When two parasite species are manipulators and have different definitive hosts, there is a potential for conflict between them. Selection may then exist for either avoiding hosts infected with conflicting parasites, or for hijacking, i.e. competitive processes to gain control of the intermediate host. The evidence for both phenomena depends largely on the study of the relative competitive abilities of parasites within their common intermediate host. We studied the effects of simultaneous infection by a fish acanthocephalan parasite, Pomphorhynchus laevis, and a bird acanthocephalan parasite, Polymorphus minutus, on the behaviour of their common intermediate host, the amphipod Gammarus pulex. We compared the reaction to light and vertical distribution of individuals infected with both parasites to those of individuals harbouring a single parasite species and uninfected ones under controlled conditions. Compared to uninfected gammarids that were photophobic and tended to remain at the bottom of the water column, P. laevis-infected gammarids were attracted to light, whereas P. minutus-infected individuals showed a modified vertical distribution and were swimming closer to the water surface. The effects of both P. laevis and P. minutus appeared to be dependent only on their presence, not on their intensity. Depending on the behavioural trait under study, however, the outcome of the antagonism between P. laevis and P. minutus differed. The vertical distribution of gammarids harbouring both parasites was half-way between those of P. laevis- and P. minutus-infected individuals, whereas P. laevis was able to induce altered reaction to light even in the presence of P. minutus. We discuss our results in relation to the occurrence of active avoidance or hijacking between conflicting manipulative parasites and provide some recommendations for future research.

A general mathematical model of a vector-borne disease involving two vertebrate host species and one insect vector species is described. The model is easily extended to other situations involving more than two hosts and one vector species. The model, which was developed from the single-host model for malaria described by Aron & May (1982), is applied to the African trypanosomiases and allows for incubation and immune periods in the two host species and for variable efficiency of transmission of different trypanosome species from the vertebrates to the vectors and vice versa. Equations are derived for equilibrium disease prevalence in each of the species involved. Model predictions are examined by 3-dimensional phase-plane analysis, which is presented as a simple extension of the 2-dimensional phase-plane analysis of the malaria model. Parameter values appropriate for the African trypanosomiases are derived from the literature, and a typical West African village situation is considered, with 300 humans, 50 domestic animals and an average population of 5000 tsetse flies. The model predicts equilibrium prevalences of Trypanosoma vivax, T. congolense and T. brucei of 47·0, 45·8 and 28·7% respectively in the animal hosts, 24·2, 3·4 and 0·15% in the tsetse vectors, and a 7·0% infection of humans with human-infective T. brucei. The contribution to the basic rate of reproduction of the human-infective T. brucei is only 0·11 from the human hosts and 2·54 from the animal hosts, indicating that in the situation modelled human sleeping sickness cannot be maintained in the human hosts alone. The animal reservoir is therefore crucial in determining not only the continued occurrence of the disease in humans, but its prevalence in these hosts as well. The effect of changing average fly density on equilibrium disease prevalences is examined, together with the effect of seasonal changes in fly numbers on disease incidence. In a seasonal situation changes in fly mortality rates affect both future population size and infection rate. Peak disease incidence lags behind peak fly numbers, and that in the less favoured host lags behind that in the more favoured host. Near the threshold fly density for disease transmission disease incidence is more changeable than at higher fly densities and may even exceed equilibrium prevalence at the same average fly density (because most hosts are susceptible at the time that fly numbers begin their annual increase). The implications of the model for disease control are discussed. Identifying the precise role of the animal reservoir may suggest that treating such animals will achieve a greater reduction of human sleeping sickness than direct treatment of the humans alone. Statistically significant results of control campaigns may also be more easily shown by monitoring the non-human reservoirs. The model provides a means by which a correct perspective view can be obtained of the complex epidemiology and epizootiology of the African trypanosomiases.

