Numerical taxonomy of Trypanozoon based on polymorphisms in a reduced range of enzymes

Development of an antigen ELISA using monoclonal antibodies against recombinant VSG for the detection of active infections of Trypanosoma evansi in animals

Trypanosoma evansi, a haemo-flagellated protozoan parasite causes chronic wasting disease in a wide range of animals. For its diagnosis, blood smear examination is useful in clinical cases for direct identification of the parasite but in latent infection the carrier animals are difficult to screen out by conventional blood smear test. Harboring low level of parasites and showing no symptom, the carrier animals for surra can act as a source of infection. The level of parasitaemia fluctuates, especially during latent infection; moreover the antibodies which are not found early in the infection may persist even after recovery or chemotherapy. In the present study a double antibody sandwich ELISA exploring, monoclonal antibodies and hyperimmune serum, raised against recombinant variable surface glycoprotein has been developed to detect circulating trypanosome antigens. The developed antigen detection ELISA (Ag-ELISA) was evaluated using 652 blood samples collected from cattle, buffalo, equine and camel. The statistical analysis of the data showed diagnostic sensitivity and specificity at 97.4% and 96.4% respectively, with a positive-negative cut-off OD value >0.28. Furthermore, the detection limit of the assay was found to 7.15 trypanosomes per mL. The present finding revealed that the developed assay can be exploited as a potential diagnostic test in the detection of circulating trypanosome antigens and also can be used as a population screening test for multiple animal species for detection of active infection for further treatment and control of the disease.

Barcoding in trypanosomes

Trypanosomes (genus Trypanosoma) are parasites of humans, and wild and domestic mammals, in which they cause several economically and socially important diseases, including sleeping sickness in Africa and Chagas disease in the Americas. Despite the development of numerous molecular diagnostics and increasing awareness of the importance of these neglected parasites, there is currently no universal genetic barcoding marker available for trypanosomes. In this review we provide an overview of the methods used for trypanosome detection and identification, discuss the potential application of different barcoding techniques and examine the requirements of the 'ideal' trypanosome genetic barcode. In addition, we explore potential alternative genetic markers for barcoding Trypanosoma species, including an analysis of phylogenetically informative nucleotide changes along the length of the 18S rRNA gene.

Genetic recombination between human and animal parasites creates novel strains of human pathogen

Genetic recombination between pathogens derived from humans and livestock has the potential to create novel pathogen strains, highlighted by the influenza pandemic H1N1/09, which was derived from a re-assortment of swine, avian and human influenza A viruses. Here we investigated whether genetic recombination between subspecies of the protozoan parasite, Trypanosoma brucei, from humans and animals can generate new strains of human pathogen, T. b. rhodesiense (Tbr) responsible for sleeping sickness (Human African Trypanosomiasis, HAT) in East Africa. The trait of human infectivity in Tbr is conferred by a single gene, SRA, which is potentially transferable to the animal pathogen Tbb by sexual reproduction. We tracked the inheritance of SRA in crosses of Tbr and Tbb set up by co-transmitting genetically-engineered fluorescent parental trypanosome lines through tsetse flies. SRA was readily transferred into new genetic backgrounds by sexual reproduction between Tbr and Tbb, thus creating new strains of the human pathogen, Tbr. There was no evidence of diminished growth or transmissibility of hybrid trypanosomes carrying SRA. Although expression of SRA is critical to survival of Tbr in the human host, we show that the gene exists as a single copy in a representative collection of Tbr strains. SRA was found on one homologue of chromosome IV in the majority of Tbr isolates examined, but some Ugandan Tbr had SRA on both homologues. The mobility of SRA by genetic recombination readily explains the observed genetic variability of Tbr in East Africa. We conclude that new strains of the human pathogen Tbr are being generated continuously by recombination with the much larger pool of animal-infective trypanosomes. Such novel recombinants present a risk for future outbreaks of HAT.

