Polyclonal antisera raised against Plasmodium knowlesi reacted with (1) NADP-specific glutamate dehydrogenase (GLDH) of P. knowlesi, (2) GLDH of P. falciparum and (3) GLDH of Proteus spp. The antisera did not react with NAD(P) GLDH from bovine liver. Polyclonal antisera raised against the GLDH of Proteus spp. cross-reacted with GLDH from P. falciparum. Monoclonal antibodies (McAbs) obtained from mice immunized with Proteus GLDH were either specific for the bacterial enzyme or cross-reacted with P. falciparum GLDH. The selected McAbs did not react with GLDI-1 from P. knowlesi, P. chabaudi or P. berghei. The GLDH of P. falciparum was shown to be a cytosolic protein (by FAT) with a subunit molecular weight of approximately 49000 Da (by immunoprecipitation) having a pre dominantly hexameric form (by sucrose density gradient). Implications of the conserved sequences of GLDHs and other enzymes are discussed.

The variable antigen repertoire expressed by metacyclic Trypanosoma brucei rhodesiense is not influenced by the anamnestic expression whereby the variable antigen type (VAT) ingested by a tsetse fly is present at high levels in early bloodstream populations of fly-infected mice. This has been demonstrated by feeding to Glossina morsitans a trypanosome line expressing a VAT which is normally a component of the metacyclic repertoire. The VAT did not constitute a significantly increased proportion of the resultant metacyclic population which would have occurred had anamnestic expression and metacyclic expression been linked. Five other metacyclic VATs were also present at control levels. We conclude that the mechanisms of expression of VATs in the fly and in the mammal are independently controlled.

A study has been made of the damage incurred by normal and Plasmodium falciparum-infected human erythrocytes following exposure to a variety of oxidant-generating systems. Hydrogen peroxide, produced by the glucose–glucose oxidase system, increased methaemoglobin formation within normal erythrocytes while normal levels of oxyhaemoglobin were maintained. Exposure to products of the xanthine–xanthine oxidase interaction did not have the same effect. Malondialdehyde measurements indicated that the host cell membranes of parasitized cells had undergone lipid peroxidation even before exposure to the oxidant-generating systems. Lipid peroxidation of normal and parasitized cell membranes was increased upon exposure to reagent-grade hydrogen peroxide and alloxan: this increase was not observed following exposure to the two enzyme–substrate systems that generated reactive oxygen intermediates. In addition, the effects of parasitism on intracellular levels of catalase and superoxide dismutase were assessed. Normal and parasitized erythrocytes were found to possess similar levels of these enzymes, which protect against oxidant-induced damage. It was therefore concluded that the increased susceptibility of infected cells to oxidant damage was probably not related to any decrease in the function of these enzymes.

Schistosomiasis is a chronic debilitating disease which accounts for considerable human mortality and morbidity and is second only to malaria in its prevalence and severity. Despite several years of concentrated effort to combat the disease with molluscicides and chemotherapy, schistosomiasis is still spreading, and it is thought that 500 million people are currently exposed to infection (larotski & Davis, 1981).

Coated metacyclics of Trypanosoma vivax exist in the hypopharynges of infected tsetse flies and are extruded in low numbers when the flies are induced to probe onto warm slides or into medium. After extensive searching of T. vivax-infected proboscides, and resort to a process for the examination of single, extruded, metacyclic trypanosomes, electron micro scopic evidence is presented that, contrary to an earlier report, metacyclic T. vivaxacquire a surface coat before contact with the mammalian host. Since T. vivax exhibits antigenic variation, the role of the surface coat in this species is likely to be functionally equivalent to the surface coat of the other tsetse-transmitted trypanosome species, T. brucei and T. congolense.

