Enzyme polymorphism and the distribution of Trypanosoma congolense isolates

Bioengineered 3D microvessels and complementary animal models reveal mechanisms of Trypanosoma congolense sequestration

In the mammalian host, Trypanosoma congolense cytoadheres, or sequesters, to the vascular endothelium. Although sequestration influences clinical outcome, disease severity and organ pathology, its determinants and mediators remain unknown. Challenges such as the variability of animal models, the only-recently developed tools to genetically manipulate the parasite, and the lack of physiologically-relevant in vitro models have hindered progress. Here, we engineered brain and cardiac 3D bovine endothelial microvessel models that mimic the bovine brain microvasculature and the bovine aorta, respectively. By perfusing these models with two T. congolense strains, we investigated the roles of flow for parasite sequestration and tropism for different endothelial beds. We discovered that sequestration is dependent on cyclic adenosine monophosphate (cAMP) signalling, closely linked to parasite proliferation, but not associated with parasite transmission to the tsetse fly vector. Finally, by comparing the expression profiles of sequestered and non-sequestered parasites collected from a rodent model, we showed gene expression changes in sequestered parasites, including of surface variant antigens. This work presents a physiologically-relevant platform to study trypanosome interactions with the vasculature and provides a deeper understanding of the molecular and biophysical mechanisms underlying T. congolense sequestration.

Investigation of Trypanosoma-induced vascular damage sheds insights into Trypanosoma vivax sequestration

Multiple blood-borne pathogens infecting mammals establish close interactions with the host vascular endothelium as part of their life cycles. In this work, we investigate differences in the interactions of three Trypanosoma species: T. brucei, T. congolense and T. vivax with the blood vasculature. Infection with these species results in vastly different pathologies, including different effects on vascular homeostasis, such as changes in vascular permeability and microhemorrhages. While all three species are extracellular parasites, T. congolense is strictly intravascular, while T. brucei is capable of surviving both extra- and intravascularly. Our knowledge regarding T. vivax tropism and its capacity of migration across the vascular endothelium is unknown. In this work, we show for the first time that T. vivax parasites sequester to the vascular endothelium of most organs, and that, like T. congolense, T. vivax Y486 is largely incapable of extravasation. Infection with this parasite species results in a unique effect on vascular endothelium receptors including general downregulation of ICAM1 and ESAM, and upregulation of VCAM1, CD36 and E-selectin. Our findings on the differences between the two sequestering species (T. congolense and T. vivax) and the non-sequestering, but extravasating, T. brucei raise important questions on the relevance of sequestration to the parasite's survival in the mammalian host, and the evolutionary relevance of both sequestration and extravasation.

Immunopathology and Trypanosoma congolense parasite sequestration cause acute cerebral trypanosomiasis

Trypanosoma congolense causes a syndrome of variable severity in animals in Africa. Cerebral trypanosomiasis is a severe form, but the mechanism underlying this severity remains unknown. We developed a mouse model of acute cerebral trypanosomiasis and characterized the cellular, behavioral, and physiological consequences of this infection. We show large parasite sequestration in the brain vasculature for long periods of time (up to 8 hr) and extensive neuropathology that associate with ICAM1-mediated recruitment and accumulation of T cells in the brain parenchyma. Antibody-mediated ICAM1 blocking and lymphocyte absence reduce parasite sequestration in the brain and prevent the onset of cerebral trypanosomiasis. Here, we establish a mouse model of acute cerebral trypanosomiasis and we propose a mechanism whereby parasite sequestration, host ICAM1, and CD4⁺ T cells play a pivotal role.

VAPPER: High-throughput variant antigen profiling in African trypanosomes of livestock

BACKGROUND

Analysing variant antigen gene families on a population scale is a difficult challenge for conventional methods of read mapping and variant calling due to the great variability in sequence, copy number, and genomic loci. In African trypanosomes, hemoparasites of humans and animals, this is complicated by variant antigen repertoires containing hundreds of genes subject to various degrees of sequence recombination.

FINDINGS

We introduce Variant Antigen Profiler (VAPPER), a tool that allows automated analysis of the variant surface glycoprotein repertoires of the most prevalent livestock African trypanosomes. VAPPER produces variant antigen profiles for any isolate of the veterinary pathogens Trypanosoma congolense and Trypanosoma vivax from genomic and transcriptomic sequencing data and delivers publication-ready figures that show how the queried isolate compares with a database of existing strains. VAPPER is implemented in Python. It can be installed to a local Galaxy instance from the ToolShed (https://toolshed.g2.bx.psu.edu/) or locally on a Linux platform via the command line (https://github.com/PGB-LIV/VAPPER). The documentation, requirements, examples, and test data are provided in the Github repository.

CONCLUSION

By establishing two different, yet comparable methodologies, our approach is the first to allow large-scale analysis of African trypanosome variant antigens, large multi-copy gene families that are otherwise refractory to high-throughput analysis.

Trypanosoma congolense: In Vitro Culture and Transfection

Trypanosoma congolense, together with T. vivax and T. brucei, causes African animal trypanosomiasis (AAT), or nagana, a livestock disease carried by bloodsucking tsetse flies in sub-Saharan Africa. These parasitic protists cycle between two hosts: mammal and tsetse fly. The environment offered by each host to the trypanosome is markedly different, and hence the metabolism of stages found in the mammal differs from that of insect stages. For research on new diagnostics and therapeutics, it is appropriate to use the mammalian life cycle stage, bloodstream forms. Insect stages such as procyclics are useful for studying differentiation and also serve as a convenient source of easily cultured, non-infective organisms. Here, we present protocols in current use in our laboratory for the in vitro culture of different life cycle stages of T. congolense-procyclics, epimastigotes, and bloodstream forms-together with methods for transfection enabling the organism to be genetically modified. © 2019 by John Wiley & Sons, Inc.

