Showing papers in "Parasitology in 1998"

Patterns of macroparasite aggregation in wildlife host populations.

Abstract: Frequency distributions from 49 published wildlife host-macroparasite systems were analysed by maximum likelihood for goodness of fit to the negative binomial distribution. In 45 of the 49 (90 %) data-sets, the negative binomial distribution provided a statistically satisfactory fit. In the other 4 data-sets the negative binomial distribution still provided a better fit than the Poisson distribution, and only 1 of the data-sets fitted the Poisson distribution. The degree of aggregation was large, with 43 of the 49 data-sets having an estimated k of less than 1 From these 19 data-sets, 22 subsets of host data were available (i.e. host data could be divided by either host sex, age, where or when hosts were sampled). In 11 of these 22 subsets there was significant variation in the degree of aggregation between host subsets of the same host-parasite system. A common k estimate was always larger than that obtained with all the host data considered together. These results indicate that lumping host data can hide important variations in aggregation between hosts and can exaggerate the true degree of aggregation. Wherever possible common k estimates should be used to estimate the degree of aggregation. In addition, significant differences in the degree of aggregation between subgroups of host data, were generally associated with significant differences in both mean parasite burdens and the prevalence of infection.

Comparative performance of species richness estimation methods.

Abstract: In most real-world contexts the sampling effort needed to attain an accurate estimate of total species richness is excessive. Therefore, methods to estimate total species richness from incomplete collections need to be developed and tested. Using real and computer-simulated parasite data sets, the performances of 9 species richness estimation methods were compared. For all data sets, each estimation method was used to calculate the projected species richness at increasing levels of sampling effort. The performance of each method was evaluated by calculating the bias and precision of its estimates against the known total species richness. Performance was evaluated with increasing sampling effort and across different model communities. For the real data sets, the Chao2 and first-order jackknife estimators performed best. For the simulated data sets, the first-order jackknife estimator performed best at low sampling effort but, with increasing sampling effort, the bootstrap estimator outperformed all other estimators. Estimator performance increased with increasing species richness, aggregation level of individuals among samples and overall population size. Overall, the Chao2 and the first-order jackknife estimation methods performed best and should be used to control for the confounding effects of sampling effort in studies of parasite species richness. Potential uses of and practical problems with species richness estimation methods are discussed.

Relative merits of nuclear ribosomal internal transcribed spacers and mitochondrial CO1 and ND1 genes for distinguishing among Echinostoma species (Trematoda).

Abstract: Cryptic species, belonging to the 37 collar-spine Echinostoma group, were distinguished using nuclear rDNA ITS (884 bases) and mtDNA CO1 (257 bases) and ND1 (530 bases) sequences. Sequences were obtained from five 37 collar-spine species, Echinostoma trivolvis, E. paraensei, E. caproni, E. revolutum and E. sp.I, a parthenogenetic isolate from Africa. Three geographic isolates of E. caproni were compared. Average sequence divergence among the 37 collar-spine species range from 2.2% in the rDNA ITS through 8% for the CO1 and 14% for the ND1. In addition, genes were sequenced from 2 non 37 collar-spine species, E. hortense and an undescribed Australian species, E. sp. (Aus). For each gene, distances of terminals from a predicted ancestral sequence were calculated. These indicated that ND1 is diverging significantly faster than the other 2 regions. In the CO1 gene most substitutions are synonymous and saturation has been reached for the majority of pairwise comparisons. The ND1 gene exhibits greater pairwise divergence but less evidence of saturation due to weaker conservation of first and second codon positions. The ITS has no amino acid coding constraints and displays no evidence of saturation. Although all 3 regions successfully distinguished the nominal species, ND1 appears to be the most informative region for investigating relationships within the 37 collar-spine group.

Modulation of immune responses to parasitoids by polydnaviruses.

Abstract: Parasitoids are parasites that invariably kill their host. Polydnaviruses are injected by parasitoid wasps into the body cavity of their insect host and cause immunosuppression, allowing the parasitoid to develop in the absence of encapsulation. One of the targets of the polydnaviruses are the haemocytes of the host, which undergo significant changes in response to entry of the virus. In some systems, haemocyte apoptosis is induced, or haemocyte clumping may be seen; in others, the cells round up and fail to adhere to a substrate. Effects on haemocytes may be transitory or permanent (cell death). Various polydnavirus gene products have been identified that interfere with normal haemocyte function. Phenoloxidase activity also is inhibited during parasitism, and the effect is inducible by polydnavirus. In some systems, venom components may act synergistically with polydnavirus in mediating the virally-induced effects on the host immune system. Polydnaviruses are powerful influences on the host immune system, which serve to permit successful development of the parasitoid without triggering the host immune response.

