Hermaphroditic mating systems profoundly influence evolution, yet in parasitic flatworms (Neodermata) they remain strikingly understudied. For decades, sweeping claims have oscillated between pervasive selfing and near-universal outcrossing, reflecting a lack of comprehensive synthesis. This review, the first in more than 40 yr, integrates nearly a century of research, from early observational studies to modern genetic analyses, to reveal a far more nuanced picture. Our meta-analysis of population-genetic data shows a sharp departure from the bimodal selfing patterns typical of plants and other hermaphroditic animals: neodermatan parasites are strongly skewed toward outcrossing. We link variation in mating systems to parasite demography and life-history traits, especially in species exhibiting mixed mating or elevated selfing. Current evidence suggests outcrossing is common, but taxonomic and life-history gaps preclude definitive conclusions. Beyond patterns and causes, we explore evolutionary consequences ranging from fitness costs such as inbreeding depression to trait evolution, including delayed selfing, sex allocation, and complex life cycles. Evidence for inbreeding depression is mixed and limited, yet emerging approaches using selfing-rate comparisons offer promising avenues for future research. Notably, demographic constraints, such as infection intensity and life-cycle architecture, often explain mixed mating without invoking selection, challenging classical models. Similarly limited in number, studies on sex allocation indicate that hermaphroditic mating systems can shape reproductive investment, with patterns consistent with local sperm competition in some taxa. By consolidating historical observations with modern genetic insights, this review provides the most comprehensive synthesis of hermaphroditic mating systems in the Neodermata to date. We highlight critical gaps in taxonomic coverage and experimental data and point to future opportunities for integrating genomic approaches with ecological and demographic frameworks. Such integration will be essential to illuminate how mating systems shape parasite evolution and to resolve long-standing questions about the persistence of mixed mating despite theoretical expectations.
Habitat preferences of gastrointestinal nematodes are well-documented in small hosts, but equine cyathostomin diversity has largely been characterized only at farm or regional levels. While organ-level site preferences are known, within-organ spatial distributions remain unexplored. We investigated the longitudinal and radial distribution of adult cyathostomins within the large-intestinal lumen. Intact large intestines from 5 Shetland ponies were frozen post-necropsy and transected into 4-cm sections. Three micro-habitat samples (2.5 cm diameter) were collected per section. We recovered 2,444 intact worms, which were identified individually to species. Data were analyzed using generalized linear mixed models (GLMMs) and Shannon-Wiener Diversity metrics. Fourteen species were identified, dominated by Cylicocyclus nassatus and Cyathostomum catinatum. Spatial modeling revealed a highly structured community hierarchy: dominant species monopolized the central lumen of the anterior ventral colon. Consequently, less abundant species exhibited strict spatial niche partitioning. When aggregated as a guild, "rare" species utilized the cecum and peripheral lumen as significant spatial refugia to avoid luminal competition (P < 0.001). Furthermore, increased host infection intensity did not force spatial expansion into marginal habitats; instead, high worm burdens significantly exacerbated crowding within the anterior central lumen (P < 0.001). These findings support interspecific niche partitioning consistent with Schad's species-flock framework: dominant species occupied the central lumen of the orad ventral colon, whereas less abundant species used peripheral/paramucosal and cecal refugia. However, increased host infection intensity did not produce density-dependent spatial expansion; instead, higher worm burdens intensified crowding within the preferred central/orad ventral-colon habitat.
Growth and reproduction in insects are strongly influenced by dietary protein and carbohydrate intake, yet parasitism may alter how hosts acquire and allocate these resources. Hairworms (Nematomorpha) develop within arthropod hosts and consume host-derived nutrients, but the effects on host nutrition and development remain poorly understood. We tested the effects of Paragordius varius (Leidy, 1851) parasitism on diet choice, growth, and reproduction in experimentally infected female house crickets, Acheta domesticus (Linnaeus, 1758). Crickets were offered complementary high-protein and high-carbohydrate diets, and we quantified food intake, nutrient preference, and host performance metrics. Infected crickets consumed similar protein-to-carbohydrate ratios as controls but ingested 14% less total food. Control crickets exhibited higher standard growth rates (SGR), whereas SGR decreased with increasing parasite intensity in infected individuals. Infected crickets produced no eggs and had significantly reduced lipid content, ovipositor growth, and fecal output relative to controls. Nutritional indices on the basis of wet mass indicated no difference in the conversion of intake to body mass; however, indices on the basis of parasite-free dry mass revealed that control crickets converted food intake 3.7 times more efficiently than infected crickets. These results indicate that hairworm parasitism reduces host growth and reproductive output and decreases the efficiency of energy assimilation, likely due to direct or indirect resource competition with developing parasites.