The Structure of a Conserved Telomeric Region Associated with Variant Antigen Loci in the Blood Parasite Trypanosoma congolense

African trypanosomiasis is a vector-borne disease of humans and livestock caused by African trypanosomes (Trypanosoma spp.). Survival in the vertebrate bloodstream depends on antigenic variation of Variant Surface Glycoproteins (VSGs) coating the parasite surface. In T. brucei, a model for antigenic variation, monoallelic VSG expression originates from dedicated VSG expression sites (VES). Trypanosoma brucei VES have a conserved structure consisting of a telomeric VSG locus downstream of unique, repeat sequences, and an independent promoter. Additional protein-coding sequences, known as “Expression Site Associated Genes (ESAGs)”, are also often present and are implicated in diverse, bloodstream-stage functions. Trypanosoma congolense is a related veterinary pathogen, also displaying VSG-mediated antigenic variation. A T. congolense VES has not been described, making it unclear if regulation of VSG expression is conserved between species. Here, we describe a conserved telomeric region associated with VSG loci from long-read DNA sequencing of two T. congolense strains, which consists of a distal repeat, conserved noncoding elements and other genes besides the VSG; although these are not orthologous to T. brucei ESAGs. Most conserved telomeric regions are associated with accessory minichromosomes, but the same structure may also be associated with megabase chromosomes. We propose that this region represents the T. congolense VES, and through comparison with T. brucei, we discuss the parallel evolution of antigenic switching mechanisms, and unique adaptation of the T. brucei VES for developmental regulation of bloodstream-stage genes. Hence, we provide a basis for understanding antigenic switching in T. congolense and the origins of the African trypanosome VES.

Mitonuclear genomics challenges the theory of clonality in Trypanosoma congolense: Reply to Tibayrenc and Ayala

We recently published the first genomic diversity study of Trypanosoma congolense, a major aetiological agent of Animal African Trypanosomiasis. We demonstrated striking levels of SNP and indel diversity in the Eastern province of Zambia as a consequence of hybridization between divergent trypanosome lineages. We concluded that these and earlier findings in T. congolense challenge the predominant clonal evolution (PCE) model. In a recent comment, Tibayrenc and Ayala claim that there are many features in T. congolense supporting their theory of clonality. While we can follow the reasoning of the authors, we also identify major limitations in their theory and interpretations that resulted in incorrect conclusions. First, we argue that each T. congolense subgroup should be analysed independently as they may represent different (sub)species rather than "near-clades". Second, the authors neglect major findings of two robust population genetic studies on Savannah T. congolense that provide clear evidence of frequent recombination. Third, we reveal additional events of introgressive hybridization in T. congolense by analysing the maxicircle coding region using next-generation sequencing analyses. At last, we pinpoint two important misinterpretations by the authors and show that there are no spatially and temporally widespread clones in T. congolense. We stand by our earlier conclusions that the clonal framework is unlikely to accurately model the population structure of T. congolense. Other theoretical frameworks such as Maynard Smith's epidemic model may better represent the complex ancestry seen in T. congolense, where clones delimited in space and time arise against a background of recombination.

Variant antigen repertoires in Trypanosoma congolense populations and experimental infections can be profiled from deep sequence data using universal protein motifs

African trypanosomes are vector-borne hemoparasites of humans and animals. In the mammal, parasites evade the immune response through antigenic variation. Periodic switching of the variant surface glycoprotein (VSG) coat covering their cell surface allows sequential expansion of serologically distinct parasite clones. Trypanosome genomes contain many hundreds of VSG genes, subject to rapid changes in nucleotide sequence, copy number, and chromosomal position. Thus, analyzing, or even quantifying, VSG diversity over space and time presents an enormous challenge to conventional techniques. Indeed, previous population genomic studies have overlooked this vital aspect of pathogen biology for lack of analytical tools. Here we present a method for analyzing population-scale VSG diversity in Trypanosoma congolense from deep sequencing data. Previously, we suggested that T. congolense VSGs segregate into defined “phylotypes” that do not recombine. In our data set comprising 41 T. congolense genome sequences from across Africa, these phylotypes are universal and exhaustive. Screening sequence contigs with diagnostic protein motifs accurately quantifies relative phylotype frequencies, providing a metric of VSG diversity, called the “variant antigen profile.” We applied our metric to VSG expression in the tsetse fly, showing that certain, rare VSG phylotypes may be preferentially expressed in infective, metacyclic-stage parasites. Hence, variant antigen profiling accurately and rapidly determines the T. congolense VSG gene and transcript repertoire from sequence data, without need for manual curation or highly contiguous sequences. It offers a tractable approach to measuring VSG diversity across strains and during infections, which is imperative to understanding the host–parasite interaction at population and individual scales.

Discovery and genomic analyses of hybridization between divergent lineages of Trypanosoma congolense, causative agent of Animal African Trypanosomiasis

Hybrid populations and introgressive hybridization remain poorly documented in pathogenic micro-organisms, as such that genetic exchange has been argued to play a minor role in their evolution. Recent work demonstrated the existence of hybrid microsatellite profiles in Trypanosoma congolense, a parasitic protozoan with detrimental effects on livestock productivity in sub-Saharan Africa. Here, we present the first population genomic study of T. congolense, revealing a remarkable number of single nucleotide polymorphisms (SNPs), small insertions/deletions (indels) and gene deletions among 56 parasite genomes from ten African countries. One group of parasites from Zambia was particularly diverse, displaying a substantial number of heterozygous SNP and indel sites compared to T. congolense parasites from the nine other sub-Saharan countries. Genomewide 5-kb phylogenetic analyses based on phased SNP data revealed that these parasites were the product of hybridization between phylogenetically distinct T. congolense lineages. Other parasites within the same region in Zambia presented a mosaic of haplotypic ancestry and genetic variability, indicating that hybrid parasites persisted and recombined beyond the initial hybridization event. Our observations challenge traditional views of trypanosome population biology and encourage future research on the role of hybridization in spreading genes for drug resistance, pathogenicity and virulence.

Identification and genetic characterization of Trypanosoma congolense in domestic animals of Fontem in the South-West region of Cameroon

To understand the circulation and the spread of Trypanosoma congolense genotypes in animals of Fontem in the southwest region of Cameroon, T. congolense forest and T. congolense savannah were investigated in 397 domestic animals in eight villages. Out of the 397 domestic animals, 86 (21.7%) were found infected by trypanosomes, using the capillary tube centrifugation test. The PCR with specific primers identified 163 (41.1%) and 81 (20.4%) animals infected by T. congolense forest and T. congolense savannah, respectively; showing for the first time the circulation of T. congolense savannah in the Fontem region. No infection with T. congolense savannah was found in pigs whereas goats and sheep were infected by T. congolense forest and/or T. congolense savannah. The prevalence of trypanosomes varied significantly amongst villages and animal species. The genotyping of T. congolense forest positive samples using microsatellites markers showed that multiple genotypes occurred in 27.2% (44/163) of animals sampled, whereas single genotypes were found in 73.8% (119/163) of samples. Some alleles were found in all animal species as well as in all villages and were responsible for major genotypes, whereas others (rare alleles) were identified only in some animals of few villages. These rare alleles were characteristic of specific genotypes, assimilated to minor genotypes which can be spread in the region through tsetse flies. The microsatellite markers show a low genetic variability and an absence of sub-structuration within T. congolense forest. The analysis of the microsatellite data revealed a predominant clonal reproduction within T. congolense forest. Pigs were the animal species with the highest number of different genotypes of T. congolense forest. They seem to play an important epidemiological role in the propagation and spread of different genotypes of T. congolense.

