In order to construct an analytical model of onchocerciasis transmission, it is necessary to elucidate the functional relationships of the various population rate processes taking place within the human and vector hosts. Two previous papers have explored the evidence for density-dependent regulation in relation to microfilarial intake by, and larval development within, the Simulium host. This paper investigates the survivorship of wild-caught blackfly samples fed on subjects with different intensities of Onchocerca volvulus microfilarial infection. Analyses were based on data for Guatemalan S. ochraceum s.l. (possessing a well-developed cibarial armature), West African S. damnosum s.l. (forest species), and South Venezuelan S. guianense (the latter two lacking a toothed cibarium). The mean survival times of samples of the 3 species, kept under laboratory conditions, decreased as parasite intake increased, the rate of mortality being dependent on the fly’s age (measured as time post-feeding) and on the worm load acquired. An empirical, timedependent hazard function was fitted to observed death rates/fly/day which rose very shortly after engorgement, declined subsequently, and rose again throughout the extrinsic incubation period of the parasite. The parameters of this hazard model were all positively correlated with the density of microfilariae in the bloodmeal. Expressions of survivorship and life-expectancy as explicit functions of time post-feeding and mean parasite intake were derived. The average expectation of life at engorgement for uninfected flies in the laboratory was estimated to be around 1 week for both, armed and unarmed blackflies. Residual life-expectancy decreased with time post-feeding and microfilarial load in both categories of vectors. This decline (resulting from age- and parasite-dependent mortality rates) was much more pronounced in those species lacking a toothed fore-gut. Whilst a fraction of heavily infected S. ochraceum was able to survive the latent period of the parasite, being therefore potentially capable of transmitting the infection, equivalent worm loads in S. guianense resulted in a drastic reduction of the expectation of infective life. These results provide additional evidence to support the hypothesis that, in the case of intrinsically susceptible vectors, unarmed simuliids are more efficient at low microfilarial loads, when the transmission rate from human to vector host is higher, and parasite-induced fly mortality is negligible. The opposite takes place in armed flies, which perform poorly at low parasite burdens and better at heavier loads, with little parasite-induced vector death.

The properties of a mathematical model of bloodstage infection with a single strain of malaria were investigated. Analysing the cell population dynamics in the absence of a host immune response we demonstrate a relationship between host and parasite parameters that defines a criterion for the successful invasion and persistence of the parasite. Important parameters are the rates of merozoite production and death and those of erythrocyte production, death and invasion. We present data from experiments designed to evaluate the erythrocyte invasion rate in a rodent malaria system. The model generates patterns of parasitaemia in good qualitative agreement with those seen in Plasmodium berghei infections. The sole force behind the rise and fall in parasitaemia in the model without immunity is the density of susceptible erythrocytes, suggesting that resource availability is an important determinant of the initial pattern of infection in vivo. When we incorporate a simple immune response into the model we find that immunity against the infected cell is much more effective at suppressing parasite abundance than immunity against the merozoite. Simulations reveal oscillating temporal patterns of parasite abundance similar to P. c. chabaudi infection, challenging the concept that antigenic variation is the sole mechanism behind recrudescing patterns of infection.

Surveys on Trichinella parasites in domestic and sylvatic animals collected in France, Italy, and in the Extremadura region of Spain showed that the distribution of Trichinella spiralis and Trichinella britovi is influenced by both environmental and human behaviour factors. In France, both Trichinella species are prevalent in the fox population from mountain areas and natural parks but are infrequent in wild boars (< 0·001%). In Italy, only T. britovi is present in sylvatic animals (foxes, wolves, and mustelids) living 500 m above sea level. This species is rare in wild boars ( < 0·001%) in that area. Sylvatic trichinellosis is found in only 24% and 34% of French and Italian territory, respectively, while lowland areas may generally be considered Trichinella-free, because the domestic cycle is absent. The ecology of T. spiralis and T. britovi in the Extremadura shows a different picture from that observed in France and Italy because of the presence of both domestic and sylvatic cycles. The domestic cycle not only allows the maintenance of T. spiralis in the domestic environment, but it also has a great impact on the prevalence in wild boar populations. It does not influence the prevalence in vulpine populations. These data suggest (1) that domestic trichinellosis occurs only in rural areas of Western Europe in association with traditional swine-rearing practices, but not in industrialized pig farms; (2) that sylvatic trichinellosis occurs only in natural habitats which, in Western Europe, are widespread in mountain areas; (3) that the fox is the primary reservoir in the sylvatic cycle, where the parasite is maintained in a closed circuit and (4) that among sylvatic animals T. spiralis is present at lower altitude than is T. britovi.

