A leptomonad parasite was found in the sap of the tomato and in one unidentified plant, but it has not yet been seen in several others on which the insect feeds.

Interesting characteristics of this Leptomonas are (a) its extreme polymorphism, (b) the development of metacyclic forms in the salivary glands of its insect host, and (c) its alternation between insect and plant host.

The tegument of Haplometra cylindracea is protoplasmic. It is organized on two levels, a surface syncytium (‘cuticle’ of the light microscopists) and tegumental cells which lie among the parenchymal cells below the muscle layers. The surface syncytium and tegumental cells are joined together by protoplasmic, tube-like, processes.

The surface syncytium is covered apically and basally by a unit plasma membrane. It contains mitochondria, vacuoles, lipid bodies, spines with a crystalline structure and dense secretory bodies. This syncytium lies on a basement membrane and a thick, fibrous, interstitial layer. The connecting protoplasmic processes, which originate from the base of the syncytium, are vacuolated and have few inclusions but, as they approach the tegumental cells, they become progressively filled with cytoplasm, cell inclusions and dense bodies.

The tegumental cells consist of individual areas of nucleated protoplasm, usually lying in groups. These cells contain dense secretory bodies, mitochondria, a few lipid droplets, crystalline inclusions, and Golgi complexes. The Golgi complexes and granular endoplasmic reticulum are jointly involved in the production of a dense, probably proteinaceous, secretory body. Some qualitative regional differences in tegumental ultrastructure are described.

The tegument of Haplometra is compared with that of other species so far described and like them must be considered both a protective and a secretory epithelium.

I should like to express my sincere thanks to the following: The Wellcome Trust and Science Research Council for grants to purchase an Akashi TRS 50 and and A.E.I. EM6B electron microscope respectively; the Royal Society for a personal grant to purchase a Reichert Ultramicrotome and a vacuum coating unit; the Wellcome Trust for a grant to employ a Research Associate.

Homologous and heterologous resistance in Echinostoma revolutum infections was studied in mice. A high level of homologous resistance was demonstrated in mice harbouring a 13-day-old primary E. revolutum infection with 9–10 and 11–15 worms, corresponding to a 70·0 and 66·7% reduction in the size of the established worm burden as compared with that of the challenge control group. A 14- and 20-day-old primary infection with 3–4 worms induced a level of resistance of 61·7 and 81·8% respectively, while higher worm levels of 9–10 and 11–15 induced almost complete resistance corresponding to a 95·1–100% reduction in the size of the established worm burden. Complete resistance was also demonstrated in mice challenged 8 days after elimination of a 20-day-old primary infection with 11–15 worms by anthelmintic treatment. A primary 43-day-old Schistosoma mansoni infection induced a 73·1% reduction in the size of the established E. revolutum challenge infection while infections of an age of 79 and 99 days conferred complete resistance to heterologous challenge with E. revolutum. Primary pre-patent S. mansoni infections and a patent S. bovis infection of an age of 56 days did not induce any resistance to challenge with E. revolutum. A primary 14- and 21-day-old infection of E. revolutum did not stimulate any significant level of resistance to heterologous challenge with S. mansoni.

Due to an increased need for new antimalarial chemotherapies that show potency against Plasmodium falciparum, researchers are targeting new processes within the parasite in an effort to circumvent or delay the onset of drug resistance. One such promising area for antimalarial drug development has been the parasite mitochondrial electron transport chain (ETC). Efforts have been focused on targeting key processes along the parasite ETC specifically the dihydroorotate dehydrogenase (DHOD) enzyme, the cytochrome bc1 enzyme and the NADH type II oxidoreductase (PfNDH2) pathway. This review summarizes the most recent efforts in antimalarial drug development reported in the literature and describes the evolution of these compounds.

