In 1915 Glaser demonstrated for the first time that a nuclear polyhedrosis of a lepidopterous larva (Lymantria dispar), was induced by an invisible filterable virus isolated from the blood of diseased larvae. Nuclear polyhedroses of the larvae of over a 100 insect species are now known, and those of them which have been investigated have been shown to be due to viruses. In these polyhedroses the nuclei of susceptible cells, hypodermis, tracheae, fat and blood cells, etc., enlarge and become packed with infectious crystalline inclusion bodies, the polyhedra, from which the diseases get their name. The pupae and imagos of these insects are not generally susceptible to polyhedral virus diseases, and so far no nuclear polyhedrosis of imaginal tissues in the larvae has been reported. This has led to the suggestion that imaginal tissues are not susceptible to polyhedral virus diseases.

Numerical taxonomy was used to review isoenzyme variation in isolates of Trypanosoma brucei obtained from cattle, tsetse, humans and wildlife from the Lambwe Valley, Kenya. From isoenzyme information alone, it was possible to classify isolates as to source through the use of linear discriminant functions analysis, with an error rate of only 2% in humans, and 14% over all groups. Differentiation was mostly dependent on patterns in the enzymes ASAT, PEPI, and ICD. Parasites from non-human sources, especially tsetse, were characterized by high isoenzyme diversity, and many unique zymodemes. Observed frequencies of genotypes for ICD, ALAT, and ASAT did not agree with expected frequencies based on random mating of a diploid organism. Deviations were particularly large for tsetse isolates, and were mostly due to a deficiency of one homozygote. Cluster analysis revealed complex relationships among isolates, with patterns evolving through time. Major human zymodemes from the 1970s clustered together with most wildlife isolates from East Africa. This chronic human-wildlife transmission cycle was characterized by ASAT pattern I. Other, minor human zymodemes were associated with a human-cattle transmission cycle characterized by ASAT pattern VII. These original chronic transmission cycles appeared to change in 1980 with the appearance of two new zymodemes in humans. These zymodemes involved changes in ALAT and/or PGM to patterns typical of tsetse and cattle isolates. A resultant epidemic was halted with repeated aerial spraying of endosulfan in 1981. Since then, a variety of new zymodemes of unknown human infectivity have appeared. The origins of these changes are discussed in terms of genetic exchange in tsetse, adaptation to human and cattle transmission cycles, and selection resulting from chronic use of insecticides.

This paper provides an overview of research on the nervous system of parasitic platyhelminths. We have emphasized studies concerned with the physiological, pharmacological and biochemical nature of the major small molecule neurotransmitters of these parasites. We have attempted to provide a critical review of the work by focusing on important unresolved issues. Finally, we have focused on some recent work in our laboratory, using patch-clamp recording techniques and quantitative fluorescence cytometry, as an example of newer methods that will hopefully resolve some of the unanswered questions concerning the nervous system of these parasites.

The structure and development of the spermatozoon of Dipetalonema viteae has been studied by means of electron microscopy. Spermatogonia are developed from a syncytium in the terminal region of the reproductive tract. The syncytium grows along the length of the testis as an anucleate rachis, carrying with it the developing germ cells. The gametes become detached from the rachis when they have become secondary spermatocytes. The chromosomes which appear in the primary spermatocytes at the onset of meiosis persist throughout all subsequent stages of development. The nucleus is not reconstructed. Cytophores are produced by the spermatids at the end of the second meiotic division. The spermatid is an elongated cell, but the mature spermatozoon, within the male tract, is amoeboid. There are only minor differences between the sperm found in the male and female tracts. The male gametes contain complex membraneous organelles which are developed from the Golgi bodies and endoplasmic reticulum of the primary spermatocytes. These organelles are suggested to have similar origins and functions to the acrosome of the typical mammalian spermatozoon.

The study of parasite population dynamics has been one of the major developments in ecology over the last 15 years (Kennedy, 1975). The seminal articles of Crofton (1971) and Anderson & May (1978, 1979; May & Anderson, 1978, 1979) began this process by illustrating the potential role of parasites in regulating or destabilizing the dynamics of wildlife host populations. Since then, a variety of empirical and theoretical studies (reviewed by Grenfell & Dobson, 1995) have explored the role of parasites in natural populations. In parallel with these population dynamical developments, a growing interest in the evolutionary ecology of parasites has also led to a large literature, examining the evolutionary impact of parasites and the importance of host-parasite coevolution (Hamilton, 1982; May & Anderson, 1990; Lively & Apanius, 1995; Read et al. 1995; Herre, this volume).

