Orgyia leucostigma haemocytes were able to encapsulate and destroy eggs of Hyposoter species, thereby preventing successful parasitism. When host larvae had previously been parasitized by Cotesia melanoscela, however, this response was not observed, and 3 different Hyposoter species could be reared to maturity in the normally non-permissive host. Appropriate injection experiments established that C. melanoscela virus and venom were both required in order for successful parasitism by Hyposoter to occur. Covalent cross-linking of C. melanoscela viral DNA led to an immune response against Hyposoter larvae, but not eggs, suggesting that an active viral genome may be required for complete development of Hyposoter species in tussock moth larvae.

The transmission mechanisms of microsporidia between generations of Operophtera brumata, a geometrid moth with an univoltine life-cycle, have been investigated. The parasites, Orthosoma operophterae, Pleistophora operophterae and Nosema wistmansi were present in moths collected from Wistman's Wood on Dartmoor. An infection rate of 100% was found in eggs laid by moderately or heavily infected female moths. Thus, 100% infection was found in eggs laid by 78 out of 90 infected females. Lower prevalences were found in eggs laid by the remaining 12 and these moths were subsequently shown to have very light infections. Male moths do not pass on this infection to progeny. The development of O. operophterae was followed during embryonation. The microsporidia do not invade the cells of the developing larva but are restricted to the yolk, where spores accumulate in abundance after earlier merogonies. When the larva is mature and ready to hatch, spores are ingested with the remains of the yolk, as the larva eats its way through the egg-shell. Spores are then found in the meconium in the lumen of the gut. These spores are infective and, once the gut of the larva has become functional, are responsible for infecting the tissues of the larvae which harbour them, as well as healthy larvae reared with them. Spores of O. operophterae held in the natural environment for 7 months from winter until mid-summer still retained infectivity and those contaminating the environment from the bodies of one generation must contribute to transmission between generations.

Groups of ten cattle were exposed to 0, 40 and 400 adult Rhipicephalus appendiculatus feeding once a week for 24 weeks. Although no differences in cumulative percentage weight gain were found at the end of the experiment, animals free of ticks performed better during the first 12 weeks. A marked recovery of the 400-tick group was recorded during weeks 13–24 and after ticks were removed. Haemoglobin and packed cell volume values were lowered in tick-infested animals but no clear trends were observed in red and white blood cells. No differences between the percentages of ticks feeding in the 2 tick-infested groups on day 2 were observed but higher numbers of females were feeding on the 40-tick group on day 5. More ticks were found on day 5 after tick application to the cattle during weeks 1–6 than in the rest of the experiment. Significant correlations were found between lymph node enlargement and ear damage when they were tested against tick load. R. appendiculatus nymphs feeding on the formerly tick-infested cattle were fewer, lighter and engorged in a shorter time than those on the control animals. The possible causes for the reduction in ticks feeding, the changes in cattle weights, blood composition and development of host resistance are discussed.

Clones of single oocysts, single sporocysts, single sporozoites, single proliferative parasites, single cysts and single cystozoites of Isospora (Toxoplasma) gondii (KB-strain) were made under visual control using a de Fonbrune micromanipulator. Cloning was successful in 28, 32, 21, 8, 54 and 7% of the trials, respectively. All clones were used for monoclonal infection in non-immune conventionally reared (CV) or specified pathogen-free (SPF) cats. Pre-patent and patent periods, sporulation percentages of excreted oocysts, mouse infectivity of sporulated oocysts, antibody response and immunity to reinfection of CV cats were determined. For these parameters almost no differences were observed between monoclonal infections and infections described with the non-cloned KB-strain. In all cats autopsied during the patent period, 5–8 days post-infection, macrogametes, microgametes and oocysts were found. Since meiosis occurs during sporulation and since all free-living and intermediate host stages proved to be bisexual, it is concluded that sex differentiation in I. (T) gondii is not determined by segregation of sex chromosomes or sex genes but is caused by some final host factor(s) that induce(s) differential gene expression in genetically identical cells.

Novel hydroxynaphthoquinones are reported with outstanding efficacy against Plasmodium, Eimeria and Theileria species. Biochemical evidence is presented for the selective toxicity of these compounds being due to inhibition of parasite respiratory systems.

High incidence of mycoses were found in the guts and malpighian tubes of Phlebokomus papatasi from the Jordan Valley and P. tobbi from Zakinthos, Greece. Infections with several different bacteria were also found in the guts of female P. tobbi. Fungi cultured from guts of laboratory reared P. papatasi that had similar mycoses were identified as Aspergillus scierotiorum and Saccharomyces cerevisiae. Fungi-infected laboratory reared P. papatasi were refractory to artificial infections with a Leishmania major strain specific to them. The crop contents of P. papatasi, where sugar meals are stored, demonstrated antibacterial activity against the following bacterial species in culture: Escherichia coli, Staphylococcus aureus, Shigella sonnei, Streptococcus group A and Pseudomonas aeruginosa. It is postulated that the bacteria-free gut normal to sandflies is effected by the bacterial inhibitor present in the crop.

