Trematode cercariae typically become unable to successfully infect a host many hours before they die. We examined the hypothesis that both time to 50% mortality and time to loss of infective capacity are controlled to the same degree by rates of energy expenditure, by determining the relative effects of temperature on both parameters. Infective capacity was assessed by exposing Echinostoma trivolvis cercariae of different ages to a suitable second intermediate host (the gastropod Biomphalaria glabrata) and counting 1–2 days later the number of metacercarial cysts formed. Temperature had a remarkably similar effect on time to 50% mortality and loss of infective capacity, supporting the hypothesis that both absolute and functional cercarial life-spans are limited by the rates at which energy stores are utilized.

1. The causal agent of tick-borne fever is transmitted by the adult female and the nymphal stage of the tick Ixodes ricinus. Two female ticks are sufficient to produce infection in a sheep. The larval stage does not appear to be infective. In a limited number of experiments the disease has not been produced in sheep by the inoculation of emulsions of presumably infective ticks.

2. The disease as produced by tick infestation is characterised by a period of incubation which varies from 3 to 13 days, after which there is a sharp rise in temperature; a “plateau” type of febrile reaction follows which lasts from 6 to 22 days, the temperature gradually subsiding to normal. Affected animals usually recover. Mortality as a direct result of infection is rare, but there is evidence to suggest that tick-borne fever may predispose affected animals to death from secondary causes.

3. The blood of a sheep which has reacted to the disease remains infective for some time after the subsidence of the thermal reaction.

4. The goat is susceptible to tick-borne fever.

5. The economic importance of tick-borne fever is discussed.

The epidemiology of nematode infections of Soay sheep on the island of St Kilda over a period of 2 years (August 1988–August 1990) spanning a host population crash is described. Infective larvae (L3) levels on pasture were high (2422±365 L3/kg D.M. grass in midsummer 1988) when host population density was high, decreasing after the sheep population declined by 70% in early 1989 (601 ±14 L3/kg D.M. in midsummer 1989). The availability of infective larvae to sheep increased during the winter of 1988–1989, probably as a result of concentration of existing larvae on grass as vegetation was destroyed by bad weather and overgrazing. Increased availability of pre-parasitic stages was accompanied by a marked increased in faecal egg counts from sheep of all ages and both sexes. Prevalence and intensity of infection (faecal egg counts) were higher in males than females throughout the 2-year study (χ2 = 208.3, P < 0.005 and F1.2000 = 304, P < 0.001 respectively), except during the lambing periods, and decreased with age in both sexes. Changes in prevalence and intensity of strongyle infections were associated with changes in host population density. Prevalence and intensity of Dictyocaulus filaria larvae in faeces increased during the host population crash. Infection intensity decreased with age (F1.203 = 44.02, P < 0.001) and was higher in males than females (F1.203 = 13.45, P < 0.001).

A mathematical model of the life-cycle of Echinococcus granulosus in dogs and sheep in New Zealand is constructed and used to discuss previously published experimental and survey data. The model is then used to describe the dynamics of transmission of the parasite, and the means by which it may be destabilized. It is found that under the conditions that prevailed in New Zealand during the late 1950s, at the time of surveys of this parasite, the dog–sheep life-cycle was not regulated by any effective density-dependent constraint. In contrast there was evidence for an effective acquisition of immunity to reinfection by cattle. The long time to maturity of the cyst in sheep, together with the practice of feeding aged sheep to dogs, provides a time delay in the intermediate host. By comparison, the time to maturity of the adult stage in dogs is short, but it is of sufficient magnitude to be a key factor in the destabilization of the system by a regular dog-dosing programme. The model used to describe the life-cycle is a linear integrodifferential equation of the Volterra type. Such equations are intrinsically unstable in that a small perturbation in parameters can drive a previous equilibrium solution to zero. At the time of the surveys, the value of the basic reproductive rate, R0 was close to 1, and it has since been reduced below 1 by control measures.

