Parasitic diseases inflict major losses on livestock production throughout the world. Currently, control of the diseases relies largely on prophylactic or therapeutic application of anti-parasitic drugs. In many instances, these measures are only partially effective. Moreover, they must be applied frequently, are therefore costly and time-consuming, and lead to the selection of drug resistance within the parasite populations. Thus, it has been recognized for several decades that effective methods of vaccination against parasitic diseases would have a major impact on livestock production. However, despite considerable efforts over the last 30 years, only a few parasite vaccines are currently in use and all of these involve the administration of live organisms.

The leucocyte responses in peripheral blood and pulmonary airways are described following vaccination of mice with radiation-attenuated parasites, and subsequent challenge with normal parasites. Percutaneous vaccination stimulated a large and sustained expansion of the circulating lymphocyte pool, more marked than after intradermal vaccination with lung schistosomula which induced comparable levels of resistance. Macrophages and lymphocytes infiltrated the pulmonary airways in response to vaccination by both routes, the lymphocytes being particularly abundant after intradermal vaccination. Exposure of mice to an equivalent number of normal cercariae induced an earlier lymphocytosis of short duration; far fewer macrophages and lymphocytes infiltrated the lungs than after vaccination. An intense but transient pulmonary eosinophilia peaked at 3 weeks after primary exposure to either normal or attenuated parasites. Percutaneous challenge of vaccinated mice elicited higher levels of circulating lymphocytes than challenge of unsensitized controls. However, whilst leucocyte numbers of all cell types were still elevated in the airways at challenge as a consequence of vaccination, no further cellular recruitment was observed coincident with parasite elimination. Our data are compatible with the hypothesis that the mechanism of immunity in once-vaccinated mice involves a T lymphocyte-macrophage interaction triggered by antigen release from lung schistosomula.

Mice infected heavily with Schistosoma mansoni cercariae were treated 6 days later with Praziquantel and the parasites studied 1 h post-treatment. Immunofluorescence experiments showed that parasite surface antigens became available for labelling in larvae harvested from Praziquantel-treated mice, but not from untreated mice. Red blood cell antigens acquired from the host were localized on all lung worms studied. The distribution of each set of antigens, as revealed by fluorescence-labelling was, however, quite different. The lungs of heavily infected, drug-treated mice, exhibited severe haemorrhages which occurred in a dose-dependent manner. Fewer haemorrhages were found when heavily infected mice were treated simultaneously with Praziquantel and Aprotinin, a proteinase inhibitor. These results indicate that Praziquantel treatment in vivo induces the exposure of parasite antigens on lung-stage worms and may cause the release of parasite-derived enzymes which induce haemorrhages in the lung tissue of the host.

The transport processes of D-glucose in Plasmodium yoelii-infected mouse erythrocytes were investigated using 2-deoxy-D-glucose (2DOG), a non-metabolizable analogue of D-glucose. Infected cells showed an increase in the uptake of 2DOG compared to uninfected controls, and an effect which was more prominent in cells with mature-stage parasites. Kinetic studies measuring the initial rates of 2DOG uptake revealed two components in infected cells with late trophozoite and schizont-stage parasites: a simple diffusion system and a carrier (transporter)-mediated system. The transporter was common for D-glucose and 2DOG and had a kinetic constant indicating a high affinity for 2DOG (the Km = 0·18 mM and the Vmax = 0·61 mmol/1010 cells/min), as compared to the constant of the mouse erythrocyte carrier (the Km = 10 mM and the Vmax = 1·8 mmol/1010 cells/min). Determination of the distribution of [3H]2DOG in infected cells and experiments with metabolic inhibitors indicated that the simple diffusion system localizes in the membrane of host cells and the transporter in the parasite plasma membrane. The parasite glucose transporter was much less sensitive to cytochalasin B than that of the host cells and the uptake of 2DOG via the transporter was dependent on energy. Based on these findings, the following features emerge: D-glucose first gains access to the cytosol of infected erythrocytes via the simple diffusion system, which appears after infection by the parasite, and an active uptake against the concentration gradient takes place at the parasite plasma membrane via the parasite glucose transporter in an energy dependent manner. Finally, an energy transduction mechanism for the transport of glucose across the parasite plasma membrane is discussed.