Northern bobwhite (Colinus virginianus) populations have experienced sustained declines across their native range, with parasitic helminths, particularly the eyeworm (Oxyspirura petrowi) and cecal worm (Aulonocephalus pennula), emerging as contributing stressors. This study assessed the long-term efficacy of a biannually administered anthelmintic feed treatment (QuailGuard®) and characterized seasonal and annual reinfection patterns in wild bobwhite populations at 2 sites (Oklahoma and Texas) from 2023 to 2025. Fecal samples were collected from wild bobwhite from March to October and analyzed via quantitative polymerase chain reaction (qPCR) to estimate infection levels. Aulonocephalus pennula infection levels were consistently reduced following spring and fall treatments, with efficacy ranging from 61 to 92%, except for 1 unsuccessful treatment in the fall of 2024 at the Texas site. Reinfection was rapid during the summer, and overwinter reinfection was more severe in 2024-2025 than the previous year. Oxyspirura petrowi prevalence was lower overall but showed similar seasonal trends. Birds under approximately 10 wk of age were less likely to have high infection levels, and the population's age structure influenced infection dynamics in late summer. A significant co-infection association between A. pennula and O. petrowi suggests overlapping parasite incidence, which is expected given their shared intermediate hosts. Although the medicated feed demonstrated sustained effectiveness, treatment outcomes varied, likely due to climate conditions and bird behavior. These results support continued biannual treatments but emphasize the need for site-specific adjustments, such as extending pre-baiting periods, to enhance efficacy. Continued monitoring will refine treatment timing, enhance cost-effectiveness, and mitigate the impacts of helminths on bobwhite populations.
Uncertainty shrouds the number and identity of the species of Sarcocystis infecting horses in the United States. Horses are intermediate hosts for Sarcocystis fayeri, for which the domestic dog is the definitive host. Clinical signs and lesions associated with S. fayeri are not defined. Granulomatous myositis, especially involving the chest and dysphagia, is one of the clinical presentations associated with S. fayeri infections. Whether S. fayeri is the sole species that forms sarcocysts in equine muscles in the United States is not clear. Little is known of the molecular diagnosis of Sarcocystis species in the muscles of horses. One of us (J.P.D.) has studied equine sarcocystosis for the past 5 decades. Here, we review the status of these aspects from a historical and personal perspective.
We herein propose Parabenedeniella Brule and Bullard n. gen. to accommodate Parabenedeniella posterocolpa n. comb., formerly of Benedeniella Johnston, 1929 (both Capsalidae: Entobdellinae). Parabenedeniella differs from all other entobdelline genera by the combination of having a U-shaped (not looped) proximal portion of the cirrus sac, a vagina that is directed posterolaterad, a tube-like proximal portion of the vagina that is approximately one-third of the total length of the vagina, a thick-walled and muscular distal portion of the vagina that is approximately two-thirds of the total length of the vagina, and a dorsal vaginal pore that is submarginal to the sinistral body margin posterior to or at the level of the testes. We collected specimens of P. posterocolpa from 5 cownose rays, Rhinoptera bonasus (Rhinopteridae), and 1 Brazilian cownose ray, Rhinoptera brasiliensis, from the Gulf of America and from 1 giant manta, Mobula birostris (Mobulidae), from an exhibit tank at a public aquarium, which also contained cownose rays. Parasite specimens were fixed in 10% neutral buffered formalin for morphology or preserved in 95% EtOH for DNA extraction. All of the collected specimens of P. posterocolpa were morphologically indistinguishable from the holotype of P. posterocolpa and were diagnosed by having anterior attachment organs, each with anteroventral grooves and a dorsal papilla, and a dorsal vaginal pore located level with or posterior to the testes. We generated 15 sequences: the ITS1, 28S, and COI from 2 specimens of P. posterocolpa from a cownose ray and 2 specimens from a giant manta plus a specimen from the Brazilian cownose ray. The only extant 28S sequence previously ascribed to P. posterocolpa was identical to each of our 28S sequences (no previous ITS1 or COI sequences for P. posterocolpa exist), and the new ITS1 sequences were identical. Two morphologically identical specimens of P. posterocolpa spaced approximately 5 mm apart on a single host specimen had distinct COI sequences (2 bp or 0.2% difference). The 28S analysis recovered P. posterocolpa as a sister to a clade comprising species of Entobdella and Neoentobdella (all Entobdellinae). The 28S and COI phylogenetic analyses recovered Entobdellinae as sister to Capsalinae; both were recovered as sister to Dionchinae (both Capsalidae). This is the first study of P. posterocolpa that combines morphology and nucleotide sequences from multiple infections and host species, the first record of P. posterocolpa infecting the Brazilian cownose ray, and the first detailed taxonomic description of specimens of P. posterocolpa infecting the giant manta.
