Bolbophorus damnificus n. sp. (Digenea: Bolbophoridae) from the channel catfish Ictalurus punctatus and American white pelican Pelecanus erythrorhynchos in the USA based on life-cycle and molecular data

Enteroparasites from the American White Pelican (Pelecanus erythrorhynchos) in Toluca Valley, Mexico

We examined 82 fresh fecal samples from American White Pelican (Pelecanus erythrorhynchos) at San Pablo Autopan, Toluca Valley, central Mexico, December 2021 to April 2022. Eggs of Contracaecum spp. were observed in 100%, Capillaria spp. in two (2.5%), Strongyloides spp. in eight (9.7%), and coccidial oocysts in 12 (15%).

Description and Phylogenetic Relationships of Anhingatrema n. gen. (Digenea: Diplostomidae) with Two New Species from New World Anhingas (Aves: Anhingidae)

PURPOSE

The Diplostomidae is a globally distributed family of digeneans that parasitize a wide variety of tetrapod definitive hosts. Recent molecular phylogenetic studies have revealed unknown diplostomid diversity in avian hosts throughout the New World. Herein, we provide descriptions of a novel genus of diplostomids with two new species.

METHODS

Two species of diplostomids belonging to the new genus were collected from anhinga birds in Mississippi (USA) and Brazil. Partial nuclear 28S ribosomal and mitochondrial cox1 genes were sequenced. Ribosomal data were used for phylogenetic inference.

RESULTS

Both species of Anhingatrema n. gen. were positioned in a 100% supported, monophyletic clade in the phylogenetic tree. The molecular phylogenetic position and a combination of morphological features (e.g., presence of pseudosuckers, testes shape and orientation) supported erection of the new genus. Anhingatrema overstreeti n. sp. and Anhingatrema cararai n. sp. are morphologically similar, but differ in size of and ratios associated with pseudosuckers. The two species differ by 2% of 28S sequences and 13.8% of cox1 sequences. Comparison of DNA sequences revealed that Diplostomidae gen. sp. in GenBank (MZ314151) is conspecific with An. overstreeti n. sp.

CONCLUSION

Anhingatrema n. gen. is the sixth genus of diplostomids known from anhingas worldwide. Anhingatrema cararai n. sp. is the first diplostomid to be reported from anhingas in South America. Combined with previous studies, the molecular phylogenies revealed at least two host switches to anhingas from other birds during the evolutionary history of the Diplostomidae.

Metacercariae in the brain of Erythrinus cf. erythrinus (Characiformes: Erythrinidae) from Iguazú National Park (Argentina): do they belong to Dolichorchis lacombeensis (Digenea, Diplostomidae)?

In Argentina, the family Diplostomidae is composed of eight genera: Austrodiplostomum Szidat & Nani; Diplostomum von Nordmann; Dolichorchis Dubois; Hysteromorpha Lutz; Neodiplostomum Railliet; Posthodiplostomum Dubois; Sphincterodiplostomum Dubois; and Tylodelphys Diesing. During a parasitological survey of fishes from the Iguazú National Park we detected diplostomid metacercariae in the brain of Erythrinus cf. erythrinus. Fish were caught using crab traps, transported alive to the field laboratory, cold-anaesthetized and euthanized by cervical dissection. Some metacercariae were heat-killed in water and fixed in 10% formalin and others were preserved in alcohol 96% for DNA extraction. They were sequenced for the partial segment of the 28S rDNA, internal transcribed spacer (ITS) rDNA and cytochrome c oxidase subunit I (COI) mtDNA genes. Phylogenetic reconstruction was carried out using Bayesian inference and the proportion (p) of absolute nucleotide sites (p-distance) was obtained. In the 28S rDNA tree, the metacercaria sequenced grouped as Dolichorchis sp. The COI mtDNA p-distance between the metacercariae with Dolichorchis lacombeensis was 0.01. There is a small number of ITS sequences for the Diplostomidae family deposited in the GenBank. The oral sucker, ventral sucker, holdfast organ and the distance between oral and ventral suckers are larger in the adult compared with the metacercariae. Additionally, hind-body length and width are larger in the adult due to the development of the genital complex. Further studies using an integrative approach will help confirm the affiliation of other species to the genus Dolichorchis.

CERCARIAL LONGEVITY AND INFECTIVITY OF BOLBOPHORUS DAMNIFICUS, WITH NOTES ON METACERCARIAL PERSISTENCE AND SITE SPECIFICITY IN CHANNEL AND HYBRID CATFISH