Hutchinson's (1957) definition of an ecological niche as a multidimensional hypervolume determined by a number of physical and biotic variables is adopted. The number of niche dimensions is very great, but as a working hypothesis it is assumed that a few are sufficient to characterize the niche of a parasite species to a high degree of accuracy. They are host species, microhabitat(s), macrohabitat(s), geographical range, sex and age of host, season, food and hyperparasites. Methods to measure niche width, in particular specificity indices, are discussed, and some examples of niche restriction are described. Proximate and ultimate causes of niche restriction are discussed, mainly using marine parasites as examples. Among proximate causes of one niche dimension, host specificity, are ecological factors restricting exposure to infection to certain host species; host-specific chemical factors that induce hatching, direct infective stages to a host and bring about settlement of a parasite; factors that lead to mortality in or on the wrong host; morphological adaptations that guarantee survival in or on the ‘correct’ host; and availability of suitable hosts. Many factors are likely to be responsible for microhabitat specificity, but have been little studied, except for some physiological and morphological adaptations to particular microhabitats. Macrohabitats and geographical range may be determined by the distribution of intermediate hosts and certain food items, and by a variety of chemical and physical factors. Hosts of different sexes may differ in feeding habits and the composition of the skin, and thus acquire parasites differentially. Hosts of different age may be differentially infected due to accumulation of parasites with age, loss of parasites due to developing resistance (or immunity), and different size and feeding habits. Among ultimate causes of niche restriction and segregation are avoidance of competition, predation and hyperparasites; facilitation of mating; reinforcement of reproductive barriers; and adaptations to environmental complexity. Few studies permit a decision on which factor or factors are responsible in particular cases. Interspecific competition may play a greater role in helminth communities of some host groups than of others, but it seems that, overall, its role has been exaggerated at least for marine parasites. Some ‘classical’ examples of microhabitat segregation explained by interspecific competition can also be explained by reinforcement of reproductive barriers. There is evidence for the importance of facilitation of mating in microhabitat restriction, and the availability of many vacant niches indicates that competition, overall, is not of great importance.

Specific sequences derived from the gene for the precursor to the major merozoite surface antigens (PMMSA) of Plasmodium falciparum have been expressed in Escherichia coli and the products have been used to produce antibodies. These antibodies, together with monoclonal antibodies, have been used to investigate the form of the PMMSA protein associated with merozoites. Polypeptide fragments derived by processing from the PMMSA protein have been detected in extracts of merozoites and assigned to locations within the PMMSA coding sequence.

Macrocyclic lactones (MLs) are highly lipophilic anthelmintics which are known to bind to and open ligand-gated ion channels. However, these anthelmintics, and particularly the avermectin members of the ML class of endectocides, are potent substrates for ABC transporters and these transporters may regulate drug concentration in both the host and the parasite. There is accumulating evidence that ivermectin (IVM), and to a lesser extent moxidectin (MOX), selects for certain alleles of P-glycoprotein and other ABC transporter genes, selects for constitutive overexpression of some of these gene products, and induces overexpression of some P-glycoproteins in nematodes. However, such mechanisms of ML resistance do not easily lend themselves to the identification of SNP markers for resistance because of the diversity of ABC transporters in nematodes, the apparent diversity of effects of different MLs, and because regulatory elements for ABC transporter gene expression are not well understood in nematodes. Another non ligand-gated ion channel gene which appears to be under IVM selection, at least in Onchocerca volvulus and Haemonchus contortus, is β-tubulin, and a simple genetic test for this selection has been described in O. volvulus. However, further work is required to elucidate a reliable marker associated with this gene in H. contortus or other parasitic nematodes of livestock. The possible involvement of ABC transporter genes and β-tubulin in ML resistance provides a start in developing our understanding of this phenotype and markers for its detection in field populations of parasitic nematodes. However, more work is required before these leads can provide practical SNP markers for ML resistance.

An area close to the Qinghai-Tibet plateau region and subject to intensive deforestation contains a large focus of human alveolar echinococcosis while sporadic human cases occur in the Doubs region of eastern France. The current review analyses and compares epidemiological and ecological results obtained in both regions. Analysis of rodent species assemblages within quantified rural landscapes in central China and eastern France shows a significant association between host species for the pathogenic helminth Echinococcus multilocularis, with prevalences of human alveolar echinococcosis and with land area under shrubland or grassland. This suggests that at the regional scale landscape can affect human disease distribution through interaction with small mammal communities and their population dynamics. Lidicker's ROMPA hypothesis helps to explain this association and provides a novel explanation of how landscape changes may result in increased risk of a rodent-borne zoonotic disease.

