Stereoscan studies of rediae, cercariae, cysts, excysted metacercariae and migratory stages of Fasciola hepatica

A cascade of biological invasions and parasite spillback in man-made Lake Kariba

Parasite spillback, the infection of a non-indigenous organism by a native parasite, is a highly important although understudied component of ecological invasion dynamics. Here, through the first analysis of the parasite fauna of lymnaeid gastropods of Lake Kariba (Zimbabwe). We illustrate how the creation of an artificial lake may lead to a cascade of biological invasions in which an invasive aquatic plant promotes the proliferation of invasive gastropods, which in turn alters the epidemiology of trematodiases of potential medical and veterinary importance. Using a new multiplex Rapid Diagnostic PCR assay, we assessed the prevalence of Fasciola sp. infections in the gastropod populations. Both gastropod hosts and trematode parasites were identified using DNA barcoding. We provide the first record of the invasive North-American gastropod Pseudosuccinea columella in Lake Kariba. This species was found at 14 out of 16 sampled sites and its abundance was strongly positively correlated with the abundance of the invasive South-American water hyacinth (Eichhornia crassipes). About 65% of the P. columella specimens analysed were infected with a hitherto unknown Fasciola species. Phylogenetic analyses indicate close affinity to Fasciola hepatica and F. gigantica, which cause fasciolosis, an important liver disease affecting both ruminants and humans. In addition, another non-native Lymnaeid species was found: a Radix sp. that clustered closely with a Vietnamese Radix species. Radix sp. hosted both amphistome and Fasciola trematodes. By linking an invasion cascade and parasite spillback, this study shows how both processes can act in combination to lead to potentially important epidemiological changes.

Detection of Galba truncatula, Fasciola hepatica and Calicophoron daubneyi environmental DNA within water sources on pasture land, a future tool for fluke control?

Background

Increasing trematode prevalence and disease occurrence in livestock is a major concern. With the global spread of anthelmintic resistant trematodes, future control strategies must incorporate approaches focusing on avoidance of infection. The reliance of trematodes on intermediate snail hosts to successfully complete their life-cycle means livestock infections are linked to the availability of respective snail populations. By identifying intermediate snail host habitats, infection risk models may be strengthened whilst farmers may confidently apply pasture management strategies to disrupt the trematode life-cycle. However, accurately identifying and mapping these risk areas is challenging.

Methods

In this study, environmental DNA (eDNA) assays were designed to reveal Galba truncatula, Fasciola hepatica and Calicophoron daubneyi presence within water sources on pasture land. eDNA was captured using a filter-based protocol, with DNA extracted using the DNeasy® PowerSoil® kit and amplified via PCR. In total, 19 potential G. truncatula habitats were analysed on four farms grazed by livestock infected with both F. hepatica and C. daubneyi.

Results

Galba truncatula eDNA was identified in 10/10 habitats where the snail was detected by eye. Galba truncatula eDNA was also identified in four further habitats where the snail was not physically detected. Fasciola hepatica and C. daubneyi eDNA was also identified in 5/19 and 8/19 habitats, respectively.

Conclusions

This study demonstrated that eDNA assays have the capabilities of detecting G. truncatula, F. hepatica and C. daubneyi DNA in the environment. Further assay development will be required for a field test capable of identifying and quantifying F. hepatica and C. daubneyi infection risk areas, to support future control strategies. An eDNA test would also be a powerful new tool for epidemiological investigations of parasite infections on farms.