Previous articles in this volume have considered the whole question of parasite vaccines from the immunological point of view. A backcloth of general concepts (Roitt, 1989) and strategies in the viral area (Schild, 1989) precedes consideration of problems specific to parasite vaccines (Mitchell, G. F. 1989). This leads on to reviews of progress to date in the development of vaccines for leishmaniasis (Modabber, 1989), malaria (Mitchell, G. H. 1989) and schistosomiasis (Sher, 1989) and in the development and use of veterinary vaccines (Morrison, 1989). I have been asked in this final chapter to take one step back from all of this and provide the alternative, chemotherapeutic, perspective. The reasons for the request are particularly pertinent, though rarely stated: antiparasite drugs already exist; if parasite vaccines are in fact developed, the way these drugs are used and the whole question of whether or not new ones are required, will have to be reconsidered. It is also a timely one: it is clear that the whole concept of a series of vaccines for the control of parasitic diseases has been so well sold, some would say oversold, that it is impacting on the resolve within the pharmaceutical industry to undertake new chemotherapeutic developments, at a time when this is already under severe strain because of financial considerations (Gutteridge, 1987 a). Hence my title: Parasite vaccines versus anti-parasite drugs: rivals or running mates?. I will try to answer it by, in turn, addressing three key questions. Why are we attempting to develop parasite vaccines? Will we be successful? If we are, will there still be a need for chemotherapy?

Histopathological data on the cellular reactions (foci) around Schistosoma mansoni schistosomula in the lungs of both irradiated (750 rad) and unirradiated, passively immunized and normal rats were consistent with the idea that a significant proportion of immune-mediated attrition in passively immunized rats occurs in the lungs. In unirradiated rats, immune serum elicited an enhanced (i.e. larger) and accelerated (i.e. more rapidly developing) inflammatory cellular infiltration around lung-stage parasites when administered 5 days post-infection, when the parasites were already in the lungs. This demonstrated the antigenicity of lung-stage schistosomula and their potential as targets for immune attack. In irradiated rats, innate immunity was decreased as judged by an increase in the number of worms recovered by portal perfusion, and was accompanied by an overall decreased percentage of trapped parasites compared with unirradiated controls, suggesting that trapping in the lungs is involved in innate, as well as acquired immunity. In contrast to the results in unirradiated rats, passive transfer of immune serum into irradiated recipients did not result in larger lung foci than in the NRS-recipients. However, there was evidence of an accelerated response resulting in an essentially similar ratio of trapped parasites (VRS- compared with NRS-recipients) in irradiated rats, as compared with unirradiated rats, reflecting the similar levels of resistance manifested in both groups of rats. This also lent credence to the notion that it was the speed of immune recognition of the migrating schistosomula and the establishment of trapping foci that were of greater importance rather than the size of the enveloping granulomata. Investigations into the cellular composition of the foci surrounding trapped parasites in unirradiated rats revealed a predominance of mononuclear cells, with equal proportions of lymphocytes and macrophages. Eosinophils represented less than 3% of the cellular composition of the foci and were typically distant from the parasites themselves, arguing against their role in specific immunity in this model. Irradiation of recipient rats resulted in a corresponding increase in the percentage of macrophages in lung foci.

In this study, the advantage of the use of chloroquine (CQ) containing liposomes (lipCQ) over free CQ in the chemotherapy of murine malaria (Plasmodium berghei) was demonstrated. The maximum permissible dose per intraperitoneal injection was 0·8 and 10 mg for CQ and lipCQ, respectively. An increase in therapeutic and prophylactic efficacy of lipCQ in comparison with free CQ at a 0·8 mg CQ dose level was found. It was possible to obtain 100% efficacy (injection at day 5 after infection; parasitaemia 4–8%) with one single intraperitoneal injection of 6 mg lipCQ. Moreover, the ability to increase the doses of CQ per injection after liposome encapsulation allowed successful treatment of infections with CQ-resistant Plasmodium berghei which could not be cured by a 7-day course with the maximum tolerable dose of free CQ of 0·8 mg/mouse/day.

