The uptake of bloodstream forms of Trypanosoma cruzi, Y and CL stocks, by mouse peritoneal macrophages and their intracellular differentiation and multiplication has been compared in vitro. After 48 h the number of macrophages showing intracellular amastigote forms was higher when the Y stock was used. The number of parasitized cells increased with the time of contact between parasites and macrophages. Prior treatment of the parasites with anti-T. cruzi antibodies and/or complement increased the number of infected macrophages, but did not interfere with their subsequent differentiation within the macrophages. The number of parasitized cells was greater when macrophages were obtained from mice previously treated with lipopolysaccharide, peptone or thioglycollate. Uptake was not appreciably affected when macrophages were pre-treated with trypsin or anti-macrophage serum, or when the parasites and macrophages were incubated in the presence of cytochalasin B. In the same experimental conditions, epimastigotes of T. cruzi were not able to differentiate into amastigotes. Their uptake was potentiated by previous treatment with specific antibodies and/or complement and was blocked by cytochalasin B. These results confirm that epimastigotes derived from T. cruzi cultures are phagocytosed and suggest that bloodstream forms penetrate actively into macrophages.

Trichinella spiralis infections influenced the establishment and maintenance of behavioural dominance among outbred male mice. Infected animals assumed a subordinate status when challenged by normal or more lightly infected conspecifics both in an unfamiliar test arena and in their established home cages. These effects were demonstrable during the acute and the chronic phases of infection. The significance of this phenomenon to the survival of the mouse host and the transmission of the parasite is discussed.

A trypanosome identified as Trypanosoma (Schizotrypanum) dionisii Bettencourt & França, 1905, has been isolated from Pipistrellus pipistrellus (Chiroptera) in England. At least five out of eight P. pipistrelhis were infected.2. In the blood of P. pipistrelhis, the parasite closely resembled T. (S.) cruzi. When grown in vitro in monophasic or diphasic media at 28°C, epimastigotes and trypomastigotes developed. The latter were of two types — very long, thin forms and less numerous shorter individuals.3. The trypanosomes multiplied as amastigotes within HeLa and mouse L cells in vitro. After 6–9 days in HeLa cells at 37°C, they transformed into small trypomastigotes and emerged from the cells. Higher infection rates (up to about 4%) were obtained in cell cultures inoculated with flagellates from older monophasic cultures, which contained more of the long, slender trypomastigotes.4. Trypanosomes also entered (or were phagocytosed by) up to 60% or more of mouse peritoneal macrophages in vitro. Multiplication occurred by binary and multiple fission of amastigotes in at least some of the parasitized macrophages and transformation into trypomastigotes was seen after 7 or more days at 37°C. Development in macrophages was less synchronous than in HeLa cells.

The egg and larval stages of Trichuris suis can be briefly characterized as follows:The egg: barrel shaped, possesses a thick shell consisting of three thick outer layers and an inner thin vitelline membrane, is operculate at each end and is unsegmented and unfertilized when freshly deposited.L. 1 within the egg: presence of an oral spear, a poorly denned oesophagus and an intestinal tract consisting of undifferentiated granulated material.L. 1 within the host: initial differentiation of an oesophagus, cell body, intestine and rectum.L. 2: further differentiation of the body organs and the appearance of the rudiments of the reproductive system.L. 3: initial development of reproductive system and development of a cloaca in the male thus distinguishing the sexes.L. 4: differentiation of reproductive system into vagina, uterus, oviduct and ovary in the female, and testis, vas deferens, ejaculatory duct, spicule and spicular muscle, sheath and tube in the male.L. 5 or adult stage: completed development of the sexual organs including formation of the vulval orifice and eggs in the female and seminal vesicle in the male.