Developmental and functional ultrastructure of Ornithodiplostomum ptychocheilus diplostomula (Trematoda: Strigeoidea) during invasion of the brain of the fish intermediate host, Pimephales promelas

We examined tegumental development of the diplostomulum of Ornithodiplostomum ptychocheilus, with respect to structural transformations that have functional relevance to the invasion, migration, and site establishment processes in the brain of the fish second-intermediate host, Pimephales promelas. Using a combination of brightfield, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and confocal microscopy (CM), we demonstrated that the diplostomula become established in the outer region of the optic lobes within 24-48 hr of penetration and continue to grow and transform over a period of 4-14 days. During this period, the J-shaped body consists of 2 distinct regions: (1) a highly motile prosoma with distinctive tegumental spines and (2) an opisthosoma, the tegument of which is elaborated into a dense uniform layer of long, thin microvilli. The prosoma is alternately invaginated into and everted from the opisthosoma, thus constituting a protrusible proboscis. By day 14 postinfection (PI), the body has lost this bipartite structure and has taken on the uniformly flattened form characteristic of metacercariae. The transitory complex structure of the diplostomula appears to be well suited to burrowing through host tissues (primarily by action of the prosoma), followed by rapid dissociation of host tissue and nutrient accumulation (primarily by action of the opisthosoma) in preparation for metacercaria encystment.

Fasciola hepatica: surfaces involved in movement of miracidia and cercariae

Rapid freezing and substitution with fixative prior to scanning electron microscopy was used to demonstrate the pattern of beat and recovery of the cilia of free swimming miracidia of Fasciola hepatica. There were stages of dexioplectic metachronal co-ordination and the power stroke was approximately 15 degrees anticlockwise from the anterior-posterior axis. Around the circumference of the body of the miracidia there were approximately 12 metachronal waves of power and recovery. Free-swimming cercariae were recorded by time-lapse photography and, after conventional fixation, by scanning electron microscopy. Cercarial tail-beats were to the posterior of the body in the lateral plane at a rate of 8 Hz. The tail has paired lateral ridges positioned to act as leading edges. There is an array of 32 sensory papillae on the mid-ventral surface of the tail. The tegument of the most distal part of the tail is described: it is free of sensory endings and the surface shows a spiral pattern.

Scanning electron microscopy of newly excysted juvenile and adult Cyclocoelum mutabile (Digenea)

In this study, changes were examined in surface structures and tegumental morphology of 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. Small papillae were scattered around the mouth region and also formed a row that encircled the anterior end of the fluke. Four groups of larger papillae, two ventrolateral and two lateral, originated from points slightly posterior to the mouth region. The two ventrolateral bands of papillae terminated slightly posterior to the acetabulum; the two lateral rows of papillae terminated about three-quarters of the length along the specimen. Six large multilobed papillae were associated with the acetabulum. Juveniles migrating through the liver lost the spines and papillae by day 6, but the anterior depressions persisted into the adult stage. The tegument of 6- and 12-day-old flukes was smooth with a varying number of transverse folds. The tegument of 15-day-old specimens from the body cavity ranged in appearance from folded and pitted to reticulate. The reticulations were more prominent in adults and the entire body was covered with a reticulate pattern of loculus-like structures that are believed to aid in the movement of the flukes within the air sacs of the coot host.

Ultrastructural observations on the redial tegument of Paramphistomum epiclitum from the planorbid snail, Indoplanorbis exustus

The morphology of the tegument in the redia of Paramphistomum epiclitum (Digenea: Paramphistomidae) resembles that shown by most larval and adult digeneans; an outer surface syncytium is in continuity with the cytoplasm of in-sunken, nucleated cytons. Although tegumental cytons usually contain a single nucleus, some display up to six nuclei. The tegumental syncytium lining the pharynx of P. epiclitum rediae lack underlying cytons. The apical membrane of the tegument is elaborated by folds and microvilli, which presumably facilitate uptake of nutrients and/or exchange of ions involved in osmoregulation. A single type of secretory body, resulting from the fusion of smaller vesicles produced at Golgi complexes in the cytons, occurs throughout the tegument. Uniciliate sensory receptors occur in the surface syncytium particularly around the oral opening.

Scanning electron microscopy of cercariae, metacercariae and adults of Pygidiopsis ardeae Køie, 1990 (Digenea, Heterophyidae)

The penetration apparatus of the cercaria of Pygidiopsis ardeae Køie, 1990 (Heterophyidae) is provided with five large preoral hooklets. Various types of presumably sensory structures surround the small oral aperture. Small, pointed spines protrude throughout the cercarial body. After parasite penetration and encystment in the fish intermediate host, the metacercarial tegument increases its absorptive area by developing irregular projections. Concurrently the pointed spines become scale-like and serrated. The tegumental outgrowths appear to have regressed in infective metacercariae. The external surface of mature worms removed from the intestine of domestic chickens does not differ from that of infective excysted metacercariae. Adults taken from experimentally infected chickens were identical to specimens obtained from naturally infected herons.

