The chromosome profiles of Trypanosoma congolense isolates from Kilifi, Kenya and their relationship to serodeme identity

Models of superinfection and acquired immunity to multiple parasite strains

Individuals in communities in which different strains of pathogen are circulating can acquire resistance by accumulating immunity to each strain. After considering susceptibility, models of infection and immunity are defined for vector-borne diseases such as malaria and trypanosomiasis. For these models the prevalence of infection, the number of infections per individual, and the mean duration of infection, increase rapidly in young individuals, but decrease in older individuals as immunity is acquired to the various strains of pathogen; the mean interval between successive infections lengthens with age. The bivariate Poisson distribution is shown to be a close approximation to some stochastic processes. The models explain observed cross-sectional patterns of age prevalence, and longitudinal patterns in which individuals typically continue to become infected as they age, albeit with decreasing frequency. In these models the time spent infected depends on parasite diversity, as well as the inoculation and recovery rates. It is shown that control measures can cause an increase in the number of infections and the prevalence of infection in older individuals, and in the average prevalence in the community, even when strain-specific immunity is life-long.

Models of superinfection and acquired immunity to multiple parasite strains

Individuals in communities in which different strains of pathogen are circulating can acquire resistance by accumulating immunity to each strain. After considering susceptibility, models of infection and immunity are defined for vector-borne diseases such as malaria and trypanosomiasis. For these models the prevalence of infection, the number of infections per individual, and the mean duration of infection, increase rapidly in young individuals, but decrease in older individuals as immunity is acquired to the various strains of pathogen; the mean interval between successive infections lengthens with age. The bivariate Poisson distribution is shown to be a close approximation to some stochastic processes. The models explain observed cross-sectional patterns of age prevalence, and longitudinal patterns in which individuals typically continue to become infected as they age, albeit with decreasing frequency. In these models the time spent infected depends on parasite diversity, as well as the inoculation and recovery rates. It is shown that control measures can cause an increase in the number of infections and the prevalence of infection in older individuals, and in the average prevalence in the community, even when strain-specific immunity is life-long.

Chromosome size polymorphisms in the genus acanthamoeba electrokaryotype by pulsed-field gel electrophoresis

Twenty-eight strains from 12 species from the genus Acanthamoeba, including five isolates from amoebic keratitis patients, were subjected to molecular karyotyping by pulsed-field gel electrophoresis. 9 to 21 chromosome-sized DNA bands ranging from 200 kb to 3 Mb in size were detected. Molecular karyotypes also showed a wide multifariousness, i.e. there existed inter- and intraspecific heterogeneity. The five isolates from amoebic keratitis patients did not exhibit characteristic molecular karyotypes distinguishable from environmental isolates. Although karyotypic heterogeneity was observed within group I amoeba, they are distinguishable from those of group II and III. Strains having identical restriction fragment length polymorphism profiles of mtDNA did not have an identical molecular karyotype, i.e. weak correlation was found between molecular karyotypes and mtDNA restriction fragment length polymorphism profiles.

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.

Long-term occurrence of Trypanosoma congolense resistant to diminazene, isometamidium and homidium in cattle at Ghibe, Ethiopia

Ten trypanosome isolates were collected at random from cattle at Ghibe, Ethiopia, in February 1993 and all shown to be savannah-type Trypanosoma congolense. When inoculated into naïve Boran (Bos indicus) calves, all 10 isolates were resistant to diminazene aceturate (Berenil), isometamidium chloride (Samorin) and homidium chloride (Novidium) at doses of 7.0 mg/kg body weight (b.w.), 0.5 mg/kg b.w. and 1.0 mg/kg b.w., respectively. In order to determine whether this multiple-drug resistance was expressed by individual trypanosomes, clones were derived from two of the isolates and characterised in mice for their sensitivity to the three compounds; by comparison to drug-sensitive populations, the two clones expressed high levels of resistance to all 3 trypanocides. In experiments to characterise the uptake kinetics of [14C]-Samorin, the maximal rates of uptake (Vmax) for 4 Ghibe isolates ranged from 9.2 to 15.0 ng/10(8) trypanosomes/min. In contrast, Vmax for the isometamidium-sensitive clone T. congolense IL 1180 was 86.7 +/- 8.6 ng/10(8) trypanosomes/min. Lastly, molecular karyotypes were determined for eight isolates: seven different chromosome profiles were observed. These data indicate that in February 1993 there was a high prevalence of drug-resistant trypanosome populations with different chromosome profiles in cattle at Ghibe. Since a similar situation existed at the same site in July 1989, this suggests that the drug-resistance phenotype of trypanosomes at Ghibe had not altered over a 4 year period.