Manipulation of a mollusc by a trophically transmitted parasite: convergent evolution or phylogenetic inheritance?

Abstract: We investigated the influence of infection by the trematode Curtuteria australis on the burrowing behaviour of its intermediate host, the bivalve Austrovenus stutchburyi. Laboratory experiments and field observations revealed that cockles, unable to bury completely or even partially under the sediment, have a reduced foot length compared with buried individuals. The ability to bury proved to be highly repeatable in field experiments: cockles found at the surface and transplanted to an experimental area did not bury themselves, and cockles found buried stayed buried when relocated. All metacercariae of C. australis were found strictly in the foot and for each of 3 samples collected in different sites, there was a negative and significant relationship between the relative length of the foot and the parasite load. A predation test conducted under natural conditions indicated that cockles with the stunted foot and the altered behaviour are significantly more susceptible to predation by aquatic birds than other cockles. Given that the definitive host of C. australis is an oystercatcher, we first discuss our results in the context of transmission strategy. Comparisons with other studies on more or less related trematode species parasitic in bivalves and evolving under similar constraints for their transmission, shed light on the origin of this adaptation in C. australis.

The coevolution of host resistance and parasitoid virulence.

Abstract: Host-parasitoid interactions are abundant in nature and offer great scope for the study of coevolution. A particularly fertile area is the interaction between internal feeding parasitoids and their hosts. Hosts have evolved a variety of means of combating parasitoids, in particular cellular encapsulation, while parasitoids have evolved a wide range of countermeasures. Studies of the evolution of host resistance and parasitoid virulence are reviewed, with an emphasis on work involving Drosophila and its parasitoids. Genetic variation in both traits has been demonstrated using isofemale line and artificial selection techniques. Recent studies have investigated the fitness costs of maintaining the ability to resist parasitoids, the comparative fitness of flies that have successfully defended themselves against parasitoids, and the degree to which resistance and virulence act against one or more species of host or parasitoid. A number of studies have examined geographical patterns, and sought to look for local adaptation; or have compared the traits across a range of species. Finally, the physiological and genetic basis of change in resistance and virulence is being investigated. While concentrating on Drosophila, the limited amount of work on different systems is reviewed, and other possible areas of coevolution in host-parasitoid interactions are briefly discussed.

Molecular characterization of Cryptosporidium from various hosts

Abstract: A 298 bp region of the Cryptosporidium parvum 18S rDNA and a 390 bp region of the acetyl-CoA synthetase gene were sequenced for a range of human and animal isolates of Cryptosporidium from different geographical areas. A distinct genotype is common to isolates from cattle, sheep and goats and also an alpaca from Peru and is referred to here as the 'calf'-derived Cryptosporidium genotype. Another genotype of 'human'-derived isolates also appears to be conserved amongst human isolates although humans are also susceptible to infection with the 'calf' Cryptosporidium genotype. Mice and pigs carry genetically distinct genotypes of Cryptosporidium. Three snake isolates were also analysed, 2 of which exhibited C. muris genotypes and the third snake isolate carried a distinct 'mouse' genotype.

Novel lineages of Giardia intestinalis identified by genetic analysis of organisms isolated from dogs in Australia.

Abstract: Infection of suckling mice with Giardia trophozoites recovered from the intestines of 11 dogs autopsied in Central and Southern Australia in each case produced an established isolate. In contrast, only 1 isolate was obtained by inoculation of faecal cysts. The organisms grew poorly in comparison with isolates from humans or non-canine animal hosts. Light microscopy revealed that the trophozoites had median bodies with the 'claw hammer' appearance typical of G. intestinalis (syn. G. duodenalis, G. lamblia) but that they differed in shape and nuclear morphology from axenic isolates of human or canine origin. Allozymic analysis of electrophoretic data representing 26 loci and phylogenetic analysis of nucleotide sequences obtained from DNA amplified from the glutamate dehydrogenase locus showed that the 11 isolates examined from Australian dogs were genetically distinct from all isolates of G. intestinalis that have been established previously from humans and animals, and also from G. muris. Both analytical methods placed 10 of the Australian canine isolates into a unique genetic lineage (designated Assemblage C) and the eleventh into a deep-rooted second branch (designated Assemblage D), each well separated from the 2 lineages (Assemblages A and B) of G. intestinalis that encompass all the genotypes known to infect humans. In contrast, 4 axenic isolates derived from dogs in Canada and Europe (the only other isolates to have been established from dogs) have genotypes characteristic of genetic Assemblages A or B. The findings indicate that the novel Giardia identified in these rural Australian dogs have a restricted host range, possibly confined to canine species. The poor success rate in establishing Giardia from dogs in vitro suggests, further, that similar genotypes may predominate as canine parasites world-wide. The absence of such organisms among isolates of Giardia that have been established from humans by propagation in suckling mice indicates that they are unlikely to infect humans. However, infection of humans by those dog-derived genotypes that grow in vitro cannot be excluded.