Genetic recombination allows transfer of harmful traits between different strains of the same pathogen and enables the emergence of genetically novel pathogen strains that the host population has not previously encountered. This can be particularly important when a pathogen acquires a virulence trait that allows it to spread beyond its normal host population. Here we show that this happens among the single-celled parasites—trypanosomes—that cause human African trypanosomiasis (HAT) or sleeping sickness carried by the tsetse fly. Genetic recombination readily occurs between the human and animal parasites when they are co-transmitted by the tsetse fly, creating new pathogen genotypes or strains. There is a single gene that confers human infectivity and each of the genotypes that inherits this gene is potentially capable of infecting humans. In this way new strains of the human pathogen can be generated by recombination between the human-infective and animal-infective trypanosomes. Such novel recombinants present a risk for future outbreaks of HAT.

Genetic diversity and population structure of Trypanosoma brucei in Uganda: implications for the epidemiology of sleeping sickness and Nagana

Background

While Human African Trypanosomiasis (HAT) is in decline on the continent of Africa, the disease still remains a major health problem in Uganda. There are recurrent sporadic outbreaks in the traditionally endemic areas in south-east Uganda, and continued spread to new unaffected areas in central Uganda. We evaluated the evolutionary dynamics underpinning the origin of new foci and the impact of host species on parasite genetic diversity in Uganda. We genotyped 269 Trypanosoma brucei isolates collected from different regions in Uganda and southwestern Kenya at 17 microsatellite loci, and checked for the presence of the SRA gene that confers human infectivity to T. b. rhodesiense.

Results

Both Bayesian clustering methods and Discriminant Analysis of Principal Components partition Trypanosoma brucei isolates obtained from Uganda and southwestern Kenya into three distinct genetic clusters. Clusters 1 and 3 include isolates from central and southern Uganda, while cluster 2 contains mostly isolates from southwestern Kenya. These three clusters are not sorted by subspecies designation (T. b. brucei vs T. b. rhodesiense), host or date of collection. The analyses also show evidence of genetic admixture among the three genetic clusters and long-range dispersal, suggesting recent and possibly on-going gene flow between them.

Conclusions

Our results show that the expansion of the disease to the new foci in central Uganda occurred from the northward spread of T. b. rhodesiense (Tbr). They also confirm the emergence of the human infective strains (Tbr) from non-infective T. b. brucei (Tbb) strains of different genetic backgrounds, and the importance of cattle as Tbr reservoir, as confounders that shape the epidemiology of sleeping sickness in the region.

Human African Trypanosomiasis (HAT) is a major health problem in Uganda, as there are recurrent sporadic outbreaks of the disease in traditionally endemic areas in south-east Uganda, and continued spread to new unaffected areas in central Uganda. In this study, we evaluate the evolutionary dynamics underpinning the origin of new disease foci and the impact of host species on parasite genetic diversity in Uganda. We found three distinct genetic clusters of T. brucei in Uganda and southwestern Kenya. Clusters 1 and 3 include isolates from central and southern Uganda, while cluster 2 contains mostly isolates from southwestern Kenya. These three clusters are not sorted by subspecies designation (T. b. brucei vs T. b. rhodesiense), host or date of collection. Our results show expansion of the disease to new foci in central Uganda occurred from the northward spread of T. b. rhodesiense. They also confirm the emergence of the human infective strains from non-infective T. b. brucei strains of different genetic backgrounds, and the importance of cattle as Tbr reservoir, as confounders that shape the epidemiology of sleeping sickness in the region.

Sero-diagnosis of surra exploiting recombinant VSG antigen based ELISA for surveillance

Trypanosoma evansi, a haemoflagellate, causes "surra" an important chronic wasting disease of a wide range of wild and domestic herbivorous and carnivorous animals including cattle, buffaloes, camels, horses, etc. The untreated recovered animal can act as a carrier without exhibiting the disease symptoms and can be a source of infection to healthy animals. The diagnosis and subsequent treatment of the carrier animals is helpful to curb the disease. As the parasitaemia in carrier animals is very scanty, the conventional blood smear examination, which is widely practiced in the field, cannot detect such condition. For this purpose improved diagnostics are very much useful for mass sero-screening test such as ELISA. In the present study, the VSG of T. evansi was expressed in prokaryotic system (E. coli) and thereafter its immunoreactivity has been evaluated in immuno blot and enzyme immuno assay. The expressed protein showed 95.6% sensitivity, 98.0% specificity and 0.93 Cohen's kappa value, when compared with standard antigens. The developed antigen has also been validated with field serum samples from bovine, camel and horse collected from different states of India. The data showed that the developed recombinant antigen can be a diagnostic tool to detect carrier animals as well as control of the disease.

