The metacyclic form of African trypanosomes is the first to express genes for the Variant Surface Glycoprotein (VSG) and it uses an unusually predictable subset of the VSG gene repertoire. We have developed a model system for the analysis of metacyclic VSG (M-VSG) gene expression and have used this to demonstrate that, for two M-VSG genes, different modes of expression operate in the insect and mammalian phases of the life-cycle. In metacyclic-derived clones, these genes are expressed in situ, whereas they are routinely activated by duplication in bloodstream trypanosomes. The expression loci for both M-VSG genes studied are structurally simple and we present a model, based on this, for the maintenance of a separate M-VSG repertoire and expression system.

A midgut trypanolysin and an agglutinin from Glossina palpalis subspecies were isolated and partially characterized using anion-exchange chromatography and polyacrylamide gel electrophoresis. FPLC fractions of midgut extracts of Glossina palpalis palpalis caused agglutination and lysis of two trypanosome species (Trypanosoma congolense and Trypanosoma brucei brucei). although Glossina palpalis gambiensis caused only agglutination. The trypanolysin and agglutinin were active only in the posterior midguts, were heat labile above 50%C, had a periodic cycle of ‘activity’ in response to bloodmeal intake and were not affected by protease inhibitors or trypsin but were inactivated by pronase. The lytic substance contained two proteins with approximate molecular weights (Mr) of 12000 and 10000 Da respectively. The agglutinin had an approximate Mr of 67000 Da. Gamma-irradiation of the two subspecies caused a temporary inhibition of trypanolytic and agglutinin activities in midgut extracts.

Praziquantel, administered over a 72 h period in the food of mature Biomphalaria glabrata harbouring 7-week-old Schistosoma mansoni infections, dramatically reduced the numbers of cercariae shed. Doses of 20–30 μg/g body weight (including shell weight) reduced shedding by 85–95% over 5 weeks before recovery was evident. Suppression of cercarial shedding in these infections was accompanied by the temporary recovery of the snail reproductive regression due to S. mansoni infection. Snail fecundity was subsequently re-suppressed 2 weeks prior to recovery of the parasite as evidenced by a resumption of cercarial shedding. Praziquantel destroyed mature and developing cercariae within the daughter sporocysts but had no apparent effect on daughter sporocysts; this may account for the eventual resumption of cercarial production. Reproductive failure of the snail is apparently related to the latter stages of cercarial development specifically. Reproductive recovery did not occur when snails were infected as juveniles or when mature-infected snails harboured 12–5 week i.e. older infections: drug treatment temporarily inhibited cercarial production but no snails produced eggs.

The hypothesis that division of Trypanosoma brucei slender bloodstream forms is dependent upon the availability of a host-derived growth factor has been tested by superimposing challenge doses of slender-form trypanosomes onto preexisting infections at a time during the primary infection when stumpy forms predominated. The challenge populations grew in the doubly-infected mice indicating that depletion of a putative growth factor by the expanding population of the primary infection had not prevented division of the trypanosomes although slight reductions in multiplication rates were observed. This effect was independent of the variable antigen type (VAT) of the trypanosomes and of their stock of origin.

The intrategumental pH in adult male Schistosoma mansoni as measured with pH-sensitive microelectrodes is between 7.0 and 7.2, a value about one pH unit more alkaline than expected for equilibrium. This value is maintained for at least 4 h after isolation in media buffered with Hepes or CO2/ with or without serum present. CCCP (1 μM) and FCCP (10 μM) cause rapid acidification. DNP (1 mM) and Na-orthovanadate (1 mM) acidify but also cause significant depolarization of the tegument. NH (20 mM) depolarizes and acidifies the tegument with no evidence of transient alkalinization. High K+ (25 mM) accelerates the acidification. Removal of the NH+4 causes a large transient further acidification with recovery requiring as long as 40 min. High K+ (25 mM) delays the early stage of the recovery. Presence of CO2/HCO in the incubation medium does not accelerate the recovery rate nor does SITS (100 μM) inhibit the recovery. Intrategumental Na+ is elevated after an acid load and amiloride (3 mM) as well as low Na+ medium interfere with recovery from the acid load indicating that a Na+–H+ exchanger may be present in the tegumental membrane.

