Response to carbon dioxide by the infective larvae of three species of parasitic nematodes

Impact of water exhaled out by visitors in show caves: a case study from the Moravian Karst (Czech Republic)

The anthropogenic impact of the water and CO2 exhaled by visitors was studied in the show caves of the Moravian Karst (Czech Republic), especially in the Balcarka and Výpustek Caves. Two alternative models based on (1) the known/presumed composition of the breathed air and physical activity of visitors and (2) the detailed monitoring microclimatic data were proposed. The CO2 fluxes of 2.4 × 10-4 and (2.0-3.9) × 10-4 mol person-1 s-1 and the water vapor fluxes of (3.2-8.9) × 10-3 and (0.6-1.2) × 10-2 g person-1 s-1 were found for a slightly increased physical load. The total attendance and cave tour duration were the main driving factors. For the available data on attendance and accessibility periods, the total mass of water vapor exhaled by visitors in all show caves in the Moravian Karst was estimated between 9.6 × 106 and 4.3 × 108 g with significant seasonality. According to the geochemical model, this mass of water is capable of dissolving 1280 to 59,038 g of calcite, assuming a mean winter and summer CO2 concentration in the cave air of 1000 and 3000 ppmv. The larger extent of water condensation can lead to the so-called condensation corrosion, whereas the lower extent of condensation probably causes a recrystallization of calcite on the surface of speleothems and rocks.

Using newly optimized genetic tools to probe Strongyloides sensory behaviors

The oft-neglected human-parasitic threadworm, Strongyloides stercoralis, infects roughly eight percent of the global population, placing disproportionate medical and economic burden upon marginalized communities. While current chemotherapies treat strongyloidiasis, disease recrudescence and the looming threat of anthelminthic resistance necessitate novel strategies for nematode control. Throughout its life cycle, S. stercoralis relies upon sensory cues to aid in environmental navigation and coordinate developmental progression. Odorants, tastants, gases, and temperature have been shown to shape parasite behaviors that drive host seeking and infectivity; however, many of these sensory behaviors remain poorly understood, and their underlying molecular and neural mechanisms are largely uncharacterized. Disruption of sensory circuits essential to parasitism presents a promising strategy for future interventions. In this review, we describe our current understanding of sensory behaviors - namely olfactory, gustatory, gas sensing, and thermosensory behaviors - in Strongyloides spp. We also highlight the ever-growing cache of genetic tools optimized for use in Strongyloides that have facilitated these findings, including transgenesis, CRISPR/Cas9-mediated mutagenesis, RNAi, chemogenetic neuronal silencing, and the use of fluorescent biosensors to measure neuronal activity. Bolstered by these tools, we are poised to enter an era of rapid discovery in Strongyloides sensory neurobiology, which has the potential to shape pioneering advances in the prevention and treatment of strongyloidiasis.

A Review of the Impact of Climate Change on the Epidemiology of Gastrointestinal Nematode Infections in Small Ruminants and Wildlife in Tropical Conditions

Climate change is causing detrimental changes in living organisms, including pathogens. This review aimed to determine how climate change has impacted livestock system management, and consequently, what factors influenced the gastrointestinal nematodes epidemiology in small ruminants under tropical conditions. The latter is orientated to find out the possible solutions responding to climate change adverse effects. Climate factors that affect the patterns of transmission of gastrointestinal parasites of domesticated ruminants are reviewed. Climate change has modified the behavior of several animal species, including parasites. For this reason, new control methods are required for controlling parasitic infections in livestock animals. After a pertinent literature analysis, conclusions and perspectives of control are given.

REVIEW OF THE DAUER HYPOTHESIS: WHAT NON-PARASITIC SPECIES CAN TELL US ABOUT THE EVOLUTION OF PARASITISM

Parasitic lineages have acquired suites of new traits compared to their nearest free-living relatives. When and why did these traits arise? We can envision lineages evolving through multiple stable intermediate steps such as a series of increasingly exploitative species interactions. This view allows us to use non-parasitic species that approximate those intermediate steps to uncover the timing and original function of parasitic traits, knowledge critical to understanding the evolution of parasitism. The dauer hypothesis proposes that free-living nematode lineages evolved into parasites through two intermediate steps, phoresy and necromeny. Here we delve into the proposed steps of the dauer hypothesis by collecting and organizing data from genetic, behavioral, and ecological studies in a range of nematode species. We argue that hypotheses on the evolution of parasites will be strengthened by complementing comparative genomic studies with ecological studies on non-parasites that approximate intermediate steps.

