The Cytochrome b (Cyt b) gene has proved to be useful for identification and classification of many mammals and plants. In order to evaluate the utility of this gene for discrimination of Leishmania parasites as well as for exploring their phylogenetic relationships, we determined the nucleotide sequences of the Cyt b gene from 13 human-infecting Leishmania species (14 strains) from the New and Old Worlds. The Cyt b genes, approximately 1080 base pairs, were found to be A/T rich, and their 5′ terminal-editing regions were highly conserved. The nucleotide sequence variation among them was enough to discriminate parasite species; 245 nucleotide positions were polymorphic and 190 positions were parsimony informative. The phylogenetic relationships based on this gene, showed good agreement with the classification of Lainson & Shaw (1987) except for the inclusion of L. (L.) major in the L. (L.) tropica complex and the placement of L. tarentolae in another genus. These data show that the Cyt b gene is useful for phylogenetic study of Leishmania parasites.

Protein degradation is regulated during the cell cycle of all eukaryotic cells and is mediated by the ubiquitin-proteasome pathway. Potent and specific peptide-derived inhibitors of the 20S proteasome have been developed recently as anti-cancer agents, based on their ability to induce apoptosis in rapidly dividing cells. Here, we tested a novel small molecule dipeptidyl boronic acid proteasome inhibitor, named MLN-273 on blood and liver stages of Plasmodium species, both of which undergo active replication, probably requiring extensive proteasome activity. The inhibitor blocked Plasmodium falciparum erythrocytic development at an early ring stage as well as P. berghei exoerythrocytic progression to schizonts. Importantly, neither uninfected erythrocytes nor hepatocytes were affected by the drug. MLN-273 caused an overall reduction in protein degradation in P. falciparum, as demonstrated by immunoblots using anti-ubiquitin antibodies to label ubiquitin-tagged protein conjugates. This led us to conclude that the target of the drug was the parasite proteasome. The fact that proteasome inhibitors are presently used as anti-cancer drugs in humans forms a solid basis for further development and makes them potentially attractive drugs also for malaria chemotherapy.

A bacterial organism is described which occurs constantly in Rhodnius prolixus. It is present in the unfed newly hatched insect inside the cells at the cardiac end of the mid-gut. Some days after feeding, the bacteria are set free into the cavity of the gut and multiply in the undigested blood in the stomach. They are ultimately digested in the intestine.

Morphologically they resemble diphtheroid bacilli in being highly pleomorphic and in giving rise to thread-like forms in old cultures.

Preliminary experiments with Lucilia larvae by Dr R. P. Hobson, suggest that when blood is infected by this micro-organism it contains “vitamin B” and becomes an adequate diet for insect growth.

These observations support the view that symbiotic organisms in exclusively blood-sucking insects provide an endogenous source of vitamin.

Outdoor observations were made on the rate of development of the free-living stages of O. ostertagi at different times of the year, and on the migratory activities and longevity of the larvae in faeces, on herbage and in soil.

The effect of temperature on the rate of development of eggs and larvae in faeces, and the effect of temperature and humidity on the longevity of eggs and larvae when separated from faeces, were studied in the laboratory.

The results of these observations are discussed in relation to translation (that is, the process whereby eggs in faeces become infective larvae on herbage available to the grazing animal) and to the transmission of infection in the field.

I wish to thank Mr J. F. Michel for providing the infected faeces, and Mr D. A. Griffiths for technical assistance.

A cross-sectional parasitological study of a population of wild olive baboons (Papio cynocephalus anubis), consisting of 5 troops, was conducted in Gombe Stream National Park. Baboons were individually recognizable. Information on age, sex, troop membership, reproductive status, social rank and life-history of each individual baboon could be related to parasite infection. Seven helminth taxa and 2 protozoan taxa were found. All baboons were parasitized by at least 1 taxon. Distributions of helminths were aggregated among hosts. There were significant differences among troops in the prevalence of all but 2 of the recorded helminths. Age had a significant impact on the prevalence and intensity of Strongyloides sp. No significant effect of sex on the prevalence of infection could be detected. There was some indication that female reproductive status was related to Trichuris egg output. In contrast to a previous study, no significant correlations between parasite infection and social rank could be found. Troop membership constituted the predominant factor contributing to heterogeneity of prevalence of infection. This suggests that spatial location and/or genetics may be important in determining levels of parasite infection.