Two recombinant Toxoplasma gondii polypeptides, H4 and H11, were tested as diagnostic antigens in enzyme-linked immunosorbent assays (ELISAs). The results obtained by ELISAs based on single H4 (H4-ELISA), on single H11 (H11-ELISA) and on a mixture of H4 and H11 (H4/H11-ELISA) were compared with results obtained by an ELISA based on traditional ELISA antigen (TEA-ELISA), an indirect fluorescent antibody test (IFAT), the Sabin-Feldman dye test (SFDT) and a direct agglutination test (DAT). A total of 306 cats from a suburban cat population were tested of which about 45% showed serological evidence of T. gondii infection. Infection rates varied from about 32% for cats kept indoors to about 55% for stray cats. Specificities > 99% were observed for all ELISAs based on the recombinant antigens (H4-ELISA, H11-ELISA and H4/H11-ELISA). The H4/H11-ELISA also reached a sensitivity of 95% which compared very favourably with those observed for the TEA-ELISA (98%) and for the IFAT (94%). Negative and positive predictive values for the H4/H11-ELISA were 96 and < 99%, respectively. Antibody titres measured by the H4/H11-ELISA also correlated well with those measured by the SFDT and the DAT. Hence, the H4/H11-ELISA appears to be a very suitable test for sero-epidemiological studies on T. gondii infections in cats.

Brain cysts from three strains of Toxoplasma gondii were purified and analysed by SDS-PAGE and immunoblotting. Western blot analysis revealed the presence of major cyst/bradyzoite antigens at Mr 25000, 36000 and 67000 common to all strains and recognized by many individual sera. There was also evidence of strain variation in cyst antigens with, for example, antigens at Mr 88000, 28000 and 26000 unique to the 17025 strain. There was some overlap in the molecular weight of cyst and tachyzoite antigens, but the major tachyzoite surface antigen (P30) could not be detected in brain cysts.

These studies were aimed at giving a clearer idea of the source of the antigens responsible for the induction of resistance by larval schistosomes. Firstly, the resistance induced by radiation-attenuated (20 krad.) infections was compared with that induced by the larval phase of unirradiated infections. The superiority of the irradiated larvae was demonstrated by infecting mice with 500 female cercariae which were either unirradiated or irradiated, and challenging them 3 weeks later with male cercariae. At perfusion, when primary and challenge worms could be distinguished by sex, significantly greater resistance was demonstrated in the groups vaccinated with irradiated larvae (56 cf. 31% and 48 cf. 26% in two experiments). Secondly, in order to establish how long immunizing larvae need to survive to stimulate optimum resistance, mice were either treated with schistosomulicidal drugs or had the site of infection excised at various intervals post-vaccination. Treatment or excision at up to 24 h did not result in significant resistance to challenge (6–16%). Similar treatments at 1 week did not prevent significant resistance developing (26–37%) but the levels of resistance were significantly lower than those manifest by vaccinated, untreated mice (51–71%). It is concluded that the immunizing larvae need to persist for between 1 and 2 weeks in order to stimulate optimum protection and that antigens released during protracted survival and/or death of the larvae in either the skin or the lungs are probably key factors in the induction of resistance.