Leptocotyle minor is a monogenean parasite specialized for living on the denticlecovered skin of the dogfish Scyliorhinus canicula. Each adult parasite attaches itself to the outer surface of a single denticle. Small postlarvae are found on the inner surfaces of the denticles. The denticles have hard enamel on both inner and outer exposed surfaces. Haptoral hooks, such as are present in other monogenean oncomiracidia, would be unable to penetrate this hard surface and are absent in postlarval and adult parasites which attach themselves by cement. The oncomiracidium possesses six haptoral spicules which may be homologues of the marginal hooks of other monogeneans, but these spicules disappear early in postoncomiracidial development.

Adult parasites feed on the host's epidermis which lies between the denticles. The position of small postlarvae on the inner surfaces of the denticles enables them to reach the epidermis for feeding purposes. Growth then permits young parasites to migrate away from the epidermis on to the outer surfaces of the denticles.

Adult parasites move from one denticle to another in a leech-like manner, and copulation occurs between parasites which meet as a result of these movements.

I am grateful to the Director and Staff of the Plymouth Laboratory for their hospitality and particularly to Mr J. E. Green for his interest and co-operation.

A microsporidium of the genus Encephalitozoon was isolated into culture from the nasal epithelium of a patient with AIDS. It was compared with in vitro isolates of Encephalitozoon cuniculi and the type isolate of Encephalitozoon hellem by SDS–PAGE and by Western blotting with murine antisera raised to E. cuniculi, E. hellem and the nasal isolate, monoclonal antibodies raised to E. cuniculi and sequential sera from the patient. All tests showed similarities between E. hellem and the nasal isolate but differences between these two isolates and E. cuniculi. Minor protein differences between E. hellem and the nasal isolate were not considered sufficient to separate them at the specific level. The new isolate is named the Wainwright isolate of E. hellem. The ultrastructure of the Wainwright isolate in vitro was similar to that of the parasite in vivo but there was a greater tendency for disruption of the parasitophorous vacuoles. The deposition of the electrondense surface coat on the sporogonic stages of E. hellem, as a uniform layer which later thickens, is in contrast to its deposition as broad bands, which later join up, in E. cuniculi. This may be a useful character in distinguishing the species without recourse to analysis of protein profiles.

RNA interference (RNAi) is a powerful research tool that has enabled molecular insights into gene activity, pathway analysis, partial loss-of-function phenotypes, and large-scale genomic discovery of gene function. While RNAi works extremely well in the non-parasitic nematode C. elegans, it is also especially useful in organisms that lack facile genetic analysis. Extensive genetic analysis of the mechanisms, delivery and regulation of RNAi in C. elegans has provided mechanistic and phenomenological insights into why RNAi is so effective in this species. These insights are useful for the testing and development of RNAi in other nematodes, including parasitic nematodes where more effective RNAi would be extremely useful. Here, we review the current advances in C. elegans for RNA delivery methods, regulation of cell autonomous and systemic RNAi phenomena, and implications of enhanced RNAi mutants. These discussions, with a focus on mechanism and cross-species application, provide new perspectives for optimizing RNAi in other species.

Faecal egg counts and peripheral blood eosinophil counts were taken from Scottish Blackface lambs following natural, predominantly Teladorsagia circumcincta infection. Peripheral eosinophil concentrations were higher in animals with lower egg counts but only in lambs that were at least 3 months of age. The reduced egg counts were due to reduced fecundity of T. circumcincta; there was no association with the number of adult T. circumcincta. Associations with the number of parasites from other species of gastrointestinal nematodes appeared to be neutral or favourable. Estimated heritabilities for eosinophil concentrations in 4- and 5-month-old lambs were 0·48±0·16 and 0·43±0·17, respectively. Therefore, under defined circumstances, eosinophil concentrations may be a useful indicator of resistance to predominantly T. circumcincta infection.