Re-examination of resistance of Toxoplasma gondii tachyzoites and bradyzoites to pepsin and trypsin digestion

Abstract: The effect of digestion in trypsin and acid pepsin on Toxoplasma gondii tachyzoites and bradyzoites was re-evaluated because of recent use of this method to distinguish tachyzoites from bradyzoites. Toxoplasma gondii tachyzoites survived better in 0.5% trypsin solution for 1 h than in 1.0% solution, and occasionally survived for 2 h in acid pepsin solution. Extracellular tachyzoites (> or = 1000) were also infectious orally to mice and cats. Bradyzoites survived equally in trypsin and acid pepsin solutions but the digestion of brain tissue in 0.5% trypsin solution was better than in acid pepsin solution. The resistance to digestion in acid pepsin solution is not a reliable method to distinguish tachyzoites from bradyzoites.

Mitochondrial genomic markers confirm the presence of the camel strain (G6 genotype) of Echinococcus granulosus in north-western China

Abstract: Twenty-eight isolates of E. granulosus, collected from humans at surgery, and a range of intermediate hosts, including sheep, cattle and camels from abattoirs in North and South Xinjiang Uygur Autonomous Region, People's Republic of China, were analysed for DNA sequence variation within regions of the mitochondrial cytochrome c oxidase I (COI) and NADH dehydrogenase subunit I (NDI) genes. The isolates were categorized into 2 distinct and uniform genotypic groupings, based on the sequences obtained, and the data clearly indicated that the camel/dog strain (G6 genotype) of E. granulosus as well as the cosmopolitan, common sheep strain (G1 genotype) occur in north Xinjiang. The presence of the camel strain has thus been confirmed in Xinjiang but it is evident from this and a previous molecular genetic survey of E. granulosus isolates from north-western China that the common sheep strain is the most predominant in the region. From the public health perspective, the majority of infected livestock will act as reservoirs of human infection there. During the course of the study, a polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) assay, based on the NDI sequence variation, was developed that allows rapid discrimination of the G1 and G6 genotypes.

Vertical transmission of Toxoplasma gondii from chronically infected house (Mus musculus) and field (Apodemus sylvaticus) mice determined by polymerase chain reaction.

Abstract: Captive-bred Mus musculus (house mice) and Apodemus sylvaticus (field mice) were each infected with 50 oocysts of Toxoplasma gondii M1 strain per os and infection in them and their offspring was assessed by polymerase chain reaction (PCR) amplification of the T. gondii B1 gene in brain tissue and by serology, using the modified agglutination test (MAT). The chronically infected female A. sylvaticus (n = 10) and M. musculus (n = 23) were mated at least 6 weeks after infection (and subsequently to produce up to 6 litters) and their pups examined 3 weeks after weaning at 6 weeks of age. By PCR, in offspring of A. sylvaticus and M. musculus respectively, vertical transmission was demonstrated in 82.7% (n = 83) and 85.0% (n = 207) of all pups (N.S., P > 0.05), 95% (n = 21) and 100% (n = 30) of all litters (N.S., P > 0.05), with a mean (+/- S.E.) proportion of each litter infected of 0.87 (0.06) and 0.86 (0.04) (N.S., P > 0.05). There was no change in any of these variables between first and subsequent litters. By serology, whilst MAT suggested 100% vertical transmission in A. sylvaticus, it under-estimated rates of infection in offspring of M. musculus. A limited series of bioassays from M. musculus tissues confirmed the good correlation of PCR and the poor correlation of MAT with mouse inoculation. These results indicate that vertical transmission in A. sylvaticus and M. musculus is extremely efficient and probably endures for the life of the breeding female. This mechanism favours parasite transmission and dispersion by providing a potential reservoir of infection in hosts predated by the cat.