Cells capable of transferring immunity to Trichinella spiralis, i.e. of accelerating adult worm expulsion, were present in the mesenteric lymph nodes of mice infected for 4, 6 or 8 days, but not in mice infected for only 2 days. The time-course of worm expulsion in mice infected on the day of transfer was similar in recipients of day 8 cells, expulsion becoming marked only when the recipients had been infected for at least 6 days. Transfer of cells 4 or 6 days after infection did not result in an accelerated worm expulsion; transfer 1 or 2 weeks before infection did not enhance the level of immunity in recipient mice. In contrast to the results obtained with mesenteric lymph node cells (MLNC) no immunity was trsnsferred when recipients were given spleen cells taken from donors infected for 8 days. It is suggested that MLNC do not cause worm expulsion directly, but cooperate with another component of the host's defence mechanism. Accelerated expulsion in recipients of cells was accompanied by a premature decline in fecundity of female worms. Evidence is presented to show that worm expulsion and impaired reproduction may represent independent aspects of the immune response to T. spiralis.

Trypanosoma (Nannomonas) congolense comprises morphologically identical but genetically heterogeneous parasites infective to livestock and other mammalian hosts; three different genotypes of this parasite have been described previously. Restriction enzyme fragment length polymorphisms (RFLPs) in both kinetoplast DNA minicircle and nuclear DNA sequences, and randomly amplified polymorphic deoxyribonucleic acid (RAPD) patterns have been used here to demonstrate the existence of another type of T. (N.) congolense that is genotypically distinct from those that have so far been characterized at the molecular level. A highly repetitive, tandemly arranged DNA sequence and oligonucleotide primers, for use in polymerase chain reaction (PCR) amplification are described, which can be used for specific identification of the trypanosome and its distinction from others within the Nannomonas subgenus.

Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

RNA interference (RNAi) is a potent gene silencing process that is playing an increasingly important role in investigations of gene function in schistosomes. Here we review what is known about the process in these parasites and provide an update on the methodology and machinery of RNAi. Data are presented to demonstrate that: (1) not all schistosome genes can be suppressed to the same extent, using the methods employed here; (2) while there is variation in the level of suppression achieved for one target gene (SmAP) in adult parasites, all individuals exhibit robust (>80%) suppression; (3) short interfering RNAs (siRNAs) can effect suppression when delivered by soaking (and not just via electroporation, as reported previously); (4) Male/female adult pairs need not be separated prior to siRNA delivery by electroporation for effective gene suppression in both genders and (5) electroporation of siRNAs in medium is as efficient as in commercial electroporation buffer. Regarding the machinery of RNAi in schistosomes, a homologue of the C. elegans multi-membrane spanning, RNA importing protein SID-1 is identified in silico. The gene encoding this protein contains 21 exons and spans over 50 kb to potentially encode a 115,556 Mr protein (SmSID-1). These analyses, and a review of the literature, permit us to derive and present here a draft of potential RNAi pathways in schistosomes.

This review examines the sexual development of Plasmodium spp. particularly relating the ultrastructural organization of their cells to the limited, though rapidly expanding, body of metabolic and biochemical studies. Thus it is hoped the article may provide a useful background of information for those undertaking studies on the sexual parasites with the objective of developing methods for the immunological and chemo-therapeutic control of malaria transmission. These objectives, however, should not dominate our clear recognition that the three phases of sexual development, gametocytogenesis, gametogenesis and fertilization contain within them examples of control and assembly of organelles without peer amongst eukaryotic cells.

Field and laboratory investigations into the effect of the parasitic isopod Anilocra pomacentri (Cymothoidae) on the population dynamics of the reef fish Chromis nitida (Pomacentridae) were carried Out at Heron Island, Great Barrier Reef, Australia. Fish carried a single adult parasite just posterior and dorsal to the eye either to the right or to the left of the midline. The adult parasite was overdispersed among fish on patch reefs (dispersion factor, k = 0·69). Sequential field observations on a single cohort of fish showed that parasites significantly depressed growth, reproduction, and survivor-ship. The von Bertalanffy growth coefficients (a measure of somatic growth) were 0·10 for parasitized fish compared with 0·17 for non-parasitized fish. Female fish carrying the parasite produced only 12% of the number of eggs produced by non-parasitized fish of the same size. In the field, the mortality of infected juvenile C. nitida (LCF 15–30 mm) was estimated to be at least 88% in the first 70 days after recruitment of the fish. The mortality of uninfected recruits over the same period was 66%. In laboratory trials, the mortality associated with the infection of juvenile fish by larval parasites ranged from 78% for small fish (mean LCF 15·0 mm) to 28% for larger fish (mean LCF 24·9 mm) within 4 days of experimental infection. This is one of the few studies that evaluates the effect of a parasite on a population of fish in the field.

