Distance decay of similarity among parasite communities of three marine invertebrate hosts

The similarity in species composition between two communities generally decays as a function of increasing distance between them. Parasite communities in vertebrate definitive hosts follow this pattern but the respective relationship in intermediate invertebrate hosts of parasites with complex life cycles is unknown. In intermediate hosts, parasite communities are affected not only by the varying vagility of their definitive hosts (dispersing infective propagules) but also by the necessary coincidence of all their hosts in environmentally suitable localities. As intermediate hosts often hardly move they do not contribute to parasite dispersal. Hence, their parasite assemblages may decrease faster in similarity with increasing distance than those in highly mobile vertebrate definitive hosts. We use published field survey data to investigate distance decay of similarity in trematode communities from three prominent coastal molluscs of the Eastern North-Atlantic: the gastropods Littorina littorea and Hydrobia ulvae, and the bivalve Cerastoderma edule. We found that the similarity of trematode communities in all three hosts decayed with distance, independently of local sampling effort, and whether or not the parasites used the mollusc as first or second intermediate host in their life cycle. In H. ulvae, the halving distance (i.e. the distance that halves the similarity from its initial similarity at 1 km distance) for the trematode species using birds as definitive hosts was approximately two to three times larger than for species using fish. The initial similarities (estimated at 1 km distance) among trematode communities were relatively higher, whereas mean halving distances were lower, compared to published values for parasite communities in vertebrate hosts. We conclude that the vagility of definitive hosts accounts for a high similarity at the local scale, while the strong decay of similarity across regions is a consequence of the low probability that all necessary hosts and suitable environmental conditions coincide on a large scale.     

The nature and evolution of the association among digeneans, molluscs and fishes

Patterns of association of digenean families and their mollusc and vertebrate hosts are assessed by way of a new database containing information on over 1000 species of digeneans for life-cycles and over 5000 species from fishes. Analysis of the distribution of digenean families in molluscs suggests that the group was associated primitively with gastropods and that infection of polychaetes, bivalves and scaphopods are all the results of host-switching. For the vertebrates, infections of agnathans and chondrichthyans are apparently the result of host-switching from teleosts. For digenean families the ratio of orders of fishes infected to superfamilies of molluscs infected ranges from 0.5 (Mesometridae) to 16 (Bivesiculidae) and has a mean of 5.6. Individual patterns of host association of 13 digenean families and superfamilies are reviewed. Two, Bucephalidae and Sanguinicolidae, are exceptional in infecting a range of first intermediate hosts qualitatively as broad as their range of definitive hosts. No well-studied taxon shows narrower association with vertebrate than with mollusc clades. The range of definitive hosts of digeneans is characteristically defined by eco-physiological similarity rather than phylogenetic relationship. The range of associations of digenean families with mollusc taxa is generally much narrower. These data are considered in the light of ideas about the significance of different forms of host association. If Manter's Second Rule (the longer the association with a host group, the more pronounced the specificity exhibited by the parasite group) is invoked, then the data may suggest that the Digenea first parasitised molluscs before adopting vertebrate hosts. This interpretation is consistent with most previous ideas about the evolution of the Digenea but contrary to current interpretations based on the monophyly of the Neodermata. The basis of Manter's Second Rule is, however, considered too flimsy for this interpretation to be robust. Problems of the inference of the evolution of patterns of parasitism in the Neodermata are discussed and considered so intractable that the truth may be presently unknowable.     

Spatial and temporal distribution of endemic entomopathogenic nematodes in a heterogeneous vegetable production landscape

