An attempt has been made to study the movements and feeding habits of juvenile forms of Fasciola hepatica as they pass through the wall of the host's intestines during the first 24 hr. of experimental infections in the mouse. Metacercarial cysts were fed to mice in large numbers (100–500) and the flukes were examined by means of sections during excystment and as free flukes and flukes making their way through various tissues. Excystment is regarded as essentially a feeding operation, the young fluke breaking through the fibrous inner cyst by the physical action of the ventral sucker in weakening the wall and the oral sucker in the actual breakthrough. Free forms were found in sections of infected intestines both in the lumen and lying between the villi, and penetrating forms were located inside the villi, in the submucosa and passing through the muscular layers and the serosa. A great deal of evidence was accumulated which indicates that the young fluke is able to break down epithelia, connective tissue and unstriped muscle fibrils and to feed upon the resultant products, thus proving itself to be a tissue feeder in the broadest sense of the term. The fluke's passage through the intestinal wall is by a devious route, tangential to the mucosa for some distance and then usually diagonally through the muscular layers, following the direction in which the circular muscle cells are oriented but cutting across the longitudinal muscle cells. This is probably due to the angle which the rim of the oral sucker makes with the longitudinal axis of the body, and it is a modification of the browsing habit which is seen when the feeding fluke lies on a plane surface. The tissue feeding proclivities of young flukes were also displayed in instances of penetration into lymph nodes and adipose tissue. These evidences support the opinion previously expressed by the writer that flukes migrating through the hepatic parenchyma, which are merely forms a little older and larger than the forms studied in the present researches, cannot be regarded as blood-feeders in the strict sense of the term but must also be regarded as tissue feeders. Attention is directed to the fact that very few digenetic trematodes have caecal outgrowths and that Fasciola and Fascioloides stand apart from all other genera in having dendritic outgrowths of the intestines. It is remarked also that the development of the ramifications of the intestines in Fasciola hepatica occurs during the first month, and largely during the second week, of juvenile life when the young flukes are eating their way through the hepatic parenchyma. During a few days the ramifications of the caeca develop to a greater extent than occurs during the entire lifespan of all other digenetic trematodes, with the exception of Fascioloides magna.

A FMRFamide-like peptide has been detected in the nematode Ascaris suum, using the peroxidase-anti-peroxidase (PAP) immunocytochemical technique. Positive reactions were obtained in both the central nervous system and the peripheral nervous system of the worm, the strongest reactions being in the anterior nerve ring, the cephalic papillary ganglia, the lateral ganglia and the dorso-rectal ganglion. Immunoreactivity was observed along the length of the main nerve cords of the worm and, to a lesser extent, in the pharyngeal nerve cords. The possible role of this neuropeptide in the physiology of the nematode is discussed.

1. The anatomy of the male and female genitalia of Parorchis acanthus is described in detail. An extensive receptaculum uterinum occurs in the female system.

2. The chromosome number in P. acanthus is twenty-two.

3. Spermatogenesis, oogenesis, and fertilization, are described in the adult worm. The chromosome number in both male and female cells is reduced to eleven at the first meiotic division.

4. Maturation in the female takes place in the “receptaculum uterinum”.

5. Cleavage of the ovum to the three cell stage has been followed.

6. The probable synonymy of P. acanthus and P. avitus is discussed.

A method of interrupting the life-cycle of the human blood fluke Schistosoma by increasing the proportion of genetically insusceptible intermediate host snails in natural populations was first proposed nearly 25 years ago. The method assumes that insusceptible snails will be at a selective advantage over susceptible snails when the schistosome parasite is present, and therefore natural selection will act to increase the proportion of alleles for insusceptibility. A major objection to the proposed technique is ‘If insusceptible snails are at a selective advantage, then why are they not predominant in natural populations that transmit disease?’ One explanation of this paradox is that insusceptibility may be associated with a disadvantageous character or a physiological defect. This study tests this hypothesis by measuring the relative reproductive success of susceptible and insusceptible snails under controlled conditions. Results indicate that insusceptible (unsuitable) snails are negatively affected in the presence of either susceptible snails or schistosome parasites. Furthermore, in the presence of both susceptible snails and schistosome parasites, insusceptible snails are selectively disadvantaged compared to susceptible snails. These results obtained under laboratory-controlled conditions suggest a plausible answer as to why insusceptible snails are not predominant in natural populations that transmit disease.