Very little is known about parasite-altered mating behaviour. This paper describes two experiments on the impact of infection and starvation on mating in Biomphalaria glabrata snails. In the first experiment, snails were infected with the parasitic trematode Schistosoma mansoni. During both the early and the late stages of infection, snails were allowed to mate with individuals of the same infection status and uninfected controls. Their mating activities were filmed and later analysed. The mating frequencies of patently infected (shedding) snails were found to be lower than those of controls. This is thought to result from stress induced by the pathology. Successfully infected snails were found to mate more often as males than as females. This is seen as a compensation for the reduced fecundity caused by trematode infection. Successfully infected snails also exhibited partner choice. More matings were found between snails of the opposite infection status than among snails of the same infection status. This may be explained by the good genes hypothesis and can be seen as a specific response to schistosome infection. In the second experiment, starved snails were allowed to mate with starved and control snails. They mated less often than controls and more often as males than as females, controls showing the opposite pattern. This may be considered to be a general stress response. No partner choice has been found with starved snails. It is suggested that partner choice in infected snails occurs in response to specific rather than general stress.

Theileria annulata infects and reversibly transforms bovine leucocytes. The parasite-transformed cells are immortalized, metastatic and express a number of metalloproteinases including matrix metalloproteinase 9 which they secrete. All the metalloproteinases observed on substrate gels are inhibited by tissue inhibitor of metalloproteinase 1 and 4 synthetic inhibitors BB94, GM6001, BRL29808AI and Ro31–4724. We have adapted an in vitro assay for metastatic behaviour that measures the ability of parasitized cells to cross reconstituted basement membrane, Matrigel™. Using this we demonstrated that macroschizont-infected cells are invasive in vitro and that their invasive properties can be almost eliminated by the same specific inhibitors of metalloproteinases as used in the substrate gels. This demonstrates that the metastatic behaviour of the infected cells is due in part to metalloproteinase activity and strongly suggests a role for the metalloproteinases we observed on gels. This is further supported by the fact that an attenuated vaccine line which shows much reduced metalloproteinase activity also exhibits a marked reduction in metastatic behaviour. We suggest that these metalloproteinases are virulence factors mediating some pathological features of the disease and their loss in the vaccine line could provide an explanation for attenuation.

As the most primitive metazoan phylum, the Platyhelminthes occupies a unique position in nervous system evolution. Centrally, their nervous system consists of an archaic brain from which emanate one or more pairs of longitudinal nerve cords connected by commissures; peripherally, a diverse arrangement of nerve plexuses of varying complexity innervate the subsurface epithelial and muscle layers, and in the parasitic taxa they are most prominent in the musculature of the attachment organs and egg-forming apparatus. There is a range of neuronal-cell types, the majority being multi- and bipolar. The flatworm neuron is highly secretory and contains a heterogeneity of vesicular inclusions, dominated by densecored vesicles, whose contents may be released synaptically or by paracrine secretion for presumed delivery to target cells via the extracellular matrix. A wide range of sense organ types is present in flatworms, irrespective of life-styles. The repertoire of neuronal substances identified cytochemically includes all of the major candidate transmitters known in vertebrates. Two groups of native flatworm neuropeptides have been sequenced, neuropeptide F and FMRFamide-related peptides (FaRPs), and immunoreactivities for these have been localised in dense-cored neuronal vesicles in representatives of all major fiatworm groups. There is evidence of co-localisation of peptidergic and cholinergic elements; serotoninergic components generally occupy a separate set of neurons. The actions of neuronal substances in flatworms are largely undetermined, but FaRPs and 5-HT are known to be myoactive in all of the major groups, and there is immuno-cytochemical evidence that they have a role in the mechanism of egg assembly.