The population dynamics of Ascaridia galli infections of poultry was studied under repeated doses of 10, 100 and 1000 eggs. Daily doses were continued for 6 weeks only. At low dose rates patency is noted by the fifth week. At higher dose rates there is an accumulation of larvae whose development has been arrested at a precise developmental stage, i.e. the third larval stage. Sporadic development of some of these larvae to adults occurs later. Arguments are produced to show that this inhibition of larval development is a resistance and not a density-dependent phenomenon. The epidemiological significance of this inhibition is discussed. This resistance phenomenon is shown to occur in single infection experiments where 1000 eggs were used. This gives rise to the development of a worm population of two facies. The whole finding is suggested to be related in some way to the phenomenon of ‘tissue phase’, which is accepted as a normal life-cycle behaviour of this parasite. A brief review of previous work on this is given and it is postulated that the ‘tissue phase’ phenomenon is not an obligatory part of the normal life-cycle of the parasite but represents a reaction to factors of resistance inherent in chickens and controlled to a large extent by a complex dynamic interplay of dose size and the chickens.

I wish to thank the Department of Technical Co-operation and the Federal Government of Nigeria for financing this project, Dr Boulton of the Poultry Research Centre, Edinburgh, for advice on nutrition, and the Department of Zoology, especially Dr J. A. Campbell, University of Edinburgh, for facilities and advice on the work. I am greatly indebted to the Ministry of Agriculture, Fisheries and Food, Poultry Research Laboratories, Lasswade, for the free supply of chickens used in this work.

Different hydrogen peroxide (H2O2)-scavenging mechanisms, and the conditions under which they operate, have been examined in promastigotes and amastigotes of Leishmania donovani. For promastigotes, the ability of the parasite to remove H2O2 was completely ablated by sonication whereas for sonicated amastigotes substantial loss of H2O2 from the phagocyte-fre¸e test system still occurred. In direct contrast, the ability of amastigotes, but not promastigotes, to remove H2O2 was markedly inhibited by aminotriazole or sodium azide. This suggested a role for haem-containing enzymes, catalase or peroxidases, as a protective H2O2-scavenging mechanism and was consistent with detection of catalase in amastigotes but not promastigotes using a spectrophotometric assay. Both forms of the parasite did, however, show reduced ability to remove H2O2 at 5–7°C indicating that additional enzymatic scavenging mechanisms may operate. Glutathione peroxidase activity was undetectable in either form of the parasite. The total thiol sink, glutathione (GSH) plus protein thiols, was greater in promastigotes but the ability to regenerate GSH via glutathione reductase was equivalent for promastigotes and amastigotes. Less temperature-dependent non-enzymatic mechanisms (e.g. an unsaturated lipid sink) also appear to contribute to removal of H2O2 by both promastigotes and amastigotes. It seems likely, nevertheless, that the difference in H2O2 sensitivity between the two forms of the parasite relates to the activity of the direct H2O2-scavenging enzyme, catalase, which appears to operate more efficiently against a bolus of reagent H2O2.

Twenty-one monoclonal antibodies, obtained after immunization of mice with erythrocytic stages of Plasmodium falciparum, produced a double dot image in IFA. Immunoelectronmicroscopy indicated that the mAbs reacted with the rhoptries. Rhoptries are pear-shaped apical organelles, believed to be involved in invasion of the host cell by the parasite. The mAbs all immunoprecipitated the high molecular weight antigen complex. Some mAbs recognized on immunoblots only 1 protein of this complex, whereas others reacted with RhopH1 and RhopH3, or RhopH2 and RhopH3 or with the 3 proteins. An additional antigen of 52 kDa was also recognized by some of the mAbs. The epitopes defined by the mAbs were present in most of the 40 P. falciparum strains or isolates studied by IFA. Interestingly, the mAbs also reacted with high titres on P. vivax and P. ovale, but produced images that did not indicate an apical location. The mAbs failed to react on the non-human malaria parasites studied, P. cynomolgi and P. inui. On P. berghei or P. chabaudi parasites, only 5 mAbs gave a positive reaction, labelling a large network outside the parasite. Finally, the mAbs did not react with P. falciparum sporozoites, indicating that the rhoptries of merozoites and sporozoites, the two invasive stages of the malaria life-cycle are equipped with distinct sets of proteins.