The trematode and cestode flatworms include numerous parasitic forms of major medical and economic importance. A better knowledge of the neuromuscular physiology of these animals could lead to development of new control measures against these parasites. Since these animals are near the stem from which all other animals have evolved, better knowledge of these animals could also yield valuable information about the early evolution of nerve and muscle systems in the animal kingdom. This review focuses on what is known about the characteristics of the somatic muscle in these animals. The anatomy of the muscles is described along with a review of current information about their electrophysiology, including descriptions of the ion channels present. Also included is a summary of recently acquired data concerning the nature of serotonin, peptide, acetylcholine and glutamate receptors on the membranes of the muscles.

Adult Trichinella spiralis were maintained in vitro using defined media and the material excreted/secreted (ES) during this time examined for proteolytic enzyme (proteinase) activity using an azocasein assay and gelatin-substrate gels. Several discrete proteinases in the size range 14–100 kDa were observed with optimal activity at pH 7·5. The use of a class-differentiating panel of proteinase inhibitors indicated that serine proteinases were predominant although some inhibition was evident in the presence of cysteine and metalloproteinase inhibitors. Of a panel of potential natural protein substrates tested, ES proteinases only degraded fibrinogen and plasminogen and degradation was, in part, susceptible to the action of serine, cysteine and aspartyl proteinase inhibitors. In addition, antibody harvested from immune but not normal mice inhibited ES proteinase activity, an observation of relevance to the immunobiology of Trichinosis.

In this study, cDNA microarray analysis of a closely related species, Leishmania major, was used as a screening tool to compare antimonial-resistant and susceptible clinical isolates of Leishmania donovani in order to to identify candidate genes on the basis of antimony resistance. Clinically confirmed resistant isolate 39 and sensitive isolate 2001 were used in this study. Many differentially regulated genes were identified whose expression levels differ in sodium antimony gluconate (SAG)-treated patients. Interestingly, genes on the array, showing changes in expression of over 2-fold revealed the identity of ABC transporters, which are known determinants of drug resistance in laboratory mutants. The functionality of the transporters was validated by flow cytometry which, being biologically informative, provides direct clues to gene function. The results suggest that isolate 39 could have developed resistance by an increased multidrug resistance protein (MRP)-like pump. This study provides preliminary clues to the role of a thiol-dependent efflux system in antimonial resistant clinical isolates of Leishmania donovani.

Neospora caninum causes economical impact in cattle-raising farms since it is implicated as the major cause of bovine abortions. Although infection by the parasite has been widely described in mammals, the role of birds in its life-cycle is still obscure. Therefore, this work aimed to evaluate the infection by N. caninum in different chicken models. Experimental infections were conducted in 7-day-old chicks, laying hens and embryonated eggs, where samples were analysed for parasite burden, IgG antibodies and lesions promoted. Chickens demonstrated an asymptomatic infection, although with seroconversion and systemic replication of the parasite. In laying hens, no signs of vertical transmission were observed. However, embryonated eggs inoculated by the allantoic cavity route demonstrated susceptibility to infection, with mortality rates around 50% independent of the inoculum dose. Additionally, dogs became infected after ingestion of different amounts of inoculated eggs, producing either oocysts or specific IgG antibodies. The results herein presented demonstrate that chickens may be intermediate hosts of N. caninum and that embryonated eggs could be a useful model to study the parasite's biology.

Recrudescing Plasmodium falciparum parasitaemia is attributed to the switching of PfEMP1, a variant antigen family encoded by the var gene repertoire, and the host's immune response. We have developed a mathematical model which incorporates var gene switching, and variant specific, non-variant specific and non-specific immunity. By conducting a sensitivity analysis of the model we have defined the parameter limits which produce chronic and recrudescing infections. We explore 3 switching mechanisms: ordered, random and uncoupled switching. We show that if var genes switch on and off independently at variable rates through the repertoire a chronic clinical infection is predicted. The fastest switching-on rate that produces a chronic infection is 0·03% per generation. The model predicts that non-variant specific immunity plays an important role in reducing disease severity. This work illustrates the complex relationship between the malaria parasite and its host and shows that var gene switching at rates substantially slower than 2% are essential for parasite survival.

