Here we examined the impact of host immunity on relationships between parasite virulence, transmission rate, intrinsic growth rate and host recovery rate in the rodent malaria parasite, Plasmodium chabaudi. Groups of naïve and immunized mice were infected with 1 of 10 cloned lines of parasites and their infection dynamics were monitored for 19 days. We found that (1) host immunity reduced the growth rate, virulence, transmission rate and infection length, with a consequent 3-fold reduction in life-time transmission potential, (2) clone means for these traits ranked similarly across naïve and immunized mice, (3) regression slopes of transmission potential on growth rate, virulence and infection length were similar in naïve and immunized mice, (4) virulence and infection length were positively correlated in immunized but not naïve mice, and (5) for a similar level of parasite growth rate and virulence, transmission potential and infection length were lower in immunized than naïve mice. Thus host immunity reduced all these fitness traits in a manner consistent with direct parasite-driven biological links among them. These results support the basic assumption underlying our theory that predicts that anti-disease vaccines will select for higher virulence in those microparasites for which virulence is integrally linked to transmission.

In Europe, the zoonotic cycle of Babesia microti has not been determined so far. Recently, B. microti was detected in Ixodes ricinus ticks in Slovenia by using molecular methods. In order to investigate the mammalian hosts of B. microti in Slovenia we collected 261 small mammals representing 11 species. They were tested for the presence of babesial parasites with a PCR assay based on the nuclear small subunit rRNA gene (nss-rDNA). The bank vole (Clethrionomys glareolus) and yellow-necked mouse (Apodemus flavicollis) were infected with B. microti. The prevalence rate was 15·9% for C. glareolus and 11·8% for A. flavicollis. Nucleotide sequences of amplified portions of B. microti nss-rDNA from C. glareolus and A. flavicollis were indistinguishable from each other and identical with those previously described in I. ricinus ticks collected in Slovenia. The results of this study represent molecular evidence of B. microti in small mammals in Europe.

Although purely vertically transmitted parasites are predicted to cause low pathogenicity in their hosts, the effects of such parasites on host fitness under stressful environmental conditions have not previously been assessed. Here, we investigate the effects of Nosema granulosis, a vertically transmitted, microsporidian parasite of the brackish water amphipod Gammarus duebeni, on host growth and survival under conditions of host–host competition and limited food. The parasite had no effect on host survival, but caused a reduction in juvenile growth. Stressful environmental conditions also led to a reduction in G. duebeni growth. However, we found no evidence to support the prediction that parasitized hosts would suffer a greater reduction in fitness than uninfected hosts under adverse environmental conditions. We interpret our results in the context of selection for successful vertical parasite transmission.

Philasterides dicentrarchi is a histophagous ciliate causing systemic scuticociliatosis in cultured turbot. This study demonstrates that turbot which survive this disease have serum antibodies that recognize ciliary antigens of this ciliate in ELISA and immobilize/agglutinate the ciliate in vitro. Mouse sera raised against ciliary antigens and integral membrane proteins are likewise capable of immobilizing/agglutinating the ciliates, indicating that P. dicentrarchi, like other ciliates, expresses surface immobilization antigens. Furthermore, the antigen agglutinating reaction induces the parasite to shed its surface antigens rapidly, replacing them with others with different specific serology. This antigen shedding and variation response is similar to that detected in other protozoan parasites. Immunization of turbot with ciliate lysate plus adjuvant or with formalin-fixed ciliates induced synthesis of agglutinating antibodies and conferred a degree of protection against challenge infection, suggesting that the response to surface antigens may play an important role in defence against this pathogen, SDS–PAGE and immunoblotting studies indicated the existence of a predominant polypeptide of about 38 kDa in the ciliary antigen and membrane protein fractions, and this may be the principal surface antigen of P. dicentrarchi.

Schistosomiasis is initiated when cercarial larvae invade human skin. Contrary to long-held assumptions, most cercariae of Schistosoma mansoni do not shed their propulsive tails as they penetrate. Scanning electron microscopy studies and infection experiments with entire human skin and differentiated, stratum corneum-like, human keratinocyte cultures, have shown that most cercarial tails enter the skin along with their bodies. We propose that this behaviour is an adaptive trait linked with concomitant immunity.

