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Trematodes of the family Cyclocoelidae are parasites mainly of the respiratory system of birds and present a cosmopolitan distribution. Although infection with these flukes can result in pathological changes and even bird death, information on their life cycles is scarce and almost entirely based on experimental infection data. Thus, the generation of knowledge on the mollusks that act as natural intermediate hosts of cyclocoelids is necessary and can aid control measures against these air sac trematodes. In the present study, gastropod mollusks collected in an urban stream from Belo Horizonte, Minas Gerais, Brazil, were subjected to the compression technique for the detection of non-emerging larval trematodes. Tailless cercariae with confluent ceca were found in 8/30 (26.7%) specimens of Biomphalaria glabrata and 3/33 (9.1%) specimen of Physella acuta. Samples of the cercariae were subjected to morphological characterization and genetic study (28S, Cox-1, and Nad-1). For comparative purposes, adult trematodes previously collected in the air sac of a common gallinule (Gallinula galeata) found dead in another waterbody from the same region were also characterized. The molecular sequences obtained revealed a high degree of similarity (100% in 28S, 99.2% in Cox-1, and 99.5% in Nad-1) between larval stages found in mollusks and adult parasites found in G. galeata and morphologically identified as Cyclocoelum mutabile. The conspecificity with this widely distributed cyclocoelid was also corroborated by phylogenetic analysis and comparison with isolates of this species previously characterized in Peru and the Czech Republic (99.4–100% and 96.7–97.0% of similarity in Nad-1, respectively). Thus, the integrative analysis carried out in the present work enabled us to identify C. mutabile in mollusks in South America for the first time. The finding of B. glabrata and P. acuta as new intermediate hosts corroborates the importance of freshwater gastropods in the transmission of C. mutabile, as well as the low specificity to the mollusk group, as previously characterized through experimental studies.

Cyclocoelidae Stossich, 1902 are medium-sized to large digenean bird parasites. Although these parasites bear few visible autapomorphic signs, and their diagnostic characters are unstable in response to the pressure applied during preparation, the numerous hitherto suggested re-classifications within the family have not been supported by any molecular analysis. We analyse here cyclocoelids found during the extensive examination of central European birds performed from 1962 to 2016, provide comparative measurements, host spectra, prevalence and intensity, and provide and analyse sequences of four DNA loci of five of the cyclocoelid species. Cyclocoleum Brandes, 1892 appears paraphyletic; thus we suggest the re-classification of Cyclocoleum obscurum (Leidy, 1887) as Harrahium obscurum (Leidy, 1887) Sitko and Heneberg comb. n. Molecular phylogenetics questioned also the validity of Cyclocoelinae Stossich, 1902 and Hyptiasminae Dollfus, 1948, which formed a single clade, whereas Allopyge Johnston, 1913, Prohyptiasmus Witenberg, 1923 and Morishitium Witenberg, 1928 formed another clade. Haematotrephinae Dollfus, 1948 are newly characterized as having a pretesticular or intertesticular ovary that forms a triangle with the testes. Analyses of non-European genera of the Cyclocoelidae and an examination of the position of families within Echinostomata La Rue, 1926 are needed.

In light of the morphological and molecular data for cyclocoelids observed from the air sacs of Mareca strepera (Linnaeus) (Anatidae, Anseriformes) caught in the southern region of the Russian Far East, we suggest new insights into the systematics of the family Cyclocoelidae. A comparative morphological and phylogenetic analyzes revealed that new cyclocoelids represented the new genus and species Paracyclocoelum lobatum. Based on the 28S rRNA gene fragment we showed the significant genetic divergence of P. lobatum from the type species of the type genus for the family, Cyclocoelum mutabile (Zeder, 1800) Brandes, 1892 and along with the confusing morphological similarity by the prepharyngeal position of the genital pore it most likely indicate homologous development of the reproductive system of Paracyclocoelum and Cyclocoelum. Here, we provide a new dichotomous key for five cyclocoelid genera from the subfamily Cyclocoelinae including Paracyclocoelumn. g. The new genus Paracyclocoelum had sister relationship to the cyclocoelin genus Circumvitellatrema. Based on the polyphyletic interrelationships of Cyclocoelum and Circumvitellatrema the Cyclocoelinae were assigned with the status sensu lato.

Based on a morphological approach, five species of cyclocoelids (Digenea: Cyclocoelidae) are revised based on material from birds in Mexico. The species studied are: Cyclocoelum cf. leidyi Harrah, 1922 from the black-necked stilt Himantopus mexicanus Müller (Recurvirostridae); Cyclocoelum mutabile (Zeder, 1800) from the northern jacana Jacana spinosa L. (Jacanidae) and the willet Tringa semipalmata Gmelin (Scolopacidae); Cyclocoelum pseudomicrostomum Harrah, 1922 from the American coot Fulica americana Gmelin (Rallidae); Selfcoelum lamothei Blend & Dronen, 2008, from the long-billed curlew Numenius americanus Bechstein (Scolopacidae); and Neohaematotrephus arayae Zamparo, Brooks, Causey & Rodriguez, 2003 from J. spinosa. New morphological data, illustrations and measurements are presented for these trematodes. Three species, insufficiently described, i.e. C. cf. leidyi, C. mutabile and C. pseudomicrostomum, are redescribed in detail. Our results indicate morphological variation in some important features used in the diagnosis of Cyclocoelum Brandes, 1892 (i.e. the presence or absence of oral sucker, the position of the genital pore relative to the pharynx, and the posterior extension of the uterus) and Selfcoelum Dronen, Gardner & Jiménez, 2006 (i.e. anterior extension of vitelline follicles). Finally, this study provides the first molecular data for the large subunit of the ribosomal RNA (28S rRNA) gene for N. arayae.

