Monte Carlo simulations were developed to assess the potential impact of parasite pathology on vector salivary function as well as of host haemostasis on transmission. Assuming that a proportion of desisting vectors switch host following failure to locate blood, we demonstrate three possible consequences: (1) infected vectors contact more hosts than non-infected hosts, (2) non-infected vectors are biased to infected hosts, independently of attraction, and (3) an exponential relationship exists between parasite load and transmission. We discuss possible epidemiological implications.

The DNA coding for parts of the repetitive amino acid sequence of the Plasmodium falciparum circumsporozoite protein has been spliced to a sequence encoding part of the precursor to the major merozoite surface antigens, to produce a hybrid gene. Expression in Escherichia coli produces a protein with antigenic determinants from both malaria proteins. Antibodies raised against the expressed material react with both a peptide derived from the circumsporozoite repeat sequence, and the merozoite surface molecule. Hybrid molecules of this type may be the basis of a malaria vaccine.

Attempts were made to immunize goats by infection with large numbers of metacyclic trypanosomes of a clone of Trypanosoma vivax, followed by chemotherapy. Five groups of 6 goats each were infected intradermally with 5 different doses of cultured metacyclics of T. vivax, ranging from 102 to 106 trypanosomes/goat. Four weeks after infection, the goats were treated with 10 mg/kg diminazene aceturate (Berenil, Hoechst A.G.). Three weeks after treatment, 3 goats in each group were challenged intradermally with 104 homologous metacyclics derived from culture. The remaining 3 goats in each group were challenged by 20 tsetse infected with the homologous clone. Five out of 30 goats were resistant to homologous challenge; 4 of the goats that had been challenged with cultured parasites, and 1 that had been challenged by tsetse. In each group 1 goat was protected. Protection was therefore not apparently influenced by the number of trypanosomes used to establish the primary infection. In another experiment, 6 goats were each infected by feeding 100 tsetse on the goats for 15 consecutive days. Three weeks after infection the goats were treated with Berenil and 3 weeks later challenged by 20 tsetse infected with the homologous clone. Three out of the 6 goats resisted challenge. The susceptible goats in both experiments, however, showed a reduction in the peak of parasitaemia following challenge compared with both challenge controls and the initial infections. Lytic antibodies to cultured metacyclics of T. vivax were detected in goats that resisted challenge after a primary infection with cultured metacyclics, and in resistant and susceptible goats after a primary infection by tsetse. All infected goats produced lytic antibodies to live bloodstream forms, as well as antibodies to bloodstream form lysates (demonstrated by ELISA). It is suggested that the immunity that had been induced in some of the experimental animals is due to antibody responses to both metacyclic and bloodstream variable antigen types (VATs) expressed during infection.

CBA/Ca mice infected with the tetrathyridia of Mesocestoides corti were exposed to Schistosoma mansoni cercariae either simultaneously, or at varying intervals after the initial M. corti infection. Cohorts were infected with either parasite alone. The dual infected mice and the mice harbouring M. corti alone were significantly heavier than either naive controls or mice carrying the S. mansoni infection only. The livers and spleens from dual infection mice were also found to be significantly heavier than those from other groups. Free M. corti tetrathyridia were reduced in number in the peritoneal cavity of dual infected mice, as compared with mice harbouring a single infection. Furthermore, the intensity of the initial M. corti infection, as measured by the number of capsules surrounding parasites in the liver, was reduced when the mice experienced an S. mansoni infection 21 days later. The mice which were exposed to M. corti only exhibited more mast cells and eosinophils around encapsulated tetrathyridia in the liver than did dual infection mice, while cells surrounding S. mansoni egg granulomas in the liver were significantly increased in dual infection mice. An increase in serum alkaline phosphatase levels was detected in both the mice receiving the dual infection and the mice with the S. mansoni only infection, but not in mice harbouring M. corti alone.

