NUTRITION AND EXCRETION OF PARASITESPart 3: TREMATODE, CESTODE, ACANTHOCEPHALA

MONOGENEAN

MONOGENEAN

The mouth and buccal funnel often have associated suckers.

Behind the buccal funnel a short prepharynx is followed by a muscular and glandular pharynx.

This powerful sucking apparatus draws food into the system.

In Entobdella soleae the pharynx can be everted and the pharyngeal lips closely applied to a host’s skin.

Pharyngeal glands secrete a strong protease that erodes host epidermis, and the worm sucks up the lysed products.

Posterior to the pharynx may be an esophagus, although it is absent from many species.

The esophagus may be simple or have lateral branches and may have unicellular digestive glands opening into it.

In most monogenes the intestine divides into two lateral crura, which are often highly branched and may even connect along their length.

If the crura join near the posterior end of the body, it is common for a single tube to continue posteriorly for some distance.

There is no anus.

The range of form of the monogenean alimentary canal.

m – mouth; ph - pharynx; int- intestinal caeca.

haptors

Buccal cavityHost tissue

Pharynx muscles

Food material

Gland cells

Esophagus

Vitellaria

Oral sucker

Pharynx

Intestine

Longitudinal section through the anterior of the monogenean Polystoma integerrimum, showing the upper alimentary tract and associated glands

Feeding habits among Monogenoidea vary along taxonomic lines:

- members of subclass Polyonchoinea (including families such as Gyrodactylidae and Dactylogyridae) feeding mainly on epidermal cells and secretions,

- members of subclass Heteronchoinea typically feeding on blood

In Polyonchoinea, the gut epithelium usually consists of a single layer of one cell type.

In blood-feeding heteronchoineans, host hemoglobin is taken up by endocytosis

As a result of digestion, hematin accumulates in gut epithelial cells, from which it is eventually released and regurgitated.

The protein is taken into a cell by pinocytosis and digested

Hematin is subsequently extruded by temporary connections between the reticular system and the gut lumen.

Indigestible particles are eliminated through the mouth in all monogenes

Can absorb neutral amino acids through its tegument, suggesting the possibility that direct absorption of low–molecular weight organic compounds could supplement its blood diet

SURFACE MORPHOLOGY OF MONOGENEAN

These microvilli may function to spread and mix secretions of the different types of head glands.

DIGENEAN

Chinese liver fluke, Clonorchis sinensis

DIGENEAN

Feeding and digestion in trematodes vary with nutrient type and habitat within their host.

Two patterns of feeding predominate: blood feeding and feeding on tissues and mucus.

For example, two lung flukes of frogs, Haematoloechus medioplexus and Haplometra cylindracea, feed predominately on blood from the capillaries.

Both species draw a plug of tissue into their oral sucker and then erode host tissue by a pumping action of their strong, muscular pharynx.

Other trematode species characteristically found in the intestine, urinary bladder, rectum, and bile ducts feed more or less by the same mechanism, although their food may consist of less blood and more mucus and tissue from the wall of their habitat, and it may even include gut contents.

In contrast S. mansoni, living in blood vessels of the hepatoportal system and immersed in its semi fluid blood food, has no necessity to breach host tissues, and, not surprisingly, this species has neither pharynx nor muscular esophagus.

THE RANGE OF FORM OF THE DIGENEAN ALIMENTARY CANAL.

int – intestinal caeca; m – mouth; oes – esophagus; o.s – oral sucker; ph – pharynx; v.s – ventral sucker

A) Gastromata – one ventral mouth and intestine in shape of sac/ bag

B) Protostomata – anterior mouth, pharynx, esophagus, caecum/ intestine (braching into 2)

C) Complex Protostomata – extensive barching of caecum/ intestine – intestinal diverticulum

A frog lung fluke, Haplometra cylindracea, has pear-shaped gland cells in its anterior end,

and a nonspecific esterase is secreted from these cells through the tegument of the oral sucker, beginning the digestive process even before food is drawn into the ceca.