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.

DNA content and molecular karyotype of trypanosomes of the subgenus Nannomonas

The relative DNA contents of representative stocks of 5 groups within the trypanosome subgenus Nannomonas (Trypanosoma simiae, Godfreyi, T. congolense Savannah, Forest and Kilifi) were measured by flow cytometry. The range of DNA contents formed a continuum. Nevertheless small differences were observed between the groups, with T. simiae/T. congolense Savannah and T. congolense Kilifi/Forest at the lower and higher ends of the range respectively. Analysis of karyotype by pulsed field gel electrophoresis showed all the 5 Nannomonas groups to have minichromosomes and variable numbers of small chromosomes in the 100-700 kb size range. The size and relative number of mini-chromosomes varied from group to group, but no correlation between molecular karyotype and DNA content was observed.

A species-specific antigen of Trypanosoma (Duttonella) vivax detectable in the course of infection is encoded by a differentially expressed tandemly reiterated gene

A monoclonal antibody that is used as a Trypanosoma vivax species-specific diagnostic reagent on antigen-trapping enzyme-linked immunosorbent assay recognized an 8-kDa peptide on western blots. The 8-kDa species-specific antigen was isolated and employed in raising rabbit polyclonal antibodies, which were used in the immunoscreening of a T. vivax cDNA library in lambda gt11.2. A clone containing a 0.8-kb insert was isolated. The cloned gene is tandemly repeated, with a monomeric unit length of 900 bp, in the genomes of all T. vivax isolates from diverse geographic locations in Africa and South America. The gene is differentially expressed, since both the transcript and antigen are present in bloodstream-stage parasites, but not in the epimastigotes of T. vivax. Although the gene is found in all T. vivax isolates so far tested, it either exists in low copy number or in a divergent form in one isolate from Kilifi at the Kenya Coast. Sequence translation revealed a remarkable degree of bias in codon usage with preference for G and C (82%) in the wobble position. Using the deduced amino acid sequence to search the databases for any structurally related peptides, revealed no significant identity with any known proteins. The function of the species-specific antigen of T. vivax is thus unknown. Nevertheless the identification and characterization of proteins released into the circulation of protozoan parasite-infected animals is important and should allow the determination of what role such molecules may play in the modulation of disease pathology.

Electrophoretic karyotyping is a sensitive epidemiological tool for studying Trypanosoma evansi infections

Thirty-six isolates of trypanozoon trypanosomes collected from camels in Northern Kenya during the dry season sporadic infections of 1986 and during the wet season epidemic infections of 1987 were identified as Trypanosoma evansi by the homogeneity of their kinetoplast DNA minicircles. Although the minicircles of all the isolates were indistinguishable, polymorphism in chromosome-sized DNA molecules detected by electrophoresis was extensive. The isolates could be grouped into eight distinct electrophoretic karyotypes which could be distinguished from three additional karyotypes identified among earlier T. evansi isolates. In one camel herd with a long history of trypanocide application, which was continued during the present study, all isolates bar one belonged to one karyotype group. From a second herd, in which trypanosomosis management was by individual treatment of proven parasitaemic cases, isolates with diverse karyotypes were obtained. Some of the karyotypes identified during the dry season sporadic infections were re-isolated in the subsequent wet season epidemic. These observations indicate that distinguishing T. evansi isolates by molecular electrophoretic karyotypes is more discriminating than kDNA analysis. Observations of karyotype patterns recurring in isolates from herds kept under chemoprophylaxis could help in the identification of drug-resistant parasites.

