During the past decade, enormous technological developments have occurred in biology that have led to significant and revolutionary advances. New techniques of DNA cloning, restriction-enzyme analyses and nucleotide sequencing are providing a mass of data concerning the genomes of a wide variety of organisms. Such insights are having a great impact upon many areas of biological investigation, and would seem to be of considerable potential value for studies of taxonomy and population biology. The application of these new approaches to the characterization and identification of parasitic helminths has only recently begun, but they promise to become powerful additional tools for this purpose. Better methods of characterization are required for more precise definition of the parasites of man and domestic animals and for determining vectors and intermediate hosts as well as possible animal reservoir hosts. Moreover, a greater understanding of the genetic diversity of parasitic organisms is required since many helminths, which are morphologically similar, show marked differences in epidemiologically significant factors such as infectivity, pathogenicity, immunogenicity and drug sensitivity.

Molecular approaches are providing new insights into the genetic diversity of schistosomes and their intermediate snail hosts. For instance, molecular tools based on the polymerase chain reaction are being developed for the diagnosis of schistosomiasis and the detection of prepatent schistosome infections in snails at transmission sites. Robust phylogenies of the different species of Schistosoma, Bulinus and Biomphalaria have been determined and novel methods are available to identify the different and cryptic taxa involved. Microsatellite analyses and mitochondrial DNA sequencing methods have been developed and are contributing to a better understanding of the genetic structure of both schistosome and snail populations. New sampling procedures to capture DNA of eggs and larval stages of schistosomes in field situations are facilitating more detailed and ethically advantageous studies on parasite heterogeneity. Knowledge of the genetic diversity of schistosome and snail populations adds a further dimension to the monitoring and surveillance of disease, and the implementation of new molecular-based approaches will be of increasing importance in helping to assess the impact of schistosomiasis control strategies.

We have examined genetic variability in parasites in the context of ecological interactions with the host. Recent research on Echinococcus, Giardia and Cryptosporidium has been used to illustrate: (i) the problems that parasite variability and species recognition pose for understanding the complex and often controversial relationship between parasite and host occurrence; (ii) the need for accurate parasite characterization and the application of appropriate molecular techniques to studies on parasite transmission if fundamental questions about zoonotic relationships and risk factors are to be answered; (iii) our lack of understanding about within-host interactions between genetically heterogeneous parasites at the inter-and intraspecific levels, and the significance of such interactions with respect to evolutionary considerations and the clinical outcome of parasite infections. If advances in molecular biology and mathematical ecology are to be realized, we need to give serious consideration to the development of appropriate species concepts and in vivo systems for testing the predictions and assumptions of theoretical models.

A gene encoding a protein of 646 amino acid residues with a molecular mass of 71·3 kDa showing homology to the cytoplasmic form of the 70 kDa heat shock protein was cloned and sequenced from the nematode parasite Trichinella britovi (Tb). The gene was expressed in vitro as a protein of 71 kDa that was immunoprecipitated by a Trichinella-infected rabbit serum. Monospecific polyclonal antibodies raised against the recombinant Tb Hsp70 expressed in Escherichia coli, recognized a protein of 70 kDa by Western blot analysis of Tb soluble antigen (muscular stage). Tb Hsp70 was located in the nuclei of the muscle larvae as determined by the indirect immunofluorescent pattern on cross-sections of the worm. The expression of this protein was not detected in adult worm nuclei suggesting a differential expression of Hsp70 between the 2 stages of Trichinella.

Random amplified polymorphic DNA (RAPD) markers were used to quantify genetic diversity within and between 5 populations of Schistosoma mansoni within its definitive host (Rattus rattus) and the 5 corresponding populations of the snail intermediate host (Biomphalaria glabrata) from a limited endemic area of murine schistosomiasis on the island of Guadeloupe. Analysis of molecular variance (AMOVA) and canonical correspondence analysis (CCA) were used to test the significance of genetic differentiation between populations. Both methods gave similar results. Of total gene diversity, 15.1% (AMOVA) and 18.8% (CCA) was partitioned between localities for S. mansoni with an absence of association between genetic and geographical distances. Geographical localities accounted for 20.5% (CCA) of the total diversity for B. glabrata populations. The genetic distances between pairs of parasite populations were not correlated with the genetic distances between the corresponding pairs of snail host populations. Such strong patterns of local differentiation of both parasite and snail populations are consistent with predictions based on metapopulation dynamics and may have implications on host–parasite susceptibility relationship through local adaptation processes.

