is found in central and west africa. impact t. brucei is a major cause of livestock disease in sub-saharan africa. it is thus of tremendous veterinary concern and one of the greatest limitations on agriculture in africa and the economic life of sub-saharan africa. evolution trypanosoma brucei gambiense evolved from a single progenitor ~10,000 years ago. it is evolving asexually and its genome shows the meselson effect. genetics there are two subpopulations of t. b. gambiense that possesses two distinct groups that differ in genotype and phenotype. group 2 is more akin to t. b. brucei than group 1 t. b. gambiense. all t. b. gambiense are resistant to killing by a serum component — trypanosome lytic factor (tlf) of which there are two types: tlf-1 and tlf-2. group 1 t. b. gambiense parasites avoid uptake of the tlf particles while those of group 2 are able to either neutralize or compensate for the effects of tlf. in contrast, resistance in t. b. rhodesiense is dependent upon the expression of a serum resistance associated (sra) gene. this gene is not found in t. b. gambiense. genome the genome of t. brucei is made up of: 11 pairs of large chromosomes of 1 to 6 megabase pairs. 3–5 intermediate chromosomes of 200 to 500 kilobase pairs. around 100 minichromosomes of around 50 to 100 kilobase pairs. these may be present in multiple copies per haploid genome. most genes are held on the large chromosomes, with the minichromosomes carrying only vsg genes. the genome has been sequenced and is available on genedb. the mitochondrial genome is found condensed into the kinetoplast, an unusual feature unique to the kinetoplastid protozoans. the kinetoplast and the basal body of the flagellum are strongly associated via a cytoskeletal structure in 1993, a new base, ß-d-glucopyranosyloxymethyluracil (base j), was identified in the nuclear dna of t. brucei. vsg coat the surface of t. brucei and other species of trypanosomes is covered by a dense external coat called variant surface glycoprotein (vsg). vsgs are 60-kda proteins which are densely packed (~5 x 106 molecules) to form a 12–15 nm surface coat. vsg dimers make up about 90% of all cell surface proteins in trypanosomes. they also make up ~10% of total cell protein. for this reason, these proteins are highly immunogenic and an immune response raised against a specific vsg coat will rapidly kill trypanosomes expressing this variant. however, with each cell division there is a possibility that the progeny will switch expression to change the vsg that is being expressed. this vsg coat enables an infecting t. brucei population to persistently evade the host's immune system, allowing chronic infection. vsg is highly immunogenic, and an immune response raised against a specific vsg coat rapidly kills trypanosomes expressing this variant. antibody-mediated trypanosome killing can also be observed in vitro by a complement-mediated lysis assay. however, with each cell division there is a possibility that one or both of the progeny will switch expression to change the vsg that is being expressed. the frequency of vsg switching has been measured to be approximately 0.1% per division. as t. brucei populations can peak at a size of 1011 within a host this rapid rate of switching ensures that the parasite population is typically highly diverse. because host immunity against a specific vsg does not develop immediately, some parasites will have switched to an antigenically distinct vsg variant, and can go on to multiply and continue the infection. the clinical effect of this cycle is successive 'waves' of parasitemia (trypanosomes in the blood). expression of vsg genes occurs through a number of mechanisms yet to be fully understood. the expressed vsg can be switched either by activating a different expression site (and thus changing to express the vsg in that site), or by changing the vsg gene in the active site to a different variant. the genome contains many hundreds if not thousands of vsg genes, both on minichromosomes and in repeated sections ('arrays') in the interior of the chromosomes. these are transcriptionally silent, typically with omitted sections or premature stop codons, but are important in the evolution of new vsg genes. it is estimated up to 10% of the t. brucei genome may be made up of vsg genes or pseudogenes. it is thought that any of these genes can be moved into the active site by recombination for expression. vsg silencing is largely due to the effects of histone variants h3.v and h4.v. these histones cause changes in the three-dimensional structure of the t. brucei genome that results in a lack of expression. vsg genes are typically located in the subtelomeric regions of the chromosomes, which makes it easier for them to be silenced when they are not being used. it remains unproven whether the regulation of vsg switching is purely stochastic or whether environmental stimuli affect switching frequency. switching is linked to two factors: variation in activation of individual vsg genes; and differentiation to the "short stumpy" stage - triggered by conditions of high population density - which is the nonreproductive, interhost transmission stage. as of 2021 it also remains unexplained how this transition is timed and how the next surface protein gene is chosen. these questions of antigenic variation in t. brucei and other parasites are among the most interesting in the field of infection. killing by human serum and resistance to human serum killing trypanosoma brucei brucei (as well as related species t. equiperdum and t. evansi) is not human infective because it is susceptible to innate immune system 'trypanolytic' factors present in the serum of some primates, including humans. these trypanolytic factors have been identified as two serum complexes designated trypanolytic factors (tlf-1 and −2) both of which contain haptoglobin related protein (hpr) and apolipoprotein li (apol1). tlf-1 is a member of the high density lipoprotein family of particles while tlf-2 is a related high molecular weight serum protein binding complex. the protein components of tlf-1 are haptoglobin related protein (hpr), apolipoprotein l-1 (apol-1) and apolipoprotein a-1 (apoa-1). these three proteins are colocalized within spherical particles containing phospholipids and cholesterol. the protein components of tlf-2 include igm and apolipoprotein a-i. trypanolytic factors are found only in a few species, including humans, gorillas, mandrills, baboons and