The success of a pathogen depends not only on its transmission to new hosts, but also on its ability to colonize and persist within its current host. Studies of within-host dynamics have focused on only a few diseases of humans, whereas little is known about the factors that influence pathogen populations as they develop inside non-human hosts. Here, we investigate pathogen dynamics occurring within bumble bees (Bombus impatiens) infected by the gut trypanosome Crithidia bombi. Infection by C. bombi showed several features characteristic of vertebrate diseases, including a rapid initial increase in infection intensity, marked oscillations in parasitaemia, and the stimulation of a systemic immune response in infected bees. Within-host dynamics generated substantial variation in the infectiousness and flower-visiting behaviour of bumble bees. Changes in bee foraging that arise from infection may influence the probability of C. bombi transmission between bees at flowers.

The flatworm nervous system employs a wide repertoire of neuroactive substances, including small chemical messengers, the so called classical transmitters, and several types of neuropeptides. A large body of research accumulated over four decades has provided a wealth of information on the tissue localization and effects of these substances, their biochemistry and, recently, their molecular modes of action in all major classes of flatworms. This evidence will be reviewed, with particular emphasis on the small (classical) transmitters and the receptors that mediate their effects. One of the themes that will emerge from this discussion is that classical transmitters regulate core activities such as movement, metabolism and transport, and thus are essential for survival of the organism. In addition, the evidence shows that flatworms have multiple neurotransmitter receptors, many with unusual pharmacological features, which make them particularly attractive as drug targets. Understanding the molecular basis of these distinctive properties, and developing new, more specific receptor agonists and antagonists will undoubtedly become a major challenge in future research.

We consider two phenomena, related to the host age-intensity profiles of parasitic infections, which have been suggested to be indicative of acquired immunity: (i) a lower age of peak intensity among more intensely infected hosts; and (ii) a decline with age in the dispersion of the distribution of parasites between hosts. We demonstrate that these phenomena occur among Kenyan schoolchildren infected with Schistosoma mansoni, although the magnitude of both is small. We also examine the mathematical models underlying these predictions and conclude that both phenomena are possible in the absence of acquired immunity or, indeed, in the absence of any density-dependent effect. In our opinion, insufficient attention has been focused upon mathematical models, describing the null hypothesis, i.e. density-independent models. In particular, we regard the usual assumptions made for the two stochastic components of these models, describing the heterogeneity between hosts and the probabilistic nature of infection and death of parasites, as too rigid and unrealistic. We demonstrate that deviation from these assumptions undermines the qualitative distinctions between models which describe acquired immunity or density dependence and those which are density-independent.

The incidence of congenital toxoplasmosis was determined by an ELISA in the litters of BALB/c mice which had been infected 8 weeks before mating, on day 12 of pregnancy, or on both these occasions. Of those mice given the infection for the first time on day 12 of pregnancy, 5 out of 6 gave birth to infected litters with approximately 50% of the individuals in each litter being infected. BALB/c mice which had been infected 8 weeks before mating did not give birth to infected litters, even if they were reinfected on day 12 of pregnancy. Following infection BALB/c mice were found to harbour significantly fewer tissue cysts than the congenic H-2 derivative BALB/K strain. However, chronically infected BALB/K mice also failed to produce infected litters, indicating that tissue cyst burden in the dam did not influence congenital infection at least on the BALB background. This study demonstrates that BALB/c dams chronically infected with Toxoplasma gondii, have immunity capable of protecting their embryos from congenital infection, even if the dams are reinfected during pregnancy. Our results demonstrate that the BALB/c mouse can be used as a model of human or ovine congenital T. gondii infection suitable for testing putative vaccines.

The effect of temperature on early Plasmodium falciparum development was examined in Anopheles stephensi. The rates of both ookinete development and bloodmeal digestion were lengthened as temperatures decreased from 27 to 21 °. However, low temperatures (21–27 °) did not significantly influence infection rates or densities of either ookinetes or oocysts. In contrast, high temperatures (30 and 32 °) significantly impacted parasite densities and infection rates by interfering with developmental processes occurring between parasite fertilization and ookinete formation, especially during zygote and early ookinete maturation. This study demonstrates clearly that temperature affects the sporogonic development of P. falciparum in anophelines by altering the kinetics of ookinete maturation. These studies not only confirm the ookinete as the key development stage affecting the probability of vector infectivity, they provide new insights into the epidemiology of P. falciparum infections.

