The structure and chemistry of the egg-shell of Hammerschmidtiella diesingi was examined. The shell consists of 5 layers: external uterine layer, internal uterine layer, vitelline layer, chitinous layer and lipid layer. The uterine layers contain discrete spaces which open to the exterior via pores. Freeze etching revealed particles of 8·0 nm diameter in the external uterine layer and fibres measuring 2·6–3·1 nm in the chitinous layer. The morphology of the reproductive system, the cytoplasmic inclusions of the mature oocyte and the formation of the layers of the egg-shell are described.

In an attempt to induce passive protection in lambs against Taenia ovis larvae that would last for the 15–20 weeks from birth to slaughter as fat lambs, one group of ewes was immunized by a series of injections of 2000, 4000, 8000, 16000 and 32000 activated oncospheres of Taenia ovis prior to parturition. Another group of ewes was not immunized. All ewes had previously grazed pasture lightly infected with T. ovis eggs. Most lambs from non-immunized ewes developed cysts after oral infection with T. ovis eggs. However, no lambs from immunized ewes developed cysts up to and including 6 weeks after birth. Between 8 and 16 weeks after birth a proportion of lambs were found to be susceptible to infection. By 18 weeks after birth all lambs were apparently susceptible. The 99% confidence band for the mean duration of demonstrable complement-fixing antibody titres was 6·2–7·8 weeks for lambs from immunized ewes. The persistence of maternal protective antibody in some lambs could possibly preclude successful active immunization of all lambs against T. ovis larvae before 18 weeks of age.

Microfilariae of Brugia malayi were obtained from the peritoneal cavities of infected gerbils and were then injected intravenously into mice. A sub-periodic, nocturnal microfilaraemia was produced. The level of microfilaraemia was proportional to the number of parasites injected, with approximately 1–3% of microfilariae being found in the peripheral circulation. The duration of microfilaraemia was proportional to the number of parasites injected; it subsided by 30 days after injection of 104 microfilariae but was still present at a low level 120 days after injection of 2 × 105 microfilariae. A transient splenomegaly developed after injection of microfilariae. Histopathological examination revealed large numbers of microfilariae free in the lumens of pulmonary small blood vessels and without any accompanying inflammatory reaction. Lesser numbers of microfilariae were seen in the cardiac blood and hepatic and renal blood vessels for the first few days after injection. There was cellular proliferation in the splenic white pulp and vascular congestion of the red pulp. Microfilariae labelled with 51Cr were injected intravenously; 57% of radioactivity was found in the lungs, 8·5% in the liver and 2·9% in the spleen. Mice developed immediate hypersensitivity reactions to B. malayi antigen by 4 weeks after injection, but Arthus and delayed hypersensitivity reactions were not seen at any time. When mice which had been injected 5 months previously were challenged with a 2nd injection of microfilariae, there was an accelerated clearance of parasites over 2 weeks and a marked peripheral blood eosinophilia developed. In contrast with natural infections, in which the continuous production of microfilariae complicates assessment, this model provides a system in which factors controlling the circulation of microfilariae in the bloodstream can be studied independently.

The egg of Syphacia obvelata is a flattened elipsoid. The egg-shell consists of 5 layers: external uterine layer, internal uterine layer, vitelline layer, chitinous layer and lipid layer. An operculum is present at one pole of the egg. The opercular groove consists of a break in the uterine layers and the modification of the chitinous layer by the deposition of lipoprotein material. On the curved side of the egg the uterine layers are modified to form alternate ridges and depressions. Discrete spaces are present in the internal uterine layer between the ridges. These are open to the exterior via pores in the external uterine layer. The structure of the uterine layers is quite different on the flattened side of the egg. The morphology of the reproductive system and the formation of the egg-shell is described. It is suggested that the complex structure of the uterine layers of oxyurids forms by a self-assembly process.

Amastigotes of 2 Leishmania species are reported from the Pakistani lizards Teratoscincus scincus (Gekkonidae) and Agama agilis (Agamidae) collected in western Baluchistan and north-central Sind, respectively. Parasites were seen only in blood cells, primarily within thrombocytes, and were detected on smears of peripheral blood. Slides made at 3-day intervals for 38 days from an infected hatchling T. scincus demonstrated an increase with time in the mean number of amastigotes/infected thrombocyte. No evidence of an infection focus in fixed cells of the viscera was found. It is suggested, in view of reports of amastigotes in circulating blood cells of hosts belonging to 5 genera, collected in 5 countries from India to France, that saurian Leishmania may behave simply as parasites of circulating blood cells, thus illustrating an early stage in the adaptation of leishmanias to the vertebrate host.