A morphological study has been carried out to determine the effect of the active sulphoxide metabolite of the benzimidazole anthelmintic, albendazole (ABZ-SO) on the adult liver fluke, Fasciola hepatica. Whole flukes were treated with ABZ-SO for 12 and 24 h at a concentration of 10 μg/ml. The changes in response to drug treatment were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tubulin immunocytochemistry (ICC). No surface changes were apparent following 12 h ABZ-SO treatment, but localized blebbing was observed after 24 h, which became more extensive towards the posterior region of both surfaces. TEM of sections from the posterior midbody region revealed that ABZ-SO caused the accumulation of secretory bodies in the tegumental cells and in their cytoplasmic connections and, after 24 h, just above the basal plasma membrane. Localized blebbing of the apical membrane also occurred. The morphology of the Golgi complexes within the tegumental cells began to change after 12 h treatment with ABZ-SO and, by 24 h, few complexes were observed. A distinct increase in tubulin immunoreactivity occurred after 12 h treatment, but this decreased after 24 h. The results obtained are consistent with those expected for microtubule inhibition. They are discussed in relation to the action of established microtubule inhibitors, as well as the benzimidazole derivative, triclabendazole.

Here we describe the purification and characterization of a vitellin (VT) degrading cysteine endopeptidase (VTDCE) from eggs of the hard tick Boophilus microplus. A homogeneous enzyme preparation was obtained by chromatographic fractionation on ion-exchange and gel filtration columns and an autolysis step. This step consisted of incubation of a semipurified enzyme (after the first ion-exchange chromatography) at pH 4·0 that dissociated the enzyme from VT, to which VTDCE is naturally tightly associated. The enzyme purity was confirmed by capillary and native gel electrophoresis, and SDS–PAGE suggested the enzyme is a dimer of 17 and 22 kDa. VTDCE was active upon several synthetic substrates, with a preference for a hydrophobic or a basic residue in P1, and a hydrophobic residue in P2. VTDCE also hydrolysed haemoglobin, albumin, gelatin and vitellin. VTDCE is inactive in the absence of DTT and was totally inhibited by E-64, indicating it is a cysteine endopeptidase. Our results suggest that VTDCE is a major enzyme involved in yolk processing during B. microplus embryogenesis.

The parasitic female of Strongyloides venezuelensis keeps invading the epithelial layer of the host intestinal mucosa. Upon invasion, it adheres to the surface of the intestinal epithelial cells with adhesion molecules secreted from the mouth. It has been demonstrated that S. venezuelensis are expelled from the intestine because mucosal mast cells inhibit the attachment of adult worms to the mucosal surface. In the present study, we generated specific antibodies against secreted adhesion molecules to investigate their function in vivo, because these molecules have been demonstrated only in vitro in spite of the importance in the infection processes. A mouse monoclonal antibody specific to S. venezuelensis adhesion molecules inhibited the attachment of adult worms to plastic dishes and the binding of adhesion molecules to rat intestinal epithelial cells. Immunohistochemical study revealed that adhesion molecules were produced by oesophageal glands and were continuously secreted in vivo to line the wall of the tunnels formed by adult worms in the intestinal mucosa. Our findings indicate that adhesion molecules play essential roles in the infection processes of S. venezuelensis in the host intestine.

This study shows that ingestion of Schistocephalus solidus coracidia was related to general activity of Macrocyclops albidus copepods at the time of exposure. The lower the activity of the host, the fewer parasites it ingested. In an earlier study it was shown that large M. albidus copepods were less likely to become infected with S. solidus than small copepods, which could potentially be caused by differential ingestion of parasites. However, the current study did not show any evidence for such an effect arising through differential ingestion. Body size was not related to ingestion of parasites, but was positively correlated to activity. So, even though size did not significantly relate to ingestion of parasites, if anything, through their higher activity large copepods rather than small copepods may have ingested more parasites. This study indicates that differences in resistance to this parasite do not come about through differential ingestion of parasites. Also, an earlier study failed to show differential elimination of the parasite from the haemocoel. This leaves avoidance of penetration through the gut wall as the most plausible candidate causing large copepods to be more resistant to this parasite than small copepods.

Cathepsin D aspartic proteases of hookworms were recently implicated in the host-specific digestion of haemoglobin by adult parasites. Ac-APR-1 from the dog hookworm, Ancylostoma caninum and Na-APR-1 from the human hookworm, Necator americanus, were shown to be expressed in the infective larval stage (L3) as well as adult worms. We now show that both proteases degraded skin macromolecules and serum proteins, some of which were cleaved more readily from permissive definitive hosts as opposed to non-permissive hosts. Na-APR-1 degraded human collagens more efficiently than did Ac-APR-1, and Ac-APR-1 degraded canine serum albumin more efficiently than did Na-APR-1. On the other hand, both enzymes degraded human serum proteins (albumin and fibrinogen) with approximately equal efficiency under the conditions of our assays in vitro. Molecular models of these 2 orthologous, aspartic proteases showed that, despite having active site clefts with identical primary sequences, residues in the S3 pocket adopted different conformations, likely accounting for different substrate preferences reported previously. Antisera raised to both proteases partially inhibited (16–26%) migration of hookworm L3 through hamster skin in vitro, further implying a connective tissue invasive role for these enzymes in addition to digestion of serum and erythrocyte proteins for nutrition.