Standard intracellular microelectrode techniques were used to determine the electrical properties of the tegument and sub-tegumental regions in male Schistosoma mansoni. Three distinct compartments of electrical potential were observed. The resting potentials recorded in these compartments of –45·9±2·5 mV (Eteg), –22·0±1·1 mV (E2) and – 4·7±0·3 mV (E3) corroborate those previously reported by Fetterer, Pax & Bennett (1980) and Bricker, Pax & Bennett (1981). Input resistance was measured in each compartment and was found to be 4·5 MΩ (tegument), 9·2 MΩ (E2) and 3·5 MΩ (E3). Time-constants for the tegument, E2 and E3 were 0·24±0·01 msec, 0·25±0·01 msec and 0·13±0·01 msec, respectively. Multiple electrode experiments revealed that the tegument and E2 compartment are electrical syncytia with similar current-spreading capabilities. Low resistance pathways also appear to connect the tegument and E2 region, since electrotonic signals initiated in either of those compartments experience only a 15–25% reduction upon passing into the other. Injecting large (> 200 nA) depolarizing current pulses into the tegument or E2 compartment often resulted in the initiation of active membrane responses. These spikes were highly variable, ranging from 4 to 75 mV in magnitude (occasionally overshooting zero potential by as much as 25 mV) and from 10–40 msec in duration. The responses were not actively propagated along the parasite, and their decay over distance was approximately equal to that predicted on the basis of length constant values obtained from electrotonic signals. The addition of a non-diffusible solute to the recording medium resulted in a significant reduction in the current-spreading capacity of both the tegument and E2 compartment. Coupling ratios between the tegument and E2 compartment were decreased, and the input resistance for both compartments increased, while resting potentials remained constant. Active responses could not be evoked in schistosomes exposed to the hyperosmotic medium.

The present studies were designed to characterize the response of sheep infected with N. spathiger to challenge infection with the same species.

The results showed that: (1) the level of worm counts attained on secondary infection with the parasite was much reduced compared with the counts recorded following initial infection; (2) the relative fecundities of parasite populations which developed in individual hosts following challenge infection, ranked in magnitude in an order which was similar to that established following initial infection; (3) as fecundity was highly correlated with the number of egg-laying females and the number of males and females was nearly equal in individual specimens, it was concluded that worm burdens were also ranked for individual animals on primary infection and following challenge; and (4) the animals of the control group remained as susceptible as the animals of groups 1 and 3 at the time of their primary infection; consequently, in the absence of previous immunological experience due to infection, there was no apparent change in susceptibility of the animals due to an effect of age during the course of the 17 weeks of the experiment.

We are greatly indebted to Dr Michael Tallis, Division of Mathematical Statistics, C.S.I.R.O., for advice and assistance with the statistical analyses, and to Dr D. F. Stewart for his interest and constructive criticism during the course of these studies. Our thanks are also due to Mr E. Teleki and Misses Helen Giller and Lindy Stothart for their valuable technical assistance.

1. The relation of host-parasite specificity to age resistance.

Conclusive demonstration of a substantial resistance developing concomitantly with the ageing of the host has been made with reference to four helminths, namely: Ancylostoma duodenale and A. caninum in the dog and Ascaridia lineata and Syngamus trachea in the chicken. A. duodenale in the dog and S. trachea in the chicken are manifestly in abnormal hosts. In contrast with this, it is to be noted that the normal hosts of these parasites, respectively man and the turkey, exhibit no appreciable age resistance. With regard to the remaining two cases, those of Ancylostoma caninum and Ascaridia lineata, the association of age resistance and a specific host-parasite mal-adjustment is not nearly so clear. In nature, both parasites enjoy a polyxenous distribution, neither having acquired the need for strictly specific host conditions for development. As a result of recent researches with A. caninum it has been brought out that the species is comprised of strains better adapted to one species of host than to another, and it has been demonstrated that a higher degree of age resistance is exhibited in the host parasitised with a foreign strain. Although no information on the matter is available, it is quite possible that age resistance will be found to be less markedly expressed in other species or races of suitable hosts, when the bionomics of both Ancylostoma caninum and Ascaridia lineata are further investigated.