Sera from 40 onchocerciasis patients from the Yemen Arab Republic with either mild localized forms of onchocerciasis, intermediate or severe localized forms of the disease or generalized forms of infection were studies with rspect to their IgG and IgM response against Onchocerca volvulus antigens. Immunoblotting, performed with SDS–PAGE-separated proteins of female O. volvulus and quantified by densitometric scanning, revealed IgG and IgM antibodies against worm components in sera of all patients. Persons with intermediate or severe localized forms of onchocerciasis had a stronger IgG response against more proteins than individuals of the other group. However, some antigens (Mr 21, 23, 30, 33 kDa) induced comparable quantities of IgG in all groups. The IgM response of patients with mild localized forms of onchocerciasis was more intensive and directed against more antigensthan in the other groups. No antigens were detected that were recognized only by individuals with low levels of microfilaridermia. In all groups, varying concentrations of antibodies against cuticle, muscle/hypodermis layer and/or uterus of female O. volvulus were detected by the indirect immunofluorescence test using frozen worm sections as antigen. The highest mean antibody titres were found in patients with intermediate and severe localized forms of disease.

The induction of schistosomulicidal activity of peritoneal macrophages by concanavalin A-stimulated supernatants from long-term T-cell clones and by interferon-gamma (IFN-γ) was investigated in detail. Optimal conditions of in vitro macrophage activation by T-cell clone supernatants were established. Macrophages from 13-week S. mansoni-infected mice responded to lymphokine activation as well as resident mnacrophages from uninfecteci mice. IFN-γ was shown to play an essential role in induction of schistosomulicidal macrophage activity: recombinant IFN-γ at high concentration could induce schistosomula killing, and an anti-IFN-γ antiserum inhibited the induction ofschistosomulicidal activity by T-cell clone supernatants. Our data also indicate that macrophage activation could be obtained by IFN-γ in synergy with other lymphokines in the supernatant of long-term T-cell clones. Macrophages from mice injected with T-cell clone supernatants were primed in vivo and triggered to kill schistosomula in vitro in the presence of lipopolysaccharide (LPS). The data demonstrate that lymphokines produced by T-cell clones and, in particular, IFN-γ can participate in the activation of schistosomulicidal macrophages.

During the past decade, enormous technological developments have occurred in biology that have led to significant and revolutionary advances. New techniques of DNA cloning, restriction-enzyme analyses and nucleotide sequencing are providing a mass of data concerning the genomes of a wide variety of organisms. Such insights are having a great impact upon many areas of biological investigation, and would seem to be of considerable potential value for studies of taxonomy and population biology. The application of these new approaches to the characterization and identification of parasitic helminths has only recently begun, but they promise to become powerful additional tools for this purpose. Better methods of characterization are required for more precise definition of the parasites of man and domestic animals and for determining vectors and intermediate hosts as well as possible animal reservoir hosts. Moreover, a greater understanding of the genetic diversity of parasitic organisms is required since many helminths, which are morphologically similar, show marked differences in epidemiologically significant factors such as infectivity, pathogenicity, immunogenicity and drug sensitivity.

Antigenic variation in the ILDar 1 serodeme of the naturally rodent-infective stock of West African Trypanosoma vivax has been investigated following cyclical transmission. The immunofluorescent and immune lysis tests were employed with a panel of 39 variant-specific mouse antisera. When antigenically homogeneous, or mixed, populations were transmitted by tsetse flies to goats, the first peak parasitaemias arising in the goats were antigenic mixtures (up to 9 major, and several minor variants being recognized in some cases) from which the ingested variant was absent. Although first peak parasitaemias in similarly infected goats showed some variants in common, there was no obvious relationship between the VAT profiles in different goats. When these populations were expanded in irradiated mice, VAT heterogeneity was maintained with a tendency towards the development of predominant variants in some, but not all, instances. Six additional variants, derived following the growth of bloodstream form ILDat 1·9 in 37°C culture, were also represented in goat and mouse populations. Two further variants, isolated after cyclical development of ILDat 1·9-derived trypanosomes in vitro, were not present in the early parasitaemias in goats and mice.