Advances in hybridization practices in U.S. catfish aquaculture have led to increased production of channel (Ictalurus punctatus) × blue catfish (Ictalurus furcatus) hybrids to capitalize on their more favorable production characteristics. However, the effects of typical channel catfish pathogens on hybrids are not well understood, including the digenean Bolbophorus damnificus, which has caused significant losses in channel catfish production. Three experiments were conducted to assess the longevity and site specificity of 2 life stages of B. damnificus impacting catfish production. The first experiment investigated the cercarial longevity and infectivity of B. damnificus over time. Channel catfish were individually challenged with 100 cercariae/fish with cercariae aged in 12-hr time intervals over 5 days (n = 5 fish/time point), with metacercarial cysts excised and enumerated 14 days postchallenge. There was a decrease in cercaria viability and encysted metacercariae over the first 36 hr, with the 12-hr time point having both the greatest cercaria survival and the highest number of metacercariae in exposed fish. The second experiment investigated the longevity of metacercariae within both channel and hybrid catfish. Fish (n = 30) were exposed to 2 treatments (75 or 150 cercariae/fish), and 2 fish from each treatment were sampled every 3 mo for 13 mo. Live metacercariae, based on motility observed after excystment, were found in both species up to 13 mo postchallenge, indicating the metacercariae of B. damnificus can persist throughout an entire growing season in both channel and hybrid catfish. The third experiment investigated the site specificity of metacercariae within both channel and hybrid catfish. Fish (n = 60/species) were challenged with 300 cercariae/fish and 9 fish/species were sampled after 90 days. Metacercariae were excised and enumerated from the anterior midsection (head and body), posterior midsection (trunk/caudal peduncle), ventral (belly), and caudal fin (tail) sections of each fish. Overall, the trunk/caudal peduncle had a 2-fold increase in the number of metacercariae excised, and although not significantly higher, results indicate this region should be the focal point of pondside assessment for the presence of B. damnificus because of ease of detection of encysted metacercariae.

Unravelling the diversity of the Crassiphialinae (Digenea: Diplostomidae) with molecular phylogeny and descriptions of five new species

Crassiphialinae Sudarikov, 1960 is a large subfamily of the Diplostomidae Poirier, 1886 with a complex taxonomic history. It includes a diversity of species parasitic in the intestines of avian and mammalian definitive hosts worldwide. Posthodiplostomum Dubois, 1936 is a large and broadly distributed crassiphialine genus notorious for its association with diseases in their fish second intermediate hosts. In this study, we generated partial 28S rDNA and cytochrome c oxidase subunit 1 (cox1) mtDNA gene sequences of digeneans belonging to seven crassiphialine genera. The 28S sequences were used to study the interrelationships among crassiphialines and their placement among other major diplostomoidean lineages. Our molecular phylogenetic analysis and review of morphology does not support subfamilies currently recognized in the Diplostomidae; therefore, we abandon the current subfamily system of the Diplostomidae. Molecular phylogenetic analyses suggest the synonymy of Posthodiplostomum, Ornithodiplostomum Dubois, 1936 and Mesoophorodiplostomum Dubois, 1936; morphological study of our well-fixed adult specimens and review of literature revealed lack of consistent differences among the three genera. Thus, we synonymize Ornithodiplostomum and Mesoophorodiplostomum with Posthodiplostomum. Our phylogenetic analyses suggest an Old World origin of Posthodiplostomum followed by multiple dispersal events among biogeographic realms. Furthermore, our analyses indicate that the ancestors of these digeneans likely parasitized ardeid definitive hosts. Four new species of Posthodiplostomum collected from birds in the New World as well as one new species of Posthodiplostomoides Williams, 1969 from Uganda are described.

Natural Concurrent Infections with Black Spot Disease and Multiple Bacteriosis in Farmed Nile Tilapia in Central Kenya

Nile tilapia (Oreochromis niloticus) is the most cultured and available fish for Kenyan consumers, and therefore, any tilapine disease deprives them the valuable source of protein. Nile tilapia farm was diagnosed with severe concurrent black spot disease and multiple bacteriosis using gross lesions and parasitological, histopathology, and standard bacteriological procedures. A total of 25 fish were sampled and inspected, and all of them had raised, macroscopic 1 mm-sized black spot lesions. The mean intensity of black spots per fish was 728 with an abundance of 2–1740 metacercariae cysts per fish. A high intensity of black spot infestation was observed in the fins (43.9%), skin and underlying muscles (18.3%), and gills (18%). In addition, histopathological data confirmed presence of a metacercaria of Neascus spp. as the aetiological agent of black spot disease. Furthermore, a thick fibrous capsule around the metacercaria, black pigment melanomacrophages, and moderate muscle atrophy were observed. The most prevalent bacteria isolated were Aeromonas, Enterobacter cloacae, Klebsiella pneumoniae, and Micrococcus luteus. Physicochemical parameters of pond water were temperature (28.2°C), dissolved oxygen (4.2 mgl⁻¹), pH (8.5), ammonia free nitrogen (15.8 mgl⁻¹), alkalinity (112 mgl⁻¹), hardness (68 mgl⁻¹), nitrites (0.058 mgl⁻¹), nitrates (58 mgl⁻¹), and phosphates (0.046 mgl⁻¹). However, the levels of nitrates, nitrites, alkalinity, and ammonia free nitrogen exceeded the recommended limits. In conclusion, these findings suggest that coinfections by these organisms coupled by water quality-related stress can be associated with low-grade mortality observed in postfingerling tilapia as well as reduced growth. The authors recommended immediate destocking, thorough disinfection, and control of piscivorous birds. Moreover, attention ought to be geared towards prevention of parasitic infestations in fish so as to minimize fish deaths related to secondary bacteriosis. Further experimental studies should be carried out to elucidate the relationship of these pathogens.