The role of carbon dioxide in nematode behaviour and physiology

Carbon dioxide (CO2) is an important sensory cue for many animals, including both parasitic and free-living nematodes. Many nematodes show context-dependent, experience-dependent and/or life-stage-dependent behavioural responses to CO2, suggesting that CO2 plays crucial roles throughout the nematode life cycle in multiple ethological contexts. Nematodes also show a wide range of physiological responses to CO2. Here, we review the diverse responses of parasitic and free-living nematodes to CO2. We also discuss the molecular, cellular and neural circuit mechanisms that mediate CO2 detection in nematodes, and that drive context-dependent and experience-dependent responses of nematodes to CO2.

Terror in the dirt: Sensory determinants of host seeking in soil-transmitted mammalian-parasitic nematodes

Infection with gastrointestinal parasitic nematodes is a major cause of chronic morbidity and economic burden around the world, particularly in low-resource settings. Some parasitic nematode species, including the human-parasitic threadworm Strongyloides stercoralis and human-parasitic hookworms in the genera Ancylostoma and Necator, feature a soil-dwelling infective larval stage that seeks out hosts for infection using a variety of host-emitted sensory cues. Here, we review our current understanding of the behavioral responses of soil-dwelling infective larvae to host-emitted sensory cues, and the molecular and cellular mechanisms that mediate these responses. We also discuss the development of methods for transgenesis and CRISPR/Cas9-mediated targeted mutagenesis in Strongyloides stercoralis and the closely related rat parasite Strongyloides ratti. These methods have established S. stercoralis and S. ratti as genetic model systems for gastrointestinal parasitic nematodes and are enabling more detailed investigations into the neural mechanisms that underlie the sensory-driven behaviors of this medically and economically important class of parasites.

Signaling in Parasitic Nematodes: Physicochemical Communication Between Host and Parasite and Endogenous Molecular Transduction Pathways Governing Worm Development and Survival

Signaling or communication between host and parasite may occur over relatively long ranges to enable host finding and acquisition by infective parasitic nematode larvae. Innate behaviors in infective larvae transmitted from the soil that enhance the likelihood of host contact, such as negative geotaxis and hypermotility, are likely mediated by mechanoreception and neuromuscular signaling. Host cues such as vibration of the substratum, elevated temperature, exhaled CO2, and other volatile odorants are perceived by mechanosensory and chemosensory neurons of the amphidial complex. Beyond this, the molecular systems that transduce these external cues within the worm are unknown at this time. Overall, the signal transduction mechanisms that regulate switching between dauer and continuous reproductive development in Caenorhabditis elegans, and doubtless other free-living nematodes, have provided a useful framework for testing hypotheses about how the morphogenesis and development of infective parasitic nematode larvae and the lifespan of adult parasites are regulated. In C. elegans, four major signal transduction pathways, G protein-coupled receptor signaling, insulin/insulin-like growth factor signaling, TGFβ-like signaling and steroid-nuclear hormone receptor signaling govern the switch between dauer and continuous development and regulate adult lifespan. Parasitic nematodes appear to have conserved the functions of G-protein-coupled signaling, insulin-like signaling and steroid-nuclear hormone receptor signaling to regulate larval development before and during the infective process. By contrast, TGFβ-like signaling appears to have been adapted for some other function, perhaps modulation of the host immune response. Of the three signal transduction pathways that appear to regulate development in parasitic nematodes, steroid-nuclear hormone signaling is the most straightforward to manipulate with administered small molecules and may form the basis of new chemotherapeutic strategies. Signaling between parasites and their hosts' immune systems also occurs and serves to modulate these responses to allow chronic infection and down regulate acute inflammatory responses. Knowledge of the precise nature of this signaling may form the basis of immunological interventions to protect against parasitism or related lesions and to alleviate inflammatory diseases of various etiologies.

Mechanisms of host seeking by parasitic nematodes

The phylum Nematoda comprises a diverse group of roundworms that includes parasites of vertebrates, invertebrates, and plants. Human-parasitic nematodes infect more than one billion people worldwide and cause some of the most common neglected tropical diseases, particularly in low-resource countries [1]. Parasitic nematodes of livestock and crops result in billions of dollars in losses each year [1]. Many nematode infections are treatable with low-cost anthelmintic drugs, but repeated infections are common in endemic areas and drug resistance is a growing concern with increasing therapeutic and agricultural administration [1]. Many parasitic nematodes have an environmental infective larval stage that engages in host seeking, a process whereby the infective larvae use sensory cues to search for hosts. Host seeking is a complex behavior that involves multiple sensory modalities, including olfaction, gustation, thermosensation, and humidity sensation. As the initial step of the parasite-host interaction, host seeking could be a powerful target for preventative intervention. However, host-seeking behavior remains poorly understood. Here we review what is currently known about the host-seeking behaviors of different parasitic nematodes, including insect-parasitic nematodes, mammalian-parasitic nematodes, and plant-parasitic nematodes. We also discuss the neural bases of these behaviors.