Potential of trypanotolerance as a contribution to sustainable livestock production in tsetse affected Africa

Tsetse transmitted trypanosomiasis is possibly the major constraint on livestock and agriculture development in Subsaharan Africa. Control of the disease has been based on vector control as well as on the use of trypanocidal drugs to treat or prevent infection in animals. Both control methods are effective but have proven not to be sustainable. Moreover, the development of a vaccine against trypanosomiasis is unlikely to be successful in the near future. On the other hand, trypanotolerant cattle, like the N'Dama can survive and produce in tsetse affected areas without interventions. This taurine breed has been indigenous to Africa for approximately 7,000 years and forms presently about 6% of the bovine population of Africa. Generally the N'Dama are kept in the rural areas by the small-scale farmer as a multi-purpose animal. Recent studies have defined management characteristics and assessed the production potential at the village level and under ranching conditions of N'Dama cattle exposed to various levels of tsetse challenge. Furthermore, experimental infections showed conclusively the superior resistance to the effects of infection of the N'Dama cattle when compared to zebu cattle and have confirmed that trypanotolerance is innate in N'Dama cattle. Studies have been conducted on development of protective humoral and cellular responses, the regulation of parasite multiplication and control of anaemia. These studies provided tools for identifying components of trypanotolerance. The ability to resist the development of anaemia in the face of infection, has shown to be correlated with the capacity to be productive; moreover, PCV values can serve as selection criterium for trypanotolerance. Subsequently, repeatabilities and heritabilities of trypanotolerance and performance traits were estimated.(ABSTRACT TRUNCATED AT 250 WORDS)

Epidemiology of bovine trypanosomiasis in the Ghibe valley, southwest Ethiopia. 3. Occurrence of populations of Trypanosoma congolense resistant to diminazene, isometamidium and homidium

In July 1989, blood samples were collected from parasitaemic cattle in the Ghibe valley, Ethiopia, frozen in liquid nitrogen and transported to Nairobi, Kenya. Twelve of the stabilates were inoculated into individual Boran (Bos indicus) calves and characterised for their sensitivity, in turn, to diminazene aceturate (Berenil), isometamidium chloride (Samorin) and homidium chloride (Novidium). All 12 stabilates produced infections which were shown to be Trypanosoma congolense and resistant to treatment with diminazene aceturate at a dose of 7.0 mg kg-1 body weight (b.w.). Eleven of the infections were also resistant to isometamidium chloride at a dose of 0.5 mg kg-1 b.w. and homidium chloride at a dose of 1.0 mg kg-1 b.w. The drug-sensitivity phenotypes of three of the same isolates were also determined in goats which were each treated with only one of the three trypanocides: all expressed the same phenotypes as the populations expressed in the aforementioned Boran calves. Five clones were derived from one of the isolates which expressed a high level of resistance to all three trypanocides; each clone expressed high levels of resistance to all three trypanocides when characterised in mice. Thus, the multi-resistance phenotype of the parental isolate was associated with expression of mutli-resistance by individual trypanosomes. Finally, molecular karyotypes and electrophoretic variants of six enzymes were determined for seven and eight of the isolates, respectively. Six different karyotypes were observed and all eight of the latter isolates belonged to different zymodemes, indicating that the multi-resistance phenotype at Ghibe was associated with many genetically distinct populations.

The molecular epidemiology of parasites

The explosion of new techniques, made available by the rapid advance in molecular biology, has provided a battery of novel approaches and technology which can be applied to more practical issues such as the epidemiology of parasites. In this review, we discuss the ways in which this new field of molecular epidemiology has contributed to and corroborated our existing knowledge of parasite epidemiology. Similar epidemiological questions can be asked about many different types of parasites and, using detailed examples such as the African trypanosomes and the Leishmania parasites, we discuss the techniques and the methodologies that have been or could be employed to solve many of these epidemiological problems.

Elusive trypanosomes

Professor Kershaw's encouragement of the development of anion-exchange separation of African trypanosomes from blood led to two decades of activity when, for the first time, considerable progress was made in the intrinsic characterization of these parasites. Such characterization depended on establishing high infections in laboratory rodents. However, the collection of samples from the field was restricted by the failure of certain trypanosomes either to infect, or to multiply adequately in, rodents. More recently, in vitro culture has come to play an increasingly important role in producing material. By obtaining procyclic forms directly from wild tsetse flies, or by transforming low numbers of bloodstream forms in field samples to the procyclic phase in experimental tsetse, trypanosomes of poor or nil infectivity to rodents were readily cultured in the large amounts required for biochemical characterization. A number of specimens of a new kind of Nannomonas, of Trypanosoma simiae, of T. grayi, and of an antigenically distinct T. brucei gambiense were found. Evidence is presented that many other kinds of trypanosome may be eluding isolation by their inability to infect rodents.