The proteinases of Trichomonas vaginalis have been analysed by electrophoresis in polyacrylamide gels containing denatured haemoglobin. Seven bands of activity were detected indicating multiple proteinases. All of the enzymes were stimulated by 1 mM dithiothreitol and had inhibitor sensitivities characteristic of cysteine proteinases. The enzymes differed significantly, however, with respect to pH optima and relative sensitivities to inhibitors.

Attenuated lines of Eimeria acervulina were isolated between 62 and 72 h post-infection from the Houghton (H) strain. The inoculation of small numbers of oocysts of precocious (HP) lines gave substantial protection to Light Sussex chicks kept on litter against challenge with the virulent H strain. The precocious trait of the 72 h HP line was shown to be stable because the kinetics of oocyst production remained unaltered after 9 consecutive passages through birds in which only late developing oocysts were used for passage. The precocious 62 h HP line was subjected to further selection for 5 consecutive passages. The resultant (62 A HP) line was shown to be more attenuated than the 72 h HP line but remained capable of immunizing chicks against challenge with the H strain.

Freshly isolated oocysts were sporulated at 29°C with constant aeration. Minimum sporulation time was determined by dosing the developing oocysts at intervals of 1 h to successive groups of chickens to assess the first occurrence of infective forms. Microscopic analysis of the sporulating oocysts allowed calculation of the time required for 50% of the oocysts to complete the process. This was considered to be the most accurate determination and the most useful for comparative purposes. For 7 species of coccidia in the fowl, the 50% sporulation times were as follows: Eimeria acervulina 11·4 h; E. brunetti 38·3 h; E. maxima 38·1 h; E. mivati 19·0 h; E. necatrix 19·7 h; E. praecox 24·8 h; E. tenella 21·2 h.

The development of microgamonts was studied in the small intestine of a 2-day-old cat after oral inoculation with 107 bradyzoites of Sarcocystis hirsuta. The cat was given bradyzoites in 2 doses at 2 h apart and killed 4 h later. Gamonts were seen in parasitophorous vacuoles of goblet cells in the epithelium of the small intestine. All stages of microgametogony, from the single nucleated meront to fully developed gametes, were seen. Microgamonts measured 6·9 × 6·0 μm and macrogamonts measured 6·0 × 5·0 μm.

By selection for early development of oocysts during serial passage through chickens the pre-patent period of the Houghton (H) strain of Eimeria mivati ( = mitis) was reduced by over 20 h. The precocious parasite is less pathogenic than the H strain and has a reduced reproductive potential. Chicks inoculated with the precocious parasite were protected against challenge with the H strain. The pathogenicity, immunogenicity and reproduction of precocious E. mivati and an attenuated egg-adapted line of E. mivati were compared. Although the precocious parasite produced more oocysts than the egg-adapted parasite in chickens, neither parasite induced loss of weight and both were immunogenic.

The serial passage of 3 strains of Eimeria from the chicken in embryonating turkey embryos is reported here. All 3 strains increased their reproduction in turkey embryos; reproduction being equal to or better than the same lines maintained in chicken embryos. Pathogenicity and immunogenicity trials conducted in chickens using these 3 strains of Eimeria showed that all 3 turkey embryo-adapted lines were significantly less pathogenic than the parent lines. Only 1 of the lines (E. tenella (PS) turkey embryo-adapted) maintained its immunogenic potential

The proteins in the soluble and membrane fractions of Plasmodium lophurae, separated by SDS-PAGE and stained with Coomassie blue, were distinct from one another and different from the soluble and membrane proteins of the host cell, the duckling erythrocyte. Pulse-labelling of malaria-infected red cells containing schizonts or trophozoites with [14C]isoleucine, followed by parasite isolation, SDS-PAGE and autoradiography, showed a pattern of incorporation into soluble proteins identical to the banding pattern of gels stained with Coomassie blue. On autoradiograms, the banding patterns of the membrane proteins of trophozoites and schizonts labelled with [14C]isoleucine were similar. No qualitative differences in the soluble proteins of trophozoites and schizonts were apparent when isoleucine was used as the radiotracer; however, the banding pattern of membrane and soluble proteins labelled with [14C]isoleucine were distinct from each other. The banding pattern of soluble proteins, after staining with Coomassie blue, was distinctly different from the array of bands seen after autoradiography of parasites derived from infected cells pulse-labelled with [14C]histidine. Only small amounts of contaminating haemoglobin in the soluble protein fraction of parasites were labelled with histidine; the membrane fraction, on the other hand, had high incorporation into a single 43 kDalton band, the histidine-rich protein. No band differences, either in the membrane or soluble fractions, were observed in autoradiograms after pulse-labelling trophozoites and schizonts with [14C]histidine. The most consistent and distinct differences in protein synthesis were seen in P. lophurae pulse-labelled with [14C]proline. Autoradiograms of both soluble and membrane fractions showed the presence of bands unique to the trophozoite and schizont–stage-specific proteins.