Where very young children come into contact with water containing schistosome cercariae, infections occur and schistosomiasis can be found. In high transmission environments, where mothers daily bathe their children with environmentally drawn water, many infants and preschool-aged children have schistosomiasis. This ‘new’ burden, inclusive of co-infections with Schistosoma haematobium and Schistosoma mansoni, is being formally explored as infected children are not presently targeted to receive praziquantel (PZQ) within current preventive chemotherapy campaigns. Thus an important PZQ treatment gap exists whereby infected children might wait up to 4–5 years before receiving first treatment in school. International treatment guidelines, set within national treatment platforms, are presently being modified to provide earlier access to medication(s). Although detailed pharmacokinetic studies are needed, to facilitate pragmatic dosing in the field, an extended ‘dose pole’ has been devised and epidemiological monitoring has shown that administration of PZQ (40 mg/kg), in either crushed tablet or liquid suspension, is both safe and effective in this younger age-class; drug efficacy, however, against S. mansoni appears to diminish after repeated rounds of treatment. Thus use of PZQ should be combined with appropriate health education/water hygiene improvements for both child and mother to bring forth a more enduring solution.

Electrophoretic variation of the enzymes glucose phosphate isomerase, 6-phosphogluconate dehydrogenase, lactate dehydrogenase and glutamate dehydrogenase (NADP-dependent) has been studied in the African murine malaria parasites Plasmodium berghei, P. yoelii, P. vinckei and P. chabaudi and their subspecies. Horizontal starch gel electrophoresis was used throughout. The number of isolates examined in each subspecies varied from 1 (P. y. nigeriensis) to 24 (P. c. chabaudi). Extensive enzyme variation was found among isolates of most of the subspecies from which more than two such isolates were available for study. It is clear that the phenomenon of enzyme polymorphism is of common occurrence among malaria parasites. With the exception of P. berghei and P. yoelii, of which all isolates share an identical electrophoretic form of lactate dehydrogenase, no enzyme forms are shared between any of the 4 species of murine plasmodia. By contrast, within each species common enzyme forms are shared among each of the subspecies. The subspecies are, nevertheless, distinguished from each other by the electrophoretic forms of at least one enzyme.

The distribution and reassortment of enzyme variation among isolates of a single subspecies is in accordance with the concept of malaria parasites as sexually reproducing organisms. The study of variation among parasites present in individual wild-caught rodent hosts demonstrates that natural malarial infections usually comprise genetically heterogeneous populations of parasites. Nevertheless, the number of genetically distinct types of parasite of any one species present in a single infected host appears to be small. Generally not more than 2 or 3 clones of parasite of distinct genetic constitution are present in a single infected animal.

As part of a survey of human leishmaniasis in Nicaragua we examined phenotypic and genotypic characteristics of 40 Leishmania isolates. We identified 3 distinct parasites associated with cutaneous disease in this area; Leishmania panamensis (40% of cases), Leishmania braziliensis (33%), and a strain which exhibits the heterozygous isoenzyme and DNA fingerprinting patterns expected of a L. panamensis/L. braziliensis hybrid (27%). There was complete correlation between the isoenzyme and DNA data for each of the putative hybrids examined. All of the ‘hybrids’ were obtained from foci in the northern region of the country where L. panamensis and L. braziliensis occur sympatrically. These observations provide strong evidence for sexual reproduction in New World Leishmania populations and suggest that it is of taxonomic and epidemiological significance.

Carefully chosen immunological measurements, informed by recent advances in our understanding of the diversity and control of immune mechanisms, can add great interpretative value to ecological studies of infection. This is especially so for co-infection studies, where interactions between species are often mediated via the host's immune response. Here we consider how immunological measurements can strengthen inference in different types of co-infection analysis. In particular, we identify how measuring immune response variables in field studies can help reveal inter-species interactions otherwise obscured by confounding processes operating on count or prevalence data. Furthermore, we suggest that, due to the difficulty of quantifying microbial pathogen communities in field studies, innate responses against broad pathogen types (mediated by pattern response receptors) may be useful quantitative markers of exposure to bacteria and viruses. An ultimate goal of ecological co-infection studies may also be to understand how dynamics within host-parasite assemblages emerge from trade-offs involving different arms of the immune system. We reflect on the phenotypic measures that might best represent levels of responsiveness and bias in immune function. These include mediators associated with different T-helper cell subsets and innate responses controlled by pattern response receptors, such as the Toll-like receptors (TLRs).