The liver fluke, Fasciola hepatica, by contrast, feeds on both tissue and blood and completes digestion of the blood meal intracellularly in the gasnodermis, passing waste iron to the excretory canals to be voided.

In Fasciola, a curious gastrodermal cell cycle has been identified, which is related to the various phases of ingestion and digestion of food

Trematodes can absorb small molecules through their tegument.

Schistosoma mansoni takes in glucose only through its tegument

Schistosomes absorb glucose both by diffusion and by a carrier-mediated system

SURFACE MORPHOLOGY OF DIGENEAN

Schistosomes ingest large quantities of host blood via the mouth and digest haemoglobin readily;

The schistosome gut is typically delineated by the presence of the black pigment, haematin, which is the result of this digestion.

In certain species they secrete some digestive enzymes: proteases, dipeptidases, an aminopeptidase, lipases, acid phosphatase, and esterases.

The proteases of schistosomes vary in their occurrence during the life cycle and may thus have stage-specific functions;

the haemoglobinase itself is only expressed in the developing schistosomulum and adult worm,

This adult protease is highly antigenic and is useful in the diagnosis of schistosomes in subclinical human cases as a prelude to chemotherapy.

Schistosome proteases may be expressed in a stage specific manner.

PART 3: CESTODES Cestodes lack any trace of a digestive tract and

therefore must absorb all required substances through their external covering.

All nutrient molecules must be absorbed across the tegument.

Mechanisms of absorption include active transport, mediated diffusion, and simple diffusion.

Whether pinocytosis is possible at the cestode surface has been the subject of some dispute, but plerocercoids of Schistocephalus solidus and Ligula intestinalis are capable of this process.

Cysticerci of Taenia crassiceps are capable of pinocytosis, and the process is stimulated by presence of glucose, yeast extract, or bovine serum albumin in the medium

Generalized diagram of a tapeworm. scolex (a), neck (b), and strobila (c).

TEGUMENT

Longitudinal section through immature proglottid of Hymenolepis diminuta, showing nature of tegumentary cortical region.

ACQUISITION OF NUTRIENTS

Glucose is the most important nutrient molecule to fuel energy processes in tapeworms.

The tapeworm transports carbohydrates by both carrier-mediated systems and by diffusion

Glycerol and glucose enter by separate carriers but both depend on sodium ion concentration.

Fatty acid transport is similarly complicated and separate systems,

POSTULATIONS

1) Choice of organics molecules - Only micromolecules are absorbed - Complex proteins and macromolecules – not

absorbed - The only carbohydrates that most cestodes can

absorb are glucose and galactose, and although some tapeworms can absorb other monosaccharides and disaccharides.

- On the surface of tegument there are numerous fingershaped tubes called microtriches

- Microtriches are similar in some respects to microvilli found on gut mucosal cells and other vertebrate and invertebrate transport epithelia, and they completely cover the worm’s surface, including its suckers

- Microtriches serve to increase absorptive area of the tegument.

Posteriorly directed microtriches on the surface of a proglottid of Hymenolepis diminuta

2) Tegument may possess some enzymes to digest some food particles

- Hymenolepis diminuta synthesizes digestive phosphohydrolases, hydrolysing phosphate esters, monoglyceride hydrolases and ribonucleases, all of which function in a digestive capacity at the tegumental surface.

- The tapeworm can also bind host digestive enzymes, such as amylases, where upon enzyme activity may become enhanced

- Conversely, tapeworms can bind and inhibit host enzymes (e.g. trypsin, chymotrypsin) and this is possibly one adaptation for parasite survival in an enzymatically hostile environment.

3) Exchange of proteins (in the form amino acids) between parasite and the host cells

- Amino acids are also actively transported and accumulated

- Presence of other amino acids in the ambient medium stimulates efflux of amino acids from the worm; therefore, the worm pool of amino acids rapidly comes to equilibrium with amino acids in the intestinal milieu.