Diversity of digenean trematode larvae in snails from Lake Victoria, Kenya: First reports and bioindicative aspects

This study investigated the occurrence of digenean trematode larvae in snails from the Kenyan part of Lake Victoria. The survey included caenogastropod snails that have received less focus in parasitological studies in Africa: their trematodes are largely unknown. Out of 1145 snail specimens, 149 (13.0%) were infected with Digenea. The highest prevalence (P) was recorded in Melanoides tuberculata (64.5%), followed by Pila ovata (15.4%), Radix natalensis (9.5%), Bulinus ugandae (9.1%), Bellamya unicolor (8.9%), Biomphalaria pfeifferi (7.3%) and Biomphalaria sudanica (4.4%). Morphological and molecular analyses revealed 17 digenean species. Contrary to reports of low diversity of Digenea in caenogastropods, P. ovata harboured 8 species - at least twice as many as in each of the pulmonates. The following taxa are reported for the first time in the Lake Victoria region: Haplorchis pumilio, Thapariella prudhoei, Nudacotyle sp., Renicola sp. and Bolbophorus sp. An unknown cercaria belonging to the genus Haematoloechus is reported from P. ovata: a xiphidiocercaria possessing a long sword-shaped stylet (47-71 µm) which does not match any available literature records. From this study, H. pumilio from M. tuberculata (P = 69.4%), Fasciola gigantica from R. natalensis (P = 1.9%) and Bolbophorus sp. from Bu. ugandae (P = 4.6%) are species of veterinary or medical importance. Snails from the study site with little direct anthropogenic influence had the highest prevalence and diversity of digenean larvae, indicating that environmental conditions influence trematode occurrence.

Odds ratios and hurdle models: a long-term analysis of parasite infection patterns in endangered young-of-the-year suckers from Upper Klamath Lake, Oregon, USA

We used odds ratios and a hurdle model to analyze parasite co-infections over 25 years on >20,000 young-of-the year of endangered Shortnose and Lost River Suckers. Host ecologies differed as did parasite infections. Shortnose Suckers were more likely to be caught inshore and 3-5 times more likely to have Bolbophorus spp. and Contracaecum sp. infections, and Lost River Suckers were more likely to be caught offshore and approximately three times more likely to have Lernaea cyprinacea infections. An observed peak shift seems likely to be due to a lower host size limit for Bolbophorus spp. (13.6 mm) compared with L. cyprinacea (23.4 mm). The large data set allowed us to generate strong hypotheses: (i) that a major marsh restoration project had unintended consequences that resulted in an increase in infections; (ii) that co-infection with Bolbophorus spp. increased the odds of infection by L. cyprinacea and Contracaecum sp.; (iii) that significant declines in the odds of infection over approximately 25 days were due to parasite-induced host mortality; (iv) that the fish's small size relative to L. cyprinacea and Contracaecum sp. might be directly lethal; (v) that the absence of L. cyprinacea infections in the early 1990s was associated with good year-class production of the suckers; and (vi) that parasites might increase the odds of vagrancy from the nursery ground.

Histopathology and external examination of heavily parasitized Lost River Sucker Deltistes luxatus (Cope 1879) and Shortnose Sucker Chasmistes brevirostris (Cope 1879) from Upper Klamath Lake, Oregon

Shortnose Sucker (Chasimistes brevirostris) and Lost River Sucker (Deltistes luxatus) are endemic to the Upper Klamath Basin of Southern Oregon and Northern California, and their populations are in decline. We used histopathology and external examination of 140 and external examination only of 310 underyearling suckers collected in 2013, 2015 and 2016 to document pathological changes, particularly those relating to parasites. The most severe infection was caused by a Contracaecum sp., infecting the atrium of 8%-33% of Shortnose Suckers. The most prevalent infections were caused by Bolbophorus sp. metacercariae in the muscle of Shortnose Suckers (21%-63%) and Lernaea cyprinacea in the skin and muscle of Lost River Suckers (30%-81%). Histology detected Bolbophorus in only 5% of cases where it was not seen externally. Three myxozoans were observed; a Parvicapsula sp. in the renal tubules (10%), a Myxobolus sp. in the intestinal mucosa (2%) and an unusual multicellular, presporogonic myxozoan in the intestinal lumen of one sucker. Severe gill epithelial hyperplasia was observed in several fish collected in 2016. Trichodinids and Ichthyobodo sp. were observed on some of the gills, but absent in many of the fish with severe lesions. A histiocytic sarcoma was observed in sucker.