Entomopathogenic nematodes within the families Steinernematidae and Heterorhabditidae (Order: Rhabditida) are potential biological control agents for many soil-dwelling vegetable pests. However, their low persistence and efficacy after field releases have resulted in limited use in pest management programs. Understanding the factors regulating natural populations of entomopathogenic nematodes may provide insight into practices to conserve populations within production systems. A series of investigations were conducted within a vegetable production area in Willard, Ohio during 2000–2003 to gain insight into the population ecology of endemic populations of entomopathogenic nematodes. A total of 440 sites across four habitats associated with the production landscape were sampled to ascertain the natural occurrence of these beneficial nematodes. Habitats included cultivated areas, grassy banks adjacent to cultivated areas, undisturbed shrub lands and forests. Twelve sites along grassy banks were monitored over a growing season to estimate associations between abiotic and biotic factors and endemic populations. Entomopathogenic nematodes were only detected along grassy banks adjacent to the cultivated areas; nematodes were recovered from 15 to 30% of sites sampled in 2001 and 2002, respectively. Two species of nematodes were isolated, Heterorhabditis bacteriophora Poinar and Steinernema feltiae Filipjev. H. bacteriophora was the most prevalent nematode species and was recovered from 60% of positive samples. Nematode populations varied temporally and spatially along grassy banks; mean population density over the growing period was 1313 infective juveniles/m². Neither macro- nor microarthropod communities nor soil temperature differed between sites at which nematodes were detected and those at which nematodes were not detected. Soil moisture, however, was associated with the occurrence and persistence of nematodes along grassy banks; mean soil moisture at sites at which nematodes were detected and those sites at which nematodes were not detected was 37.3 and 26.8%, respectively. Water management is an important component of vegetable production and our results suggest that soil moisture manipulation would be important in the establishment and sustained presence of entomopathogenic nematode populations within cultivated areas over the growing season.     

The Prevalence of Angiostrongylus cantonensis/mackerrasae Complex in Molluscs from the Sydney Region

Angiostrongylus cantonensis and Angiostrongylus mackerrasae are metastrongyloid nematodes that infect various rat species. Terrestrial and aquatic molluscs are intermediate hosts of these worms while humans and dogs are accidental hosts. Angiostrongylus cantonensis is the major cause of angiostrongyliasis, a disease characterised by eosinophilic meningitis. Although both A. cantonensis and A. mackerrasae are found in Australia, A. cantonensis appears to account for most infections in humans and animals. Due to the occurrence of several severe clinical cases in Sydney and Brisbane, the need for epidemiological studies on angiostrongyliasis in this region has become apparent. In the present study, a conventional PCR and a TaqMan assay were compared for their ability to amplify Angiostrongylus DNA from DNA extracted from molluscs. The TaqMan assay was more sensitive, capable of detecting the DNA equivalent to one hundredth of a nematode larva. Therefore, the TaqMan assay was used to screen molluscs (n=500) of 14 species collected from the Sydney region. Angiostrongylus DNA was detected in 2 of the 14 mollusc species; Cornu aspersum [14/312 (4.5%)], and Bradybaenia similaris [1/10 (10%)], which are non-native terrestrial snails commonly found in urban habitats. The prevalence of Angiostrongylus spp. was 3.0% ± 0.8% (CI 95%). Additionally, experimentally infected Austropeplea lessoni snails shed A. cantonensis larvae in their mucus, implicating mucus as a source of infection. This is the first Australian study to survey molluscs using real-time PCR and confirms that the garden snail, C. aspersum, is a common intermediate host for Angiostrongylus spp. in Sydney.     

昆虫病原线虫感染寄主行为研究进展

昆虫病原线虫斯氏属Steinernema和异小杆属Heterorhabditids线虫是新型的生物杀虫剂。其感染期幼虫是惟一能够侵染寄主昆虫的虫态。这类线虫感染寄主的行为分为寻找寄主、识别寄主和侵染寄主。文章综述昆虫病原线虫感染寄主昆虫的行为以及在感染寄主过程中的影响因素。     