Cell extracts of an entero-invasive protozoon of squirrel monkeys, Tritrichomonas mobilensis, contained relatively high proteolytic activity, measured on hide powder azure (HPA). Multiple proteinase forms, optimally active at pH 5–7, were detected by electrophoretic analysis in gelatin-containing polyacrylamide gels. Three major proteinase bands of apparent low molecular weights, Mr 18, 23 and 30 kDa, were seen on gels. Inhibition-activation studies suggest that only cysteine proteinases were involved in HPAase and gelatinolytic activities of T. mobilensis cell extracts.

All pairwise combinations of three cloned stocks of Trypanosoma brucei (STIB 247L, STIB 386AA and TREU 927/4) were co-transmitted through tsetse flies (Glossina morsitans) and screened for the production of hybrid trypanosomes. Clones of metacyclic and bloodstream trypanosomes from flies harbouring mature infections containing hybrid trypanosomes were established and screened for several isoenzyme and restriction fragment length polymorphisms. For each of the three combinations of parents, some progeny clones were observed to be of a phenotype and genotype indicating that genetic exchange had occurred during development of the trypanosomes in flies. These hybrid clones shared three salient features: (1) where the parents were homozygous variants the progeny were heterozygous, (2) where one of the parents was heterozygous, allelic segregation was observed and (3) the progeny clones were shown to be recombinant when two or more markers for which one of the parents was heterozygous were examined. These results are consistent with the progeny being an F1 in a diploid mendelian genetic system involving meiosis and syngamy. Our observations show that all possible combinations of the three stocks may undergo genetic exchange. A marker analysis of a series of clones each derived from single metacyclic trypanosomes showed that individual flies transmit a mixture of trypanosome genotypes corresponding to F1 progeny and to parental types, indicating that genetic exchange was a non-obligatory event in the life-cycle of the trypanosome. In addition, a preliminary analysis of the phenotype of procyclic stage trypanosomes derived from flies infected with two stocks, indicates that genetic exchange is unlikely to occur at this stage.

Two panels of H-2 recombinant strains of mice were used in an attempt to map the H-2-linked genes which control resistance to infection with Trichuris muris. Response phenotypes could be related to the presence of ‘resistance’ (q, b) or ‘susceptibility’ (k, d) alleles at I-A. The influence of these genes was modulated by other alleles, particularly q or d alleles, at the D end of the H-2. Absence of I-E molecules correlated with resistance to infection in some but not all strains studied. Thus the (B10.BR × B10.G) Fl strain which expressed I-Ek gene products was resistant to infection. A study of the time-course of infection in strains of mice expressing q alleles throughout the H-2 on 4 different genetic backgrounds (NIH, SWR, DBA and B10) revealed that most strains were resistant to infection. However, the DBA/1 strain exhibited differential responsiveness, 4 out of 6 individuals harbouring mature adult parasites on day 35 post-infection.

Oxytetracycline (OTC) significantly inhibited cytochrome c oxidase activity in bovine lymphocytes infected with Theileria parva and in uninfected mitogen-stimulated lymphocytes. The inhibitory effect was detected in vitro within 24 h of treatment with drug concentrations as low as 1 µg/ml. Following mitogen stimulation of lymphocytes, concentrations of 3 and 10 µg/ml OTC completely inhibited an increase in cytochrome c oxidase activity for 48–72 h. This inhibitory activity was considered to be due to a direct effect on lymphoblast mitochondrial protein synthesis. As a consequence, adenosine triphosphate activity was significantly reduced in lymphocytes stimulated either by infection with T. parva sporozoites or by mitogen and then treated with OTC. The results also indicated that parasite mitochondrial protein synthesis was inhibited by OTC. The activity of OTC reported in this study could explain the suppression of disease following ‘infection and treatment’ immunization against East Coast fever and the in vitro drug-inhibition of schizont development.