Regulation of Life Cycle Checkpoints and Developmental Activation of Infective Larvae in Strongyloides stercoralis by Dafachronic Acid

The complex life cycle of the parasitic nematode Strongyloides stercoralis leads to either developmental arrest of infectious third-stage larvae (iL3) or growth to reproductive adults. In the free-living nematode Caenorhabditis elegans, analogous determination between dauer arrest and reproductive growth is governed by dafachronic acids (DAs), a class of steroid hormones that are ligands for the nuclear hormone receptor DAF-12. Biosynthesis of DAs requires the cytochrome P450 (CYP) DAF-9. We tested the hypothesis that DAs also regulate S. stercoralis development via DAF-12 signaling at three points. First, we found that 1 μM Δ7-DA stimulated 100% of post-parasitic first-stage larvae (L1s) to develop to free-living adults instead of iL3 at 37°C, while 69.4±12.0% (SD) of post-parasitic L1s developed to iL3 in controls. Second, we found that 1 μM Δ7-DA prevented post-free-living iL3 arrest and stimulated 85.2±16.9% of larvae to develop to free-living rhabditiform third- and fourth-stages, compared to 0% in the control. This induction required 24-48 hours of Δ7-DA exposure. Third, we found that the CYP inhibitor ketoconazole prevented iL3 feeding in host-like conditions, with only 5.6±2.9% of iL3 feeding in 40 μM ketoconazole, compared to 98.8±0.4% in the positive control. This inhibition was partially rescued by Δ7-DA, with 71.2±16.4% of iL3 feeding in 400 nM Δ7-DA and 35 μM ketoconazole, providing the first evidence of endogenous DA production in S. stercoralis. We then characterized the 26 CYP-encoding genes in S. stercoralis and identified a homolog with sequence and developmental regulation similar to DAF-9. Overall, these data demonstrate that DAF-12 signaling regulates S. stercoralis development, showing that in the post-parasitic generation, loss of DAF-12 signaling favors iL3 arrest, while increased DAF-12 signaling favors reproductive development; that in the post-free-living generation, absence of DAF-12 signaling is crucial for iL3 arrest; and that endogenous DA production regulates iL3 activation.

cGMP and NHR signaling co-regulate expression of insulin-like peptides and developmental activation of infective larvae in Strongyloides stercoralis

The infectious form of the parasitic nematode Strongyloides stercoralis is a developmentally arrested third-stage larva (L3i), which is morphologically similar to the developmentally arrested dauer larva in the free-living nematode Caenorhabditis elegans. We hypothesize that the molecular pathways regulating C. elegans dauer development also control L3i arrest and activation in S. stercoralis. This study aimed to determine the factors that regulate L3i activation, with a focus on G protein-coupled receptor-mediated regulation of cyclic guanosine monophosphate (cGMP) pathway signaling, including its modulation of the insulin/IGF-1-like signaling (IIS) pathway. We found that application of the membrane-permeable cGMP analog 8-bromo-cGMP potently activated development of S. stercoralis L3i, as measured by resumption of feeding, with 85.1 ± 2.2% of L3i feeding in 200 µM 8-bromo-cGMP in comparison to 0.6 ± 0.3% in the buffer diluent. Utilizing RNAseq, we examined L3i stimulated with DMEM, 8-bromo-cGMP, or the DAF-12 nuclear hormone receptor (NHR) ligand Δ7-dafachronic acid (DA)--a signaling pathway downstream of IIS in C. elegans. L3i stimulated with 8-bromo-cGMP up-regulated transcripts of the putative agonistic insulin-like peptide (ILP) -encoding genes Ss-ilp-1 (20-fold) and Ss-ilp-6 (11-fold) in comparison to controls without stimulation. Surprisingly, we found that Δ7-DA similarly modulated transcript levels of ILP-encoding genes. Using the phosphatidylinositol-4,5-bisphosphate 3-kinase inhibitor LY294002, we demonstrated that 400 nM Δ7-DA-mediated activation (93.3 ± 1.1% L3i feeding) can be blocked using this IIS inhibitor at 100 µM (7.6 ± 1.6% L3i feeding). To determine the tissues where promoters of ILP-encoding genes are active, we expressed promoter::egfp reporter constructs in transgenic S. stercoralis post-free-living larvae. Ss-ilp-1 and Ss-ilp-6 promoters are active in the hypodermis and neurons and the Ss-ilp-7 promoter is active in the intestine and a pair of head neurons. Together, these data provide evidence that cGMP and DAF-12 NHR signaling converge on IIS to regulate S. stercoralis L3i activation.