Intermediate Hosts of Angiostrongylus cantonensis in Tenerife,Spain

The nematode Angiostrongylus cantonensis is the causative agent of human angiostrongyliasis, the main clinical manifestation of which is eosinophilic meningitis. Although this parasite has been found recently in its definitive rat host in Tenerife (Canary Islands, Spain), showing a widespread distribution over the north-east part of the island, there are no available data regarding which snail and/or slug species are acting as intermediate hosts on this island. Consequently, the objective of this work was to determine the possible role of three mollusc species, Plutonia lamarckii, Cornu aspersum and Theba pisana, as intermediate hosts of A. cantonensis in Tenerife. Between 2011 and 2014, 233 molluscs were collected from five biotopes where rats had been found previously to harbor either adult worms or antibodies against A. cantonensis, and the identification was carried out on the basis of morphological features and a LAMP technique. The prevalence of A. cantonensis larvae in the mollusc samples, based on morphological identification, was 19.3%, whereas 59 out of the 98 individuals (60.2%) analyzed by LAMP were positive. Positive results were obtained for the three mollusc species analyzed and two of the positive samples, both obtained from P. lamarckii, were confirmed as positive by 18S rRNA and ITS1 PCR. Sequence analysis of 18S rRNA PCR products showed 100% similarity with previously published A. cantonensis sequences. These results may be relevant from a public health point of view, since all the biotopes from which the samples were obtained were in inhabited areas or areas with human activity, but it is also important from the perspective of a possible transmission to other accidental hosts, such as dogs and horses, animals that are present in some of the areas analyzed.     

Trematodes and nematodes parasitizing the benthic insect community of an Andean Patagonian stream,with emphasis on plagiorchiid metacercariae

In freshwater systems, parasitological studies have mainly been carried out on vertebrates and molluscs, but little is known about parasites of aquatic insects. We describe the trematodes and nematodes parasitizing the benthic insects of an Andean Patagonian stream and the presence of parasites in the terrestrial adult stages. Members of 3 of 20 insect taxa were found to be parasitized by larval nematodes, and members of six taxa harbored metacercariae of digeneans. In benthic samples, chironomids, simuliids (Order Diptera), and baetids (Order Ephemeroptera) harbored mermithid larvae (Nematoda). The stonefly Antarctoperla michaelseni (Order Plecoptera), the caddisfly Smicridea annulicornis (Order Trichoptera), a watersnipe fly (Order Diptera: Athericidae), and three species of leptophlebiid mayflies (Order Ephemeroptera) were parasitized by encysted plagiorchiid metacercariae (Order Plagiorchiida). Most metacercariae were found in the three species of mayflies with prevalences ranging 15–63% and mean intensities ranging 1.2–4.9. Prevalence declined from summer to early winter, probably because of the emergence of infected nymphs and the recruitment of uninfected new cohorts. The imagos had live metacercariae with higher prevalences and intensities of infection than nymphs. We suggest that these plagiorchiids have an allogenic life cycle, involving a terrestrial definitive host.     

To complete their life cycle, pathogenic nematode-bacteria complexes deter scavengers from feeding on their host cadaver

The life cycle of commercially used molluscicidal rhabditid nematodes Phasmarhabditis hermaphrodita and entomopathogenic steinernematid nematodes is similar: infective stages carry symbiotic bacteria, which kill their host. Nematodes complete their life cycle feeding on the proliferating symbiont and the host tissue. After 1-2 weeks, new infective stages carrying the bacteria leave the host cadaver in search of new hosts. The removal of invertebrate cadavers by scavengers is extremely fast and represents a severe threat to the developing nematodes.Two-choice trials were used to assess prey choice of the generalist predator/scavenger Pterostichus melanarius (Coleoptera: Carabidae) between Deroceras reticulatum (Mollusca: Agriolimacidae) slugs or wax moth Galleria mellonella (Lepidoptera: Pyralidae) larvae killed by infection of P. hermaphrodita/Steinernema affine and control killed by freezing. We demonstrate that the presence of either of the two nematodes tested deters the beetles from consuming infected cadavers. As P. hermaprodita cannot infect an insect host, we hypothesise the deterrent effect being an evolutionary adaptation of the nematode/bacteria complex rather than the ability of the beetles to avoid potentially infective cadavers.     

