- Trypanosomes are flagellated protozoan parasites of the genus Trypanosoma, responsible for various diseases in humans and animals.
- The most notable diseases caused by trypanosomes are African trypanosomiasis and American trypanosomiasis.
- Different insect vectors transmit these parasites and have distinct life cycles and clinical manifestations.
Habitat
Trypanosomes are extracellular parasites that typically reside in the bloodstream or tissues of their vertebrate hosts. The habitat of the parasite depends on the species and the disease it causes:
- African Trypanosomes (Trypanosoma brucei): Found in the blood of humans and animals in sub-Saharan Africa. The disease is transmitted by the tsetse fly (genus Glossina).
- American Trypanosomes (Trypanosoma cruzi): Found in the blood of humans and animals in Central and South America. It is transmitted by triatomine bugs (known as kissing bugs, genus Triatoma).
Trypanosomes also live in the insect vectors, undergoing certain developmental stages, including multiplication and maturation.
Epidemiology
The epidemiology of trypanosomiasis is largely dependent on the region of the world and the specific type of trypanosome.
- African Trypanosomiasis (Sleeping Sickness):
- Caused by Trypanosoma brucei species (specifically T. b. gambiense and T. b. rhodesiense).
- Endemic areas: Primarily sub-Saharan Africa, with some outbreaks in regions of Central Africa and East Africa.
- Vectors: Transmitted by the tsetse fly, which lives in rural, forested, or savanna areas.
- Human-to-human transmission is possible, particularly with T. b. gambiense through the bite of an infected fly or from mother to child during pregnancy or childbirth.
- American Trypanosomiasis (Chagas Disease):
- Caused by Trypanosoma cruzi.
- Endemic areas: Found in rural areas of Latin America, including countries like Brazil, Argentina, Mexico, and Bolivia.
- Vectors: Transmitted by triatomine bugs (kissing bugs) found in cracks and crevices of homes.
- Transmission can also occur via blood transfusion, organ transplantation, or from mother to child during pregnancy.
Morphology
Trypanosomes exhibit characteristic morphological forms during their life cycle. These forms are based on the location within the host or vector:
- Trypomastigote (blood stage in mammals and tsetse fly stage):
- This is the infective form that circulates in the bloodstream of mammals.
- It has a long, slender body, a single flagellum that extends along the body, and a prominent kinetoplast (a DNA-containing structure in the mitochondrion).
- The flagellum is located at the anterior end, extending as a free flagellum or attached along the body.
- Epimastigote (intermediate stage in the vector):
- This form is found in the midgut of the tsetse fly or triatomine bug.
- It has a shorter body with a flagellum extending from the anterior end.
- The kinetoplast is located near the anterior region of the cell.
- Amastigote (tissue stage in mammals):
- Found in tissues of the mammalian host (e.g., heart, liver, spleen).
- It is oval-shaped and non-flagellated.
- Amastigotes multiply by binary fission in tissue macrophages.
Life Cycle
The life cycle of trypanosomes is complex and involves two hosts: a vertebrate host (usually a mammal) and an insect vector (either the tsetse fly for African trypanosomes or the triatomine bug for American trypanosomes). The life cycle consists of several stages, including transformation from one form to another.
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In the Mammalian Host:
- The trypomastigote is injected into the bloodstream when an infected insect vector bites the host.
- The trypomastigotes circulate in the bloodstream and invade various tissues, including the heart, liver, spleen, and lymph nodes.
- Sometimes, the trypomastigotes transform into amastigotes within the host’s tissues, multiplying by binary fission.
- The amastigotes later differentiate into trypomastigotes, which re-enter the bloodstream.
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In the Insect Vector:
- The vector (tsetse fly or triatomine bug) becomes infected by ingesting trypomastigotes during a blood meal.
- The trypomastigotes transform into epimastigotes in the insect’s gut.
- The epimastigotes multiply and later transform into trypomastigotes.
- These infective trypomastigotes migrate to the salivary glands or the hindgut of the insect, ready to be transmitted back to a mammalian host during the next bite.
Pathogenesis
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African Trypanosomiasis (Sleeping Sickness):
- T. b. gambiense causes the chronic form of the disease, which progresses slowly over several years, while T. b. rhodesiense causes the acute form, leading to more rapid progression.
- The parasite is initially localized in the bloodstream, causing fever, headaches, and lymphadenopathy (swollen lymph nodes).
- As the disease progresses, neurological symptoms appear, such as drowsiness, confusion, and coma, due to the invasion of the central nervous system (CNS). The patient eventually falls into a coma, hence the name sleeping sickness.
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American Trypanosomiasis (Chagas Disease):
- The parasite initially infects the heart, gut, and nervous system.
- In the acute phase, symptoms include fever, swelling at the bite site (chagoma), and lymphadenopathy.
- In the chronic phase, the infection can lead to severe complications such as heart failure, dilated cardiomyopathy, megaesophagus, and megacolon.
The parasites evade the immune system through antigenic variation, meaning they periodically change the surface proteins recognized by the host immune system.
Laboratory Diagnosis
Diagnosis of trypanosomiasis involves direct identification of the parasite in clinical specimens and serological and molecular tests.
Microscopy:
- Blood smears: Thin or thick blood smears can be examined under a microscope for trypomastigotes. Giemsa staining is commonly used to highlight the parasites.
- Cerebrospinal fluid (CSF): In suspected sleeping sickness, CSF examination can reveal trypomastigotes or an increase in white blood cells.
- Tissue biopsies (for Chagas disease): In suspected cases of Chagas disease, biopsies from the heart or digestive tract can be examined for amastigotes.
Serology:
- ELISA (Enzyme-Linked Immunosorbent Assay) and Indirect Hemagglutination Assay (IHA) are commonly used to detect antibodies specific to Trypanosoma species.
- Card agglutination test (CATT) for African trypanosomiasis.
PCR:
- Polymerase Chain Reaction (PCR) can detect Trypanosoma DNA in blood, cerebrospinal fluid, or tissue samples. PCR helps with species identification and can detect infections in the early stages before antibodies are produced.
Xenodiagnosis:
- This involves allowing a vector insect (such as a tsetse fly or triatomine bug) to feed on a suspected patient and then examining the insect for trypomastigotes.