The transmission of Theileria parva by nymphal and adult Rhipicephalus appendiculatus was compared by the assessment of salivary gland infections in tick batches fed on the same group of infected cattle at the same time. When larval and nymphal R. appendiculatus Muguga ticks were fed concurrently on cattle undergoing acute infection with T. parva Muguga, the resultant nymphae developed a slightly lower prevalence of infection than did the adult ticks. The abundance of infection was 5–20 times higher in the adult ticks than in the nymphae. When larval and nymphal R. appendiculatus Muguga and R. appendiculatus McIlwaine were fed to repletion on cattle infected with T. parva Boleni, a parasite causing subacute infection, resultant adult tick batches had a relatively high prevalence of infection, but infection was not detected in resultant nymphal batches. When cattle that were carriers of 2 stocks of T. parva, Marikebuni and Kiambu 5, were used as the source of infection, the infections developing in adult R. appendiculatus Muguga ticks were much higher than those developing in nymphae. The structure of salivary glands differed between nymphal ticks, adult males and adult females, and this is considered to be an important factor affecting the infection levels. The morphology of the type III acini, the target acini for sporogony, was similar, but the mean numbers of type III acini were different, with 87 in nymphae, 1346 in males and 1736 in females. This difference was correlated with the different infection levels produced in the various instars and sexes. While the process of sporogony in the different tick instars and sexes was similar, the rate of sporogony was fastest in feeding nymphae, taking on average 2–3 days, compared to 3–4 days in females and an irregular period in the males. These results are discussed in relation to the epidemiology of T. parva.

The lobate ctenophore Mnemiopsis leidyi is a periodically abundant and voracious plankton predator in coastal waters along the east coast of the United States. In the 1980s it was accidentally introduced to the Black Sea where it caused a dramatic reduction in fisheries. We investigated how M. leidyi is affected by infestation with parasitic larvae of the sea anemone Edwardsia lineata. Infested M. leidyi contained 1–30 (median 7) E. lineata larvae. Within M. leidyi most larvae had their mouth in the gastrovascular system near the aboral end of the pharynx. Parasitic E. lineata ingested all food previously ingested and pre-digested by M. leidyi. Non-infested M. leidyi had higher growth rates than infested individuals, which had zero or negative growth rates. Egg production was similar for infested and non-infested M. leidyi of similar size. Simulation based on the empirical data suggests that growing, non-infested, M. leidyi are expected to have a larger life-time egg production than infested shrinking individuals. E. lineata could be at least partially responsible for the sharp decline of M. leidyi populations in fall in US coastal waters. Advantages and disadvantages of E. lineata as a potential candidate for the control of the artificially introduced M. leidyi population in the Black Sea are discussed.

A study on the immune and mating interactions between Schistosoma intercalatum and S. mansoni has been carried out. The immunologically mediated interactions and mating patterns observed between the two species demonstrate that S. mansoni is competitively dominant over S. intercalatum. When S. mansoni females are absent, S. mansoni males are more competitive than S. intercalatum males at pairing with S. intercalatum females. The use of a mathematical model shows how the abundance of S. mansoni worms per host could dramatically affect the mating probability of S. intercalatum females with their homologous males. It appears that the competitive exclusion between these two species of schistosomes may probably be an important factor restricting the distribution of S. intercalatum in Africa.

The A subunit of NF-Y nuclear factor from Schistosoma mansoni was expressed in E. coli fused to a histidine tag and purified by affinity chromatography using a Ni2+-Agarose matrix. Antibodies against the recombinant protein were prepared and used for Western blot and immunolocalization. The presence of SMNF-YA in all stages of the parasite life-cycle was determined by RT-PCR and Western blot analysis. The immunolocalization of SMNF-YA showed the presence of this factor in a parenchymal cell population of cercariae and adult worms and in embryos within eggs. The expression of SMNF-YA was demonstrated to decrease in maturating spermatozoites whereas an accumulation of this factor was observed in the nucleus from oocytes during their maturation processes.

In this study, we have analysed enzyme polymorphism among a group of protozoan parasites of the genus Endotrypanum (Kinetoplastida: Trypanosomatidae). Seventeen stocks of Endotrypanum spp. isolated from sloths (Choloepus didactylus and C. juruanus) in the Amazon Region of Brazil were analysed by enzyme electrophoresis, and their electromorphic profiles were compared with reference strains reported previously. The 16 enzymic loci were analysed, and the strains were classified into zymodemes, each representing parasites with unique enzyme profiles. Each zymodeme was considered as an elementary taxon, and using numerical analyses (cladistic, agglomerative hierarchical and ordination techniques) the genus was shown to be monophyletic and the 12 zymodemes characterized could be divided into 3 groups (A, B, C). The heterogeneous population (which may represent a complex of parasite species or strains variants) showed, however, no correlation with the origin (i.e. host species involved or geographic area of isolation) of Endotrypanum stocks. Eight isolates of Endotrypanum sp. from Rondonia State (Brazil) and a parasite strain from Panama were clustered together into a zymodeme, which was phenetically closely related to the E. monterogeii from Costa Rica. The data indicate that E. schaudinni is a species complex.

