Life cycle and paratenesis of American gordiids (Nematomorpha: Gordiida)

To complete their life cycle, gordiids must make a transition from aquatic to terrestrial environments. However, epibenthic aquatic larvae and their terrestrial definitive hosts do not overlap in habitat. This has led many investigators to suggest that infections are acquired through the ingestion of insects, which become infected as aquatic larvae with gordiid cysts and subsequently carry gordiids to land. This proposed life cycle was experimentally tested using 3 common American species of gordiids: Gordius robustus, Paragordius varius, and Chordodes morgani. Cysts of all 3 species survived the metamorphosis of Tanytarsus sp., a midge. Subsequent infection trials of definitive hosts with cysts from imagos show that cysts surviving the metamorphosis of insects remained viable and free of host internal defense reaction. Data from naturally infected mayflies, Callibaetus sp., show that encystment and survival of gordiids within aquatic insects occur in nature. Paratenesis between paratenic hosts was also shown to be possible in these 3 species. This latter finding appears to indicate that cysts formed in spurious paratenic hosts may not be lost but may eventually transfer to normal paratenic hosts.     

Physid snails as sentinels of freshwater nematomorphs.

Freshwater nematomorphs, or gordiids, are parasitic as larvae, but free-living in aquatic environments as adults. Studies based on the collection of adults have reported gordiids to be widespread, but discontinuous in distribution. However, a relatively short adult life span and unknown life history make the detection of adults difficult. An alternative approach to investigate gordiid distribution is to use cysts. Of all paratenic hosts, snails were chosen because they lacked internal defense reactions to the cysts and become easily infected. Here, it is reported that the occurrence of gordiids on the basis of the cyst stage is much more common than previously reported, thus altering the perception of how common these worms are. Using this modified survey procedure, gordiid cysts were found at 70% of sites examined, in an area where extensive sampling over 3 yr yielded adults only at a single site. Of 1,000 snails dissected, 395 were infected with gordiids (intensity range: 1-465). Furthermore, different types of human-modified landscapes did not affect gordiid distribution, suggesting that as urban and suburban areas sprawl, human encounters or pseudoparasitism with nematomorphs may increase. The results of this study indicate that use of organismal-specific sampling techniques can be critical in studies of parasite distribution and biodiversity.     

Hyperparasitism by Paragordius varius (Nematomorpha: Gordiida) larva of Monostome redia (Trematoda: Digenea)

Gordiid larvae enter and encyst within a large variety of aquatic vertebrates and invertebrates. Cysts of a single species of gordiid have been found in such diverse hosts as insect larvae, snails, crustaceans, fish, and many others. One particularly surprising host report was made nearly 100 yr ago, i.e., cysts within adult trematodes. The origin of these cysts remained unclear, but it was suggested that the gordiid larvae gained entry into the adult flukes via their definitive, vertebrate host. In the present study, the finding is reported of gordiid cysts within monostome redia within a Physa gyrina snail. This is the first time gordiid cysts have been reported within redia, and this provides an alternative explanation for the presence of cysts within adult trematodes.     

Transmammary infection of paratenic and definitive hosts with Alaria marcianae (trematoda) mesocercariae

Transmammary infection with Alaria marcianae mesocercariae was demonstrated using mice as model, paratenic hosts. Prenatal transmission was ruled out because neonates removed immediately postpartum from infected dams were never infected. Mesocercarial distribution in virgin females and in females examined immediately postpartum showed no marked preference for the mammary glands. In contrast, infection of neonates that were allowed to suckle on infected dams was absolute, and the number of mesocercariae in the mammary glands of postparturient dams that suckled their young was increased significantly. These experimental observations were coupled with other observations on paratenic hosts to outline the pathways open to mesocercariae in gravid hosts. The term amphiparatenic host is coined for those host species that are paratenic hosts as adults, but as juveniles can serve as definitive hosts.     

