Aquatic molluscs as auxiliary hosts for terrestrial nematode parasites: implications for pathogen transmission in a changing climate

Characterisation of Nematoda and Digenea in selected Australian freshwater snails

Freshwater snails are integral to local ecosystems as a primary food source for various vertebrate species, thereby contributing significantly to ecological food webs. However, their role as intermediate hosts also makes them pivotal in the transmission of parasites. In Australia, research on freshwater snails has predominantly focused on their role as intermediate hosts for livestock parasites, while there has been limited exploration of the impact of these parasites on snail health and population dynamics. The aim of this study was to determine parasitic infection in freshwater snails. This study was conducted in the south-eastern region of Australia, in 2022. A total of 163 freshwater snails from four different species were collected and examined in the Murrumbidgee catchment area in the southeastern part of Australia during the Southern Hemisphere summer and autumn months (February to May). The species included Isidorella hainesii, Glyptophysa novaehollandica, Bullastra lessoni (endemic species), and Physella acuta (an introduced species). Through the analysis of sequence data from the various regions of the nuclear ribosomal DNA, we determined that the Digenea species in this study belonged to three distinct species, including Choanocotyle hobbsi, Petasiger sp. and an unidentified species belonging to Plagiorchioidea. Additionally, analysis of the sequences from Nematoda found in this study, revealed they could be categorized into two separate taxa, including Krefftascaris sp. and an unidentified nematode closely associated with plant and soil nematodes. This research holds significant implications for the future understanding and conservation of Australian freshwater ecosystems. Most parasites found in the present study complete their life cycle in snails and turtles. As many of freshwater snail and turtle species in Australia are endemic and face population threats, exploring the potential adverse impacts of parasitic infections on snail and turtle health, is crucial for advancing our understanding of these ecosystems and also paving the way for future research and conservation efforts. While none of the native snail species in the present study have been listed as endangered or threatened, this may simply be attributed to the absence of regular population surveys.

Aquaculture of Animal Species: Their Eukaryotic Parasites and the Control of Parasitic Infections

Parasites are very diverse and common in both natural populations and in stocks kept in aquacultural facilities. For most cultured species, there are important bacteria and viruses causing diseases, but eukaryotic parasites are also very important. We review the various combinations of aquacultured species and eukaryotic parasitic groups and discuss other problems associated with aquaculture such as eutrophication, zoonotic species, and invasive species, and we conclude that further development of aquaculture in a sustainable manner must include a holistic approach (One Health) where many factors (e.g., human health, food safety, animal health and welfare, environmental and biodiversity protection and marketability mechanisms, etc.) are considered.

The impact of climate change and pollution on trematode-bivalve dynamics

Coastal ecosystems and their marine populations are increasingly threatened by global environmental changes. Bivalves have emerged as crucial bioindicators within these ecosystems, offering valuable insights into biodiversity and overall ecosystem health. In particular, bivalves serve as hosts to trematode parasites, making them a focal point of study. Trematodes, with their life cycles intricately linked to external factors, provide excellent indicators of environmental changes and exhibit a unique ability to accumulate pollutants beyond ambient levels. Thus, they act as living sentinels, reflecting the ecological condition of their habitats. This paper presents a comprehensive review of recent research on the use of bivalve species as hosts for trematodes, examining the interactions between these organisms. The study also investigates the combined impact of trematode infections and other pollutants on bivalve molluscs. Trematode infections have multifaceted consequences for bivalve species, influencing various aspects of their physiology and behavior, including population-wide mortality. Furthermore, the coexistence of trematode infections and other sources of pollution compromises host resistance, disrupts parasite transmission, and reduces the abundance of intermediate hosts for complex-living parasites. The accumulation process of these parasites is influenced not only by external factors but also by host physiology. Consequently, the implications of climate change and environmental factors, such as temperature, salinity, and ocean acidification, are critical considerations. In summary, the intricate relationship between bivalves, trematode parasites, and their surrounding environment provides valuable insights into the health and sustainability of coastal ecosystems. A comprehensive understanding of these interactions, along with the influence of climate change and environmental parameters, is essential for effective management and conservation strategies aimed at preserving these delicate ecosystems and the diverse array of species that rely on them.