Anthelmintic resistance is a growing threat to the treatment and control of parasitic nematode infections in both humans and animals. In the canine hookworm, Ancylostoma caninum, multi-anthelmintic drug-resistant (MADR) isolates have been identified, but the molecular mechanisms underlying resistance and the potential for these mechanisms to promote cross-resistance between drug classes remain unclear. In this study, we report evidence for a previously uncharacterized association between resistance to macrocyclic lactones (MLs) and benzimidazoles (BZs) in A. caninum. In a wild-type isolate with no history of drug exposure, serial passaging without treatment led to an unexpected rise in F167Y allele frequency, accompanied by increased resistance to ivermectin (ML) but not to thiabendazole (BZ). Separately, 3 rounds of combination treatment of a triple-resistant isolate with moxidectin (ML) and pyrantel (tetrahydropyrimidine) led to a substantial increase in the frequency of the β-tubulin F167Y mutant allele, despite the absence of BZ selection pressure. These findings suggest that ML resistance in A. caninum may be associated with canonical BZ resistance mutations. Furthermore, the F167Y mutation does not appear to confer any negative fitness effects; instead, the mutation correlated with increased egg output and prolonged infection. This surprising observation raises concerns about the appearance, persistence, and spread of resistance alleles in populations not exposed to drugs. These results have critical implications for resistance management strategies and highlight the urgent need for surveillance tools capable of detecting early cross-resistance in hookworm populations.
We describe Parorchis laffertyi n. sp., a trematode infecting the California horn snail, Cerithideopsis californica (Haldeman, 1840), as a first intermediate host. The first intermediate host stages of this species were previously cryptic with Parorchis catoptrophori Dronen & Blend, 2008, which also infects C. californica. For both species, we describe the parthenita colonies, soldier rediae, reproductive rediae, cercariae, metacercariae, and sexual adults by using DNA sequencing and material from natural and experimental infections. Our study of all life stages was conducted with material originating from naturally infected, lab-maintained horn snails from San Diego, California. Cercariae that were repeatedly shed from specific, labeled snails were used as material for experimental infections, developmental studies, and DNA sequencing. In all cases, data were tracked to their source individual first intermediate host colonies. These same colonies were dissected to describe the morphology, behavior, and demographics of reproductive and soldier rediae. Metacercaria descriptions came from cercariae shed and encysted in the laboratory; we also document a metacercaria developmental time series. We experimentally exposed final host chickens (Gallus gallus domesticus Linnaeus, 1758) and ducks (Anas platyrhynchos domesticus Linnaeus, 1758) to metacercariae per cloaca and successfully obtained and described the sexual adults of P. catoptrophori and an immature adult of P. laffertyi from the chickens. Both cytochrome c oxidase subunit I (COI) and 28S rDNA phylogenetic analyses supported the hypothesis that P. catoptrophori and P. laffertyi are 2 species of Parorchis Nicoll, 1907. As indicated in its original description, adults of P. catoptrophori can be readily distinguished from adults of other Parorchis species by their possession of a greater number of collar spines (∼80 vs. ∼60 in most other species); a bipartite pharynx; a pharynx to oral sucker ratio of ∼1:2; presence of esophageal diverticula; and deeply lobed testes. The immature adult of P. laffertyi can be distinguished from adults of other Parorchis species by the collar spine number (∼55); bipartite pharynx; a pharynx to oral sucker ratio of ∼1:2; presence of esophageal diverticula; and distinct COI and 28S sequence data. Mature adult specimens of P. laffertyi are needed to parse additional morphological characteristics. We used our morphometric data and additional published information to provide an emended diagnosis for the genus Parorchis. There are now 18 formally named Parorchis species, 4 of which have had their first intermediate host stages described and 4 of which have available DNA sequence information.
