Parasites Of Genus Research Articles

The role of specialist parasites in structuring host communities

AbstractNatural enemies can be a powerful force when structuring natural communities, and in facilitating or preventing species coexistence depending on the nature of the trophic interaction. In particular, “keystone” predators can promote species coexistence, provided they preferentially attack the competitively dominant species. However, it is not clear whether parasites can play a similar structuring role; parasites typically form chronic associations with their victims, reducing their fitness (i.e., fecundity) rather than survival, and allowing infected hosts to remain viable competitors within the community. Therefore the density‐dependent suppression of the host is likely to be more subtle than that due to predation. Using a series of simple population‐dynamic models we show that specialist parasites can facilitate species coexistence, although possibly less so than predators. These results contrast with those typically found with models of generalist parasites, which can reduce the likelihood of species coexistence through apparent competition. In addition, we show that the likelihood of parasite‐facilitated species coexistence depends greatly on the specific type of parasite. In particular, macroparasites (e.g., parasitic helminths) may be less likely to facilitate species coexistence than microparasites (e.g., viruses or bacteria) due to their typically highly aggregated distribution amongst their hosts. Furthermore, species coexistence is more likely if the parasite is relatively benign to its host. Parasitism by apparently “harmless” specialist parasites may provide an important but overlooked factor in the maintenance of species diversity, facilitating species invasions into new communities and the emergence of novel infectious diseases.

Prevalence and intensity of pentastomid infection in two species of snakes from northeastern Brazil

This study aimed to evaluate the infection rates of snakes by pentastomids in the semi-arid region of Brazil. Fifteen snakes (four Micrurus ibiboboca (Merrem, 1820) and eleven Philodryas nattereri Steindachner, 1870) were collected between January and April of 2005, in the municipality of Crato (07 degrees 14' S and 39 degrees 24' W), State of Ceará, Brazil. Laboratorial analysis of the respiratory tracts of the sampled snakes indicated differences in host infection rates: four individuals of P. nattereri (36.4%) were infected by Cephalobaena tetrapoda Heymons, 1922 (mean infection intensity 1.5 +/- 0.28, 1-2) and three specimens (27.3%) by Raillietiella furcocerca (Diesing, 1863) (2.3 +/- 1.32, 1-5). Only one individual of M. ibiboboca (25%) was infected by a non-identified species of Raillietiella sp. These are the first data on pentastomid infection in snakes in Northeastern Brazil and both snake species comprise new host records for the pentastomids. The results also indicate that the generalist parasites C. tetrapoda and R. furcocerca share their definitive hosts.

ENDANGERED LIGHT‐FOOTED CLAPPER RAIL AFFECTS PARASITE COMMUNITY STRUCTURE IN COASTAL WETLANDS

An extinction necessarily affects community members that have obligate relationships with the extinct species. Indirect or cascading effects can lead to even broader changes at the community or ecosystem level. However, it is not clear whether generalist parasites should be affected by the extinction of one of their hosts. We tested the prediction that loss of a host species could affect the structure of a generalist parasite community by investigating the role of endangered Light-footed Clapper Rails (Rallus longirostris levipes) in structuring trematode communities in four tidal wetlands in southern California, U.S.A. (Carpinteria Salt Marsh, Mugu Lagoon) and Mexico (Estero de Punta Banda, Bahia Falsa-San Quintin). We used larval trematode parasites in first intermediate host snails (Cerithidea californica) as windows into the adult trematodes that parasitize Clapper Rails. Within and among wetlands, we found positive associations between Clapper Rails and four trematode species, particularly in the vegetated marsh habitat where Clapper Rails typically occur. This suggests that further loss of Clapper Rails is likely to affect the abundance of several competitively dominant trematode species in wetlands with California horn snails, with possible indirect effects on the trematode community and changes in the impacts of these parasites on fishes and invertebrates.