As part of an integrated programme investigating human schistosomiasis, work which involves epidemiological surveys and detailed immunological studies as well as biochemical investigations, we have, over the last three years, been cloning schistosome genes in a variety of plasmid and lambda vector systems. In this lecture we present a review of some selected aspects of work primarily aimed at production of experimental vaccines against the disease but which, on a broader front, is also concerned with developmental regulation of gene expression around the parasite's life-cycle. Specifically, we are interested in cloning three groups of genes. First, those encoding surface antigens; second, those associated with sexual maturity and egg production; and third, antigens which may provide a basis for a specific immunodiagnostic test.

Autoradiography of compressed mouse tissues has been used to estimate the numbers of Se-labelled schistosomula present in different mouse organs. Day 7 parasites extracted from the lungs of donor mice were delivered by injection to the lungs, systemic organs and liver of recipient mice as a discrete pulse. The numbers detected in various locations with time post-injection were then used to analyse the dynamics of intravascular migration. Approximately 98% of cercaria-associated label was lost during the first 14 days of parasite life, two-thirds of this in the first 7 h post-infection. Nevertheless, 99–113% of schistosomula could be detected 30 min post-injection into the locations chosen. The efficiency of the parasite delivery system was 95%. The time required for the number of foci in the lungs to decline to 50%, after injection of parasites via the femoral vein, was 55 h. Adjustment of this data to allow for parasites returning to the lungs after passage round the systemic vasculature gave a value of 30–35 h for the true mean time of lung transit. The distribution of parasites to systemic organs after their exit from the lungs was proportional to the fractional distribution of cardiac output. The probability (P) of a schistosomulum being distributed to splanchnic beds was estimated at 0·32 and its P of being trapped in the hepatic portal distributaries within the liver as 0·72–0·86. On this basis, the entire hepatic portal population of adult schistosomes would be recruited during 2–3 circuits of parasites around the pulmonary-systemic vasculature. The mean transit time of schistosomula through intestinal capillaries was 6·5 h whilst that through other systemic organs combined (muscles, kidneys, brain, etc) was 16 h, considerably more rapid than lung transit. The time taken for schistosomula to pass between organs, in arterial and venous blood, was shown to be less than 30 min in both cases (probably much less).

Daughter sporocysts (SpII) of Schistosoma bovis produced by the mother sporocyst (SpI) can direct their production along three different pathways during intramolluscal development. (i) Direct sporocystogenesis–tertiary sporocysts (SpIII) are directly produced by non-cercariae producing daugher sporocysts (SpIII). (ii) Simultaneous sporocystogenesis and cercariogenesis–SpIII are produced by Spil in which cercariae are being produced. (iii) Sporocystogenesis after cercariogenesis-Spill are produced by SpIl in which cercariae are undergoing degeneration. These mechanisms can be interpreted in terms of demographic strategy leading swiftly to the taking over and optimum exploitation of the host's hiotic capacity.

The migration of the schistosomula of Schistosoma mansoni labelled with [75Se] has been followed from the skin to the hepatic portal system. Parasites were detected in all mouse tissues by compressed organ autoradiography. Two separate experiments were performed to track parasites in normal mice, and in mice previously vaccinated with irradiated cercariae. In normal mice, the profile of numbers of autoradiographic foci detected in the skin, lungs, systemic and splanchnic organs was described with time post-infection. The distribution of parasites to systemic organs, following exit from the lungs, paralleled the fractional distribution of cardiac output. Accumulation of schistosomula in the hepatic portal system was complete by day 21 post-infection. Only 2–3 passes of parasites around the vascular system would be required to produce the hepatic portal population. No significant decline in total foci was detected in the first 12 days post-infection. The majority of parasite elimination appeared to occur in the lungs as late as day 21, with lesser proportions in the systemic organs and skin infection site. The pattern of migration in vaccinated mice was similar to that in normal animals. One difference observed was the longer duration of stay in the skin; however, the majority of parasites eventually reached the lungs. The systemic phase of migration occurred on a reduced scale, as did accumulation of parasites in the hepatic portal system. The decline in total foci in vaccinated mice commenced approximately 7 days earlier than in normal mice and proceeded to a lower end-point. Again the majority of parasite elimination appeared to occur in the lungs with lesser proportions in the systemic organs and skin infection site. It is suggested that resistance to reinfection in vaccinated mice has two additive components which combine to retard the migration of schistosomula within the vasculature, preventing them from reaching the hepatic portal system.