Validity of the Diplostomoidea and Diplostomida (Digenea, Platyhelminthes) upheld in phylogenomic analysis

Higher systematics within the Digenea, Carus 1863 have been relatively stable since a phylogenetic analysis of partial nuclear ribosomal markers (rDNA) led to the erection of the Diplostomida Olson, Cribb, Tkach, Bray, and Littlewood, 2003. However, recent mitochondrial (mt) genome phylogenies suggest this order might be paraphyletic. These analyses show members of two diplostomidan superfamilies are more closely related to the Plagiorchiida La Rue, 1957 than to other members of the Diplostomida. A recent phylogeny based on partial cytochrome c oxidase I also indicates one of the groups implicated, the Diplostomoidea Poirier, 1886, is non-monophyletic. To determine if these results were robust to additional taxon sampling, we analyzed mt genomes from seven diplostomoids in three families. To choose between phylogenetic alternatives based on mt genomes and the prior rDNA-based topology, we analyzed hundreds of ultra-conserved genomic elements assembled from shotgun sequencing. The Diplostomida was paraphyletic in the mt genome phylogeny but supported in the ultra-conserved genomic element phylogeny. We speculate this mitonuclear discordance is related to ancient, rapid radiation in the Digenea. Both ultra-conserved genomic elements and mt genomes support the monophyly of the Diplostomoidea and show congruent relationships within it. The Cyathocotylidae Mühling, 1898 are early diverging descendants of a paraphyletic clade of Diplostomidae Poirier, 1886, in which are nested members of the Strigeidae Railliet, 1919; the results support prior suggestions that the Crassiphialinae Sudarikov, 1960 will rise to the family level. Morphological traits of diplostomoid metacercariae appear to be more useful for differentiating clades than those of adults. We describe a new species of Cotylurus Szidat, 1928, resurrect a species of Hysteromorpha Lutz, 1931, and find support for a species of Alaria Schrank, 1788 of contested validity. Complete rDNA operons from seven diplostomoid species are provided as a resource for future studies.

Life History, Systematics and Evolution of the Diplostomoidea Poirier, 1886: Progress, Promises and Challenges Emerging From Molecular Studies

Members of the Diplostomoidea mature in amniotes and employ vertebrates, annelids and molluscs as second intermediate hosts. Diplostomoid life cycles generally follow a three-host pattern typical of digeneans, but novelties have arisen in some species, including obligate four-host life cycles, vertical transmission, and intracellular parasitism. In this review, we summarize the basic biology of diplostomoids with reference to molecular studies, and present challenges, gaps and areas where molecular data could address long-standing questions. Our analysis of published studies revealed that most molecular surveys find more diplostomoid species than expected, but this tendency is influenced by how much effort goes into examining specimens morphologically and the number of sequenced worms. To date, molecular work has concentrated disproportionately on intraspecific or species-level diversity of larval stages in the Diplostomidae in temperate northern regions. Although the higher taxonomy of the superfamily is recognized to be in need of revision, little molecular work has been conducted at this level. Our phylogenetic analysis indicates several families and subfamilies require reconsideration, and that larval morphotypes are more reflective of evolutionary relationships than definitive hosts. The host associations of adult diplostomoids result from host-switching processes, whereas molecular surveys indicate that larval diplostomoid metacercariae have narrow ranges of second intermediate hosts, consistent with coevolution. Molecular data are often used to link diplostomoid developmental stages, and we provide data from adult Neodiplostomum and Mesoophorodiplostomum that correct earlier misidentifications of their larval stages and propose alternatives to collecting definitive hosts.

Trematode diversity in freshwater fishes of the Globe II: 'New World'

We provide a summary overview of the diversity of trematode parasites in freshwater fishes of the 'New World', i.e. the Americas, with emphasis on adult forms. The trematode fauna of three regions, South America, Middle America, and USA and Canada (North America north of Mexico), are considered separately. In total, 462 trematode species have been reported as adults from the Americas. The proportion of host species examined for parasites varies widely across the Americas, from a high of 45% in the Mexican region of Middle America to less than 5% in South America. North and South America share no adult species, and one exclusively freshwater genus, Creptotrema Travassos, Artigas & Pereira, 1928 in the Allocreadiidae Looss, 1902 is the most widely distributed. Metacercariae of strigeiforms maturing in fish-eating birds (e.g. species of the Diplostomidae Poirier, 1886) are common and widely distributed. The review also highlights the paucity of known life-cycles. The foreseeable future of diversity studies belongs to integrative approaches and the application of molecular ecological methods. While opportunistic sampling will remain important in describing and cataloguing the trematode fauna, a better understanding of trematode diversity and biology will also depend on strategic sampling throughout the Americas.

Fossils of parasites: what can the fossil record tell us about the evolution of parasitism?

Parasites are common in many ecosystems, yet because of their nature, they do not fossilise readily and are very rare in the geological record. This makes it challenging to study the evolutionary transition that led to the evolution of parasitism in different taxa. Most studies on the evolution of parasites are based on phylogenies of extant species that were constructed based on morphological and molecular data, but they give us an incomplete picture and offer little information on many important details of parasite-host interactions. The lack of fossil parasites also means we know very little about the roles that parasites played in ecosystems of the past even though it is known that parasites have significant influences on many ecosystems. The goal of this review is to bring attention to known fossils of parasites and parasitism, and provide a conceptual framework for how research on fossil parasites can develop in the future. Despite their rarity, there are some fossil parasites which have been described from different geological eras. These fossils include the free-living stage of parasites, parasites which became fossilised with their hosts, parasite eggs and propagules in coprolites, and traces of pathology inflicted by parasites on the host's body. Judging from the fossil record, while there were some parasite-host relationships which no longer exist in the present day, many parasite taxa which are known from the fossil record seem to have remained relatively unchanged in their general morphology and their patterns of host association over tens or even hundreds of millions of years. It also appears that major evolutionary and ecological transitions throughout the history of life on Earth coincided with the appearance of certain parasite taxa, as the appearance of new host groups also provided new niches for potential parasites. As such, fossil parasites can provide additional data regarding the ecology of their extinct hosts, since many parasites have specific life cycles and transmission modes which reflect certain aspects of the host's ecology. The study of fossil parasites can be conducted using existing techniques in palaeontology and palaeoecology, and microscopic examination of potential material such as coprolites may uncover more fossil evidence of parasitism. However, I also urge caution when interpreting fossils as examples of parasites or parasitism-induced traces. I point out a number of cases where parasitism has been spuriously attributed to some fossil specimens which, upon re-examination, display traits which are just as (if not more) likely to be found in free-living taxa. The study of parasite fossils can provide a more complete picture of the ecosystems and evolution of life throughout Earth's history.

