CESTODES I

The tapeworms, the Cestoda, are one of the groups of Platyhelminthes and physiologically they are a very interesting group of parasites. The longest animal in the world is a tapeworm, Polygonoporus from the whale, 40 metres long. Adult tapeworms are all endoparasites and with the exception of a few forms which become sexually mature whilst still larvae, adult cestodes only occupy one habitat, the vertebrate intestine. Those species of cestode that are not found in the gut lumen itself, occur in the bile duct, gall bladder or pancreatic duct. So the habitat of the adult cestode is restricted to the alimentary canal or its immediate derivatives.

Major features of the Cestoda are:

1. Platyhelminths
2. Adults are all endoparasites
3. No gut at any stage of the life cycle. This lack of a gut may be one of the factors that restrict tapeworms to the lumen of the intestine
4. They have a characteristic larva called a hexacanth. This is a little larva with six hooks, arranged in three pairs

In the egg the embryophore arises as a layer of surface cells that become separated from the developing larva. In some groups of tapeworm - Pseudophyllideans, it remains cellular and is cilliated, in other groups it becomes a protective membrane. Some people have suggested that the embryophore represents the ectoderm, and that the surface of the adult tapeworm is, in fact, endoderm.

Adult tapeworm, long ribbon like body, divided into three regions. The scolex ( plural scoleces) which is usually held to be the anterior end and is concerned with attachment to the gut wall.

A neck region (germinative region) where the proglottides are budded off.

The strobila main part of the body, made up of maturing proglottides (singular proglottis)

The lack of a gut in cestodes is of considerable physiological importance, since it means that the external surface of the parasite must function, not only for protection, locomotion and as a sensory surface (as in all other organisms), but also as a metabollicaly active layer taking up nutrients from the environment and a layer through which secretions and excretions must pass.

Ultra-structural studies have shown that the external body covering of the adult tapeworm consists of a naked cytoplasmic layer, there is no resistant cuticle. This outer layer is a syncitium, there are no cell walls and it is usually referred to as a tegument to distinguish it from non-living cuticles of the type that occur for example in nematodes.

Rhabditiform organelles, not equivalent to rhabdites of turbellaria, occur in the tegument. Muscle layers in cestodes are usually much reduced when compared with free-living Platyhelminthes.

The micro-villi on the tapeworm surface, increase the surface area of the parasite by about 20 times. A structural analogy can be drawn between the cestode tegument and a typical intestinal cell.

In the vertebrate the lining of the intestine is constantly being renewed, the cells at the base of the villi are constantly dividing, the cells migrate up the villi and are shed from the tip (4-5 days) - (intracellular stages of parasites such as Coccidia which inhabit these cells must complete their development within this time or slow cellular turnover). The surface layer of cestodes is also turning over at a similar sort of rate. But the tegument grows, not by cell division, but by recruitment. Cells from the parenchyma come up and attach to the tegument and vesicles etc pass up into the tegument.

Cestodes do not produce digestive enzymes of any sort, instead they rely entirely on the digestive enzymes of their hosts to breakdown nutrients into low molecular weight compounds, which they can then absorb. Tapeworms have evolved a whole repertoire of transport mechanisms in their tegument which are able to effectively compete with the uptake mechanisms of the hosts intestine. Cestode transport systems have been characterized for amino acids, sugars, fatty acids, purines and pyrimidines and vitamins. Most soft-bodied aquatic invertebrates are able to take up nutrients across their body wall, but this ability has been greatly elaborated in the cestodes.

One peculiarity of tapeworms is that they have an absolute requirement for carbohydrate in their diet, moreover they can only use glucose or the closely related galactose (vertebrates can metabolise a much wider range). If the host is kept on a low carbohydrate diet the tapeworms appear small and stunted, and competition for carbohydrate may be one of the key factors in the crowding effect. This is the observation that worms from high density infections, where there are a lot of tapeworms are smaller and less fecund (produce fewer eggs) than worms from low density infections.

Glucose does not act as an adequate replacement for starch in the host diet. A possible reason for this is that starch is digested relatively slowly and so provides free glucose in the intestine for a long period of time.

The Scolex - Generally held to be the anterior end. Three basic types of scolex found in cestodes: acetabulate, bothriate and bothridiate.

Acetabulate - The acetabulate type have suckers, similar in structure to those of Digenea. Acetabulate scoleces can be further divided into non-penetrative and penetrative. Within the non-penetrative type the suckers enclose a group of villi, but the scolex does not penetrate into the mucosa. In the penetrative type, the scolex penetrates down into the crypts of Lieberkuhn at the base of the villi. The scolex may be armed with hooks, and in the Taenia tapeworms there is a rostellum, which is like a little proboscis and usually has hooks.