The distribution and abundance of the cestode Bothriocephalus rarus infecting red-spotted newts, Notophthalmus viridescens, were studied for over 1 year in an artificial pond in West Virginia, USA. Macrocyclops ater and Mesocyclops edax were found to harbour B. rarus procercoids experimentally, and M. ater was confirmed as a natural intermediate host for this cestode. The recruitment of B. rarus by newts coincided with the peak density of M. ater, indicating that B. rarus may have adapted itself to the copepod life-cycle to increase the probability of successful parasite transmission. Infection was more frequent and of higher intensity in larval newts. The observed frequency distribution of cestodes in all segments of the newt population closely fits the negative binomial. The data indicated seasonal cycles in population structure, intensity of infection, maturation and reproduction. Recruitment of worms by adult newts occurred in June, July and August, at the time when the density of adult worms decreased.

The mean parasite burden of a population of Tribolium confusum is shown to rise to a plateau as the exposure density of infective eggs of Hymenolepis diminuta increases. The level of this plateau is shown to be dependent on the nutritional status of the host population, being depressed from approximately 18 cysticercoids/beetle in hosts which have been starved prior to experimentation, to approximately 2 cysticercoids/beetle in satiated hosts. A simple model is used to describe the shape of this infection functional response in terms of the predator–prey interaction between hosts (T. confusum) and parasite infective stages (H. diminuta eggs). The distribution of successful infections/host is shown to be over-dispersed, even when hosts are exposed to infective stages arranged in a uniform spatial pattern. The over-dispersion of parasite numbers/host is shown to become more severe as the spatial pattern of infective stages changes from under-dispersed, through random, to over-dispersed. Experimental results are discussed in relation to the dynamics of parasite–host interactions, in which infection takes place by host ingestion of a free-living infective stage.

Two 8-week experiments were conducted to determine the relationships between nutritional stress and pathophysiological changes in male Holstein calves infected with Sarcocystis cruzi. Calves were infected by oral inoculation with 200000 S. cruzi sporocysts. In the first experiment weight gain reduction was greatest in inoculated calves during weeks 4 and 5 after inoculation. Feed intake was reduced during the 5th week. Erythrocyte count was reduced during week 5 and haemoglobin was reduced during week 6. The 24-h excretion of urinary and urea nitrogen from the inoculated calves was increased by treatment. In the 2nd experiment, both the feed-restricted and inoculated calves lost weight during weeks 4 and 5; feed intake was lower from week 5 to 8 inclusive. Urine volume from inoculated calves was lower during week 8. Lower urine excretions of sodium and potassium resulted from S. cruzi inoculation. There was a non-significant trend for higher urinary zinc excretion in the inoculated group during week 4. Urine nitrogen excretion from inoculated calves was higher during weeks 4 and 6; nitrogen excretion from the feed-restricted group was higher during week 6. The urinary excretion of 3-methylhistidine from the inoculated calves was higher during week 4 and excretion of guanine was higher during weeks 4, 5 and 8. S. cruzi has several specific pathophysiological effects on calves beyond those induced by nutritional stress.

Infective larvae of homogonic Strongyloides ratti grown in faecal culture with 32P or 75Se acquired a significant amount of radioactivity which was firmly attached to them. Heating removed most of the 32P but left 75Se in place. Subcutaneous injection of virgin and nursing mother rats with living and heat-killed radioactive larvae resulted in a pattern of labelling in the small intestine of injected animals and, in the case of 75Se, those of suckling pups, which can only be explained if labelled worms follow the natural migratory routes. The use of this tool in migratory studies is discussed, with precautions to allow for flaws in the technique.

Antigenic relationships of 4 stocks of Trypanosoma congolense from different parts of Africa were examined by immunofluorescence (IFAT) and neutralization tests. Antisera to each stock were obtained from rabbits infected with trypanosomes transmitted by Glossina morsitans. Trypanosomes for use as antigens were obtained from local skin reactions developing on rabbits infected with 2 of the stocks. Using the IFAT and antisera at end-point dilutions approximately 40% of the trypanosomes fluoresced strongly and a further 30% less intensely with homologous antisera, indicating antigenic heterogeneity among the trypanosomes developing in the skin. Using antisera at low dilutions some samples gave cross-reactions with trypanosomes of heterologous stocks, but at higher dilutions there were no cross-reactions with either of the antigens. The lack of cross-reactions at high antiserum dilutions was interpreted as indicating antigenic differences between the 4 trypanosome stocks. Using neutralization tests only homologous antisera reduced the infectivity of trypanosome suspensions. Overall, these observations indicated that there were at least 3 different strains of T. congolense among the 4 stocks examined.