Chronic pathology and longevity of Drepanocephalus spathans infections in juvenile Channel Catfish

Drepanocephalus spathans (Digenea: Echinostomatidae) is a common parasite of the double-crested cormorant Phalacrocorax auritus. The cercariae of D. spathans have been shown infective to juvenile Channel Catfish Ictalurus punctatus. The developing metacercariae concentrate in the cranial regions, often occluding blood vessels at the base of the branchial arch, occasionally resulting in death. The purpose of this study was to determine how long metacercariae of D. spathans persist in experimentally challenged Channel Catfish. Two separate infectivity trials were conducted. In both trials, metacercariae persisted at least 49 d postinfection, although prevalence and intensity of infection decreased over time. In the first trial, juvenile catfish (1-3 g) were exposed over three consecutive days to 100, 100, and 80 cercariae/fish/d, respectively. Fish were sampled 7 d after the final exposure, and metacercariae were observed in 83.3% (five of six) of challenged fish. At 21 d postexposure, metacercariae were present in only 50% of exposed fish (three of six). No metacercaria were observed in fish sampled at 35 d, however, metacercariae were present in one of six (16.7%) fish sampled 49 and 70 d postexposure, respectively. A second challenge consisted of a 24-h pooled exposure of 500 cercariae per fish. Again, metacercariae were present in most (six of seven; 85.7%) fish at 7 d postexposure. At 21 d postexposure, metacercariae were only evident in one of seven (14.3%) sampled fish. No metacercariae were present in any fish at 35 d postchallenge, yet one of seven (14.3%) fish was positive at 49 d postchallenge. The second study was terminated at 63 d postchallenge, as all fish sampled (n = 14) were negative for metacercariae. These data suggest that cercariae of D. spathans are infective to juvenile Channel Catfish, although the infection appears short lived as metacercariae rarely persisted longer than 2 months.

Comparative susceptibility of Channel Catfish, Blue Catfish, and their hybrid cross to experimental challenge with Bolbophorus damnificus (Digenea: Bolbophoridae) cercariae

The digenetic trematode Bolbophorus damnificus has been implicated in significant losses in catfish aquaculture since the late 1990s. The complex life cycle sequentially involves the American white pelican Pelecanus erythrorhynchos, the marsh rams horn snail Planorbella trivolvis, and Channel Catfish Ictalurus punctatus. Research supports anecdotal reports from the industry, suggesting that the hybrid of Channel Catfish×Blue Catfish I. furcatus is less susceptible to disease agents that have been historically prohibitive to Channel Catfish production, namely the gram-negative bacteria Edwardsiella ictaluri and Flavobacterium columnare, as well as the myxozoan parasite Henneguya ictaluri. This current research compared the susceptibility of Channel Catfish, Blue Catfish, and their hybrid cross to an experimental challenge by B. damnificus. Fish were exposed to 0, 100, 200, and 400 B. damnificus cercariae per fish, and the numbers of metacercariae per fish were determined 14 d postchallenge. Metacercariae were recovered from all challenged fish. There were no significant differences among fish groups challenged with the same dose, suggesting Channel and Blue Catfish and their hybrid are comparably susceptible to B. damnificus infection. As such, it is recommended that producers raising hybrid catfish remain diligent in controlling populations of the snail intermediate host to prevent production losses attributed to B. damnificus, especially when loafing pelicans have been observed at the aquaculture operation.

