Trypanosomes are unicellular eukaryotic parasites equipped with a flagellum and are also classified as protozoa. Found worldwide, trypanosomes have slender cell bodies and are classified by the exit point of their flagella. Characteristic of these agents of some tropical diseases, such as sleeping sickness, is the obligatory host switching between an invertebrate vector and a vertebrate.
What are trypanosomes?
Trypanosomes are unicellular, flagellated parasites that are also classified among the protozoa because of their nucleus and other organelles. Of the several hundred species of the genus Trypanosoma, only a few are pathogenic to humans and cause diseases such as sleeping sickness in West and East Africa and Chagas disease in Central and South America. Trypanosomes have slender cell bodies and are characterized by obligate host switching, between an invertebrate vector, also called a vector, and a vertebrate, which includes reptiles, birds, and fish. Since many species are highly host-specific, the corresponding species of trypanosomes can only be found in the distribution range of the intermediate host and the “final host”. Trypanosomes can be divided into trypomastigote, epimastigote, and amastigote forms with respect to the point of attachment of their flagella. In trypomastigote trypanosomes the flagella originate at the posterior end of the cell, in epimastigote in the middle and in amastigote forms no flagella can be seen externally. A further distinction can be made with respect to the route of infection. Trypanosomes that multiply in the terminal part of the insect intestine and are excreted in the feces are called Sterocoraria, and those that are transmitted by the proboscis while sucking blood are called Salivaria.
Occurrence, distribution and characteristics
Trypanosomes are distributed worldwide, although species pathogenic to humans are largely restricted to tropical Africa and Central and South America. Pathogens pathogenic to humans include Trypanosoma brucei (African sleeping sickness) and Trypanosoma cruzi (Central American Chagas disease). Sleeping sickness is transmitted by the tsetse fly when it bites with its proboscis, while the causative agent of Chagas disease is transmitted by the feces of predatory bugs. The smallest skin lesions are sufficient to give Trypanosoma cruzei access to the human organism and blood vessels. In vertebrates, trypanosomes usually live in the blood plasma, lymph, or even cerebrospinal fluid. The pathogens of sleeping sickness have developed a sophisticated system of alternating antigen expression on their surface. Once the adaptive immune system has adjusted to the antigen type, it is confronted with an altered antigen to which the immune system must first readjust in an elaborate process. Trypanosoma cruzi takes a different route to evade the immune response. The pathogen changes into an amastigote form and multiplies inside the host cells to escape the attention of the immune system. In the case of trypanosomes transmitted by biting flies, a swelling, also known as a trypanosome chancre, typically develops at the site of injection. About two weeks after infection, the pathogens enter the bloodstream and lymph nodes. The lymph nodes swell and, if left untreated, periodic episodes of fever occur. In some cases, the pathogens can cross the blood-brain barrier, sometimes only after years, and cause meningitis in the central nervous system (CNS). In principle, a distinction must be made between East African sleeping sickness and West African sleeping sickness because of the different host changes. Strictly speaking, Trypanosoma brucei rhodesiense (East African sleeping sickness) is the causative agent of a zoonosis, since animals such as antelopes, springboks, and other savanna dwellers are the main reservoirs without contracting the disease themselves. Tsetse flies then become infected primarily on the wild animals and pass the pathogen on to humans without undergoing the usual change of generations in the tsetse fly. De facto, this is an infection from wild or farm animals to humans. Remarkably, both the female and male tsetse fly act as vectors. In contrast, the transmission of malaria pathogens to humans occurs exclusively through the female Anopheles mosquito.
Diseases and ailments
Of the multitude of trypanosome species that exist worldwide, only three occur as pathogens to humans. Specifically, these are the causative agents of West African and East African sleeping sickness and the causative agent of Chagas disease, which is common in countries of Central America and northern South America. The risk of contracting trypanosome infection is limited exclusively to regions where tsetse flies are native and to Central America. The causative agents of Chagas disease are not transmitted by flies or mosquitoes, but by a certain species of predatory bugs, which, however, do not transmit the sporozoites during the blood meal, but excrete them with the feces. The sporozoites can enter the body via smear infection, where they infect the heart muscles, nerve support tissue (neuroglia), and certain cells of the immune system. Chagas disease, if untreated, progresses in several stages and is fatal to about 10 percent of infected individuals. There is an increased risk of infection for young children and for people with naturally or artificially weakened immune systems. After the incubation period of about three weeks, the first symptoms appear, such as skin lesions, fever that is constant or occurs in episodes, and swollen lymph nodes. The symptoms during this acute phase are very similar to those of an influenza infection. A local skin reaction called a chagoma develops at the site of entry of the pathogen.
