Both verbal and mathematical models of parasite virulence predict that genetic diversity of microparasite infections will influence the level of costs suffered by the host. We tested this idea by manipulating the number of co-existing clones of Plasmodium mexicanum in its natural vertebrate host, the fence lizard Sceloporus occidentalis. We established replicate infections of P. mexicanum made up of 1, 2, 3, or >3 clones (scored using 3 microsatellite loci) to observe the influence of clone number on several measures of parasite virulence. Clonal diversity did not affect body growth or production of immature erythrocytes. Blood haemoglobin concentration was highest for the most genetically complex infections (equal to that of non-infected lizards), and blood glucose levels and rate of blood clotting was highest for the most diverse infections (with greater glucose and more rapid clotting than non-infected animals). Neither specific clones nor parasitaemia were associated with virulence. In this first experiment that manipulated the clonal diversity of a natural Plasmodium-host system, the cost of infection with 1 or 2 clones of P. mexicanum was similar to that previously reported for infected lizards, but the most complex infections had either no cost or could be beneficial for the host.

Several lines of indirect evidence suggest that subjects with latent infection of the coccidian parasite Toxoplasma gondii have a higher concentration of testosterone than uninfected controls. Here, we searched for direct evidence of latent toxoplasmosis-associated differences in testosterone concentration among a population of 174 female and 91 male students screened for Toxoplasma infection. We have found Toxoplasma-infected men to have a higher concentration of testosterone and Toxoplasma-infected women to have a lower concentration of testosterone than Toxoplasma-free controls. The opposite direction of the testosterone shift in men compared to women can explain the observed gender specificity of behavioural shifts in Toxoplasma-infected subjects.

A surface agglutination/immobilization antigen was purified from the novel agglutination/immobilization serotype (serotype G37) of the ciliated protozoan Cryptocaryon irritans, a parasite of seawater fishes. Serum from fish immunized with C. irritans theronts had agglutination/immobilization activity against theronts in vitro. However, fish and rabbit antisera raised against serotype G32 (reported previously) caused little agglutination/immobilization of serotype G37 theronts. Immunological analysis indicated that the 37 kDa theront surface membrane protein may be the agglutination/immobilization antigen of this serotype. The full-length 37 kDa antigen cDNA contained 1171 base pairs, encoding a 331–amino acid protein with hydrophobic N- and C-termini, which are characteristically found in proteins containing a C-terminal glycosylphosphatidylinositol anchor. In addition, the genetically characterized nucleotide sequences of the first internal transcribed spacer region of ribosomal DNA of these 2 serotypes were compared. The internal transcribed spacer rDNA sequence of serotype G32 was identical to that of isolates from Pingtung, Taiwan, and from the USA. On the other hand, the sequences of serotype G37 were not identical to those of any C. irritans isolate.

Cryptosporidium parvum is an intracellular protozoan parasite belonging to the phylum Apicomplexa, and a major cause of waterborne gastroenteritis throughout the world. Invasive zoites of apicomplexan parasites, including C. parvum, are thought to have characteristic organelles on the apical apex; however, compared with other parasites, the cytoskeletal ultrastructure of C. parvum zoites is poorly understood. Thus, in the present study, we ultrastructurally examined C. parvum sporozoites using electron microscopy to clarify the framework of invasive stages. Consequently, at the apical end of sporozoites, 3 apical rings and an electron-dense collar were seen. Two thick central microtubules were seen further inside sporozoites and extended to the posterior region. Using anti-α and -β tubulin antibodies generated from sea urchin and rat brain, both antibodies cross-reacted at the apical region of sporozoites in immunofluorescent morphology. The molecular mass of C. parvum α tubulin antigen was 50 kDa by Western blotting and the observed apical cytoskeletal structures were shown to be composed of α tubulin by immunoelectron microscopy. These results suggested that C. parvum sporozoites were clearly different in their cytoskeletal structure from those of other apicomplexan parasites.