Specimens of Contracaecum spp. from Phalacrocorax auritus and Pelecanus occidentalis from Florida were characterized by sequencing of the small subunit of the mitochondrial ribosomal RNA gene (rrnS) and by PCR-based RFLP analysis of the same gene and of the internal transcribed spacers (ITS) of nuclear ribosomal DNA. Analyses of the rrnS sequence data using the MP and UPGMA approaches yielded trees with similar topologies, delineating 3 main clusters. Specimens from Ph. auritus, morphologically assigned to C. rudolphii (s.l.), were part of the cluster comprising also the other 2 species of the C. rudolphii complex (A and B), but representing a genetically distinct group, potentially corresponding to a distinct lineage within the complex, provisionally named as C. rudolphii C. The second cluster comprised 5 individuals from P. occidentalis, which formed a genetically relatively homogeneous group. The rrnS data indicate that these specimens (indicated as Contracaecum sp. 1) are clearly genetically different from the morphologically most closely related species, i.e. C. rudolphii (s.l.) and C. microcephalum, and could represent a new species. The third cluster comprised 7 specimens from P. occidentalis morphologically assigned to C. multipapillatum (s.l.). These were shown to be genetically homogeneous and related to but quite distinct from C. multipapillatum from Greece, although additional studies are needed to assess their status. PCR-RFLP based markers for the quick identification of these taxa are provided.

Currently, annual mass treatments with albendazole (ABZ) plus ivermectin (IVM) or diethylcarbamazine (DEC) are administered under the Global Programme to Eliminate Lymphatic Filariasis (GPELF). Drug resistance against both ABZ and IVM is prevalent in nematodes of veterinary importance, raising awareness that if anthelmintic resistance were to develop among Wuchereria bancrofti populations, this would jeopardize GPELF's goals. Genetic structure was incorporated into an existing transmission dynamics model for lymphatic filariasis (LF) to investigate the potential development of concurrent resistance to ABZ and IVM. The resultant models explore the impact of different inheritance modes of resistance to ABZ and IVM on the likely risk of treatment failure under our model assumptions. Results indicate that under ABZ+IVM combination, selection for resistance to one drug is enhanced if resistance to the other drug is already present. Excess parasite homozygosity may increase selection for dominant IVM resistance via enhancing the frequency of recessive ABZ resistance. The model predicts that if multiple resistance genes are associated with different efficacy properties of a drug combination, then examining changes at single loci may be misleading. Sampling schemes in genetic epidemiological surveys investigating the frequency of an allele under selection should consider host age, as individuals of different ages may acquire parasites at different rates.

The successful cellular immune response of larvae of D. melanogaster against A. tabida is characterized by an increase in the number of blood cells, a premature transformation of spherical plasmatocytes to flattened, disc-shaped lamellocytes, and the lysis of crystal cells. The plasmatocytes and transformed lamellocytes rapidly aggregate around the eggs of the parasite to form a cellular capsule, while the lysing crystal cells release substances causing the melanization of the capsule. The early appearance of large numbers of transformed blood cells in the haemolymph prior to their involvement in capsule formation suggests that transformation is independent of cell contact with the parasite and may be due instead to biochemical stimuli emitted from the parasite and/or by hormonal changes. Quantitative blood cell studies of hosts exhibiting no encapsulation and melanization response indicate that successfully developing parasites are recognized by the immune system but are able sufficiently to block the blood cell transformation and thus prevent capsule formation.

Uptake of radioactively labelled purine bases and nucleosides by suspensions of Trypanosoma brucei and Trypanosoma congolense in bicine buffer was determined at 37 °C. With T. brucei, the rate of uptake of adenosine was much greater than that of the other compounds tested, the uptake of which decreased in the order adenine, inosine, guanosine and hypoxanthine. With T. brucei, adenosine uptake increased with concentration in a manner suggesting two mechanisms, one with high and the other with low affinity for adenosine. The uptake of adenine increased with concentration only up to about 1·5 µM while the uptake of guanosine increased little with concentration and that of inosine and hypoxanthine not at all. In both species adenosine strongly inhibited the uptake of both of the other nucleosides and of both purine bases. In T. brucei, guanosine and inosine caused small increases in adenosine uptake which was, however, inhibited by them in T. congolense. In T. brucei, each of the purine bases adenine and hypoxanthine inhibited its own uptake maximally but that of each other less effectively. Hypoxanthine was more effective than adenine in inhibiting the uptake of the nucleosides guanosine and inosine, but neither base effected marked inhibition of adenosine uptake. The uptake of adenosine by T. brucei was inhibited by dipyridamole and its analogue, compound RA–233, strongly at 100µM and slightly at 10 µM. The other dipyridamole analogues, VK–744 and VK–774, were ineffective.