Gas sensing in nematodes

Nearly all animals are capable of sensing changes in environmental oxygen (O2) and carbon dioxide (CO2) levels, which can signal the presence of food, pathogens, conspecifics, predators, or hosts. The free-living nematode Caenorhabditis elegans is a powerful model system for the study of gas sensing. C. elegans detects changes in O2 and CO2 levels and integrates information about ambient gas levels with other internal and external cues to generate context-appropriate behavioral responses. Due to its small nervous system and amenability to genetic and genomic analyses, the functional properties of its gas-sensing microcircuits can be dissected with single-cell resolution, and signaling molecules and natural genetic variations that modulate gas responses can be identified. Here, we discuss the neural basis of gas sensing in C. elegans, and highlight changes in gas-evoked behaviors in the context of other sensory cues and natural genetic variations. We also discuss gas sensing in other free-living nematodes and parasitic nematodes, focusing on how gas-sensing behavior has evolved to mediate species-specific behavioral requirements.

Should I stay or should I go?

Most animals inhabit environments in which resources are heterogeneous and distributed in patches. A fundamental question in behavioral ecology is how an animal feeding on a particular food patch, and hence depleting it, decides when it is optimal to leave the patch in search of a richer one. Optimal foraging has been extensively studied and modeled in animals not amenable to molecular and neuronal manipulation. Recently, however, we and others have begun to elucidate at a mechanistic level how food patch leaving decisions are made. We found that C. elegans leaves food with increasing probability as food patches become depleted. Therefore, despite its artificial laboratory environment, its behavior conforms to the optimal foraging theory, which allowed us to genetically dissect the behavior. Here we expand our discussion on some of these findings, in particular how metabolism, oxygen and carbon dioxide regulate C. elegans food leaving behavior.

Chemosensory behaviors of parasites

Many multicellular parasites seek out hosts by following trails of host-emitted chemicals. Host seeking is a characteristic of endoparasites such as parasitic worms as well as of ectoparasites such as mosquitoes and ticks. For host location, many of these parasites use CO(2), a respiration byproduct, in combination with host-specific chemicals. Recent work has begun to elucidate the behavioral responses of parasites to CO(2) and other host chemicals, and to unravel the mechanisms of these responses. Here we discuss recent findings that have greatly advanced our understanding of the chemosensory behaviors of host-seeking parasites. We focus primarily on well-studied parasites such as nematodes and insects, but also note broadly relevant findings in a few less well studied parasites.

Olfaction shapes host-parasite interactions in parasitic nematodes

Many parasitic nematodes actively seek out hosts in which to complete their lifecycles. Olfaction is thought to play an important role in the host-seeking process, with parasites following a chemical trail toward host-associated odors. However, little is known about the olfactory cues that attract parasitic nematodes to hosts or the behavioral responses these cues elicit. Moreover, what little is known focuses on easily obtainable laboratory hosts rather than on natural or other ecologically relevant hosts. Here we investigate the olfactory responses of six diverse species of entomopathogenic nematodes (EPNs) to seven ecologically relevant potential invertebrate hosts, including one known natural host and other potential hosts collected from the environment. We show that EPNs respond differentially to the odor blends emitted by live potential hosts as well as to individual host-derived odorants. In addition, we show that EPNs use the universal host cue CO(2) as well as host-specific odorants for host location, but the relative importance of CO(2) versus host-specific odorants varies for different parasite-host combinations and for different host-seeking behaviors. We also identified host-derived odorants by gas chromatography-mass spectrometry and found that many of these odorants stimulate host-seeking behaviors in a species-specific manner. Taken together, our results demonstrate that parasitic nematodes have evolved specialized olfactory systems that likely contribute to appropriate host selection.