Expressed truncated N-terminal variable surface glycoprotein (VSG) of Trypanosoma evansi in E. coli exhibits immuno-reactivity

The variant surface glycoprotein (VSG) of trypanosome is an important part of its body surface coat, which is expressed in early, middle and late stages of infection contributing a major diagnostic value. In the present study, the 5' end of the partial VSG gene sequences (681 bp) encoding N-terminal protein of RoTat 1.2 VSG (227 amino acid) was amplified, cloned into pET32a vector, and expressed in prokaryotic system. The fused His-tagged expressed VSG protein (43 kDa) of the Trypanosoma evansi was characterized in SDS-PAGE and immunoblotting using hyperimmune/immune sera raised against buffalo, dog, lion and leopard isolates of T. evansi. The expressed protein remained immunoreactive with all the sera combinations. The animals immunized with whole cell lysate or recombinant protein showed similar antibody reactions in ELISA and CATT (Card Agglutination Test for Trypanosomiasis). This study suggests the expressed recombinant truncated VSG is having its importance for its possible use in sero-diagnosis of surra.

A spectrum of disease in human African trypanosomiasis: the host and parasite genetics of virulence

For over 50 years it has been known that there are considerable differences in the severity and rate of progression of both Trypanosoma brucei rhodesiense and T. b. gambiense infection between individuals. Yet research into the factors, whether parasite or host, which control virulence in Human African trypanosomiasis is in its infancy. In this paper we review the clinical evidence for virulence variation and the epidemiological and experimental data that give clues as to the mechanisms involved. Evidence will be presented for both asymptomatic forms of T. b. gambiense infection and low virulence forms of T. b. rhodesiense infection in humans. While in both cases the mechanisms remain to be elucidated, the overall infection virulence phenotype is determined by both parasite and host genotype.

Genetic diversity of Trypanosoma evansi in beef cattle based on internal transcribed spacer region

This study was focused on genetic diversity of Trypanosoma evansi which is a widely distributed haemoflagellate of veterinary importance that infects a variety of larger mammals including horses, mules, camels, buffalo, cattle and deer. The genetic diversity of T. evansi of beef cattle LAM19 was accomplished by using phylogenetic analysis based on internal transcribed spacer region (ITS). Blood sample was collected from a naturally infected beef cattle LAM 19 and parasitemia was raised by mouse inoculation. The parasites were collected and isolated by using DE 52 DEAE cellulose anion exchange column prior to DNA extraction. Upon PCR amplification of ITS region, the product of 1300bp in size was obtained. The ITS nucleotide sequences were analyzed and revealed that it could demonstrate the genetic diversity of T. evansi of beef cattle LAM19. Based on the ITS tree, beef cattle LAM 19 T. evansi were categorized into two main groups where the genetic diversity occurred within Group 1. The data could be applicable for the survey of parasite dynamics, epidemiological studies as well as prevention and control of the disease.

