Classical studies of non-immune individuals infected with Plasmodium falciparum reveal that the infection may be regulated for long periods at a relatively stable parasite density, despite the enormous growth potential of a parasite that continually replicates within host erythrocytes. This suggests that the parasite population may be controlled by density-dependent mechanisms, and in theory the most obvious of these is competition between parasites for host erythrocytes. Here we evaluate the role of this mechanism in the regulation of parasitaemia, by modelling the basic population interaction between parasites and erythrocytes in a form that allows all the essential parameters to be estimated from clinical data. Our results show that competition cannot account for the total regulation of P. falciparum, but when combined with immune mechanisms it may play a more important role than is generally supposed. Further analysis of the model indicates that in the long term, parasite replication at low parasite densities can contribute significantly to the high degree of anaemia observed in natural infection, a conclusion which is not obvious from simple clinical observation.

Small vessels showing inflammatory cell infiltrates were invariably observed in the vicinity of intact Toxoplasma tissue cysts within the brains of mice congenitally infected with the protozoan. Lymphocytes were observed in intimate contact with the luminal aspect of the endothelium, penetrating into the thickened basal lamina and in the perivascular area, which also contained macrophages and neutrophilic granulocytes. Rarely, lymphocytes were observed attached to the outer membrane of the host neurone which contained a Toxoplasma tissue cyst and within the inflammatory infiltrate associated with a disintegrating cyst. An hypothesis is presented which combines these morphological observations to explain the events associated with tissue cyst associated recrudescence of latent Toxoplasma infection in mouse brain.

Isoenzyme typing was used to study a number of oocyst isolates of Cryptosporidium parvum from different geographical locations and of human or animal origin. All isolates showed identical enzyme motility when glucose phosphate isomerase (GPI; 23 isolates tested) or lactate dehydrogenases (LDH; 20 isolates tested) was assayed. However, two isoenzyme forms were observed with phosphoglucomutase (PGM; 9 animal isolates showed one form, while 8/9 human isolates showed a second form) and hexokinase (HK; 4 human isolates showed one form and 6 animal isolates showed a second form). Thus, PGM and HK each exhibit 2 isoenzymes corresponding to 2 parasite populations associated with separate hosts. The data from this study, plus supportive evidence obtained by different methods and by independent researchers, lend support to the hypothesis that separate cycles of transmission of C. parvum may exist within human and animal hosts.

Finding and recognition of snail second intermediate hosts was studied in cercariae of 3 echinostome species. The cercariae of the 3 species accumulated in snail-conditioned water (SCW) with 2 types of orientation mechanisms and responded to different small molecular weight (< 500 Da) components of SCW. Pseudechinoparyphium echinatum and Echinostoma revolutum cercariae returned by swimming an arc, when swimming in decreasing concentration gradients of SCW (turnback swimming). The stimulating cues of SCW were identified as hydrophilic organic molecules, probably posessing amino groups. Amino acids contributed to the attractivity of SCW, at least in P. echinatum, but they could not account for the complete attractivity of SCW. Hypoderaeum conoideum were directed chemotactically and swam along increasing concentration gradients of small peptides within SCW, but in decreasing SCW gradients they showed no turn-back swimming. Chemotactic orientation in H. conoideum only started 1 h after emission, which may assist the cercariae to leave the immediate area of their first intermediate host snails and to disperse. Attachments occurred specifically to snail hosts in the 3 species and were stimulated by macromolecular mucus compounds, probably mainly by viscoelastic properties of the mucus. The results of this study show, that host-finding mechanisms and the stimulating host cues of snail invading echinostome cercariae differ considerably from those of schistosome miracidia.