Integrated characterisation of Daubaylia burnupiae n. sp. (Nematoda: Daubayliidae) from a freshwater gastropod in South Africa, with comments on the biology of Daubaylia spp

Gastropod-nematode associations are underreported worldwide. In the present study, juvenile and adult nematodes were found in the freshwater gastropod Burnupia stenochorias (Melvill & Ponsonby, 1903), from the Vaal River, South Africa. The nematodes were confirmed to belong to the genus Daubaylia chitwood & chitwood, 1934 (Daubayliidae). This is the first report of Daubaylia from a snail belonging to the family Burnupiidae, and the first report of this nematode taxon in southern Africa. Like D. pearsoni and D. malayanum from Australia and Malaysia respectively, adult females of the current species possess multiple well-developed eggs in the uteri, with larvae developing in utero. Morphological and molecular characteristics showed that the nematodes are distinct from all the described species of Daubaylia. Thus, they are considered a new species, Daubaylia burnupiae n. sp. The species differs from its congeners based on spicule shape, the short tail of the male, an anal cuticular knob-like protrusion on the female, and oesophagi with short isthmi and short glandular basal bulbs in both sexes. Three club-shaped pharyngeal lobes, extending slightly above the surface of the cephalic lips in both sexes and a pre-cloacal median papilla on the male were described using scanning electron microscopy, the first of such observations for the daubayliids. Genetic analyses showed that partial sequences of D. burnupiae n. sp. differed from species for which genetic data are available, by at least 26 and 9 base pair differences for 28S and 18S rDNA, respectively. Our results show that low prevalence and abundance of nematodes in the snails, corresponded with increased pollution in the river. We suspect that exposure to pollutants reduces the viability of the infective gravid female nematode during transmission. Therefore, the nematode is a potential bioindicator for aquatic pollution.

Evaluation of terrestrial gastropods as possible intermediate hosts of Gurltia paralysans in southern Chile

Gurltia paralysans is the causal agent of gurltiosis in domestic cats in South America. Although the life cycle of G. paralysans is unknown, it is thought that gastropods could act as intermediate hosts (IHs), as is the case for several nematodes in the Angiostrongylidae family. The aim of this study was to search for G. paralysans larvae in terrestrial gastropods and determine their role in the life cycle of this nematode species. Terrestrial gastropod samples (n=835) were collected in Punucapa, Valdivia, southern Chile, where cases of gurltiosis had been reported before. The samples included species from the families Arionidae, Limacidae, Helicidae and Milacidae. All gastropods were subjected to enzymatic digestion to isolate G. paralysans larvae. Ten percent of the gastropod samples were analyzed using seminested PCR targeting the 28S rRNA gene, while 2.6% were analyzed by histopathological examination. The results indicated the absence of G. paralysans when using any of the three methods. In conclusion, further studies are needed to evaluate specific species of aquatic or native gastropods acting as possible IHs (in this geographic location).

Invasion and Dispersal of Biomphalaria Species: Increased Vigilance Needed to Prevent the Introduction and Spread of Schistosomiasis