Sixteen monoclonal antibodies (MABs) were selected for their reactivity with adult schistosome tegument. The distribution of target epitopes in different tissues of the adult and in various developmental stages was investigated by indirect immunofluorescence. The distinct patterns of reactivity of the MABs permitted their classification into 9 groups. The distribution of epitopes in larvae, particularly 3 h schistosomula, generally mirrored that in adults. A change in distribution of epitopes coincided with transformation from cercaria to schistosomulum providing a marker for this process. Two MABs reacted with surface membranes of intact cercariae, and 5 with the surfaces of intact 3 h schistosomula. The target epitopes of these 5 MABs were present in the tegument of adults, but not accessible externally. These observations suggest masking of antigens, not shedding, in the course of development. Indeed, no MAB reacted with intact lung or liver worms unless they were damaged. Three MABs reacted with membranes of the tegument and most other tissues, implying properties in common. Evidence for shared functions between epithelia was provided by the reactivity of 2 MABs with the tegument and protonephridia. Eight MABs bound both to the tegument and its cell bodies. Since the latter contains the machinery for protein synthesis, it is difficult to explain why 5 MABs bound to the tegument alone. Little cross-reactivity was found with the tissues of the tapeworm Hymenolepis, rather more with the closely related Schistosoma japonicum. It was concluded that tegument antigens are to a degree tissue specific but definitely not stage specific.

Blastocrithidia triatomae parasitizes vectors of Chagas' disease and is very difficult to cultivate in conventional media. However, co-cultivation with a cell line of its host Triatoma infestans (TI-32; in Schneider's Drosophila medium supplemented with 20% foetal calf serum and 10% tryptose phosphate broth) led to vigorous growth at 24 or 28 °C without an adaptation phase. More than 60 primary cultures were initiated successfully without any failures. Subcultures could be started immediately at weekly intervals. The doubling time was similar in both the primary cultures (41 h) and the 33rd subculture (39 h). The importance of the reduviid cells for B. triatomae became clear after removal of the insect cells, when multiplication of epimastigotes stopped and mainly cysts were formed. Cysts produced in vitro were infective for reduviids. Scanning electron microscopy showed that B. triatomae attached to the host cells, inserted its flagellum into them and destroyed them.

The localization and distribution of neuropeptides in the nervous system of the liver fluke, Fasciola hepatica at different stages in the development of the adult fluke have been determined by an indirect immunofluorescence technique, using antisera to 19 vertebrate peptides and the invertebrate neuropeptide, FMRFamide. Positive immunoreactivity was obtained with antisera to pancreatic polypeptide (PP), peptide tyrosine tyrosine (PYY), substance P (SP) and FMRFamide. Cell bodies and nerve fibres immunoreactive to the 4 peptides are present in the anterior ganglia and the 3 pairs of longitudinal nerve cords and their commissures in the central nervous system. In the peripheral nervous system, immunoreactivity occurs in the nerve plexuses supplying the subtegumental musculature, the oral and ventral suckers, and the muscular lining of the male and female reproductive ducts, including the ootype, uterus, cirrus pouch and gonopore. Cells displaying immunoreactivity to PYY and FMRFamide lie amongst the Mehlis' gland cells that surround the ootype. Processes from these cells extend into the wall of the ootype. One group of PP-immunoreactive cells occurs at the junction of the vitelline and ovovitelline ducts, whilst another group is situated at the entrance to the uterus from the ootype. The results are discussed in relation to the possible roles of the peptides in the neurophysiology and egg production of the fluke.