The localization and distribution of SALMFamide immunoreactivity (IR), SI(GFNSALMFamide), in the nervous system of both the adult and larval stages of the trematode Schistosoma mansoni has been determined by an indirect immunofluorescent technique in conjunction with confocal scanning laser microscopy (CSLM). Immunostaining was widespread in the nervous system of adult male and female S. mansoni. In the central nervous system (CNS), IR was evident in nerve cells and fibres in the anterior ganglia, cerebral commissure and dorsal and ventral nerve cords. In the peripheral nervous system (PNS), IR was apparent in nerve plexuses associated with the subtegmental musculature, oral and ventral suckers, the lining of the gynaecophoric canal, and in fine nerve fibres innervating the dorsal tubercles of the male worm. In the reproductive system of male and female worms, Sl-IR was only observed around the ootype/Mehlis' gland complex in the female. Immunostaining was also evident in the nervous system of both miracidium and cercarial larval stages. A post-embedding, IgG-conjugated colloidal gold immunostaining technique was employed to examine the subcellular distribution of SALMFamide-IR in the CNS of S. mansoni. Gold labelling of peptide was localized over dense-cored vesicles within nerve cell bodies and fibres constituting the neuropile of the anterior ganglia, cerebral commissure and nerve cords of the CNS. Antigen pre-absorption studies indicated that the results obtained do suggest S1-like immunostaining and not cross-reactivity with other peptides, in particular FMRFamide.

By incubating total protein extracts of Schistosoma mansoni with 3H-17-β-estradiol and 20-hydroxyecdysone, steroid binding proteins were detected in both male and female worms. The interaction of nuclear proteins with a restriction fragment of the gender and stage-specific gene F-10 was investigated using the ‘band-shift’ technique. Male and female nuclear proteins bound in a distinct way to the fragment of this gene containing putative regulatory consensus motifs. Among the nuclear proteins, only those rich in cysteine residues bound to DNA. In vitro incubation of live worms with the oestrogen antagonist Tamoxifen, altered the pattern of the DNA binding proteins, producing in females a profile similar to that obtained with male worm protein extracts. This effect of Tamoxifen could not be correlated to inhibition of protein biosynthesis. These results suggest that the regulation of transcription of the F-10 gene might involve steroid receptors.

The mouse type 2 Alu (B2) sequence was detected in both DNAs of Schistosoma mansoni and S. japonicum except for the cercarial stage by the polymerase chain reaction (PCR). Using several kinds of mouse STMS (sequence tagged microsatellite site) primer sets, PCR products related to the host were found in the DNAs of S. mansoni as well as of S. japonicum. Products could be detected only in the DNA of S. japonicum using certain STMS primer sets. The fact that no products could be amplified from the DNAs of both parasites when other kinds of STMS primer sets were used suggests unequal incorporation of the host DNA into the schistosomes. Furthermore, the sequence of the N-terminal domain of H-2, the mouse major histocompatibility complex (MHC), was detected in the DNAs from S. mansoni miracidium, male adult and S. japonicum adults, whereas the sequence of the C2 domain of H-2 was found only in the DNAs of S. japonicum adults. This evidence that host DNA sequences, including the class I MHC, exist heterogeneously in the DNAs of schistosomes might provide an important insight for further understanding of host-parasite immune interactions.

We have isolated a cDNA (SKvl. 1) encoding a Shaker-related K+ channel from an adult cDNA library of the human parasitic trematode Schistosoma mansoni. The deduced amino acid sequence (512 aa, 56·5 kDa) contains 6 putative membrane-spanning domains (S1–S6) and a pore-forming domain (H5). SKv1.1 is grouped in the Shaker family, but forms a unique branch within this family, on the basis of dendrogram analysis. SKv1.1 shows significant sequence identity with most other Shaker channels, with 64—74% identity in the core region (S1–S6). It has the highest sequence identity with the K+ channel (Ak01a) from Aplysia. Northern blot analysis detected a single primary transcript of 2·8 kb. Southern blot analysis indicated that SKv1.1 is present as a single copy in the genomic DNA of S. mansoni. Expression of SKv1. 1 in Xenopus oocytes produced a rapidly activating and inactivating outward K+ current which is highly sensitive to 4-aminopyridine, but is insensitive to tetraethylammonium, mast cell degranulating peptide, dendrotoxin and charybdo-toxin. The presence of a Shaker homologue in Schistosoma suggests that Sh subfamilies may exist in other lower invertebrates as well as platyhelminths.