Seven microfilaraemic and five amicrofilaraemic cats which had been repeatedly infected with Brugia pahangi were challenged along with normal cats 28, 14 and 1 day before autopsy. The lymphatics of the amicrofilaraemic cats contained no female adult worms originating from the repeat infections and only two adult males (both from the same cat). Only 5·2 % of the worms in the control cats were recovered from the amicrofilaraemic cats. Most of the challenge worms were killed in the first 24 h. The microfilaraemic cats all contained fertile adult male and female worms derived from the repeated infection but in such low numbers as to indicate considerable resistance to infection. Compared to their controls 26·4% of the challenge worms were recovered. Analysis of the life-cycle stages recovered showed that in both groups there was attrition of all stages and that although a number of worms reached L5 these were all killed later in the amicrofilaraemic cats.

Eimeria mitis and E. acervulina affect different areas of the intestine. Young chicks received E. mitis with or without E. acervulina in doses ranging from 2 to 128 × 104 oocysts. Mixed infections produced significantly more severe effects on growth than single infections of either species. The ratio of villus height to total mucosal thickness was reduced throughout the intestine in birds which received the mixed infection. E. mitis and E. brunetti affect the same area of the intestine. Single infections of E. brunetti in a range of doses from 1.25 to 20 × 103 oocysts produced a similar reduction in growth. When 20 × 103 oocysts of E. mitis were inoculated simultaneously, the effect on growth was not so severe and the oocyst output of both species was reduced. E. brunetti, superimposed 1, 2 or 3 days after an E. mitis infection, produced no greater effect on growth than E. mitis alone. Similarly, infections of E. mitis, superimposed on E. brunetti had no greater effect on growth than the latter species alone. Oocyst output of E. brunetti in superimposed infections was not reduced but peak output was delayed.

In order to determine whether a given H-2 haplotype has similar effects on responses to schistosomiasis mansoni on different genetic backgrounds, mice of 2 pairs of congenic strains (H-2b and H-2k on BALB/c and C57BL/10 backgrounds) were infected. Worm burdens, mortality, splenomegaly, tissue and faecal egg counts, and antibody titres to worm and egg antigens were measured. The genetic background had a major effect on the genesis of splenomegaly, on the deposition of eggs in the spleen, the maximum faecal egg count, the antibody titre to egg and worm antigens and the rate of generation of antibody response. The H-2 haplotype was shown to consistently influence the maximum faecal egg count and the antibody titres. Worm burden was not influenced by genetic differences between strains and mortality differences were not significant. The data presented here indicate that the effect of the major histocompatibility complex on responses to infection is greatly influenced by the genetic background on which it is expressed.

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.

Six samples of rodent blood infected with malaria parasites were isolated from Cameroon. Of these, 2 contained mixed infections of Plasmodium vinckei and P. yoelii, 3 contained P. vinckei alone and 1 P. chabaudi alone. Each isolate was cloned and the resulting lines examined for morphology of blood and mosquito forms, and for electrophoretic variation in enzymes. The P. chabaudi and P. yoelii lines were morphologically and enzymically identical to isolates of the Central African Republic. Similarly, 1 P. vinckei line was identical to an isolate of the Central African Republic. The remaining 4 P. vinckei lines showed considerable variation, some enzymes being like those in isolates of surrounding regions, while others were unique to Cameroon.

The development of second generation schizonts of Eimeria necatrix and E. tenella was studied with the electron microscope. Invasion of the crypt epithelial cells by merozoites of the first generation schizonts caused changes in the morphology of the infected cells and stimulated their migration into the lamina propria through breaks which appeared in the basement membrane of the crypts. Second generation schizonts developed in the lamina propria within these crypt cells whose epithelial origin was confirmed by their interconnection by desmosomes and tight junctions and by their possession of characteristic microvilli. A comparison is made between this invasion of the lamina propria by parasitized cells and invasion of connective tissue by malignant epithelial cells; the possible mechanisms involved are discussed.