The host—parasite relationships of two geographical isolates of Schistosoma haematobium in CBA mice are described and compared to previous reports on this parasite in other experimental hosts and in man. The mean percentage establishment of worms in mice was 17% and was not affected by the age or sex of the host. Adult worm burdens remained constant over 20 weeks, but were reduced after 18 months of infection. Male and female worms reached mean maximum lengths of 4·78 and 5·9 mm respectively. Egg laying commenced 9·5 weeks after infection and eggs accumulated in the tissues throughout the period of infection. A large increase in the rate of egg accumulation occurred coincidental with the appearance of eggs in the bladder of some mice. Faecal eggs were first observed in some mice at 12·5 weeks and most mice excreted a few eggs by 17 weeks p.i. (post-infection). Eggs were not found in the urine of infected mice. Excreted eggs and eggs isolated from the livers of infected mice hatched, but the resulting miracidia were unable to infect appropriate snail hosts. The development of hepatic granulomas and egg-induced pathology in the bladder of mice is described.

A preparation highly enriched in tegument membranes from adult worms has been used to probe rodent responses to schistosome surface antigens. All exposure regimes in mice stimulated levels of anti-tegument IgG, measurable by ELISA. Plateau levels of circulating antibody attained were similar after mixed-sex and male-only infections, and vaccination with tegument membranes. Female-only infections and exposure to irradiated cercariae stimulated lower levels of anti-tegument antibody. Responses in rats were similar, but occurred more rapidly, probably due to the large initial parasite exposure. Although both mice and rats could be stimulated to produce high levels of anti-tegument antibody by multiple vaccination, consistent protection against a challenge was not obtained. Paradoxically, rats and mice vaccinated with irradiated cercariae developed low levels of antibody to tegument membrane antigens but were highly resistant to challenge. It was concluded that, in the range of sera examined, antibody responses to schistosome tegument membranes did not correlate with the immune status of the host.

Transmission of the monogenean Pseudodiplorchis americanus is restricted to the brief period when its host, the desert toad Scaphiopus couchii enters temporary water to breed. This study followed parasite recruitment into one host population in Arizona, USA, during the spawning season (July). Torrential rainfall triggered 3 successive assemblies. Amongst males, which may enter each assembly, the 3 pulses of invasion led to an approximate doubling of mean worm burdens at each exposure, culminating in 100% prevalence and a mean intensity of over 100 larvae/host. Females, which generally enter only one assembly, acquired a mean of about 40 larvae/host. Each exposure is limited to a maximum of 7 h by strictly nocturnal host activity, and the force of infection increased exponentially: around 10% of recruitment occurred in the first 3 h, and more than 30% in the last 1 h. Correlation of recruitment into males with parasite reproductive potential suggests that individual oncomiracidia had a 30% chance of invasion. Although invasion of the desert host relies on an aquatic infective stage, host behaviour ensures that targets are concentrated in space and time, and the parasite's annual reproductive output is synchronized with a period of host vulnerability totalling less than 24 h/year.

Several recent studies have demonstrated predisposition of certain humans to heavy gastro-intestinal nematode infections. This finding has been used to support the concept of community control of nematodes based on selective treatment of the few heavily infected individuals in the community. A mouse model has been used in this study to determine whether those individuals found to be ‘predisposed’ to heavy infection continue to regain heavy nematode infections following repeated anthelmintic treatments. Re-infection was examined over 3 intervals of 8 weeks in 52 mice and individual worm burdens attained following drug-induced expulsion were analysed. In the case of Heligmosomoides polygyrus, parasite numbers obtained after first treatment were only significantly correlated with those obtained after the second treatment. Re-infection levels in mice during the second and third re-infection periods did not correlate with parasite loads at the beginning of the study. In the case of Aspiculuris tetraptera, correlations after each re-infection period remained significant, although the magnitude of the correlation coefficient decreased with successive re-infection intervals. The data suggest that a selective control programme based on repeated screening to identify heavily infected individuals prior to each treatment would reduce the number of H. polygyrus in the host population 17–6% more than a single screening at the beginning of the control programme, and would reduce the number of A. tetraptera by 14% more than a single screening. Re-infection rates for H. polygyrus were shown to decrease with increasing mouse age whereas re-infection rates for A. tetraptera increased with increasing mouse age.