A double clone of Trypanosoma platydactyli Catouillard, 1909, derived from a single trypomastigote from the blood of the Moorish gecko, Tarentola mauritanica, was grown in vitro. Morphogenesis of the parasites led to a stable population of promastigotes which were identical, in general morphology, ultrastructure and the electrophoretic mobility of 8 enzymes, to those cultures previously considered to be Leishmania tarentolae Wenyon, 1921. These cultures included the strain isolated from the type locality in Algeria by Parrot (1949) and later used as a laboratory model for the genus Leishmania. T. platydactyli and L. tarenlolae are synonymized and the present status of saurian Leishmania parasites is discussed.

The Trypanosoma brucei metacyclic population in the salivary glands of the tsetse fly displays a characteristic set of variable antigen types (VATs) which represents only a restricted part of the parasite's total VAT repertoire. After introduction into the mammalian host by fly bite, the metacyclics transform into bloodstream forms which retain expression of the metacyclic VATs. Specific antibodies, both polyvalent and monoclonal, have been used to neutralize separately 4 individual VATs from metacyclic populations. Control experiments and visual observation confirmed lysis of each VAT. On injection of the surviving trypanosomes, after washing, into mice each neutralized VAT was nevertheless expressed within a few days. Simultaneous neutralization of 2 metacyclic VATs which usually switch to one another in bloodstream infections did not prevent expression of either on subsequent injection into mice. Expression of neutralized VATs was not influenced by the antigenic composition of the population originally ingested by the tsetse fly. Metacyclic forms and their immediate successors thus appear to switch rapidly to expression of other metacyclic VATs in bloodstream populations.

A collection of stocks of Trypanosoma brucei rhodesiense isolated in Kenya have been examined for electrophoretic variation in 20 enzymes. The results obtained have been analysed in order to determine whether these trypanosomes are diploid and undergo mating and to determine the genetic distance between T. b. rhodesiense, T. b. brucei and T. b. gambiense. The enzyme electrophoretic markers were further used in experiments involving cyclically transmitted mixtures of stocks aimed at detecting genetic exchange in the laboratory. No genetic exchange was detected. Two novel features of the enzyme electro phoretic results were found. Firstly, the stocks of T. b. rhodesiense were considerably more homogeneous than equivalent collections of stocks of T. b. brucei and secondly, all the stocks examined were heterozygous for two alleles of alkaline phosphatase and showed an excess of heterozygotes at the phosphoglucomutase locus. The degree to which these features are typical of T. b. rhodesiense has been examined in relation to previously published data. The results obtained strongly support the view that T. b. rhodesiense is a set of variants of T. b. brucei rather than a subspecies and a working hypothesis as to the relationship between T. b. brucei and T. b. rhodesiense is proposed to explain the enzyme electrophoretic data obtained.

Non-specific immunostimulants were used in an attempt to protect baboons from infection by schistosomes. Subcutaneous vaccination with cord factor (4·50 mg) and muramyl dipeptide (4·56 mg) 6 days before percutaneous exposure to 3000 Schistosoma haematobium cercariae/baboon (c.p.b.) failed to protect naive baboons: baboons with a 7-month-old, 5000 c.p.b. S. haenzatobium primary infection had developed too strong a natural immunity to detect any protection attributable to vaccination. Subcutaneous vaccination with 0·4 ml of Bacillus Calmette-Guerin (BOG, 1–8 x 108 colony forming units7sol;ml) 4 days before exposure to 1000 c.p.b. S. mansoni gave a significant (38%) reduction in worm load compared with controls. However, vaccination with 0·8 (intramuscular) and 0·2 (intradermal) ml of BOG 11 days before exposure to S. mansoni 800 c.p.b. did not protect naive baboons, nor did it significantly reduce challenge worm recovery from baboons with a 13-week-old, 500 c.p.b. S. mansoni primary infection. Obvious pathology was seen at the site of vaccination in the first but not the second BCG experiment. These results partly support the findings in mice that non-specific macrophage and monocyte activators give partial protection against schistosome infections but they also illustrate that rodents and primates do not necessarily react identically. Hence, findings from rodent models should be extrapolated to man with some caution.