The glycogen stores of adult Schistosoma mansoni worms could be labelled by incubation of the worms, after an initial reduction of their glycogen content, in the presence of [6-14C]glucose. Subsequent breakdown of the labelled glycogen by the parasite revealed that glycogen was degraded to lactate and carbon dioxide. The degradation of glycogen, as compared to that of glucose, resulted in slightly different ratios of these two end-products. This indicates that glycogen breakdown did not replace glucose breakdown to the same extent in all cells and that Krebs-cycle activity was not uniformly distributed throughout the cells of this parasite. Both fructose and mannose could replace glucose as an energy source and the rate of glycogen synthesis from either of these two carbohydrates was higher than from glucose. No indications for glyconeogenesis from C3-units were found. Glycogen metabolism of S. mansoni was not influenced by hormones of the mammalian host. It is regulated by the external glucose concentration and by the level of the endogenous glycogen stores. Studies on paired and unpaired worms showed that no interaction between male and female was necessary for the synthesis of glycogen by female worms.

This meeting included talks which covered both the principles and problems of vaccine development and others that extensively reviewed the present state of progress towards the goal of antigenically defined parasite vaccines for human and veterinary use. Several speakers, particularly Drs Morrison and Gutteridge, pointed out that in comparison with bacteria and viruses there are few vaccines available against parasites and these consist of attenuated organisms for veterinary infections. Therefore, despite the intensity of effort, better defined vaccines for parasites still elude us.

A study of Leptomonas lygaei in Lygaeus pandurus is described. Flagellates with promastigote configuration were found in the midgut, ileum and rectum. Cysts and encysting stages (straphangers) were found in the rectum either attached to the flagella of promastigotes or free in the lumen. In the posterior midgut, the crypts were frequently filled with flagellates, but no attachment to host epithelium was observed. A close association between the flagellates and extracellular membrane layers was observed. In the rectum the flagellates frequently attached by hemidesmosomes to the cuticle of the gland cells and less frequently to the rest of the rectal wall. Interflagellar desmosomes between the expanded sheaths of flagella of adjacent parasites were also observed. Straphanger cysts attached to parental flagella by the formation of zonular desmosomes. Differences were apparent between the organelles of the encysting stages and the parental flagellates. In mature cysts, the cellular organelles were unrecognizable. A comparison between the cysts and flagellates of L. lygaei and Blastocrithidia familiaris is made.

The ribosomal RNA (rRNA) gene units of Schistosoma mansoni and S. rodhaini, of the lateral spined egg group, have been studied. The schistosome rRNA gene unit consists of a regular interspersion of the two genes encoding the large and small rRNA units with two spacers. The large spacer is not transcribed while the small spacer is part of the transcription unit. Variation in the rRNA gene unit between the two species is demonstrated to take the form of loss or gain of restriction sites within the non-transcribed and transcribed spacers. This variation has been demonstrated to enable the differentiation of S. mansoni from S. rodhaini by Southern hybridization analysis. In addition, a DNA clone representing a female specific, tandemly repeated sequence, is demonstrated to enable differentiation of S. mansoni and S. rodhaini using dot blot hybridization analysis.

The effects of pharmacological concentrations of several gastrointestinal hormones on the rate of sucker activity and the frequency and the amplitude of spontaneous longitudinal muscle contractions have been examined in adult Fasciola hepatica. Caerulein and serum decrease the rate of oral sucker activity; motilin decreases and CCK–PZ increases ventral sucker activity when compared to controls. Caerulein, serum and motilin significantly inhibit the frequency of contractions while bile, caerulein and motilin decrease the amplitude of contractions. These results suggest that F. hepatica can recognize and respond to certain gastrointestinal hormones and there may be adaptive value in these behavioural responses.

Normal human serum (HS) contains trypanolytic activity and agglutinins to Trypanosoma equiperdum, while such activities are not found in sera from a range of animals susceptible to infection. HS given to T. equiperdum-infected mice caused a rapid decrease in the number of circulating trypanosomes and protection from lethal infection. Trypanolytic activity of human serum was found to be associated, after DEAE chromatography and Sephadex G-200 gel filtration, with the fraction containing 19S antibodies. Immunofluorescence assays confirmed a binding of human IgM and C1q complement component onto the surface of T. equiperdum. Anti-T. equiperdum activity of HS was specifically directed to T. equiperdum surface components and not to some mouse serum components adsorbed on parasites during the growth in the host, because HS adsorbed in vivo in CD-1 mice retained full protective and agglutinating properties. Trypanocidal activity appears in human serum about the 7th month after birth and persists until late in life. On the contrary, human purified high-density lipoprotein had no significant in vitro or in vivo trypanocidal activity. In conclusion, strong natural anti-T. equiperdum activity in human serum was mainly mediated by natural antibodies of the IgM class. The presence of natural IgM active against T. equiperdum in HS could represent one of the natural mechanisms of resistance of refractory hosts against trypanosome infections. This phenomenon provides further evidence that host specificity of trypanosomes may be partly conditioned by the presence of natural antibodies.