Interspecific associations among larval helminths in fish

Various processes can generate associations between the larvae of different helminth species in their fish intermediate or paratenic host. We investigated the pairwise associations among larval helminth species in eight different fish populations, using two different coefficients of associations, in order to determine in what situations they are strongest. All helminth species included use the fish studied as either their second intermediate host or their paratenic host, and are acquired by the fish when it ingests an infected first intermediate host. The intensity of infection correlated positively with fish length for most helminth species. Pairs of species which both exhibited positive correlations with fish length tended to be more strongly associated with one another, although this tendency was not pronounced. Similarity in life cycle had a more important influence on pairwise associations. Among the 62 pairwise associations that could be computed, pairs of helminth species that shared both first intermediate hosts and definitive hosts were the most strongly associated, followed by pairs that shared only one other host, and finally by pairs that did not share other hosts. The results suggest that assemblages of larval helminth parasites in fish are not random collections of locally available species, but rather structured packets of larval parasites that travel together along common transmission routes.     

Evidence that flatheaded mayflies (Heptageniidae) serve as the likely paratenic host for Chordodes morgani (Nematomorpha) in Nebraska

Hairworms (Nematomorpha) have indirect life cycles that require a terrestrial arthropod definitive host and an aquatic arthropod host that serves as a paratenic host, transferring the hairworm from the aquatic to the terrestrial environment. The life cycles for most hairworms remain unknown, especially the paratenic host. The hairworm Chordodes morgani lays its eggs on sticks, which are susceptible to the activity of aquatic insects such as larvae of the flatheaded mayfly (Heptageniidae), which feed by scraping algae and detritus. To test the likelihood that scrapers serve as the paratenic host for C. morgani, we collected sticks and their accompanying invertebrates from three sites near Lincoln, NE. We noted the presence of eggs of C. morgani on each stick, placed them in labeled resealable bags, and examined the invertebrates microscopically for the presence of hairworm cysts. We predicted that scrapers found on sticks with eggs of C. morgani will harbor the highest number of cysts. To confirm that larvae of the flatheaded mayfly serve as the aquatic host, we fed field-collected specimens to three captive-reared wood roaches (Parcoblatta pensylvanica). Of the three exposed roaches, one yielded nine individuals of C. morgani 63 days after exposure. Understanding the life cycle of this horsehair worm will allow researchers to rear the species in the lab, which could become a model for parasite research.     

Morphometric analysis of nonadult characters of common species of American gordiids (Nematomorpha: Gordioidea)

The nonadult stages, egg strings, eggs, larvae, and cysts of Gordius robustus, Paragordius varius, and Chordodes morgani are described morphometrically. The goal was to document the differences between species and to evaluate the usefulness of morphometrics in species identification. In concert, multivariate analysis of variance (MANOVA) and univariate analysis of variance (ANOVA, a posteriori contrasts) statistical tests demonstrated that each species is morphometrically distinguishable from all others. Additionally, discriminant function analysis indicated that postseptum length, pseudointestine length, and stylet width were the most important variables in the discrimination of species based on larval characters. Finally, differences in oviposition behaviors among these 3 species were found. It is suggested that ovipositioning differences may place larvae into distinct niches and may ultimately lead to the use of different paratenic hosts by different gordiid species.     

Cryptic speciation and patterns of phenotypic variation of a highly variable acanthocephalan parasite

An investigation of a parasite species that is broadly host- and habitat-specific and exhibits alternative transmission strategies was undertaken to examine intraspecific variability and if it can be attributed to cryptic speciation or environmentally induced plasticity. Specimens of an acanthocephalan parasite, Leptorhynchoides thecatus, collected throughout North America were analysed phylogenetically using sequences of the cytochrome oxidase I gene and the internal transcribed spacer region. Variation in host use, habitat use, and transmission were examined in a phylogenetic context to determine if they were more likely phylogenetically based or due to environmental influences. Results indicated that most of the variation detected can be explained by the presence of cryptic species. The majority of these species have narrow host and microhabitat specificities although one species, which also may comprise a complex of species, exhibits broad host and habitat specificity. Alternate transmission pathways only occurred in two of the cryptic species and correlate with host use patterns. Taxa that mature in piscivorous piscine hosts use a paratenic fish host to bridge the trophic gap between their amphipod intermediate host and piscivorous definitive host. One potential example of environmentally induced variation was identified in three populations of these parasites, which differ on their abilities to infect different host species.     