The uptake and distribution of radioselenium in the larvae of Fasciola hepatica and its snail host Lymnaea columella

Radiolabeled metacercariae of Fasciola hepatica were obtained in vivo by incubating infected Lymnaea columella snails with 20 muCi radioselenomethione (75Se-M) per snail in 5 ml of water for 5 h, or in vitro by incubating a batch of unlabeled F. hepatica metacercariae with 75Se-M for 24 h. Radioassay showed that only 5% of the 75Se-M was incorporated into maritas (juvenile flukes) from the in vivo labeled metacercariae. The inner cyst wall of in vivo labeled metacercariae contained 46% of the total activity, of which 21% was dissolved in the excysting medium. The outer, tan-colored cyst wall contained 49% of the radioactivity. Through diffusion/attachment, maritas from in vitro labeled metacercariae could occasionally be labeled with 0.4% of the total radioactivity. However, the activity was lost after inoculation into the body of mice. The outer and inner cyst walls of in vitro labeled metacercariae contained 92% and 7.6%, respectively, of the total activity. Microautoradiography demonstrated that 75Se-M was evenly distributed in the body of marita and the cyst wall of inner and outer layers from an in vivo labeled metacercaria. A 9 X 4 micron rectangularly-shaped aggregate of Ag degree grains was present on the outer periphery of the inner cyst wall. Microautoradiography of in vitro labeled metacercariae demonstrated a significant concentration of Ag degree grains on the cyst walls. The ventral plug contained fewer Ag degree grains per unit area compared to the other portion of the inner cyst wall. Uptake and distribution of 75Se-M in the snail host were also studied. It appeared that rediae and cercariae tended to concentrate the label in the foot, the mantle and the digestive gland. Little or no radioactivity was present in the areas where F. hepatica larvae were not found.

Scanning electron microscopy and chemical excystation of Fascioloides magna (Trematoda) metacercariae

Scanning electron microscopy was used to study encysted metacercariae and newly excysted juveniles of Fascioloides magna. The outer cyst was rough, coarse and discontinuous in the ventral aspect; the inner cyst was smooth. The newly excysted metacercaria was plump and contained numerous tegumentary spines; large dome-shaped papillae were prominent around the oral sucker and on the rim of the acetabulum. Encysted metacercariae with outer cysts were excysted in an alkaline bile salts-trypsin medium at an elevated temperature in the absence of acid saline or acid pepsin pretreatment. Pretreatment in acid saline slightly decreased subsequent excystation, while pretreatment in acid pepsin slightly enhanced subsequent excystation in the alkaline bile salts-trypsin medium.

The ultrastructural characterization of the tegument of Clonorchis sinensis (Cobbold, 1875) cercaria

The tegumental ultrastructure of the cercaria of the liver fluke, Clonorchis sinensis (Cobbold, 1875), was studied by scanning and transmission electron microscopy. The body surface is almost encircled by many rows of regularly arranged spines. The tegumental syncytium of the body contains many rod-shaped dense granules and central electron-lucent bodies, neither of which are present in the tail tegument. There are four rows of hooked teeth and modified spines on the oral cone. These teeth are differentiated morphologically and probably functionally from the other body spines. Disc-shaped papillae with long or short cilia are distributed on the body in a bilaterally symmetrical pattern dorsally and ventrally. There are 30 to 37 papillae with much longer cilia laterally. Four pairs of papillae were found on the tail. From their structure and location these papillae appear to have a mainly tango- or rheoreceptive function. Another type of sheathed papillae is situated around the oral sucker. The cuticular tegument is expanded laterally at the base of the tail, forming a characteristic sac-like structure.