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.

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.

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.

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.

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.

Isoelectric focusing studies on enzyme variation between populations of the snail Bulinus senegalensis revealed that parasitic infections in the snails contributed additional bands of enzyme activity, particularly in the glucose phosphate isomerase (GPI) and malate dehydrogenase (MDH) systems. The patterns due to the parasite enzymes were, in most cases, clearly distinct from those of the host and different from each other. Parasites encountered included Schistosoma haematobium, S. bovis, Paramphistomum microbothrium, another amphistome probably belonging to the group which infect amphibians, Echinostoma revolutum, another echinostome (probably Echinoparyphium sp.), strigeids, xiphidiocercariae (these were resolved into 3 distinct types by the enzyme data) and ciliate protozoa. The 7 host populations which were examined showed marked differences in both the prevalence and variety of their parasitic infections and these variations were tentatively related to environmental differences in their respective habitats and to the nature of human contact patterns. Seasonal changes in the parasite fauna were also noted and some of the implications of the parasite load on the host population are briefly mentioned.

Trypanosomes were studied from the blood of 6 species of small British fish caught in the River Lee. Morphologically the trypanosomes from the blood of Nemacheilus barbatulus L., Phoxinus phoxinus L., Cottus gobio L., Gobio gobio L., Gasterosteus aculeatus and Pungitus pungitus L. were indistinguishable. Cross-transmission experiments using syringe passage of culture forms and also the leech vector Hemiclepsis marginata showed that the trypanosomes were not host specific. The isoenzyme patterns of culture forms from N. barbatulus and P. phoxinus were identical for 11 enzymes studied. The trypanosomes from the 6 species of fish previously classified as separate species are, on the basis of these results, regarded as a single species, Trypanosoma cobitis (Mitrophanow, 1883).

The prevalence of Trypanosoma cobitis was recorded in species of small fish from the River Lee at Enfield Lock during 33 months of sampling. Trypanosome infections were more numerous during the summer months with 50–70% of bottom-living ‘inactive’ fish, bullheads (Cottus gobio), stone loach (Nemacheilus barbatulus) and gudgeon (Gobio gobio) infected, compared to only 6 and 18% of the more active pelagic fishes, stickle-backs (Gasterosteus aculeatus) and minnows (Phoxinus phoxinus).

The freeze fracture technique has been used to study the internal cyto-architecture of the surface membranes of the parasite and erythrocyte in Plasmodium knowlesi infections. Six fracture faces, derived from the plasma membrane and 2 pellicular membranes, have been identified at the surface of the free merozoite. The apposed leaflets of the 2 pellicular membranes show the characteristic features of E fracture faces, a result compatible with the view that the pellicular membranes line a potential cisterna. There is evidence to suggest that there may be changes in the distribution and density of the integral proteins in the merozoite plasma membrane at invasion. Furthermore, vesicles consisting of stacked membranes occur within and around the erythrocyte invagination at invasion; it is suggested that these vesicles are released from the merozoite rhoptries. Formation of the parasitophorous vacuole is accompanied by dramatic changes in the density and distribution of intra-membraneous particles (IMP) in the vacuolar membrane. Initially there is a great reduction in particle numbers, but subsequently the particles reappear and show reversed polarity. The possible causes and implications of these changes are discussed. The intra-erythrocytic parasite synthesizes new transmembrane proteins as development proceeds, and the trophozoite and schizont stages of development are characterized by the appearance of circular, particle-free regions in the parasite plasmalemma. There is a decrease in the density of transmembrane proteins in the erythrocyte plasma membrane during parasite maturation, and the P face IMP show the characteristic features of aggregation.

Attachment, engorgement and subsequent development of successive infestations of Ixodes trianguliceps larvae and nymphs on natural hosts, Apodemus sylvaticus, and unnatural hosts, laboratory mice, are compared. In laboratory mice, primary infestations above a threshold level of about 10 ticks elicit an immunological response which reduces, in a density-dependent manner, the rate of successful tick engorgement during subsequent infestations. In contrast, in A. sylvaticus successive infestations of larvae result in unchanged or slightly improved survival through to nymphs. The relevance of these results to the concept of host-parasite co-evolution and to tick population regulation is discussed.

An experiment is described in which the effects of age, previous infection, pregnancy and lactation on some reactions of cattle to infection with Ostertagia ostertagi were studied. It was found that an acquired resistance to the establishment of worms developed more rapidly in 20-month-old heifers than in calves, that it was unaffected by pregnancy of the host but that it was largely lost by heifers in early lactation. The rate at which populations were turned over, i.e. the mean life-span of worms through the late 4th and 5th stages was unaffected by the factors studied. Although, in the conditions of the experiment, development of the worms was not arrested in susceptible calves, both age of the host and its previous experience of infection were significant causes of arrest, and in previously infected 20-month-old cattle 86% of the worms of a challenge infection were arrested. Pregnancy did not affect the proportion of worms that was arrested but in lactating heifers only marginally more worms were arrested than in calves. Worms that were not arrested grew more rapidly in calves and in lactating heifers than in empty heifers or those in mid-pregnancy.

A comparative study has been made of the factors influencing the assessment of anticoccidial potency in vitro against Eimeria tenella using established anticoccidials and arprinocid and some of its analogues. Drugs whose potency depended upon medium composition were amprolium, lasalocid and halofuginone. There was a difference in strain sensitivity with robenidine. Host cell type had an important effect on potency of monensin, decoquinate, arprinocid and its analogues. Arprinocid was active in chick liver cell systems but totally inactive in chick kidney cell systems, although its N-oxide was active in both cell types. Arprinocid-containing medium, conditioned by supporting the growth of chick embryo liver cell cultures, had an anticoccidial effect on E. tenella, growing in chick kidney cells. It is deduced that the anticoccidial activity of arprinocid in the chick is due to a metabolite.