A sensory code for host seeking in parasitic nematodes

Parasitic nematode species often display highly specialized host-seeking behaviors that reflect their specific host preferences. Many such behaviors are triggered by host odors, but little is known about either the specific olfactory cues that trigger these behaviors or the underlying neural circuits. Heterorhabditis bacteriophora and Steinernema carpocapsae are phylogenetically distant insect-parasitic nematodes whose host-seeking and host-invasion behavior resembles that of some devastating human- and plant-parasitic nematodes. We compare the olfactory responses of Heterorhabditis and Steinernema infective juveniles (IJs) to those of Caenorhabditis elegans dauers, which are analogous life stages. The broad host range of these parasites results from their ability to respond to the universally produced signal carbon dioxide (CO(2)), as well as a wide array of odors, including host-specific odors that we identified using thermal desorption-gas chromatography-mass spectroscopy. We find that CO(2) is attractive for the parasitic IJs and C. elegans dauers despite being repulsive for C. elegans adults, and we identify a sensory neuron that mediates CO(2) response in both parasitic and free-living species, regardless of whether CO(2) is attractive or repulsive. The parasites' odor response profiles are more similar to each other than to that of C. elegans despite their greater phylogenetic distance, likely reflecting evolutionary convergence to insect parasitism.

Larval migration in PERL chambers as an in vitro model for percutaneous infection stimulates feeding in the canine hookworm Ancylostoma caninum

Background

Ancylostoma caninum third-stage larvae are the non-feeding infective stage of this parasite and are able to infect potential hosts via different infection routes. Since percutaneous infection is one of the most important routes and skin penetration is the first step into parasitic life, an existing in vitro model for percutaneous migration was modified and evaluated. The main parameter used to evaluate migration was the migration ratio (migrated larvae as a percentage of total number of larvae recovered). Additionally, the skin lag was calculated, expressing the percentage of larvae remaining in the skin and therefore not being recovered. Since initiation of feeding is proposed to be an important step in the transition from free-living to parasitic A. caninum larvae, feeding assays were performed with in vitro percutaneously migrated larvae. Additionally, infective larvae of A. caninum were activated via serum-stimulation and feeding behaviour was analysed and compared between percutaneously migrated and serum-stimulated larvae.

Results

Maximum skin migration levels of infective larvae were observed at temperatures above 32°C when larvae were placed on the epidermal side of skin for more than 12 hours. The medium beneath the skin had no effect on migration ratio, and no significant difference between the migration ratios through fresh and frozen/thawed skin was observed.

Maximum feeding levels of 93.2% were observed for percutaneously migrated larvae after 48 h incubation, whereas serum-stimulated larvae reached the maximum of 91.0% feeding larvae after 24 h.

Conclusions

The PERL chamber system was optimised and standardised as an in vitro model for percutaneous migration. The larvae recovered after percutaneous migration showed characteristic signs of activation similar to that of serum-stimulated larvae. The observed difference in time course of resumption of feeding indicates that percutaneously migrated larvae are not identical to serum-stimulated larvae, which are currently representing the model for early parasitic stages.

Assessment of skin penetration of third-stage larvae of Strongyloides ratti

Strongyloides stercoralis infection is caused by skin penetration of third-stage larvae (L3s). We studied skin penetration of L3s of Strongyloides ratti using an in vitro assay that has been used previously to study Angiostrongylus cantonensis, an agarose membrane with a temperature gradient, and scanning electron microscopy. Our results revealed that skin penetration of L3s depended on host skin temperature. When the target temperature of the outer liquid was 37°C, more than 80% of L3s penetrated the skin, but penetration was only 60% when the target temperature was 20°C. Thirdstage larvae moved rapidly on the agarose membrane toward optimum temperature area for this parasite, which indicates that L3 has a sensor that is sensitive to temperature changes. Penetration rate for hosts such as cat (36%), dog (32%), and bird (13%) were significantly lower than that for rat (82%). Although we could not establish the reason, L3s seemed to have an ability to differentiate these hosts at the time of penetration. By using scanning electron microscopy, penetration of L3s could be observed within 10 min. We demonstrated thermotaxis of L3 of S. ratti, and this peculiar characteristic seemed to have a close relationship with the process of searching for the host.

Differences in transcription between free-living and CO2-activated third-stage larvae of Haemonchus contortus

BACKGROUND

The disease caused by Haemonchus contortus, a blood-feeding nematode of small ruminants, is of major economic importance worldwide. The infective third-stage larva (L3) of this gastric nematode is enclosed in a cuticle (sheath) and, once ingested with herbage by the host, undergoes an exsheathment process that marks the transition from the free-living (L3) to the parasitic (xL3) stage. This study explored changes in gene transcription associated with this transition and predicted, based on comparative analysis, functional roles for key transcripts in the metabolic pathways linked to larval development.