The life cycle of Trypanosoma (Nannomonas) congolense in the tsetse fly

Background

The tsetse-transmitted African trypanosomes cause diseases of importance to the health of both humans and livestock. The life cycles of these trypanosomes in the fly were described in the last century, but comparatively few details are available for Trypanosoma (Nannomonas) congolense, despite the fact that it is probably the most prevalent and widespread pathogenic species for livestock in tropical Africa. When the fly takes up bloodstream form trypanosomes, the initial establishment of midgut infection and invasion of the proventriculus is much the same in T. congolense and T. brucei. However, the developmental pathways subsequently diverge, with production of infective metacyclics in the proboscis for T. congolense and in the salivary glands for T. brucei. Whereas events during migration from the proventriculus are understood for T. brucei, knowledge of the corresponding developmental pathway in T. congolense is rudimentary. The recent publication of the genome sequence makes it timely to re-investigate the life cycle of T. congolense.

Methods

Experimental tsetse flies were fed an initial bloodmeal containing T. congolense strain 1/148 and dissected 2 to 78 days later. Trypanosomes recovered from the midgut, proventriculus, proboscis and cibarium were fixed and stained for digital image analysis. Trypanosomes contained in spit samples from individually caged flies were analysed similarly. Mensural data from individual trypanosomes were subjected to principal components analysis.

Results

Flies were more susceptible to infection with T. congolense than T. brucei; a high proportion of flies infected with T. congolense established a midgut and subsequent proboscis infection, whereas many T. brucei infections were lost in the migration from foregut to salivary glands. In T. congolense, trypomastigotes ceased division in the proventriculus and became uniform in size. The trypanosomes retained trypomastigote morphology during migration via the foregut to the mouthparts and we confirmed that the trypomastigote-epimastigote transition occurred in the proboscis. We found no equivalent to the asymmetric division stage in T. brucei that mediates transition of proventricular trypomastigotes to epimastigotes. In T. congolense extremely long epimastigotes with remarkably elongated posterior ends were observed in both the proboscis and cibarium; no difference was found in the developmental stages in these two organs. Dividing trypomastigotes and epimastigotes were recovered from the proboscis, some of which were in transition from trypomastigote to epimastigote and vice versa. It remains uncertain whether these morphological transitions are mediated by cell division, since we also found non-dividing cells with a variously positioned, juxta-nuclear kinetoplast.

Conclusions

We have presented a detailed description of the life cycle of T. congolense in its tsetse fly vector. During development in the fly T. congolense shares a common migratory pathway with its close relative T. brucei, culminating in the production of small metacyclic trypanosomes that can be inoculated with the saliva. Despite this outward similarity in life cycle, the transitional developmental stages in the foregut and mouthparts are remarkably different in the two trypanosome species.

The origins of the trypanosome genome strains Trypanosoma brucei brucei TREU 927, T. b. gambiense DAL 972, T. vivax Y486 and T. congolense IL3000

The genomes of several tsetse-transmitted African trypanosomes (Trypanosoma brucei brucei, T. b. gambiense, T. vivax, T. congolense) have been sequenced and are available to search online. The trypanosome strains chosen for the genome sequencing projects were selected because they had been well characterised in the laboratory, but all were isolated several decades ago. The purpose of this short review is to provide some background information on the origins and biological characterisation of these strains as a source of reference for future users of the genome data. With high throughput sequencing of many more trypanosome genomes in prospect, it is important to understand the phylogenetic relationships of the genome strains.

Post eclosion age predicts the prevalence of midgut trypanosome infections in Glossina

The teneral phenomenon, as observed in Glossina sp., refers to the increased susceptibility of the fly to trypanosome infection when the first bloodmeal taken is trypanosome-infected. In recent years, the term teneral has gradually become synonymous with unfed, and thus fails to consider the age of the newly emerged fly at the time the first bloodmeal is taken. Furthermore, conflicting evidence exists of the effect of the age of the teneral fly post eclosion when it is given the infected first bloodmeal in determining the infection prevalence. This study demonstrates that it is not the feeding history of the fly but rather the age (hours after eclosion of the fly from the puparium) of the fly when it takes the first (infective) bloodmeal that determines the level of fly susceptibility to trypanosome infection. We examine this phenomenon in male and female flies from two distinct tsetse clades (Glossina morsitans morsitans and Glossina palpalis palpalis) infected with two salivarian trypanosome species, Trypanosoma (Trypanozoon) brucei brucei and Trypanosoma (Nannomonas) congolense using Fisher's exact test to examine differences in infection rates. Teneral tsetse aged less than 24 hours post-eclosion (h.p.e.) are twice as susceptible to trypanosome infection as flies aged 48 h.p.e. This trend is conserved across sex, vector clade and parasite species. The life cycle stage of the parasite fed to the fly (mammalian versus insect form trypanosomes) does not alter this age-related bias in infection. Reducing the numbers of parasites fed to 48 h.p.e., but not to 24 h.p.e. flies, increases teneral refractoriness. The importance of this phenomenon in disease biology in the field as well as the necessity of employing flies of consistent age in laboratory-based infection studies is discussed.

Trypanosome genetics: populations, phenotypes and diversity

In the last decade, there has been a wide range of studies using a series of molecular markers to investigate the genotypic diversity of some of the important species of African trypanosomes. Here, we review this work and provide an update of our current understanding of the mechanisms that generate this diversity based on population genetic analysis. In parallel with field based studies, our knowledge of the key features of the system of genetic exchange in Trypanosoma brucei, based on laboratory analysis, has reached the point at which this system can be used as a tool to determine the genetic basis of a phenotype. In this context, we have outlined our current knowledge of the basis for phenotypic variation among strains of trypanosomes, and highlight that this is a relatively under researched area, except for work on drug resistance. There is clear evidence for 'strain'-specific variation in tsetse transmission, a range of virulence/pathogenesis phenotypes and the ability to cross the blood brain barrier. The potential for using genetic analysis to dissect these phenotypes is illustrated by the recent work defining a locus determining organomegaly for T. brucei. When these results are considered in relation to the body of research on the variability of the host response to infection, it is clear that there is a need to integrate the study of host and parasite diversity in relation to understanding infection outcome.

