fall into the trypomastigote and epimastigote morphological categories. trypanosomatids show several different classes of cellular organisation of which two are adopted by t. brucei at different stages of the life cycle: epimastigote, which is found in tsetse fly. its kinetoplast and basal body lie anterior to the nucleus, with a long flagellum attached along the cell body. the flagellum starts from the centre of the body. trypomastigote, which is found in mammalian hosts. the kinetoplast and basal body are posterior of nucleus. the flagellum arises from the posterior end of the body. flagellar structure these names are derived from the greek mastig- meaning whip, referring to the trypanosome's whip-like flagellum. the trypanosome flagellum has two main structures. it is made up of a typical flagellar axoneme, which lies parallel to the paraflagellar rod, a lattice structure of proteins unique to the kinetoplastids, euglenoids and dinoflagellates. the microtubules of the flagellar axoneme lie in the normal 9+2 arrangement, orientated with the + at the anterior end and the − in the basal body. the cytoskeletal structure extends from the basal body to the kinetoplast. the flagellum is bound to the cytoskeleton of the main cell body by four specialised microtubules, which run parallel and in the same direction to the flagellar tubulin. the flagellar function is twofold — locomotion via oscillations along the attached flagellum and cell body in human blood stream and tsetse fly gut, and attachment to the salivary gland epithelium of the fly during the epimastigote stage. the flagellum propels the body in such a way that the axoneme generates the oscillation and a flagellar wave is created along the undulating membrane. as a result, the body moves in a corkscrew pattern. in flagella of other organisms, the movement starts from the base towards the tip, while in t. brucei and other trypanosomatids, the beat originates from the tip and progresses towards the base, forcing the body to move towards the direction of the tip of the flagellum. life cycle life cycle t. brucei completes its life cycle between tsetse fly (of the genus glossina) and mammalian hosts, including humans, cattle, horses, and wild animals. in stressful environments, t. brucei produces exosomes containing the spliced leader rna and uses the endosomal sorting complexes required for transport (escrt) system to secrete them as extracellular vesicles. when absorbed by other trypanosomes these evs cause repulsive movement away from the area and so away from bad environments. in mammalian host infection occurs when a vector tsetse fly bites a mammalian host. the fly injects the metacyclic trypomastigotes into the skin tissue. the trypomastigotes enter the lymphatic system and into the bloodstream. the initial trypomastigotes are short and stumpy (ss). they are protected from the host's immune system by producing antigentic variation called variant surface glycoproteins on their body surface. once inside the bloodstream, they grow into long and slender forms (ls). then, they multiply by binary fission. some of the daughter cells then become short and stumpy again. some of them remains as intermediate forms, representing a transitional stage between the long and short forms. the long slender forms are able to penetrate the blood vessel endothelium and invade extravascular tissues, including the central nervous system (cns) and placenta in pregnant women. sometimes, wild animals can be infected by the tsetse fly and they act as reservoirs. in these animals, they do not produce the disease, but the live parasite can be transmitted back to the normal hosts. besides preparation to be taken up and vectored to another host by a tsetse fly, transition from ls to ss in the mammal serves to prolong the host's lifespan – controlling parasitemia aids in increasing the total transmitting duration of any particular infested host. in tsetse fly unlike anopheline mosquitos and sandflies that transmit other protozoan infections in which only females are involved, both sexes of tsetse flies are blood feeders and equally transmit trypanosomes. the short and stumpy trypomastigotes (ss) are taken up by tsetse flies during a blood meal. survival in the tsetse midgut is one reason for the particular adaptations of the ss stage. the trypomastigotes enter the midgut of the fly where they become procyclic trypomastigotes as they replace their vsg with other protein coats called procyclins. because the fly faces digestive damage from immune factors in the bloodmeal, it produces serpins to suppress the infection. the serpins including gmmsrpn3, gmmsrpn5, gmmsrpn9, and especially gmmsrpn10 are then hijacked by the parasite to aid its own midgut infection, using them to inactivate bloodmeal trypanolytic factors which would otherwise make the fly host inhospitable.: 346 the procyclic trypomastigotes cross the peritrophic matrix, undergo slight elongation and migrate to the anterior part of the midgut as non-proliferative long mesocyclic trypomastigotes. as they reach the proventriculus, they became thinner and undergo cytoplasmic rearrangement to give rise to proliferative epimastigotes. the epimastigotes divide asymmetrically to produce long and short epimastigotes. the long epimastigote cannot move to other places and simply die off by apoptosis. the short epimastigote migrate from the proventriculus via the foregut and proboscis to the salivary glands where they get attached to the salivary gland epithelium. even all the short forms do not succeed in the complete migration to the salivary glands as most of them perish on the way–only up to five may survive. in the salivary glands, the survivors undergo phases of reproduction. the first cycle in an equal mitosis by which a mother cell produces two similar daughter epimastigotes. they remain attach to the epithelium. this phase is the main reproduction in first-stage infection to ensure sufficient number of parasites in the salivary gland. the second cycle, which usually occurs in late-stage infection, involves unequal mitosis that produces two different daughter cells from the mother epimastigote. one daughter is an epimastigote that remains non-infective and the other is a trypomastigote. the trypomastigote detach from the epithelium and undergo transformation into short and stumpy trypomastigotes. the surface procyclins are replaced with vsgs and become the infective metacyclic trypomastigotes. complete development in the fly takes about 20 days. they are injected into the mammalian host along with the saliva on biting, and are known as salivarian. in the case of t. b. brucei infecting glossina palpalis gambiensis, the parasite changes the proteome contents