We provide evidence from isoenzyme analysis, hybridization with repetitive DNA probes, behavioural studies and morphometrics that 4 trypanosome isolates from Glossina morsitans submorsitans in The Gambia constitute a new species now named Trypanosoma (Nannomonas) godfreyi. The bloodstream trypomastigotes of T. (N.) godfreyi are relatively small with a mean length of 13·7 μm (range: 9·1–21·8 μm) and a mean width of 1·65 μm (range: 0·65–2·69 μm). There is no free flagellum and the marginal kinetoplast is subterminal to a rounded posterior end; the undulating membrane is usually conspicuous. As with other Nannomonas, T. godfreyi developed in the midgut and proboscis of Glossina and infections matured in 21–28 days in laboratory G. m. morsitans. In The Gambia the normal vertebrate host appears to be the warthog, Phacochoerus aethiopicus, although elsewhere other wild and domestic suids may also be implicated in the life-cycle. T. godfreyi was identified unequivocally using a 380 bp DNA probe specific for a major genomic repeat sequence; its isoenzyme profile distinguished it clearly from T. simiae and three strain groups of T. congolense: savannah, riverine-forest and kilifi.

The present revision of the morphology, taxonomy and affinities of trypanosomes causing Surra in the Old World was undertaken in order to determine the status of the diverse geographical and hostal strains, the exact relation of which to Trypanosoma evansi and allied species was hitherto a subject of controversy. The material for this study was represented by thirty strains from diverse mammalian hosts and various countries, extending from Morocco to the Philippines.

One of the problems dealt with is the occurrence of polymorphism in T. evansi. When single samples of typical (monomorphic) strains from naturally infected animals were examined, only negligible numbers of stumpy forms (0·05–0·40 %) were found in a minority of cases. But in strains kept under continuous observation for several years the proportion of stumpy forms occasionally rose higher (2–25%), while one strain exhibited a remarkable and persistent degree of polymorphism in the course of 17 years, the proportion of stumpy forms—including numerous posteronuclear ( = posterior-nuclear) ones—fluctuating from 0 to 60%. It was shown that—as in the case of the Brucei group—the length distributions in polymorphic samples of T. evansi could be statistically resolved into distinct curves, corresponding to the three morphological components (stumpy, intermediate, slender).

Circulating haemocytes from Echinostoma paraensei-infected M line Biomphalaria glabrata snails, or from age- and sizematched control snails, were studied on plastic slides with phase-contrast optics. Granulocytes, hyalinocytes, and round cells were consistently present; granulocytes were further categorized as ‘fully spread' (FS) or ‘partially spread' (PS). Among control snails, the relative percentage and estimated number/mm3 of round cells declined significantly with increased snail size, and the corresponding values for both categories of granulocytes increased. At 1 day post-infection (p.i.) with E. paraensei, overall composition of the haemocyte population was relatively unaffected, but by 8 days p.i. infected snails had significantly higher relative percentages of round cells and PS granulocytes than controls. Because a marked increase in the number of circulating haemocytes is also evident by 8 days p.i., infected snails had approximately 12 times more round cells and PS granulocytes/mm3 of haemolymph than did controls. At 30 days p.i. the relative and absolute abundance of PS granulocytes was still significantly elevated, but otherwise haemocyte populations did not differ from control snails. Alterations in granulocyte size in infected snails were also noted. Infection with E. paraensei has a striking impact of circulating haemocyte populations and also increases the relative concentration of haemocytes with less ability to adhere to a foreign surface.

Steinernematid nematodes form a developmentally arrested infective juvenile (IJ) stage at the second moult, when conditions inside the insect host are no longer suitable for further reproduction. In a liquid culture micro-assay two environmental cues were shown to influence the formation of Steinernema feltiae IJs. High nematode population density induced IJ formation, suggesting the presence of a nematode pheromone. Bacterial food and soluble nutrients acted competitively to reduce the frequency of IJ formation. Frequency of IJ formation was greatest when lst-stage juveniles were subjected to IJ-inducing conditions. The optimum temperature range for the IJ induction response was 25–30°C. These findings suggest that maximal IJ production in large-scale liquid culture will depend on the correct balance of nematode population density and nutrient availability at the peak of egg hatching.

The survival characteristics of Discocotyle sagittata larvae are both age- and temperature-dependent. In laboratory studies at constant temperatures of 6, 10, 13, 18 and 22°C, oncomiracidia had a maximum life-span of 96 h at 6°C, declining with increasing temperature to 26 h at 22°C. Larval swimming activity was also age dependent, and continued for a large proportion of survival time. The relationship between time at which larvae stop swimming as a proportion of total survival time was statistically significant between temperatures. Proportionally, oncomiracidia swam for longer periods at lower temperatures. As oncomiracidia age, they become progressively less active, spending less time in the water column. In contrast to observations recorded in the literature, D. sagittata larvae are infective soon after emergence (within the minimum of 2 h studied), and feed on blood from the gill capillaries within 2 h post-exposure. The temperature-dependent characteristics of the larval stage are likely to have important implications for seasonal changes in parasite transmission.