Resolution of the species problem in African trypanosomes

There is a general assumption that eukaryote species are demarcated by morphological or genetic discontinuities. This stems from the idea that species are defined by the ability of individuals to mate and produce viable progeny. At the microscopic level, where organisms often proliferate more by asexual than sexual reproduction, this tidy classification system breaks down and species definition becomes messy and problematic. The dearth of morphological characters to distinguish microbial species has led to the widespread application of molecular methods for identification. As well as providing molecular markers for species identification, gene sequencing has generated the data for accurate estimation of relatedness between different populations of microbes. This has led to recognition of conflicts between current taxonomic designations and phylogenetic placement. In the case of microbial pathogens, the extent to which taxonomy has been driven by utilitarian rather than biological considerations has been made explicit by molecular phylogenetic analysis. These issues are discussed with reference to the taxonomy of the African trypanosomes, where pathogenicity, host range and distribution have been influential in the designation of species and subspecies. Effectively, the taxonomic units recognised are those that are meaningful in terms of human or animal disease. The underlying genetic differences separating the currently recognised trypanosome taxa are not consistent, ranging from genome-wide divergence to presence/absence of a single gene. Nevertheless, if even a minor genetic difference reflects adaptation to a particular parasitic niche, for example, in Trypanosoma brucei rhodesiense, the presence of a single gene conferring the ability to infect humans, then it can prove useful as an identification tag for the taxon occupying that niche. Thus, the species problem can be resolved by bringing together considerations of utility, genetic difference and adaptation.

Characterization of Trypanosoma brucei s.l. subspecies by isoenzymes in domestic pigs from the Fontem sleeping sickness focus of Cameroon

Though it has been established that domestic animals (especially the pig) are potential reservoir hosts for Trypanosoma brucei gambiense in West Africa, there is little data to this effect concerning Central Africa. Instead, some previous authors report the absence of Trypanozoon type trypanosomes in domestic animals in Cameroon. Thirty-two domestic pigs were sampled by KIVI (kit for in vitro isolation) of trypanosomes in the northern region (Bechati) of the Fontem sleeping sickness focus of Cameroon. Twenty-one of these were found positive, from 15 of which 17 isolates were successfully obtained. Isoenzyme characterization revealed that isolates from 4 of the 15 pigs belonged to zymodemes associated with T. brucei gambiense group 1. The prevalence of this disease in the local human population is, however, very low. It is evident from this study that the domestic pig may be a potential reservoir host for T. brucei gambiense in the Fontem focus. There is, however, need for an extensive study on domestic animals in Cameroon and other neighbouring countries for a better comprehension of the epidemiology of sleeping sickness within the Central African region.

Trypanosoma evansi: cloning and expression in Spodoptera frugiperda [correction of fugiperda] insect cells of the diagnostic antigen RoTat1.2

A complementary DNA encoding the variant surface glycoprotein (VSG) of Trypanosoma evansi Rode Trypanozoon antigenic type (RoTat)1.2, currently used for experimental serological diagnosis of T. evansi infection in livestock, was cloned as a recombinant plasmid and sequenced. A recombinant baculovirus containing the coding region of RoTat1.2 VSG was constructed to express the protein in Spodoptera frugiperda [corrected] insect cells. From this, sufficient quantities of the recombinant protein are being produced for empirical and wide-scale objective assessment of the diagnostic potential of this antigen. The gene encoding the RoTat1.2 VSG was shown by PCR to be present in the genomes of many different cloned isolates of T. evansi, but not T. brucei, from geographically separate regions of Africa, Asia, and South America. With the recombinant RoTat1.2 at hand, it is now possible to investigate the extent to which epitopes on this VSG are conserved among different T. evansi isolates.

General expression of RoTat 1.2 variable antigen type in Trypanosoma evansi isolates from different origin

The variable surface glycoprotein of Trypanosoma evansi RoTat 1.2 variable antigen type (VAT) is used as an antigen in different antibody detection assays for T. evansi. To obtain more information on the predominant character of RoTat 1.2 and its diagnostic potential in antibody detection tests, we checked its expression in 10 different T. evansi stocks and clones from different parts of the world. Cryostabilates were injected into mice and the trypanosomes of the first peak parasitaemia were screened for the presence of RoTat 1.2 by VAT specific immunofluorescence. To monitor the appearance of RoTat 1.2 specific antibodies during infection, rabbits were infected and serologically tested at different time intervals with VAT specific immune trypanolysis, CATT/T. evansi, LATEX/T. evansi and ELISA/T. evansi. Test results confirm the predominant character of RoTat 1.2.

