rRNA and a heterologous cloned rDNA probe have been used to detect the rRNA genes of Eimeria species which infe the chicken, and has allowed the isolation and preliminary characterization of cloned rDNA sequences from a genomic DNA library of Eimeria tenella. It is demonstrated that rRNA and rDNA probes can be used to identify individual Eimeria species by the restriction fragment patterns detected after Southern hybridization. In addition, studies have shown that the large and small subunit rRNAs are expressed throughout sporulation.

Intracellular differentiation of Leishmania promastigotes to amastigotes is a critical step in the establishment of infection. In this report three related features of mexicana subspecies amastigotes were used to follow the differentiation of the parasites within macrophages. Early after infection, (a) parasites did not contain ultrastructurally recognizable megasomes, (b) cysteine proteinase activity of parasite lysates was not detected in gelatin-containing acrylamide gels, and (c) parasites were essentially resistant to L-leucine-methyl ester (Leu-OMe). Typical megasomes were first identified on the 5th day, were more prevalent on day 7, and underwent swelling in macrophages exposed to Leu-OMe. Cysteine proteinase activity was first detected on day 3 and increased thereafter. Susceptibility to Leu-OMe of parasites studied in situ or isolated from infected macrophages increased with time of intracellular residence and by 7 days approached that of amastigotes isolated from mouse lesions. In contrast, parasites derived from either promastigotes or amastigotes were equally susceptible to another leishmanicidal compound, tryptophanamide (Trp-NH2). The results provide additional support for the involvement of megasomes and their cysteine proteinases in parasite killing by Leu-OMe, and highlight the slow pace of the intracellular differentiation of L. amazonensis promastigotes to amastigotes.

The cellular responses in regional lymphoid organs of C57B1/6 mice were examined, following protective immunization with 20 krad.-irradiated cercariae of S. mansoni. Marked changes in total cell number were observed, with peak increases of 13·5-fold in the skin-draining (axillary) lymph nodes and 6·9-fold in the lung-draining (mediastinal) lymph nodes. In contrast, cellular responses were small in the spleen and undetectable in the brachial lymph nodes. The temporal pattern of responses was coincident with the kinetics of parasite migration, events in the mediastinal lymph node being apparent only after day 7. Phenotypic analysis of the cellular changes revealed an increase both in T lymphocytes and, to a greater extent, in B lymphocytes. The changes in Thyl+ cells comprised an increase in both L3T4+ and Lyt2+ populations. A comparison of mice exposed to non-immunizing parasites (normal or 80 krad.-irradiated cercariae) with protected animals, revealed smaller and more transient cellular changes in the axillary lymph nodes of the former. We suggest that the successful immunization of mice with attenuated parasites depends upon the prolonged priming of lymphocytes within the lymph nodes draining the skin-exposure site and that the persistence of 20 krad.-irradiated parasites within these nodes provides the requisite stimulus.

When cercariae of Schistosoma mansoni and of S. japonicum were irradiated with various levels of u.v. light at 254 nm, their development to perfusable worms was reduced to below 1% at about 200 μW min cm−2. Cercariae attenuated with about 300 μW min cm−2 induced partial resistance against an homologous challenge infection in mice. No differences were observed between the two schistosome species when the same treatment was given to the cercariae. Thus the same u.v. dose can confer immunizing ability to cercariae of both S. mansoni and S. japonicum.

Sex ratios of adult schistosomes in mice are almost invariably different from 1·0 and are biased towards males. The bias applies to wild rats infected with Schistosoma japonicum and trapped in an endemic area of the Philippines (male: female ratio = 1·7). It also applies to cercariae of snails collected in such areas and assessed by infection of laboratory mice using cercariae from individual snails (male: female ratio may approach 6·0). Experiments were designed to determine if duration of infection in the mammalian host was a factor that influenced the sex ratio of miracidia used for infecting snails and subsequently mice. BALB/c and C57BL/6 mice were infected with 100 cercariae of S. mansoni, and liver eggs harvested at early and late time points for infection of snails and production of cercariae. Two phenomena were demonstrated: firstly, a more pronounced male bias when eggs were harvested late compared with early in infection; secondly, a reduced apparent hatchability of eggs in BALB/c compared with C57BL/6 livers. The possibility is raised by the data that female miracidia within eggs of chronically infected individuals may be more prone to immune damage than male miracidia with important epidemiological consequences.