Rickettsia-like organisms (RLO) from tsetse midguts and mosquito cell cultures showed high levels of endochitinase activity. A line of Glossina morsitans morsitans highly susceptible to midgut trypanosome infection and with high incidence of RLO infection showed significantly greater chitinolytic activity than G. austeni which had low RLO incidence and were correspondingly refractory to midgut infection. Midgut infection rates of Trypanosoma brucei rhodesiense in G. m. morsitans showed a dose-related increase when flies were fed N-acetyl-D-glucosamine (GlcNAc) in the infective meal and for 4 subsequent days. A model is proposed for susceptibility to trypanosome infection based on the generation of GlcNAc by RLO endochitinase activity in tsetse pupae inhibiting midgut lectin in teneral flies.

T cell-deprived mice acutely infected with S. mansoni suffer microvesicular hepatocyte damage which is not seen in infected, immunological intact animals. A cationic fraction (CEF6) of the PBS-soluble portion of S. mansoni eggs (SEA) induces antibodies which, on passive transfer, prevent hepatocyte damage. CEF6 contains 2 antigens, ω1 and α1, and has also been shown to be a useful serodiagnostic reagent. This paper describes the purification and characterization of the 2 antigens present in CEF6. ω1 is a monomeric glycoprotein with a pI > 9·0 and a molecular weight of 31 kDa. α1 consists of two immunologically cross-reactive dimers, 41 and 36 kDa in non-reducing conditions, each of which consists of one unique and one common glycoprotein subcomponent. In ELISA with mouse and human infection sera ω1 is shown to be S. mansoni specific and is better able to distinguish S. mansoni infections from other schistosome infections than are unfractionated SEA, CEF6 or α1. Passive transfer of monospecific anti-ω1 sera into S. mansoni infected, T cell-deprived mice completely prevented the occurrence of microvesicular hepatocyte damage in these animals. Monospecific anti-α1 serum had no hepatoprotective capacity.

Pig faeces were deposited on experimental plots in the spring, summer, autumn and winter to study development and survival of Ascaris suum and Trichuris suis eggs under outdoor conditions. Faeces were placed either in short grass or 2 cm below the surface of bare soil, imitating pastures used by nose-ringed, grazing pigs or normally rooting pigs, respectively. The numbers and developmental stages of the eggs were recorded in faeces and soil for up to 50 weeks post-deposition. Embryonation took place only during the summer months and seemingly was independent of the microclimate. The majority of A. suum and T. suis eggs, which are generally considered to be extremely resistant and long-lived, seems to disappear rather fast. The disappearance rate for A. suum eggs was higher than for T. suis eggs, and both egg types disappeared significantly faster in the summer months than in the winter months, and when placed in short grass than when buried in soil (less exposed). We discuss how knowledge on egg development and survival may be used in the planning of pasture strategies for control of helminth infections in outdoor pigs.

The survival characteristics of free-living cercarial populations of Transversotrema patialensis were described and shown to be age-dependent. The maximum life-span was found to be 44 h with a 50% survival at 26 h. Activity and infectivity of the larvae were also characterized by age-dependence, and were demonstrated to be closely correlated with one another. For individual cercariae, both activity and infectivity had dropped to extremely low levels many hours before death occurred. An attempt was made to interrelate activity and infectivity, in a theoretical manner, with the availability of energy reserves.

Conceptual understanding of the biological processes involved was aided by the formulation of simple mathematical models.