One of the major threats to biodiversity involves biological invasions with direct consequences on the stability of ecosystems. In this context, the role of parasites is not negligible as it may enhance the success of invaders. The red-eared slider, Trachemys scripta elegans, has been globally considered among the worst invasive species. Since its introduction through the pet trade, T. s. elegans is now widespread and represents a threat for indigenous species. Because T. s. elegans coexists with Emys orbicularis and Mauremys leprosa in Europe, it has been suggested it may compete with the native turtle species and transmit pathogens. We examined parasite transfer from American captive to the two native species that co-exist in artificial pools of a Turtle Farm in France. As model parasite species we used platyhelminth worms of the family Polystomatidae (Monogenea) because polystomes have been described from American turtles in their native range. Phylogenetic relationships among polystomes parasitizing chelonian host species that are geographically widespread show patterns of diversification more complex than expected. Using DNA barcoding to identify species from adult and/or polystome eggs, several cases of host switching from exotic to indigenous individuals were illustrated, corroborating that parasite transmission is important when considering the pet trade and in reintroduction programmes to reinforce wild populations of indigenous species.

To determine whether the relative abundance of immature Ixodes dammini (the vector of Lyme disease and human babesiosis) is related to habitat structure, we examined tick burdens on their main host, the white-footed mouse (Peromyscus leucopus), in 4 structurally diverse sites on Great Island, Massachusetts, USA. Vegetation structure at each site was quantified with respect to 25 habitat variables. Principal components analysis was used to reduce this set of habitat variables to seven new and orthogonal variables. Immature tick abundance varied widely among grids. Regression analysis of tick burdens on the habitat principal components showed that larval burdens were related strongly to the density of woody vegetation and negatively to herbaceous vegetation. Nymphal burdens were related negatively to herbaceous vegetation, but the relationship was not as strong as in the case of larvae. An experimental reduction in the abundance of white-tailed deer (Odocoileus virginianus), the main host of adult ticks, substantially reduced tick burdens and altered their relationships to habitat structure. Nymphal burdens were unrelated to habitat structure following deer removal. Manipulating habitat structure may have utility as a control strategy against this important vector.

Various authors have recorded the seasonal occurrence of different species of nematode parasites in sheep. Some of these (Ross & Gordon, 1936; Gordon, 1950; Taylor, 1934) have been mainly interested in the epidemiology of outbreaks of parasitic disease, but others have recorded the incidence of different species in flocks in which no outbreak has occurred (Gordon, 1948, 1950; Morgan & Oldham, 1934; Morgan & Sloan, 1947; Morgan, Parnell & Rayski, 1950; Parnell, 1954; Parnell, Rayski, Dunn & Mackenzie, 1954; Stewart & Douglas, 1938; Tetley, 1935, 1941, 1949; Wilson, Morgan, Parnell & Rayski, 1953).

The cuticle of the third-stage larva of Nippostrongylus brasiliensis consists of seven layers: an outer triple-layered membrane, a double-layered outer cortex, an inner cortex, a matrix layer, a striated layer and two fibril layers. In each ‘annule’ two fibres run transversely around the nematode and lie between the inner cortex and the matrix layer. There is no basement lamella.

The hypodermis is a thin layer between the muscles and the cuticle, but expands to form the dorsal, ventral and lateral cords. The nerves lie between the plasma membrane of the hypodermis and the basement membrane or between the plasma membrane of the hypodermis and the sarcolemma of the muscles. The muscle cells are typical of those described previously for nematodes. The ‘myofibrils’ are apparently similar to those of Ascaris.

An excretory canal is present in each lateral cord and is enclosed by the basement membrane but is not embedded in the hypodermal tissue. Numerous small vesicles appear to move across the wall of the excretory canal and open into the central lumen.

I am grateful to Dr P. Tate, Dr R. W. Horne and Dr K. A. Wright for helpful discussions, to Professor C. P. Read and Dr A. Enders for the use of facilities at Rice University, Houston, Texas and to Professor J. D. Boyd for permission to use the electron microscope in the Department of Anatomy. Thanks are also due to Mr A. J. Page for technical assistance.