Abomasal nematodes of the red deerCervus elaphusin north-eastern Italy

The composition of the abomasal helminth fauna and parasite diversity were studied in 298 red deer collected during 1997-2000 from three different sectors (Bolzano, Trento and Sondrio provinces) of the Stelvio National Park, one of the main protected areas of north-eastern Italy. The association between parasite burdens and geographical areas of the hosts was assessed using the negative binomial regression. A variety of abomasal helminths, both host specific and generalist, was found in all sectors. The most commonly observed parasites were the Spiculopteragia spiculoptera morph spiculoptera and Ostertagia leptospicularis morph leptospicularis, with prevalences of 79.5% and 40.9%, respectively. The minor morphs S. spiculoptera morph mathevossiani (prevalence 31.9%) and O. leptospicularis morph kolchida (18.8%) occurred at lower prevalences. Teladorsagia circumcincta morph circumcincta, Marshallagia marshalli morph marshalli and Haemonchus contortus were rarer, at prevalences of 1, 1.3 and 1.3%, respectively. Deviance analysis of the negative binomial regression model shows that the geographical area is significantly related to parasite burdens (P = 0.001). Prevalences of hosts with parasites were greater in the Sondrio (odds ratio = 1.31; 95% confidence interval: 0.16-10.85) and smaller in the Trento (odds ratio = 0.62; 95% confidence interval: 0.20-1.96) sectors with respect to Bolzano, but these differences were not statistically significant. Possible cross-infections by more generalist parasites between wild and domestic animals were also suggested, as deer and domestic ruminants (Bovinae, Caprinae and Ovinae) used the same feeding areas of the park sectors.

Chemical deterrent enables a socially parasitic ant to invade multiple hosts

Social parasites are involved in a coevolutionary arms race, which drives increasing specialization resulting in a very narrow host range. The Formicoxenus ants are a small group of social parasites with a xenobiotic lifestyle. Formicoxenus quebecensis and Formicoxenus provancheri are highly specialized ants using chemical mimicry to blend into their respective Myrmica ant host colonies. However, Formicoxenus nitidulus is unique in being able to survive in over 11 different ant host species. We observed that when live or dead F. nitidulus adults are seized by their host they are immediately dropped undamaged, despite possessing a cuticular hydrocarbon profile that differs markedly from its host. Hexane extracts of the F. nitidulus cuticle made previously acceptable prey items unattractive to their Formica host, indicating a chemical deterrent effect. This is the first time that a social parasite has been shown to exploit the generalized deterrence strategy to avoid host aggression over long periods of time. This supports the idea that coevolved and generalist diseases or parasites require fundamentally different defence mechanisms. We suggest that F. nitidulus uses its cuticular chemistry, possible alkadienes, as a novel deterrent mechanism to allow it to switch hosts easily and so become a widespread and abundant social parasite.

Impacts of agriculture on the parasite communities of northern leopard frogs (Rana pipiens) in southern Quebec, Canada

Given that numerous amphibians are suffering population declines, it is becoming increasingly important to examine the relationship between disease and environmental disturbance. Indeed, while many studies relate anthropogenic activity to changes in the parasitism of snails and fishes, little is known of the impact on the parasites of amphibians, particularly from agriculture. For 2 years, the parasite communities of metamorphic northern leopard frogs from 7 agricultural wetlands were compared with those from 2 reference wetlands to study differences in parasite community diversity and abundance of various species under pristine conditions and 3 categories of disturbance: only agricultural landscape, only pesticides, and agricultural landscape with pesticides. Agricultural (and urban) area was negatively related to species richness, and associated with the near absence of adult parasites and species that infect birds or mammals. We suggest that agriculture and urbanization may hinder parasite transmission to frogs by limiting access of other vertebrate hosts of their parasites to wetlands. The only parasite found at all localities was an unidentified echinostome infecting the kidneys. This parasite dominated communities in localities surrounded by the most agricultural land, suggesting generalist parasites may persist in disrupted habitats. Community composition was associated with dissolved organic carbon and conductivity, but few links were found with pesticides. Pollution effects may be masked by a strong impact of land use on parasite transmission.

Partial host fidelity in nest selection by the shiny cowbird (Molothrus bonariensis), a highly generalist avian brood parasite

Obligate avian brood parasites can be host specialists or host generalists. In turn, individual females within generalist brood parasites may themselves be host specialists or generalists. The shiny cowbird Molothrus bonariensis is an extreme generalist, but little is known about individual female host fidelity. We examined variation in mitochondrial control region sequences from cowbird chicks found in nests of four common Argentinean hosts. Haplotype frequency distributions differed among cowbird chicks from nests of these hosts, primarily because eggs laid in nests of house wrens Troglodytes aedon differed genetically from those laid in nests of the other three hosts (chalk-browed mockingbird Mimus saturninus, brown-and-yellow marshbird Pseudoleistes virescens, and rufous-collared sparrow Zonotrichia capensis). These differences in a maternally inherited marker indicate the presence of a nonrandom laying behaviour in the females of this otherwise generalist brood parasite, which may be guided by choice for nest type, as house wrens nest in cavities whereas the other three species are open cup nesters.