Several approaches were adopted for the disruption and removal of the tegumental surface from protoscoleces of the horse strain of the hydatid organism, Echinococcus granulosus. The effectiveness of each method and the purity of subsequent microthrix-enriched fractions obtained by differential centrifugation were evaluated by electron microscopy, by the amount of protein released and by the degree of enrichment of surface plasma membrane marker enzymes. Incubation in saponin for 10 min produced the purest microtriche preparation, but in low yield; freeze/thawing, incubation in Triton X-100 for 10 mm or in saponin for 20 min produced fractions containing significant amounts of relatively pure microtriches, but mild homogenization was a poor method for surface disruption and subsequent isolation of microtriches. Phosphodiesterase, adenosine triphosphatase (total and ouabain-inhibited), leucine aminopeptidase and glutamyltransferase were active in the protoscoleces but none were enriched in any of the microthrix fractions. In contrast, alkaline phosphatase, acid phosphatase, 5′ nucleotidase and maltase were enriched significantly in all of the isolated microtriche preparations, which suggests that these enzymes are predominantly surface membrane bound. The protein profiles of the microthrix-enriched fractions, following SDS—PAGE, were basically similar, although there were some qualitative and quantitative differences in the proteins released by each isolation procedure. Three major PAS-staining components were present in all the preparations and these probably originated from the glycocalyx. One of these PAS-positive components, with an approximate molecular weight of 110 kDa, may be a glycoprotein specific to the horse strain of E. granulosus.

The fluorescent probe 1-anilino-8-naphthalene suiphonic acid (ANS) was used to investigate the effect of temperature on the physical state of the mitochondrial membranes of adult and larval schistocephalus solzdus together with that of their hosts Gasterosteus aculeatus and Gallus domesticus. Arrhenius plots of ANS/membrane fluorescence for S. solidus plerocercoids was linear over the temperature range 15 to 58 °C, while that for the adult was biphasic with a discontinuity at 39·9 °C. This was interpreted as a physical change which occurred in the adult membrane but not in the plerocercoid membrane and pointed to an alteration in membrane composition during infection. Gasierosteus aculeatus showed a linear Arrhenius plot for membrane fluorescence, irrespective of acclimation temperature. Gallus domesticus showed a discontinuity in the Arrhenius plot for membrane fluorescence at 46·9 °C, outside the normal physiological temperature range.

Restriction endonuclease digestion of genomic DNA generates DNA fragments of unique size, dependent upon the particular base sequence. Following fractionation by agarose gel electrophoresis, repetitive DNA can be visualized as distinct bands in stained gels and the restriction fragment length of such bands used as diagnostic characters. Restriction fragment length differences were detected between species within the genera Trichinella, Caenorhabditis, Romanomermis, Steinernema (syn. Neoaplectana) and Meloidogyne. This technique provides a new tool for the taxonomist, which is independent of phenotypic variation and it enables the rapid and reliable separation of closely related species.

Cytochemical staining with diaminobenzidine (DAB) has been used to locate cytochrome oxidase activity in mitochondria from major tissues of three intestinal parasitic nematodes, Nippostrongylus brasiliensis, Haemonchus contortus and Ascaridia galli. Deposition of DAB reaction product was uniform and relatively intense in mitochondria from all tissues studied in N. brasiliensis. However, mitochondria from different tissues of H. contortus and A. galli varied considerably in their cytochemical staining, decreasing in the order hypodermis < muscle < gut < reproductive tissue. Mitochondria in gut and reproductive tissue of A. galli possessed little or no cytochrome oxidase activity. These results are compatible with respect to oxygen availability and the extent and localization of the mammalian-type aerobic mitochondrial electron transport pathway in intestinal parasitic nematodes.

Food intake, body weight changes, serum protein and amino acid concentrations were measured during the course of primary infections of Nippostrongylus brasiliensis in rats fed on either a 2% protein (casein) diet or a 16% protein diet (Oxoid 41 B). Total serum protein concentration declined from 77·84±5·35 mg/ml in uninfected well-nourished rats to 54±3·29 mg/ml in rats at 9 days post-infection (p.i.). A reduction from 47·80±2·78 to 40·38±5·62 mg/ml had occurred in protein-malnourished rats by day 6 p.i. The hypoproteinaemia was accompanied by significant hypoalbuminaemia in the protein-malnourished rats and concentrations fell from approximately 33 to 19 mg/ml at the time of peak infection. Six days after inoculation, a significant increase was detected in the concentration of serum amino acids in both well-nourished and protein-malnourished rats; the effect was more prolonged for non-essential than for essential amino acids. The results are discussed briefly in relation to recent work on the control of protein metabolism during malnutrition.