Trichinella spiralis larvae infect and develop within skeletal muscle cells causing major changes to their mechanical properties. The aim of this investigation was to determine the effects of T. spiralis on the power output and fatigue resistance of the mammalian diaphragm under conditions simulating in vivo operation and to relate these to respiratory performance. Infection with T. spiralis leads to major reductions in mechanical stress, work, power output and fatigue resistance. These changes are associated with the number of larvae present in the muscle and the duration of infection. However, the initial decline in mechanical performance occurs during the onset of infection when there are few larvae observed within the muscle cells, indicating that T. spiralis may affect the properties of muscle before encapsulation. This may correspond to the host's inflammatory response and the effects of larval excretory/secretory products. The decline in mechanical performance will have a profound effect on respiration both at rest and during exertion. This must influence the behaviour of the host and increase its chance of capture by predators, which is likely to benefit the parasite by facilitating its transmission.

Our understanding of the neurobiology of the Platyhelminthes has come in large part from free-living turbellarians. In addition to providing considerable information about the capabilities of the rudimentary nervous system present in all members of the phylum, turbellarians have provided the most definitive information about the variety of ion channels present in the membranes of neurones and muscle cells, and about the physiology and pharmacology of those channels. Furthermore, preparations of single, viable muscle cells have provided some of the most conclusive evidence about the variety of transmitters present, and the types of response they evoke. Here, we review what is known about the physiology and pharmacology of the turbellarian neuromuscular system. Particular attention is given to the triclad flatworm Bdelloura Candida, the best studied species in this respect, but other species are included where relevant.

A large database on the transmission of a stabilate of the Theileria parva Muguga stock from one breed of cattle using two stocks of Rhipicephalus appendiculatus, Muguga and Ol Pejeta was developed and analysed. Factors associated with the ticks and cattle, and the infections developing in cattle were studied in relation to the infection variables in the tick batches harvested daily from cattle. Generalized Linear Interactive Modelling (GLIM) was used to determine the importance of factors and interactions in influencing the levels of tick infection variables using Type I and Type III sums of squares analyses. Analysis of the 6 variables, prevalence (percentage of ticks infected), abundance (mean number of infected salivary gland acini per tick examined) and intensity (mean number of infected salivary gland acini per infected tick) in batches of 30 male and 30 female ticks showed that 24 covariates, factors or interactions had a significant effect (P> 0·05). Certain covariates and factors were particularly important for all 6 tick infection variables; parasitaemia of animal on the day of tick harvest, stabilate dilution administered to animal, month in which tick batch was harvested, minimum packed cell volume of animal over the sampling period, age of animal, and the minimum leukocyte count of the animal over the sampling period. The GLIM analyses were found to be a useful tool in identifying factors that influence infection levels and in devising methods of producing tick batches with more predictable infections.

Circulating antigen levels and antibody responses in Schistosoma bovis-infected West African Dwarf goats were evaluated during infection and following treatment with praziquantel (60 mg/kg) 13 weeks post-infection. One day, 1 week and 4 weeks post-treatment, subgroups of goats were sacrificed and perfused for worm recovery. For comparison, parasite-free control animals were included. Blood and faecal samples were collected biweekly. Two gut-associated schistosome antigens, circulating cathodic and circulating anodic antigen (CCA and CAA) and 3 specific antibody responses (total Ig, IgG and IgM) were measured. For specific antibody detection, crude S. bovis adult worm and egg homogenates were used. The level of CCA in the infected groups was significantly elevated from the time of onset of egg excretion onwards. However, following treatment, the CCA litres dropped to control levels within 1 week post-treatment. Strong positive correlations were found between CCA levels and worm counts and faecal egg counts during peak egg excretion. The correlations of CAA and specific antibody litres to egg and worm counts were poor. The antibody responses were all significantly elevated in the infected goats during patency, but only marginally affected by the treatment. Hence, CCA proved to be superior by correlating strongly to the level of infection and by being a sensitive indicator of the effect of treatment.