昆虫病原线虫共生细菌共生性的分子生物学研究进展

嗜线虫致病杆菌属Xenorhabdus和发光杆菌属Photorhabdus细菌隶属肠杆菌科Enterobacteriaceae,对多种害虫致病能力强,分别与斯氏属Steinernema和异小杆属Heterorhabditis昆虫病原线虫互惠共生。该两属共生细菌既存在对昆虫寄主的病原性,又存在与线虫寄主的共生性。共生细菌与其线虫寄主的共生性主要表现以下4方面：（1）细菌产生食物信号诱导滞育不取食的感染期线虫恢复;（2）细菌为线虫生长与繁殖提供营养;（3）细菌能于感染期线虫的肠道定殖与生长;（4）细菌产生杀线虫毒素杀死非共生线虫。本文综述了共生菌以上4方面的共生性及其相关的分子机制。     

Public health importance of Angiostrongylus cantonensis and its relatives

Species of Angiostrongylus are fairly common nematode parasites of a variety of small mammals, but some species - particularly A. cantonensis and A. costaricensis - are increasingly seen as human infections. The worms make use of a range of aquatic, amphibious or terrestrial gastropod molluscs as intermediate hosts, which become infected by ingestion of eggs shed in the faeces of infected mammals. Porotenic hosts, including crustaceans, amphibians and reptiles, are known for most Angiostrongylus species, and can also be a source of the infective third-stage larvae that initiate mammalian infections.     

A novel ascaroside controls the parasitic life cycle of the entomopathogenic nematode Heterorhabditis bacteriophora

Entomopathogenic nematodes survive in the soil as stress-resistant infective juveniles that seek out and infect insect hosts. Upon sensing internal host cues, the infective juveniles regurgitate bacterial pathogens from their gut that ultimately kill the host. Inside the host, the nematode develops into a reproductive adult and multiplies until unknown cues trigger the accumulation of infective juveniles. Here, we show that the entomopathogenic nematode Heterorhabditis bacteriophora uses a small-molecule pheromone to control infective juvenile development. The pheromone is structurally related to the dauer pheromone ascarosides that the free-living nematode Caenorhabditis elegans uses to control its development. However, none of the C. elegans ascarosides are effective in H. bacteriophora, suggesting that there is a high degree of species specificity. Our report is the first to show that ascarosides are important regulators of development in a parasitic nematode species. An understanding of chemical signaling in parasitic nematodes may enable the development of chemical tools to control these species.     

Why do fish have so few roundworm (nematode) parasites?

Synopsis Although they are the oldest and most diverse members of the subphylum, the fishes have relatively few nematode parasites in comparison with other vertebrate classes. It is hypothesized that this paucity of parasite species has occurred because nematode parasites first evolved in terrestrial hosts and only a few lines of these parasites were able to transfer to fish after the appearance of heteroxeny (use of intermediate hosts) and paratenesis (use of transport hosts). The inability of nematodes to initiate parasitism in aquatic ecosystems restricted fish parasites mainly to forms first adapted to terrestrial vertebrates and at the same time deprived large groups of aquatic invertebrates such as the crustaceans, annelids and molluscs of a nematode parasite fauna.Invited editorial     

Preferential infectivity of entomopathogenic nematodes in an envenomed host

Entomopathogenic nematodes and parasitoid wasps are used as biological control agents for management of insect pests such as the Indian meal moth, Plodia interpunctella. The parasitoid wasp Habrobracon hebetor injects a paralytic venom into P. interpunctella larvae before laying eggs. A previous study reported that the entomopathogenic nematode Heterorhabditis indica preferentially infects P. interpunctella that have been envenomed by H. hebetor while results in this study showed a similar preference by the entomopathogenic nematode, Steinernema glaseri. We therefore tested four hypotheses for why nematode infection rates are higher in envenomed hosts: (1) elevated CO₂ emission from envenomed hosts attracts nematodes, (2) paralysis prevents hosts from escaping nematodes, (3) volatile chemicals emitted from envenomed hosts attract nematodes and increase infection, and (4) reduced immune defenses in envenomed hosts increase nematode survival. Results showed that envenomed P. interpunctella larvae emitted lower amounts of CO₂ than non-envenomed larvae. Physical immobilization of P. interpunctella larvae did not increase infection rates by S. glaseri but did increase infection rates by H. indica. Emissions from envenomed hosts were collected and analyzed by thermal desorption gas chromatography/mass spectrometry. The most abundant compound, 3-methyl-3-buten-1-ol, was found to be an effective cue for S. glaseri attraction and infection but was not an effective stimulus for H. indica. Envenomed P. interpunctella exhibited a stronger immune response toward nematodes than non-envenomed hosts. Altogether, we conclude that different mechanisms underlie preferential infection in the two nematode species: host immobilization for H. indica and chemical cues for S. glaseri.     