Biological invasion is a matter of great concern from both public health and biodiversity perspectives. Some invasive snail species may trigger disease emergence by acting as intermediate hosts. The geographic distribution of Schistosoma mansoni depends on the presence of susceptible species of Biomphalaria freshwater snails that support the parasite's transformation into infective stages. Biomphalaria spp. have shown strong local and global dispersal capacities that may increase due to the global warming phenomenon and increases in the development of agricultural and water projects. Should intermediate hosts become established in new areas then this will create potential transmission foci. Examples of snail invasions that have had an impact on schistosomiasis transmission include the introduction of Biomphalaria tenagophila to Congo and B. glabrata to Egypt. The current spread of B. straminea in China is causing concern and needs to be monitored closely. An understanding of the mode of invasion and distribution of these snails as well as their experimental susceptibility to S. mansoni will predict the potential spread of schistosomiasis. Here we review the invasion patterns of Biomphalaria snails and factors that control their distribution and the impact that invasion may have on intestinal schistosomiasis transmission. In addition, we propose some possible surveillance responses for optimum control strategies and interventions. Whenever possible, swift action should be taken to contain any new occurrence of these intermediate snail hosts.

Alternative pathways in Angiostrongylus cantonensis (Metastrongyloidea: Angiostrongylidae) transmission

In order to elucidate the infection pathways of third stage larvae (L3) of Angiostrongylus cantonensis, we performed experiments to assess: (i) the shedding of L3 from two species of experimental veronicellid slugs drowned in water and the ratio of emerged larvae, (ii) the transmission of viable L3 from drowned terrestrial gastropods to aquatic snails, and (iii) the transmission of viable L3 between terrestrial snails. Molluscs were experimentally infected by first stage larvae (L1) of A. cantonensis. Significantly more L3 larvae were released from Veronicella cubensis than from Veronicella sloanei. Numerous L3 were observed in the muscular foot, and also in the connective tissue between internal organs. Experimental exposure of P. maculata to L3 of A. cantonensis liberated from other gastropod species led to their infection and the infectivity of larvae after intermediesis was demonstrated by infection of laboratory rats (Rattus norvegicus). The transmission of L3 was observed in three out of four experiment replications and L3 were retrieved from 6 out of 24 Subulina octona snails. The infected synanthropic molluscs represent a key component in the epidemiology of human infections by A. cantonensis. Escape of L3 larvae from bodies of dead snails or slugs and their ability to infect further gastropod hosts (intermediesis) represents a public health risk. Thus, control of molluscs living in peri-domestic environment is an essential part of prevention of human infections.

The Mediterranean Mussel (Mytilus galloprovincialis) as Intermediate Host for the Anisakid Sulcascaris sulcata (Nematoda), a Pathogen Parasite of the Mediterranean Loggerhead Turtle (Caretta caretta)

Sulcascaris sulcata (Anisakidae), a pathogenic nematode of sea turtles, may cause ulcerous gastritis with different degrees of severity. Previous studies demonstrated a high prevalence of infection in the Mediterranean loggerhead turtle (Caretta caretta), although no data on the potential intermediate hosts of this nematode has been published thus far from the Mediterranean basin. Here, using molecular analyses, we demonstrated that the cross sections of nematode larvae observed histologically in Mediterranean mussels (Mytilus galloprovincialis) collected from a farm along the Tyrrhenian coast of southern Italy belong to S. sulcata. The BLAST analysis of sequences at the ITS2 region of rDNA and mtDNA cox2 gene loci here obtained from samples of two Mediterranean mussels containing nematode larvae showed 100% homology with those at the same gene loci from the adults of S. sulcata collected from the Mediterranean Sea and deposited in GenBank. To our knowledge, this study is the first to present data on a potential intermediate host of S. sulcata in the Mediterranean basin and to report a nematode parasite from the Mediterranean mussel.