of the fly's head and causes behavioral changes such as unnecessarily increased feeding frequency, which increases transmission opportunities. this is related to altered glucose metabolism that causes a perceived need for more calories. (the metabolic change, in turn, being due to complete absence of glucose-6-phosphate 1-dehydrogenase in infected flies.) monoamine neurotransmitter synthesis is also altered: production of aromatic l-amino acid decarboxylase involved in dopamine and serotonin synthesis, and α-methyldopa hypersensitive protein was induced. this is similar to the alterations in other dipteran vectors' head proteomes under infection by other eukaryotic parasites of mammals. reproduction binary fission trypanosome cell cycle (procyclic form) the reproduction of t. brucei is unusual compared to most eukaryotes. the nuclear membrane remains intact and the chromosomes do not condense during mitosis. the basal body, unlike the centrosome of most eukaryotic cells, does not play a role in the organisation of the spindle and instead is involved in division of the kinetoplast. the events of reproduction are: the basal body duplicates and both remain associated with the kinetoplast. each basal body forms a separate flagellum. kinetoplast dna undergoes synthesis then the kinetoplast divides coupled with separation of the two basal bodies. nuclear dna undergoes synthesis while a new flagellum extends from the younger, more posterior, basal body. the nucleus undergoes mitosis. cytokinesis progresses from the anterior to posterior. division completes with abscission. meiosis in the 1980s, dna analyses of the developmental stages of t. brucei started to indicate that the trypomastigote in the tsetse fly undergoes meiosis, i.e., a sexual reproduction stage. but it is not always necessary for a complete life cycle. the existence of meiosis-specific proteins was reported in 2011. the haploid gametes (daughter cells produced after meiosis) were discovered in 2014. the haploid trypomastigote-like gametes can interact with each other via their flagella and undergo cell fusion (the process is called syngamy). thus, in addition to binary fission, t. brucei can multiply by sexual reproduction. trypanosomes belong to the supergroup excavata and are one of the earliest diverging lineages among eukaryotes. the discovery of sexual reproduction in t. brucei supports the hypothesis that meiosis and sexual reproduction are ancestral and ubiquitous features of eukaryotes. infection and pathogenicity main article: african trypanosomiasis the insect vectors for t. brucei are different species of tsetse fly (genus glossina). the major vectors of t. b. gambiense, causing west african sleeping sickness, are g. palpalis, g. tachinoides, and g. fuscipes. while the principal vectors of t. b. rhodesiense, causing east african sleeping sickness, are g. morsitans, g. pallidipes, and g. swynnertoni. animal trypanosomiasis is transmitted by a dozen species of glossina. in later stages of a t. brucei infection of a mammalian host the parasite may migrate from the bloodstream to also infect the lymph and cerebrospinal fluids. it is under this tissue invasion that the parasites produce the sleeping sickness. in addition to the major form of transmission via the tsetse fly, t. brucei may be transferred between mammals via bodily fluid exchange, such as by blood transfusion or sexual contact, although this is thought to be rare. newborn babies can be infected (vertical or congenital transmission) from infected mothers. chemotherapy there are four drugs generally recommended for the first-line treatment of african trypanosomiasis: suramin developed in 1921, pentamidine developed in 1941, melarsoprol developed in 1949 and eflornithine developed in 1990. these drugs are not fully effective and are toxic to humans. in addition, drug resistance has developed in the parasites against all the drugs. the drugs are of limited application since they are effective against specific strains of t. brucei and the life cycle stages of the parasites. suram is used only for first-stage infection of t. b. rhodensiense, pentamidine for first-stage infection of t. b. gambiense, and eflornithine for second-stage infection of t. b. gambiense. melarsopol is the only drug effective against the two types of parasite in both infection stages, but is highly toxic, such that 5% of treated individuals die of brain damage (reactive encephalopathy). another drug, nifurtimox, recommended for chagas disease (american trypanosomiasis), is itself a weak drug but in combination with melarsopol, it is used as the first-line medication against second-stage infection of t. b. gambiense. historically, arsenic and mercuric compounds were introduced in the early 20th century, with success particularly in animal infections. german physician paul ehrlich and his japanese associate kiyoshi shiga developed the first specific trypanocidal drug in 1904 from a dye, trypan red, which they named trypanroth. these chemical preparations were effective only at high and toxic dosages, and were not suitable for clinical use. animal trypanosomiasis is treated with six drugs: diminazene aceturate, homidium (homidium bromide and homidium chloride), isometamidium chloride, melarsomine, quinapyramine, and suramin. they are all highly toxic to animals, and drug resistance is prevalent. homidium is the first prescription anti-trypanosomal drug. it was developed as a modified compound of phenantridine, which was found in 1938 to have trypanocidal activity against the bovine parasite, t. congolense. among its products, dimidium bromide and its derivatives were first used in 1948 in animal cases in africa, and became known as homidium (or as ethidium bromide in molecular biology). alternative therapy some phytochemicals have shown research promise against the t. b. brucei strain. aderbauer et al., 2008 and umar et al., 2010 find khaya senegalensis is effective in vitro and ibrahim et al., 2013 and 2008 in vivo (in rats). ibrahim et al., 2013 find a lower dose reduces parasitemia by this subspecies and a higher dose is curative and prevents injury. distribution t. brucei is only found in the blue areas t. brucei is found where its tsetse fly vectors are prevalent in continental africa. that is to say, tropical rainforest (af), tropical monsoon (am), and tropical savannah (aw) areas of continental africa. hence, the equatorial region of africa is called the "sleeping sickness" belt. however, the specific type of the trypanosome differs according to geography. t. b. rhodesiense is found primarily in east africa (botswana, democratic republic of the congo, ethiopia, kenya, malawi, tanzania, uganda and zimbabwe), while t. b. gambiense