African trypanosomes: celebrating diversity

Recent advances in molecular identification techniques and phylogenetic analysis have revealed the presence of previously unidentified tsetse-transmitted trypanosomes in Africa. This is surprising in a comparatively well-known group of pathogens that includes the causative agents of human and animal trypanosomiasis. Despite levels of genetic divergence that warrant taxonomic recognition, only one of these new trypanosomes has been named as a new species; the increased diversity is largely ignored or regarded as an inconvenient complication. Yet, some of these trypanosomes have demonstrated pathogenicity, whereas others are closely related to known pathogens, and might share this trait. We should first acknowledge that these novel trypanosomes exist and then take steps to investigate their host range, pathogenicity to livestock and response to chemotherapy.

Trypanosoma vivax displays a clonal population structure

African animal trypanosomiasis, or Nagana, is a debilitating and economically costly disease with a major impact on animal health in sub-Saharan Africa. Trypanosoma vivax, one of the principal trypanosome species responsible for the disease, infects a wide host range including cattle, goats, horses and donkeys and is transmitted both cyclically by tsetse flies and mechanically by other biting flies, resulting in a distribution covering large swathes of South America and much of sub-Saharan Africa. While there is evidence for mating in some of the related trypanosome species, Trypanosoma brucei, Trypanosoma congolense and Trypanosoma cruzi, very little work has been carried out to examine this question in T. vivax. Understanding whether mating occurs in T. vivax will provide insight into the dynamics of trait inheritance, for example the spread of drug resistance, as well as examining the origins of meiosis in the order Kinetoplastida. With this in mind we have identified orthologues of eight core meiotic genes within the genome, the presence of which imply that the potential for mating exists in this species. In order to address whether mating occurs, we have investigated a sympatric field population of T. vivax collected from livestock in The Gambia, using microsatellite markers developed for this species. Our analysis has identified a clonal population structure showing significant linkage disequilibrium, homozygote deficits and disagreement with Hardy-Weinberg predictions at six microsatellite loci, indicative of a lack of mating in this population of T. vivax.

Species-specific probes for the identification of the African tsetse-transmitted trypanosomes

The first step in studying the epidemiology of a disease is the accurate identification of the pathogen. Traditional reliance on morphological identification has given way to the use of molecular methods for the detection and identification of pathogens, greatly improving our understanding of epidemiology. For the African tsetse-transmitted trypanosomes, the growth of PCR methods for identification of trypanosomes has led to increased appreciation of trypanosome genetic diversity and discovery of hitherto unknown trypanosome species, as well as greater knowledge about the number and type of trypanosome infections circulating in mammalian hosts and vectors. Sequence data and phylogenetic analysis have provided quantitative information on the relatedness of different trypanosome species and allowed the new trypanosome genotypes discovered through the use of species identification methods in the field to be accurately placed in the phylogenetic tree.

Discovery of mating in the major African livestock pathogen Trypanosoma congolense

The protozoan parasite, Trypanosoma congolense, is one of the most economically important pathogens of livestock in Africa and, through its impact on cattle health and productivity, has a significant effect on human health and well being. Despite the importance of this parasite our knowledge of some of the fundamental biological processes is limited. For example, it is unknown whether mating takes place. In this paper we have taken a population genetics based approach to address this question. The availability of genome sequence of the parasite allowed us to identify polymorphic microsatellite markers, which were used to genotype T. congolense isolates from livestock in a discrete geographical area of The Gambia. The data showed a high level of diversity with a large number of distinct genotypes, but a deficit in heterozygotes. Further analysis identified cryptic genetic subdivision into four sub-populations. In one of these, parasite genotypic diversity could only be explained by the occurrence of frequent mating in T. congolense. These data are completely inconsistent with previous suggestions that the parasite expands asexually in the absence of mating. The discovery of mating in this species of trypanosome has significant consequences for the spread of critical traits, such as drug resistance, as well as for fundamental aspects of the biology and epidemiology of this neglected but economically important pathogen.

Endemic type of animal trypanosomiasis is not associated with lower genotype variability of Trypanosoma congolense isolates circulating in livestock

In order to verify whether the low impact on livestock production in endemic areas is related to a low number of trypanosome strains circulating in livestock, 37 Trypanosoma congolense isolates collected from cattle in 11 sites in an endemic trypanosomiasis area in Eastern Zambia were characterised for genotype variability using a modified amplified fragment length polymorphism technique (AFLP). Isolates were further cloned to evaluate the occurrence of mixed infections in individuals. The results obtained revealed a high genotype diversity (94.6%) among these isolates. Apart from one site, all isolates gave different AFLP profiles in each of the sites. When clones were compared, three (8%) of the 37 isolates had mixed infections. These results indicate the circulation of a high number of strains in this trypanosomiasis endemic area despite the low impact the disease has on livestock production.

First report of canine African trypanosomosis in the UK

A six-year-old neutered male Jack Russell terrier was presented two years after importation into the UK from southern Africa with severe anaemia and abdominal distension. Abdominal ultrasonography revealed the presence of hepato-splenomegaly and ascites. A diagnosis of trypanosomosis was made by blood smear examination. Shortly after admission the dog collapsed and died. PCR analysis revealed a single infection with Trypanosoma congolense savannah type. This is the first reported case of canine African trypanosomosis in Europe and suggests that chronic trypanosomosis may allow importation of the disease in apparently asymptomatic animals, even with extended quarantine periods.

A multiplex PCR that discriminates between Trypanosoma brucei brucei and zoonotic T. b. rhodesiense

Two subspecies of Trypanosoma brucei s.l. co-exist within the animal populations of Eastern Africa; T. b. brucei a parasite which only infects livestock and wildlife and T. b. rhodesiense a zoonotic parasite which infects domestic livestock, wildlife, and which in humans, results in the disease known as Human African Trypanosomiasis (HAT) or sleeping sickness. In order to assess the risk posed to humans from HAT it is necessary to identify animals harbouring potentially human infective parasites. The multiplex PCR method described here permits differentiation of human and non-human infective parasites T. b. rhodesiense and T. b. brucei based on the presence or absence of the SRA gene (specific for East African T. b. rhodesiense), inclusion of GPI-PLC as an internal control indicates whether sufficient genomic material is present for detection of a single copy T. brucei gene in the PCR reaction.