RESULTS

Totals of 101,305 (L3) and 105,553 (xL3) expressed sequence tags (ESTs) were determined using 454 sequencing technology, and then assembled and annotated; the most abundant transcripts encoded transthyretin-like, calcium-binding EF-hand, NAD(P)-binding and nucleotide-binding proteins as well as homologues of Ancylostoma-secreted proteins (ASPs). Using an in silico-subtractive analysis, 560 and 685 sequences were shown to be uniquely represented in the L3 and xL3 stages, respectively; the transcripts encoded ribosomal proteins, collagens and elongation factors (in L3), and mainly peptidases and other enzymes of amino acid catabolism (in xL3). Caenorhabditis elegans orthologues of transcripts that were uniquely transcribed in each L3 and xL3 were predicted to interact with a total of 535 other genes, all of which were involved in embryonic development.

CONCLUSION

The present study indicated that some key transcriptional alterations taking place during the transition from the L3 to the xL3 stage of H. contortus involve genes predicted to be linked to the development of neuronal tissue (L3 and xL3), formation of the cuticle (L3) and digestion of host haemoglobin (xL3). Future efforts using next-generation sequencing and bioinformatic technologies should provide the efficiency and depth of coverage required for the determination of the complete transcriptomes of different developmental stages and/or tissues of H. contortus as well as the genome of this important parasitic nematode. Such advances should lead to a significantly improved understanding of the molecular biology of H. contortus and, from an applied perspective, to novel methods of intervention.

Urocanic acid is a major chemoattractant for the skin-penetrating parasitic nematode Strongyloides stercoralis

Host-seeking behavior by parasitic nematodes relies heavily on chemical cues emanating from potential hosts. Nonspecific cues for Strongyloides stercoralis, a nematode that infects humans and a few other mammals, include carbon dioxide and sodium chloride; however, the characteristic species specificity of this parasite suggested the existence of other, more specific cues. Here we show that the infective larva of S. stercoralis is strongly attracted to an extract of mammalian skin and that the active component in this skin extract is urocanic acid. Urocanic acid, a histidine metabolite, is particularly abundant in mammalian skin and skin secretions, suggesting that it serves as an attractant specific to mammalian hosts. The attractant activity of urocanic acid is suppressed by divalent metal ions, suggesting a possible strategy for preventing infection.

Anthropogenic CO2-flux into cave atmosphere and its environmental impact: A case study in the Císarská Cave (Moravian Karst, Czech Republic)

The evolution of CO2 levels was studied in the ventilated and unventilated Nagel Dome chamber (the Císarská Cave) with- and without human presence. Based on a simplified dynamic model and CO2/Rn data (222Rn considered as a conservative tracer), two types of CO2-fluxes into the chamber were distinguished: (1) the natural input of (2-4) x 10(-6) m3 s(-1), corresponding to a flux of (8.5-17) x 10(-10) m3 m(-2) s(-1) and (2) an anthropogenic input of (0.6-2.5) x 10(-4) m3 s(-1), corresponding to an average partial flux of (4.8-7.7) x 10(-6) m3 s(-1) person(-1). The chamber ventilation rates were calculated in the range from 0.033 to 0.155 h(-1). Comparison of the chamber CO2-levels with chamber dripwater chemistry indicates that the peak CO2-concentrations during stay of persons (log p(CO2) approximately -2.97, -2.89, and -2.83) do not reach the theoretical values at which dripwater carbonate species and air CO2 are at equilibrium (log p(CO2[DW]) approximately -2.76 to -2.79). This means that CO2-degassing of the dripwaters will continue, increasing supersaturation with respect to calcite (dripwater saturation index defined as SI(calcite) = a(Ca2+)a(CO3(2-))/10(-8.4) varied in the range from 0.76 to 0.86). The p(CO2[DW]) values, however, would easily be exceeded if the period of person stay in the chamber had been slightly extended (from 2.85 to 4 h under given conditions). In such case, the dripwater CO2-degassing would be inverted into CO2-dissolution and dripwater supersaturation would decrease. Achieving the threshold values at which water become aggressive to calcite (log p(CO2[EK]) approximately -1.99, -2.02, and -1.84) would require extreme conditions, e.g., simultaneous presence of 100 persons in the cave chamber for 14 h. The study should contribute to a better preservation of cave environment.

Parasitic worms: strategies of host finding, recognition and invasion

Many parasitic worms enter their hosts by active invasion. Their transmission success is often based on a mass production of invasive stages. However, most stages show a highly specific host-finding behaviour. Information on host-finding mechanisms is available mainly for trematode miracidia and cercariae and for nematode hookworms. The larvae find and recognise their hosts, in some cases even with species specificity, via complex sequences of behavioural patterns with which they successively respond to various environmental and host cues. There is often a surprisingly high diversity of host-recognition strategies. Each parasite species finds and enters its host using a different series of cues. For example, different species of schistosomes enter the human skin using different recognition sequences. The various recognition strategies may reflect adaptations to distinct ecological conditions of transmission. Another question is how, after invasion, parasitic worms find their complex paths through their host's tissues to their often very specific microhabitats. Recent data show that the migrating parasite stages can follow local chemical gradients of skin and blood compounds, but their long-distance navigation within the host body still remains puzzling. The high complexity, specificity and diversity of host-recognition strategies suggest that host finding and host recognition are important determinants in the evolution of parasite life cycles.