Evidence for the existence of 2 first intermediate host-specific sibling species of the 45 collar-spined echinostome Echinoparyphium recurvatum is presented. Experimental studies on their life-cycles were carried out under controlled laboratory conditions. The two entities were found to be morphologically indistinguishable in all major respects, yet they exhibited distinct biological characteristics. One of the sibling species utilizes the freshwater lymnaeid pulmonate snail Lymnaea peregra as first intermediate host, and the adults occur in the anterior small intestine of the wildfowl experimental definitive host Anas platyrhynchos. The other utilizes the freshwater mesogastropod prosobranch snail Valvata piscinalis as first intermediate host, and the adults occur in the posterior small intestine and rectum of A. platyrhynchos. The existence of the two sibling species in sympatry may be explained by the fact that the life-cycles of the two forms represent two distinct cycles of transmission serving to reduce excessive competition between them.

Transmission of cercariae of the echinostome digenean Echinoparyphium recurvatum was observed in experimental populations of the second intermediate host snail Lymnaea peregra. Populations of 100 snails were distributed among plastic mesh cages in an infection arena to produce different patterns of host dispersion. Cercarial transmission was investigated in host populations showing three separate patterns of host dispersion; Ideal Regular, Random and Contagious (‘Clumped’). Levels of cercarial transmission in populations showing each type of dispersion pattern were found to be significantly different. The highest rate of cercarial transmission occurred in the contagiously dispersed host population, and the lowest in the randomly dispersed population. Analysis of results from the randomly dispersed population also showed that both the mean percentage prevalence of infection, and also the mean number of cysts recovered per snail, increased significantly with the number of snails per cage. The positive relationship between the mean number of cysts per snail and the number of snails per cage was found to be best described by an exponential function. Chemosensory location of L. peregra snail hosts by E. recurvatum cercariae may provide an explanation for the patterns of cercarial transmission observed.

The hypothesis that division of Trypanosoma brucei slender bloodstream forms is dependent upon the availability of a host-derived growth factor has been tested by superimposing challenge doses of slender-form trypanosomes onto preexisting infections at a time during the primary infection when stumpy forms predominated. The challenge populations grew in the doubly-infected mice indicating that depletion of a putative growth factor by the expanding population of the primary infection had not prevented division of the trypanosomes although slight reductions in multiplication rates were observed. This effect was independent of the variable antigen type (VAT) of the trypanosomes and of their stock of origin.

Cell extracts of an entero-invasive protozoon of squirrel monkeys, Tritrichomonas mobilensis, contained relatively high proteolytic activity, measured on hide powder azure (HPA). Multiple proteinase forms, optimally active at pH 5–7, were detected by electrophoretic analysis in gelatin-containing polyacrylamide gels. Three major proteinase bands of apparent low molecular weights, Mr 18, 23 and 30 kDa, were seen on gels. Inhibition-activation studies suggest that only cysteine proteinases were involved in HPAase and gelatinolytic activities of T. mobilensis cell extracts.

Two panels of H-2 recombinant strains of mice were used in an attempt to map the H-2-linked genes which control resistance to infection with Trichuris muris. Response phenotypes could be related to the presence of ‘resistance’ (q, b) or ‘susceptibility’ (k, d) alleles at I-A. The influence of these genes was modulated by other alleles, particularly q or d alleles, at the D end of the H-2. Absence of I-E molecules correlated with resistance to infection in some but not all strains studied. Thus the (B10.BR × B10.G) Fl strain which expressed I-Ek gene products was resistant to infection. A study of the time-course of infection in strains of mice expressing q alleles throughout the H-2 on 4 different genetic backgrounds (NIH, SWR, DBA and B10) revealed that most strains were resistant to infection. However, the DBA/1 strain exhibited differential responsiveness, 4 out of 6 individuals harbouring mature adult parasites on day 35 post-infection.