Sarcocystis species are heteroxenous protozoan parasites of major veterinary and public health relevance, yet the role of wild felids in their natural transmission cycles remains poorly understood. In this study, we investigated Sarcocystis infections in 189 bobcats (Lynx rufus) harvested from Minnesota during 2024-2025. Intestinal homogenates of 179 were examined microscopically, and Sarcocystis sporocysts were found in 38 bobcats (21.2%). DNA was extracted from all 38 positive samples, of which 19 were PCR-positive, and 14 yielded high-quality sequences for molecular characterization using partial 18S rRNA, 28S rRNA, and mitochondrial COI markers. Molecular results confirmed the presence of Sarcocystis bovifelis and Sarcocystis svanai-like. Due to inconsistent amplification and partial overlap among nuclear markers, only COI was suitable for phylogenetic analysis. The COI-based phylogenetic tree resolved strongly supported clades corresponding to these species (bootstrap 99-100%). These findings provide strong molecular evidence that bobcats in Minnesota serve as natural definitive hosts for S. bovifelis and S. svanai-like.
This study reports the prevalence, morphological, and molecular identification of Sarcocystis species in the tongues of Florida panthers (Puma concolor coryi). By light microscopy, sarcocysts were found in 46 of 108 (42.6%) panthers and appeared to represent at least 2 morphological types. One morphological type had serrations on the sarcocyst wall and resembled Sarcocystis felis reported from domestic and wild felids; molecular characterization using the ITS1 gene confirmed its identity. The second kind of sarcocyst had a smooth sarcocyst wall without any visible protrusions. The possibility of other unidentified Sarcocystis species is discussed.
Avian haemosporidians (Haemosporida) commonly infect terrestrial birds worldwide and can have negative consequences for individuals. We studied avian haemosporidian infections and mitochondrial cytochrome-b lineages in a southern Indiana breeding population of cerulean warbler (Setophaga cerulea; n = 51) over 2 yr with traditional microscopy and standard molecular methods. We investigated potential effects of infection on individual body condition, associations of host specificity and levels of infection (parasitemia) according to haemosporidian lineage genus, and geographic distributions of lineages. Additionally, we aimed to provide the first documentation of haemosporidian lineages for the species and to estimate population prevalence and parasitemia for comparison with earlier studies of the species. Parasitemia was low (<1% of cells infected) for most individuals, indicative of chronic infections, and we did not find any negative effects of infection on body condition of males (n = 49); however, mean body condition of individuals with Plasmodium sp. infections was higher than that of uninfected individuals, which could reflect survivorship bias in the sample. Neither host specificity nor parasitemia varied by haemosporidian genus, and for some lineages, we inferred likely geographic regions of transmission to Neotropical-Nearctic migrants, including the cerulean warbler. Prevalence was moderate at 31.4% across both years, and we documented 11 unique lineages in the genera Haemoproteus and Plasmodium, 2 of which were novel. These findings advance knowledge of host-parasite interactions for a declining avian species and add to the broader field of avian haemosporidian biology, including novel host-parasite associations.