Praziquantel is widely used for the treatment of human schistosomiasis. However, in recent years, there has been increasing concern about the resistance of Schistosoma species to praziquantel. The study described here was designed to evaluate the current susceptibility to praziquantel in S. japonicum in China. During the non-transmission period of schistosomiasis, a random sample of 4760 subjects from the main endemic foci of China were examined using parasitological stool examination. In total, 584 subjects were identified as being infected with S. japonicum, with a prevalence rate of 12 27%. Among them, 565 stool-egg-positive subjects were treated with praziquantel in a single oral dose of 40 mg/kg. Six weeks post-treatment, among the 505 villagers re-examined, 480 (95 05%) had no detectable S. japonicum eggs. Twenty-one subjects still excreting eggs after the first treatment were treated with praziquantel for the second time. All stool samples, including those from those participants with second treatment were re-examined 6 weeks after the second treatment, and no stool-egg-positives were found. The results indicate that the current efficacy of praziquantel against S. japonicum is still high and has not changed after more than 2 decades of repeated, expanded chemotherapy in the main endemic areas of China. It is suggested that no evidence of tolerance or resistance to praziquantel in S. japonicum was detected in China.

Reproductive success of Gyrodactylus salaris from River Lierelva was compared experimentally on 3 stocks of salmon (12·5°C±0·2). Isolated fish from 2 susceptible Norwegian stocks (Rivers Lier and Alta) and 1 resistant Baltic stock of salmon (River Neva) were infected with a single gravid worm in order to record the temporal sequence of births and age at death of individual parasites. Establishment success (proportion of worms surviving to give birth) was generally low and mortality high, but significantly fewer worms survived on Neva (45% with mean survival of 3·5 days) compared to Alta and Lier fish (60%, mean survival 7·9 and 5·2 days, respectively). There was a dramatic difference in parasite fecundity between the host stocks: only 2 births occurred on Neva fish compared to third and fourth births on both Alta and Lier hosts. The timing of the first birth was more variable on Neva hosts and was significantly extended (mean 2·3 days) relative to that on Alta and Neva fish (1·8 days). However, timing of the second birth did not vary on any of the 3 salmon stocks. Age-specific mortality and fecundity data are consistent with exponential population growth of G. salaris on Alta and Lier fish but eventual extinction on Neva hosts. This is the first demonstration that gyrodactylids maintained on different host stocks exhibit variations in fecundity, development and mortality, which may in turn account for the variable virulence noted when G. salaris infects different salmonid hosts.

The blackcap (Sylvia atricapilla) is a common Palearctic migratory warbler, and haemosporidian parasites are common in this species. However, genetic and phenotypic diversity of haemosporidians in warblers has been insufficiently investigated and poorly linked. We addressed this issue by combining molecular and microscopy data for detection of pigment-forming haemosporidians of the genera Haemoproteus and Plasmodium. Blood samples from 498 blackcaps were collected at 7 different sites in Europe and investigated for these parasites by polymerase chain reaction (PCR)-based techniques and microscopic examination. In all, 56% of the birds were infected by at least 1 out of 25 distinct mitochondrial cytochrome b (cyt b) gene lineages of these haemosporidians. It is concluded that the blackcap is infected not only with blackcap specific haemosporidians, but also with Haemoproteus majoris, which is a host generalist and common in birds belonging to the Paridae. Haemoproteus pallidulus sp. nov. is described based on morphology of its blood stages and segments of the cyt b and dihydrofolate reductase/thymidylate synthase (DHFR-TS) genes. This study provides evidence that genetic diversity of haemosporidian parasites might be positively correlated with migratory strategies of their avian hosts; it also contributes to the value of both microscopy and molecular diagnostics of avian blood parasites.

1. Studies, using serial sections and a method of numerical assessment of the, numbers of cercariae in the various tissues, have shown that cercarial entry occurs through the skin at any point along the length of the fish and also through the gills and pharynx in the head region. The distribution of penetration along the length of the body is not uniform but exhibits a marked concentration in the head region.

2. As the time after infection increases, this initial unimodal distribution is maintained and becomes even more restricted.

3. Nearly all the cercariae have localized in the eye 12–24 hr. after infection. The few that remain outside the eye after this period appear to be moribund and undergoing phagocytosis.

4. The majority of migrating cercariae are present in connective tissue and muscle. Very few occur in the blood system and even fewer in the other organs of the body. The numbers in the blood system show a local anterior concentration in the blood vessels anterior to the heart and in the region of the gills. It seems probable that the blood system does not form the major route of migration.