The migration of schistosomula through the lungs of mice has been investigated at the ultrastructural level. On Days 4 and 7 post-infection schistosomula were located only within the vasculature. Newly arrived Day 4 schistosomula were highly convoluted and completely occluded vessels measuring approximately 15–25μm in diameter. By Day 7 parasites were located in pulmonary capillaries, often causing considerable distension. The subtegumentary fibrous interstitial layer was absent in elongated lung schistosomula. The minimum diameter of such parasites was 8μm, and their tegument was usually unridged and flattened against the vascular endothelium, thus resulting in close apposition of endothelial and tegumental membranes. The tight fit of schistosomula in the pulmonary capillaries implies that migration through them is both a strenuous and slow process. The migratory delay in the lungs after arrival from the skin occurs because dimensional changes are necessary for successful capillary migration.

Sporozoites of P. falciparum and other Plasmodia appear to be fairly simple antigenically, in that there is a dominant antigen, the circumsporozoite (CS) protein that forms the sporozoite surface coat (Potocnjak, Yoshida, Nussenzweig & Nussensweig, 1980; Santoro et al. 1983). Consequently, the CS protein and the gene encoding it have now been studied in considerable detail (Ellis et al. 1983; Godson et al. 1983; Ozaki et al. 1983; Dame et al. 1984; Enea et al. 1984). In contrast to sporozoites, the asexual blood stages of P. falciparum are antigenically complex. Two-dimensional gel analyses of immunoprecipitated, biosynthetically labelled antigens indicate that repeated infection with P. falciparum results in the synthesis of antibodies against a large number of distinct antigens (Perrin & Dayal, 1982; Brown et al. 1981, 1983). In further contrast to the sporozoite, the asexual blood stages of different P. falciparum isolates exhibit a high degree of antigenic heterogeneity (Brown et al. 1983; Hall et al. 1983; McBride, Walliker & Morgan, 1982). Much of this antigenic diversity is no doubt due to allelic differences but clonal populations of parasites may also have the capacity to undergo antigenic variation (Hommel, David & Oligino, 1983).

The transmission of infection with Giardia muris among CD-I mice was quantified by determining the duration of the latent period and intensity of cyst release during the acute phase of the infection. Housing of experimentally inoculated mice with uninfected mice in the same cage resulted in 100% transmission of infection. The dissemination of the infection was faster if experimentally inoculated hosts were passing high numbers of cysts at the time of housing with uninfected animals. The course of ‘naturally’ acquired and experimentally induced infections was similar. We conclude that G. muris has a high transmission potential and that the number of cysts randomly consumed does not influence the course and eventual outcome of the infection.