Pathobiology and first report of larval nematodes (Ascaridomorpha sp.) infecting freshwater mussels (Villosa nebulosa, Unionidae), including an inventory of nematode infections in freshwater and marine bivalves

Little information is available on host-parasite relationships between bivalves and larval nematodes. Herein, we describe nematode larvae (likely stage 2) in the infraorder Ascaridomorpha infecting the foot, intestine, and mantle of a freshwater mussel (Alabama rainbow, Villosa nebulosa [Conrad, 1834]) and detail histopathological changes to infected tissues. A total of 43 live mussels from the South Fork of Terrapin Creek, Alabama, were collected between 2010 and 2014, with 14 sectioned for histopathology and 29 dissected. Of the 14 sectioned mussels, 5 appeared to be uninfected, and 7, 1, and 1 had histozoic infections observed in the foot and intestine, intestine only, and mantle edge and foot, respectively. Twenty-three of 29 (79%) of the mussels dissected were infected by live nematodes, and mean nematode abundance was 8.3 (CL = 5.23–13), with 2 mussels infected with >100 nematodes each. Thus, with a total of 32 of the 43 collected mussels observed with nematodes, overall infection prevalence was 74.4% (CL = 0.594–0.855). The 18S rDNA of this nematode was 99% similar to that of several ascaridids (species of Kathlaniidae Lane, 1914 and Quimperiidae Baylis, 1930) that mature in aquatic/semi-aquatic vertebrates; the recovered 18S phylogenetic tree indicated this nematode from V. nebulosa shares a recent common ancestor with Ichthyobronema hamulatum (Ascaridomorpha: Quimperiidae; GenBank Accession Number KY476351). Pathological changes to tissue associated with these infections comprised focal tissue damage, but a cellular response was not evident. The Alabama rainbow possibly represents an intermediate or paratenic host. Given these results, the nematode is likely not pathogenic under normal stream conditions; however, high intensity infections in the foot could inhibit pedal extension and retraction; which would have demonstrable health consequences to a freshwater mussel. Based on our review of the bivalve mollusc parasite literature, a collective biodiversity of 61 nematodes reportedly exhibit some degree of symbiosis (from commensal to parasitic) with 21 bivalves (28 nematode spp. from 17 marine bivalve spp.; 33 nematode spp. from 4 freshwater bivalve spp.); only four records exist of putatively parasitic nematodes from Unionida. The present study represents the first description of a nematode species that invades the tissues of a Unionidae species.

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Pathological changes comprised focal tissue damage but a cellular response to the parasites was not evident.

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Nematodes from infected tissues appear to be second stage larvae of Ascaridomorpha.

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The small subunit rDNA sequence of this nematode was 99% similar to species of Kathlaniidae and Quimperiidae.

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The nematode is likely not pathogenic under normal stream conditions.

Angiostrongylus cantonensis: a review of its distribution, molecular biology and clinical significance as a human pathogen

Angiostrongylus cantonensis is a metastrongyloid nematode found widely in the Asia-Pacific region, and the aetiological agent of angiostrongyliasis; a disease characterized by eosinophilic meningitis. Rattus rats are definitive hosts of A. cantonensis, while intermediate hosts include terrestrial and aquatic molluscs. Humans are dead-end hosts that usually become infected upon ingestion of infected molluscs. A presumptive diagnosis is often made based on clinical features, a history of mollusc consumption, eosinophilic pleocytosis in cerebral spinal fluid, and advanced imaging such as computed tomography. Serological tests are available for angiostrongyliasis, though many tests are still under development. While there is no treatment consensus, therapy often includes a combination of anthelmintics and corticosteroids. Angiostrongyliasis is relatively rare, but is often associated with morbidity and sometimes mortality. Recent reports suggest the parasites' range is increasing, leading to fatalities in regions previously considered Angiostrongylus-free, and sometimes, delayed diagnosis in newly invaded regions. Increased awareness of angiostrongyliasis would facilitate rapid diagnosis and improved clinical outcomes. This paper summarizes knowledge on the parasites' life cycle, clinical aspects and epidemiology. The molecular biology of Angiostrongylus spp. is also discussed. Attention is paid to the significance of angiostrongyliasis in Australia, given the recent severe cases reported from the Sydney region.