The role of phylogeny and ecology in experimental host specificity: insights from a eugregarine-host system

The degree to which parasites use hosts is fundamental to host-parasite coevolution studies, yet difficult to assess and interpret in an evolutionary manner. Previous assessments of parasitism in eugregarine-host systems suggest high degrees of host specificity to particular host stages and host species; however, rarely have the evolutionary constraints on host specificity been studied experimentally. A series of experimental infections were conducted to determine the extent of host stadium specificity (larval vs. adult stage) and host specificity among 6 tenebrionid host species and 5 eugregarine parasite species. Eugregarines from all host species infected both the larva and adult stages of the host, and each parasite taxa colonized several host species (Tribolium spp. and Palorus subdepressus). Parasite infection patterns were not congruent with host phylogeny, suggesting that host phylogeny is not a significant predictor of host-parasite interactions in this system. However, the 2 host stages produced significantly different numbers of parasite propagules, indicating that ecological factors may be important determinants of host specificity in this host-parasite system. While field infections reflect extant natural infection patterns of parasites, experimental infections can demonstrate potential host-parasite interactions, which aids in identifying factors that may be significant in shaping future host-parasite interactions.     

Infectivity of two nematode parasites, Camallanus lacustris and Anguillicola crassus, in a paratenic host, the three-spined stickleback Gasterosteus aculeatus

Three-spined sticklebacks Gasterosteus aculeatus are frequent paratenic hosts of the nematode parasites Anguillicola crassus and Camallanus lacustris. As paratenic hosts, sticklebacks could spread infection by carrying high numbers of infective stages. In contrast, low infective ability of either parasite for the paratenic host could hinder the spread of infection. In the present study, G. aculeatus was, for the first time, infected under controlled laboratory conditions with defined doses of the parasites. Sticklebacks were exposed to 6, 12, 18 and 24 parasite larvae to determine the infective ability of the 2 nematode species. There were significantly higher infection rates for C. lacustris (18 to 49%) than for A. crassus (4 to 14%) at each exposure dose. In C. lacustris-infected sticklebacks, infection rates tended to be highest after exposure to 12 C. lacustris larvae and lowest after exposure to 24 parasites. In A. crassus-infected sticklebacks, no effect of parasite exposure dose on infection rates was observed. Immunity parameters such as respiratory burst activity and lymphocyte proliferation of head kidney leukocytes recorded 18 wk post exposure were not significantly affected by either parasite or exposure dose. Granulocyte:lymphocyte ratios were elevated only within the stickleback group showing the highest infection intensity of C. lacustris, i.e. to those exposed 18 parasites.     

Global patterns in helminth host specificity: phylogenetic and functional diversity of regional host species pools matter

Host specificity has a major influence on a parasite's ability to shift between human and animal host species. Yet there is a dearth of quantitative approaches to explore variation in host specificity across biogeographical scales, particularly in response to the varying community compositions of potential hosts. We built a global dataset of intermediate host associations for nine of the world's most widespread helminth parasites (all of which infect humans). Using hierarchical models, we asked if realised parasite host specificity varied in response to regional variation in the phylogenetic and functional diversities of potential host species. Parasites were recorded in 4–10 zoogeographical regions, with some showing considerable geographical variation in observed versus expected host specificity. Parasites generally exhibited the lowest phylogenetic host specificity in regions with the greatest variation in prospective host phylogenetic diversity, namely the Neotropical, Saharo‐Arabian and Australian regions. Globally, we uncovered notable variation in parasite host shifting potential. Observed host assemblages for Hydatigera taeniaeformis and Hymenolepis diminuta were less phylogenetically diverse than expected, suggesting limited potential to spillover into unrelated hosts. Host assemblages for Echinococcus granulosus, Mesocestoides lineatus and Trichinella spiralis were less functionally diverse than expected, suggesting limited potential to shift across host ecological niches. By contrast, Hyd. taeniaeformis infected a higher functional diversity of hosts than expected, indicating strong potential to shift across hosts with different ecological niches. We show that the realised phylogenetic and functional diversities of infected hosts are determined by biogeographical gradients in prospective host species pools. These findings emphasise the need to account for underlying species diversity when assessing parasite host specificity. Our framework to identify variation in realised host specificity is broadly applicable to other host–parasite systems and will provide key insights into parasite invasion potential at regional and global scales.     