The Bothriate scolex consists of a pair of shallow sucking grooves called bothria, in some species, the margins of the groves are fused to form tubes.

The Bothridiate type of scolex has four leaf like outgrowths called bothridia. The morphology of the bothridia is often closely adapted to the morphology of the hosts' intestine. Bothridiate scoleces can also be armoured with retractable proboscides.

The function of the scolex is clearly attachment. It has been suggested that penetrative scoleces may also have a specialised nutritional function. The scolex of tapeworms also contains glands of unknown function.

Excretory System. This is a typical platyhelminth system with protonephridia or flame cells which open into a series of collecting vessels. There are usually 2 or 4 longitudinal vessels, with a transverse canal connecting the longitudinal canal at the posterior part of each segment. The longitudinal canals open to the outside at the extreme posterior end of the animal. Some tapeworms have up to 20 longitudinal canals.

In a number of tapeworms there is a separate dorsal and ventral network of excretory canals. These dorsal and ventral systems do not interconnect except in the scolex where there is an elaborate network of anastomosing vessels. It is claimed that in these anastomosing canals are valve like structures and that there is a directional flow of fluid in the canals.

It has also been suggested that the excretory system could also function as a circulatory system for the tapeworm. Many tapeworms are extremely long and the scolex may be in a very different environment from the end of the proglottides. The main excretory product in cestodes is ammonia. Micropuncture of the nephridial canals has shown that they do indeed contain ammonia and that injected glucose is rapidly reabsorbed. So the canal system does seem to be involved in the excretion of nitrogenous wasted by a mixture of filtration and selective reabsorbion. Although the main function of the protorephridial system is probably osmoregulation.

Most cestodes behave like osmometers when placed in solutions of different osmotic pressure all swell in dilute solutions and shrink in concentrated solutions.

Tapeworms appear to be osmoconformers with little if any ability to regulate their body volume. However, despite this limited osmoregulatory ability Cestodes all appear to be slightly hypotonic to their surroundings in vivo.

An interesting situation occurs in elasmobranchs, where a substantial part of the osmotic pressure in the tissues is due to urea, up to 300mm, which makes their tissues isotonic with sea water.

Cestodes from sharks fall into two types. Some of them contain up to 1% wet weight of urea, and rapidly reach equilibrium with urea in the medium.

In contrast, the other group of shark tapeworms, of which Lacistorhynchus is an example, do not behave like this. They are not permeable to urea and have an active urease in their tissues which rapidly breaks down urea to CO₂ and ammonia, so removing any urea that does diffuse in. Presumably these tapeworms use some other osmotic agent, possibly the amino acid taurine.

Nervous system. The nervous system of cestodes is difficult to study because the nerve cords are not surrounded by a clear connective tissue sheath, which makes histology difficult. Nevertheless, cestodes have a fairly typical invertebrate nervous system.

There is a superficial nerve net under the tegument, but it is diffuse. There are also quite a lot of what appear to be sensory nerve endings in the tegument, and peg like sense organs. Pharmacologically the nervous system of cestodes is very different from that of mammals. In mammals the excitatory transmitter released at the nerve muscle junction is acetylcholine, this stimulates the muscle to contract. In cestodes, as in many invertebrates, there is a double innervation, each group of muscle fibres receive input from an excitatory nerve and an inhibitory nerve, one causes the muscle to contract, the other causes it to relax. In cestodes the excitatory neurotransmitter is the amino acid glutamate, whilst the inhibitory neurotransmitter is acetylcholine. In mammals, in the nerve, the resting potential is a K⁺ potential, that is it is dependent on the difference in concentration of potassium ions inside and out, whilst the action potential is a Na⁺ potential. In cestodes the resting potential is again a K⁺ potential, but the action potential is a Ca²⁺ potential. One of the best anti-tapeworm drugs Paraziquantel works by opening the calcium channels, thus causing the worm to die in a contracted state.

Neurosecretory cells have been demonstrated in the scolex of tapeworms and their activity has been correlated with strobilization. A range of neuropeptides has also been isolated from tapeworms. Their function may be either as neurotransmitters in their own right, or as neuromodulators, that is they reconfigure the nervous system, in particular altering the sensitivity of inhibitory and excitatory synapses, or they may act as neurohormones.

So although you have a structurally simple nervous system, biochemically it is complex.

The neuropeptides that have been isolated from tapeworm are often similar to those found in mammals, but tend to differ in detail. This has led to the suggestion that parasites can produce analogues of host regulatory peptides which can modify the host physiology (some plerocercoids for example cause gigantism or castration of their hosts).