The localization and distribution of the serotoninergic and peptidergic elements of the nervous system of larval Schistosoma mansoni have been investigated using an indirect immunofluorescence technique in conjunction with confocal scanning laser microscopy (CSLM). A range of antisera was used, raised to the biogenic amine, 5-hydroxytryptamine (5-HT, or serotonin), the vertebrate peptides pancreatic polypeptide (PP), peptide YY (PYY) and neuropeptide Y (NPY) and to the native invertebrate peptide, FMRFamide; all these antisera were shown previously to be immunopositive in the adult worm. No immunoreactivity to 5-HT was detected in any of the larval stages, but both miracidia and cercariae were consistently immunoreactive to all 4 peptides. The peptidergic nervous system of the miracidium is relatively simple, taking the form of a central neural mass with associated paired anterior and posterior nerve tracts. The cercarial peptidergic nervous system comprises a central commissure joining paired anterior ganglia, from which emanate paired dorsal and ventral nerve tracts, which terminate at the body/tail junction. The excretory bladder region of the tail is also immunoreactive for the 4 peptides, and a fine pair of nerve tracts extends the length of the tail shaft. Immunoreactive nerve cell bodies are also evident in the midbody region of the intrasprocystic cercariae, these same structures being immunoreactive for the neuronal marker, neurone-specific enolase (NSE). The organization of the larval peptidergic nervous system is compared to that of the cholinergic nervous system and contrasted with the peptidergic system in the adult worm. The absence of immunoreactivity to 5-HT is discussed in relation to the proposed development of the aminergic nervous system upon establishment in the definitive host.

Six stocks of Trypanosoma congolense were cloned from 17 stocks isolated from Eastern Zambia and used to initiate insect-form in vitro cultures producing metacyclic trypanosomes. Serological assays were then developed using these in vitro-derived metacyclics as a reference collection of antigens. Monoclonal antibodies recognized 8 metacyclic variable antigen types (M-VATs) of one stock, T. congolense TREU 1885, representing 70–80% of that stock's M-VAT repertoire, and in an indirect fluorescent antibody test (IFAT) there were no cross-reactions between them and the metacyclic trypanosomes of the other 5 stocks. Cross-protection assays between the 6 stocks in mice showed that the stocks cultured in vitro were serologically distinct. In order to facilitate serological typing for serodeme characterization, an IFAT was developed using formalin-fixed metacyclic trypanosomes to identify VAT specific immune responses using 21 day post-infection antisera. The cultured stocks reacted only with their homologous antisera thus confirming the results obtained in the cross-protection assays. No cross-reactions were observed with the 6 cloned stocks and antisera against the 11 stocks of T. congolense isolated in the same area at the same time suggesting that these stocks were different from the reference collection of cultured metacyclics. Hence, at least 7 serodemes of T. congolense have been identified from the 17 stocks isolated.

Spermatogenesis and the fine structure of the mature spermatozoon of Fasciola hepatica have been studied by transmission electron microscopy. The primary spermatogonia display a typical gonial morphology and occupy the periphery of the testis. They undergo 3 mitotic divisions to give rise to 8 primary spermatocytes forming a rosette of cells connected to a central cytophore. The primary spermatocytes undergo 2 meiotic divisions, resulting in 32 spermatids that develop into spermatozoa. Intranuclear synaptonemal complexes in primary spermatocytes confirm the first meiotic division. The onset of spermiogenesis is marked by the formation of the zone of differentiation which contains 2 basal bodies and a further centriole derivative, the central body. The zone extends away from the spermatid cell to form the median process; into this migrates the differentiated and elongate nucleus. Simultaneously, 2 axonemes develop from the basal bodies. During development, they rotate through 90° to extend parallel to the median process. The migration of the nucleus to the distal end of the median process coincides with the fusion of the axonemes to the latter to form a monopartite spermatozoon. The mature spermatozoon possesses 2 axonemes of the 9 + ‘1’ pattern typical of parasitic platyhelminths, 2 elongate mitochondria and a variable array of peripheral microtubules. The nuclear region of the spermatozoon is immotile. The value of sperm ultrastructure as a taxonomic tool in platyhelminth phylogeny is discussed.