Blastocystis spp. pathogenic potential remains unclear as these anaerobic parasitic protozoa are frequently isolated from stools of both symptomatic and asymptomatic subjects. In silico analysis of the whole genome sequence of Blastocystis subtype 7 revealed the presence of numerous proteolytic enzymes including cysteine proteases predicted to be secreted. To assess the potential impact of proteases on intestinal cells and gut function, we focused our study on two cysteine proteases, a legumain and a cathepsin B, which were previously identified in Blastocystis subtype 7 culture supernatants. Both cysteine proteases were produced as active recombinant proteins. Activation of the recombinant legumain was shown to be autocatalytic and triggered by acidic pH, whereas proteolytic activity of the recombinant cathepsin B was only recorded after co-incubation with the legumain. We then measured the diffusion of 4-kDa FITC-labelled dextran across Caco-2 cell monolayers following exposition to either Blastocystis culture supernatants or each recombinant protease. Both Blastocystis culture supernatants and recombinant activated cathepsin B induced an increase of Caco-2 cell monolayer permeability, and this effect was significantly inhibited by E-64, a specific cysteine protease inhibitor. Our results suggest that cathepsin B might play a role in pathogenesis of Blastocystis by increasing intestinal cell permeability.

As more facts emerge regarding the ways in which E. multilocularis-derived molecules trigger the host immune response and modulate the host-parasite interplay, it becomes possible to envisage how the parasite can survive and proliferate in its intermediate host, while in other hosts it dies out. Through effects on cells of both the innate and adaptive arms of the immune response, E. multilocularis can orchestrate a range of outcomes that are beneficial not only to the parasite, in terms of facilitating its intrahepatic proliferation and maturation, and thus life cycle over all, but also to its intermediate host, in limiting pathology. The present review deals with the role of metacestode surface molecules as well as excretory/secretory (E/S) metabolic products of the parasite in the modulation of the host responses such as to optimize its own survival.

Leishmania infantum and Leishmania donovani both pertain to the L. (L.) donovani complex and are responsible for visceral leishmaniasis. To explore the L. donovani complex, we focused our study on cysteine protease B (cpb) and especially on 2 cpb copies: cpbE and cpbF. We selected cpb genes because of their phylogenetic interest and host–parasite interaction involvement. Sequencing these 2 copies revealed (i) that cpbE is specific to L. infantum and cpbF is specific to L. donovani and (ii) that these 2 copies are different in length and sequence.Nucleotide sequence data reported in this paper are available in the GenBank database under Accession numbers AY896776 AY896777, AY896778, AY896779, AY896780, AY896781, AY896782, AY896783, AY896784, AY896785, AY896786, AY896787, AY896788, AY896789, AY896790, AY896791. Phylogenetic analysis and protein predictions were carried out in order to compare these copies (i) with other trypanosomatid cpb, especially L. mexicana, and (ii) within the L. donovani complex. Our results revealed patterns specific to the L. donovani complex such as the COOH-terminal extension, potential epitopes and N-glycosylation sites. Moreover, phylogenetic analysis revealed different levels of polymorphism between L. infantum and L. donovani and confirmed the ancestral status of the latter. L. infantum has a shorter sequence and a deleted sequence responsible for modifications in protein conformation and catalytic triad. Considering the clinical aspect, L. infantum dermotropic strains appeared more polymorphic than L. infantum viscerotropic strains.

