This group of flagellates are characterized by having more complex life cycles involving 2 hosts. One of the host is usually a blood feeding invertebrate vector such as an insect or leach. Most members of this group are heteroxenous. They live in blood or fixed tissues of the vertebrate host and usually in the intestine of the insect vector host. Reproduction is generally regarded to be by binary fission although there is some indirect evidence for genetic exchange between trophozoites.
The trophozoites range from being elongate with a single flagellum to being round with a short non-protruding flagellum.
The flagellum arises from a structure called the kinetosome and may be attached to part or all of the body length forming an undulatory membrane. Near to the kinetosome lies the kinetoplast. This structure is unique to the group. It is sausage or disc shaped and contains mitochondrial DNA and a single mitochondrion arises from it. The mitochondrial DNA, known as kDNA, is different in arrangement from the nuclear DNA, composed of linked circles of 2 types, mini circles of which there are around 20,000 and maxi circles of which there are from 20-50. The maxi circles are equivalent to the mitochondrial DNA found in other organisms and code for similar functions. The function of the mini circles, however, is still obscure.
The relative position of the kinetosome, kinetoplast and flagella are characteristic for different species of the group but also different stages within the life cycles of some species.
Two different genuses will be considered Trypanosoma and Leishmania.
There are two distinct groups of trypanosomes, posterior station or stercorarian group and the anterior station or salivarian group. The nomenclature refers to the method by which the flagellate leaves the insect vector to be transmitted to the vertebrate host.
The main elements of the trypanosome are shown below, the flagellum arising from the anterior of the cell and remaining attached for the whole length of the protozoan. The kinetoplast is associated with the single the prominent mitochondria, the complexity of which changes with different phases of the life cycle. There is a single nucleus extensive endoplasmic reticulum and prominent Golgi apparatus. Under the pellical there is an extensive arrangement of subpellicular microtubules which give rigidity and form to the organism.
The life cycle of trypanosomes is punctuated by distinct morphological forms, both in the vertebrate host, where it is located in the blood stream, and in the gut of the insect vector.
Blood stream forms: The blood stream forms have a distinctive long and slender shape, and the mitochondria has sparse tubular cristae and has no functional citric acid cycle enzymes or cytochromes. Only the glycolytic pathway is present in the cell, catabolizing glucose to pyruvate which is excreted. These enzymes are located in specialised structures or microbodies known as glycosomes. The slender forms also have a prominent glycoprotein surface coat called the glycocalyx.
Intermediate forms: These forms reproduce by binary fission.The mitochondrial cristae lengthen and are tubular in form.
As the name suggests the trypanosomes becomes short and stumpy and the flagella no longer extend past the end of the cell body. In the mitochondrion, many tubular cristae appear and it has partially functioning citric acid cycle enzymes but still no cytochromes. This is the infective or transmission stage of the parasite. Only these forms will survive when ingested by the intermediate host the tsetse fly (Glossina sp.).
Within one hour in the insect host the trypanomastigote loses its surface coat. In the cardia and posterior region of the midgut the trypanomastigote undergoes multiplication for about ten days then migrates to the anterior midgut for another 8- 10 days before finally migrating via the oesophagus, pharynx and hypopharynx to the salivary glands. During this period the trypanosomes have a fully functional mitochondrion which contains numerous plate like cristae which is associated with the presence of functional cytochromes.
Once in the salivary glands they transform into the epimastigote stage and remain free in the lumen or attach to host cells. After a number of asexual cycles they transform into a stumpy metacyclic form with reduced tubular cristae, which are presumed also to have lost their cytochrome activity.
Until recently it was generally assumed that trypanosomes only reproduced asexually, but recently, DNA analysis as revealed that the metacyclic trypanosomes have a haploid amount of DNA compared with the blood stream forms. This suggests that meiosis occurs during development in the insect with syngamy occurring shortly after cyclic transmission. A deviation of this classical life cycle pattern has been claimed by several authors who have observed amastigote forms inside the cells of the chorioidea during the first 48 hours of infection. Following this period they emerge to give rise to the slender blood forms.
