Haematoloechus medioplexus

Haematoloechus medioplexus have been collected from regions extending from the eastern coast of Mexico to the central United States, as far north as Nebraska. (Leon-Regagnon, et al., 1999)

Haematoloechus medioplexus generally inhabit aquatic environments (freshwater only), where their aquatic and amphibian hosts are likely to be found.

Normally the egg of H. medioplexus is swallowed by the Planorbula armigera snail, indicating high first intermediate host-specificity. Second intermediate hosts are restricted to the nymphs of Libellula lydia. This high second intermediate host-specificity may have an effect on H. medioplexus's definitive host range, since the adult flukes are normally found in Lithobates pipiens. Other frogs can also serve as the flukes' definitive hosts. (Leon-Regagnon, et al., 1999; Roberts and Janovy, Jr., 2000)

Haematoloechus medioplexus go through several morphologically distinct developmental stages that include the egg, the miracidium, the sporocyst, the redia, the cercaria, the metacercaria, and the adult.

Adult H. medioplexus have transparent, leaf-shaped bodies that are up to 8 mm long and 1.2 mm wide. Belonging to the Phylum Platyhelminthes, or flatworms, they retain the dorsoventrally flattened body, which is composed largely of parenchyma, or a mesh of loose fibers interspersed with various specialized cell types. As in other digeneans, H. medioplexus adults possess a tegument consisting of a layer of anucleated living tissues known as the distal cytoplasma, which lies atop a nucleated layer. Being amphistomic, they possess an acetabulum, or ventral sucker, in addition to an oral sucker, which lies anterior to a muscular pharynx. Posterior to the pharynx is a short esophagus that branches into a pair of lateral cecal chambers (or gut) that are lined with a layer of cells known as the gastrodermis. The nervous system is of the characteristic orthogon type found within the Class Trematoda: it is basically composed of a pair of ganglia at the anterior end with three longitudinal nerve trunks running posteriorly along the length of the body. Being monoecious, H. medioplexus possess both male and female sex organs. The female system includes a large ovary, an extensive uterus that is normally filled with eggs, and masses of vitelline cells (which contribute the yolk to the yolk-less egg); the metraterm, which lies at the distal end of the uterus, functions as a vagina. In the male system, a pair of testes is connected to the vas deferens via a pair of vas efferens, which in turn is connected to a copulatory organ called the cirrus, located at the genital pore.

The egg of H. medioplexus is technically a developing embryo that is housed within a protective shell. The embryo eventually exits the eggshell via a cap-like structure known as the operculum, which is found at one end of the shell. The resulting miracidium, resembling a protozoan, is pyriform in shape and is covered with a layer of nucleated epithelial cells bearing a mat of cilia. A group of posterior germ balls (which enable asexual reproduction) is also present in the miracidium. After the cilial layer is shed, the resulting sporocyst is quite simple in structure. Within the sporocyst, asexual reproduction may take place to give rise to multiple rediae. At this stage, a rudimentary digestive system, consisting of a mouth, a pharynx, and an unbranched cecum, become present. The redia then reproduces asexually into cercariae, which, in H. medioplexus, is classified under the grade xiphidiocercariae for having possessed a simple tail and an oral stylet. A branched cecum and an oral sucker (i.e., acetabulum) appear at this stage. Finally, intermediate between the cercaria and the adult stage is the metacercaria stage, which for the most part is dormant, thus undergoing little morphological development. (Humphries, et al., 1997; Roberts and Janovy, Jr., 2000; Snyder and Janovy, Jr., 1994)

Haematoloechus medioplexus goes through a complex life cycle that involves seven developmental stages: the egg, the miracidium, the sporocyst, the redia, the cercaria, the metacercaria, and the adult. These stages cycle between the environment and three different hosts, two of which are intermediate hosts and one is a definitive host. Sexual reproduction only occurs at the adult stage, within the definitive host. Asexual reproduction may occur at the sporocyst and the redia stages, within the first intermediate host.

Adult H. medioplexus are hermaphroditic (i.e., having both male and female systems), meaning that each can produce eggs. Prodigious numbers of eggs are laid inside the lung of frogs (i.e., the definitive host), where the adults live and feed. By ciliary action, the egg makes its way up the respiratory tract of the frog and is swallowed. Passing through the frog's digestive tract and out into the environment, the egg may be swallowed by a snail (the first intermediate host), within whose body the egg hatches into a miracidium. The miracidium then develops into a sporocyst. At this stage, asexual reproduction may occur, giving rise to multiple rediae. Rediae then asexually reproduce cercariae, which escape from the snail's body into the aquatic environment. Swimming freely, a cercaria may encounter a dragonfly nymph (the second intermediate host) and is drawn into its branchial basket (a structure serving respiratory functions, located at the posterior end of the nymph). The cercaria then encysts itself within nearby tissues. At this stage, the cercaria has become a metacercaria. The metacercaria remains within the dragonfly's body throughout its nymphal and adult stages. When the adult dragonfly is eaten by a frog, the metacercaria encysts within the frog's stomach, after which it excysts (or hatch), migrates up the digestive tract, and down to the lungs via the frog's respiratory tract. There the adult fluke dwells and produces copious amount of eggs via sexual reproduction. (Roberts and Janovy, Jr., 2000; Snyder and Janovy, Jr., 1997)

Haematoloechus medioplexus go through a complex life cycle that involves seven developmental stages: the egg, the miracidium, the sporocyst, the redia, the cercaria, the metacercaria, and the adult. These stages cycle between the environment and three different hosts, two of which are intermediate hosts and one is a definitive host. Sexual reproduction only occurs at the adult stage, within the definitive host. Asexual reproduction may occur at the sporocyst and the redia stages, within the first intermediate host.