In June of 2022, 3 creole wrasses, Bodianus parrae, from a public display aquarium were submitted for diagnostic examination at Mississippi State University's College of Veterinary Medicine. Fresh myxospores recovered from the intestines were morphologically consistent with previous accounts of Enteromyxum leei (Diamant, Lom & Dyková, 1994). Molecularly, myxospores from the present study were 99.7-100% and 99.7-99.8% similar to publicly available E. leei sequences at 18S small subunit ribosomal (18S) and 28S large subunit ribosomal (28S) genes, respectively. Internal transcribed spacer region 1 (ITS1), 5.8S ribosomal rRNA gene (5.8S), and internal transcribed spacer region 2 (ITS2) sequences were 95.8-98.0%, 100%, and 99.9% similar, respectively, to the only publicly available data from these regions from E. leei. Eukaryotic elongation factor 2 (EEF2) sequences were 98.8% similar to the available EEF2 sequence of E. leei in GenBank. Lastly, cytochrome c oxidase subunit 1 (COI) sequences were 98.6% similar to previously published data. Bayesian phylogenetic analyses using both single locus 18S sequence data, concatenated 18S, 28S, and EEF2 data, and concatenated 18S, 28S, and COI amino acid sequences from each of the 3 wrasses grouped E. leei from the present study with previously published E. leei sequences with high Bayesian and maximum likelihood support. This is the first account of E. leei from B. parrae. Histological assessment reveals pathological changes to intestinal epithelia consistent with previous accounts of E. leei, with 18S, ITS1, 5.8S, ITS2, 28S, EEF2, and COI sequence data providing greater phylogenetic resolution of this enigmatic species. These morphological and molecular data from an enteromyxosis outbreak in a mixed-species exhibit at a public aquarium offer robust species-level identification of the causative agent, E. leei.
Gonatodes humeralis is a small gecko common in the Amazon, and little is known about its parasite community. The present work assessed for the first time in detail the parasite community of G. humeralis in 2 conservation areas of the municipality of São Luís, State of Maranhão, Brazil, and the factors associated with it. A total of 132 lizards were collected. We found only 3 parasite taxa: the nematode Skrjabinelazia galliardi in the stomach, the digenean Mesocoelium sp., and the cestode Oochoristica sp., both in the small intestine. Skrjabinelazia galliardi was the most dominant, prevalent, and abundant, followed by Oochoristica sp.; Mesocoelium sp. was rare, infecting only 2 female lizards from one of the collection sites. The parasite community was depauperate, noninteractive, and parasites were highly aggregated, which is typical of lizard hosts. Host body length was a strong factor shaping the parasite community, whereas sex, ontogeny, and collection site exerted a weak influence on it. Because all parasites are trophically transmitted, the host diet is likely an important indicator of parasite transmission in the present community. The local anthropic activities possibly influence the environmental quality, which may impact the existence of intermediate hosts for Mesocoelium sp. and its transmission. Gonatodes humeralis harbored only adult parasites, indicating that this species does not play a significant role as intermediate/paratenic host in the area but represents an important definitive host at least for S. galliardi and Oochoristica sp.
This study reports the automation and evaluation of the Giardia chemiluminescence assay (GCA), an assay designed to detect infectious Giardia duodenalis present in veterinary stool samples. The automated GCA (auto-GCA) enabled stringent step pacing and significant reduction of hands-on time for the assay operator, and it yielded assay performance comparable with the manual assay in activity profiles, dose-dependent ranges and half-maximal effective concentrations. In a study with clinic-collected canine stool specimens, 115 samples analyzed by auto-GCA had a sensitivity of 95.1% and a specificity of 90.7% compared with a standard G. duodenalis ELISA. In another study, 81% of 62 canine samples that were identified as borderline by ELISA were classified definitively as positive or negative for G. duodenalis by auto-GCA. To our best knowledge, the auto-GCA is the first system in the parasite diagnosis area to use full automation and chemiluminescence, and it is expected to improve work quality for individual researchers and for laboratories.