- Purines and pyrimidines are absorbed by facilitated diffusion

Amino acid and purine/pyrimidine transport are complex processes: there are six separate amino acid carriers; four transporting neutral amino acids, one for acidic and one for basic amino acids; and at least three purine/pyrimidine carriers with multiple binding capacity.

EXCRETION AND OSMOREGULATION OF CESTODES In many families of cestodes the main excretory canals run

the length of the strobila from the scolex to the posterior end.

These are usually in two pairs, one ventrolateral and the other dorsolateral on each side

Most often the dorsal pair is smaller in diameter than is the ventral pair

The canals may branch and rejoin throughout the strobila or may be independent.

Usually a transverse canal joins the ventral canals at the posterior margin of each proglottid.

DIAGRAM SHOWING THE TYPICAL ARRANGEMENTOF DORSAL (D) AND VENTRAL (V) OSMOREGULATORY CANALS.

The dorsal and ventral canals unite in the scolex, often with some degree of branching.

Posteriorly, the two pairs of canals merge into an excretory bladder with a single pore to the outside.

When the terminal proglottid of a polyzoic species detaches, the canals empty independently at the end of the strobila.

Rarely the major canals also empty through short, lateral ducts.

In some orders, such as Pseudophyllidea, canals form a network that lacks major dorsal and ventral ducts.

Embedded throughout the parenchyma are flame cell protonephridia, whose ductules feed into the main canals.

The flagella of a flame cell provide motive force to the fluid in the system.

Protonephridia of tapeworms show the weir construction.

*weir – is formed by rods from both the terminal flagellated cell

DIAGRAM OF TERMINAL ORGAN OF FLAME CELL PROTONEPHRIDIUM IN HYMENOLEPIS DIMINUTA.

The flame is composed of approximately 50 - 100 flagella.

The excretory ducts are lined with microvilli thus suggesting that the duct linings serve a transport function.

Therefore, functions of the system might include active transport of excretory wastes and ionic regulation of the excretory fluid.

Fluid from the excretory canals of H. diminuta contains glucose, soluble proteins, lactic acid, urea, and ammonia but no lipid.

Low-magnification electron micrograph of excretory duct of Hymenolepis diminuta showing beadlike microvilli (MV).

The principal end products of cestode energy metabolism, short-chain organic acids, are probably excreted through the tegument, either by diffusion.

Osmoregulation is another function of the tegumental surface.

With little ability to regulate their body volume in media of differing osmotic concentrations,

Hymenolepis diminuta can osmoregulate between 210 and 335 mOsm/L in a balanced salt solution if 5 mM glucose is present.

The worms rapidly lose water at pH 7.4 and 300 mOsm/L without glucose.

Water balance in H. diminuta is closely related to excretory acid concentration and pH of the medium.

ACANTHOCEPHALA: THORNY-HEADED WORMS

Acanthocephala is a small group of obligate parasites that utilize arthropods and vertebrates in a conserved two-host life cycle.

Inhabit the intestine of fishes, amphibians, reptiles (rarely), birds, and mammals, in which they occasionally cause serious disease.

The acanthocephalan body consists of an anterior proboscis, a neck, and a trunk

Examples of different types of acanthocephalan proboscides

(a) Octospiniferoides australis; (b) Sphaerechinorhynchus serpenticola; (c) Oncicola spirula; (d) Acanthosentis acanthuri; (e) Pomphorhynchus yamagutii; (f) Paracanthocephalus rauschi; (g) Mediorhynchus wardae; (h) Palliolisentis polyonca; (i) Owilfordia olseni.

Quadrigyrus nickoli, illustrating basic acanthocephalan morphology.(a) Female; (b) male.