Acid proteases hydrolysing haemoglobin, first described by Timms & Bueding (1959), were studied to investigate the interspecific and intraspecific distribution in adult parasitic helminths. Acid protease activity was detected in a variety of adult helminths, irrespective of their habitats, namely, parasitic nematodes, Angiostrongylus cantonensis, Dirofilaria immitis, Trichuris muris and Ascaris suum, a trematode, Paramphistomum sp. and cestodes, Diphyllobothrium erinacei and Hymenolepis nana. Much higher specific activity was observed in the blood nematodes, A. cantonensis and D. immitis than in the gastro-intestinal helminths examined. The intraspecific distribution of the acid protease activity within organs was studied in those nematodes from which the organs could be anatomically separated. The highest specific activity was observed in the intestine of A. cantonensis, D. immitis and A. suum. The activity was also found in other organs of these nematodes, namely, the reproductive organs and the body wall, but to a much lesser extent. The pH optima for the hydrolysis of haemoglobin were found to be in the pH range of 3·1 to 4·6. Each of the enzyme solutions from the 7 species of helminths hydrolysed haemoglobin faster than the other proteins examined but some activity on myoglobin, casein, albumin and other proteins was demonstrated. Pepstatin, a potent inhibitor of carboxyl proteases, inhibited the hydrolysis of haemoglobin by the acid proteases of the 7 species of the parasites. Acid protease activity from A. cantonensis and A. suum was not inhibited in the presence of thiol-, serine- and metallo-protease inhibitors at a concentration of 10−6 M. This fact, together with the inhibitory effect of pepstatin, suggests that the enzyme activity is likely to be due to carboxyl protease(s). Acid proteases such as cathepsin D and pepsin were simultaneously used for comparison of some enzymological characteristics. The substrate specificity of the enzyme solutions from the helminths seemed to resemble that of cathepsin D rather than that of pepsin. The effects of pH and pepstatin on the acid protease activity of the helminths were similar to those on pepsin and cathepsin D.

Ixodid female ticks take one comparatively large bloodmeal which they convert to a single large egg mass and then they die. To examine the outcome of interrupted feeding, equal numbers of male and female Rhipicephalus appendiculatus adult ticks were fed on guinea pigs (host 1) for either 2, 4, or 6 days, or to engorgement (8 days). All of the fully engorged (D8) females laid a single large egg mass (80–160 mg/tick), while 85% of the day 6-fed (D6) female ticks (n=20) each laid a small egg mass (6·1 mg/tick). None of the females that had fed for 2 or 4 days oviposited. Ninety percent (n=20) of the day 2-fed (D2) females survived for 4 weeks after their feeding was interrupted, whereas 65% (n=20) of the day 4-fed (D4) females survived. All of the surviving partially fed female ticks (D2 and D4) attached to a second guinea pig (host 2) and attained engorged body weights that were not significantly different from those of the control females (P<0·05). Female ticks that engorged following interrupted feeding layed egg masses comparable to the controls, indicating that engorgement on host 2 was successful. The salivary gland protein profile of female ticks changed constantly during feeding. However, when feeding was interrupted, the protein expression pattern switched back to that of the non-parasitic state, presumably to enable the partially fed ticks to survive and reattach on the new host. This observation indicates that female ixodid ticks have a natural ability to survive and re-establish successful feeding on a new host if the first attempt at feeding is unsuccessful. Such an interrupted feeding mechanism supports the hypothesis that partially engorged ticks may play a role in tick-borne pathogen transmission.

The acanthocephalan parasite Acanthocephalus dirus induces a colour change in the intermediate host, the aquatic isopod Caecidotea intermedius, which increases transmission to definitive hosts (creek chub, sunfish). We examined the potential for conflict to occur between infective (cystacanth) and non-infective (acanthor, acanthella) stages of A. dirus over the level of colour modification that should be induced when these stages share a host. Using a field survey, we showed that host sharing by infective and non-infective stages was relatively common and that infective and non-infective stages differed in their effects on colour modification. Non-infective stages induced a colour change over 40% of the body, whereas infective stages induced a colour change over 80%. Thus, conflict could occur between stages over the level of modification that should be induced. We then showed that mixed-stage infections induced a colour change in the host that was consistent with the level of modification induced by the infective stage. We discuss the potential significance of these results to patterns of host modification and their effects on stage-related survival in nature.

Infection of the snail, Biomphalaria glabrata, by the free-swimming miracidial stage of the human blood fluke, Schistosoma mansoni, and its subsequent development to the parasitic sporocyst stage is critical to establishment of viable infections and continued human transmission. We performed a genome-wide expression analysis of the S. mansoni miracidia and developing sporocyst using Long Serial Analysis of Gene Expression (LongSAGE). Five cDNA libraries were constructed from miracidia and in vitro cultured 6- and 20-day-old sporocysts maintained in sporocyst medium (SM) or in SM conditioned by previous cultivation with cells of the B. glabrata embryonic (Bge) cell line. We generated 21 440 SAGE tags and mapped 13 381 to the S. mansoni gene predictions (v4.0e) either by estimating theoretical 3′ UTR lengths or using existing 3′ EST sequence data. Overall, 432 transcripts were found to be differentially expressed amongst all 5 libraries. In total, 172 tags were differentially expressed between miracidia and 6-day conditioned sporocysts and 152 were differentially expressed between miracidia and 6-day unconditioned sporocysts. In addition, 53 and 45 tags, respectively, were differentially expressed in 6-day and 20-day cultured sporocysts, due to the effects of exposure to Bge cell-conditioned medium.