Climate projections for the Southwestern United States predict increases in temperature, drought intensity and frequency, and wildfire intensity and frequency. Such climatic change is expected to impact communities and species of the region, including Crotalus atrox (western diamond-backed rattlesnake) and its helminth communities. We hypothesized that C. atrox in New Mexico has experienced measurable changes in body size, diet composition, and helminth communities over the past century. We examined specimens of C. atrox from 3 time periods: 1947-1960 (1950s, n = 34), 1980-1990 (1980s, n = 53), and 2011-2021 (2010s, n = 57). We dissected specimens of C. atrox and recorded snout-vent length (SVL), sex, age class (juvenile or adult), prey items from gastrointestinal (GI) contents, and helminth prevalence. We then tested for differences in body size, diet, and helminth prevalence between sexes of C. atrox and across time periods. To better understand the environmental breadth of the helminths in C. atrox, we also performed a literature search to reconstruct in what coarse-scale habitat conditions the helminths identified in our study have previously been reported. Over the 3 time periods we studied, body size and diet composition have remained stable; however, helminth community composition appears to have shifted significantly. Kalicephalus inermis, Mesocestoides sp., and a nematode in the family Pharyngodonidae were recovered from snakes from the 1950s, followed by Hexametra boddaertii, Mesocestoides sp. and Oochoristica osheroffi from the 1980s. Finally, only O. osheroffi was recovered from snakes from the 2010s. We also found that O. osheroffi appears to tolerate areas that often lack reliable moisture levels. We suggest 3 hypotheses explaining the observed shift in helminth community composition: changes to climate, undetected changes in the diet of C. atrox, or other untested factors. Museum vouchers of both the hosts and the helminths recovered herein are available for verification, additional analyses or in-depth species reevaluations.
Understanding and controlling vectors and vector-borne diseases is critical for maintaining public health in Singapore. Although mosquitoes, ticks, and the diseases they cause have attracted strong attention from researchers in recent years, fleas and flea-borne diseases remain largely neglected in Singapore. To stimulate research on this neglected topic, we examine trends in flea-borne rickettsiosis in Singapore from 2016-2022. Trends in these data indicate that the majority of cases are locally acquired and are more common in males, migrant workers, and teens/young adults from ages 15-34. In addition, the bat flea Thaumapsylla breviceps is recorded from Singapore for the first time along with the first records of Ctenocephalides felis from Paradoxurus hermaphroditus and Canis familiaris in Singapore. A checklist of the fleas of Singapore with a host index is presented and the present state of knowledge on the diversity of fleas and flea-borne diseases in Singapore is discussed. Finally, a roadmap is presented to provide a direction for future research and public policy to more effectively address the challenges associated with fleas and flea-borne diseases in Singapore.
Trematodes in the family Echinostomatidae (Looss, 1899) are globally distributed parasites of both humans and wildlife. In North America, echinostomes are nearly ubiquitous in freshwater systems, where they commonly parasitize snails as their first intermediate hosts, aquatic hosts such as snails or tadpoles as second intermediate hosts, and waterfowl and semi-aquatic mammals as their definitive hosts. Our study expands on sampling efforts in the midwestern United States and contributes novel ND1, ITS2, and 28S sequences from 12 lineages of echinostomes in the genera Echinostoma, Echinoparyphium, and Hypoderaeum. Additionally, COI barcoding and phylogenetic analysis of snail first intermediate hosts elucidated novel host-parasite combinations. Finally, we demonstrate that echinostomes are often not specific to 1 species of snail first intermediate host but rather to a family of aquatic snails, and we suggest that first intermediate hosts may drive echinostome speciation.
Reexamination of the holotype of the louse species Esthiopterum tataupa Carriker, 1936 demonstrated that the species belongs to the genus Furnariphilus Price and Clayton, 1995 and is conspecific with Furnariphilus parkeri Price and Clayton, 1995. The Tataupa tinamou Crypturellus t. tataupa (Temminck, 1815), originally listed as the type host of this species, is not the true host. This association likely resulted from contamination during Carriker's 1934 Bolivian collections, when a furnariid (Sclerurus albigularis albicollis Carriker, 1935) was obtained at the same locality. The present study refines both the taxonomy and host association of this louse species and underscores the importance of continued critical evaluation of historical type specimens and refined morphological analysis. It also underscores the value of well-documented collections of hosts and parasites with associated field notes for assessing taxonomy and documenting ecological and evolutionary relationships of hosts and their parasites.