The parameters involved in the induction of resistance against Schistosoma mansoni by injection of irradiated, artificially transformed schistosomula were studied in mice. Single intramuscular injections of 500 schistosomula exposed to radiation doses in the range 2·3 to 160 krad. resulted in significant protection (in the range 20 to 50% as assessed by reduced worm burdens) against a challenge infection administered at intervals from 3 to 24 weeks post-vaccination. However, schistosomula irradiated with 20 krad. consistently resulted in better protection than those exposed to either higher or lower radiation doses despite the persistence of stunted adults from the infections irradiated with 2·3 krad. Vaccination with 40 krad. schistosomula resulted in significant protection in terms of reduced worm and tissue egg burdens and increased survival following lethal challenge. Varying the number of irradiated schistosomula, the frequency and route of their administration, the site of challenge and the strain of host all failed to enhance the level of resistance. However, percutaneously applied, irradiated cercariae were found to be more effective in stimulating resistance (60%) than intramuscularly injected, irradiated schistosomula (40%).

The development of Filaroides hirthi has been shown to proceed through all 5 stages in the lungs of experimental dogs and the attendant morphological changes are described and illustrated. These observations are interpreted as proof that the unhatched larva of F. hirthi is the 1st stage and that the 1st stage is infective. Moults occurred at approximately 1, 2, 6 and 9 days after infection in the lung tissue. Larvae also developed in vitro to the doubly ensheathed 3rd stage which is the stage achieved by heteroxenous metastrongyloids in their molluscan intermediate hosts. The arrival of 1st-stage larvae in the lungs as early as 6h after oral administration and the anatomical distribution of larvae during the first 24 h indicates that larvae are transported from the alimentary tract to the lungs by way of the hepatic portal circulation, the mesenteric lymphatic drainage, or both. First-stage larvae and eggs containing 1st-stage larvae, when injected into the jugular vein, developed into adult F. hirthi lung- worms in the usual 5 week pre-patent period. This was interpreted as demonstrating that the canine alimentary canal, although undoubtedly the normal portal of entry, is not a site of reactions indispensable to the activation or development of the larvae. Eggs containing 1st-stage larvae, when introduced by jugular and portal venous and mesenteric lymphatic routes, yielded equivalent numbers of adult lungworms indicating that neither the hepatic capillary bed nor the mesenteric lymph nodes presented quantitatively significant impediments to their passage. The question is raised whether the mesenteric lymphatics really are the exclusive migration pathways of certain other species of metastrongyloid larvae. The suggestion is made that conventional studies based on the anatomical/time distribution of larvae be supplemented by intravascular experiments like those described in this report, to provide an independent line of evidence.

Quantitative histological methods were applied in a study of the migratory route of schistosomula within the definitive mammal host. The observations are consistent with an entirely intravascular mode of migration in the direction of blood flow. They do not support a trans-diaphragm route. Schistosomula can be identified in low numbers in systemic organs, in the left side of the heart and in the venous compartment of the pulmonary circulation. They were not observed penetrating the diaphragm or the capsule of the liver. No histopathological evidence was found to suggest that the tissue responses of previously unexposed hosts affect migrating schistosomula at any stage during migration. These non-specific tissue responses were marked only in the skin phase of migration.

Administration of the immunostimulants Corynebacterium parvum, Bacillus Calmette-Guérin (BCG) or Bordetella pertussis prior to, or at the same time as, challenge with Trypanosoma congolense significantly increased survival times in mice, both of trypano-susceptible (A/J) and trypano-resistant (C57Bl) strains. The increased survival time was associated with significant alterations in parasitaemia, which included lengthening of the pre-patent period, a delay in the time taken to reach the first peak of parasitaemia and a reduction in the level of parasitaemia. Similar results were obtained when these strains of mice were challenged with Trypanosoma brucei following pre-treatment with C. parvum. Thus, by the use of immunostimulants it was possible to reduce the susceptibility of mice to trypanosomiasis and the hope is that this can also be achieved with domestic livestock.

Infections with irradiated Schistosoma mansoni were established by intramuscular (i.m.) injection of mechanically transformed schistosomula. A dose of 2·3 krad. allowed persistence of a small proportion of worms to adulthood, and of these survivors the majority of the female worms were sexually sterile. However, a small proportion of 2·3 krad. irradiated females and a larger proportion of similarly irradiated males were capable of pairing successfully with non-irradiated partners. Radiation in the range 2·3 to 10 krad. resulted in slightly reduced peak recoveries from the lungs while 20 krad. resulted in a much reduced and 40 krad. a virtual absence of survival to the lung stage. Increasing doses of radiation in the range 2·3 to 10 krad. resulted in successively fewer parasites reaching the liver. Thus, the major sites of the radiation-induced mortality appeared to be as follows: 2·3 krad., mainly in the liver; 4 krad., in the lungs and liver; 10 krad., mainly in the lungs; 20 krad., at the injection site and in the lungs and 40 krad., mainly at the injection site. The infections studied here showed reduced survival following exposure to high doses of radiation compared with the infections, established as percutaneously applied cercariae, which have been reported by other workers. Possible reasons for the disparity are discussed.