A complete developmental sequence of Leishmania mexicana has been produced in axenic culture for the first time. This was achieved by manipulation of media, pH and temperature conditions over a period of 16 days. All experiments were initiated with lesion amastigotes that were transformed to multiplicative promastigotes by culture in HOMEM, 10% foetal calf serum, pH 7·5, at 25 °C. Metacyclogenesis was induced by subpassage in Schneider's Drosophila medium, 20% foetal calf serum, pH 5·5, and the resulting forms transformed to axenically growing amastigotes by subpassage in the same medium and raising the temperature to 32 °C. Parasites from each day were characterized with respect to their general morphology using light microscopy of Giemsa-stained smears, and biochemically by analysis of total protein content, proteinases, nucleases and secretory acid phosphatase. The results demonstrated that the three main stages identified - amastigotes, multiplicative promastigotes and metacyclic promastigotes - each exhibited the expected suite of biochemical properties. Further, the changes in morphology observed as the developmental sequence proceeded from stage to stage were accompanied by appropriate changes in biochemical properties. These results provide both useful biochemical markers and a culture system in which to examine the regulation of differentiation and transformation during the Leishmania life-cycle.

Developmentally arrested malarial gametocytes undergo gamete formation in the mosquito midgut immediately after ingestion of the infected bloodmeal. In the rodent malaria parasite Plasmodium berghei male gametogenesis (exflagellation) can be induced in vitro by a temperature decrease (from 39°C in the vertebrate host to 20°C) and a concomitant pH increase (from 7·3 in mouse blood to 8·0). We report the presence of additional Gametocyte Activating Factor(s) (GAF) present in Anopheles stephensi tissue extracts, which induce both male and female gametogenesis at the otherwise non-permissive pH of 7·3 in vitro but are unable to overcome the low temperature requirement. All constituent cellular events of microgametogenesis studied here are induced by the same triggers in vitro. A temperature decrease is also required for exflagellation in the mosquito midgut. The possible role of GAF as a second obligatory natural trigger of gametogenesis is discussed.

Although Babesia divergens is the the principal confirmed zoonotic Babesia sp. in Europe, there are gaps in our knowledge of its biology and transmission by the tick Ixodes ricinus. In order to reproduce the part of the parasite cycle that occurs in the vector, an in vitro animal skin feeding technique on blood containing in vitro cultivated B. divergens was developed. Parasite DNA was detected in all samples of salivary glands of nymphs and adults that had fed on parasitized blood as larvae and nymphs, respectively, indicating acquisition as well as a transtadial persistence of B. divergens. PCR performed on eggs and larvae produced by females that had fed on parasitized blood demonstrated the existence of a transovarial transmission of the parasite. Gorging B. divergens infected larvae on non-infected gerbils showed persistance of the parasite over moulting into the resulting nymphs. These results indicate that the parasitic stages infective for the vector (i.e. the sexual stages) can be produced in vitro. To our knowledge, this is the first report of artificial feeding of I. ricinus via membrane as well as in vitro transmission of B. divergens to its vector. The opportunities offered by the use of such a transmission model of a pathogen by I. ricinus are discussed.

In a fishing community on Lake Albert in Uganda the pattern of intensity of Schistosoma mansoni infection 6 months after treatment with praziquantel was found to be very similar to reinfection patterns seen in previously studied endemic communities: the profile peaks sharply at around the age of 10 years falling away rapidly to much lower levels in adults. This is in stark contrast to the patterns of water contact, which differ greatly between fishing and non-fishing communities. On Lake Albert, adults appear to be more heavily exposed than children. From these observations we conclude that adults are physiologically (perhaps immunologically) more resistant to infection after treatment than children.

Experiments are described which show that in a recently isolated field strain of the cattle nematode, Ostertagia ostertagi, the primary stimulus to the induction of inhibited development at the early fourth larval stage (EL4) in the host is chilling of the infective larvae (L3). Thus exposure of L3 to a temperature of 4 °C for 8 weeks prior to their oral administration to calves resulted in a mean of 66% of the established worm burden becoming inhibited at the EL4 stage. This provides an explanation for the seasonal occurrence in Britain of inhibited EL4 stages of O. ostertagi in cattle during the cold climate of late autumn and winter.

If the L3 stages were exposed to chilling for a prolonged period (> 19 weeks), prior to infection of calves, then the prevalence of inhibited EL4 stages amongst the worms established declined. After 33 weeks chilling the worm population consisted almost entirely of adults and only 2% inhibited EL4 stages. This suggested that a selective mortality of inhibition prone L3 was taking place and explained why under natural grazing conditions inhibition prone L3 were not available on the pasture in spring.