We describe a variety of restriction site polymorphisms in the introns of Ascaris nuclear genes and in the ribosomal DNA spacers. We use these markers, in addition to previously described mitochondrial variation, to clarify our understanding of the epidemiology of Ascaris in Guatemalan villages where humans and pigs occur in sympatry and to describe the genetic structure of host-associated Ascaris populations from world-wide locations. Intron sequences were amplified from individual worms and alleles defined by endonuclease digestion. Two loci were monomorphic, while 4 length variants and 22 point mutations were detected in the other 7 loci. Within sympatric Guatemalan populations no single locus from either the nuclear or mitochondrial genome was fixed for alternative alleles, although allele frequencies were significantly different at many loci. Phenograms constructed from multilocus nuclear genotypes of individual worms failed to reveal a single case of cross-infection, and demonstrate that divergent mtDNA genotypes are segregating within host-associated populations. On a world-wide scale, the data suggest that extant worm populations result from a single host shift, although characterization of genetic variation in additional loci will be necessary to confirm this. The direction and the geographical origin of the host shift were unresolved. Overall 65% of nuclear genetic variation was found within populations, host (human or pig) explained 18%, while geographical variation within host-associated populations explained 17%. The results (a) demonstrate the utility of introns for studying the epidemiology of parasites showing limited allozyme variation (b) suggest that programmes aiming to control Ascaris infection in the human population can safely ignore zoonotic infection from pigs and (c) illustrate the problems inherent in using single genetic markers to make inferences about the epidemiology of closely related parasite taxa.

Over 39000 species of arthropods parasitize humans, domestic animals and wildlife. Despite their medical, veterinary and economic importance, most aspects of the population genetics and evolution of the vast majority of parasitic arthropods are poorly understood. Mitochondrial genomes are a rich source of markers for studies of population genetics and evolution. These markers include (1) nucleotide sequences of each of the 37 mitochondrial genes and non-coding regions; (2) concatenated nucleotide sequences of 2 or more genes; and (3) genomic features, such as gene duplications, gene rearrangements, and changes in gene content and secondary structures of RNAs. To date, the mitochondrial genomes of over 700 species of multi-cellular animals have been sequenced entirely, however, only 24 of these species are parasitic arthropods. Of the mitochondrial genome markers, only the nucleotide sequences of 4 mitochondrial genes, cox1, cob, rrnS and rrnL, have been well explored in population genetic and evolutionary studies of parasitic arthropods whereas the sequences of the other 33 genes, and various genomic features have not. We review current knowledge of the mitochondrial genomes of parasitic arthropods, summarize applications of mitochondrial genes and genomic features in population genetic and evolutionary studies, and highlight prospects for future research.

This study represents an analysis of trypanosome strains circulating within a confined location over a short period of time during a sleeping sickness epidemic in S.E. Uganda. A large number of Trypanosoma brucei isolates (88) were collected from a variety of hosts (man, cattle, pigs and tsetse) from villages within a 10 km radius and were analysed for variation in isoenzyme patterns, restriction fragment length polymorphism (RFLP) in repetitive DNA sequences and susceptibility to human serum. The human infective stocks form a clearly distinguishable population when compared with other stocks circulating in the domestic cattle reservoir. The data here support the occurrence of genetic exchange between the cattle stocks while an ‘epidemic’ population structure involving limited genetic exchange is a characteristic of the human infective stocks. Furthermore, it is shown that when both RFLP and isoenzyme analysis are carried out most stocks appear to have individual genotypes. Stocks which were formerly grouped as zymodemes are better considered as a collection of distinct individuals.

Serine protease inhibitors (serpins) are a superfamily of structurally conserved proteins that inhibit serine proteases and play key physiological roles in numerous biological systems such as blood coagulation, complement activation and inflammation. A number of serpins have now been identified in parasitic helminths with putative involvement in immune regulation and in parasite survival through interference with the host immune response. This review describes the serpins and smapins (small serine protease inhibitors) that have been identified in Ascaris spp., Brugia malayi, Ancylostoma caninum Onchocerca volvulus, Haemonchus contortus, Trichinella spiralis, Trichostrongylus vitrinus, Anisakis simplex, Trichuris suis, Schistosoma spp., Clonorchis sinensis, Paragonimus westermani and Echinococcus spp. and discusses their possible biological functions, including roles in host-parasite interplay and their evolutionary relationships.