Of the 7 genetic assemblages of the parasite Giardia duodenalis only 2 (A and B) are known to cause infections in humans. These assemblages have been characterized in detail at the genomic level but few studies have examined differences in the proteins expressed. Employing one and two-dimensional PAGE we have identified an assemblage A-specific protein of human infective G. duodenalis; alpha 2 giardin. The protein difference was evident using both electrophoretic techniques. Alpha 2 giardin is known to be a structural protein and associates with the caudal flagella and the plasma membrane; however, its exact function is unknown. Although several proteins unique to assemblage B were also observed, we were unable to identify these proteins due to a lack of genomic data available for assemblage B isolates. Together, these proteins represent distinct phenotypic differences between the human infective assemblages of G. duodenalis and support the need to revise the taxonomy of this parasite.

For the evaluation of the epidemiology of Theileria equi and Babesia caballi in a herd of 510 horses in SW Mongolia, several mathematical models of the transmission dynamics were constructed. Because the field data contain information on the presence of the parasite (determined by PCR) and the presence of antibodies (determined by IFAT), the models cater for maternal protection with antibodies, susceptible animals, infected animals and animals which have eliminated the parasite and also allow for age-dependent infection in susceptible animals. Maximum likelihood estimation procedures were used to estimate the model parameters and a Monte Carlo approach was applied to select the best fitting model. Overall, the results are in line with previous experimental work, and add evidence that the epidemiology of T. equi differs from that of Babesia spp. The presented modelling approach provides a useful tool for the investigation of some vector-borne diseases and the applied model selection procedure avoids asymptotical assumptions that may not be adequate for the analysis of epidemiological field data.

To date, large-scale production of Cryptosporidium parvum oocysts has only been achieved by amplification in neonatal calves and sheep. Many laboratories currently depend on supplies from external sources and store oocysts for prolonged periods which results in progressive loss of viability. Six to 8-week-old interferon γ receptor knockout (IFNγR-KO) mice on a C57BL/6 background were inoculated by gavage (2000 oocysts/animal). Fecal pellets were collected daily from 7 days post-infection (p.i.) up to 2 weeks p.i. Intestinal oocyst yield was assessed at days 11, 12 and 14 p.i. by homogenization of intestinal tissues. Ether extraction and one or more NaCl flotations were used to purify oocysts. Total recoveries averaged 2·6×106 oocysts/mouse from fecal material and 3·8×107 oocysts/mouse from intestinal tissues. Overall, 2·3×109 purified oocysts were obtained from 60 mice. Recovered oocysts were capable of sporulation and were shown to be infectious both in vitro and in vivo. Oocyst amplification was achieved in only 11–14 days with minimal expense. The simplicity of this method presents a practical alternative for the routine passage, maintenance and storage of C. parvum in biomedical laboratories.

Clinical immunity to Plasmodium falciparum malaria develops after repeated exposure to the parasite. At least 2 P. falciparum variant antigens encoded by multicopy gene families (var and rif) are targets of this adaptive antibody-mediated immunity. A third multigene family of variant antigens comprises the stevor genes. Here, 4 different stevor sequences were selected for cloning and expression in Escherichia coli and His6–tagged fusion proteins were used for assessing the development of immunity. In a cross-sectional analysis of clinically immune adults living in a malaria endemic area in Ghana, high levels of anti-STEVOR IgG antibody titres were determined in ELISA. A cross-sectional study of 90 nine-month-old Ghanaian infants using 1 recombinant STEVOR showed that the antibody responses correlated positively with the number of parasitaemia episodes. In a longitudinal investigation of 17 immunologically naïve 9-month-old infants, 3 different patterns of anti-STEVOR antibody responses could be distinguished (high, transient and low). Children with high anti-STEVOR-antibody levels exhibited an elevated risk for developing parasitaemia episodes. Overall, a protective effect could not be attributed to antibodies against the STEVOR proteins chosen for the study presented here.