The duration of inhibition at the EL4 stage in calves infected with chilled L3 was consistently between 16 and 18 weeks and maturation of the majority of the inhibited larvae occurred spontaneously after such a period and irrespective of season. This period is equivalent to the duration of a normal winter in Britain when conditions unsuitable for the free living development of O. ostertagi larvae prevail.

The induction of inhibition by the environmental stimulus of chilling, the fixed period for the duration of inhibition within the host and the spontaneous maturation of the inhibited larvae are characteristic of a true diapause.

Each year the study of Diptera reveals some new and often unexpected facts concerning the mode of life of their early stages. The rich entomological literature dealing with this subject clearly shows that the diversity of conditions under which the larvae of this order of insects live surpasses that of any other order of the Animal Kingdom.

To study population kinetics during primary Ascaris suum infections, 3 groups of 52 pigs each were inoculated with 100, 1000, or 10000 infective eggs. In all groups, the majority of larvae was found in the liver on day 3 post inoculation (p.i.) and in the lungs on day 7 p.i. Liver white spots, caused by migrating larvae, were most numerous at day 7 p.i., whereafter they gradually healed, and only low numbers of granulation-tissue type white spots and lymphonodular white spots persisted at days 21–56 p.i. Independent of dose level, 47–58% of the inoculated eggs were recovered as larvae in the small intestine on day 10 p.i., but most larvae were eliminated at days 17–21 p.i. This elimination started earlier and removed a higher percentage of the worms with increasing inoculation dose, resulting in small strongly aggregated worm populations by day 28 p.i. (k of the negative binomial distribution was low: 0·2–0·4) without significant differences between groups. Thus, overdispersion, which is a characteristic of both porcine and human ascarosis, is found here under experimental conditions where aggregation factors like host behaviour, transmission rate, host status etc have been partly or totally controlled.

A novel method for the isolation of schistosome eggs and miracidia from livers of mice infected with Schistosoma japonicum or S. mansoni is described. The method employed collagenase B to degrade the interstitial matrix of mouse liver tissue, after which the schistosome eggs were separated from the liver cells by 2 single-step density centrifugations through Percoll. Using this procedure sufficient quantities of miracidia were obtained to generate a cDNA library. Southern blot analysis demonstrated that miracidia isolated by this method were free from contaminating host DNA.

Endosymbiotic bacteria of the genus Wolbachia infect a number of invertebrate species in which they induce various alterations in host reproduction, mainly cytoplasmic incompatibility (CI). In contrast to most other maternally transmitted parasites, manipulation of host reproduction makes the spread of Wolbachia possible even if they induce a physiological cost on their hosts. Current studies have shown that fitness consequences of Wolbachia infection could range from positive (mutualist) to negative (parasitic) but, in most cases, Wolbachia do not have strong deleterious effects on host fitness and the status of association remains unclear. Here, we show that in the Drosophila parasitoid wasp Leptopilina heterotoma, Wolbachia infection has a negative impact on several host fitness traits of both sexes. Fecundity, adult survival and locomotor performance are significantly reduced, whereas circadian rhythm, development time and offspring sex-ratio are not affected. Although the cost of bacterial infection can be overcome by effects on host reproduction i.e. cytoplasmic incompatibility, it could influence the spread of the bacterium at the early stages of the invasion process. Clearly, results underline the wide spectrum of phenotypic effects of Wolbachia infection and, to our knowledge, Wolbachia infection of L. heterotoma appears to be one of the most virulent that has ever been observed in insects.

Trypanosoma cruzi specific sequences were amplified by the polymerase chain reaction from total blood of human chagasic patients and normal individuals. A 330 bp fragment originating from kinetoplast DNA was specifically detected in most chagasic individuals. We tested the sensitivity and specificity of this method in normal and affected individuals attending the Evandro Chagas Hospital, Rio de Janeiro. The results of these tests were compared with serological diagnosis performed using standard techniques, and in some cases with xenodiagnosis. We found that none of the serologically negative individuals gave any specific amplification product, whereas 55 out of 61 patients previously serodiagnosed as chagasic were positive using the PCR method (sensitivity: 90%). Xenodiagnosis, which is currently considered to be the most sensitive parasitological technique for Chagas' disease diagnosis, detected only 12 out of 28 serologically positive patients (sensitivity: 43%). The usefulness of the PCR method was further investigated with chagasic patients who had received anti-parasite treatment with benznidazole. It has always been difficult to evaluate the incidence of cure in such cases by serology, since a humoral response against T. cruzi antigens may remain for years even in the absence of the parasite. We observed a positive amplification result in only 9 out of 32 treated patients who remained reactive when tested using classical serology. These observations suggest that PCR is the most sensitive technique available for direct detection of T. cruzi in chagasic patients and that it can be a very useful instrument for the follow-up of patients after specific treatment.