Parasite species richness in carnivores: effects of host body mass, latitude, geographical range and population density

ABSTRACTAim Comparative studies have revealed strong links between ecological factors and the number of parasite species harboured by different hosts, but studies of different taxonomic host groups have produced inconsistent results. As a step towards understanding the general patterns of parasite species richness, we present results from a new comprehensive data base of over 7000 host–parasite combinations representing 146 species of carnivores (Mammalia: Carnivora) and 980 species of parasites.Methods We used both phylogenetic and non‐phylogenetic comparative methods while controlling for unequal sampling effort within a multivariate framework to ascertain the main determinants of parasite species richness in carnivores.Results We found that body mass, population density, geographical range size and distance from the equator are correlated with overall parasite species richness in fissiped carnivores. When parasites are classified by transmission mode, body mass and home range area are the main determinants of the richness of parasites spread by close contact between hosts, and population density, geographical range size and distance from the equator account for the diversity of parasites that are not dependent on close contact. For generalist parasites, population density, geographical range size and latitude are the primary predictors of parasite species richness. We found no significant ecological correlates for the richness of specialist or vector‐borne parasites.Main conclusions Although we found that parasite species richness increases instead of decreases with distance from the equator, other comparative patterns in carnivores support previous findings in primates, suggesting that similar ecological factors operate in both these independent evolutionary lineages.

Relative number of generations of hosts and parasites does not influence parasite local adaptation in coevolving populations of bacteria and phages

A potential consequence of host-parasite coevolution in spatially structured populations is parasite local adaptation: local parasites perform better than foreign parasites on their local host populations. It has been suggested that the generally shorter generation times of parasites compared with their hosts contributes to parasites, rather than hosts, being locally adapted. We tested the hypothesis that relative generation times of hosts and parasites affect local adaptation of hosts and parasites, using the bacterium Pseudomonas fluorescens and a lytic phage as host and parasite, respectively. Generation times were not directly manipulated, but instead one of the coevolving partners was regularly removed and replaced with a population from an earlier time point. Thus, one partner underwent more generations than the other. Manipulations were carried out at both early and later periods of coevolutionary interactions. At early stages of coevolution, host and parasites that underwent relatively more generations displayed higher levels of resistance and infectivity, respectively. However, the relative number of generations that bacteria and phages underwent did not change the level of local adaptation relative to control populations. This is likely because generalist hosts and parasites are favoured during early stages of coevolution, preventing local adaptation. By contrast, at later stages manipulations had no effect on either average levels of resistance or infectivity, or alter the level of local adaptation relative to the controls, possibly because traits other than resistance and infectivity were under strong selection. Taken together, these data suggest that the relative generation times of hosts and parasites may not be an important determinant of local adaptation in this system.

SPECIFICITY AND SPECIALIZATION OF CONGENERIC MONOGENEANS PARASITIZING CYPRINID FISH

Patterns and likely processes connected with evolution of host specificity in congeneric monogeneans parasitizing fish species of the Cyprinidae were investigated. A total of 51 Dactylogyrus species was included. We investigated (1) the link between host specificity and parasite phylogeny; (2) the morphometric correlates of host specificity, parasite body size, and variables of attachment organs important for host specificity; (3) the evolution of morphological adaptation, that is, attachment organ; (4) the determinants of host specificity following the hypothesis of specialization on more predictable resources considering maximal body size, maximal longevity, and abundance as measures of host predictability; and (5) the potential link between host specificity and parasite diversification. Host specificity, expressed as an index of host specificity including phylogenetic and taxonomic relatedness of hosts, was partially associated with parasite phylogeny, but no significant contribution of host phylogeny was found. The mapping of host specificity into the phylogenetic tree suggests that being specialist is not a derived condition for Dactylogyrus species. The different morphometric traits of the attachment apparatus seem to be selected in connection with specialization of specialist parasites and other traits favored as adaptations in generalist parasites. Parasites widespread on several host species reach higher abundance within hosts, which supports the hypothesis of ecological specialization. When separating specialists and generalists, we confirmed the hypothesis of specialization on a predictable resource; that is, specialists with larger anchors tend to live on fish species with larger body size and greater longevity, which could be also interpreted as a mechanism for optimizing morphological adaptation. We demonstrated that ecology of host species could also be recognized as an important determinant of host specificity. The mapping of morphological characters of the attachment organ onto the parasite phylogenetic tree reveals that morphological evolution of the attachment organ is connected with host specificity in the context of fish relatedness, especially at the level of host subfamilies. Finally, we did not find that host specificity leads to parasite diversification in congeneric monogeneans.