For hundreds of years, the unmanaged Soay sheep population on St Kilda has survived despite enduring presumably deleterious co-infections of helminth, protozoan and arthropod parasites and intermittent periods of starvation. Important parasite taxa in young Soay sheep are strongyles (Trichostrongylus axei, Trichostrongylus vitrinus and Teladorsagia circumcincta), coccidia (11 Eimeria species) and keds (Melophagus ovinus) and in older animals, Teladorsagia circumcincta. In this research, associations between the intensity of different parasite taxa were investigated. Secondly, the intensities of different parasite taxa were tested for associations with variation in host weight, which is itself a determinant of over-winter survival in the host population. In lambs, the intensity of strongyle eggs was positively correlated with that of Nematodirus spp. eggs, while in yearlings and adults strongyle eggs and coccidia oocysts were positively correlated. In lambs and yearlings, of the parasite taxa tested, only strongyle eggs were significantly and negatively associated with host weight. However, in adult hosts, strongyles and coccidia were independently and negatively associated with host weight. These results are consistent with the idea that strongyles and coccidia are exerting independent selection on Soay sheep.

A study carried out on a sheep farm examined whether Toxoplasma gondii foetal infection and associated abortion occur in successive lambings. We identified 29 ewes that gave birth to lambs on at least 2 successive years over our study period, 2000–2003. Tissue samples from the progeny of these ewes were analysed by PCR to determine infection status with T. gondii. T. gondii-infected lambs were born in 31% of successive pregnancies. T. gondii-positive lambs were aborted in successive pregnancies in 21% of lambings during study period, 2000–2003. The frequency of successive abortions within this flock over the period 1992–2003 was 18%. If a lamb was congenitally infected there was a high risk (69%) that the successive lamb from that ewe would also be congenitally infected. Similarly, if a lamb was aborted there was a high risk (55%) of abortion in the next lamb produced. These data suggest that life-long immunity to T. gondii infections may not always be acquired following an initial infection and raises the question as to whether the mechanisms of T. gondii transmission prior to and during ovine pregnancies are fully understood.

A Minchinia sp. (Haplosporidia: Haplosporidiidae) parasite was identified infecting rock oysters and morphologically described by Hine and Thorne (2002) using light microscopy and transmission electron microscopy (TEM). The parasite was associated with up to 80% mortality in the host species and it is suspected that the parasite would be a major impediment to the development of a tropical rock oyster aquaculture industry in northern Western Australia. However, attempts to identify the parasite following the development of a specific probe for Haplosporidium nelsoni were unsuccessful. The SSU region of the parasite's rRNA gene was later characterized in our laboratory and an in situ hybridization assay for the parasite was developed. This study names the parasite as Minchinia occulta n sp. and morphologically describes the parasite using histology, scanning electron microscopy and transmission electron microscopy. The non-spore stages were unusual in that they consisted primarily of uninucleate stages reminiscent of Bonamia spp. The parasite's spores were ovoid to circular shaped and measured 4·5 μm–5·0 μm×3·5–4·1 μm in size. The nucleus of the sporoplasm measured 1·5–2·3 μm and was centrally located. The spores were covered in a branching network of microtubule-like structures that may degrade as the spore matures.

An indirect ELISA and an immunoblot were standardized to detect anti-Toxoplasma gondii antibodies in sheep, and were compared with a commercial ELISA as reference. Sensitivity was 92·0 and 96·0%, and specificity 88·0 and 75·0%, respectively. Then the serum samples of 103 sheep on a ranch located in the Eastern region of Mexico were analysed. A frequency ranging from 77 to 84% was observed, with a heterogeneous pattern among the animals by immunoblotting. Ten months later 56 sheep were sampled and tested again. Six animals became negative while 1 case was negative the first time and strongly positive 10 months later. Considering this to be a new case we calculated an incidence rate of 2·1% (CI95% 0·6–4·8%). IgG avidity ELISA was performed on 36 positive samples, 33 being of high-avidity at both times; slight increases in 2 samples and conservation of low-avidity in 1 sheep were also observed. Higher prevalence rates of toxoplasmosis in a moist warm compared to a cold atmosphere was attributed to the long viability of T. gondii oocysts. This may explain the high frequency of T. gondii in this region, which apparently has favourable climatic conditions for the transmission of this protozoan, besides the presence of both domestic and wild cats.

