Flow cytometric analysis of the binucleated protozoan parasite Giardia intestinalis gave DNA histograms with a broad Gl peak and a definable G2 + M peak with twice the DNA content of Gl. Twenty-four hour treatment with metronidazole arrested cell cycle progression of susceptible trophozoites in the G2 + M phase, but had no effect, even at toxic doses, on the DNA histogram of a line selected for resistance to metronidazole. Furazolidone was inhibitory to both stocks, causing an arrest in the S and G2 + M phases. Inhibitors of the mammalian cell cycle were also tested. Hydroxyurea, which blocks mammalian cells in Gl/S, and razoxane, which blocks in G2 + M, arrested trophozoites in the G2 + M phase whereas colchicine and gamma-irradiation had little or no effect on the cell cycle of G. intestinalis. These results suggest that the cell cycle of G. intestinalis may be controlled in a different manner from mammalian cells.

The prevalence of schistosome infections in intermediate host snails varies with snail age. The relationship between age and prevalence, the age—prevalence curve, is complex and may vary in space and time, and among parasite—host species. Field studies show that the shape of the age—prevalence curve may be seasonally variable, and that at some times there may be a decline in prevalence among older snails. This paper attempts to explain these observations in terms of the underlying epidemiological processes. A discrete-time version of Muench's catalytic model for age-dependent infection is developed. Model simulations were carried out using life-history and epidemiological parameters derived from studies of Schistosoma haematobium—Bulinus globusus in Zimbabwe. Analysis of model behaviour identifies aspects of the schistosome— snail interaction that affect the shape of the age—prevalence curve. The following features can result in a decline in prevalence among older snails. (1) A decrease in the survival rate of patent infected snails with age. (2) A decrease in the force of infection with age. (3) A high rate of loss of infection. (4) A heterogeneity in the snail population such that the probability of infection is correlated with snail fecundity. (This would occur if there existed a spatial correlation between force of infection and fecundity, or if there were a correlation between fecundity and susceptibility.) The evidence for the occurrence of these features in the field is assessed. Survival rate is related more closely to the duration of patent infection than to age per se. The evidence for age-dependent force of infection is equivocal. Significant loss-of-infection rates have yet to be demonstrated. Heterogeneities in force of infection and fecundity have been reported and, for the Zimbabwe data, this mechanism can explain seasonally in the age—prevalence curve as a function of known seasonal variation in the force of infection and snail fecundity.

Effects of lithium chloride on growth, differentiation and respiration of Herpetomonas samuelpessoai, cultivated in a synthetic medium were studied both at 28 and 37 °C. Low concentration of lithium chloride (15 mM) stimulated growth at 37 °C. In addition, the protozoon tolerated high concentrations (60–150 mM) of the salt at both incubation temperatures. In general, 15 mM lithium chloride increased and 150 mM decreased oxygen uptake when glucose, glutamic acid and proline were used as substrates. However, at 28 °C after incubation for 96 h, the highest concentration increased oxygen uptake in the presence of glucose. Sodium butyrate induced cell differentiation in H. samuelpessoai both at 28 and 37 °C. High concentration (150 mM) of lithium chloride inhibited cell differentiation of H. samuelpessoai induced by both controlled growth conditions and butyrate addition. The results obtained are described in this paper.

Genetic exchange is now known to occur during the life-cycle of many parasitic protozoa, including malaria parasites, coccidia and trypanosomes. The process is studied by making deliberate crosses between cloned organisms differing in clearly defined markers. In malaria parasites, crosses have been made between parasites differing in characters such as isoenzymes, antigens and other proteins, drug sensitivity, and chromosome and other DNA polymorphisms. Crosses are made by transmitting a mixture of gametes of each clone through mosquitoes to allow cross-fertilization to take place, and examining the resulting progeny by cloning for organisms exhibiting non-parental combinations of characters. The inheritance of many characters, such as antigen and protein variants, is in accordance with Mendelian expectations for a haploid organism. Recombination occurs at a higher than expected frequency. Studies on chromosomes have show that crossing-over events commonly occur following meiosis of hybrid zygotes. Repetitive DNA and subtelomeric regions of chromosomes appear to be particularly susceptible to such recombination events. In trypanosomes, crosses between clones of Trypanosoma brucei have shown that hybrids are formed during tsetse fly transmission. The organism appears to be mainly diploid, but some characters including certain chromosomes seem to be inherited in a non-Mendelian manner.