The activities of glycolytic and related enzymes and the tricarboxylic acid cycle enzymes were measured in freshly isolated 1st- (Li), 2nd- (L2) and 3rd-stage (L3) larvae of both Ancylostoma tubaeforme and Haemonchus contortus. All enzymes of the glycolytic pathway were present in all developmental stages of both strongylid nematodes although higher levels of activities were obtained in the pre-infective 1st- and 2nd-stage larvae than in the infective 3rd stage. However, the pre-infective larvae contained lower levels of pyruvate kinase (PK) than the infective larvae. Consequently, the pyruvate kinase to phosphoenolpyruvate carboxykinase (PEPCK) ratios were 0·23 and 0·26 for the L1s and L2s for A. tubaeforme and 0·36 and 0·21 for those of H. contortus respectively. High levels of activity of glucose-6-phosphate dehydrogenase obtained in the bacteriophagous pre-infective larvae were consistent with high rates of morphogenesis and substrate synthesis characteristic of the pre-infective stages. All the tricarboxylic acid cycle enzymes were present in the infective larvae of both nematodes while in the pre-infective Li and L2 stages, the enzymes at the beginning of the cycle, namely aconitate hydratase and NAD-linked isocitrate dehydrogenase, were not detected. A scheme was proposed for the energy metabolism of these developing larvae. In this scheme, the pre-infective larvae were shown to operate an anaerobic metabolic pathway involving the carboxylation of phosphoenolpyruvate (PEP) by phosphoenolpyru vate carboxykinase (PEPCK) to form oxaloacetate (OAA), whereas in the infective larvae the metabolic pathway favouring the direct dephosphorylation of PEP, as in vertebrate tissues, was followed.

Spermatogenesis and sperm maturation in Nematospiroides dubius were studied using electron microscopy. The testis is telegonic and germ cells in the zones of mitosis, growth and meiosis are connected by a central anucleate mass of cytoplasm, the rachis. The early part of spermatogenesis is dominated by the synthesis and growth of membrane-bound vesicles called membranous organelles, which originate from RER-associated Golgi bodies. Following meiosis the spermatids separate from the rachis and their chroinatin, which is no longer bounded by a nuclear envelope, condenses into an arrow-head shape and is extruded to form a tail-like structure. After insemination spermatozoa undergo a profound change called activation. The cytoplasmic region which was previously long and cylindrical becomes spherical and the membranous organelles which lined its perimeter fuse with the plasma membrane and become confined to the posterior hemisphere of the sperm, close to the nuclear tail. The anterior half of the sperm is devoid of organelles but contains many filaments organized into clumps and chains; this region being responsible for amoeboid locomotion of the sperm.

Formation of the tegument and alimentary tract in developing Fellodistomum fellis cercariae has been examined by electron microscopy. Morphogenesis of the tegument is a two-stage event: (1) ectodermal cells at the germ ball periphery coalesce beneath a closely apposed and transitory primitive epithelium of sporocyst origin, forming a syncytial boundary of nucleated tegument. The primitive epithelium is then lost and the nuclei in the surface layer of the early tegument degenerate; (2) cells in the mesenchyme of the cercarial embryo differentiate as secretory cells and connect with the anucleate layer to form the true tegument. The foregut develops as an apical invagination of early nucleated tegument, with the primordial digestive cells differentiating behind the oesophagus. When fully formed, the surface anucleate layer of oesophagus tegument extends posteriorly over the apposed surfaces of the developing digestive cells, thus isolating them from each other and from the gut lumen. An apical cavity develops in each of the digestive cells and later communicates with the gut lumen through a pore in the overlying caecal tegument. Lamellated extensions of digestive cell cytoplasm project into the cavity, and development of GER and a Golgi apparatus in the cell mark the onset of digestive secretion.

Over half of all living species of plants and animals are parasitic, which by definition involves intimate association with and unfavourable impact on hosts (Price, 1980). This paper will only consider parasites whose ‘unfavourable impact’ adversely affects the birth and/or mortality rates of their hosts (Anderson, 1978). Most organisms are potential hosts and must deal with the problem of parasitism. The probability of parasitic infection of a host is influenced by both environmental and genetic factors. Traditionally it was assumed that a host was either resistant or susceptible to a particular parasite and therefore the interaction between a parasite and potential host had only two possible outcomes: either the resistant host rebuffed the parasitic attack and remained uninfected or the parasite successfully invaded and significantly reduced the reproductive success of the susceptible host. This approach, however, ignored the intraspecific genetic variation present within both host and parasite populations (Wakelin, 1978). Since the outcome is determined by the interaction of a finite set of host genes and parasite genes, genetic variation in host susceptibility and parasite infectivity (Richards, 1976; Wakelin, 1978) suggests that more than two outcomes are possible. Variation in host and parasite genomes does not begin and end at the susceptibility/infectivity loci. Other genes may also influence the outcome of host–parasite interactions by altering the life-history patterns of hosts and parasites, and lead to a variety of outcomes.