The number of parasites colonizing a host (termed ‘multiple infection’) is an important determinant of host-parasite interactions. In theory, multiple infection is determined by random mass action in genetically and spatially homogeneous populations of host and parasite. In real populations, deviations from these assumptions may strongly influence levels of multiple infection. We carried out inoculation experiments in microcosms of the freshwater protozoan Paramecium caudatum and its bacterial parasite Holospora undulata. Increasing parasite dose produced higher levels of (multiple) infection; more susceptible host genotypes also were more multiply infected. An overall pattern of parasite aggregation (excess of uninfected individuals and of individuals carrying larger numbers of parasites) indicated deviations from random mass-action transmission. Homogenizing spatial distributions of parasite and host in our microcosms did not affect aggregation, whereas aggregation was more pronounced in old than in new host clones. Thus, variation in susceptibility may arise over time within clonal populations. When sequentially inoculated, already established infections increased the probability of additional infection in generally resistant host clones, but decreased it in more susceptible clones. Hence, the role of multiple infection as a driver of epidemiological or evolutionary processes may vary among populations, depending on their precise genetic composition or infection history.

The physiological and immunological state of an animal can be influenced by current infections and infection history. Consequently, both ongoing and previous infections can affect host susceptibility to another parasite, the biology of the subsequent infection (e.g. infection length) and the impact of infection on host morbidity (pathology). In natural populations, most animals will be infected by a succession of different parasites throughout the course of their lives, with probably frequent concomitant infections. The relative timing of different infections experienced by a host (i.e. the sequence of infection events), and the effects on factors such as host susceptibility and host survival, can only be derived from longitudinal data on individual hosts. Here we review some of the evidence for the impact of co-infection on host susceptibility, infection biology and pathology focusing on insights obtained from both longitudinal studies in humans and experiments that explicitly consider the sequence of infection. We then consider the challenges posed by longitudinal infection data collected from natural populations of animals. We illustrate their usefulness using our data of microparasite infections associated with field vole (Microtus agrestis) populations to examine impacts on susceptibility and infection length. Our primary aim is to describe an analytical approach that can be used on such data to identify interactions among the parasites. The preliminary analyses presented here indicate both synergistic and antagonistic interactions between microparasites within this community and emphasise that such interactions could have significant impacts on host-parasite fitness and dynamics.

While BALB/c mice survive infection with blood stages of Plasmodium chabaudi chabaudi (AS), 70% of DBA/2 mice die by day 9–11 of infection, both strains controlling parasitaemia. We describe here that infection of DBA/2 mice results in extensive, multifocal hepatocyte death. Antibody neutralization of TNF-α prevents both liver damage and death.

Prevalence and abundance of Cryptosporidium spp. and Giardia spp. infections were studied over the 8-year period in 3 species of rodents in N.E. Poland (bank vole Myodes glareolus-1523; yellow-necked mouse Apodemus flavicollis- 638; common vole Microtus arvalis- 419). Prevalence was 53·8, 28·1 and 62·3% respectively for Cryptosporidium spp. and 58·3, 24·4 and 74·2% respectively for Giardia spp. Prevalence and abundance of infection varied markedly across 8 years of the study with 1998 and 2002 being years of higher prevalence and abundance, following changes in the densities of host species. The distribution of intestinal protozoa in forest rodents did not vary in the 3 isolated sites during the 4-year study. In the case of Cryptosporidium, fewer older animals carried infection and infections of the oldest bank and common voles were relatively milder. In the case of Giardia in yellow-necked mice, infections were more common in older age classes (2 and 3). The two species showed significant co-occurrence and in animals carrying both species there was a strong significant positive correlation between abundance of infection with each. These data are discussed in relation to the parasite genotypes identified in this region and in respect of the role of various ecological factors in shaping of intestinal protozoa communities.