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.

A modified AFLP for Trypanosoma congolense isolate characterisation

The amplified fragment length polymorphism (AFLP) technique is a reliable and powerful DNA fingerprint tool for genetic characterisation and analysis. In this paper, we described a modified AFLP with high resolution for Trypanosoma congolense using one enzyme and agarose or Elchrom gel electrophoresis. Eleven allopatric and fourteen sympatric isolates of T. congolense savannah were used to assess the resolution of the method and its ability to characterise T. congolense isolates. Two enzymes (Eco RI or Bgl II) and corresponding non-selective and selective primers were used to identify the most appropriate combination. Patterns generated by Bgl II enzyme and a single selective primer A, C, G or T produced clear profiles. Each of the four selective primers produced different profiles for all the 25 T. congolense isolates. Due to the reduction in the number of bands, profiles could be analysed using agarose or Elchrom gels. Although comparison of a great number of samples could benefit from software help, this technique did not require flurochrome detection methods. The results of the present study demonstrated that this modified AFLP makes the characterisation of T. congolense easier while maintaining high resolution.

Species concepts for trypanosomes: from morphological to molecular definitions?

The way species and subspecies names are applied in African trypanosomes of subgenera Trypanozoon and Nannomonas is reviewed in the light of data from molecular taxonomy. In subgenus Trypanozoon the taxonomic importance of pathogenicity, host range and distribution appear to have been inflated relative to actual levels of genetic divergence. The opposite is true for subgenus Nannomonas, where current taxonomic usage badly underrepresents genetic diversity. Data from molecular characterisation studies are revealing a growing number of genotypes, which may represent distinct taxa. Unfortunately few of these genotypes are yet supported by sufficient biological data to be recognized taxonomically. But we may be missing fundamental epidemiological information, because of our inability to distinguish these trypanosomes in host blood morphologically or in tsetse by their developmental cycle. Molecular taxonomy has led the way in identifying these new genotypes and now offers the key to elucidating the biology of these organisms.

Identification of Trypanosoma brucei circulating in a sleeping sickness focus in Côte d'Ivoire: assessment of genotype selection by the isolation method

Genetic studies of Trypanosoma brucei have been mainly based on rodent inoculation (RI) for isolation of trypanosome strains. However, Trypanosoma brucei gambiense is difficult to grow in rodents. The development and use of the Kit for In Vitro Isolation (KIVI) of trypanosomes has led to a better isolation success. However, some authors report a genetic monomorphism in T. b. gambiense, and the extensive use of the KIVI was suspected as being responsible for this low genetic diversity. In the present work, trypanosome stocks were isolated from both humans and pigs in an active sleeping sickness focus in Côte d'Ivoire. Two methods were simultaneously used for this purpose: KIVI and rodent inoculation. None of the human stocks grew in rodents. Some of the stocks originating from pigs could be isolated with both methods. Each of these stocks (from the same pig) showed a different isoenzymatic pattern according to the isolation method used. All the human stocks identified belonged to the major zymodeme 3 of T. b. gambiense group 1, whereas the stocks isolated from pigs belonged to a new group of zymodemes even if they were genetically closely related. These observations may have significant implications when analysing the population structure of T. brucei, and also raise again the question of the importance of the animal reservoir in Human African Trypanosomiasis (HAT).

Comparative pathogenicity of three genetically distinct types of Trypanosoma congolense in cattle: clinical observations and haematological changes

The pathology of African bovine trypanosomosis was compared in Zebu cattle subcutaneously inoculated with three clones of trypanosomes corresponding to the three genetically distinct types of Trypanosoma congolense; savannah-type, west African riverine/forest-type and kilifi-type. All inoculated animals became parasitaemic between 7 and 11 days post-infection (dpi). The savannah-type showed consistently higher levels of parasitaemia and lower packed red cell volume percentages and leukocyte counts than the other two types. The syndrome was also more severe in the savannah-type and led inexorably to death between 29 and 54 dpi while animals with the forest or the kilifi-types recovered from earlier symptoms and haematological alterations after 3 months of infection. By the end of the experiment, the animals self-cured from the forest-type infection and the kilifi-type passed under control. The results of the present study indicated clear difference in pathogenicity between the three types of T. congolense; the savannah-type was virulent while the forest-type was of low pathogenicity and the kilifi-type was non-pathogenic.

Genetic characterization of Trypanosoma brucei gambiense and clinical evolution of human African trypanosomiasis in Côte d'Ivoire

Human African trypanosomiasis is a parasitic infection caused by protozoa belonging to Trypanosoma brucei subspecies. The clinical evolution of this disease is complex and might be because of the parasite itself, as genetic diversity has been observed in T. brucei ssp. We investigated the relationship between the genetic diversity of trypanosomes and the diversity of clinical patterns in Côte d'Ivoire. We studied clinical sleeping sickness cases, and genetically analysed the trypanosomes isolated from these patients. An important genetic monomorphism among stocks isolated in Côte d'Ivoire was observed by using various markers: isoenzymes electrophoresis, random amplified polymorphism DNA and PCR of microsatellite sequences. At the same time, the diversity of clinical patterns and evolutions was confirmed by clinical analysis. The existence of an individual susceptibility to disease (human trypanotolerance) should be taken into account even if our genetic conclusions might be distorted because the isolation success rates were particularly poor. In fact, we observed that the isolation success rate varied significantly depending both on the focus of origin (P=0.0002) and on the ethnic group (P=0.0317) of the patient. Further investigations are required in order to study a possible selective impact of the use of the kit for in vitro isolation of trypanosomes as an isolation technique.

Comparative pathogenicity of three genetically distinct Trypanosoma congolense-types in inbred Balb/c mice

Inbred Balb/c mice were infected with three clones of Trypanosoma congolense (Sam.28.1, Dind.3.1 and K60.1A) corresponding, respectively, to the three genetically distinct types (savannah, forest and kilifi) defined within this species, for the purpose of comparing their pathogenicity for a better understanding of the epidemiology of African trypanosomosis. Another clone of savannah type, IL 3000, was also tested simultaneously to study a probable strain variation. Both the clones of savannah type were found of extreme virulence with loss of appetite, rough hair, rapid respiration, lethargy, and all mice died within a week. Parasitaemias evolved rapidly to the first peak by day 3-5 post-inoculation without any remission and the course of disease was correlated positively with the prepatent period. The clones of the forest type and the kilifi type were of low virulence with chronic infection and symptoms progressively less patent throughout the infection; only one mouse died in each experimental group.