The commonly held view that the kinetoplastida, and in particular trypanosomes, are asexual is largely derived from the principle that an organism is asexual until proved sexual. If the basis for this view is examined in detail, it largely arises from the lack of morphologically distinguishable gametes, the difficulties encountered in visualizing chromosomes and a few experiments, using drug-resistant stocks, in which no recombination between stocks could be demonstrated. While it is clear that these organisms are able to reproduce asexually, the existence of a sexual cycle was, until recently, an entirely open question. The early work strongly suggests that any sexual process (in the species examined extensively at the morphological level) does not involve classical well-differentiated gametes and so must involve fusion of morphologically very similar cells. These findings taken together with the inability to visualize chromosomes and thereby identify meiosis, mean that classical methods are unable to detect any sexual process even if it did occur. This review examines the evidence provided by the experimental approaches which have been applied recently to the question of kinetoplastid sexuality. These approaches include isoenzyme studies and the analysis of possible genetic exchange by the use of selective markers (e.g. drug resistance). The results which these techniques have produced make it clear that the kinetoplastid protozoans cannot be regarded as a totally asexual group of organisms.

The interactions in vivo between Plasmodium berghei sporozoites and Kupffer cells in rat livers were studied by transmission electron microscopy. By 10 and 15 min after inoculation, sporozoites were both free in the liver sinusoids and inside endocytotic vacuoles of the Kupffer cells. The latter cells were very active in phagocytosing sporozoites, bacteria and red blood cells. The sporozoites retained their integrity inside the endocytotic vacuoles and no signs of lysosomal digestion were observed. Sporozoites seen within endocytotic vacuoles 1 h after inoculation were still morphologically intact, although bristle-coated vesicles fused with the vacuole membrane. Evidence is presented which suggests that Kupffer cells transport sporozoites towards the space of Disse and adjacent hepatocytes. No sporozoites were seen to penetrate an endothelial cell or its narrow fenestrae. It is proposed that Kupffer cell passage, rather than gaps in the sinusoidal lining, represents the normal route that sporozoites take to circumvent the endothelial barrier. The localization of exo-erythrocytic forms was made easier by the use of Brown Norway rats in which many more parasites develop than in the Wistar rats. The distribution pattern of the parasites was found to be mainly around the ‘periportal’ zones of the acini of liver tissue.

The microfibrillate component of the laminated layer of Echinococcus granulosus equinus contains, except for the zone adjacent to the germinal layer, aggregates of electron-dense bodies displaying a sub-structure of electron-lucent spheres. The tegumentary syncytial cytoplasm contains randomly distributed electron-dense granules, many occurring near the apical plasmalemma, although exocytosis was rarely seen. Granules, similar in size and sub-structure to the bodies of the aggregates, also occur in the internuncial connexions and tegumentary cytons, suggesting that they may be produced in the cytons and released into the laminated layer via the internuncial connexions and tegumentary cytoplasm. Cysts incubated for 0·5–2·5 h in serum- and non-serum-containing media showed differences from non-incubated cysts. The distal half of the syncytium contained a progressive increase in the number of granules (distal:proximal 1·9:1 at 2·5 h; cf. 1:1 in non-incubated cysts), and exocytosis of granules into the laminated layer adjacent to the cyst had occurred. This cannot be attributed wholly to serum proteins. Cysts incubated for 21 h appeared ‘normal’, suggesting re-establishment of an equilibrium. Since the matrix of the laminated layer is considered homologous to the glycocalyx of other cestodes, the possible protective role played by the granules/bodies, characteristic of Echinococcus spp., is discussed.

The number of schistosomula in lungs was determined by compressed organ autoradiography at intervals up to 14 days after exposure of mice to 75Se-labelled cercariae by tail immersion. Probit analysis of compressed lung autoradiogram focus counts, expressed as percentages of initial infection level, yielded estimates of the average time of arrival, peak accumulation in the lungs and average time of departure of schistosomula: 4·5±0·87, 6·3 ± 0·45 and 11±0·58 days, respectively. At peak accumulation 92±3·5 % of the initial number of schistosomula were found in the lungs. It thus appears that little or no significant attrition of schistosomula occurred in the skin and, instead, that most of the 50–70% of penetrant cercariae that fail to reach adulthood are lost somewhere between the pulmonary and hepatic phases of development. Loss of 75Se label from schistosomula during the first 14 days was exponential, with an average half-life of 4·5±0·81 days. However, the high sensitivity of autoradiography tended to compensate for this rather rapid rate of label loss. It was pointed out that autoradiographic detection of schistosomula as discrete loci of radioactivity can also be expected to overcome the problem posed by the accumulation in such tissues as liver and kidney of 75Se label that has become separated from larvae.