The cell-mediated resistance of human leucocytes to Trypanosoma dionisii, a bat parasite related to T. cruzi, was investigated. Human peripheral blood neutrophils and monocytes were cytotoxic to T. dionisii as assessed by electron microscopy and by induction of 99mTc release from trypanosomes pre-labelled with [99mTc]pertechnetate. The enhancement of cytotoxicity by specific antiserum varied considerably from one individual to another. Neither blood lymphocytes nor blood eosinophils induced 99mTc release from T. dionisii. The trypanosomes were readily phagocytosed by neutrophils and monocytes even in the absence of added antiserum but the rate was enhanced when antiserum was present. Eosinophils also phagocytosed T. dionisii but only in the presence of antiserum. Investigation by electron microscopy revealed that T. dionisii is rapidly destroyed in the phagocytic vacuole of neutrophils and monocytes and by eosinophils. Phagocytosis, ultrastructural damage and induction of 99mTc release occurred more rapidly in neutrophils than in monocytes.

Eimeria flavescens and E. irresidua from the domestic rabbit are redescribed. The relatively smaller ovoidal oocysts of E. flavescens which measure on average 31·7 × 21·4 μm, possess a wide micropyle at the broad end. First-generation schizonts of this species develop deep in the glands of the lower small intestine. Merozoites migrate to the caecum and colon where second, third and fourth-generation schizonts develop in the superficial epithelium followed by the fifth-generation schizonts and gametocytes which form in the glands. In young Dutch rabbits E. flavescens is very pathogenic; low doses of oocysts produce a severe enteritis with high mortality and morbidity. The larger, broadly ellipsoidal oocysts of E. irresidua measure on average 38·4 × 23·2 μm and often possess a very small cryptic oocyst residuum. The endogenous stages develop in the small intestine only; first-generation schizonts in the glands and second-generation schizonts in the lamina propria whilst third and fourth-generation schizonts and gametocytes develop in the villous epithelium. E. irresidua is not pathogenic in young Dutch rabbits; even heavy infections produce only a transient pause in weight gain.

The paper draws together a large and scattered body of empirical evidence concerning the prevalence of snail infection with schistosome parasites in field situations, the duration of the latent period of infection in snails (and its dependence on temperature), and the mortality rates of infected and uninfected snails in field and laboratory conditions. A review and synthesis of quantitative data on the population biology of schistosome infections within the molluscan host is attempted and observed patterns of infection are compared with predictions of a schistosomiasis model developed by May (1977) which incorporates differential snail mortality (between infected and uninfected snails) and latent periods of infection. It is suggested that the low levels of prevalence within snail populations in endemic areas of schistosomiasis are closely associated with high rates of infected snail mortality and the duration of the latent period of infection within the mollusc. In certain instances, the expected life-span of an infected snail may be less than the duration of the latent period of infection. Such patterns generate very low levels of parasite prevalence. A new age prevalence model for schistosome infections within snail populations is developed and its predictions compared with observed patterns.

The implications of this study of observed and predicted patterns of snail infection within molluscan populations are discussed in relation to the overall transmission dynamics of schistosomiasis.

Eight species of coccidia were recognized in 596 faeces samples from 3 commercial rabbitries in South East England. The level of infection was related to management methods and at one site it was reduced by an outbreak of mucoid enteritis and/or its treatment with oxytetracycline. In samples from rabbits managed conventionally in wire cages over droppings-pits, 96% contained oocysts and of these, 60% had 1000–10000 oocysts/g. Mixed infections were common, 67% of the animals carrying 2–4 different species. Eimeria media, E. magna and E. perforans occurred most frequently; E. coecicola, E. irresidua and E. flavescens were less common and E. intestinalis and E. piriformis were relatively rare. E. stiedai was not recorded.

A microsporidian parasite, Nosema eurytremae, was fed to Lymnaea trunculata infected with Fasciola hepatica. Microsporidian infection of the snail was always light, though spores were present in all tissues, but the rediae became heavily infected in the parenchyma. The proportion of infected rediae increased with time and was especially high in ageing infections. Although rediae were only lightly infected when examined before cercarial release began, the infections became progressively heavier, so that towards the end of the life of the snails a high proportion of rediae were totally destroyed and contained no cercariae. Cercarial output was significantly depressed over a long period of active shedding by snails which had been hyperinfected with 1 × 10 6 spores and some of the liberated cercariae were themselves infected.