Sleeping sickness: a tale of two diseases

Sleeping sickness presents clinically as two distinct diseases, reflecting the fact that two very different trypanosomes are responsible. The African Rift separating East and West Africa defines the distribution of the two diseases. In this review, Susan Welburn, Eric Fèvre, Paul Coleman, Martin Odiit and Ian Maudlin discuss the biology and distribution of these two diseases in relation to the evolution of hominids in Africa.

Novel species specific antigens of trypanosoma congolense and their different localization among life-cycle stages

Seven monoclonal antibodies (mAbs) were raised against Trypanosoma congolense procyclic form (PCF). Localization of the antigens recognized by the mAbs was determined in bloodstream form (BSF), PCF, epimastigote form (EMF) and metacyclic form (MCF) by confocal laser scanning microscopy (CLSM). Two mAbs (10F9 and 20H12) showed different fluorescent patterns among different life-cycle stages of the parasite. The 10F9 recognized a 76 kDa antigen of all life-cycle stages of the parasite and the antigen localization corresponded with that of a mitochondrion. While the 20H12 recognized 119 and 122 kDa antigens of all the life-cycle stages and the antigen localization corresponded with a flagellum in BSF and MCF, tip of a flagellum in PCF, and part of cytoplasm in EMF. Moreover, the 20H12 did not react to T. brucei gambiense, T. b. rhodesiense and T. evansi antigens in both CLSM and immunoblotting. Therefore, the antigens recognized by the 20H12 seem to be T. congolense specific. Although, further studies will be required for a full characterization of the T. congolense specific 119 and 122 kDa antigens, the mAb 20H12 and the specific antigens may be useful in not only establishment of T. congolense specific diagnosis methods but also studies on molecular mechanisms regulating differentiation of the parasite during life-cycle.

Genetic exchange in the trypanosomatidae

The only trypanosomatid so far proved to undergo genetic exchange is Trypanosoma brucei, for which hybrid production after co-transmission of different parental strains through the tsetse fly vector has been demonstrated experimentally. Analogous mating experiments have been attempted with other Trypanosoma and Leishmania species, so far without success. However, natural Leishmania hybrids, with a combination of the molecular characters of two sympatric species, have been described amongst both New and Old World isolates. Typical homozygotic and heterozygotic banding patterns for isoenzyme and deoxyribonucleic acid markers have also been demonstrated amongst naturally-occurring T. cruzi isolates. The mechanism of genetic exchange in T. brucei remains unclear, although it appears to be a true sexual process involving meiosis. However, no haploid stage has been observed, and intermediates in the process are still a matter for conjecture. The frequency of sex in trypanosomes in nature is also a matter for speculation and controversy, with conflicting results arising from population genetics analysis. Experimental findings for T. brucei are discussed in the first section of this review, together with laboratory evidence of genetic exchange in other species. The second section covers population genetics analysis of the large body of data from field isolates of Leishmania and Trypanosoma species. The final discussion attempts to put the evidence from experimental and population genetics into its biological context.

Trypanosoma brucei s.l: evolution, linkage and the clonality debate

The Index of Association (IA) has been proposed by Maynard Smith et al. (1993) as a general method for characterizing the population structures of microorganisms as either: clonal, epidemic, cryptic species or panmictic. With reference to the current debate surrounding the mode of reproduction in parasitic protozoa, this study explores (i) the suitability and limitations of the IA for characterizing populations of Trypanosoma brucei s.l., and (ii) the idea that the significance of genetic differences between populations may be better understood if the evolution, spread and temporal stability of certain parasite genotypes are also considered. Four populations of T. brucei from Côte d'Ivoire, Uganda and Zambia are analysed using the IA and a complementary test for linkage disequilibrium, test f of Tibayrenc, Kjellberg & Ayala (1990). The two populations from Uganda are characterized as epidemic, while the others appear more or less clonal; the merits of the two methods are compared. The implications of the various population classifications are discussed with reference to genotype longevity in each region; the evolution and biomedical consequences of the genetic non-homogeneity of T. brucei are reviewed.

