent upon examination of a patient’s blood if LAMP is effective? In other words, what contribution does LAMP have in diagnosing HAT that a simple blood examination does not?

    1. Hi Matt! So, I’ll answer your last two questions first. The use of LAMP to diagnose HAT in a way that is accessible to all is still a fairly new concept (note that the paper was published in 2015). Prior to this paper, LAMP kits were available under a company called Eiken Chemical Co Ltd, but they were very expensive, and therefore did not make sense for an under-equipped laboratory to use. The authors of the paper that discusses LAMP as an effective way to diagnose HAT actually were attempting to make their own LAMP protocol without access to the tightly guarded secrets of the company, and just using their understanding of the process. It looks like they had success, so it is possible that LAMP will be used more often to diagnose HAT more quickly. That being said, although it is effective, an examination of the blood is what 100% confirms trypanosomiasis- seeing the trypanosomes is clear validation, so if you want to be absolutely sure, you still have to look at the blood.

      To answer your question about LAMP cutting out the chilling of the samples, the reason is actually fairly simple, but ingenious: LAMP uses dried blood samples, so there is no need to chill them.

  2. Hi Gabby,
    I found this article to be very informative. I think one area of improvement would be to include transitions of some kind, particularly in the “Affected Biomolecules” section. I think the figure you have demonstrating how TNF signaling is great, and it could be improved by adding something showing that in the diseased state, TNF production is blocked. Similarly, a figure showing the interplay between hemoglobin, haptoglobin, and the parasite. Out of curiosity, how do trypanosomes scavenge iron from heme and is it necessary for parasite survival?

    1. Hello Elia, thank you very much for your feedback! I agree, I think additional figures would be helpful, and I will try to find them in order to make the page as accessible as possible.

      I am glad you asked your questions, because I believe it made me realize a flaw in my understanding. To answer your questions, trypanosomes acquire iron from heme by transferring the protein (transferrin) that carries it into the lysosome. There, iron is freed, and the transferrin is degraded (O’Brien et al, 2008). I previously thought that trypanosome obtained iron from heme, but a closer look at the literature leads me to believe that it actually endocytocizes iron-bound transferrin for its iron needs, and heme is used for other processes. From my research, I am under the impression that heme is critical for the proper functioning of the parasite, but it is never directly stated that the trypanosome will perish without heme. It seems likely that some major functions will be impaired, such as PUFA biosynthesis, which uses the heme protein cytochrome b5. The function of cytochrome c in the host mitochondria would also be inhibited.

      From this evidence, although the literature never specifically states that the trypanosome will die without heme, I believe that blocking the trypanosome’s uptake of heme could have severely negative consequences for the parasite.

  3. Gabby,

    You’ve made a great article which is quite informative. You mention that HAT has been referenced as early as the 14th century; however, little context is currently given. Do we have any idea what people believed caused or cured HAT during earlier centuries? Did this change with European occupation of Africa? Please put some additional sources for the history section, I’d be interested in viewing what you find.

    1. Hi Cyrus! Thanks for checking out my page- I’m glad you enjoyed it. While I was unable to find anything about what people believed caused HAT in earlier centuries, I do know that people did not believe there was a cure. It was acknowledged that some people do recover, but with lasting damage. See this quote (said by John Atkins, a British Naval Surgeon, in 1734):

      “The Sleepy Distemper (common among the Negroes) gives no other previous Notice, than a want of Appetite 2 or 3 days before; their sleeps are sound, and Sense and Feeling very little; for pulling, drubbing or whipping will scarce stir up Sense and Power enough to move; and the Moment you cease beating the smart is forgot, and down they fall again into a state of Insensibility, drivling constantly from the Mouth as in deep salivation; breathe slowly, but not unequally nor snort. Young people are more subject to it than the old; and the Judgement generally pronounced is Death, the Prognostik seldom failing. If now and then one of them recovers, he certainly loses the little Reason he had, and turns Ideot… (Strong, 1944)”

      The Arabian historian Ibn Khaldun gave a report of trypanosomiasis even earlier (mid-late 1300s). He discussed an emperor of Mali who had been struck by the disease:

      “He told me that Jata [the empreror] had been smitten by the sleeping illness, a disease which frequently afflicts the inhabitants of that climate, especially the chieftains who are habitually affected by sleep. Those afflicted are virtually never awake or alert. The sickness harms the patient and continues until he perishes (Steverding, 2008).”

      One of the main impetuses for trying to figure out the cause of HAT was the slave trade. Slavers noticed that the humans they were trafficking were dying of a mysterious sleeping sickness, and they didn’t want their business to be hurt (heartwarming, isn’t it…). Still, what actually lead to the discovery of the vector of HAT was the discovery of the vector for nagana, the animal form of HAT. Having figured this out, microbiologist David Bruce went on to discover the vector of HAT. The species is named after him (Trypanosoma brucei) (Steverding, 2008).

      1. Fascinating subject with an interesting history! Great response, thanks Gabby.

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