Relationship between intermediate host taxon and infection by nematodes of the genus Rhabdochona

We describe the intermediate and definitive hosts of the fish nematodes Rhabdochona coronacauda and R. denudata honshuensis and discuss the relationships between parasitism and the feeding habitats of their intermediate hosts. We found that the principal intermediate hosts of the two nematodes were filter-feeding mayflies of the genera Ephemera, Photamanthus and Isonychia. Ephemera strigata seemed to be the most important intermediate host of these nematodes. Adult R. coronacauda were found mainly in Hemibarbus longirostris and Rhinogobius flumineus, which are benthic fishes that feed on benthic aquatic insects, including E. strigata. For R. coronacauda, therefore, the feeding habits of the definitive hosts facilitate host alternation by this species. However, adult R. denudata honshuensis were found in cyprinids. In particular, Zacco temmincki was the principal natural definitive host in our study area. Since Z. temmincki is a swimming predator, E. strigata nymphs that burrow in the substrate are not the main prey of this species. This indicates that the transmission of R. denudata honshuensis hardly occurs from E. strigata nymph to Z. temmincki, suggesting another, unknown transmission route.     

Aggregative group behavior in insect parasitic nematode dispersal

Movement behavior of foraging animals is critical to the determination of their spatial ecology and success in exploiting resources. Individuals sometimes gain advantages by foraging in groups to increase their efficiency in garnering these resources. Group movement behavior has been studied in various vertebrates. In this study we explored the propensity for innate group movement behavior among insect parasitic nematodes. Given that entomopathogenic nematodes benefit from group attack and infection, we hypothesised that the populations would tend to move in aggregate in the absence of extrinsic cues. Movement patterns of entomopathogenic nematodes in sand were investigated when nematodes were applied to a specific locus or when the nematodes emerged naturally from infected insect hosts; six nematode species in two genera were tested (Heterorhabditis bacteriophora, Heterorhabditis indica, Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri and Steinernema riobrave). Nematodes were applied in aqueous suspension via filter paper discs or in infected insect host cadavers (to mimic emergence in nature). We discovered that nematode dispersal resulted in an aggregated pattern rather than a random or uniform distribution; the only exception was S. glaseri when emerging directly from infected hosts. The group movement may have been continuous from the point of origin, or it may have been triggered by a propensity to aggregate after a short period of random movement. To our knowledge, this is the first report of group movement behavior in parasitic nematodes in the absence of external stimuli (e.g., without an insect or other apparent biotic or abiotic cue). These findings have implications for nematode spatial distribution and suggest that group behavior is involved in nematode foraging.     

江西井冈山自然保护区陆生贝类多样性

2011年4～8月调查了江西井冈山自然保护区陆生贝类资源,共采得陆生贝类67种和亚种(含9个未定种),隶属4目19科31属,其中有1新种,即龙潭弯螺(Sinoennea sp.nov.)(另文报道);14种为江西省陆生贝类新纪录种;优势种为长柱倍唇螺(Diplommatina paxillus longipalatalis)、细锥倍唇螺(D.apicina)、灰尖巴蜗牛(Bradybaena ravida ravida)、双线巨蓬蛞蝓(Meghimatium bilineatum)。区系组成以东洋界成分为主,占种类总数的74.14%。阔叶林、灌木丛和农田生境陆生贝类种类较丰富,竹林和苔藓生境种类较少。根据调查数据,分别计算井冈山自然保护区5种不同生境类型中陆生贝类群落的多样性、丰富度和均匀度,结果表明,灌木丛生境陆生贝类的丰富度指数和多样性指数均最高,苔藓生境的均匀度指数最高。与邻近自然保护区比较,井冈山自然保护区陆生贝类物种较丰富,且与江西齐云山陆生贝类物种相似系数较高,与广东南岭物种相似系数较低。     