Trying to predict and explain the presence of African trypanosomes in tsetse flies

Trypanosome infections identified by polymerase chain reaction on field-caught tsetse flies from various locations were analyzed with respect to factors intrinsic and extrinsic to the trypanosome-tsetse association. These factors were then simultaneously analyzed using artificial neural networks (ANNs) and the important factors were identified to predict and explain the presence of trypanosomes in tsetse. Among 4 trypanosome subgroups (Trypanosoma brucei s.l., T. congolense of the 'savannah' and of the 'riverine-forest' types, and T. simiae), the presence of the 2 types of T. congolense was predictable in more than 80% of cases, suggesting that the model incorporated some of the key variables. These 2 types of T. congolense were significantly associated in tsetse. Among all the examined factors, it was the presence of T. congolense savannah type that best explained the presence of T. congolense riverine forest type. One possible biological mechanism would be 'hitchhiking,' as previously suspected for other parasites. The model could be improved by adding other important variables to the trypanosome tsetse associations.

Application of PCR and DNA probes in the characterisation of trypanosomes in the blood of cattle in farms in Morogoro, Tanzania

Polymerase chain reaction (PCR) and deoxyribonucleic acid (DNA) probes were used to characterise trypanosomes from cattle in Morogoro region of Tanzania. Blood samples collected from 390 beef and dairy cattle in selected farms in Morogoro region were examined for presence of trypanosomes using the buffy coat technique (BCT) and blood smears (BSs). Fifty-two animals were found infected: 40 with Trypanosoma congolense, 10 with T. vivax and two with both T. congolense and T. vivax. DNA extracted from all the parasitologically positive and 62 randomly selected parasitologically negative samples were subjected to PCR amplification using primers specific for different trypanosome species. Using a set of seven specific-pairs of primers on the parasitologically positive samples, we detected only T. congolense, either the Savannah- or the Kilifi-type, as single or mixed infections. With the PCR, trypanosome DNA could be detected in 27 (43%) out of 62 samples that were parasitologically negative. DNA hybridisation using probes specific for Savannah- or Kilifi-types T. congolense, or T. vivax, confirmed the presence of these parasites in cattle kept on some farms in Morogoro region of Tanzania. From these studies, it is clear that there is a need to undertake molecular epidemiological studies to determine the distribution of trypanosome species and subspecies, and to assess the economic impact of these parasites in the productivity of livestock in Tanzania. In particular, it would be desirable to verify the assumed association between the different presentations of trypanosomosis on one hand and genotypes of T. congolense on the other.

Characterization of trypanosome isolates from naturally infected horses on a farm in Kenya

Following an outbreak of trypanosomosis in horses on a farm in Kenya, 18 trypanosome isolates were collected from the infected animals over a period of one and a half years and cryopreserved for characterization. The characterization was done on the basis of morphology using Giemsa-stained blood and buffy coat smears, infectivity to mice, recombinant DNA hybridization, and chromosome separation by orthogonal field alternation gel electrophoresis (OFAGE). Morphologically, all the trypanosome isolates were identified as belonging to the subgenus Nannomonas, and a total of 16 out of the 18 isolates grew in mice. Using the recombinant DNA hybridization technique, the isolates were further classified as the 'savannah' type of Trypanosoma congolense. Furthermore, chromosome separation by OFAGE, carried out on six clones derived from different isolates, exhibited a profile characteristic of T. congolense, 'savannah' type. However, there were differences in the number and positions of the medium-sized and minichromosomes indicating a diversity of serodemes within the isolates. Hence the infecting trypanosomes in this disease outbreak were T. congolense, 'savannah' type, and comprised several serodemes or strains.

Evolutionary genetics of Trypanosoma and Leishmania

We review recent advances in the study of population structure and phylogenetic diversity of parasites belonging to the genera Trypanosoma and Leishmania. In all species properly analyzed, these parasites exhibit a basically clonal population structure, with occasional bouts of genetic exchange or hybridization, and a strong structuration of their populations into discrete evolutionary lineages. On an evolutionary scale, the impact of sex appears to be greater in African than in American trypanosomes. The taxonomic status of some Leishmania 'species' is questionable.

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.

The mini-exon gene of Trypanosoma (Nannomonas) simiae: sequence variation between isolates and a distinguishing molecular marker

The mini-exon gene repeats from two isolates of Trypanosoma (Nannomonas) simiae were amplified by polymerase chain reaction (PCR). The products from each isolate differed in size, however they cross-hybridised in Southern blots. The nature of the size variation was revealed by comparison of the DNA sequence of each repeat: relative to the BAN7 strain, the mini-exon gene of KETRI 2431 contained three apparent deletions that were flanked by short (> or = 6-bp) direct repeats. Furthermore, one of the cloned repeats was used as a hybridisation probe against DNA from other closely-related African trypanosomes. The lack of hybridisation of the T. (N.) simiae mini-exon gene to genomic DNA from the Forest, Kilifi and Savannah subgroups of T. (N.) congolense and T. (N.) godfreyi indicate that this PCR-hybridisation assay may be useful for distinguishing T. (N.) simiae from other members of the subgenus Nannomonas.

Trypanosome infections and survival in tsetse

The effect of trypanosome infection on vector survival was observed in a line of Glossina morsitans selected for susceptibility to trypanosome infection. The differential effects of midgut and salivary gland infections on survival were examined by exposing flies to infection with either Trypanosoma congolense which colonizes midgut and mouthparts or Trypanosoma brucei rhodesiense which colonizes midgut and salivary glands. A comparison of the survival distributions of uninfected flies with those exposed to infection showed that salivary gland infection significantly reduces tsetse survival; midgut infection had little or no effect on the survival of tsetse. The significance of these findings is discussed in relation to the vectorial capacity of wild flies.