Distribution of substance P reveals a novel subdivision in the hippocampus of parasitic South American cowbirds

Parasitic cowbirds monitor potential hosts' nests and return to lay when appropriate, a task that is likely to involve spatial recall. Seasonal and sexual behavioral variations in the cowbirds correlate with anatomical changes in the hippocampal formation. During the breeding season, parasites have larger hippocampal formations than nonparasites. In parasitic species in which females alone perform nest bookkeeping, females have larger hippocampal formations than males. We investigated the distribution of the neuropeptide substance P (SP) in three sympatric cowbirds: two obligate parasites (shiny cowbird and screaming cowbird) and one nonparasite (bay-winged cowbird). Distribution of SP was similar to that in other songbirds, except for a previously undescribed field of dense SP-rich terminals within the hippocampus that we call the hippocampal SP terminal field (SPh). We found robust species differences in the volume of this new area, measured relative to the remainder of the telencephalon. SPh was largest in the generalist parasite (shiny cowbird) and smallest in the nonparasitic species (bay-winged cowbird). In the specialist parasite (screaming cowbird), SPh was smaller than in the generalist parasite but larger than in the nonparasitic species. SPh overlaps with two subdivisions described in the pigeon that have been related to the mammalian dentate gyrus and subiculum. The area containing SPh receives a major input from the lateral mammillary nucleus, which is probably the avian equivalent of the mammalian supramammillary nucleus (SUM), the main source of extrinsic SP input to mammalian hippocampus. SPh may be the termination of a pathway homologous to the SP-rich projection from SUM to the hippocampus in mammals.

Generalist cuckoo bees (Hymenoptera: Apoidea: Sphecodes) are species-specialist at the individual level

Intensive and incessant arms races between a parasite and its host are generally expected to lead to parasite specialization. Nevertheless, some parasitic species still successfully attack wide spectra of hosts. One of the solutions to the evolutionary enigma of the long-term existence of generalist parasites is their specialization at an individual level, a phenomenon well known, e.g., in European common cuckoo. Over its range, it parasitizes a number of bird species; however, individual females are mostly specialists possessing adaptations to a particular host species. In this study, we test the possibility of individual specialization in generalist cuckoo bees, the insect counterparts of avian cuckoos. Females of cuckoo bees lay each egg into a single brood cell in the nests of other bee species. The host’s offspring is destroyed by the parasitic female or later by her larvae, which feed on pollen supplies accumulated by the host. Both studied cleptoparasitic bees (Sphecodes ephippius and Sphecodes monilicornis) are widely distributed in Europe, where they have been reported to use broad host spectra. We recorded several host species (including some previously unknown) for both cuckoo bee species, and confirmed that these parasites are indeed generalist even at a small local scale. However, we demonstrate that exactly as in the avian cuckoos, each female in both species of generalist bee parasites tends to attack just one host species.

Parasitic plants indirectly regulate below-ground properties in grassland ecosystems

Parasitic plants are one of the most ubiquitous groups of generalist parasites in both natural and managed ecosystems, with over 3,000 known species worldwide. Although much is known about how parasitic plants influence host performance, their role as drivers of community- and ecosystem-level properties remains largely unexplored. Parasitic plants have the potential to influence directly the productivity and structure of plant communities because they cause harm to particular host plants, indirectly increasing the competitive status of non-host species. Such parasite-driven above-ground effects might also have important indirect consequences through altering the quantity and quality of resources that enter soil, thereby affecting the activity of decomposer organisms. Here we show in model grassland communities that the parasitic plant Rhinanthus minor, which occurs widely throughout Europe and North America, has strong direct effects on above-ground community properties, increasing plant diversity and reducing productivity. We also show that these direct effects of R. minor on the plant community have marked indirect effects on below-ground properties, ultimately increasing rates of nitrogen cycling. Our study provides evidence that parasitic plants act as a major driver of both above-ground and below-ground properties of grassland ecosystems.