Effects of four tanniferous plant extracts on thein vitroexsheathment of third-stage larvae of parasitic nematodes

The anthelmintic properties of tanniferous plants and of their secondary metabolites represent one possible alternative to chemotherapy that is currently being explored as a means of achieving sustainable control of gastrointestinal nematodes in ruminants. Previousin vivoandin vitroresults suggest that tanniferous plants can have direct anti-parasitic effect against different stages of nematodes. However, the mode of action of the bioactive plant compounds remains obscure. The objectives of the current study were (1) to examine the hypothesis that extracts of tanniferous plants might interfere with the exsheathment of third-stage infective larvae (L3); (2) to assess the role of tannins in the process by examining the consequence of adding an inhibitor of tannins (polyethylene glycol: PEG) to extracts. The effects of 4 tanniferous plant extracts on exsheathment have been examined on L3 ofHaemonchus contortusandTrichostrongylus colubriformis. Artificial exsheathment was inducedin vitroby adding hypochloride solution to larval suspension. The evolution of exsheathment with time was measured by repeated observations at 10-min interval for 60 min. The selected plants were: genista (Sarothamnus scoparius), heather (Erica erigena), pine tree (Pinus sylvestris), and chestnut tree (Castanea sativa), with tannin contents ranging from 1·5 to 24·7% of DM. Extracts of a non-tanniferous plant (rye grass, tannin content: 0·3% of DM) were included in the assay as negative controls. The extracts were tested at the concentration of 600 μg/ml and the effects were compared to the rate of exsheathment of control larvae in PBS. No statistical differences in the pattern of exsheathment was observed after addition of rye grass or genista extracts for both nematode species and with heather extracts forT. colubriformis. In contrast, pine tree extracts on larvae of both species and heather extracts withH. contortusinduced a significant delay in exsheathment. Last, contact with chest nut extracts led to a total inhibition of the process for both nematodes. These results suggest that extracts of tanniferous plants might affect a key process in the very early stages of larval invasion of the host. In most cases, the addition of PEG led to a total or partial restoration towards control values. This suggests that tannins are largely involved in the inhibitory process. However, other secondary metabolites may also interfere with the process that would help to explain some of the differences in response observed between the two nematode species.

Strongyloides ratti: chemokinesis of glycolytic enzyme- and lectin-treated third-stage infective larvae in vitro

The infective third-stage larvae (L3s) of Strongyloides ratti, a parasitic nematode in rodents, showed two types of chemokinesis on a gradient of sodium chloride (NaCl) in an in vitro agarose tracking assay. The types were a consistent directional avoidance behavior under unfavorable environmental conditions and a reduced avoidance behavior under favorable conditions. We examined the effects of treatments with glycolytic enzymes and lectins by analyzing the avoidance behavior. L-Fucose dehydrogenase, hyaluronidase, beta-glucosidase, alpha-mannosidase, beta-galactosidase, concanavalin A, wheat germ agglutinin and soybean agglutinin exhibited inhibitory or enhancive effects on chemokinesis. We also confirmed the sites of the amphids of L3s aside from the mouth at the anterior end by scanning electron microscopy, and that concanavalin A-binding sites existed in the vicinity of the amphids using lectin-histochemistry. The carbohydrate moieties in the amphids of S. ratti L3s may play an important role as chemosensors in perceiving environmental cues.

Strongyloides ratti: thermokinesis of glycolytic enzyme- and lectin-treated third-stage infective larvae in vitro

The infective third-stage larvae (L3s) of a parasitic nematode of rodents, Strongyloides ratti, showed three types of thermokinesis on a temperature gradient using an in vitro agarose tracking assay method. These depended both on the pattern of gradient temperature and the prior culture temperature. Most L3s (> or = 80%) isolated from rat feces cultured at 25 degrees C and placed on a gradient at temperatures between 30 degrees C and 37 degrees C showed no directional response, at 22-29 degrees C more than 50% of the L3s showed positive thermokinesis, at 21 degrees C L3s showed positive, negative and no directional responses in the same ratio, while at 18-20 degrees C, L3s showed negative thermokinesis (approx. 40%) or no directional response (approx. 60%) as in our previous study. The present study describes the effects of glycolytic enzyme- and lectin-treated positive thermokinesis of L3s. alpha-Glucosidase or concanavalin A significantly exhibited inhibitory effects on thermokinesis.