Trypanosoma cruzi, the agent of Chagas disease is associated with a very high clinical and epidemiological pleomorphism. This might be better understood through studies on the evolutionary history of the parasite. We explored here the value of antigen genes for the understanding of the evolution within T. cruzi. We selected 11 genes and 12 loci associated with different functions and considered to be involved in host-parasite interaction (cell adhesion, infection, molecular mimicry). The polymorphism of the respective genes in a sample representative of the diversity of T. cruzi was screened by PCR-RFLP and evolutionary relationships were inferred by phenetic analysis. Our results support the classification of T. cruzi in 2 major lineages and 6 discrete typing units (DTUs). The topology of the PCR-RFLP tree was the one that better fitted with the epidemiological features of the different DTUs: (i) lineage I, being encountered in sylvatic as well as domestic transmission cycles, (ii) IIa/c being associated with a sylvatic transmission cycle and (iii) IIb/d/e being associated with a domestic transmission cycle. Our study also supported the hypothesis that the evolutionary history of T. cruzi has been shaped by a series of hybridization events in the framework of a predominant clonal evolution pattern.

Understanding genetic relationships amongst the life-history traits of parasites is crucial for testing hypotheses on the evolution of virulence. This study therefore examined variation between parasite isolates (the bacterium Pasteuria ramosa) from the crustacean Daphnia magna. From a single wild-caught infected host we obtained 2 P. ramosa isolates that differed substantially in the mortality they caused. Surprisingly, the isolate causing higher early mortality was, on average, less successful at establishing infections and had a slower growth rate within hosts. The observation that within-host replication rate was negatively correlated with mortality could violate a central assumption of the trade-off hypothesis for the evolution of virulence, but we discuss a number of caveats which caution against premature rejection of the trade-off hypothesis. We sought to test if the characteristics of these parasite isolates were constant across host genotypes in a second experiment that included 2 Daphnia host clones. The relative growth rates of the two parasite isolates did indeed depend on the host genotype (although the rank order did not change). We suggest that testing evolutionary hypotheses for virulence may require substantial sampling of both host and parasite genetic variation, and discuss how selection for virulence may change with the epidemiological state of natural populations and how this can promote genetic variation for virulence.

A panel of microsatellites mapped to the Leishmania genome might make it possible to find associations between specific loci and phenotypic traits. To identify such loci, a Perl programme was written that scans the sequence of a genome and writes all loci containing microsatellites to a MySQL database. The programme was applied to the sequences of the L. braziliensis, L. infantum and L. major genomes. The database is publicly available over the internet: http://www.genomics.liv.ac.uk/tryps/resources.html ‘Microsatellite Locus Extractor’, and allows the selection of mapped microsatellites that meet user-defined criteria from a specified region of the selected genome. The website also incorporates a primer design pipeline that will design primers to amplify the selected loci. Using this pipeline 12 out of 17 primer sets designed against the L. infantum genome generated polymorphic PCR products. A tailed primer protocol was used to label all microsatellite primers with a single set of labelled primers. To avoid the culture of parasites prior to genotyping, sets of nested PCR primers were developed to amplify parasite DNA eluted from microscope slides. The limit of detection was approximately 1·6 parasite equivalents. However, only 6/56 DNA from slides stored at ambient temperature for over 6 months gave positive PCR results.

Post-kala-azar dermal leishmaniasis (PKDL) is an unusual dermatosis that develops as a sequel in 5–15% of cured cases of kala-azar (KA) after months or years of treatment in India. Molecular differences are reported to exist between the KA and PKDL isolates which may underlie the diversity in clinical manifestations of the disease. Here, arbitrary primed-PCR (AP-PCR) has been used for genetic fingerprinting of parasite isolates from dermal lesions of PKDL patients (n=14) and compared with bone-marrow derived parasites from KA patients (n=3). All isolates showed an identical AP-PCR pattern with 4 arbitrary primers. Further, AP-PCR was exploited to identify the stage regulated genes of the parasite. Six polymorphic fragments were identified in PKDL in comparison with KA isolates, and were subjected to Northern blot analysis. Five polymorphic fragments represented transcribed sequences; 4 out of 5 drew differential expression in pro- and amastigote stages, although the expression was comparable between PKDL and KA isolates. The study led to the identification of genes, which exhibit stage-regulated expression in Leishmania donovani derived from PKDL or KA patients, including a putative phosphodiesterase, DEAD box RNA helicase, iron superoxide dismutase b (fesodb) and a hypothetical protein. Demonstration of transcripts of DEAD box RNA helicase in PKDL and KA diseased tissues implicates its role in disease pathogenesis.