Detection of linkage disequilibrium in Trypanosoma brucei isolated from tsetse flies and characterized by RAPD analysis and isoenzymes

This study analyses the different populations of Trypanosoma brucei spp. which may coexist within the midgut of wild tsetse flies (Stevens et al. 1994). Cloned trypanosome populations characterized by multilocus enzyme electrophoresis (MLEE) were further analysed by the random amplified polymorphic DNA (RAPD) technique, allowing detection of genetic variation at a finer level than that possible by MLEE. Genetic distance matrices derived from the results of each of the two biochemical methods were calculated and compared using a computer program based on the method of Mantel (1967). The observed correlation was used to investigate the degree of linkage disequilibrium (LD) in the data, association between unrelated polymorphic markers providing a measure of the departure from panmixia. The potential of each biochemical method to detect linkage was evaluated by an extended Mantel test. The MLEE/RAPD correlation test evidenced significant LD within the population, suggesting a predominantly clonal method of reproduction for these West African trypanosomes. Analysis of RAPD data by the extended Mantel test also showed significant LD, while the results with MLEE data were less conclusive, providing an indication of the relative potential of the two techniques to detect fine genetic variation.

Discrimination of Leishmania isolates using a limited set of enzymatic loci

The characterization of more than 200 Leishmania isolates, using 18 enzymatic loci, indicated that only a limited number of enzymes produces diagnostic alleles which are potentially useful in the identification of species or zymodemes. 6-Phospho-gluconate dehydrogenase was the most polymorphic enzyme, containing enzyme, containing the majority of diagnostic alleles, and could be used to separate strains of the subgenus Leishmania from those of Viannia. It appears that just a few enzymatic loci, chosen a priori, are all that are needed in many taxonomic and epidemiological studies of Leishmania.

The origins of parasitism in the protists

The origins of parasitism among the protists are, like the group itself, polyphyletic. Probably the majority of present-day parasitic forms evolved from free-living ancestors which were ingested as part of the food of their hosts, though origins from ectoparasitic forms and via a phase of facultative parasitism are possibilities, particularly among the ciliated protozoa and (for ectoparasitism) the Kinetoplasta. Sporozoan parasites most probably developed via a stage which was ingested and became adapted to life in the host's gut. Further developments in parasitism involved deeper penetration into the host's tissues and the adoption of more than one host in the life cycle, thus avoiding entirely the potentially hazardous phase of existence outside the host.

The persistence of genetic homogeneity among Trypanosoma brucei rhodesiense isolates from patients in north-west Tanzania

Trypanosomes isolated during 1991 from nine patients with Rhodesian sleeping sickness in north-west Tanzania were genetically characterized by electrophoresis of ten enzymes. Eight isolates were allocated to a known zymodeme (Z306); another had an enzyme profile (Z379) not previously encountered. An example of Z306 has been previously isolated in 1971, nearby in a part of Rwanda adjacent to the border with Tanzania; in addition, a closely related isolate, in Z307, was collected in 1959 from a patient in north-west Tanzania. The new zymodeme (Z379) was 94% similar to Z306, and both had a close similarity of 89% to Z307. All these isolates belonged to the zambezi strain group of related zymodemes, and evidence is presented that other examples of the group have been collected from man in Tanzania since 1959. Such apparent long term genetic stability is similar to circumstances further south in an endemic area of Zambia, where 12 examples of Z306 and two of Z307 were acquired over a period of 12 years from patients. The similar genetic homogeneity among trypanosomes in endemic parts of both Tanzania and Zambia contrasted markedly with the heterogeneity described to the north of Tanzania in that different strain groups circulate in epidemic areas of Kenya and Uganda.