Killing of Gyrodactylus salaris (Platyhelminthes, Monogenea) mediated by host complement.

Abstract: Gyrodactylus salaris, an important pathogen of Atlantic salmon Salmo salar, has been shown to be highly sensitive to factors in host serum and mucus, being killed rapidly (50% within 1 h) by serum at a dilution of 1:200. The time needed for killing was inversely proportional to serum concentration. Similar effects were noted using host mucus, which contained approximately 1/20th of the anti-Gyrodactylus activity of serum. Serum activity was abolished completely by heating at 45 degrees C for 30 min, and by addition of EDTA, but not by EGTA + 1 mM magnesium ions. Activity was not dependent on whether the serum was from infected or naive fishes, nor was it species specific. Attempts to pre-coat parasites in salmon anti-Gyrodactylus antibodies also failed to enhance the activity of fresh serum. These observations suggest that killing is due to the complement system of the host, acting via the alternate pathway. G. salaris appears to be exceptionally sensitive to complement, being killed at concentrations which could be experienced in vivo. The role of complement in the protection of fishes against gyrodactylid infection therefore deserves further investigation.

In vitro stimulation of metacyclogenesis in Leishmania braziliensis, L. donovani, L. major and L. mexicana

Abstract: Promastigotes of Leishmania braziliensis, L. donovani, L. major and L. mexicana recently derived from tissue amastigotes were cultured in Schneider's Drosophila medium supplemented with 20% (v/v) heat-inactivated foetal calf serum and 25 micrograms gentamicin sulfate/ml at pH 5.5. These cultures produced more metacyclic promastigotes in their stationary-phase populations than others cultured at pH 7.0. Metacyclic promastigotes possessed a short (< or = 8 microns) and narrow (< or = 1.5 microns) cell body with a flagellum twice or more the length of the cell body. Promastigotes from acidic cultures were more resistant to complement-mediated lysis and more infective in vivo than those grown at neutral pH. These results demonstrate that induction of metacyclogenesis by acidic pH is a response conserved across a variety of species of Leishmania.

Molecular characterization of a Toxocara variant from cats in Kuala Lumpur, Malaysia

Abstract: The ascaridoid nematode of cats from Kuala Lumpur, Malaysia, previously identified morphologically as Toxocara canis, was characterized using a molecular approach. The nuclear ribosomal DNA (rDNA) region spanning the first internal transcribed spacer (ITS-1), the 5.8S gene and the second internal transcribed spacer (ITS-2) was amplified and sequenced. The sequences for the parasite from Malaysian cats were compared with those for T. canis and T. cati. The sequence data showed that this taxon was genetically more similar to T. cati than to T. canis in the ITS-1, 5.8S and ITS-2. Differences in the ITS-1 and ITS-2 sequences between the taxa (9.4-26.1%) were markedly higher than variation between samples within T. canis and T. cati (0-2.9%). The sequence data demonstrate that the parasite from Malaysian cats is neither T. canis nor T. cati and indicate that it is a distinct species. Based on these data, PCR-linked restriction fragment length polymorphism (RFLP) and single-strand conformation polymorphism (SSCP) methods were employed for the unequivocal differentiation of the Toxocara variant from T. canis and T. cati. These methods should provide valuable tools for studying the life-cycle, transmission pattern(s) and zoonotic potential of this parasite.

Wolbachia as a possible means of driving genes into populations.

Abstract: Cytoplasmic incompatibility consists of sterility in cross matings, the crossing type being maternally inherited. It can be explained by the action of Wolbachia symbionts which are transmitted through the egg cytoplasm and leave an imprint on the sperm which prevents it fertilizing unless it is 'rescued' by the action of the same type of Wolbachia in the egg. Thus matings between infected males and uninfected females are sterile, but the reciprocal matings are fertile. Hence uninfected females are at risk of failing to transmit their uninfected cytoplasm if they cross mate, but infected females are at no such risk. Therefore natural selection favours the infected state and in two wild insect populations the infection has been observed spreading. If a gene for inability to transmit malaria could be introduced into Wolbachia and if this could be introduced into Anopheles (where these symbionts appear not to occur naturally), release of a limited number of such insects should trigger a process of displacement of malaria vectors, by the non-vector type. A simple model is used to demonstrate the limitations to this process which would be introduced by immigration.