A two-colour flow cytometry technique was used to assess killing of Giardia muris trophozoites by rabbit anti-trophozoite antibodies and complement. Binding of rabbit antibody to trophozoites and killing of trophozoites were documented by flow cytometry, after incubating the organisms with fluorescein-conjugated anti-rabbit IgG and propidium iodide (PI). Percentages of PI+ (dead) trophozoites ranged from >80%, after incubation with rabbit antiserum and complement, to <30%, after incubation with complement alone. The assay technique may be applicable to studies aimed at determining whether intestinal antibodies from Giardia-infected mammals can kill Giardia trophozoites.

The nuclear DNA of Trypanosoma congolense contains a family of highly conserved 369 base pair (bp) repeats. The sequences of three cloned copies of these repeats were determined. An unrelated family of 177 bp repeats has previously been shown to occur in the nuclear DNA of Trypanosoma brucei brucei (Sloof et al. 1983a). Oligonucleotides were synthesized which prime the specific amplification of each of these repetitive DNAs by the polymerase chain reaction (PCR). Amplification of 10% of the DNA in a single parasite of T. congolense or T. brucei spp. produced sufficient amplified product to be visible as a band in an agarose gel stained with ethidium bromide. This level of detection, which does not depend on the use of radioactivity, is about 100 times more sensitive than previous detection methods based on radioactive DNA probes. The oligonucleotides did not prime the amplification of DNA sequences in other trypanosome species nor in Leishmania, mouse or human DNAs. Amplification of DNA from the blood of animals infected with T. congolense and/or T. brucei spp. permitted the identification of parasite levels far below that detectable by microscopic inspection. Since PCR amplification can be conducted on a large number of samples simultaneously, it is ideally suited for large-scale studies on the prevalence of African trypanosomes in both mammalian blood and insect vectors.

The paper examines non-linear dynamical phenomena in host—parasite interactions by reference to a series of different problems ranging from the impact on transmission of control measures based on vaccination and chemotherapy, to the effects of immunological responses targeted at different stages in a parasite's life-cycle. Throughout, simple mathematical models are employed to aid in interpretation. Analyses reveal that the influence of a defined control measure on the prevalence or intensity of infection, whether vaccination or drug treatment, is non-linearly related to the magnitude of control effort (as defined by the proportion of individuals vaccinated or treated with a drug). Consideration of the relative merits of gametocyte and sporozoite vaccines against malarial parasites suggests that very high levels of cohort immunization will be required to block transmission in endemic areas, with the former type of vaccine being more effective in reducing transmission for a defined level of coverage and the latter being better with respect to a reduction in morbidity. The inclusion of genetic elements in analyses of the transmission of helminth parasites reveals complex non-linear patterns of change in the abundance of different parasite genotypes under selection pressures imposed by either the host immunological defences or the application of chemotherapeutic agents. When resistance genes are present in parasite populations, the degree to which abundance can be suppressed by chemotherapy depends critically on the frequency and intensity of application, with intermediate values of the former being optimal. A more detailed consideration of the impact of immunological defences on parasite population growth within an individual host, by reference to the erythrocytic cycle of malaria, suggests that the effectiveness of a given immunological response is inversely related to the life-expectancy of the target stage in the parasite's developmental cycle.

A Babesia divergens merozoite antigen was purified by affinity chromatography using a monoclonal antibody. Silver staining of SDS-PAGE gels revealed 2 bands of Mr 50–60 kDa and Mr 24–29. It is proposed that the Mr 24–29 kDa band represents the native protein and that it is complexed, perhaps as a dimer or perhaps to host protein to give the Mr 50–60 kDa band. Peptide analysis of the antigen followed by Western blotting with the monoclonal antibody revealed only an Mr 20–24 kDa band. After affinity purification the antigen was precipitated with acetone and inoculated subcutaneously, together with complete Freund's adjuvant, into gerbils. Two inoculations were given 3 weeks apart, after which the immunized animals and a group of control animals were challenged with 5 × parasitized red cells from a virulent microaerophilus stationary phase (MASP) culture of an homologous strain of B. divergens. Parasitaemias rose rapidly in control animals and all were dead by the fifth day. Two of the vaccinated animals survived the challenge. Parasitaemias in the remaining two rose slowly but both animals succumbed on the seventh day. Western blotting of merozoites and of the purified antigen, using sera from the surviving animals showed strong reactivity with the Mr 24–29 kDa band.