This paper describes the structure of the microfilarial reservoir of Loa loa in an endemic population of central Cameroon. The possible effects of age and sex on the prevalence and intensity of microfilaraemia have been explored. Logistic analysis showed that the prevalence of microfilaraemia increased significantly with age, reaching 60% in the oldest males. This result suggests that the figure commonly reported, according to which only one third of the infected individuals were microfilaraemic, should be reconsidered; in addition, as part of surveys of loiasis, crude microfilaraemia prevalence values should be replaced by adjusted ones. The intensity of infection did not show any age-specific change. As a result, even if the oldest members of the male population are clearly the most at risk of developing post-ivermectin serious adverse reactions, especially Loa-encephalopathy, the other members of the population are not risk-free. Therefore, in those areas where the African Programme for Onchocerciasis Control is undertaking regular mass distributions of ivermectin for onchocerciasis control, and where loiasis is co-endemic, no subpopulation should be excluded from surveillance and monitoring during community directed treatments with ivermectin.

In sub-Saharan Africa, over 22 million people are estimated to be co-infected with both helminths and HIV-1. Several studies have suggested that de-worming individuals with HIV-1 may delay HIV-1 disease progression, and that the benefit of de-worming may vary by individual helminth species. We conducted a systematic review and meta-analysis of the published literature to determine the effect of treatment of individual helminth infections on markers of HIV-1 progression (CD4 count and HIV viral load). There was a trend towards an association between treatment for Schistosoma mansoni and a decrease in HIV viral load (Weighted mean difference (WMD)=−0·10; 95% Confidence interval (CI): −0·24, 0·03), although this association was not seen for Ascaris lumbricoides, hookworm or Trichuris trichiura. Treatment of A. lumbricoides, S. mansoni, hookworm or T. trichiura was not associated with a change in CD4 count. While pooled data from randomized trials suggested clinical benefit of de-worming for individual helminth species, these effects decreased when observational data were included in the pooled analysis. While further trials are needed to confirm the role of anthelmintic treatment in HIV-1 co-infected individuals, providing anthelmintics to individuals with HIV-1 may be a safe, inexpensive and practical intervention to slow progression of HIV-1.

Utilization of chemical pesticide to control monogenean diseases is often restricted in many countries due to the development of pesticide resistance and concerns of chemical residues and environmental contamination. Thus, the use of antiparasitic agents from plants has been explored as a possible way for controlling monogenean infections. Extracts from Cinnamomum cassia were investigated under in vivo conditions against Dactylogyrus intermedius in goldfish. The two bioactive compounds, cinnamaldehyde and cinnamic acid, were identified using nuclear magnetic resonance and electrospray ionization mass spectrometry. The 48 h median effective concentrations (EC50) for these compounds against D. intermedius were 0·57 and 6·32 mg L−1, respectively. The LD50 of cinnamaldehyde and cinnamic acid were 13·34 and 59·66 mg L−1 to goldfish in 48 h acute toxicity tests, respectively. These data confirm that cinnamaldehyde is effective against D. intermedius, and the cinnamaldehyde exhibits potential for the development of a candidate antiparasitic agent.

Epidemiological rate parameters of host generalist parasites are difficult to estimate, especially in cases where variation in parasite performance can be attributed to host species. Such cases are likely common for generalist parasites of sympatric grazing mammals. In this study, we combined data from experimental exposures in cattle and sheep and natural infections in elk to compare the recruitment, morphology and reproduction of adult Dicrocoelium dendriticum, a generalist trematode that has emerged in sympatric grazing hosts in Cypress Hills Provincial Park, Alberta. Overall, there were no significant differences in the recruitment of metacercariae and in the pre-patency period of adults in experimentally exposed cattle and sheep. All flukes reached reproductive maturity and the degree of reproductive inequality between individual flukes within each infrapopulation was moderate and approximately equal among the three host species. Neither fluke size nor per capita fecundity was constrained by density dependence. Thus, fitness parameters associated with growth and reproduction were approximately equivalent among at least three species of definitive host, two of which are sympatric on pastures in this Park. The generalist life-history strategy of this trematode, which is known to extend to other stages of its life cycle, has likely contributed to its invasion history outside its native range in Europe.