Longevity of Bolbophorus damnificus infections in channel catfish

The digenean Bolbophorus damnificus infects commercial channel catfish Ictalurus punctatus, causing mortality, lower feed consumption, and reduced growth in surviving fish. The purpose of this study was to determine the length of time for which B. damnificus prodiplostomulum metacercariae (juvenile trematode stage that infects fish) would remain viable (parasite appearing to be intact or exhibiting movement) in channel catfish. Fish (n = 210) were infected with molecularly confirmed B. damnificus cercariae harvested from naturally infected marsh rams-horn snails Planorbella trivolvis. During the first sampling (at 20 d postinfection), 8.3 +/- 3.6 metacercariae/fish (mean +/- SD) were found in the host muscle and visceral organs. The channel catfish were then acclimated to a water temperature of either 18 degrees C or 28 degrees C. After 11 months, 6.8 +/- 3.5 and 5.9 +/- 3.0 metacercariae/fish were found in groups held at 18 degrees C and 28 degrees C, respectively. The mean number of parasites per fish did not significantly differ between fish held at the two temperatures and did not significantly decline over time at either temperature. Fish examined from 13 to 30 months postinfection all contained viable metacercariae that were morphologically and molecularly identified as B. damnificus. At 18 months, 12 metacercariae (of which 11 were intact and 10 displayed movement) were found in the one fish sampled; at 30 months, the last fish sampled contained three intact metacercariae (one displayed slight movement). Our results indicate that B. damnificus metacercariae can remain viable in channel catfish for at least an 18-30-month production cycle during which they have the potential to affect fish growth; in addition, infected fish may serve as intermediate hosts for these metacercariae for at least 2.5 years postinfection.

Linking larvae and adults of Apharyngostrigea cornu, Hysteromorpha triloba, and Alaria mustelae (Diplostomoidea: Digenea) using molecular data

Because the taxonomy of trematodes is based on adults, the larval stages of most digeneans cannot be identified to species based on morphology alone. Molecular data provide a means of linking larval stages to known adults. We obtained sequences from the barcode region of cytochrome oxidase I (CO1) from adult and larval parasites of fish, frogs, birds, and mammals across North America. Sequences from adult Apharyngostrigea cornu, Hysteromorpha triloba, and Alaria mustelae (Diplostomoidea: Digenea) from definitive hosts matched those of meta- and mesocercariae from fish and frogs. These data provided new information on the distributions of all 3 parasite species. Metacercariae of A. cornu, which have not been previously reported in North American hosts, were found in Notemigonus crysoleucas, Pimephales notatus, and Catostomus commersonii in the St. Lawrence River. Metacercariae of H. triloba are reported in Canadian waters and in N. hudsonius for the first time. Alaria mustelae is reported for the first time in frogs from Quebec, Canada, and an additional species of Alaria was detected in California. Sequences of internal transcribed spacer rDNA from a subset of specimens support the same species boundaries indicated by CO1 divergence. There was little divergence in CO1 sequences from an unidentified diplostomid species sampled at a large spatial scale.

A duplex real-time polymerase chain reaction assay for differentiation between Bolbophorus damnificus and Bolbophorus type II species cercariae

A duplex quantitative real-time polymerase chain reaction (qPCR) assay was developed to differentiate between Bolbophorus damnificus and Bolbophorus type II species cercariae. Both trematode species are prevalent throughout the commercial catfish industry, as both infect the ram's horn snail, Planorbella trivolvis, which is commonly found in catfish ponds. Identification of cercaria to species is important in catfish disease challenge experiments, as only B. damnificus has been shown to have negative impacts on channel catfish. Oligonucleotide primers and fluorescence resonance energy transfer hydrolysis probes were designed to amplify the 18S small subunit ribosomal DNA gene of each species. The quantification cycle indicative of the number of cercariae in the sample prep was determined, and standard curves correlating to cercaria numbers were established. For both species, the assay was found to be highly repeatable and reproducible, with a linear dynamic range covering 7 orders of magnitude. The sensitivity limit of the assay was approximately 1/256th of a cercaria, regardless of species, and there was no remarkable interference between the 2 assays when run simultaneously within the same reaction. In a field study, identification of cercaria by the duplex real-time qPCR assay was in complete agreement with previously established end-point PCR protocols, demonstrating the assay to be a more rapid, quantifiable means of parasite identification.

A novel Henneguya species from channel catfish described by morphological, histological, and molecular characterization

Channel catfish Ictalurus punctatus from a commercial farming operation in the Mississippi Delta were submitted for examination for the presence of infection by the trematode Bolbophorus damnificus. The fish were instead found to possess skin nodules suggestive of Henneguya pellis, a species previously described in the blue catfish I. furcatus. Despite the dermal location and distribution of lesions, morphological characteristics of the myxospores were inconsistent with H. pellis. Spores possessed a lanceolate spore body 15.4 +/- 1.5 microm (mean +/- SD; range = 12.2-19.3 microm) in length and 5.5 +/- 0.6 microm (range = 4.5-6.8 microm) in width in valvular view, and 4.7 +/- 0.2 microm (range = 4.2-5.0 microm) in width in sutural view. Polar capsules were pyriform and unequal in both length and width and contained polar filaments with six coils. Polar capsules measured 6.1 +/- 0.8 microm (range = 4.0-7.9 microm) long and 1.7 +/- 0.3 microm (range = 1.0-2.2 microm) wide. The caudal appendages were 50.5 +/- 8.3 microm (range = 34.8-71.4 micorm) long and the total length of the spore was 65.9 +/- 8.6 microm (range = 48.2-90.0 microm). The "blister like" plasmodia were round or ovoid, up to 2 mm in diameter, and randomly distributed throughout the epidermis of the fish. Histologically, plasmodia were confined to the dermis and elicited no inflammatory reaction from the fish. A blast search of the 18S small subunit rDNA sequence obtained by polymerase chain reaction amplification resulted in no identical sequence matches but indicated a close relationship to H. gurlei, H. ictaluri, and H. exilis. The unique host record, spore morphology, and novel genetic sequence derived from this isolate lead us to propose this isolate as a novel species, H. sutherlandi.