Outbred MF1 mice were characterized with respect to their susceptibility to infection with Heligmosomoides polygyrus (Nematoda) on the basis of faecal egg counts after 8 weeks of repeated infection (50 larvae/week). Selective breeding for resistance and susceptibility was carried out over 3 generations. The H-2 type of a sample of the mice was determined, and antigen recognition assessed on the basis of Western blots against adult and larval H. polygyrus homogenate. The selective breeding programme yielded very strong evidence for the heritability of susceptibility to infection. The results were consistent with a model of single gene control with resistance dominant over susceptibility. The presence of the H-2k haplotype was significantly associated with susceptibility, as was the recognition of a 17 kDa antigen in blots against both larval and adult worm homogenate. The proportion of mice phenotypically susceptible to infection, the proportion bearing the H-2k haplotype, and the proportion recognizing the 17 kDa antigen, were all approximately 0.25.

The population dynamics of outbred laboratory mice in indoor enclosures in the absence and presence of a naturally transmitted direct life-cycle nematode Heligmosomoides polygyrus Dujardin 1845 were reported previously. This manuscript presents further information on the age and sex structure of the populations, results of experiments designed to estimate the density-dependent effect of the parasite on host survival and reproduction, and a mathematical model of both uninfected and infected mouse populations. In the uninfected mouse population, survival of female mice was age- and density-independent, survival of male mice was age-dependent and density-independent, and recruitment was density-dependent. Independent experiments revealed that the parasite had no density-dependent effect on mouse reproduction, but had density-dependent effects on both acute and chronic survival of mice. An age-structured Leslie matrix model captured the exponential growth and plateau of the uninfected mouse population. Modification of the model to incorporate the effects of the parasite provided a good fit to the data from the infected populations, supporting the hypothesis that density-dependent effects of the parasite on host survival could lead to regulation of host abundance.

The nematode-trapping fungus Arthrobotrys oligospora traps and invades all the free-living juvenile stages of the tri-chostrongyle nematode Trichostrongylus colubriformis. The processes of capture and invasion of the 1st-stage juveniles are described using differential interference contrast optics and 3-D reconstruction techniques. The adhesive responsible for capture is well preserved using a freeze-substitution technique for scanning electron microscopy. The invasion process of the ensheathed 3rd-stage juvenile of T. colubriformis takes much longer than in the 1st- or 2nd-stage juvenile and involves the formation of secondary infection pegs between the sheath and the cuticle which appear to penetrate the cuticle by physical pressure.

Removal of the sheath of the ensheathed infective juvenile of Trichostrongylus colubriformis prevents capture by the nematophagous fungus Arthrobotrys oligospora. Exposure of the trap hyphae to a variety of saccharides, which may block a recognition system based on lectin/carbohydrate binding, failed to prevent capture but some saccharides did inhibit penetration and invasion by the fungus. Capture and penetration thus appear to be two distinct processes with capture being less specific than penetration. Carbohydrate residues were absent from the outer surface of the cuticle and the sheath but were present on the inner surface of the sheath. The limited accessibility of these lectin-binding sites may explain the slow process of infection of the infective juvenile by the fungus. The sheath does not protect the infective juvenile against attack by this nematophagous fungus.

Mean villus height, crypt depth and the number of 5-HT-positive enterochromaffin (EC) cells have been examined in two regions of the small intestine (20–30° and 60–70° distance from the pylorus) of male, 6 to 8-week-old, C57 mice following a 5-cysticercoid infection of the rat tapeworm, Hymenolepis diminuta. Test mice and sham-infected controls were autopsied 0, 4, 8, 10, 14 and 28 days post-primary infection (p-1°-i) and 2, 4, 5, 7 and 14 days post-secondary infection (p-2°-i), administered 28 days p-1°-i. Morphometric analysis revealed a statistically significant increase in crypt depth in the 60–70°o intestine region in infected mice during both primary and secondary infections; no significant deviation from the control was observed for villus height in infected mice. Statistical analysis showed a significant increase in the number of 5-HT-positive EC cells in infected mice. This response occurred in the lower portion of the intestine on days 10-p-1°-i and 5-p-2°-i, and was not due to increased mucosal surface area in this region. Results are discussed with reference to murine cestode rejection and the possible involvement therein of the neuroendocrine system.