Various mechanisms that enable and improve transmission success of myxozoan actinospore stages towards fish hosts are described, based upon a combination of experimental data and functional analysis of morphological characters. For this purpose, laboratory-reared actinospores of Myxobolus cerebralis, Myxobolus parviformis, Henneguya nuesslini and Myxobolus pseudodispar were employed to exemplarily investigate aspects of host attachment and invasion. The process of polar filament discharge of M. cerebralis actinospores was analysed, showing that full discharge occurs in less than 10 msec. Additionally, a mechanism that rapidly contracts the discharged filament after discharge is described for the first time. Its purpose is most likely to bring the actinospore apex rapidly into intimate contact with the surface of the host. Unlike M. cerebralis, M. parviformis actinospores did not discharge polar filaments after mechanical and chemical stimulation, suggesting a different mode of triggering. For H. nuesslini actinospores, experimental results indicated that polar filament discharge is independent of external calcium-ion concentration but is influenced by osmolality. After attachment of an actinospore and prior to penetration into the host, an ensheathed unit (‘endospore’), containing the sporoplasm, was emitted from the valves in a manner which varied from species to species. Experimentally induced sporoplasm emission was time-dependent and was found to be independent of polar filament discharge in H. nuesslini. Remarkably, it could be concluded that the sporoplasm is able to recognize host-stimuli while still within the intact spore. An updated summary of the sequential course of events during host recognition and invasion by actinospores is given.

The construction of the Diama dam on the Senegal River and the ensuing ecological changes have led to a massive outbreak of Schistosoma mansoni infection in Northern Senegal, associated with very high intensity of infections, due to extremely intense transmission. The vectorial capacity of Biomphalaria pfeifferi from Ndombo, near Richard-Toll was investigated in order to assess the role of the snail–parasite relationship in this particular epidemiological situation. The results revealed an unusually high compatibility between the Senegalese S. mansoni strain and its local snail intermediate host, B. pfeifferi. The snail infection rate after exposure to a single miracidium per snail was 87%. The cercarial production of infected snails was very high, with a mean total production of 50456 cercariae per snail. No significant difference was found in the total cercarial output between snails exposed to 1 miracidium and those exposed to 5 miracidia. The increase in the rate of cercarial output was significantly greater in snails exposed to 5 miracidia, but there was a higher mortality in this group. The chronobiological cercarial production pattern showed a peak around mid-day. The implications of these findings on the epidemiology of schistosomiasis in Northern Senegal are discussed.

Plasmodium and Theileria parasites are obligate intracellular protozoa of the phylum Apicomplexa. Theileria infection of bovine leukocytes induces transformation of host cells and infected leukocytes can be kept indefinitely in culture. Theileria-dependent host cell transformation has been the subject of interest for many years and the molecular basis of this unique phenomenon is quite well understood. The equivalent life cycle stage of Plasmodium is the infection of mammalian hepatocytes, where parasites reside for 2–7 days depending on the species. Some of the molecular details of parasite-host interactions in P. berghei-infected hepatocytes have emerged only very recently. Similar to what has been shown for Theileria-infected leukocytes these data suggest that malaria parasites within hepatocytes also protect their host cell from programmed cell death. However, the strategies employed to inhibit host cell apoptotic pathways appear to be different to those used by Theileria. This review discusses similarities and differences at the molecular level of Plasmodium- and Theileria-induced regulation of the host cell survival machinery.