Common sculpin Cottus gobio as a natural paratenic host of Proteocephalus longicollis (Cestoda: Proteocephalidae), a parasite of salmonids, in Europe.

Common sculpins Cottus gobio L. (Pisces: Cottidae), from the Mlynsky Brook near Ceské Zleby in the Sumava National Park, southwestern Bohemia, Czech Republic, were found to harbour in their intestines juvenile cestodes Proteocephalus longicollis (Zeder, 1800), a common parasite of holarctic salmonids, with a prevalence of 60% and intensity of 1 to 11 (mean 5) parasites per fish; undoubtedly, these prey fish serve as paratenic hosts. In this locality, the definitive host of P. longicollis is the brown trout Salmo trutta m. fario L., large specimens of which apparently acquire infection of this parasite by feeding on infected sculpins. C. gobio is the first known natural paratenic host of P. longicollis in Europe.     

The role of the European bitterling (<Emphasis Type="Italic">Rhodeus amarus,</Emphasis> Cyprinidae) in parasite accumulation and transmission in riverine ecosystems

In aquatic ecosystems, fish play a key role in parasite accumulation and transmission to predacious animals. In the present study, realized on seven populations of a small cyprinid fish species, the European bitterling Rhodeus amarus, we investigated (1) the role of the European bitterling as a potential intermediate or paratenic host, (2) the ability of the fish to accumulate parasites with similar final host group, and (3) its significance as a potential source of parasite infection in the ecosystem in respect to habitat characteristics. A total of 36 parasite species were recorded; 31 species (90% of all parasite specimens) were classified as endoparasites. Most of the endoparasites were found in the larval life stage, using bitterling as an intermediate or paratenic host. In particular, parasite community structure showed significantly higher proportions of allogenic parasites in comparison with autogenic. The supposed co-occurrence of parasite species with identical final host groups showed only a weak association. The adjacent reservoir areas were a significant determinant of both the total and infracommunity parasite species richness and for the mean parasite abundance. No relationship between the distance of sampling site from the adjacent reservoir and parasite community characteristics was found. As a small-sized fish with a wide distribution range and high local abundances, the European bitterling can represent a natural prey for a wide range of piscivorous predators. Due to its susceptibility to the number of larval endoparasites, this fish species may therefore fulfill the role as important transmitter of parasites to their final hosts.     

The distribution and incidence of larval trematodes in the freshwater fauna of the Wentloog level, South Wales

The Wentloog level, in South Wales, is a low-lying coastal plain dissected by a network of drainage channels. The latter contain a rich aquatic invertebrate fauna which harbour a wide range of larval digenetic trematodes. Few of the host species examined were not infected with larval trematodes. Miracidia together with their associated sporocysts, rediae and cercariae were more host specific than metacercariae but many of the miracidia which developed in pulmonate molluscs could do so in more than one genus. Metacercarial host specificity is less rigid, and usually species are only restricted to members of one phylum although several were found which could develop in two or more phyla. Most larval stages showed some host preference when they occurred in several host species and this was attributed to ecological or physiological factors or to mechanical barriers. Seasonal variation of infection rate occurs in all larval stages: infection with developing cercariae showed peaks during June and August, but metacercarial infection rates varied according to the host. In molluscs there may be two peaks of infection with metacercariae (as in the cercarial stages), but for many arthropods infections show a gradual increase through the year. The factors mainly responsible for these seasonal changes are considered to be the life-cycle of the intermediate hosts and the environmental temperature, the latter acting on the rate of development of the trematodes. Rainfall, and patterns of egg production of adult trematodes in the definitive hosts are also important.     