5. Entry into the eye may occur at any point.

6. Cercarial migration causes extensive tissue damage, but this is fatal only when major blood vessels are perforated and severe internal haemorrhage results. Interference with the nervous system and lateral line system may be the cause of the orientation disturbances which are sometimes noted within 12 hr. after infection.

7. It is suggested that migration through the tissues is achieved mainly by the action of the anterior spination and that the function of the penetration gland cells may be lubricatory and/or adhesive rather than lytic.

8. The restricted tissue distribution during migration and the retention of an essentially unimodal distribution of cercariae along the length of the body through-out the whole period of migration suggest that localization in the lens is not achieved by chance migration.

Widespread use of ivermectin (IVM) as part of the Onchocerciasis Control Program (OCP) in West Africa could influence the evolution of the human filarial parasite Onchocerca volvulus. Use of IVM, in some areas for 15 years, may have restricted genetic diversity of O. volvulus, resembling effects attributed to a population bottleneck. Large population-based chemotherapy programmes, such as the OCP, may impose strong selection pressure on parasites and an examination of possible genetic selection by IVM in O. volvulus is warranted. IVM is a substrate for P-glycoprotein; a homologue from O. volvulus (OvPGP) has been linked with IVM sensitivity. Linkage disequilibrium (LD) patterns of 28 genetic markers spanning the OvPGP locus were examined in 4 O. volvulus populations from the Volta Region of Ghana, West Africa. Reduced gene diversity, increased heterozygosity and an increase in the number of markers not in Hardy-Weinberg equilibrium were associated with increasing IVM treatment. The number of regions in LD decreased with treatment and with time. However, between 1999 and 2002, seven regions of OvPGP were always in complete LD, while surrounding areas showed a reduction in genetic variation. The use of IVM for onchocerciasis control has imposed strong selection on O. volvulus populations, reducing genetic variation and disrupting LD.

Using data sets derived from published literature, the contribution of congeneric species to helminth component community richness is evaluated. Consideration of the frequency distribution of congeners in relation to host and parasite groups reveals that the distributions are unimodal, that singletons are the commonest class and that the frequency of occurrence of congeners decreases with increasing number of species per genus. Congeners may be found in any group of hosts or parasites, but are more common amongst cestodes of aquatic birds. Two patterns of occurrence of congeneric species are recognized: one in which from a few to multiple congeners are found within a single helminth genus, exemplified by dactylogyrid monogeneans and cloacinid nematodes, and the other in which there is a simultaneous occurrence of congenerics and confamilials such that there are several genera involved, but each represented by fewer species, exemplified by strongyles in horses. The question of whether these patterns can be considered examples of species flocks is discussed. We conclude that multiple congeners and species flocks are interesting phenomena but, except in isolated cases, they make insignificant contributions to community richness.

The body wall of Polymorphus minutus has been studied with the electron microscope and the structure of the various layers has been described.

The layers are the same in number as those seen with the light microscope, and pores have been found which penetrate the cuticle. Thus, the structure of the surface layers is such as would facilitate the absorption of nutrients.

It has been found that the cuticle and striped layer extend over the trunk spines, a feature which increases the area of the absorptive surface of the parasite.

The structure of the striped layer of the praesoma supports the theory that the praesoma body wall and lemnisci are involved in the absorption of fat.

Mitochondria have been detected in the felt and radial layers of the body wall and in the circular and longitudinal muscles.

The body wall of this acanthocephalan worm is entirely different from the body wall of trematodes, cestodes and parasitic nematodes.

We are grateful to Dr P. Tate for helpful discussions, Dr R. J. Skaer for criticism of the manuscript 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.