Information relating to the characterization of cestode surface macromolecules is limited. This is especially the case with Taenia crassiceps, a well-recognized model for the study of larval cestodiasis. Here, the protein and glycoprotein composition of the tegumental surface and cyst fluid of the metacestode have been investigated using radio-isotope labelling, immunoprecipitation, SDS–PAGE and lectin affinity chromatography. A restricted number of surface proteins was labelled with the 125I/Iodogen method although the majority were immunogenic; in contrast an array of cyst fluid antigens were labelled. Host serum proteins, including immunoglobulins, were identified on the surface and in the cyst fluid. Some of the 125I-labelled surface proteins, including a 37 kDa molecule, have been shown to be glycoproteins and probably contain-D-mannose and/or D-glucose; there is limited or no N-acetylglucosamine and no terminal galactose present on these components. A 37 kDa surface molecule, possibly the same glycoprotein, was also precipitated by infection sera and this may endorse the theory that highly immunogenic carbohydrates are continuously shed by T. crassiceps as a mechanism for diverting the immune response of the host. Radio-iodinated and biosynthetically labelled T. crassiceps antigens were highly cross-reactive with antibody raised to other cestodes and not one antigen was identified as a possible candidate for use in specific immunodiagnosis of any of the important taeniid infections.

In common with other taeniid cestodes, host or host-like proteins, especially immunoglobulins, occur on the surface and in the cyst fluid of Taenia crassiceps metacestodes. Here, several approaches have been used to determine the origin of the immunoglobulins present on the tegument. Indirect IFAT showed that IgG was almost totally lost from the surface of bladders after 6 days culture in vitro. There was a rapid reacquisition of immunoglobulins following incubation of the cultured metacestodes with either normal mouse serum or mouse anti-T. crassiceps antiserum. Immunoprecipitation of in vitro translation products and biosynthetically labelled T. crassiceps proteins with a panel of anti-IgG antisera failed to positively identify any molecule with homology to mammalian immunoglobulins. These results suggest strongly that the immunoglobulins located on the surface of T. crassiceps are of host rather than parasite origin. The occurrence of a relatively low abundance receptor in the surface of the bladders, which binds non-specific host immunoglobulin, together with surface-bound specific anti-T. crassiceps antibodies can account for the presence of these host proteins. Freshly obtained bladders and metacestodes cultured in vitro for 6 days were transplanted into naive mice and the survival and development of the resulting parasites compared. In some individual mice there was a decrease in the number and volume of metacestodes and an increase in encapsulated parasites arising from cultured bladders. This was probably not related to the loss of host immunoglobulins from the parasite surface during culture as the reacquisition of these proteins after transplantation is likely to be far more rapid than any immune response could evoke in a naive host.

Observed field data from a range of geographically distinct human communities suggest a consistent non-linear relationship between prevalence and mean intensity of Ascaris lumbricoides infection. Utilizing the negative binomial distribution as a description of observed aggregation, maximum-likelihood analysis reveals that the degree of aggregation is a negative linear function of mean worm burden. The factors responsible for this relationship in human populations require further study but may involve some combination of (i) density-dependent reduction in worm numbers within individuals, (ii) density-dependent parasite-induced host mortality or (iii) self-treatment by heavily infected hosts. Variability in the degree of aggregation appears dependent on the level of infection in a community and independent of geographical differences in the host or parasite populations.

A comparison has been made of the interactions between passively transferred and actively acquired immunity in regulating populations of Taenia hydatigena and T. ovis. When ewes were grazed prior to parturition under a high infection pressure, immunity was transferred to their offspring for up to 8 weeks. A qualititative difference between the species was the destruction of larval T. ovis prior to their establishment (‘pre-encystment immunity’) and that of T. hydatigena after they had become established (‘post-encystment immunity’) in the challenged lambs. The major difference in terms of population regulation between the two parasites was that infection occurred with T. hydatigena but not with T. ovis in those lambs reared from birth for 16 weeks under high infection pressure. Passive, like active immunity, is a density-dependent constraint. It plays an important role in the population regulation of T. ovis, but not of T. hydatigena. This is discussed in terms of transmission in the natural environment, an hypothesis on humoral protection and the need to elucidate pathways of protection when immunization schedules are being evaluated for controlling the taeniid zoonoses.

A protocol for the routine laboratory maintenance of the life-cycle of Diphyllobothrium dendriticum, using herring gulls (Larus argentatus), Cyclops abyssorum and rainbow trout Oncorhynchus mykiss ( = Salmo gairdneri) as the definitive, first and second intermediate hosts respectively, is described.