Morphologic, Pathologic, and Genetic Investigations of Bolbophorus Species Affecting Cultured Channel Catfish in the Mississippi Delta

Trematodes belonging to the genus Bolbophorus have recently been reported as the cause of substantial morbidity and mortality in cultured channel catfish Ictalurus punctatus in Mississippi and Louisiana. Previous investigators identified only a single species, B. confusus. In this investigation, genetic techniques were used to identify all stages of the parasite in all of its hosts. The 18s rRNA genes from specimens collected in Mississippi were sequenced and compared; this analysis revealed that there are two distinct species, B. damnificus (previously identified as B. confusus) and another, undescribed species. (Phylogenetic analysis indicated that a third species, B. levantinus, is also closely related to the Mississippi species.) Species-specific polymerase chain reaction assays capable of identifying and differentiating between these two parasites were developed. Both species were found to infect the first intermediate host (the ram's horn snail Planorbella trivolvis) in commercial channel catfish ponds, but only B. damnificus was recovered from the fish themselves. The new, unidentified Bolbophorus species was determined to be highly pathogenic to a number of fish species. The contribution of B. damnificus to disease in cultured channel catfish remains undetermined. Future investigations of these parasites must now take into account the presence of two distinct species.

A review of Polylekithum Arnold, 1934 and its familial affinities using morphological and molecular data, with description of Polylekithum catahoulensis sp. nov

The type material of Polylekithum ictaluri, P. halli, and Maculifer chandleri was examined from the United States National Parasite Museum, and we determined that the material was conspecific, making P. halli and M. chandleri junior subjective synonyms of P. ictaluri. Polylekithum catahoulensis sp. nov. was described from material collected from catfishes at the Catahoula Wildlife Refuge, LaSalle Parish, Louisiana, USA, and compared with P. ictaluri collected from catfishes in Reelfoot Lake, Obion County, Tennessee, USA, and the Pearl River, Hancock County, Mississippi, USA. Polylekithum catahoulensis had smaller eggs (77–88 μm long by 51–63 μm wide vs. 94–108 μm by 52–76 μm) and a longer forebody (35–41% of overall body length vs. 29–34%). Comparison of more than 2,400 bp long fragments of nuclear ribosomal DNA (complete ITS and partial 28S regions) strongly supported the status of P. catahoulensis as a new species. Molecular phylogenetic analysis of 28S rDNA gene sequences from Polylekithum as well as representative species from Allocreadiidae, Atractotrematidae, Brachycoeliidae, Callodistomidae, Dicrocoeliidae, Encyclometridae, Gorgoderidae, Haploporidae, Opecoelidae, Plagiorchiidae, and Telorchiidae rooted by Monorchiidae and Lissorchiidae demonstrated that of the families tested, Polylekithum was most closely related to Encyclometridae as a gorgoderoid and not to Allocreadiidae as previously reported. Morphological features of three South American allocreadiids, Allocreadium patagonicum, P. percai, and A. pichi were inconsistent with generic diagnoses of Allocreadium and Polylekithum, so we suggested they belonged in a single unnamed genus similar to Creptotrema. Polylekithum catlai from India was assessed from the description and failed to conform to the generic diagnosis of Polylekithum. Morphology of Caudouterina suggested a close relationship with Polylekithum and not Allocreadiidae.

Advances and Trends in the Molecular Systematics of the Parasitic Platyhelminthes

The application of molecular systematics to the parasitic Platyhelminthes (Cestoda, Digenea and Monogenea) over the last decade has advanced our understanding of their interrelationships and evolution substantially. Here we review the current state of play and the early works that led to the molecular-based hypotheses that now predominate in the field; advances in their systematics, taxonomy, classification and phylogeny, as well as trends in species circumscription, molecular targets and analytical methods are discussed for each of the three major parasitic groups. A by-product of this effort has been an ever increasing number of parasitic flatworms characterized genetically, and the useful application of these data to the diagnosis of animal and human pathogens, and to the elucidation of life histories are presented. The final section considers future directions in the field, including taxon sampling, molecular targets of choice, and the current and future utility of mitochondrial and nuclear genomics in systematic study.

The Use and Implications of Ribosomal DNA Sequencing for the Discrimination of Digenean Species

In just over a decade, the use of molecular approaches for the recognition of parasites has become commonplace. For trematodes, the internal transcribed spacer region of ribosomal DNA (ITS rDNA) has become the default region of choice. Here, we review the findings of 63 studies that report ITS rDNA sequence data for about 155 digenean species from 19 families, and then review the levels of variation that have been reported and how the variation has been interpreted. Overall, complete ITS sequences (or ITS1 or ITS2 regions alone) usually distinguish trematode species clearly, including combinations for which morphology gives ambiguous results. Closely related species may have few base differences and in at least one convincing case the ITS2 sequences of two "good" species are identical. In some cases, the ITS1 region gives greater resolution than the ITS2 because of the presence of variable repeat units that are generally lacking in the ITS2. Intraspecific variation is usually low and frequently apparently absent. Information on geographical variation of digeneans is limited but at least some of the reported variation probably reflects the presence of multiple species. Despite the accepted dogma that concerted evolution makes the individual representative of the entire species, a significant number of studies have reported at least some intraspecific variation. The significance of such variation is difficult to assess a posteriori, but it seems likely that identification and sequencing errors account for some of it and failure to recognise separate species may also be significant. Some reported variation clearly requires further analysis. The use of a "yardstick" to determine when separate species should be recognised is flawed. Instead, we argue that consistent genetic differences that are associated with consistent morphological or biological traits should be considered the marker for separate species. We propose a generalised approach to the use of rDNA to distinguish trematode species.