Isolation of an aspartic proteinase precursor from the egg of a hard tick, Boophilus microplus

Abstract: An aspartic proteinase precursor, herein named BYC (Boophilus Yolk pro-Cathepsin) was isolated from eggs of the hard tick, Boophilus microplus. As judged by electrophoresis on sodium dodecyl sulfate polyacrylamide slab gel (SDS-PAGE), purified BYC presented 2 bands of 54 and 49 kDa, bearing the same NH2-terminal amino acid sequence. By Western blot analysis, BYC was also found in the haemolymph, indicating an extraovarian site of synthesis. Several organs were incubated in culture medium with [35S]methionine, and only the gut and fat body showed synthesis of BYC polypeptides. Protein sequencing of both the NH2-terminal and an internal sequence obtained after cyanogen bromide (CNBr) cleavage of BYC revealed homology with several aspartic proteinase precursors. Incubation at pH 3.5 resulted in autoproteolysis of BYC, which produced the mature form of the enzyme, that displayed pepstatin-sensitive hydrolytic activity against haemoglobin. Western blot analysis using anti-BYC monoclonal antibodies showed proteolytic processing of BYC during embryogenesis and suggested activation of the enzyme during development. A role of BYC in degradation of vitellin, the major yolk protein of tick eggs, is discussed.

The processes influencing the distribution of parasitic nematodes among naturally infected lambs

Abstract: The impact of mixed, nematode infection upon a group of animals will depend upon the number of nematodes present, how they are distributed among hosts and whether individuals that are heavily parasitized with one species are more likely to be heavily parasitized with other species. A survey of over 500 six-month-old, Scottish Blackface lambs from a single farm in Southwest Strathclyde identified 7 different categories of nematodes in the abomasum and small intestine. There were considerable differences among years and among nematodes in the prevalence and mean intensity of infection. Ostertagia circumcincta was present in nearly all lambs and judged by prevalence and intensity is one of the most successful of all parasitic nematodes. Each category of nematodes had a skewed distribution; most animals had relatively few worms but a small proportion had many worms. The variance of the number of nematodes in each category were approximately equal to the square of the mean. The counts of adult O. circumcincta followed a negative binomial distribution, but the negative binomial distribution did not provide a good description of the observed values for the other species. These other species had a lower prevalence and possibly some sheep were not exposed to infection. There was no significant genetic variation among lambs in the number of nematodes present and therefore the differences among these lambs were unlikely to be a consequence of genetic differences in host susceptibility. Lambs with increased numbers of one species were more likely to be have increased numbers of the other species, but the correlations were weak and may reflect covariation in exposure to different parasites.

The gp63 gene locus, a target for genetic characterization of Leishmania belonging to subgenus Viannia.

Abstract: In the present study the gp63 gene locus was used as a target for genetic characterization of Leishmania parasites by 2 methods: (i) RFLP analysis with several restriction enzymes (gp63-RFLP), and (ii) intra-genic PCR amplification coupled with restriction analysis (PCR-RFLP). Both methods were applied to a large number of natural isolates belonging to 4 species of the subgenus Viannia, namely L. (V.) braziliensis, L. (V.) peruviana, L. (V.) guyanensis and L. (V.) lainsoni: reference stocks of subgenus Leishmania were included as outgroups. Multilocus isoenzyme typing (MLEE) was used as a reference. On the one hand gp63-RFLP evidenced an extensive polymorphism and revealed specific markers for subgenus, species and geographical populations: congruence with MLEE was demonstrated statistically. The particular interest of gp63-RFLP was illustrated by infra-specific polymorphism, because of the possible relationship with phenotype diversity. On the other hand intra-genic amplification was less resolutive than gp63-RFLP, but also allowed discrimination of the 2 subgenera (PCR alone) and all the species tested in the subgenus Viannia (PCR-RFLP). PCR-RFLP presents an important operational advantage as it allows genetic characterization of minute amounts of parasites, using Leishmania specific primers. The polymorphism revealed by gp63-RFLP and PCR-RFLP illustrates the very high genomic and genetic plasticity of gp63 genes.