Identification of trypanosomes isolated by KIVI from wild mammals in Côte d'Ivoire: diagnostic, taxonomic and epidemiological considerations

In Côte d'Ivoire, a comparative study was carried out on 122 wild mammals by parasitological and serological examination and by in vitro isolation of trypanosomes from fresh blood (KIVI). Thirteen isolated stocks were studied by isoenzymes and compared with Trypanosoma congolense and T. brucei bouaflé group reference stocks. Of the 122 animals, only 22 were positive on blood smears while 88 were KIVI positive and 92 were CATT/T. b. gambiense positive. For six stocks identified by isoenzymes as T. congolense, the agreement between ELISA and CATT was good (75%). As compared with CATT, antigen detection ELISA was not satisfactory for T. brucei (20%). Out of 18, 16 stocks represented a separate zymodeme (seven T. congolense and nine T. brucei) and a high genetic heterogeneity was observed. For T. congolense, savanna, kilifi and forest groups were represented by one zymodeme each. The four remaining zymodemes while put into this T. congolense group, were strongly independent of each other. Morphology indicated that those new zymodemes correspond to T. congolense. In the other hand, five new zymodemes fit into T. brucei classification.

Molecular characterization of trypanosome isolates from naturally infected domestic animals in Burkina, Faso

A total of 33 trypanosome cryostabilates isolated from domestic animals (bovine and dogs) were analysed using the polymerase chain reaction (PCR). The PCR was undertaken on diluted and treated buffy coat solutions according to an easy protocol of purification, using primers specific to Trypanosoma (Nannomonas) congolense of Savannah, Riverine-Forest, Kilifi and Tsavo types, T. (N) simiae, T. (Trypanozoon) brucei and T. (Duttonella) vivax. The results showed a lack of PCR sensitivity when target solutions were simply diluted, probably a reflection of the inaccuracy of the dilution procedure at very low trypanosome numbers. Nine mixed infections were found in purified samples whereas only three were detected in diluted crude solutions. T. congolense Savannah-type was present in all stabilates. Double infections involving this type with the Riverine-Forest type, T. vivax or T. brucei, were found. One stabilate was found to be infected with the three trypanosome types, namely T. congolense Savannah and Riverine-Forest genotypes and T. vivax. No infection attributable to T. congolense Kilifi and Tsavo types or T. simiae was detected in these stabilates. This work confirmed the abundance of mixed infections in the field, which could not have been detected by the classical parasitological methods. Amongst the T. congolense infections, the Savannah genotype was found to be predominant over the Riverine-Forest type; that could be a consequence of differences in genotype virulence in cattle. The detection of T. congolense Riverine-Forest type in vertebrate hosts living in wet areas could be confirmation of the suspected affinity of relationships between this taxa and the riverine forest tsetse fly species.

Comparison of the susceptibility of different Glossina species to simple and mixed infections with Trypanosoma (Nannomonas) congolense savannah and riverine forest types

Teneral Glossina morsitans mositans, G.m.submorsitans, G.palpalis gambiensis and G.tachinoides were allowed to feed on rabbits infected with Trypanosoma congolense savannah type or on mice infected with T.congolense riverine-forest type. The four tsetse species and subspecies were also infected simultaneously in vitro on the blood of mice infected with the two clones of T.congolense via a silicone membrane. The infected tsetse were maintained on rabbits and from the day 25 after the infective feed, the surviving tsetse were dissected in order to determine the infection rates. Results showed higher mature infection rates in morsitans-group tsetse flies than in palpalis-group tsetse flies when infected with the savannah type of T.congolense. In contrast, infection rates with the riverine-forest type of T.congolense were lower, and fewer flies showed full development cycle. The intrinsec vectorial capacity of G.m.submorsitans for the two T.congolense types was the highest, whereas the intrinsic vectorial capacity of G.p.gambiensis for the Savannah type and G.m.morsitans for the riverine-forest type were the lowest. Among all tsetse which were infected simultaneously with the two types of T.congolense, the polymerase chain reaction detected only five flies which had both trypanosome taxa in the midgut and the proboscis. All the other infections were attributable to the savannah type. The differences in the gut of different Glossina species and subspecies allowing these two sub-groups of T.congolense to survive better and undergo the complete developmental cycle more readily in some species than other are discussed.

A repetitive DNA sequence specific for Trypanosoma (Nannomonas) godfreyi

The satellite DNA sequence of Trypanosoma (Nannomonas) godfreyi, a recently described parasite of Suidae, was determined. The sequence is 373bp in length, and contains two imperfect internal repeats of approximately 170bp. Like other trypanosome satellite DNAs, it has no extensive open reading frames and is probably non-coding. There is no significant homology with other major repetitive DNAs within subgenus Nannomonas. We have developed a PCR test that is specific for T. godfreyi and used it to identify the parasite in natural tsetse infections from Zimbabwe and Côte d'Ivoire. This test shows no cross reaction with non-target trypanosomes, even within subgenus Nannomonas, and will be invaluable in studies of the prevalence and distribution of T. godfreyi.

Distribution and abundance of trypanosome (subgenus Nannomonas) infections of the tsetse fly Glossina pallidipes in southern Africa

Over 10 000 Glossina pallidipes tsetse flies were collected during two field studies in the Zambezi Valley, Zimbabwe and one in the Luangwa Valley, Zambia. These were screened for mature trypanosome infections and 234 dot-blot preparations were made of infected midguts, which were screened using DNA probes or PCR with primers specific to different species or types of the trypanosome subgenus Nannomonas. Over 70% of midgut infections were successfully identified as either Trypanosoma godfreyi, T. simiae or three types of T. congolense, savannah, riverine-forest and Kilifi. The relative abundance of species and types did not vary significantly between study locations, habitat, season or tsetse age or sex, although there were differences between DNA probe and PCR results. Mixed species and/or mixed type infections were common and were more often detected using PCR. The distribution of infections among flies was highly aggregated, but there was no tendency for multiple infections to accumulate in older flies, implying that sequential superinfection may be uncommon. Possible explanations for these patterns are discussed.