This study analyses the different populations of Trypanosoma brucei spp. which may coexist within the midgut of wild tsetse flies (Stevens et al. 1994). Cloned trypanosome populations characterized by multilocus enzyme electrophoresis (MLEE) were further analysed by the random amplified polymorphic DNA (RAPD) technique, allowing detection of genetic variation at a finer level than that possible by MLEE. Genetic distance matrices derived from the results of each of the two biochemical methods were calculated and compared using a computer program based on the method of Mantel (1967). The observed correlation was used to investigate the degree of linkage disequilibrium (LD) in the data, association between unrelated polymorphic markers providing a measure of the departure from panmixia. The potential of each biochemical method to detect linkage was evaluated by an extended Mantel test. The MLEE/RAPD correlation test evidenced significant LD within the population, suggesting a predominantly clonal method of reproduction for these West African trypanosomes. Analysis of RAPD data by the extended Mantel test also showed significant LD, while the results with MLEE data were less conclusive, providing an indication of the relative potential of the two techniques to detect fine genetic variation.

Studies were conducted on the role of small mammals in maintaining toxocariasis foci in urban, rural and montane biotopes. The lowest relative density of small mammals was recorded in the urban locality and the highest in the rural and montane localities. Anti-Toxocara antibodies were most frequently detected in synanthropic and hemisynanthropic species Mus musculus, Apodemus agrarius and Micromys minutus – 32·0, 30·4 and 25·0%, respectively. The highest seropositivity was found in small mammals from the urban and rural localities – 22·2 and 21·6%, respectively. Toxocara canis was most prevalent in urban stray dogs (75·0%) and least prevalent in foxes from the montane locality (7·0%). The prevalence of Toxocara cati in cats at the urban, rural and montane localities was 66·2, 65·2 and 76·9%, respectively. In clinically healthy human populations, the highest seroprevalence was detected in the rural locality (14·0%). Children of the same area were 3 times more seropositive (12·9%) than those from the urban and montane localities (4·3 and 4·0%). Our studies suggest an important role for small mammals as paratenic hosts – reservoirs of Toxocara larvae – in maintaining toxocariasis foci. In this respect toxocariasis may be classified as an anthropopurgic focal zoonosis.

We report experimental data comparing the effects of Mesostigmatid mites and Ischnoceran lice on the reproductive performance of a single group of captive rock doves (Columba livia). Several components of host reproductive success were compared for the two groups, including number of eggs laid, hatching success, nestling growth rates, fledging success, post-fledging body mass and survival. Adult body mass and survival were also compared. There was a dramatic difference in the effects of the mites and lice. The former drove host reproductive success to zero, mainly by agitating adults and causing them to incubate eggs less faithfully. Nestling growth rates and post-fledging survival were also significantly reduced by mites. Lice showed no effect on reproductive success whatsoever, even though the feather damage they cause is known to have energetic consequences (Booth, Clayton & Block, 1993). Neither parasite had a significant effect on adult birds. Although Ischnocera are found on most species of birds, our results for lice constitute the first experimental test of the impact of Ischnocera on avian reproductive success (preliminary report by Clayton & Tompkins, 1994). We discuss reasons for the different effects of mites and lice, including the relationship of horizontal (mites) and vertical (lice) transmission to the evolution of virulence.

The importance of host cues to three species of steinernematid nematodes (Rhabdita: Steinernematidae) with different foraging strategies was compared. We presented host materials to nematodes in series to test responses to combinations of host cues. If a fixed hierarchy of cues is followed during foraging, parasites should respond most strongly to cues offered in natural order. Steinernema carpocapsae, an ambush forager, aggregated at the source of volatile host cues only after attachment to host cuticle. They also parasitized hosts more efficiently after contact with cuticle. Steinernema glaseri, a cruise forager, was unaffected by exposure to combinations of host cues. Steinernema feltiae, a nematode with characteristics of both ambushing and cruising, was affected by cue hierarchies when either contact or volatile cues were presented first. Host-associated materials encountered out of the context may not qualify as host cues for the ambush forager, S. carpocapsae. Perhaps the order in which cues are encountered is more predictable for ambushers than for cruisers. Therefore an ambusher's response to host materials has a more fixed contextual framework.