Gastropod-Borne Helminths: A Look at the Snail-Parasite Interplay

More than 300 million people suffer from a range of diseases caused by gastropod-borne helminths, predominantly flatworms and roundworms, whose life cycles are characterized by a diversified ecology and epidemiology. Despite the plethora of data on these parasites, very little is known of the fundamental biology of their gastropod intermediate hosts, or of the interactions occurring at the snail-helminth interface. In this article, we focus on schistosomes and metastrongylids of human and animal significance, and review current knowledge of snail-parasite interplay. Future efforts aimed at elucidating key elements of the biology and ecology of the snail intermediate hosts, together with an improved understanding of snail-parasite interactions, will aid to identify, plan, and develop new strategies for disease control focused on gastropod intermediate hosts.

The ecology, evolution, impacts and management of host-parasite interactions of marine molluscs

Molluscs are economically and ecologically important components of aquatic ecosystems. In addition to supporting valuable aquaculture and wild-harvest industries, their populations determine the structure of benthic communities, cycling of nutrients, serve as prey resources for higher trophic levels and, in some instances, stabilize shorelines and maintain water quality. This paper reviews existing knowledge of the ecology of host-parasite interactions involving marine molluscs, with a focus on gastropods and bivalves. It considers the ecological and evolutionary impacts of molluscan parasites on their hosts and vice versa, and on the communities and ecosystems in which they are a part, as well as disease management and its ecological impacts. An increasing number of case studies show that disease can have important effects on marine molluscs, their ecological interactions and ecosystem services, at spatial scales from centimeters to thousands of kilometers and timescales ranging from hours to years. In some instances the cascading indirect effects arising from parasitic infection of molluscs extend well beyond the temporal and spatial scales at which molluscs are affected by disease. In addition to the direct effects of molluscan disease, there can be large indirect impacts on marine environments resulting from strategies, such as introduction of non-native species and selective breeding for disease resistance, put in place to manage disease. Much of our understanding of impacts of molluscan diseases on the marine environment has been derived from just a handful of intensively studied marine parasite-host systems, namely gastropod-trematode, cockle-trematode, and oyster-protistan interactions. Understanding molluscan host-parasite dynamics is of growing importance because: (1) expanding aquaculture; (2) current and future climate change; (3) movement of non-native species; and (4) coastal development are modifying molluscan disease dynamics, ultimately leading to complex relationships between diseases and cultivated and natural molluscan populations. Further, in some instances the enhancement or restoration of valued ecosystem services may be contingent on management of molluscan disease. The application of newly emerging molecular tools and remote sensing techniques to the study of molluscan disease will be important in identifying how changes at varying spatial and temporal scales with global change are modifying host-parasite systems.

Diverse gastropod hosts of Angiostrongylus cantonensis, the rat lungworm, globally and with a focus on the Hawaiian Islands

Eosinophilic meningitis caused by the parasitic nematode Angiostrongylus cantonensis is an emerging infectious disease with recent outbreaks primarily in tropical and subtropical locations around the world, including Hawaii. Humans contract the disease primarily through ingestion of infected gastropods, the intermediate hosts of Angiostrongylus cantonensis. Effective prevention of the disease and control of the spread of the parasite require a thorough understanding of the parasite's hosts, including their distributions, as well as the human and environmental factors that contribute to transmission. The aim of this study was to screen a large cross section of gastropod species throughout the main Hawaiian Islands to determine which act as hosts of Angiostrongylus cantonensis and to assess the parasite loads in these species. Molecular screening of 7 native and 30 non-native gastropod species revealed the presence of the parasite in 16 species (2 native, 14 non-native). Four of the species tested are newly recorded hosts, two species introduced to Hawaii (Oxychilus alliarius, Cyclotropis sp.) and two native species (Philonesia sp., Tornatellides sp.). Those species testing positive were from a wide diversity of heterobranch taxa as well as two distantly related caenogastropod taxa. Review of the global literature showed that many gastropod species from 34 additional families can also act as hosts. There was a wide range of parasite loads among and within species, with an estimated maximum of 2.8 million larvae in one individual of Laevicaulis alte. This knowledge of the intermediate host range of Angiostrongylus cantonensis and the range of parasite loads will permit more focused efforts to detect, monitor and control the most important hosts, thereby improving disease prevention in Hawaii as well as globally.