Sleeping sickness in Zaire: a nested polymerase chain reaction improves the identification of Trypanosoma (Trypanozoon) brucei gambiense by specific kinetoplast DNA probes

Blood samples collected in the sleeping sickness focus of Boma, Zaire, from human patients and domestic animals were analysed by polymerase chain reaction (PCR) for the presence of trypanosome DNA. The comparison of PCR and miniature anion exchange centrifugation technique (m-AECT) results clearly shows that in domestic animals mixed infections (Trypanozoon/Trypanosoma [Nannomonas] congolense) are more frequently diagnosed by PCR than by m-AECT. Trypanozoon positive blood samples were further analysed for Trypanosoma (Trypanozoon) brucei gambiense. For that purpose amplified minicircle kinetoplast DNA (minicircle kDNA) was differentiated in gambiense and non-gambiense by hybridization with DNA probes. To analyse blood samples, especially those with low parasite numbers, the amplification step had to be improved by a nested PCR. Subsequent hybridization was done with kDNA probes generated by PCR from blood samples which had been obtained from a human patient infected with T.(T.) b. gambiense and a pig infected with Trypanozoon. The hybridization results clearly show that at least two genotypes of Trypanozoon parasites occur in the sleeping sickness focus of Boma, Baz-Zaire. One obviously corresponds to T.(T.) b. gambiense and was present in humans and two domestic animals (pig, dog). The other genotype seems to be associated with T.(T.) b. brucei and could be detected only in the blood of domestic animals. This is the first time that field samples could be analysed by a technique which facilitates the molecular identification of T.(T.) b. gambiense without prior cloning, propagation, and/or isolation of the parasites. Therefore, this technique seems to be a promising tool to elucidate the significance of the animal reservoir for the epidemiology of the gambiense sleeping sickness in Africa.

Production and characterization of a monoclonal antibody specific for Trypanosoma simiae

Monoclonal antibodies (MoAb) were produced against invariant antigens of vector forms of Trypanosoma simiae. X63/AG8.653, NSI/1AG401 and Sp20Ag14 myeloma cells were fused with splenic lymphocytes from BALB/c mice that had been immunized with various preparations of T. simiae procyclics. A T. simiae-specific MoAb [KNS7/14.X (IgG1)] was detected in the hybridoma culture supernatants, which were screened for antibody activity by micro-plate and dot ELISA. Immunofluorescence studies showed that KNS7/14.X stained cytoplasmic antigens in T. simiae procyclics. Proteinase-K digestion and periodate oxidation studies revealed that KNS7/14.X binds to a carbohydrate antigenic determinant in glycoprotein or glycolipid. Cross-reactivity studies using vector forms of T. brucei, T. vivax, T. congolense, T. simiae and T. grayi showed that KNS7/14.X only reacted with T. simiae. Attempts to generate other T. simiae-specific MoAb, using 107-, 75- or 41.7-43.6-kDa peptides selected by western blotting analysis, did not yield positive results.

Molecular characterization of trypanosome species and subgroups within subgenus Nannomonas

Restriction fragment length polymorphism (RFLP) analysis of both genomic and kinetoplast DNA from representative stocks from 3 Trypanosoma congolense subgroups (Savannah, Forest, and Kilifi), T. simiae and T. godfreyi, was used to investigate the relatedness of the different groups within subgenus Nannomonas. DNA probes for beta-tubulin and the ribosomal DNA (rDNA) locus were isolated from a T. congolense Savannah genomic library; additional probes were generated by PCR amplification of mini-exon and glutamate and alanine rich protein (GARP) gene sequences. Our results provide evidence that at the molecular level the T. congolense Savannah and Forest groups are the most closely related groups within the subgenus Nannomonas: the Savannah and the Forest groups had mini-exon gene repeats of identical size, which shared homology, had mini-circles of the same size and had a high level of similarity (63%) when the banding patterns produced with a tubulin and rDNA probe were subjected to numerical analysis. All other pairwise combinations of groups have very low percentage similarities of < 10%, suggesting that the Kilifi group trypanosomes, are as distantly related to the T. congolense Savannah and Forest groups as they are to T. simiae or T. godfreyi. The conservation of the GARP gene between the Savannah, Forest and Kilifi groups provides the only evidence linking the Kilifi trypanosomes to the other groups in T. congolense. We find no evidence for the presence of the GARP gene in the T. simiae or T. godfreyi group trypanosomes.

The kinetics of maturation of trypanosome infections in tsetse

Estimates of the time delay between the infective bloodmeal and maturation (incubation or maturation time) for 4 trypanosome stocks (2 Trypanozoon and 2 Trypanosoma congolense) show that maturation time in tsetse is not a parasite species-specific constant. The mean incubation time of a Trypanosoma brucei rhodesiense stock (EATRO 2340 - 18 days) was not significantly different from one T. congolense stock (SIKUDA88 - 15.5 days) but was significantly greater than another (1/148 FLY9 - 12.5 days). There was no significant difference in incubation times between male and female Glossina morsitans morsitans for any of the stocks but in both of the Trypanozoon stocks the proportion of female flies producing mature infections was significantly less than in males. However, estimates of gene frequency, assuming a model in which maturation is controlled by an X-linked recessive allele, gave inconsistent results indicating that maturation cannot be controlled by a single sex-linked gene. Maturation was shown to be a tsetse sex-dependent phenomenon in Trypanozoon but not in T. congolense infections. Incubation time was quite variable even for a single trypanosome stock (e.g., standard deviation of 5 days for one Trypanozoon stock); we discuss how this variability can affect disease transmission, and the interpretation of age-prevalence data.

Multiple trypanosome infections in wild tsetse in Côte d'Ivoire detected by PCR analysis and DNA probes

Trypanosomes were isolated from the midguts of Glossina palpalis palpalis, G. pallicera pallicera and G. nigrofusca nigrofusca captured around the village of Guediboua, South West of Daloa in Côte d'Ivoire. Seventy of the 124 isolates, obtained from 688 flies, were examined for four different kinds of trypanosome using the Polymerase Chain Reaction (PCR). Prevalences were: Trypanozoon 46%, riverine-forest T. congolense 86% and savannah T. congolense 54%. Only 29 samples were examined for T. simiae but it was not detected. Just 30% of the infections involved a single kind of trypanosome; the remainder were mixtures either of two (37%) or all three (27%) of the target organisms. 30 of the 70 isolates examined by PCR were successfully amplified to provide material for DNA probe hybridization. To a large extent, DNA probes confirmed the PCR results; all (28/28) of the riverine-forest and 82% (18/22) of the savannah T. congolense infections were identified. However, only 8% (1/13) of the PCR positives for Trypanozoon hybridized with the appropriate DNA probe. No T. simiae or T. godfreyi infections were identified using DNA probes but a large proportion (97%) (29/30) of the probed midguts were shown to contain Kilifi T. congolense. Four isolates out of 70 could not be identified by any method. There was no obvious association between the different species of flies and the infecting trypanosomes.