Seasonal changes in the Plasmodium falciparum population in individuals and their relationship to clinical malaria: a longitudinal study in a Sudanese village

Abstract: Residents of Daraweesh village in Sudan were monitored for Plasmodium falciparum infection and malaria morbidity in 3 malaria seasons from 1993 to 1996. Malaria parasites were detected microscopically and by polymerase chain reaction (PCR) in a series of cross-sectional surveys. PCR revealed submicroscopical infections during the dry season, particularly among individuals who had recovered from a malaria episode following successful drug treatment. Clinical and subclinical infections were contrasted by assaying for allelic polymorphism at 2 gene loci, MSP-1 and GLURP and 2 hypotheses examined with reference to these data: that clinical malaria is associated with infection with novel parasite genotypes not previously detected in that host, or alternatively, that clinical malaria episodes are associated with an increased number of clones in an infection. We detected more mixed infections among clinical isolates, but people carrying parasites during the dry season were not found to have an increased risk of disease in the following malaria season. There was a clear association of disease with the appearance of novel parasite genotypes.

In vitro cultivation and developmental cycle in culture of a parasitic dinoflagellate (Hematodinium sp.) associated with mortality of the Norway lobster (Nephrops norvegicus) in British waters.

Abstract: Dinoflagellates are common and often important parasites of aquatic organisms, but their developmental cycles are poorly known and have not been established in in vitro culture. The parasitic dinoflagellate (Hematodinium sp.) associated with mortality of the Norway lobster (Nephrops norvegicus) in British waters has been cultivated in vitro in 10% foetal calf serum in a balanced Nephrops saline. In culture the parasite undergoes a characteristic cycle of development. Circulating sporoblasts from the host's haemolymph in vitro generate 2 kinds of flagellated uninucleate dinospore, macrospores and microspores, either of which will, after 5 weeks in fresh medium, germinate to produce multinucleate unattached filamentous trophonts. These trophonts multiply by fragmentation and growth and may be serially subcultured in this form, at 2 week intervals, indefinitely. If not subcultured, the filamentous trophonts give rise to colonies of radiating filaments ('gorgonlocks') which subsequently attach to the substratum to form flattened web-like 'arachnoid' multinucleate trophonts. Arachnoid trophonts become arachnoid sporonts when they synthesize trichocysts and flagellar hairs and may give rise to secondary arachnoid sporonts or to dinospores which initiate a new cycle.

Intra- and inter-specific variation in nuclear ribosomal internal transcribed spacer 1 of the Schistosoma japonicum species complex.

Abstract: The first internal transcribed spacer (ITS1) of the nuclear ribosomal DNA repeat was sequenced for members of the Schistosoma japonicum species complex (S. malayensis, S. mekongi and 2 geographical isolates of S. japonicum). The ITS1 is composed of 3 distinct regions: the 5' end (23 nucleotides); a tract of approximately 90–140 nucleotides, which occurs up to 7 times in tandem, the number varying even within an individual in all species investigated in this study; the 3′ region (378 nucleotides), which lacks repeats. There is size and sequence variation among copies of the ITS1 repeat within a single individual. The relative abundances of size variants of ITS1 in S. japonicum have been ascertained by hybridizing genomic digests with an ITS1 probe. Multiple repeats and intra-individual variation in numbers and abundance of these is a feature of the Asian schistosomes, but not generally of African schistosomes. Possible reasons for this difference in ITS1 between African and Asian schistosomes are discussed. The ITS1 repeat sequences described for African schistosomes are different to, and cannot be aligned with, those from the Asian species described here, whereas the remainder of the ITS1 can be aligned quite easily.

Evidence for strategic egg production in a hermaphroditic cestode

Abstract: The cestode Schistocephalus solidus is a simultaneous hermaphrodite that grows in 2 intermediate hosts and reproduces rapidly within a few days in the gut of a bird. Reproduction takes place by self- or cross-fertilization. Here, it was tested whether egg production differs between S. solidus that reproduce alone and those that are allowed to reproduce in pairs. Egg production in an in vitro system was found to depend on the cestodes' social situation. When kept alone, larger cestodes produced larger eggs. This was not so when kept in pairs--the difference between these 2 reproductive modes being highly significant in this respect. Further experiments revealed that, within the first 3 days, these hermaphrodites produced a larger total egg mass when kept alone than when kept in pairs. This was also reflected by the energy contents of the cestodes after this time-span: selfers had used up more energy than paired worms. Furthermore, S. solidus appeared to adjust its investment per egg depending on whether the offspring will be the result of self- or cross-fertilization. Selfers produced larger numbers of eggs, but these eggs were smaller and contained even smaller embryos per given egg size than eggs of potentially outbreeding cestodes. Selfed eggs reached lower hatching rates. Although this is to be expected from inbreeding depression it may also be an effect of the reduced maternal investment per egg. The observed phenotypic plasticity in the reproduction of S. solidus is discussed within 4 evolutionary frameworks: local mate competition adjusted for hermaphrodites, the hermaphrodite's dilemma, bet-hedging, and sib-competition.