sooty mangabeys. this appears to be because haptoglobin-related protein and apolipoprotein l-1 are unique to primates. this suggests these genes originated in the primate genome 25 million years ago-35 million years ago. human infective subspecies t. b. gambiense and t. b. rhodesiense have evolved mechanisms of resisting the trypanolytic factors, described below. apol1 apol1 is a member of a six gene family, apol1-6, that have arisen by tandem duplication. these proteins are normally involved in host apoptosis or autophagic death and possess a bcl-2 homology domain 3. apol1 has been identified as the toxic component involved in trypanolysis. apols have been subject to recent selective evolution possibly related to resistance to pathogens. the gene encoding apol1 is found on the long arm of chromosome 22 (22q12.3). variants of this gene, termed g1 and g2, provide protection against t. b. rhodesiense. these benefits are not without their downside as a specific apol1 glomerulopathy has been identified. this glomerulopathy may help to explain the greater prevalence of hypertension in african populations. the gene encodes a protein of 383 residues, including a typical signal peptide of 12 amino acids. the plasma protein is a single chain polypeptide with an apparent molecular mass of 42 kilodaltons. apol1 has a membrane pore forming domain functionally similar to that of bacterial colicins. this domain is flanked by the membrane addressing domain and both these domains are required for parasite killing. within the kidney, apol1 is found in the podocytes in the glomeruli, the proximal tubular epithelium and the arteriolar endothelium. it has a high affinity for phosphatidic acid and cardiolipin and can be induced by interferon gamma and tumor necrosis factor alpha. hpr hpr is 91% identical to haptoglobin (hp), an abundant acute phase serum protein, which possesses a high affinity for hemoglobin (hb). when hb is released from erythrocytes undergoing intravascular hemolysis hp forms a complex with the hb and these are removed from circulation by the cd163 scavenger receptor. in contrast to hp–hb, the hpr–hb complex does not bind cd163 and the hpr serum concentration appears to be unaffected by hemolysis. killing mechanism the association of hpr with hemoglobin allows tlf-1 binding and uptake via the trypanosome haptoglobin-hemoglobin receptor (tbhphbr). tlf-2 enters trypanosomes independently of tbhphbr. tlf-1 uptake increases when haptoglobin level is low. tlf-1 overtakes haptoglobin and binds free hemoglobin in the serum. however the complete absence of haptoglobin is associated with a decreased killing rate by serum. the trypanosome haptoglobin-hemoglobin receptor is an elongated three a-helical bundle with a small membrane distal head. this protein extends above the variant surface glycoprotein layer that surrounds the parasite. the first step in the killing mechanism is the binding of tlf to high affinity receptors—the haptoglobin-hemoglobin receptors—that are located in the flagellar pocket of the parasite. the bound tlf is endocytosed via coated vesicles and then trafficked to the parasite lysosomes. apol1 is the main lethal factor in the tlfs and kills trypanosomes after insertion into endosomal / lysosomal membranes. after ingestion by the parasite, the tlf-1 particle is trafficked to the lysosome wherein apol1 is activated by a ph mediated conformational change. after fusion with the lysosome the ph drops from ~7 to ~5. this induces a conformational change in the apol1 membrane addressing domain which in turn causes a salt bridge linked hinge to open. this releases apol1 from the hdl particle to insert in the lysosomal membrane. the apol1 protein then creates anionic pores in the membrane which leads to depolarization of the membrane, a continuous influx of chloride and subsequent osmotic swelling of the lysosome. this influx in its turn leads to rupture of the lysosome and the subsequent death of the parasite. resistance mechanisms: t. b. gambiense trypanosoma brucei gambiense causes 97% of human cases of sleeping sickness. resistance to apol1 is principally mediated by the hydrophobic β-sheet of the t. b. gambiense specific glycoprotein. other factors involved in resistance appear to be a change in the cysteine protease activity and tbhphbr inactivation due to a leucine to serine substitution (l210s) at codon 210. this is due to a thymidine to cytosine mutation at the second codon position. these mutations may have evolved due to the coexistence of malaria where this parasite is found. haptoglobin levels are low in malaria because of the hemolysis that occurs with the release of the merozoites into the blood. the rupture of the erythrocytes results in the release of free haem into the blood where it is bound by haptoglobin. the haem is then removed along with the bound haptoglobin from the blood by the reticuloendothelial system. resistance mechanisms: t. b. rhodesiense trypanosoma brucei rhodesiense relies on a different mechanism of resistance: the serum resistance associated protein (sra). the sra gene is a truncated version of the major and variable surface antigen of the parasite, the variant surface glycoprotein. however, it has little similarity (low sequence homology) with the vsg gene (<25%). sra is an expression site associated gene in t. b. rhodesiense and is located upstream of the vsgs in the active telomeric expression site. the protein is largely localized to small cytoplasmic vesicles between the flagellar pocket and the nucleus. in t. b. rhodesiense the tlf is directed to sra containing endosomes while some dispute remains as to its presence in the lysosome. sra binds to apol1 using a coiled–coiled interaction at the apol1 sra interacting domain while within the trypanosome lysosome. this interaction prevents the release of the apol1 protein and the subsequent lysis of the lysosome and death of the parasite. baboons are known to be resistant to t. b. rhodesiense. the baboon version of the apol1 gene differs from the human gene in a number of respects including two critical lysines near the c terminus that are necessary and sufficient to prevent baboon apol1 binding to sra. experimental mutations allowing apol1 to be protected from neutralization by sra have been shown capable of conferring trypanolytic activity on t. b. rhodesiense. these mutations resemble those found in baboons, but also resemble natural mutations conferring protection of humans against