The life cycle of Paracardicoloides yamagutii Martin, 1974 (Digenea: Sanguinicolidae)

The sanguinicolids Paracardicoloides yamagutii Martin, 1974 and Plethorchis acanthus Martin, 1975 were obtained from their definitive hosts, Anguilla reinhardtii Steindachner and Mugil cephalus Linnaeus (respectively) in the tributaries of the Brisbane River, Queensland, Australia. Two putative sanguinicolid cercariae were collected from a hydrobiid gastropod, Posticobia brazieri Smith, in the same waters. The two cercariae differ markedly in size and the form of their sporocysts. Both putative cercariae develop in the digestive gland of Po. brazieri. The ITS2 rDNA region from these sanguinicolids and a Clinostomum species (utilised as an outgroup due to the close morphological similarities between the cercarial stages of the Clinostomidae and the Sanguinicolidae) were sequenced and aligned. Comparison of the ITS2 sequences showed one cercaria to be that of P. yamagutii. This is the first sanguinicolid life history determined by a molecular method. P. yamagutii is the fourth sanguinicolid known to utilise a freshwater hydrobiid gastropod as its intermediate host. ITS2 rDNA is effective in distinguishing sanguinicolids at the species level.

Defeating diplostomoid dangers in USA catfish aquaculture

Diplostomoid digenean metacercariae have caused widescale mortalities of channel catfish, Ictalurus punctatus (Rafinesque), at aquaculture farms in Louisiana, Mississippi and Arkansas, USA. Originally, based on a tentative diagnosis, the industry considered the primary harmful agent to be an introduced species from Europe, Bolbophorus confusus (Krause, 1914), frequently reported from the American white pelican, Pelecanus erythrorhynchos Gmelin. Our group has now shown, using ITS 1-2 plus three more-conservative gene fragments, that two sympatric species of Bolbophorus exist in the American white pelican. One, B. damnificus Overstreet et Curran, 2002, infects the musculature of catfish, and the other, probably not B. confusus, does not infect catfish. However, at least four other pathogenic diplostomoids and a clinostomoid infect the catfish, and they use at least four different snail hosts, including the planorbids Planorbella trivolvis (Say) and Gyraulus parvus (Say), the physid Physella gyrina (Say) and a lymnaeid. Two metacercariae, B. damnificus and Bursacetabulus pelecanus Dronen, Tehrany et Wardle, 1999, infect the catfish and mature in the pelican; two others, Austrodiplostomum compactum (Lutz, 1928) and Hysteromorpha cf. triloba (Rudolphi, 1819), mature in cormorants; one, Diplostomum sp., matures in seagulls and at least one, Clinostomum marginatum (Rudolphi, 1819), matures in herons, egrets and other wading birds. Consequently, management of catfish ponds relative to digenean infections requires considerable biological information on the fish, bird, and snail hosts as well as the parasites.

Molecular systematics of some North American species of Diplostomum (Digenea) based on rDNA-sequence data and comparisons with European congeners

The systematics of Diplostomum species, common intestinal parasites of piscivorous birds, has long been problematic, owing to phenotypic plasticity and the paucity of morphological features that are often subject to age- and host-induced variation. We sequenced the ITS1–5.8S–ITS2 regions of the rDNA from adult Diplostomum huronense, Diplostomum indistinctum, and Diplostomum baeri obtained from experimentally infected ring-bill gulls (Larus delawarensis) and compared them with partial ITS1 sequences from several species of Diplostomum in GenBank. The three North American species were distinguishable on the basis of ITS sequences. Sequences from D. huronense differed from those of D. indistinctum at 12 sites in ITS1 and 4 sites in ITS2, supporting morphological and morphometric data that indicate the two are distinct species. Sequences of D. huronense and D. indistinctum differed from those of D. baeri at 27 and 24 sites, respectively, in ITS1 and 15 and 12 sites, respectively, in ITS2. Phylogenetic analysis of partial ITS1 sequences revealed that the North American and European species of Diplostomum formed separate groups, with the former being basal to the latter. The results indicated that D. huronense and D. indistinctum from North America are distinct from Diplostomum spathaceum and other similar species from Europe. Furthermore, sequences from specimens identified as D. baeri from North America differed from those of D. baeri from Europe by 3.8% in ITS1 (23 sites). While morphologically similar, the two are not conspecific. Sequences of the North American species have been deposited in GenBank (AY 123042–123044).