Outbred MF1 mice were characterized with respect to their susceptibility to infection with Heligmosomoides polygyrus (Nematoda) on the basis of faecal egg counts after 8 weeks of repeated infection (50 larvae/week). Selective breeding for resistance and susceptibility was carried out over 3 generations. The H-2 type of a sample of the mice was determined, and antigen recognition assessed on the basis of Western blots against adult and larval H. polygyrus homogenate. The selective breeding programme yielded very strong evidence for the heritability of susceptibility to infection. The results were consistent with a model of single gene control with resistance dominant over susceptibility. The presence of the H-2k haplotype was significantly associated with susceptibility, as was the recognition of a 17 kDa antigen in blots against both larval and adult worm homogenate. The proportion of mice phenotypically susceptible to infection, the proportion bearing the H-2k haplotype, and the proportion recognizing the 17 kDa antigen, were all approximately 0.25.

A great deal of published experimental evidence shows that parasite-induced changes in host choice, and biting and survival rates of bloodsucking insects are an important source of bias in estimates of Vectorial Capacity. I argue here that, because of the difficulties of handling these and other biases in practice, indices (based on the Vectorial Capacity) which purport to measure transmission rate are best used in a comparative rather than an absolute way. They should attempt to measure, not transmission rate, but changes in transmission rate. In this case, the less restrictive assumption that parasites are responsible for constant bias (rather than no bias) leads to relatively small errors. The results encourage a more optimistic view of the performance of classical entomological indices such as the parous rate. They should also help to redirect the principal epidemiological question about vector–parasite interactions: future studies should be concerned with the impact of parasites, not directly on Vectorial Capacity, but on the comparative measures which are actually used to evaluate transmission rate.

The population dynamics of outbred laboratory mice in indoor enclosures in the absence and presence of a naturally transmitted direct life-cycle nematode Heligmosomoides polygyrus Dujardin 1845 were reported previously. This manuscript presents further information on the age and sex structure of the populations, results of experiments designed to estimate the density-dependent effect of the parasite on host survival and reproduction, and a mathematical model of both uninfected and infected mouse populations. In the uninfected mouse population, survival of female mice was age- and density-independent, survival of male mice was age-dependent and density-independent, and recruitment was density-dependent. Independent experiments revealed that the parasite had no density-dependent effect on mouse reproduction, but had density-dependent effects on both acute and chronic survival of mice. An age-structured Leslie matrix model captured the exponential growth and plateau of the uninfected mouse population. Modification of the model to incorporate the effects of the parasite provided a good fit to the data from the infected populations, supporting the hypothesis that density-dependent effects of the parasite on host survival could lead to regulation of host abundance.

Isolates of avian trypanosomes from nestling sparrowhawks (Accipiter nisus) and from Simulium latipes were compared by isoenzyme electrophoresis with the previously described avian trypanosomes Trypanosoma corvi and T. everetti. Simulium isolates developed into trypomastigotes in semi-defined medium at 37 °C confirming that they belong to the genus Trypanosoma. Cellulose acetate electrophoresis (CAE) and polyacrylamide gel electrophoresis (PAGE) methods were employed. A total of 11 enzymes was examined, of which 8 gave satisfactory results for all lysates (MDH, ICD, PGM, 6PGD, SOD, PEP I, PEP II and PEP D). The zymograms of 3 isolates from Simulium and 4 from A. nisus were similar indicating that they were the same organism and the isoenzyme patterns coincided with those of T. corvi. It is concluded that A. nisus and S. latipes are infected with T. corvi. Flagellate infections were found in the midgut, hindgut and rectum of Simulium where large numbers of epimastigotes were found in the lumen and attached to the cuticular intima of the rectal ampullae by hemidesmosomes at the distal end of an expanded flagellum. Out of 285 nestling sparrowhawks 13 (5%) were infected with trypanosomes. In view of their age and inability to preen themselves it is suggested they became infected via infected prey.