Effect of Entomopathogenic Nematode Concentration on Survival during Cryopreservation in Liquid Nitrogen

Entomopathogenic nematodes are used for biological control of insect pests. A method for improved cryopreservation of infective juvenile stage nematodes has been developed using Steinernema carpocapsae and Heterorhabditis bacteriophora. Optimum survival for both species was achieved with 12,000 infective juveniles/ml in glycerol and 7,500/ml in Ringer''s solution. For S. carpocapsae, maximum survival also was observed with 60,000 infective juveniles/ml in glycerol and 25,000/ml in Ringer''s solution. These concentrations resulted in 100% post-cryopreservation survival of S. carpocapsae and 100% retention of original virulence to Galleria mellonella larvae. This is the first report of achieving 100% survival of an entomopathogenic nematode after preservation in liquid nitrogen. Maximum survival of H. bacteriophora following cryopreservation was 87%.     

The slug parasitic nematode Phasmarhabditis hermaphrodita associates with complex and variable bacterial assemblages that do not affect its virulence

Phasmarhabditis hermaphrodita is a nematode parasite of slugs that is commercially reared in monoxenic culture with the bacterium Moraxella osloensis and sold as a biological molluscicide. However, its bacterial associations when reared in vivo in slugs are unknown. We show that when reared in vivo in slugs, P. hermaphrodita does not retain M. osloensis and associates with complex and variable bacterial assemblages that do not influence its virulence. This is in marked contrast to the entomopathogenic nematodes that form highly specific mutualistic associations with Enterobacteriaceae that are specifically retained during in vivo growth.     

Foraging Ants as Scavengers on Entomopathogenic Nematode-Killed Insects

Ants were the most apparent invertebrate scavengers observed foraging on entomopathogenic nematode-killed insects (i.e., insect cadavers containing entomopathogenic nematodes and their symbiotic bacteria) in the present study. Workers of the Argentine ant,Linepithema humile(Mayr), scavenged nematode-killed insects on the surface and those buried 2 cm below the soil surface. Ant workers scavenged significantly more steinernematid-killed (60–85%) than heterorhabditid-killed (10–20%) insects. More 4-day-postinfected cadavers (hosts died within 48 h after exposure to nematodes) were scavenged than 10-day-postinfected cadavers. Ten-day-postinfected hosts contained live infective juvenile nematodes therefore ants may serve as phoretic agents. Other ant species, includingVeromessor andrei(Mayr),Pheidole vistanaForel,Formica pacificaFrancoeur, andMonomoriom ergatogynaWheeler, also scavenged nematode-killed insects. These ant species removed or destroyed about 45% of the steinernematid-killed insects. These results suggest that survival of steinernematid nematodes may be more significantly impacted by invertebrate scavengers, especially ants, than that of heterorhabditid nematodes, and placement of steinernematid-killed insects in the field for biological control may be an ineffective release strategy. Because entomopathogenic nematodes kill insects with the help of symbiotic bacteria, we tested the role of these bacterial species in deterring invertebrate scavengers by injecting bacteria (without nematodes) into insects and placing the cadavers in the field. None of the insects killed by the symbiotic bacterium,Photorhabdus luminescens(Thomas and Poinar) fromHeterorhabditis bacteriophoraPoinar, were scavanged, whereas 70% of the insects killed by the symbiotic bacterium,Xenorhabdus nematophilus(Poinar and Thomas) fromSteinernema carpocapsae(Weiser), and 90% of the insects killed byBacillus thuringiensisBerliner were scavenged by the Argentine ant. We conclude thatP. luminescensis responsible for preventing ants from foraging on heterorhabditid-killed hosts.