The Unusual Sleeping Sickness Parasite or Trypanosoma

Trypanosoma is a microscopic and unicellular parasite of humans and other animals. It can produce some very serious effects, but it’s a fascinating organism with some unusual characteristics. In humans, two species of Trypanosoma cause the disorder known as sleeping sickness. One of the symptoms of this disease is daytime sleepiness and disruption of sleep patterns at night. The disorder can be treated, although the drugs that are used are often quite toxic. If it’s not treated, the disease is very often deadly.

Trypanosoma sp.
Photo credit: CDC/Dr. Myron G. Schultz

Sleeping Sickness

Sleeping sickness is technically known as trypanosomiasis. It’s sometimes called African sleeping sickness because it occurs in the continent of Africa. Two subspecies of Trypanosoma brucei cause the illness: T. b. rhodesiense (which causes East African sleeping sickness) and T. b. gambiense (which is responsible for West Africa sleeping sickness). The latter organism causes most cases of the disease.

The parasite requires both a human and a tsetse fly (genus Glossina) in order to complete its life cycle. One of the common symptoms of sleeping sickness is a sore in the area where the fly bites, which is sometimes known as a chancre. Other common symptoms include a fever, headache, aching muscles and joints, a skin rash, and swollen lymph nodes. If the parasite invades the central nervous system, confusion, personality changes, and extreme fatigue may appear.

The effects are slightly different in the two types of sleeping sickness. The East African variety develops faster and if untreated can lead to an earlier death. A doctor’s diagnosis is required to confirm the presence of either form of the disease and to prescribe treatment.

The Trypanosoma Parasite and the Kinetoplast

In its mature form, Trypanosoma has an elongated cell with a tapered shape. It moves by beating its flagellum, or the whip-like appendage extending from its cell. It exists in specific forms in its insect host and in its human host and has a complex life cycle. Tsetse flies feed on human blood and pass the parasite to people while feeding on them.

Trypanosoma is sometimes referred to as a kinetoplastid. Kinetoplastids are organisms that contain a special structure in their mitochondrion known as a kinetoplast. Trypanosoma has a single and very large mitochondrion. The mitochondria are the organelles that produce energy. They are the only cellular location of DNA outside the nucleus in either the parasite or us. Unlike the parasite, we have numerous mitochondria in our cells.

The kinetoplast looks like a large and very dense granule in electron micrographs. It contains kinetoplast DNA or kDNA. This exists as a network of interlocking large and small mitochondrial DNA rings. The interesting arrangement is often said to resemble chain mail. RNA is also present in the kinetoplast. The nature, behaviour, and function of the structure are still being investigated.

Evading the Immune System With a Protein Coat

One reason why Trypanosoma is such a potent pathogen is it has the ability to evade our immune system. Its cells are covered by identical proteins. Our immune system recognizes these proteins as inappropriate for our body and destroys the parasite. This sounds like it should be the end of the story and the problem, but the parasite has a very useful feature that protects it from the immune system.

The proteins borne by the parasite are known as variable surface glycoproteins, or VSGs. A glycoprotein is a protein with attached carbohydrates. The proteins cover the cell membrane of the parasite in a dense, tightly-packed layer that prevents the immune system from attacking the membrane.

Occasionally, the parasite changes the identity of its VSGs. As is true for other proteins, the code for making VSGs is found in the genes located in the DNA of the nucleus. Trypanosoma has many VSG genes, each coding for a different protein. During an infection, parasite cells that haven’t yet been killed by the immune system activate a different VSG gene and make different surface proteins. One researcher likens the process to putting on a different coat. The antibodies produced by the immune system don’t recognize the new coat and the parasite concentration builds up again. Understanding the details of the process could be useful in improving the treatment of the disease.

Dealing With the Disease and the Parasite

Ideally, sleeping sickness either wouldn’t exist or every incidence of the disease would be quickly and successfully treated. From a biological point of view, however, the parasite that causes the disease is a very interesting entity. The scientific study of its complex life cycle, cell structure, and behaviour might help us to understand not only the parasite's biology but also the biology of other cells, including those of humans.

References

Sleeping sickness from the CDC (Centers for Disease Control and Prevention)

Facts about kinetoplasts from Tulane University

Trypanosoma camouflage from the phys.org news service