Metazoan parasites of African annual killifish (Nothobranchiidae): abundance,diversity, and their environmental correlates

Estimates of biodiversity and its global patterns are affected by parasite richness and specificity. Despite this, parasite communities are largely neglected in biodiversity estimates, especially in the tropics. We studied the parasites of annual killifish of the genus Nothobranchius that inhabit annually desiccating pools across the African savannah and survive the dry period as developmentally arrested embryos. Their discontinuous, non‐overlapping generations make them a unique organism in which to study natural parasite fauna. We investigated the relationship between global (climate and altitude) and local (pool size, vegetation, host density and diversity, and diversity of potential intermediate hosts) environmental factors and the community structure of killifish parasites. We examined metazoan parasites from 21 populations of four host species (Nothobranchius orthonotus, N. furzeri, N. kadleci, and N. pienaari) across a gradient of aridity in Mozambique. Seventeen parasite taxa were recorded, with trematode larval stages (metacercariae) being the most abundant taxa. The parasites recorded were both allogenic (life cycle includes non‐aquatic host; predominantly trematodes) and autogenic (cycling only in aquatic hosts; nematodes). The parasite abundance was highest in climatic regions with intermediate aridity, while parasite diversity was associated with local environmental characteristics and positively correlated with fish species diversity and the amount of aquatic vegetation. Our results suggest that parasite communities of sympatric Nothobranchius species are similar and dominated by the larval stages of generalist parasites. Therefore, Nothobranchius serve as important intermediate or paratenic hosts of parasites, with piscivorous birds and predatory fish being their most likely definitive hosts.     

Species-specific prevalence of mermithid parasites in populations of six congeneric host caddisflies of Ecnomus McLachlan, 1864 (Trichoptera: Ecnomidae)

Mermithid nematodes are entomophagous parasites and, despite being present in diverse aquatic insects, studies of caddisflies acting as definitive hosts are few and the ecological impacts on host populations are largely unknown. During a four-year study in a stream in southeastern Australia, parasitic mermithid worms were commonly found inside adult caddisflies, but only species of the genus Ecnomus McLachlan, 1864 (Ecnomidae). Ecnomus were the definitive host and parasite prevalence ranged from <1% to nearly 50% across six species. Species-specific prevalence varied little between years and was typically higher in males than in females. Parasite intensity ranged from 1 to 6 (median = 1), but did not vary between species or sexes. Infected hosts could fly, but were castrated and died when worms emerged. High prevalence and virulence (reproductive failure and death of the host) suggests that these parasites could potentially play a role in the population dynamics of some Ecnomus spp.     

两种颚口线虫流行病学调查和刚刺颚口线虫幼虫对四十种动物感染力的研究

本文报告刚刺颚口线虫Gnathostoma hispidum和棘颚口线虫Gnathostoma spinigerum流行学和动物实验。证明我国有40种动物充当它们的第一、二中间宿主和转续宿主,其中30种是这两种病源共同宿主。首次报告猕猴Macaca mulatta可作刚刺颚口线虫的第二中间宿主和转续宿主。用刚刺颚口线虫晚第三期幼虫经皮肤感染家猫和小白鼠均得阳性。调查和实验结果表明刚刺颚口线虫和棘颚口线虫的生物学和流行学特性十分相似,显示它们都是人兽共患的寄生虫。文中讨论刚刺颚口线虫的传播途径和人体感染问题。     

Probing into the diversity of trypanosomatid flagellates parasitizing insect hosts in South-West China reveals both endemism and global dispersal

Flagellates of the class Kinetoplastea are known to frequently parasitize insects. We have collected 67 isolates from 407 Heteroptera hosts captured in several locations of South-West China. Their splice leader (SL) RNA gene repeats and small subunit (SSU) rRNA genes were PCR amplified from the infected tissue samples. In most cases, parasites were found in the midgut, rarely the infection was confined to the Malpighian tubes. Phylogenetic analysis of the obtained sequences has significantly expanded the known diversity of these monoxenous parasites. Fifteen typing units were found among these isolates including 11 potentially new species. Four typing units matched the previously known typing units from the Neotropics indicating a global distribution of the respective parasite species. At the same time, new clades appeared, testifying for a certain level of endemism. The host record of the parasites found indicated a variable specificity level of the host–parasite association including several cases of a very broad host range. Our results disprove the “one host – one parasite” paradigm and show that although the global diversity of monoxenous parasites is high, it is not as enormous as suggested earlier. Moreover, phylogenetic analysis revealed the presence, among the isolated strains, of a new Phytomonas species, which is the first documentation of this potentially pathogenic dixenous parasite of plants in China.