Cyclocoelum mutabile, un digeneo de la familia Cyclocoelidae, fue hallado parasitando los sacos aéreos de una polla de agua común (Gallinula chloropus), proveniente de alrededores del Refugio de Vida Silvestre Pantanos de villa, localizada en el distrito de Chorrillos en Lima, Perú. Un total de 7 parásitos fueron colectados e identificados por métodos morfológicos como C. mutabile. El diagnóstico fue confirmado por análisis molecular, amplificando los genes mitocondriales citocromo c oxidasa subunidad 1 (cox1) y deshidrogenasa NADH subunidad 1 (nad1). Las secuencias de nucleótidos de los aislados se compararon con secuencias previas de GenBank, y mostraron una similitud entre ellas (> 96%). Este hallazgo constituye el primer registro de C. mutabile para el Perú. Además, el trabajo realiza una breve descripción del parásito, así como la discusión de sus hospederos y distribución geográfica en Sudamérica.

Six specimens representing a new genus of cyclocoelid, Psophiatrema greineri n. gen., n. sp. (Digenea: Cyclocoelidae: Cyclocoelinae), were collected from the air sacs of a common or grey-winged trumpeter, Psophia crepitans (Gruiformes: Psophiidae), from Guyana on 19 January 2001. The bird was being maintained in an aviary at Emerald Forest Bird Gardens, Fallbrook, California. The new genus has an intertesticular ovary forming a triangle with the testes, placing it in Cyclocoelinae. There are currently 2 genera recognized in Cyclocoelinae: Cyclocoelum and Selfcoelum. The new genus is most similar to Selfcoelum by having a postpharyngeal genital pore, but differs from both Cyclocoelum and Selfcoelum by having the vitelline fields confluent posteriorly. A revised key to the Cyclocoelinae is provided to include the new genus, and the subfamily diagnosis is emended to reflect that the vitelline fields can be either confluent or not posteriorly.

Fischthal and Nasir (1974) reported Neohaematotrephus brasilianum (as Cyclocoelum brasilianum) in the spotted-sandpiper Actitis macularia from Venezuela. Three voucher specimens from that report, deposited in the United States National Parasite Collection, however, differ from N. brasilianum by having the cirrus sac on the sinistral side of the body, which resembles N. facioi, N. arayae, and N. gendrei. The new species is similar to N. brasilianum by having vitelline follicles extending well anterior to the intestinal bifurcation and by having a short and laterally displaced cirrus sac whose posterior end does not reach the intestinal bifurcation, whereas all other members of Neohaematotrephus have a cirrus sac that is medially oriented. Neohaematotrephus gendrei and N. facioi have cirrus sacs that extend to the level of the intestinal bifurcation, and N. arayae has a cirrus sac that extends well posterior to the posterior margin of the ceca. By having the ovary on the sinistral side of the body, the new species is similar to N. brasilianum, N. gendrei, and N. arayae but differs from N. facioi, in which the ovary is dextral.

In this study, changes were examined in surface structures and tegumental morphology or newly excysted, migrating, and adult Cyclocoelum mutabile obtained from experimentally infected coots (Fulica americana). Newly excysted juveniles were spinous and had two large depressions situated at the anterior tip of the body and a large acetabulum.

Infectivity of Cyclocoelum mutabile miracidia to 9 species and up to 4 size classes of pulmonate snails at 14, 16, and 20 C was studied under laboratory conditions. Of the 9 species examined, 6 (Stagnicola elodes, Lymnaea stagnalis, Gyraulus parvus, Gyraulus circumstriatus, Promenetus exacuous, and Armiger crista) were highly susceptible (infection success ≥25%), 2 (Physa jennessi and Helisoma trivolvis) had low susceptibility (infection success <25%, >0), and 1 (Physa gyrina) was not susceptible to infection. Within highly susceptible species, snail size was negatively related to susceptibility and temperature had variable effects. Infection success was not affected by temperature or snail size in species with low susceptibility. Production of cercariae was negatively correlated with susceptibility among snails of different sizes and species, but was not influenced by snail size for a given species. Among species, metacercariae production was typically higher in lymnaeids than in either planorbids or physids. Results of experiments where miracidia were provided with a choice of 2 different snails suggest that they do not discriminate between species with high and low susceptibility.

The transmission of Cyclocoelum mutabile to snails was examined under natural conditions by sampling the snail communities of 4 natural ponds that had been exposed experimentally to infection by laboratory-infected coots (Fulica americana). Five of 6 snail species in the ponds, Physa jennessi, Promenetus exacuous, Armiger crista, Gyraulus parvus, and Stagnicola elodes, became infected. No natural infections were found in the few Helisoma trivolvis examined. The second most abundant species Promenetus exacuous was infected most often, whereas Physa jennessi, the most abundant species present, was rarely infected. The temporal pattern of infections in the snail community suggests the transmission window of this parasite in southern Manitoba is limited by both the 14 C hatching threshold of the fluke eggs and the seasonality of ovigerous infections in the coot host. No naturally overwintering infections were found in snails from these ponds, which were examined the following spring. None of the 1,120 laboratory-infected snails placed in cages and held overwinter in the ponds survived, whereas 14 of the 1,120 uninfected control snails kept in the same cages survived. These results are consistent with the hypothesis that C. mutabile must be reestablished in northern waterfowl breeding areas each spring.