Different stocks and stabilates within a stock of Theileriaparva were analysed for genotypic differences and for their effect on the expression of host cell surface antigens following infection of BoT8+ T lymphocyte clones. The parasites were characterized in vitro by hybridization of T. parva- specific DNA probes to Southern blots of endonuclease-digested DNA from the infected T cell clones. Phenotypic changes in the host lymphoblastoid cells before and after infection were examined using lineage-specific monoclonal antibodies which reacted with the differentiation antigens BoT2, BoT4, Bo6, BoT8 and a null cell marker on bovine T cells. Expression of Class I and Class II major histocompatibility complex (MHC) antigens on the cell populations was also assessed. Results of this study indicate that genotypically different parasites exist among and within T. parva stabilates and that the expression of Bo6, BoT8 and the null cell marker was differentially altered by infection with parasites from different stocks or from different stabilates of the same stock. Expression of Class II antigens was significantly increased after infection. Moreover, clones that were derived from the same cell line but had genotypically distinct T. parva parasites, also showed differences in expression of Bo6 and BoT8 and the null cell marker.

Rates at which Trypanosoma brucei rhodesiense trypanosomes switch from expression of one variable antigen type (VAT) to that of another have been determined in cloned populations that have been recently tsetse-fly transmitted. Switching rates have been determined between several, specific pairs of VATs in each population. High rates of switching were observed in 2 cloned trypanosome lines, each derived from a separate cyclical transmission of the same parental stock and each expressing a different major VAT. Five estimates of the switching rate between one particular pair of VATs were consistently high (approximately 1 × 103 switches/cell/generation). These high switching rates were similar both in bloodstream populations of mice and in populations confined to subcutaneously implanted growth chambers in mice, thus indicating that the interaction of the bloodstream population with other trypanosome populations in the lymphatics or extravascular sites in systemic infections did not influence the estimates of the rate of switching. Fourteen estimates were made of VAT-specific switching rates in bloodstream infections involving 8 combinations from among 6 VATs. Switching rate estimates were VAT-specific and showed considerable variation between different combinations of VATs — from 1.9 × 10−6 to 6.9 × 10−3 switches/cel/generation. The rates of switching to different metacyclic-VATs were, however, very similar. Summation of between 3 and 5 VAT-specific switching rate values in each of 4 experiments conducted in bloodstream infections has provided minimum estimates of the overall rate of antigenic variation: 2.0−9.3 × 10−3 switches/cell/generation. These values are between 20 and 66000-fold higher than previously published estimates. It is likely that at least 1 in every 100 trypanosomes switches its VAT expression every generation.

During the last decade there has been a great increase in the number of countries that have endorsed the primary health care (PHC) policy at national level, set up national guidelines for it and launched its large-scale implementation. In addition, there have been many important developments with regard to appropriate, cost-effective technologies, training concepts and approaches to securing community participation. These achievements have produced numerous encouraging results. However, although the control of parasitic infections integrated into PHC systems has often been initially successful, these achievements could often not be sustained. Using case studies, mainly concerning schistosomiasis, as examples, control technologies and their applicability within PHC are discussed at three levels; the identification of public health priorities, the community-based implementation of control and the process of evaluation and monitoring. There is a great potential for the integration of a substantial part of control activities, particularly morbidity control, into PHC, provided that the aims and sequences of control activities are well matched with the felt needs of the communities concerned. This implies that the biomedical researcher, the epidemiologist and the health planner need to consider the indigenous health perspectives of the affected community. For example, recent progress in the laboratory in the development of vaccines against parasites needs to be complemented by field studies that continuously validate, standardize and assess the applicability of the proposed measures. This kind of interplay will form the basis for participatory approaches in health planning and make it possible for control activities to be integrated into existing PHC structures and to respond to the needs of the communities concerned.