During trans-dermal invasion of the vertebrate host, larval schistosomes (cercariae) transform into schistosomula and become enveloped by a double lipid bilayered, tegumental membrane. The glucose transporter protein SGTP4 is found exclusively in these host-interactive tegumental membranes and in membranous vesicles proposed to be their precursor. In this study, we monitored the appearance and migration of this tegumental marker protein during larval transformation to test the current model of tegumental membrane biosynthesis in parasitic blood flukes. Only minutes after transformation was initiated, SGTP4 began accumulating in a previously unrecognized, bilaterally symmetrical, ‘cyton network’ beneath the peripheral muscle. Approximately 30min after the initiation of transformation the marker protein was seen in tubules connecting the network to the surface and erupting onto the surface in discrete patches. After 1h the patches were regularly arrayed over the schistosomula body and began to cover the anterior organ. By 3h the staining has largely resolved into a contiguous layer of fluorescence covering most of the worm surface. These findings confirm earlier suggestions, based on electron microscopy, that the parasite's surface tegumental membranes are derived from the migration of membranous vesicles produced within cytons and reveal a new subtegumental architecture interconnecting the cytons.

Since the appearance of Johnston's Catalogue of the British Non-parasitical Worms in the Collection of the British Museum—a work which although not published until 1865 had been completed some ten years earlier and consequently embodies observations made more than half a century ago—the Hirudinea of the British Islands have been largely neglected. Apart from this the interest now being taken in leeches by students of their protozoan parasites and the absence of any recent work upon this group of our fauna seems sufficient apology for the revised descriptive catalogue attempted in the following pages.

Analyses of laboratory-based helminth-rodent model systems have been immensely useful in delineating the workings of the mammalian immune system. Investigations in the 1970s–1980s on the fate of the rat tapeworm, Hymenolepis diminuta, in rats and mice and the systemic and local responses evoked following infection have contributed directly to our knowledge of how permissive and non-permissive hosts respond to the challenge of infection with a helminth parasite. This convenient laboratory model system has, in the authors' opinion, regrettably received considerably less attention in recent years. With the goal of highlighting the utility of this model system, data is presented on: (1) the immune and enteric responses of rats and mice to infection with H. diminuta; (2) the ability of excretory or secretory products derived from H. diminuta to significantly reduce T cell and macrophage activation in vitro; and (3) how assessment of H. diminuta-rodent models can be used to identify immune effector or regulatory mechanisms that can be translated into novel treatments for inflammatory and autoimmune disorders.

The present study compared the genetic variation among 14 different geographical isolates of Opisthorchis viverrini sensu lato from Thailand and Lao PDR using sequence data for 2 mitochondrial DNA genes, the subunit 1 of NADH dehydrogenase gene (nad1) and cytochrome c oxidase gene (cox1). Four different nad1 haplotypes were detected among isolates, all of which were identical at the amino acid sequence level. Nucleotide sequence variation among 14 isolates ranged from 0 to 0·3% for nad1. Two different cox1 haplotypes were detected among isolates. These two haplotypes differed at 2 nucleotide positions, one of which resulted in a change in the amino acid sequence. Nucleotide sequence variation among isolates for cox1 ranged from 0 to 0·5%. Comparison of cox1 sequences of O. viverrini to those of other trematodes revealed nucleotide differences of 13–31%. A phylogenetic analysis of the cox1 sequence data revealed strong statistical support for a clade containing O. viverrini and 2 other species of opisthorchid trematodes; O. felineus and Clonorchis sinsensis.

Toxoplasma gondii infections are common in humans and animals worldwide. Domestic free-range chickens (Gallus domesticus) are excellent sentinels of environmental contamination with T. gondii oocysts because they feed on the ground. Chickens can be easily infected with T. gondii; however, clinical toxoplasmosis is rare in these hosts. Chickens are comparatively inexpensive and thus are good sentinel animals for T. gondii infections on the farms. Here, the authors reviewed prevalence, the persistence of infection, clinical disease, epidemiology and genetic diversity of T. gondii strains isolated from chickens worldwide for the past decade. Data on phenotypic and molecular characteristics of 794 viable T. gondii strains from chickens are discussed, including new data on T. gondii isolates from chickens in Brazil. This paper will be of interest to biologists, epidemiologists, veterinarians and parasitologists.