The numerical distributions of Echinococcus granulosus in an experimental dog population are described. At all dose rates of protoscoleces from 10 to 175000 the distribution of worms was over-dispersed. Host age had no effect. There was a direct proportionality between the infective-stage density and rate of infection, and between the latter and the index of clumping. The worm burdens were significantly higher in the proximal than distal portions of the small intestine. Lengths of the 3- and 4-segmented worms increased from 4 to 10 and 4 to 8 weeks of age, respectively. Thereafter apolysis was asynchronous and could not be determined. Eggs were first detected in the faeces at 6 weeks and the mean age at oogenesis was 7·26 weeks. Retarded growth of the whole population of worms was observed in some dogs. For the first few infections, worm burdens varied widely in the same dog, but by the 6th infection 50% of the dog population had developed a relative insusceptibility to infection. Growth or oogenesis of the worms were not affected. A short-acting immune response was artificially induced in some dogs following the parenteral injection of activated embryos of E. granulosus, Taenia hydatigena, T. ovis, T. multiceps, T. pisiformis and T. serialis. The response affected either the number of worms established, growth or oogenesis or all three parameters. There was a strong positive correlation between numbers and lengths of worms in dogs with acquired and induced immunity, indicating that no ‘crowding’ effects were involved. In sheep populations the mean number of cysts which established was directly proportional to the number of eggs given, implying that there was no negative feedback mechanism operating at this stage of the life-cycle. The distribution of the larval population in sheep was over-dispersed and the index of clumping increased with the size of the egg dose from 25 to 2500 eggs. Protoscoleces were first observed in cysts at 2 years and the proportion producing them increased with age, with an estimate of 50% of cysts containing protoscoleces at 6 years. No deaths were observed in dogs or sheep even when high parasite burdens were present, implying that E. granulosus does not regulate the population of its hosts.

Peritoneal leucocytosis, with an increased percentage of eosinophils, was found in mice which had been infected with Schistosoma mansoni for 7 weeks or longer. Specific IgG against worm and egg antigens increased in peritoneal fluids and their corresponding sera respectively 5 and 7 weeks after infection. An intraperitoneal challenge with schistosomula elicited neutrophilia in all mice regardless of immune status, as well as infiltration of eosinophils and macrophages in infected mice. The secondary eosinophilia occurred in mice previously infected for 1 week or longer, whereas the infiltration of macrophages occurred only after worms from the primary infection had started laying eggs. Unlike the eosinophilia the macrophage response required infection with bisexual populations of cercariae. Injection of previously infected mice with Escherichia, Trichinella or Toxocara failed to increase the proportions of eosinophils and macrophages. Schistosomula-induced eosinophilia could be elicited in passively sensitized mice. Intraperitoneal injection of PBS extract of adult worms elicited eosinophilia in infected mice and neutrophilia in normal mice. Two chromatographic fractions induced eosinophilia and the third only neutrophilia. The relevance of these observations to host response to S. mansoni infections is discussed.

Previously quoted figures for the frequency of antigen switching in Trypanosoma brucei are based on incorrect assumptions. In order to determine the correct switching frequency, an equation was derived that takes the growth rates of the newly expressed antigen types into consideration as well as the proportion of switched trypanosomes and the number of generations since the population was antigenically homogeneous. When this equation was applied to published in vitro data, variable values were obtained for the switching frequency in clonal populations originally expressing one antigen type. The calculated most likely switching frequencies ranged from 1·4×10−7 to 3·5×10−6. This variation was probably caused by differences in the growth rates of the new antigen types in the population and failure to detect slow growing variants. To overcome these problems, an experimental procedure was developed to analyse the switching frequency in vitro. Trypanosomes were cloned and grown in parallel cultures. After an appropriate number of generations, cells expressing the original antigen type were destroyed and, from the proportion of cultures that contained new antigen types, the switching frequency was calculated. The technique minimized subculturing or other procedures that could distort the results. Although the method was optimized for analysing switching frequency, the values differed between experiments, ranging from 2·2×10−7 to 2·6×10−6 for one variant. Possible causes for the variations in switching frequency are discussed.

Smallpox has been eradicated by immunization with live vaccinia virus. Now, using genetic engineering, infectious vaccinia virus recombinants are being constructed which express genes from other pathogens. These viruses can stimulate specific immunological responses against the foreign antigen in vaccinated animals and can protect against disease caused by the corresponding pathogen. In this paper we describe how recombinant vaccinia viruses are constructed and illustrate the potential of this vector system for expression of genes from parasitic pathogens.