trypanosome

A trypanosome is a type of parasitic protozoan which can cause a number of serious diseases in people and domestic animals, including African sleeping sickness and leishmaniasis.

The trypanosome is able to remain within a host's body for a very long time because it regularly changes the proteins (i.e. antigens) on its outer surface, so that the host's immune system must constantly develop new antibodies in order to continue to recognize and destroy it. The trypanosome uses a large number of different genes which can be used in many combinations to come up with the different surface proteins. As a result, the host is never able to completely eliminate the trypanosome without the aid of antibiotics.

From the science dictionary at http://biotech.icmb.utexas.edu/

Trypanonsome - a group of flagellated single cellular organisms (protozoa) which cause disease in vertebrate hosts

The causative agents of many tropical diseases remained a mystery until the development of microscopes made it possible to see them in the blood of infected people and animals. The first trypanosome to be associated with disease was named Lieshmania donovani after Leishman and Donovan who discovered the flagellate responsible for kala-azar, now called Leishmaniasis, in 1900.

The discovery of other socially important trypanonsomes followed. These include Trypanosoma brucei which causes African sleeping sickness, Trypanosoma vivax (a cattle pathogen in Brazil and Bolivia which spread from the native water buffalo), Trypanosoma congolense which causes nagana in cattle in Africa and T. cruzi, the parasite responsible for Chagas disease. Trypanosomes are generally found in Central and Southern America, Africa and the Middle East.

Trypanosomes have complex life cycles, passing through and reproducing in insects and mammals, both in the blood stream and in the tissues.

Life cycle

The parasite goes through a number of developmental phases

• Trypomastigote - This is a long, leaf-shaped form of the parasite which enters the mammal from the insect. It has a long tail or flagellum stretching from the tip, back along the length of the cell, allowing it to quickly swim through the blood, lymph or spinal fluid. The trypomastigote shortens slightly (intermediate form), undergoes asexual reproduction by binary fission in the blood, and then changes to an amastigote as it enters the tissues.

• Amastigote - This in the intracellular form of the organism in the vertebrate host. When the trypomastigote enters a cell it becomes round and non-flagellated and it is less vulnerable to attack from the immune system. It multiplies, bursts out of the cell and reinfects other cells. Amastigotes may also become stumpy trypomastigotes which are infective to biting insect vectors

• Epimastigote - This is the form of trypanosome which lives in the insect host. Within an hour of ingestion by the insect, the trypomastigote loses its protein coat, transforms to an epimastigote and migrates to the gut where it multiplies many times before changing back into a trypomastigote. In some species of trypanosome the epimastigote moves to the salivary glands of the insect and is passed on by biting. In Chagas diseases it stays in the gut and is deposited in the feaces of the bug.

It was always thought that trypanosomes only multiplied asexually but recent developments in DNA analysis have shown that some sexual reproduction does take place within the insect stage of the life cycle.

Refs include: http://www.urbanfischer.de/journals/protist/content/issue2/0002.pdf
http://martin.parasitology.mcgill.ca/jimspage/biol/trypan.htm
http://www.uta.edu/chagas/html/biolTcru.html