Attachment, engorgement and subsequent development of successive infestations of Ixodes trianguliceps larvae and nymphs on natural hosts, Apodemus sylvaticus, and unnatural hosts, laboratory mice, are compared. In laboratory mice, primary infestations above a threshold level of about 10 ticks elicit an immunological response which reduces, in a density-dependent manner, the rate of successful tick engorgement during subsequent infestations. In contrast, in A. sylvaticus successive infestations of larvae result in unchanged or slightly improved survival through to nymphs. The relevance of these results to the concept of host-parasite co-evolution and to tick population regulation is discussed.

Clones of animal-infective bloodstream forms of Trypanosoma brucei (stocks S.427 and LUMP 227) were made by transferring a single organism from bloodstream-form cultures into each well of Microtest II Tissue Culture Plates containing bovine fibroblast-like feeder cells. When the number of trypanosomes increased to 102–103/well on days 4–16, they were transferred into plastic T-25 culture flasks also containing feeder cells and fresh medium. Cultures were thereafter maintained by partially replacing the trypanosome suspension with the same volume of fresh medium (diluting the density to 2–5 × 105 trypanosomes/ml) every 24 h. Sub-cultivations could be made by transferring 1–2 ml of the trypanosome suspension to a new culture flask at 4–5 day intervals. A total of 42 clones in the 3 series TC221, TC52 and TC227, carrying variable antigen types (VATs) 221, 052 and ILTat 1·4, respectively, have been established. Average population doubling times for clones of TC221, TC52 and TC227 were 8·7, 14·5 and 15·5 h respectively. Of 35 populations examined, 34 clones retained the original specificity of their VATs for at least 8–32 days from cloning. One cloned population of TC52 consisted of 99·8% VAT 052 and 0·2% VAT 221 at the time when the first VAT test was made on day 18.

We tested the null hypothesis that populations of hosts trapped in isolated neighbouring locations showing comparable habitat quality, should support similar helminth parasite communities. The study was undertaken in a 2-week period in late summer in NE Poland in a single year, thereby eliminating seasonal and between-year variation in parasite burdens. A total of 139 Clethrionomys glareolus (bank vole) were sampled from 3 forest sites of similar habitat quality. Total species richness was 11 (6 nematodes and 5 cestodes) with 85·6% of the voles carrying at least 1 species and an overall mean species richness of 1·4. At the component community level, the fewest species of helminths were recorded from site 2 (n = 6, compared with 9 at each of the other sites), but site 3 had the lowest Berger-Parker Dominance Index and the highest Simpson's Index of Diversity. At the infracommunity level, site 3 had the highest mean no. of helminth species/vole, the highest mean Brillouin's Index of Diversity but the lowest mean no. of helminths/vole. Voles from sites 1 and 3 differed in the nematodes that were most common (site 1, Heligmosomum mixtum – 95%; site 3, Heligmosomoides glareoli – 79·3%). At site 2 no species exceeded 50% but prevalence of Syphacia petrusewiczi was higher than at the other sites. The prevalence of cestodes was too low to test reliably (12·9%), but the highest prevalence of adult cestodes was recorded at site 1 (22·5% compared with 4·9 and 1·7% for sites 2 and 3 respectively). Host sex did not influence infection, but mean species richness increased with age. The different sites were responsible for most of the variation in our data, and the intrinsic factors (sex and age) were less important in shaping the component community structure of helminths. We conclude that even locations in relative close proximity to one another (13–25 km), selected on the basis of similar habitat quality, have rodent populations that differ in their helminth parasite communities, although for reasons other than the factors quantified in the present study.

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.