In an operational Schistosoma mansoni field-study in an area of about 20 km (population approximately 8000), transmission detection by simple snail sampling was compared with cercariometry. Between 1985 and 1987, 62 field sites were sampled at fortnightly intervals. Of a total of 2758 field observations, 89.8% gave full snail data; 64.4% full cercarial data; and 61.7% complete data for both methods. The complete data sets showed significant but not strong correlations between Biomphalaria pfeifferi (total and infected with S. mansoni or other trematodes) and cercarial (S. mansoni and non-human) recoveries. Non-human (but not S. mansoni) cercarial recovery decreased with deteriorating cercariometry filter quality. Both snail and cercarial recoveries diminished significantly with increasing water flows at the time of collection. Many samples yielded infected snails or cercariae, but not both, and neither method detected significantly more transmission sites. The method of choice for detecting transmission in a large-scale field-study depends on logistical and financial considerations. Relatively simple snail sampling allows quick, cheap and widespread data collection adequate for most purposes but more complicated cercariometry is still valuable for specific, small-scale studies.

Proterometra macrostoma and P. edneyi infect the same snail host, Goniobasis semicarinata, but different fish hosts in their life-cycles. Cercariae of P. macrostoma complete development in sunfish, those of P. edneyi in darters; fish become infected when they ingest free-swimming cercariae as ‘prey’. Laboratory and field experiments were designed to test the hypothesis that light: dark (L: D) cycling regulates emergence of both species. Under L: D cycling conditions, P. macrostoma emerged in the dark and P. edneyi in the light. These emergence patterns resulted from differential sensitivity to light and dark. In the laboratory and field, reversing the light and dark periods resulted in corresponding alterations in emergence patterns of both species. Both species emerged in constant light and constant dark, but their emergence patterns were altered. Emergence patterns may represent adaptations that make the cercariae more susceptible to ‘predation’ by their respective fish hosts.

Throughout evolution, enzymes and their metabolites have been highly conserved. Parasites are no exception to this and differ most markedly by the absence of metabolic pathways that are present in the mammalian host. In general, parasites are metabolically lazy and rely on the metabolism of the host both for a supply of prefabricated components such as purines, fatty acids, sterols and amino acids and for the removal of end-products. Nonetheless, parasites are metabolically highly sophisticated in that (1) they retain the genetic capacity to induce many pathways, when needed, and (2) they have developed complex mechanisms for their survival in the host. Certain unique features of the metabolism of trypanosomes, leishmania, malaria and anaerobic protozoa will be discussed. This will include (1) glycolysis and electron transport with reference to the unique organelles: the glycosome and the hydrogenosome, (2) purine salvage, pyrimidine biosynthesis and folic acid metabolism and (3) polyamine and thiol metabolism with special reference to the role of the unique metabolite of trypanosomes and leishmanias, trypanothione.

Naive CBA/Ca mice and CBA/Ca mice infected 12 weeks previously with 20 normal cercariae of Schistosoma mansoni were challenged percutaneously with isotopically labelled parasites. Challenge worm migration was followed, in 5 separate experiments, by means of compressed organ autoradiography. In three experiments, 43.1 % of challenge parasites did not arrive in the lungs of infected mice on day 6 as compared to 7.7 % in naive controls, thereby indicating that pre-lung sites constitute the first barrier in resistance to reinfection. A further 15% of the challenge worm burden was lost in the lungs or en route to the liver in the immune animals, and portal perfusion revealed that 25.4% of the challenge was lost in the liver. Two other experiments revealed no comparable phases of pre-liver attrition however; instead resistance was only evident at final perfusion on days 28 or 35. These data reveal the variable generation of specific acquired immunity in mice harbouring a chronic schistosome infection and thus clarify current discrepancies in the literature. The results are discussed in relation to documented evidence for the nature of specific and non-specific immune mechanisms reported to operate at different sites in the infection model of schistosomiasis mansoni.

Steinernematid nematodes form a developmentally arrested infective juvenile (IJ) stage at the second moult, when conditions inside the insect host are no longer suitable for further reproduction. In a liquid culture micro-assay two environmental cues were shown to influence the formation of Steinernema feltiae IJs. High nematode population density induced IJ formation, suggesting the presence of a nematode pheromone. Bacterial food and soluble nutrients acted competitively to reduce the frequency of IJ formation. Frequency of IJ formation was greatest when lst-stage juveniles were subjected to IJ-inducing conditions. The optimum temperature range for the IJ induction response was 25–30°C. These findings suggest that maximal IJ production in large-scale liquid culture will depend on the correct balance of nematode population density and nutrient availability at the peak of egg hatching.