We have used a number of molecular genetic approaches to characterize the cervid strain (‘northern form’) of Echinococcus granulosus. PCR–RFLP analysis of the nuclear ITS1 region of the rDNA repeat can readily distinguish the cervid form from other strains of E. granulosus. The complexity of the RFLP patterns obtained suggests that a number of distinct ITS1 types are present in this strain which may represent an inter-strain E. granulosus hybrid. Mitochondrial CO1 sequence of the cervid genotype was ambiguous at 18 positions and closely resembles a cluster of previously characterized E. granulosus genotypes, G1 (common, domestic sheep)/G2 (Tasmanian sheep)/G3 (buffalo). In contrast, mitochondrial ND1 sequence, although unique, suggests that the cervid form is most similar to strains represented by the G6 (camel)/G7 (pig) genotypes. We assume that the CO1 and ND1 sequences obtained for the cervid genotype are linked in a single mitochondrial genome although this is difficult to explain if conventional molecular genetics of mitochondrial DNA are assumed. Based on its unique ND1 sequence and ITS1 PCR–RFLP pattern, the cervid strain appears to represent a distinct genotype (designated G8) of E. granulosus.

Schistosomiasis and food-borne trematodiases are chronic parasitic diseases affecting millions of people mostly in the developing world. Additional drugs should be developed as only few drugs are available for treatment and drug resistance might emerge. In vitro and in vivo whole parasite screens represent essential components of the trematodicidal drug discovery cascade. This review describes the current state-of-the-art of in vitro and in vivo screening systems of the blood fluke Schistosoma mansoni, the liver fluke Fasciola hepatica and the intestinal fluke Echinostoma caproni. Examples of in vitro and in vivo evaluation of compounds for activity are presented. To boost the discovery pipeline for these diseases there is a need to develop validated, robust high-throughput in vitro systems with simple readouts.

The microsporidial species Enterocytozoon bieneusi is found among immunocompromised, particularly HIV-infected, patients with chronic diarrhoea, and rarely also among immunocompetent persons with self-limited diarrhoea. Only recently, E. bieneusi was detected in 4 pigs in Switzerland raising the question of a potential zoonotic nature of this parasite. We examined faecal samples of 109 pigs, 24 cows, horses and red foxes each for the presence of E. bieneusi by PCR and compared these isolates with isolates obtained from stool samples of 13 HIV-infected patients living in Switzerland. In animals, E. bieneusi was only identified in pigs with a prevalence of 35%. Analysis of the rDNA internal transcribed spacer (ITS) sequence allowed the classification of E. bieneusi from 28 pigs into 4 distinct genotypes which grouped very closely (identity 96·3–98·8%) together with 2 of the 3 human-derived E. bieneusi genotypes. Hence, E. bieneusi seems to be a common parasite in swine, but no genotypes were identified that were found in humans. Nevertheless, swine might serve as a new animal model for enterocytozoonosis.

Unicellular flagellates of the family Trypanosomatidae are obligatory parasites of invertebrates, vertebrates and plants. Dixenous species are aetiological agents of a number of diseases in humans, domestic animals and plants. Their monoxenous relatives are restricted to insects. Because of the high biological diversity, adaptability to dramatically different environmental conditions, and omnipresence, these protists have major impact on all biotic communities that still needs to be fully elucidated. In addition, as these organisms represent a highly divergent evolutionary lineage, they are strikingly different from the common ‘model system’ eukaryotes, such as some mammals, plants or fungi. A number of excellent reviews, published over the past decade, were dedicated to specialized topics from the areas of trypanosomatid molecular and cell biology, biochemistry, host–parasite relationships or other aspects of these fascinating organisms. However, there is a need for a more comprehensive review that summarizing recent advances in the studies of trypanosomatids in the last 30 years, a task, which we tried to accomplish with the current paper.