The clinical course of the infection is characterized by an increase in trypanosome numbers, followed by a crash or sudden decrease in the population which is repeated over a number of cycles. If untreated, these cycles of remission followed by high parasitaemia continue until the host dies.
The remission coincides with an increase in host protective antibody against specific surface expressed antigens on the trypanosome, located in the surface coat or glycocalyx. The cyclic nature appears to occur due to an almost endless change in the Variant Antigen Type (VAT). As a result, just as the host appears to be winning the battle, the parasite changes its variant antigen coating and evades the host immune response allowing a resurgence of the infection. The antigen recognised by the host is known as a Variant-specific Surface Glycoprotein or VSG.
These proteins are released through the flagellar pocket at the base of the flagellum and completely cover the surface of the trypanosome, including the flagellum.
Each individual trypanosome possesses a large number of basic copy (BC) genes which code for the VSG, of which there are up to 2000. Only one gene is expressed at a time but the sequence of expression is predictable.
Depending on the species, trypanosomes are found in a number of locations within the vertebrate host, including blood, lymph nodes, spleen and cerebrospinal fluid, and the degree of pathogenicity is dependent on the species of trypanosome.
African Sleeping Sickness
So named because people become weak listless and appear to drop off to sleep. It is caused by two species of trypanosomes:
West & Central African form of Trypanosomiasis (Humans are the principal reserviour host)
T. brucei gambiense: During the first 1-2 weeks of infection a patient experiences fevers, chills, headaches and loss of appetite. This is followed by an enlargement of the spleen, liver and lymph nodes. As the infection proceeds and the nervous system becomes infected it can lead to meningoencephalomyolytis and haemolysis. The progressive symptoms are weakness, apathy and headaches, finally culminating in coma and death.
East African form of Trypanosomiasis (Reserviour hosts, domestic cattle and wild animals)
T. brucei rhodesiensis: These infections result in an acute form of sleeping sickness. The disease progresses so rapidly that classical symptoms of sleeping sickness never develop and the patient dies very soon after being infected.
The principal method of diagnosis is through the detection of trypanosomes in the body fluids. In addition serological tests have also been developed. The main early clinical signs are enlarged lymph glands. Early diagnosis of T. b. gambiense is difficult because specific clinical signs are absent
The drugs of choice for the treatment of sleeping sickness include Suramin, pentamidine and Berenil, which can cure the infection if given prior to invasion of the nervous system. More recently, difluoromethylornithine (DMFO) has been found to be efficacious, even if the parasite has invaded the brain.
Epidemiology and Control
Trypanosomiasis is of immense importance, both as a parasite of humans and live stock, and has effectively prevented the use of large areas of central Africa for cattle ranching. The exception is where resistant or tolerant breeds of native cattle are farmed, which is usually in West Africa. It is a disease which is mainly rural in nature but also very focal with discrete areas of infection coincident with the distribution of the vector. It is estimated that between 55-60 million people are exposed to the risk of infection. In some regions prevalence is greater than 50%.
The principle methods of control are spraying with DDT, clearance of brush to produce brush-free belts to isolate the area. Tsetse flies are poor flyers and can only fly short distances. They are also larviperous and solitary releasing their larvae in burrows underground, usual under the protection of the brush. They are therefore difficult to treat with insecticide. Other methods of control include removing reservoir host from the area and breeding resistant stock animals.
It is perhaps a testament to the difficulties in vector control that after over 80 years of tsetse eradication programs there is little observable impact on the distribution of this parasite.
In addition to infecting humans trypanosomes infect a number of different mammalian hosts and are transmitted by different vectors. Glossina sp transmits a number to trypanosome species and there is a progression of parasitism within the tsetse, which some authors equate with the length of time the vector and parasite has been in association shown below.
In addition to tsetse flies, T. evansi and T. equiperdum are transmitted mechanically by the blood sucking tabanids. In a species affecting horses, T. equiperdum there is no insect vector involved. The parasite is transmitted during coitus and the disease is known as Dourine. The various diseases and species of trypanosomes are shown below.
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