129/Ola mice resemble WEHI 129J mice in that around 70% of the individuals in any given population resist a primary infection with Schistosoma mansoni. Squashed-organ autoradiographic tracking of [Se]selenomethionine-labelled parasites has shown that the kinetics of worm migration in 129/Ola mice follows the expected pattern, and that all rodents harbour essentially similar numbers of worms on day 14 post-infection. Combined lung and liver worm recovery techniques have revealed, however, that segregation of mice into ‘permissive’ and ‘non-permissive’ individuals can first be detected on day 20. ‘Non-permissive’ mice are characterized by the absence of schistosome eggs at all times in the liver parenchyma and, in consequence, lack the attendant manifestations of pathology; they do, however, harbour a few stunted worms in the liver and significant numbers of adult schistosomes in the pulmonary vasculature. Histological analysis of sectioned lung tissue from such animals indicates that some lung-located schistosomes feed, pair and lay eggs. Nevertheless, eosinophil-enriched inflammatory reactions develop around such worms and the parasites themselves exhibit various manifestations of trauma, ranging from minor vacuolation to gut herniation and extrusion. The phenomenon of ‘non-permissiveness’ thus involves retardation of worm development in the liver and, in consequence, relocation of the parasites to the lungs, where they become subject to host effector responses.

Hybridization between Schistosoma mansoni, with a diurnal cercarial emergence rhythm and S. rodhaini, with a nocturnal cercarial shedding pattern leads to F1 and F2 generations, hybrid schistosomes whose chronobiological phenotype of cercariae is characterized by two unequal emergence peaks, one diurnal and the other nocturnal. The relative importance of diurnal and nocturnal peaks depends upon which S. mansoni strain (early or late) is used for the hybridization with S. rodhaini. The results are compared and discussed with those resulting from crosses between intraspecific sympatric and allopatric chronobiological variants (early and late) of S. mansoni. The genetic determinism of the cercarial emergence of schistosomes and the significant differences observed between cercarial shedding patterns of parental species and their hybrids allow the use of this behavioural marker in biological and genetical studies in schistosome populations.

A comparison was made of the development, survival and reproduction of 5 isolates of Trichinella spiralis in inbred mice. Low responder C57BL/10 mice allowed worms of all isolates to survive longer and reproduce more successfully than did high responder NIH mice, suggesting that host immunity exerted a dominant influence upon infectivity. One isolate (Is-5 (W) — an arctic isolate) had a markedly lower infectivity than all other isolates, and was selected for more detailed study, together with isolate Is-1 (S) (a temperate isolate) which showed high infectivity. The lower infectivity of Is-5 (W) reflected a more rapid onset of immunity in mice infected with this parasite, immunity reducing the reproductive potential of female worms and causing an early expulsion from the intestine. No evidence of a dose-dependent suppression of immunity was found to explain the higher infectivity of Is-1 (S). Is-5 (W) provided a very rapid mucosal mastocytosis in infected mice, whereas Is-1 (S) caused no measurable response. In contrast, Is-1 (S) elicited higher levels of circulating parasite-specific antibodies than did Is-5 (W). These results are discussed in relation to the interplay of parasite immunogenicity and host immune responsiveness in determining infectivity, and point to the importance of identifying those immunodominant parasite molecules which control the balance of the host—parasite relationship.

The complex life-cycles of parasitic animals are a product of exploiting the process of development to generate organisms with different biological potentials within a species. Successive stages of the parasite adapt to functions such as host invasion and transmission, and to the colonization of a variety of niches, often involving different hosts, tissues or cells. Understanding the molecular basis of adaptive biology among parasities is a major challenge that lies at the heart of research in contemporary parasitology. Differences in scale at the levels of genomic complexity and cell biology exist between most parasitic protozoa and helminths, rendering a cautionary note to making generalized observations. The two principal approaches used to gain insights into adaptive specializations are to start with a biological activity and identify the molecule, or select molecules with particular properties and deduce function. Both have their part to play, with their inherent advantages and limitations, and both are illustrated with examples of studies on adaptive biology among parasitic nematodes.