CESTODES II

Reproduction. With the exception of one family Dioicocestus that has separate sexes, tapeworms are all hermaphrodite. Protandry is the usual situation, where the male reproductive organs mature first and the fully mature proglottides are little more than an enlarged uterus packed with eggs. New proglottides are budded off from the neck region and the reproductive system differentiates progressively as you go down the strobila. The proglottides do not divide, they are budded off individually from the germinative region. The rate of maturation of the proglottides is under environmental control, in some species parasitic in poikilotherms, maturation is triggered by the sudden temperature rise in summer. The rate of proglottis development is also influenced by the availability of carbohydrate, and proglottis production is greatly reduced in high-density infections. Tapeworms also have a characteristic length. All of which suggests that there is some form of feed back loop between the mature proglottides and the germinative region, whether this is nervous or hormonal-possibly involving neuropeptides is unknown.

The male and female reproductive systems follow the general platyhelminth pattern, based on a series of tubes and sacs.

The detailed form of the reproductive system is used in classification.

In some groups of tapeworm, the Pseudophyllideans the proglottides are not continuously shed, but instead there is an opening from the uterus to the outside and eggs are produced continuously.

In the Cyclophyllideans the eggs are retained in the uterus which expands as the proglottis matures, so that the mature segments just contain a greatly expanded uterus.

These gravid segments are shed, either singly or in short chains. In some cestodes the segments rupture in the intestine so you just find eggs in the faeces (Hymenolepis, Moniezia). But in others, such as the Taeniids the whole proglottis pass out in the faeces. These proglottides have well developed cortical musculature, not reduced as is usual in parasites, and are able to crawl actively (T. pisiformis of cat.) Some of these active proglottides have developed a secondary opening to the uterus, so as they crawl, they shed eggs. These active proglottides represent a solution to a problem which faces all parasites which use the faecal route to eliminate their eggs or infective stages. Animals rarely graze near their own faeces, so the infective stages are faced with having to get from the faeces to uncontaminated pasture. Some use insects which feed on dung either as intermediate hosts, as in the case of Hymenolepis or some use insects just to hitch hike (phoresy), many nematodes temporarily attach themselves to the legs or bodies of passing flies or beetles. In one nematode Dictyocaulus (causes husk in cattle), when the cow pat becomes covered with a fungus Pilobolus that has explosive sporangiophores, the nematode migrates up into the developing spore and is then shot into the surrounding vegetation. Other infective parasite stages, like the Teaniid proglottids are sufficiently mobile to migrate on their own.

Some of the Taeniid proglottides, do not wait for the host to defecate, they can migrate out of the anus. Proglottides are not released continuously, they may follow a diurnal rhythm.

In the platyhelminth type of reproductive system, the ootype, which is the muscular sac , which moulds the egg shell as it hardens, forms a bottle neck in egg production. It has been estimated that it probably takes at least a minute to make an egg with this system, a maximum of 1,440/day. For a parasite this is not a high rate of egg production. The nematode Ascaris lumbricoides produces something like 200,000 eggs/day. In the digeneans, the flukes, they increase their reproductive potential by sexual multiplication in the mollusc host. Sporocysts can give rise to daughter sporocysts. Each sporocyst can give rise to many redia which in turn can produce daughter redia, each of which can produce many cercariae. The total reproductive capacity of a single liver fluke - Fasciola hepatica has been estimated as 4 x 10⁸ offspring. In cestodes, the reproductive potential has been increased by serial repetition of the gonads. Taenia solium for example produces about 15 proglottides per day, each containing about 10,000 eggs.

Because they are protandrous hermaphrodites, tapeworms can be self-fertilizing, they can fold back on themselves. There is some limited evidence that suggests that they tend to cross fertilize rather than self fertilize where possible. However, with some of the larger tapeworms, the Taeniids you get a phenomenon called premunition, where you only ever get one tapeworm at any one time in the host. It is not really an immunological phenomenon, since if you loose your tapeworm, you can immediately become re-infected. But the presence of the tapeworm prevents others establishing. Competition for nutrients or space is probably involved and possibly the production of inhibitory secretory products. But of course if you only have one worm it has to self fertilize, so you have a population of parasites which are clones and do not interbreed. So there is no free-flow of genes and the definition of a species becomes problematical.