In affluent societies the prevalences of so-called ‘Western’ diseases such as atherosclerosis, allergies and autoimmune disorders appear to have increased, while many diseases caused by communicable infections are now relatively less common. To test whether there may be a causal relationship we examined the effects of Schistosoma mansoni infections in mice that develop cardiovascular pathology as a result of a genetic deficiency in apolipoprotein E (apoE−/−). The development of atherosclerotic lesions in the aortic arch and brachiocephalic artery of the apoE−/− mice was reduced by approximately 50% in mice with the parasitic infection, when comparison was made with uninfected control mice fed the same diet. Observations on S. mansoni-infected conventional laboratory mice indicate that patent schistosome infections could be counteracting the effects of an atherogenic diet by modulating host lipid metabolism and inducing a reduction in blood total cholesterol concentrations.

Survival and infectivity characteristics are described for cercariae of the echinostome Echinoparyphium recurvatum at 10, 15, 20, 25 and 30°C. Cercarial survival was markedly age- and temperature-dependent, maximum survival time being reduced from 68 h at 10°C to 12 h at 30°C, 50% survival times falling from 47·9 h at 10°C to 8·4 h at 30°C. The mean instantaneous per capita death rate of cercariae increased exponentially with temperature. Cercarial infectivity was also temperature dependent, the main effect of temperature being upon the rate at which infectivity diminished with increasing age of cercariae. Infectivity increased from zero at 10°C to a maximum at 25°C after which it declined to a low level at 30°C. Overall transmission efficiency of cercariae at each experimental temperature was calculated as the ratio of the mean instantaneous per capita death rate (μ) to the mean instantaneous per capita rate of infection (β). Transmission efficiency of cercariae was found to be maximal at 20°C and at a relatively high level between 10 and 25°C. Transmission efficiency was zero at 10°C and at a very low level at 30°C. These results suggest that the cercariae of E. recurvatum show transmission optima at water temperatures likely to be encountered in natural habitats in Britain and Europe.

All pairwise combinations of three cloned stocks of Trypanosoma brucei (STIB 247L, STIB 386AA and TREU 927/4) were co-transmitted through tsetse flies (Glossina morsitans) and screened for the production of hybrid trypanosomes. Clones of metacyclic and bloodstream trypanosomes from flies harbouring mature infections containing hybrid trypanosomes were established and screened for several isoenzyme and restriction fragment length polymorphisms. For each of the three combinations of parents, some progeny clones were observed to be of a phenotype and genotype indicating that genetic exchange had occurred during development of the trypanosomes in flies. These hybrid clones shared three salient features: (1) where the parents were homozygous variants the progeny were heterozygous, (2) where one of the parents was heterozygous, allelic segregation was observed and (3) the progeny clones were shown to be recombinant when two or more markers for which one of the parents was heterozygous were examined. These results are consistent with the progeny being an F1 in a diploid mendelian genetic system involving meiosis and syngamy. Our observations show that all possible combinations of the three stocks may undergo genetic exchange. A marker analysis of a series of clones each derived from single metacyclic trypanosomes showed that individual flies transmit a mixture of trypanosome genotypes corresponding to F1 progeny and to parental types, indicating that genetic exchange was a non-obligatory event in the life-cycle of the trypanosome. In addition, a preliminary analysis of the phenotype of procyclic stage trypanosomes derived from flies infected with two stocks, indicates that genetic exchange is unlikely to occur at this stage.

Regulation of ungulate populations by parasites relies on establishing a density-dependent relationship between infection and vital demographic rates which may act through the effect of parasites on body condition. We examine evidence for parasite impacts in 285 red deer (Cervus elaphus) harvested during 1991 and 1992 on the Isle of Rum. In the abomasa, prevalence of nematodes was 100% and the most abundant genus observed were Ostertagia species, however, mean intensity of infection was low (less than 1000) relative to other studies. Additional species, also present in low numbers, included Nematodirus spp., Capillaria spp., Cooperia spp., Monieza expanza, Oesophagostomum venulosum and Trichuris ovis. Lungworm (Dictyocaulus spp.) and tissue worm (Elaphostronygylus cervi) larvae were also observed in faecal samples. There was no evidence for acquired immunity to abomasal nematodes. Despite low levels of infection, both adult male and female deer showed significant negative correlation between indices of condition (kidney fat index, dressed carcass weight and larder weight) and intensity of Ostertagia spp. infection. However, there was no evidence that pregnancy rate in females was related to intensity of infection. For calves, there was no relationship between body condition and intensity of infection. The apparent subclinical effects of low-level parasite infection on red deer performance could alternatively be due to animals in poorer nutritional state being more susceptible to infection. Either way the results suggest that further studies of wild populations are justified, in particular where high local host densities exist or alternative ungulate hosts are present, and, where experimental treatments are tractable.