Species abundance and the distribution of specialization in host–parasite interaction networks

1 Recent studies have evaluated the distribution of specialization in species interaction networks. Species abundance patterns have been hypothesized to determine observed topological patterns. We evaluate this hypothesis in the context of host–parasite interaction networks. 2 We used two independent series of data sets, one consisting of data for seven sites describing interactions between freshwater fish and their metazoan parasites and another consisting of data for 25 localities describing interactions between fleas and their mammalian hosts. We evaluated the influence of species abundance patterns on the distribution of specialization in these host–parasite interaction networks with the aid of null models. 3 In parallel with recent studies of plant–animal mutualistic networks, our analyses suggest that host–parasite interactions in these systems are highly asymmetric: specialist parasites tend to interact with hosts with high parasite richness, whereas hosts with low parasite richness tend to interact mainly with generalist parasites. 4 The observed distribution of specialization was predicted by a null model that assumed that species-specific probabilities of being assigned a link during the randomization process were roughly proportional to their relative abundance. Thus, abundant hosts tend to harbour richer parasite faunas, with a high proportion of rare specialists.

Mechanisms of Fungal Speciation

The objective of this review is to provide a synthesis of speciation theory, of what is known about mechanisms of speciation in fungi and from this, what is expected, and of ideas on how speciation can be elucidated in more fungal systems. The emphasis is on process rather than pattern. Phylogeographic studies in some groups, such as the agarics, demonstrate predominantly allopatric speciation, often through vicariance, as seen in many plants and animals. The variety of life history factors in fungi suggests, however, a diversity in speciation mechanisms that is borne out in comparison of some key examples. Life history features in fungi with a bearing on speciation include genetic mechanisms for intra- and interspecies interactions, haploidy as monokaryons, dikaryons, or coenocytes, distinctive types of propagules with distinctive modes of dispersal, as well as characteristic relationships to the substrate or host as specialized or generalist saprotrophs, parasites or mutualists with associated opportunities and selective pressures for hybridization. Approaches are proposed for both retrospective, phylogeographic determination of speciation mechanisms, and experimental studies with the potential for genomic applications, particularly in examining the relationship between adaptation and reproductive isolation.

The effect of host heterogeneity and parasite intragenomic interactions on parasite population structure

Understanding the processes that shape the genetic structure of parasite populations and the functional consequences of different parasite genotypes is critical for our ability to predict how an infection can spread through a host population and for the design of effective vaccines to combat infection and disease. Here, we examine how the genetic structure of parasite populations responds to host genetic heterogeneity. We consider the well-characterized molecular specificity of major histocompatibility complex binding of antigenic peptides to derive deterministic and stochastic models. We use these models to ask, firstly, what conditions favour the evolution of generalist parasite genotypes versus specialist parasite genotypes? Secondly, can parasite genotypes coexist in a population? We find that intragenomic interactions between parasite loci encoding antigenic peptides are pivotal in determining the outcome of evolution. Where parasite loci interact synergistically (i.e. the recognition of additional antigenic peptides has a disproportionately large effect on parasite fitness), generalist parasite genotypes are favoured. Where parasite loci act multiplicatively (have independent effects on fitness) or antagonistically (have diminishing effects on parasite fitness), specialist parasite genotypes are favoured. A key finding is that polymorphism is not stable and that, with respect to functionally important antigenic peptides, parasite populations are dominated by a single genotype.

Population dynamics and avian brood parasitism: persistence and invasions in a three‐species system

Summary Avian brood parasites include species that are host specialists and others that are generalists. The impact of each kind of parasite on the persistence of the host population is studied by means of a population dynamics model. Our model examines conditions for coexistence and invasions in a community of three South American cowbirds, the shiny cowbird Molothrus bonariensis (a generalist parasite), the screaming cowbird M. rufoaxillaris (a specialist parasite), and the bay‐winged cowbird Agelaiodes badius (a nonparasite that hosts the other two). Three biologically realistic characteristics not previously included in brood parasitism models are explored and shown to be crucial for the stability of the system. These characteristics are: (i) female parasites take at least a day to produce an egg and cannot store eggs for delayed laying – this is modelled by means of a type II functional response; (ii) parasites often remove or puncture (destroy) host eggs when visiting a nest; and (iii) hosts desert nests when the total clutch (host plus parasite) exceeds some threshold. These characteristics have a direct impact on parasite population renewal and reduce dramatically the stable coexistence conditions. Comparing the stability conditions of the host–specialist system with those of the three‐species system shows the impact of the arrival of a generalist parasite on the persistence of the host–specialist system when the three characteristics are present. The stability boundaries are restricted when the generalist is absent and change little for a realistic density of generalist. The study of invasion by a specialist into a host–generalist stable community shows that the parameter region for coexistence and invasion coincide. Comparison of our model against its precursors using field data for the parameters when available, shows that the three‐species cowbird model system is stable for empirically realized parameter values, unlike a previous model by May & Robinson (1985; American Naturalist, 126, 475–494) where none of the three characteristics were included.