Levels of 2 Schistosoma circulating antigens, the circulating anodic antigen (CAA) and the circulating cathodic antigen (CCA), were determined in serum samples collected, on a monthly basis over a period of 1·5 years, from 32 farm animals of different ages and from 12 tracer calves exposed to Schistosoma mattheei infection on a Zambian farm. Faecal egg counts were monitored in all animals and worm burdens in tracers determined after perfusion. Antigen determination tests in serum, with sensitivities between 95 and 100% in heifers and adult cows, proved to be excellent tools for the diagnosis of cattle schistosomiasis. Also in young calves, some infections could be demonstrated earlier by CCA determination than by faecal egg examination. A poor correlation was seen between the data for faecal egg counts and for CAA and CCA levels. It therefore appears that circulating antigen measurements in serum are of limited value as indicators of the pathogenesis of infection in cattle. Although all tracer calves were found infected at perfusion, large variations were recorded in antigen levels. An unexpected finding was the observation in farm animals of a clear seasonal pattern in CAA levels, with significant increase between August and October during the second half of the dry season, when animals are subjected to heavy physical and nutritional stress. It therefore appears that, although circulating antigen determination may provide an indication of the worm burden in ageing infections, possible variations of antigen clearance rate with the physiological condition of the host may complicate the interpretation of the results.

A longitudinal investigation on natural populations of Ascaris in humans and pigs and an investigation of soil contamination with Ascaris eggs were carried out from June 1993 to June 1994 in 2 villages, Manhu area, Xinjian County, Jiangxi Province, China. Results from these studies indicate that although human ascariasis is endemic there is significant fluctuation in both prevalence and the mean number of eggs/g faeces (epg) of the communities. Fluctuation of age-stratified prevalence and mean epg was detected in children but not in most adult groups. Most cases of human ascariasis were judged to involve low intensities of infection and a typical overdispersion distribution pattern was observed through the year. It was estimated that during the year, nearly half of the eggs discharged in the environment came from infections in children aged between 2 and 15 years which accounted for about 30% of the total population. Soil in and around houses and in vegetable gardens was found to be contaminated by Ascaris eggs and this situation remained relatively stable throughout the year. Monthly developmental rate of Ascaris eggs in soil was detected and the results suggest that the fluctuation in prevalence observed during the year should be directly attributed to the effect of seasonality of egg development. Features of Ascaris infection in pigs were found to be similar to those in humans except for a lower mean intensity of infection. The possibility of cross-infection of Ascaris between human and pig hosts is discussed.

Sporozoite invasion of bovine lymphocytes by Theileria parva is a pH-dependent process that occurs without the need for de novo protein synthesis. The process was inhibited by RGD(S) peptides, fibronectin and, in the presence of serum, by antibodies reactive with fibronectin. Invasion was also blocked by a range of sulphated glycoconjugates, but treatment of lymphocytes with heparitinase did not inhibit entry. Enzymic modifications of the lymphocyte surface demonstrated that trypsin-insensitive glycoproteins containing O- and N-linked carbohydrates as well as phospholipase-sensitive molecules on the host cell surface were critical to sporozoite entry. Modification of the lymphocyte surface with NEM and DTT had only marginal effects on sporozoite binding but blocked parasite internalization. Invasion was also blocked by several antibodies which cross-reacted with sporozoite surface molecules. While only a few experimental conditions specifically blocked sporozoite binding, a wider range of reagents and treatments inhibited parasite entry. The reasons for this are discussed in terms of the nature of the zippering process that facilitates sporozoite internalization.

The localization of the 28 kDa Schistosoma mansoni glutathione S-transferase (Sm28 GST) has been investigated using immunohistochemistry and electron microscopy and the results compared with previously published data. This study confirms the wide distribution of this antigen in the parasite. In male and female worms, Sm28 GST is localized in the tegument, the parenchyma, the oesophageal epithelium and in genital organs. Sm28 GST was clearly detected in germinal and sustentacular cells. The decrease of staining intensity during the differentiation of germinal cells suggests a down-regulated expression of the molecule. At the ultrastructural level, this antigen was abundant in nuclei and less present in the cytoplasm. The marked heterogeneity observed in the staining of individual worms indicates that Sm28 GST seems to be closely associated with the parasite’s metabolism. The results are discussed in relation to the biological and protective functions of the protein.

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.