Isoenzyme comparison of Trypanozoon isolates from two sleeping sickness areas of south-eastern Uganda

The study characterized 151 Trypanozoon isolates from south-east Uganda by isoenzyme electrophoresis. Stocks were from a range of hosts, including man, cattle, pigs, dogs and Glossina fuscipes fuscipes: 104 isolates were from the Busoga area, 47 were from the Tororo district. Stocks were characterized on thin layer starch gel using eight enzyme systems: ALAT, ASAT, ICD, MDH, ME, NHD, NHI, PGM. Enzyme profiles were generally typical of East Africa; new patterns for ICD and ME were detected. Trypanosomes were classified on the basis of their profile by similarity coefficient analysis and the unweighted pair-group method using arithmetic averages (UPGMA). The majority of trypanosomes were classified in one or other of two genetically distinct groups which corresponded to the strain groups busoga and zambezi, both of which are associated with Rhodesian sleeping sickness in East Africa. Contingency table analyses indicated associations between certain isoenzymes of ICD and PGM, according to host and geographical origin. Significant relationships between trypanosome strain group and geographic origin were also demonstrated for some host groups.

Molecular variation in trypanosomes

Several species of the genus Trypanosoma cause parasitic diseases of considerable medical and veterinary importance throughout Africa, Asia and the Americas. These parasites exhibit considerable intra-species genetic diversity and variation, which has complicated their taxonomic classification. This diversity and variation can be defined at the level of both the genome and of individual genes. The nuclear genome shows considerable inter- and intra-species plasticity in terms of chromosome number and size (molecular karyotype). The mitochondrial (kDNA) genome also varies considerably between species, especially in terms of minicircle size and organization. There is also considerable intra-specific sequence diversity in minicircles and within the Variable Region of the maxicircle. Restriction enzyme analysis of this diversity has lead to the concept of 'schizodemes'. At the gene level, isoenzyme analysis has proven very useful for strain and isolate identification, with the classification into numerous 'zymodemes'. Considerable antigenic diversity has also been identified in T. cruzi and T. brucei, with the development of 'serodemes' in the latter. In addition to this inter-strain diversity, African trypanosomes (T. brucei, T. congolense, and T. vivax) exhibit the phenomenon of antigenic variation, where individual parasites are able to express any one of hundreds of different copies of the Variant Surface Glycoprotein gene at any particular time. The molecular mechanisms underlying antigenic variation are now understood in considerable detail. The implication of this molecular diversity and variation are discussed in terms of trypanosome taxonomy and disease control.

Genetic processes within an epidemic of sleeping sickness in Uganda

Reproductive processes within the current Ugandan epidemic of sleeping sickness are investigated. Genotype frequencies derived from isoenzyme patterns in 44 stocks of Trypanosoma brucei s.l. collected in 1988 from Tororo, south-east Uganda are analysed by single and multiple loci methods. In the single locus method, the hypothesis of random mating is tested by agreement with Hardy-Weinberg equilibrium. The multiple loci method uses a contingency table approach to detect non-random associations between pairs of loci; this equates to the detection of disequilibrium. The results do not support the concept of a randomly mating population of T. brucei within the current epidemic. Results from the epidemic data set are discussed in relation to the broader problem of genetic exchange in Trypanozoon.

A simplified method for identifying subspecies and strain groups in Trypanozoon by isoenzymes

To characterize trypanosomes from the subgenus Trypanozoon, 272 stocks in 111 zymodemes were analysed by the polymorphisms seen in a rationalized range of nine enzymes, resolved by electrophoresis, mostly on cellulose acetate. Several highly polymorphic or invariant enzymes used previously were omitted, while two new enzymes, NHD and SOD were included; the isoenzymes seen for SOD were interpreted as two separate enzymes, SODA and SODB. Isoenzyme band patterns were analysed by two complementary numerical methods to elucide taxonomic relationships within the subgenus; groups of zymodemes corresponding to subspecies and strain groups were defined, which agreed closely with previous studies. Except for one zymodeme, Trypanosoma evansi could not be clearly distinguished from the bouaflé strain group. This strain group had enzymic features that overlapped to some extent those of the busoga group. Trypanosoma brucei gambiense and the zambezi, kakumbi, kiboko and sindo groups were clearly defined. Eight zymodemes could not be classified. A rapid identification system using a limited number of enzymes is presented.