A single exposure to radiation-attenuated cercariae of Schistosoma mansoni induces a high level of protective immunity in C57BL/6 mice, which is mediated by Th1 responses. Events in the skin and/or draining lymph nodes early after exposure are crucial for the induction of protection, and we have investigated the interactions of vaccinating parasites with host leucocytes in these 2 locations. We observed extensive lateral spreading of cercarial secretions along layers of the stratum corneum but not between keratinocytes. There was little direct contact with host leucocytes during the first 1–2 days when the parasites lay at the base of the epidermis, but cells accumulated in the underlying dermis. In contrast to normal parasites, attenuated larvae persisted in the dermis for >10 days, often surrounded by aggregates of macrophages/dendritic cells. Whilst cells bearing MHC II, CD11b or CD11c markers were present in the lymph nodes, particularly in the periphery and paracortical areas, no obvious redistribution was seen as a result of parasite residence there for 5–15 days. However, ultrastructural observations revealed numerous cells with macrophage/dendritic morphology in the vicinity of parasites, in some instances closely adherent to the tegument. The observations strongly suggest that the tegument is a potent source of the antigens which prime the immune system in the lymph nodes of vaccinated mice for a protective response.

We compared the relevance of ibuprofen, vitamins C and E to control oxidative/nitrosative stress and heart disease in mice infected by Trypanosoma cruzi. Swiss mice were randomized into five groups: control, uninfected; infected without treatment; and infected treated with vitamins C, E or ibuprofen. Animals were inoculated with 2000 trypomastigote forms of T. cruzi. After 20 days, infected mice presented reduced vitamin C and E tissue levels, high cytokines (interferon gamma, tumour necrosis factor-α, interleukin 10 and chemokine ligand 2), prostaglandin F2α (PGF2α) and nitric oxide (NO) cardiac production, intense myocarditis and reactive tissue damage, which was directly correlated with the intensity of the inflammatory infiltrate and the degree of pathological cardiac remodelling. Vitamins C and E supplementation were irrelevant to counteract reactive tissue damage and myocarditis in infected animals. Conversely, ibuprofen reduced tissue levels of cytokines, PGF2α and NO, as well as lipid and protein oxidation, antioxidant enzyme activity and the cardiac damage, without interfering with heart parasitism. Our results do not support the applicability of vitamin C and E supplementation in the management of acute Chagas cardiomyopathy. By controlling the inflammatory infiltrate, anti-inflammatory-based therapy proved to be a more rational strategy than a direct antioxidant therapy in attenuating oxidative/nitrosative stress and cardiac damage.

Helminth immunology is a field which has changed beyond recognition in the past 30 years, transformed not only by new technologies from cDNA cloning to flow cytometry, but also conceptually as our definition of host immune pathways has matured. The molecular revolution defined key nematode surface and secreted antigens, and identified candidate immunomodulators that are likely to underpin parasites' success in eluding immune attack. The immunological advances in defining cytokine networks, lymphocyte subsets and innate cell recognition have also made a huge impact on our understanding of helminth infections. Most recently, the ideas of regulatory immune cells, in particular the regulatory T cell, have again overturned older thinking, but also may explain immune hyporesponsiveness observed in chronic helminth diseases, as well as the link to reduced allergic reactions observed in human and animal infections. The review concludes with a forward look to where we may make future advances towards the final eradication of helminth diseases.

We compared 19 stocks of Trypanosoma brucei rhodesiense collected in 1991 and 1994 from Tanzania with representative stocks from other foci of Rhodesian sleeping sickness in Zambia, Kenya and Uganda. Stocks were characterized by isoenzyme electrophoresis, restriction fragment length polymorphisms in variant surface glycoprotein genes and random amplification of polymorphic DNA; the banding patterns obtained were coded for numerical analysis. In addition, the Tanzanian stocks were compared by pulsed field gel electrophoresis. Overall the Tanzanian stocks formed a homogeneous group and the predominant genotype isolated in 1991 was still present in the 1994 sample, although at a reduced level. The Tanzanian stocks were distinct from representative stocks from other East African foci. This observation does not support the proposal that there are northern and southern strains of T. b. rhodesiense, but is consistent with the view that T. b. rhodesiense stocks form a mosaic of different genotypes varying from focus to focus in East Africa.