Haemoflagellates of the genus Trypanosoma are prevalent in freshwater fishes and are transmitted by leeches as vectors. As demonstrated by sequence comparisons of nuclear small subunit rRNA genes, trypanosomes isolated from several fish species at different localities can be divided into at least 2 closely related types, designated Type A and Type B. A clone derived from a Type A isolate from carp (Cyprinus carpio) was used to study the anti-parasite immune response in specified pathogen-free outbred carp. Infection leads to an initial rise in parasitaemia in the blood followed by a sharp decline in all fish (acute phase). Thereafter, in some carp, parasites become undetectable both in the blood and in internal organs while, in others, low numbers can be found in the blood for up to 1 year (chronic phase). Fish that have controlled an acute infection with the clone are not only protected against an homologous challenge infection, but also against the infection with parasite lines derived from carp in the chronic phase of infection. Passive immunization experiments with IgM purified from serum of recovered carp indicate that the infection is controlled by antibodies. The anti-parasite antibody level in recovered carp remains high for many months although the parasitaemia is controlled at very low levels and the half life of IgM, t1/2=22·5 days, is comparatively short. The effective control of trypanosomes in laboratory infections is in contrast to the high prevalence in natural and farmed freshwater fish populations.

A group of 27 variable antigen type (VAT)-specific monoclonal antibodies (McAbs) have been made against metacyclic forms of a cloned stock of Trypanosoma brucei rhodesiense. In combination, these labelled in immunofluorescence 99·3% of trypanosomes in salivary probes from tsetse flies. The 0·7% of unlabelled trypanosomes were believed to be uncoated forms. The ability of a mixture of antibodies to kill metacyclics in vitro by complement-mediated lysis, thus neutralizing their infectivity for mice, was tested. The antibody mixture consisted of 24 McAbs plus 3 VAT-specific rabbit antisera. In 12 replicate experiments this mixture of antibodies prevented infection of mice. Parallel controls showed that neutralization was probably antibody-mediated and VAT specific. However, we have not been able to repeat these results on a long-term basis; this may be due to a loss of neutralizing activity by one of the McAbs. The successful neutralization experiments indicate that the number of VATs in the metacyclic repertoire of one stock of T. b. rhodesiense is limited to at most 27.

The great French helminthologist, Dujardin, appears to have been the first to suggest a grouping of the genus “Ascaris” according to the structure of the alimentary canal. As is, unhappily, only too well known to modern helminthologists, little attention was paid to internal structures by the earlier workers, whose numerous descriptions and classifications were based almost entirely upon external characters. It is now recognized that a natural system must be based on a survey of the whole structure of the animal, both external and internal, and a re-examination of many of the older species becomes extremely important.

Parasites are generally found aggregated among hosts. In this paper we propose a method for measuring aggregation at different scales in the host population. We use the method to characterize the pattern of aggregation of the tick Ixodes uriae on chicks of its seabird host, the Kittiwake Rissa tridactyla. We found evidence of aggregation at the among-nest scale, but not among chicks within nests. This shows that the processes leading to aggregation occur at a higher scale than the nest. The methods we develop provide a way to compare parasite aggregation at different scales in a quantitative fashion and can be applied in a large number of epidemiological studies.

The morphology of a micro-organism, Cytœcetes microti, new genus, new species, which invades the granulocytes and occasionally the lymphocytes, is described. It occurs naturally in the vole, Microtus pennsylvanicus, is transmissible to white-footed mice, Peromyscus leucopus and Peromyscus eremicus, the common tame mouse, the white rat, but inoculations of guinea-pigs. domestic rabbits, a kitten and young chickens have resulted negatively. The only pathological effect attributable thus far to this organism is an increase in the number of granulocytes found in certain organs such as the spleen and adrenals, and degenerative changes in the individual cell invaded. Splenectomy performed soon after the microscopic disappearance of Cytœcetes from the circulating blood may result in its reappearance, when it may occur in a considerable proportion of the granulocytes, but nevertheless in smaller numbers than at the height of early infection. Overwhelming infections have in no instance followed splenectomy. The probable relationship of the organism to Ægyptianella pullorum (Carpano, 1928) is pointed out, but grounds for placing it in a separate genus are presented.