The ultrastructure of the formation of the first-stage cuticle within the egg of Trichostrongylus colubriformis is described. The first indication of cuticle formation was the formation of the epicuticle external to the epidermal plasma membrane. This becomes thickened to form the epicuticle of the fully formed cuticle. Microtubules and microfilaments associated with the epidermal plasma membrane were observed during the early phases of cuticle formation. The basal zone was secreted into the space between the epicuticle and the epidermal plasma membrane, followed by the formation of the cortical zone. The basal zone had a striated appearance during the earliest stages of its formation.

The study examines the rate of re-acquisition of Trichuris trichiura infection after treatment in two populations, one of mixed age and the other of children with known preintervention infection intensity. A population living in a Caribbean village was treated with mebendazole and the rate of reacquisition of infection of four age classes (2–4, 5–10, 11–30, 30+ years) monitored over a 20-month period. The reinfection rate was higher in the child age-classes indicating either that children are more exposed to infection or that adults slowly develop a partially effective acquired immunity. A cohort of children (mean age 4·5 years) was separated into 3 intensity categories on the basis of expelled worm burdens and their rate of reacquisition of infection monitored over a 12-month period. The rate of reinfection was directly and positively associated with initial infection status. This may indicate that children with low intensity infections are consistently less exposed to infection or that they have effective immune responses. The latter conclusion, however, would imply that they had acquired this immunity early in life, and so appears to contradict the suggestion that resistance is only slowly acquired by adults. Reconciling these two conclusions may require more sophisticated immunological models than have been suggested previously for geohelminthiases.

Excretions and secretions (ES) and somatic components of 4, 8, 12 and 16-week-old Taenia saginata metacestodes were biosynthetically radio-isotope labelled by incubating the larvae in the presence of [35S]methionine. Despite their small size, 4-week-old metacestodes produced as much isotope-labelled ES/parasite as older metacestodes, indicating a proportionately greater metabolic activity of the parasite at this age. In situ the 4-week-old metacestodes were surrounded by a marked granulomatous cellular infiltrate which had largely resolved around 8-week-old metacestodes. Examination of the isotope-labelled ES by SDS-PAGE revealed distinct age-specific components from 4- and 12-week-old metacestodes and other ES components which were produced by all the ages of metacestodes examined. In comparison the labelled somatic components were conserved. Antigenic characterization of the ES by immunoprecipitation against a panel of clinically defined bovine sera combined with SDS-PAGE analysis, identified some highly immunogenic parasite products and others which did not elicit an antibody response demonstrable by immunoprecipitation. These components are of interest in relation to the host/parasite relationship, to the construction of diagnostic assays for the detection of T. saginata cysticercosis, and to the immunity that cattle develop against this parasite.

The paper provides an overview of recent studies on the use of 75Se to tag larval schistosomes and to monitor their migration and distribution patterns in naive mice and those previously exposed to cercariae. The principles and techniques of radioassay and autoradiography in studying various aspects of 75Se-labelled larval schistosomes are described. The main shortcoming of radioassay in monitoring location and movement of labelled schistosomula is that some of the label dissociates from the schistosomula and accumulates in host tissues, notably the liver. Dissociated label is indistinguishable from schistosomula-bound label making monitoring of parasite migration extremely difficult. This difficulty is overcome by compressed tissue autoradiography where labelled schistosomula can be seen as reduced silver foci on an autoradiographic film, whereas dissociated label is too diffusely distributed to produce such reduced silver foci. Furthermore, using autoradiography, quantitative information on parasite migration in normal and immunized laboratory animals can be obtained that would be impossible using traditional recovery techniques. In addition to using 75Se tracer in migration studies, the radio-isotope has potential for elucidating various aspects of schistosome transmission ecology and snail population dynamics in natural waters.