Behavioural strategies used by the hookworms Necator americanus and Ancylostoma duodenale to find, recognize and invade the human host

The infective third-stage larvae of the hookworms Necator americanus and Ancylostoma duodenale infect their human hosts by active skin invasion, but A. duodenale is in addition capable of oral infection. The behaviour of the larvae when crawling on surfaces has already been described. Here we analyse in various in vitro systems the other behavioural invasion phases: activation, penetration, and orientation within the host. The larvae normally remained in a motionless, energy-saving, resting posture. An activation to sinusoidal locomotion was stimulated in both species by similar cues such as touch, vibration, water currents, heat, light, and chemicals. Human breath in addition stimulated searching and waving ("nictating") behaviour, which facilitates a change-over to the host. Activating cues in air streams were warmth and moisture; CO2 activated only in combination with warmth and/or moisture. Penetration behaviour in both species was stimulated by warmth and skin extracts. The stimulating components of skin extracts were fatty acids, but their stimulating characteristics differed from those inducing schistosome cercarial skin penetration. After penetration into agar substrates, both species showed thermo-orientation, but only A. duodenale followed gradients of serum. The directing serum cues were not amino acids and glucose (the supposed cues for schistosome blood vessel localization), but Ringer's solution attracted the larvae. The host-finding and host-invasion behaviour of both hookworm species is well adapted to the invasion of the human skin, and there seems to be no particular adaptation of A. duodenale behaviour to the oral infection mode. Hookworm host-finding behaviour is not as complex as that of schistosome cercariae but seems well adapted to the ecological conditions in the transmission sites.

Infective larvae of the human hookworms Necator americanus and Ancylostoma duodenale differ in their orientation behaviour when crawling on surfaces

The infective third stage larvae of hookworms infect their hosts by active skin invasion, and they find and recognize their hosts by the behavioural phases of activation, directed crawling, and penetration. Here we analyse the orientation of the infective larvae of the human hookworms Necator americanus and Ancylostoma duodenale when crawling on surfaces. Their behaviour differed from that of the larvae of the dog hookworm Ancylostoma caninum, but the two species also differed from each other. N. americanus crawled towards light, but A. duodenale did not. Both species migrated towards the warm ends of thermal gradients, and this response was more sensitive than in other skin-invading helminths (threshold 0.09 degrees C/cm). However, A. duodenale turned back and accumulated at higher temperatures than N. americanus [turn-back 45.7 (44.5-49.9) vs 41.5 (38.5-43.9) degrees C; accumulation 43.6 (41.6-46.0) vs 39.5 (37.9-43.0) degrees C]. In contrast to other skin-invading helminths, both species showed no chemo-orientation towards skin compounds when crawling on surfaces. This behaviour may reflect adaptations for reaching the skin surface from hairs or adhering material, but the differences in the orientation of the two species could not be attributed to differing transmission strategies.

An investigation of chemotaxis in the insect parasitic nematode Heterorhabditis bacteriophora

We tested the chemotactic responses of dauer juvenile stages (DJs) of the insect parasitic nematode Heterorhabditis bacteriophora to a variety of compounds that are known to be highly attractive or highly repellent to Caenorhabditis elegans. While H. bacteriophora DJs respond to alcohols and some aromatic compounds as well as to host metabolites such as uric acid and CO2, the most notable difference in the responses of these two nematodes is that H. bacteriophora DJs are unresponsive to a large number of compounds which C. elegans finds highly attractive. The latter compounds are typical by-products of bacterial metabolism and include aldehydes, esters, ketones and short-chain alcohols. While C. elegans finds long-chain alcohols (e.g. 1-heptanol and 1-octanol) repellent and short-chain alcohols highly attractive, H. bacteriophora DJs are strongly attracted to 1-heptanol, 1-octanol and 1-nonanol and find short-chain alcohols to be only slightly attractive. Parasitic-stage H. bacteriophora nematodes show a very weak chemotactic response to volatile molecules that DJs find highly attractive. Our results suggest that, associated with the adoption of a parasitic mode of life by Heterorhabditis, there was an adaptive change in chemotactic behaviour of the infective stages, resulting in a decreased sensitivity to volatile by-products of bacterial metabolism and an increased sensitivity towards long-chain alcohols and other insect-specific volatiles and possibly also to herbivore-induced plant volatiles.