Mating was observed in populations of Gyrodactylus turnbulli from guppies. Copulation took place between older (post-1st birth) flukes and resulted in the insemination of spermatozoa into the seminal receptacle. New-born flukes, and flukes which had only recently given birth for the first time (in which the penis was absent or developing), were never inseminated. Flukes were promiscuous, copulating with several partners in succession. The occurrence of mating was related to the size, age and crowding of the G. turnbulli population, taking place most frequently when the parasites were most densely crowded upon the peduncle of the fish on days 5 and 6 of the infection. Maximum population size was positively correlated with the number of flukes initiating an infection. As a result, mating began earlier, and continued for longer in populations founded by several parasites rather than those begun by single flukes. These observations are discussed with respect to the population biology of G. turnbulli: it is likely that sexual and asexual reproduction alternate, with sex being important only during phases of epidemic population growth. The significance of sex in gyrodactylids is discussed, and the potential importance of continuous sexual reproduction in G. turnbulli infections on cultivated guppies is noted.

Three DNA fragments from the genome of the rickettsia-like organism (RLO) symbiotic in tsetse flies (Glossina spp.) have been cloned. One of these fragments represents a family of highly conserved repeats, which occurs in RLO from all species of tsetse examined and has been amplified in the original RLO culture from which the DNA is cloned. This fragment serves as a highly sensitive and specific probe for the detection of RLO in tsetse midguts. As few as 30 organisms were unequivocally identified by dot blot hybridization of homogenized midgut preparations. Since the presence of RLO within tsetse midguts is associated with susceptibility to trypanosome infection, this technique provides a rapid and reliable method of assessing the potential susceptibility of a tsetse population to Trypanosoma brucei s.l. and T. congolense infections.

Schistosoma mansoni cercariae in water were shown to possess a largely aerobic energy metabolism, the Krebs cycle being the main terminal of carbohydrate breakdown. A metabolic transition towards a more anaerobic breakdown of carbo-hydrate could be achieved by incubation conditions which also stimulated biological transformation. Incubation of cercariae in a simple salt medium containing 5 mM glucose induced such a metabolic transition: beside carbon dioxide large amounts of lactate and pyruvate were excreted. The results indicate that the production of pyruvate was coupled to electron transfer in the respiratory chain. Some aspects of this unusual pyruvate production are discussed. The observed change in the end-product pattern of carbohydrate breakdown is very rapid: most of the switch occurred within 2 h. Our results show that the metabolic transition was triggered by the biological transformation itself, or by the same event that induces the biological transformation. The metabolic and the biological changes proceeded synchronously.

Trypanosoma musculi are readily killed when phagocytosed by mononuclear phagocytes but the nature of the mediators of this cytotoxicity is unclear. Among the most potent mediators are oxygen-derived species. The generation of chemilumine-scence (CL) by peritoneal macrophages from 12 day T. musculi-infected mice, which phagocytose and kill parasites when opsonizing antibodies are present, was recorded in the presence of antibody-coated trypanosomes. Taurine, a specific quencher of hypochlorous acid (HOCl) inhibited CL production by peritoneal macrophages, showing that HOCl is produced during phagocytosis of T. musculi. In vitro, HOCl alone exerted a powerful trypanocidal activity which was inhibited in the presence of specific quenchers. The role of HOCl generated by phagocytes in trypanosome killing was studied using granulocytes which produce more oxygen-derived species than macrophages when stimulated. Phorbol myristate acetate-triggered granulocytes can destroy T. musculi and trypanosome killing is inhibited in the presence of taurine. These data demonstrate that HOC1 produced by phagocytes can effectively destroy T. musculi.