Scanning electron micrographs of Leptorhynchoides thecatusNote some of the major anatomical features of acanthocephalans. P, proboscis; H, hook; N, neck; T, trunk

ACQUISITION AND USE OF NUTRIENTS

Have no gut and obtain all nutrient molecules through their body surfaces and is facilitated by a syncytial epidermis and a lacunar system of circulatory channels.

Lacunar system refers to a network of fluid-filled channels in the body wall.

The acanthocephalan surface is physiologically comparable although morphologically quite different from the platyhelminths.

Acanthocephalans can absorb at least some triglycerides, amino acids, nucleotides, and sugars.

The surface of Moniliformis moniliformis contains peptidases, which can cleave several dipeptides, and the amino acid products are then absorbed by the worm.

In several other species, lysine is absorbed across the metasomal tegument, especially the anterior portion, and accumulates in nuclei and the outer muscle belt.

Tegument of Moniliformis moniliformis

(a)Diagram of transverse section to show layers. The felt-fiber zone contains many vesicles and mitochondria with poorly developed cristae. Lacunar canals are in the radial fiber zone.

(b) Electron micrograph showing the major features of the striped zone. The worm is coated with a finely filamentous surface coat (SC). Numerous surface crypts (C) appear as large scattered vesicular structures with elements occasionally appearing to course to the surface of the helminth. The crypts are separated by patches of moderately electronopaque material (*), giving the zone its striped appearance under the light microscope. Mitochondria (M), glycogen particles, microtubules, and other cytoplasmic details are evident in the inner portion of the striped zone. Bundles of fine cytoplasmic filaments (f) extend between this region and the deeper cytoplasm of the body wall

(a) Midmetasomal region; (b) region near neck, with presomal lacunar system not indicated; (c) near posterior end of metasoma. DLC, dorsal longitudinal channel; HC, hypodermal canal (in radial fiber zone); MLC, medial longitudinal channel; PRC, primary ring canal; RC, radial canal; SRC, secondary ring canal; VLC, ventral longitudinal channel.

Organization of lacunar system in Macracanthorhynchus hirudinaceus

The uptake started in the anterior half of the proboscis

M. moniliformis has an absolute dependence on host dietary carbohydrate for growth and energy metabolism as an adult.

The worm can absorb glucose, mannose, fructose, and galactose, as well as several glucose analogs.

M. moniliformis can grow and mature in the host fed a diet containing fructose as the sole carbohydrate source.

Absorption of glucose is through a single transport locus, whereas transport of mannose, fructose, and galactose is mediated both by the glucose locus and the fructose site.

Maltose and glucose-6-phosphate (G6P) are absorbed also, but first they are hydrolyzed to glucose by enzymes in or on the tegumental surface.

Acanthocephalans also accumulate a variety of nonorganic molecules, including heavy metals.

M. moniliformis took up more lead and cadmium than their rat hosts, concentrating both mainly in female worms, the lead especially in eggs.

Acanthocephalan species parasitizing fish take up so much heavy metal—up to 200 times as much as their hosts—that the worms are potentially useful as bioindicators of pollution

At least 16 different elements are taken up.

In some cases worms compete with one another and with their fish hosts for these substances.

EXCRETORY SYSTEM Excretion in most species appears to be effected by diffusion

through the body wall.

However, members of Oligacanthorhynchidae, one family in class Archiacanthocephala, are unique in possessing two protonephridial excretory organs.

Each comprises many anucleate flame bulbs with tufts of flagella and may or may not be encapsulated, depending on species.

In males these organs are attached to the vas deferens and empty through it;

In females they are attached to the uterine bell and empty into the uterus.

Acanthocephalans show little ability to osmoregulate, swelling in hypotonic, balanced saline or sucrose solutions and becoming flaccid in hypertonic solutions.

They take up sodium and potassium, swelling in hypertonic solutions of sodium chloride or potassium chloride at 37°C.

In balanced saline they lose sodium and accumulate potassium against a concentration gradient.