Genetic variation in sympatric Ascaris populations from humans and pigs in China.

Abstract: It has recently been shown using genetic markers that Ascaris in humans and pigs in Central America comprise reproductively isolated populations. We present a similar analysis for a region of China in which close association between pigs and humans has been the norm for thousands of years, and agricultural practices will result in frequent exposure to eggs from both sources. DNA fragments from selected regions of mitochondrial and ribosomal DNA were amplified by PCR and allelic forms identified following digestion with a panel of restriction enzymes, using DNA from a total of 115 individual worms from both people and pigs from 2 neighbouring villages. Significant frequency differences in both mtDNA haplotypes and the rDNA spacer were found between the 2 host-associated populations, indicating that they represented reproductively isolated populations. Mitochondrial haplotype frequencies were different from those observed in Guatemala and also from other Asian Ascaris populations, suggesting low levels of gene flow between populations. However, we found no evidence for significant heterogeneity in the genetic composition of Ascaris infrapopulations in either humans or pigs, possibly indicative of agricultural practices in China which have resulted in a random distribution of alleles within the parasite populations.

Ultrastructure of early stages of infections in mice fed Toxoplasma gondii oocysts.

Abstract: Transmission electron microscopy was used to study Toxoplasma gondii infections in the small intestines of Swiss-Webster mice at 2-48 h post-feeding of oocysts (p.f.). Sporozoites passed through intestinal epithelial cells (enterocytes and goblet cells) and infected all cells except red blood cells in the lamina propria. Parasites in intestinal epithelial cells or in cells in the lamina propria were located within a single type of parasitophorous vacuole, which contained exocytosed electron-dense material and well-developed tubulovesicular membranous networks. Sporozoites did not infect intraepithelial lymphocytes (IELs), but at 48 h p.f. IELs had become infected with tachyzoites arising from those that had developed in the lamina propria. At 48 h p.f., the lamina propria contained numerous tachyzoites, much cellular debris, and few intact cells. The intestinal epithelium exhibited limited cytopathological changes except for villar fusion, slight vacuolation, and cell separation at the bases of enterocytes.

Heterogeneities in schistosome transmission dynamics and control.

Abstract: We review the theoretical framework for exploring the impact of individual and spatial heterogeneities in patterns of exposure and contamination and on the basic reproduction number, R0, for human schistosomes. Analysis of water contact data for 5 communities in Zimbabwe and Mali suggests that the impact is substantial, increasing R0 by factors of up to 6.5, mostly due to highly overdispersed distributions of contact rates among individuals. Several practical conclusions emerge: concentration of contacts at a single site should be avoided; the impact of control targeted at certain sites cannot be predicted without knowledge of how individuals' contacts are distributed among sites; control programmes targeted at individuals or sites contributing most to transmission can be very efficient but, conversely, will be ineffective if any of these individuals or sites are missed.

In vivo selection of a population of Trypanosoma cruzi and clones resistant to benznidazole.

Abstract: A benznidazole-resistant population of Trypanosoma cruzi, Y strain, was selected after 25 successive passages (8 months) in mice treated with a single high drug dose. Initially, the resistant parasites produced a low parasitaemia level and low mortality rate in infected mice. Thereafter, the parasitaemia level and mortality rate increased to the same value obtained for mice infected with the wild-type strain. Long-term treatment with benznidazole (100 mg/kg/day) cured 71-80% of mice infected with the wild-type strain. No cure was observed in mice infected with the selected resistant parasite population. Treatment with 500 mg/kg of benznidazole at peak parasitaemia cleared all blood parasites from mice infected with wild-type parasites. No effect on parasitaemia level was observed in mice infected with the selected parasites. Benznidazole-resistant parasites showed cross-resistance to different drugs. Contrary to wild type, all clones analysed from the resistant T. cruzi population were resistant to benznidazole. Without drug pressure the resistance phenotype of clones was far more stable than that presented by the resistant population. This work demonstrates, for the first time, the in vivo selection of a population and clones of T. cruzi resistant to benznidazole, and makes available an experimental model for the study of mechanisms of drug resistance in T. cruzi.