Adult H. medioplexus are hermaphroditic (i.e., having both male and female systems), meaning that each can produce eggs. Prodigious numbers of eggs are laid inside the lung of frogs (i.e., the definitive host), where the adults live and feed. (Roberts and Janovy, Jr., 2000; Snyder and Janovy, Jr., 1997)

More research has been done on cercarial behavior than on any other developmental stages of digenean flukes. This special interest is to be expected since the fitness of the individual fluke depends on whether suitable host species can be infected at every stage of their complex life cycles. Since the cercarial stage is free-swimming, the fluke's reproductive success is largely contingent upon the successful location and infection of the host by the cercaria. For this reason, cercariae are shown to exhibit more variety in sensory capabilities than flukes at other stages in development, including sensitivity to light, chemical gradients, pressure, gravity, and so on. Moreover, because cercariae are free-swimming, they are potentially more vulnerable to predation and other environmental stresses. Hence, cercariae may exhibit a suite of behavioral adaptations in response to these selective pressures.

Haematoloechus medioplexus cercariae have been found to paratisize only dragonfly nymphs of the species Libellula lydia. A link has been made between the cercariae's narrow range in host-specificity and their behavior. It is found that a cercaria must be sucked into the nymph' branchial basket in order to infect it. For this reason, the cercariae display no attachment behavior towards their hosts, nor do they drill into the host's exoskeleton. Also, the cercariae do not have the capacity to recognize encounters with their hosts. It is likely that they do possess the capacity to optimize the probability of encountering a host. Presumably, the probability of host encounter is proportional to the probability of infection. (Haas, 1992; Roberts and Janovy, Jr., 2000; Snyder and Janovy, Jr., 1994; Snyder and Janovy, Jr., 1997)

More research has been done on cercarial behavior than on any other developmental stages of digenean flukes. The fitness of the individual fluke depends on whether suitable host species can be infected at every stage of their complex life cycles. Since the cercarial stage is free-swimming, the fluke's reproductive success is largely contingent upon the successful location and infection of the host by the cercaria. For this reason, cercariae are shown to exhibit more variety in sensory capabilities than flukes at other stages in development, including sensitivity to light, chemical gradients, pressure, gravity, and so on. (Haas, 1992; Roberts and Janovy, Jr., 2000; Snyder and Janovy, Jr., 1994; Snyder and Janovy, Jr., 1997)

Adult H. medioplexus are endoparasites that live within the lungs of frogs (the definitive hosts) and feed on blood extracted from the host's capillaries. Using the oral sucker for attachment, a plug of lung tissue is sucked into the mouth through the pumping action of the muscular pharynx. This pumping action eventually damages the epithelial layer, resulting in the rupturing of superficial capillaries; at the same time, blood is being pumped into the fluke's gut for digestion. Within the gastrodermis lined gut, the blood cells are then ruptured to release their contents, which are broken down with a suite of lysosomal proteases secreted by the gastrodermis. Absorption of soluble blood components into the fluke's cells then occurs through the dense mat of gastrodermal microvilli, which serve to expand the surface area of the gut. Insoluble components, which consist mostly of the iron-containing molecules found in hemoglobin, are then expelled from the gut via periodic regurgitation.

The free-living larval stages of H. medioplexus--the miracidium and the cercacia--are non-feeding. But the redia, which parasitizes a snail (the first intermediate host), feeds on host tissue either by direct absorption across the tegument or via the sucking action of the rudimentary mouth and pharynx. And the metacercaria, after having encysted within a dragonfly host (the second intermediate host), can normally subsist on host tissue. (Bogitsh, 1985; Halton, 1967; Roberts and Janovy, Jr., 2000)

These animals are probably not preyed on directly but are ingested. Egg and larval mortality are high since the parasites often do not reach appropriate hosts.

Normally the egg of H. medioplexus is swallowed by the Planorbula armigera snail, indicating high first intermediate host-specificity. Second intermediate hosts are restricted to the nymphs of Libellula lydia. This high second intermediate host-specificity may have an effect on H. medioplexus's definitive host range, since the adult flukes are normally found in Lithobates pipiens. Other frogs can also serve as the flukes' definitive hosts. (Leon-Regagnon, et al., 1999; Roberts and Janovy, Jr., 2000)

Haematoloechus medioplexus are not important in either human or veterinary medicine since they have such a narrow range of host-specificity and parasitize neither bird nor mammal. They are also of negligible economic importance. (Roberts and Janovy, Jr., 2000)

There is no conservation status for H. medioplexus.