Detection of Onchocerca volvulus larvae in vector populations is of prime importance in the assessment of the effectiveness of onchocerciasis control programmes. Traditionally, detection of larvae is attained by the dissection of flies, but this time-consuming method cannot easily discriminate between species of Onchocerca. The genome of all Onchocerca species has a unique 150 bp repeat, which can be amplified by PCR, and O. volvulus-specific DNA probes can detect these products by Southern blot (SB). This study optimizes a PCR/SB assay, and compares it with fly dissection to estimate the prevalence (p) and intensity of infection (m) in the local vector population of a Mexican community that has become hypoendemic as a result of 7 years of treatment with ivermectin and nodulectomy. The PCR detected 1 infected fly in a pool of 99 uninfected flies, but the optimal pool size was 50 flies. At the community level, 1 out of 10550 flies was positive (p=0·0095%, 95% confidence intervals CI=0·00024–07·05280%; m=0·00027 larvae/parous fly, CI=−0·00026–0·00081) by PCR, and 4 out of 10772 flies (p=0·0371%, CI=0·01012–0.09505%; m=0·00107 larvae/parous fly, 95% CI=0·00002–0·00212) by dissection (observed m=0·0005). Both methods produce statistically similar estimates of the prevalence and intensity, indicating that pool screening is a viable alternative for entomological surveillance in areas where the intensity of transmission is becoming extremely low as a result of control interventions.

A cDNA library derived from mRNA of tachyzoites of Neospora caninum (NC-Liverpool strain) was screened with antisera from a cow naturally infected with N. caninum. The DNA sequence of 1 recombinant isolated predicted a significant protein sequence homology of the gene product to the 28 kDa (GRA2) antigen of Toxoplasma gondii. Studies on the N. caninum gene coding for this antigen demonstrated the presence of a single intron flanked by 2 exons; the gene was also highly expressed in culture-derived tachyzoites. The antigen was expressed in Escherichia coli; when injected into mice it stimulated the production of antibodies which detected a 29 kDa antigen of N. caninum. Secondary structure predictions made for the N. caninum protein showed support for several amphipathic helices separated by loops and turns. The available evidence indicates maintenance of protein secondary structure, and not DNA or amino acid sequence, has occurred during the evolution of GRA2 proteins in N. caninum and T. gondii.

Parasitic nematodes are important and abundant parasites adapted to live a parasitic lifestyle, with these adaptations all aimed at facilitating their survival and reproduction in their hosts. The recently sequenced genomes of four Strongyloides species, gastrointestinal parasites of humans and other animals, alongside transcriptomic and proteomic analysis of free-living and parasitic stages of their life cycles have revealed a number of protein families with a putative role in their parasitism. Many of these protein families have also been associated with parasitism in other parasitic nematode species, suggesting that these proteins may play a fundamental role in nematode parasitism more generally. Here, we review key protein families that have a putative role in Strongyloides’ parasitism – acetylcholinesterases, astacins, aspartic proteases, prolyl oligopeptidases, proteinase inhibitors (trypsin inhibitors and cystatins), SCP/TAPS and transthyretin-like proteins – and the evidence for their key, yet diverse, roles in the parasitic lifestyle.

In recent years, the isolation of parasitic DNA from faecal samples and PCR techniques, have been improved and simplified. Moreover, the introduction of real-time PCR has made it possible to multiplex different targets into one reaction. These new technical possibilities make it feasible to introduce PCR with its unsurpassed sensitivity and specificity in a routine laboratory setting for the diagnosis of intestinal parasites. Detection rates of the parasitic infections included in the PCR are increased significantly compared with microscopy. Molecular diagnostics, especially in children, reveal a possible cause of the gastrointestinal complaints in many more cases compared with conventional methods. Usually in GP patients no other pathogenic parasites are detected using microscopy and in the returning travellers additional parasites are found with microscopy in a minority of cases only. Multiplex real-time PCR offers a highly sensitive and specific diagnostic alternative for labour intensive microscopy in clinical laboratory practice. Additional diagnostic methods for the detection of parasitic infections that are not included as PCR target can be limited to a selected group of patients.