Associations among multiple geohelminth species infections in schoolchildren from Pemba Island

Abstract: In order to estimate the potential benefits of interventions against multiple geohelminth species in endemic areas, an improved understanding of the population biology of multiple infections is required. This paper presents a detailed analysis of the associations among Ascaris lumbricoides, Trichuris trichiura and hookworm infections in 1539 schoolchildren on Pemba Island, Tanzania, where 58% of the sampled children carried infections of all 3 parasites at the time of the study. Infection intensities of different species were positively correlated, and individuals with single-species infections had generally lower species-specific egg counts than individuals with multiple-species infections. There was no age- or sex-related clustering of infections. A weak clustering of intense infections among individuals with multiple-species infections was observed, which became more pronounced as the threshold defining an intense infection increased for each species. The results suggest that individuals with multiple species infections are likely to be at highest risk of geohelminth-related morbidity, not only because of the number of infections they harbour, but also because they generally carry heavier infections of each species.

Acidic compartments and rhoptry formation in Toxoplasma gondii

Abstract: DAMP (3-(2,4-dinitroanilino)-3′amino-N-methyldipropylamine), which differentially accumulates in acidic compartments, was used to identify such compartments in Toxoplasma gondii tachyzoites at the electron microscope level. In both free tachyzoites and dividing intracellular parasites the only sites of DAMP accumulation were mature and forming rhoptries. No labelling of other secretory organelles (micronemes and dense granules), the ER, Golgi or any other membrane-bounded organelles or anything resembling a lysosomal system was observed. Labelling of the forming rhoptries was higher and more homogenous than in mature rhoptries in which labelling was confined to the expanded ends of each organelle. The acid pH-dependent accumulation of DAMP in the forming and mature rhoptries was blocked by ammonium chloride and monensin, reagents known to abolish intracellular pH gradients. Estimates of rhoptry pH, based on the level of DAMP accumulation, show that the intralumenal pH of forming rhoptries is more acidic (pH 5·5–3·5) than the mature rhoptries (pH 7·0–5·0).

An analysis of the temperature effects of fever on the intra-host population dynamics of Plasmodium falciparum

Abstract: Observations that growth of Plasmodium falciparum in vitro is inhibited by high temperatures have led to hypotheses that malaria fever may influence the parasite population dynamics, regulating parasite density and synchronizing parasite growth. In order to investigate the fever hypotheses, we have developed an age-structured coupled Markov chain model that describes the parasite erythrocyte cycle and its interaction with the host fever response. We estimated the model parameters using data collected from laboratory parasite cultures that were exposed to febrile or normal temperature. Using the experimental parameter values, quantitative predictions were made of the effect of fever in determining the parasite population dynamics. It was concluded from the model behaviour that, during the primary infection of a non-immune host, a typical episode of fever can effect density-dependent regulation of the parasite population, maintaining cycles of parasitaemia and promoting synchronous parasite growth.

Displaced tick-parasite interactions at the host interface

Abstract: Reciprocal interactions of parasites transmitted by blood-sucking arthropod vectors have been studied primarily at the parasite-host and parasite-vector interface. The third component of this parasite triangle, the vector-host interface, has been largely ignored. Now there is growing realization that reciprocal interactions between arthropod vectors and their vertebrate hosts play a pivotal role in the survival of arthropod-borne viruses, bacteria, and protozoa. The vector-host interface is the site where the haematophagous arthropods feeds. To obtain a blood meal, the vector must overcome the host's inflammatory, haemostatic, and immune responses. This problem is greatest for ixodid ticks which may imbibe as much as 15 ml blood whilst continuously attached to their host for 10 days or more. To feed successfully, the interface between tick and host becomes a battle between the host's mechanisms for combating the tick and the tick's armoury of bioactive proteins and other chemicals which it secrets, via saliva, into the feeding lesion formed in the host's skin. Parasites entering this battlefield encounter a privileged site in their vertebrate host that has been profoundly modified by the pharmacological activities of their vector's saliva. For example, ticks suppress natural killer cells and interferons, both of which have potent antiviral activities. Not surprisingly, vector-bone parasites exploit the immunomodulated feeding site to promote their transmission and infection. Certain tick-bone viruses are so successful at this that they are transmitted from one infected tick, through the vertebrate host to a co-feeding uninfected tick, without a detectable viraemia (virus circulating in the host's blood), and with no untoward effect on the host. When such viruses do have an adverse effect on the host, they may impede their vectors' feeding. Thus important interactions between ticks and tick-borne parasites are displaced to the interface with their vertebrate host-the skin site of blood-feeding and infection.