Ectoparasites of livestock are of great economic and social importance but their effective control remains difficult. The feasibility of vaccination as a novel control measure was established over a decade ago with the commercial release of a recombinant vaccine against the cattle tick Boophilus microplus. Since then, research has continued on ticks and other ectoparasites. While some ectoparasite species will undoubtedly be refractory to immunological control, for others there has been a steady accumulation of knowledge of partially protective antigens, now accelerating through the application of genomic technologies. Nevertheless, progress towards usable, commercially available vaccines has been limited by a number of factors. The number of highly effective antigens is still very small. Although some classes of antigen have been investigated in more detail than others, we have no systematic knowledge of what distinguishes an effective antigen. Much hope has been placed on the potential of multi-antigen mixtures to deliver the efficacy required of a successful vaccine but with little experimental evidence. The application of current knowledge across parasite and host species needs to be explored but little has been done. In most cases, the path to commercial delivery is uncertain. Although many constraints and challenges remain, the need for vaccines and our capacity to develop them can only increase.

The initiation of genome projects on helminths of medical importance promises to yield new drug targets and vaccine candidates in unprecedented numbers. In order to exploit this emerging data it is essential that the user community is aware of the scope and quality of data available, and that the genome projects provide analyses of the raw data to highlight potential genes of interest. Core bioinformatics support for the parasite genome projects has promoted these approaches. In the Brugia genome project, a combination of expressed sequence tag sequencing from multiple cDNA libraries representing the complete filarial nematode lifecycle, and comparative analysis of the sequence dataset, particularly using the complete genome sequence of the model nematode C. elegans, has proved very effective in gene discovery.

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.

Macrogyrodactylus spp. from the gills of Clarias gariepinus in Zimbabwe and Kenya, and C. anguillaris in Senegal were identified using haptoral sclerite morphology and by sequencing the nuclear ribosomal DNA internal transcribed spacers (ITS) 1 and 2, partial 18S and the complete 5·8S rRNA gene. A molecular phylogeny was constructed using all sequenced Macrogyrodactylus species to date. Based on morphology, Macrogyrodactylus congolensis, M. heterobranchii, M. clarii, and M. karibae were identified, with one specimen from Zimbabwe displaying morphological features that were intermediate between M. heterobranchii and M. clarii. In the intermediate form, the partial 18S and ITS1 sequence was identical to that of M. clarii while the remaining ITS1 and complete ITS2 region was almost identical to M. heterobranchii as was the partial cox1 fragment, thus strongly suggesting a hybrid origin. At present, the catfish host of M. heterobranchii and M. clarii do not co-occur in southern Zimbabwe; this hybridization event is therefore proof of historical sympatry of both fish species.

Little is known regarding the diversity, distribution or host-parasite associations of Trypanosoma spp. in Australian wildlife. Here we report on an investigation based on divergence of the 18S rRNA gene of trypanosomes isolated from a range of hosts and varied geographical locations. A total of 371 individuals representing 19 species of native animals from 14 different locations were screened. In total, 32 individuals from 9 different species tested positive for the parasite. Phylogenetic analysis revealed considerable parasite diversity with no clear geographical distribution and no evidence of host specificity. In general, it appears that Australian Trypanosoma spp. are widespread, with several genotypes appearing in multiple host species and in varied locations including both mainland areas and offshore islands. Some host species were found to be susceptible to multiple genotypes, but no individuals were infected with more than a single isolate.

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.

Periods of low host density impose a constraint on parasites with direct transmission, challenging their permanence in the system. The microsporidium Octosporea bayeri faces such constraint in a metapopulation of its host, the cladoceran Daphnia magna, where ponds frequently lose their host population due to ponds drying out in summer and freezing in winter. We conducted experiments aimed to investigate the mechanisms of transmission of O. bayeri, and discuss how these mechanisms could contribute to the parasite's permanence in the system. Spores accumulate in the fat cells and the ovaries of the host, and vary in morphology, possibly corresponding to 3 different spore types. Horizontal transmission occurred through the release of spores from dead hosts, with the proportion of infected hosts depending on the spore dose. Further, spores are able to persist outside the host both in dry and wet conditions. Vertical transmission occurred to both parthenogenetic and sexual offspring. The former were invariably infected, while the sexually produced resting eggs (=ephippia) had a less efficient transmission. The parasite may be carried by the ephippia, and thus disperse to new ponds together with the host. Together, these mechanisms may allow the parasite to endure periods of harsh environmental conditions both outside and inside the host.