Impatiens capensis (Balsaminaceae) Meerb. is a Necessary Nurse Host for the Parasitic Plant Cuscuta gronovii (Cuscutaceae) Willd. ex J.A. Schultes in Southeastern Michigan Wetlands

Abstract Generalist parasites have been shown to have the potential to substantially affect the structure of the communities they inhabit. In order to predict the potential effects of these parasites, understanding the relationships they have with their host species is critical. In this study, the host range of Cuscuta gronovii was determined at two different times during the growing season, which corresponded with two life stages of parasitic individuals, seedling and adult. Field observations suggest that most of the successful infections by seedlings occurred upon only Impatiens capensis, one of the many species that it infects as an adult. This paper reports three tests of the hypothesis that I. capensis is a necessary nurse host for the parasite C. gronovii. In early summer survey plots, I. capensis was found to be infected by C. gronovii seedlings significantly more than predicted by the null hypothesis that all species in the plot were equally likely to be infected. In a late winter survey, in whic...

Parasite Infection Status of Potamopyrgus Antipodarum

Outside its native range, the invasive species Potamopyrgus antipodarum (New Zealand Mud snail; NZMS) has seldom been reported to harbor parasites. To test this intriguing observation, 7 sites along the Snake River and Polecat Creek in the Grand Teton National Park/John D Rockefeller Memorial Parkway area (W yoming, USA) were surveyed for NZMS, native aquatic snails and parasites: digenean trematodes (specialist parasites of snails) and nematomorphs (horse hair worms, generalist parasites of aquatic invertebrates), in July 2005. Non-destructive examination of 96 native snails at each site revealed abundant presence of digeneans (parasites that specialize in snails) at 6 sites. Within 2 hours, up to 12.4% of lymnaeid snails shed furcocercous cercaria or xiphidiocercaria, and up to 50% of physid snails released furcocercous cercaria or echinostome cercaria. A total of ~900 NZMS (collected from 5 of the sites) were investigated for infection with digeneans by overnight shedding. One NZMS yielded amphistome­like metacercariae that had encysted in the observation tray. The dissection of 150 ethanol-fixed NZMS (30/site) revealed several types of internal metacercarial stages of digeneans in 5 snails (all from Polecat Creek). PCR-based assays targeting parasite rDNA sequences in DNA extracted from pools of 150 snails each also indicated associations between digenean parasites and NZMS. All parasites were detected in NZMS from Polecat Creek. NZMS were not observed to harbor nematomorph parasites with any of the techniques used. In conclusion, the sites tested contained multiple native snail-digenean parasite associations, and the capacity of invasive NZMS outside the native range to serve as first and second intermediate host for digenean parasites may have been underestimated previously.

TWO NEW NEMATODE SPECIES, ORIENTATRACTIS CAMPECHENSIS N. SP. AND ORIENTATRACTIS CHIAPASENSIS N. SP. (NEMATODA: ATRACTIDAE) FROM CICHLID FISHES IN SOUTHERN MEXICO AND NICARAGUA

Two new nematode species, Orientatractis campechensis n. sp. and Orientatractis chiapasensis n. sp. (Atractidae, Cosmocercoidea), are described from the intestine of cichlid fishes collected in localities of southeastern Mexico and Nicaragua. These atractid nematodes are characterized by the presence of a cephalic end armed with 4 marked, Y-shaped, well-sclerotized pieces, consisting of 2 "horns" extending outward and downward and immediately below a single-horned structure. Both new species differ from the other 2 species of Orientatractis (Orientatractis leiperi and Orientatractis levanhoai) mainly in the body measurements, presence of a vulvar appendix, lengths of spicules and of the gubernaculum, distribution of caudal papillae, and the number of anterior sclerotized structures. Morphological and biometrical variations were evident among specimens from different hosts and localities, with the larger organisms being collected from Vieja bifasciata and Cichlasoma pearsei. Orientatractis campechensis and O. chiapasensis represent the first 2 species of this genus parasitizing freshwater fishes; the other species have been found in amphibians and reptiles.