Cases of human Ascaris infection occur sporadically in areas such as N. America and Western Europe, where this parasite is thought to be non-endemic. Clinical case histories suggest that many of these cases may be cross-infections from pigs. I describe patterns of variation in the ribosomal DNA of Ascaris from 9 such cases. For comparative purposes, I also describe patterns of variation in parasites obtained from pigs and humans from worldwide locations. A Hae III restriction site distinguishes two classes of rDNA repeats; repeats bearing this restriction site were found in > 96% of parasites from pig populations worldwide and in all 9 worms from humans in N. America. In contrast, repeats bearing this restriction site were detected in < 2% of parasites from humans in endemic areas. The molecular data clearly incriminate pigs as the source of infection in the N. American cases. I discuss evolutionary and public health implications of incomplete host fidelity in Ascaris.

Patterns of genetic subdivision in parasite populations can provide important insights into transmission processes and complement information obtained using traditional epidemiological techniques. We describe mitochondrial sequence variation in 265 Ascaris collected from 62 individual hosts (humans and pigs) from 35 households in 3 Guatemalan locations. Restriction mapping of individual worms revealed 42 distinct mitochondrial genotypes. We ask whether the mitochondrial genotypes found in worms from individual hosts, from families of hosts and from villages represent random samples from the total Ascaris population. Patterns of genetic subdivision were quantified using F-statistics, while deviations from the null hypothesis of randomness were evaluated by a simple resampling procedure. The analysis revealed significant deviations from panmixia. Parasite populations were strongly structured at the level of the individual host in both humans and pigs: parasites bearing the same mitochondrial genotype were found more frequently than would be expected by chance within hosts. Significant heterogeneity was also observed among populations from different villages, but not from different families within a village. The clustering of related parasites within hosts suggests a similar clustering of related infective stages in the environment and may explain why sex ratios in Ascaris are female-biased. We discuss aspects of Ascaris biology which may lead to the observed patterns.

The carbohydrate constituents of whole eggs and fractions derived from the eggs of Hymenolepis diminuta were partly characterized. Whole eggs contained 9·6 ng of carbohydrate (phenol-sulphuric acid-positive material) per egg, and 3·6 ng of glucose. Analysis of hydrolysed eggs by HPLC demonstrated the presence of glucose, galactose, glycerol, N-acetylgalactosamine, and 2-deoxyribose. Isolated egg shells and the KOH-stable, ethanol-precipitable fraction (putative glycogen) of eggs contained 26 and 76%, respectively, of the total carbohydrate associated with eggs. Glucose and galactose were present in both of these samples, but only glucose was present in the KOH-stable, ethanol-precipitable fraction isolated from chemically deshelled eggs; thus, the source of the galactose in the KOH-stable, ethanol-precipitable fraction isolated from whole eggs was the egg-shells. These data confirm earlier reports that the shells contain carbohydrate; the data demonstrate further that only two monosaccharides (glucose and galactose) are present in the shells, and that they are present as high molecular weight polymers. The ethanol-soluble fraction of eggs contained 2·5% of the total carbohydrate in eggs, and glucose accounted for < 1% of the ethanol-soluble carbohydrate. Analysis of the ethanol-soluble fraction by HPLC demonstrated that the predominant monosaccharides were glycerol and mannose, with smaller quantities of 2-deoxyribose and 2-deoxyglucose. The absence of a ‘free pool’ of glucose in eggs, and the presence of large amounts of glycerol and mannose, suggest that the pathways of intermediary carbohydrate metabolism in eggs might be very different from those in adult tapeworms.