Parasites in ungulates of Arctic North America and Greenland: a view of contemporary diversity, ecology, and impact in a world under change

Parasites play an important role in the structure and function of arctic ecosystems, systems that are currently experiencing an unprecedented rate of change due to various anthropogenic perturbations, including climate change. Ungulates such as muskoxen, caribou, moose and Dall's sheep are also important components of northern ecosystems and are a source of food and income, as well as a focus for maintenance of cultural traditions, for northerners. Parasites of ungulates can influence host health, population dynamics and the quality, quantity and safety of meat and other products of animal origin consumed by people. In this article, we provide a contemporary view of the diversity of nematode, cestode, trematode, protozoan and arthropod parasites of ungulates in arctic and subarctic North America and Greenland. We explore the intricate associations among host and parasite assemblages and identify key issues and gaps in knowledge that emerge in a regime of accelerating environmental transition.

The complete mitochondrial genome of the rodent intra-arterial nematodes Angiostrongylus cantonensis and Angiostrongylus costaricensis

The two rodent intra-arterial nematodes, Angiostrongylus cantonensis and Angiostrongylus costaricensis, can cause human ill-health. The present study aimed to characterize and compare the mitochondrial (mt) genomes of these two species, and clarify their phylogenetic relationship and the position in the phylum Nematoda. The complete mt genomes of A. cantonensis and A. costaricensis are 13,497 and 13,585 bp in length, respectively. Hence, they are the smallest in the class of Chromadorea characterized thus far. Like many nematode species in the class of Chromadorea, they encode 12 proteins, 22 transfer RNAs, and two ribosomal RNAs. All genes are located on the same strand. Nucleotide identity of the two mt genomes is 81.6%, ranging from 77.7% to 87.1% in individual gene pairs. Our mt genome-wide analysis identified three major gene arrangement patterns (II-1, II-2, and II-3) from 48 nematode mt genomes. Both patterns II-1 and II-2 are distinct from pattern II-3, which covers the Spirurida, supporting a closer relationship between Ascaridida and Strongylida rather than Spirurida. Thymine (T) was highly concentrated on coding strands in Chromadorea, but balanced between the two strands in Enoplea, probably due to the gene arrangement pattern. Interestingly, the gene arrangement pattern of mt genomes and phylogenetic analysis based on concatenated amino acids indicated a closer relationship between the order Ascaridida and Rhabditida rather than Spirurida as indicated in previous studies. These discrepancies call for new research, reassessing the position of the order of Ascaridida in the phylogenetic tree. Once consolidated, the findings are important for population genetic studies and target identification.

Effect of temperature on activity of third-stage larvae of Angiostrongylus vasorum

To evaluate the effect of temperature on the activity and mortality of the L3 of Angiostrongylus vasorum, 1,500 L3 were isolated from experimentally infected snails and distributed into five equal groups. Three groups were incubated at 37°C, 27°C, and 5°C. The remaining two groups were incubated at 27°C and 5°C for 10 days, at which time the temperature for the 27°C group was reduced to 5°C and the 5°C group increased to 27°C. Larva activity was observed daily and inactive larvae were removed. At 37°C, larvae survived up to 8 days. At 27°C, larvae were active until day 6. When subjected to a reduction in temperature from 27°C to 5°C beginning on day 10, the number of active larvae increased until day 13. Only on day 17 did the number of active larvae decline to zero. At 5°C, larvae remained active until day 15, surviving to 24 days. When temperature was increased from 5°C to 27°C beginning on day 10, larvae were found active until day 12 and maintained an intermediate level of activity to day 21. Survival of larvae was greater at lower temperatures, while high temperatures were associated with higher mortality.