This paper uses simple mathematical models and statistical estimation techniques to analyse the frequency distribution of microfilariae (mf) in blood samples from human populations which are endemic for lymphatic filariasis. The theoretical analysis examines the relationship between microfilarial burdens and the prevalence of adult (macrofilarial) worms in the human host population. The main finding is that a large proportion of observed mf-negatives may be ‘true’ zeros, arising from the absence of macrofilarial infections or unmated adult worms, rather than being attributable to the blood sampling process. The corresponding mf distribution should then follow a Poisson mixture, arising from the sampling of mf positives, with an additional proportion of ‘true’ mf-zeros. This hypothesis is supported by analysis of observed Wuchereria bancrofti mf distributions from Southern India, Japan and Fiji, in which zero-truncated Poisson mixtures fit mf-positive counts more effectively than distributions including the observed zeros. The fits of two Poisson mixtures, the negative binomial and the Sichel distribution, are compared. The Sichel provides a slightly better empirical description of the mf density distribution; reasons for this improvement, and a discussion of the relative merits of the two distributions, are presented. The impact on observed mf distributions of increasing blood sampling volume and extraction efficiency are illustrated via a simple model, and directions for future work are identified.

The nematode-trapping fungus Arthrobotrys oligospora traps and invades all the free-living juvenile stages of the tri-chostrongyle nematode Trichostrongylus colubriformis. The processes of capture and invasion of the 1st-stage juveniles are described using differential interference contrast optics and 3-D reconstruction techniques. The adhesive responsible for capture is well preserved using a freeze-substitution technique for scanning electron microscopy. The invasion process of the ensheathed 3rd-stage juvenile of T. colubriformis takes much longer than in the 1st- or 2nd-stage juvenile and involves the formation of secondary infection pegs between the sheath and the cuticle which appear to penetrate the cuticle by physical pressure.

The uptake of glucose by Acanthocheilonema viteae was studied in vitro. The process was selective for the D-isomer and saturatable with a Km of 2 mM. The rate of glucose transport/utilization was inhibited by 2-deoxyglucose, mannose, 5-thioglucose and dipyridamole but, unlike mammalian systems, was not impaired by cytochalasin B, phloretin, phloridzin, 3-O-methylglucose and 4, 6-ethylideneglucose. A potential chemotherapeutic advantage of selectively inhibiting filarial glucose transport exists for the following reasons. (1) The glucose transporter present in A. viteae was shown to be different from the one present in some mammalian systems. (2) Incubation under glucose-free conditions led to glycogen depletion, loss of motility and worm death. (3) Worms maintained in vitro for more than 18 h without glucose did not survive when implanted into gerbils.

Removal of the sheath of the ensheathed infective juvenile of Trichostrongylus colubriformis prevents capture by the nematophagous fungus Arthrobotrys oligospora. Exposure of the trap hyphae to a variety of saccharides, which may block a recognition system based on lectin/carbohydrate binding, failed to prevent capture but some saccharides did inhibit penetration and invasion by the fungus. Capture and penetration thus appear to be two distinct processes with capture being less specific than penetration. Carbohydrate residues were absent from the outer surface of the cuticle and the sheath but were present on the inner surface of the sheath. The limited accessibility of these lectin-binding sites may explain the slow process of infection of the infective juvenile by the fungus. The sheath does not protect the infective juvenile against attack by this nematophagous fungus.