Life cycles Three different basic types of life cycle:

First a Pseudophyllidean life cycle, these are the more primitive group of tapeworms and have a free-swimming larva. There are two major cestode groups, Pseudophyllideans and Cyclophyllideans, and seven minor groups. Diphyllobothrium latum, broad tapeworm of man, 10 meters long (33ft). Infects perhaps as many as 10 million people, mostly round the Baltic, but has been introduced with immigrants into the great lakes region of North America. It has a life cycle involving two intermediate hosts. The eggs pass out in the faeces, they are operculate eggs with a little cap. Under the influence of light, the eggs hatch to release a ciliated free swimming coracidium larva. This is then eaten by an intermediate crustacean host, where it penetrates the gut wall. In the crustacean haemocoel it develops into a procercoid. This is a solid larva. The infected crustaceans are eaten by fresh water fish, the proceroid is released, burrows through the gut wall and develops in the body cavity into a plerocercoid.

Man becomes infected by eating raw or imperfectly cooked fish. If another fish eats the infected fish, the plerocercoid can re-invade the second host, and the second fish can act as a transport host or paratenic host. The adult Diphyllobothrium does not shed proglottides but produces eggs continuously.

The ability to move from one second intermediate host to another increases the tapeworms chance of infecting its final host. A related genus Spirometra is particularly good at this. The adults occur in dogs or cats or their relations - small carnivores and the plerocercoids occur in frogs or snakes, or in some species in mammals. Man can become infected, acting as a secondary host, and develops a disease called sparganosis. One of the more unusual ways of contracting sparganosis, is from the practice in some parts of the Far East of using a poultice made from raw frog meat to treat injuries, particularly inflamed eyes, and plerocercoids in the frog tissue migrate into the wound. Sparganoses can be quite a serious disease as the plerocercoids have the habit of migrating round the body and they can be quite large.

Cyclophyllidean life cycles. These are the more advanced tapeworm group, and there are two basic life cycles, depending on whether the intermediate host is an arthropod or a mammal.

Taenia solium pork tapeworm, 6-12ft long, infects perhaps 2.5 million people. The adult lives in the small intestine, the ripe proglottides are shed as short chains and are active outside the host, the eggs are released when the segment disintegrates. The egg is a fairly resistant structure composed of keratin blocks.

Inside the pig, the hexacanths hatch in the gut and bore through into the blood stream where they are carried to the muscles. The keratin blocks of the striated layer are held together by a cement substance and this is dissolved by the hosts digestive enzymes (the presence of a cement substances has been disputed by some electron microscopists). The actual larva is activated by temperature (37^oC) and the presence of specific bite salts in the gut.

In the muscles the hexacanth develops into a cysticercus or bladder worm, this is a fluid filled larva with an inverted scolex.

When man eats raw or insufficiently cooked pork the worm evaginates and attaches to the intestinal wall. This involves removal of the outer membrane, which is brought about by proteases in the digestive juices and activation of the larva that is again dependent on temperature (37^oC) and the presence of specific bile salts.

Taenia solium is particularly unpleasant because man can act both as the primary and the secondary host. Accidental contamination with the eggs can result in cysticercoses, where the cysts can develop in virtually any organ in the body.

Taenia saginata, beef tapeworm similar life cycle, but with the cow as the intermediate host, much larger worm (12 meters, 39 feet) can live up to 10 years, more widespread than the pork worm - 39 million cases. But man cannot act as an intermediate host.

One other species we ought to mention, because of its economic and human importance, Echinococus granulosus. The adult is a small worm in dogs, eggs pass out and are picked up by the normal host the sheep, where they develop into a cystic stage called an hydatid. Dogs become infected when they scavenge the carcass of infected sheep. Cycles in different parts of the world can involve dog/cow or dog/camel cycles as well as dog/sheep. Humans can also become infected accidentally, and this can be a serious problem. Different strains of Echinococus differ in their infectiveness to humans, the strains found in Australia and New Zealand are very pathogenic to man. In the UK there appear to be two strains, one which is primary a dog/horse cycle, but infective to sheep but not to humans, and a dog/sheep cycle which is. The later is concentrated in an area around Brecon, where the incidence of human hydatid disease is twice the national average, 0.6/million as compared with 0.3/ million nationally. This area has been the target for an eradication campaign now for over 30 years. This is based on regular worming of farm dogs backed up by an extensive educational campaign. It takes about 8 weeks between infection and the tapeworm producing eggs, so worming all dogs on a 6 week basis, prevents dogs from becoming infective. Hydatid disease used to be very high amongst leather tanners in the Lebanon, probably because they used dog faeces in the tanning process

Hymenolepis diminuta - rat tapeworm, intermediate host is a beetle, a much used experimental parasite. The proglottides break down in the gut, so only eggs are found in the faeces. The eggs have a thin embryophore and when they are eaten by the beetle, the insect mouthparts crack the capsule and the albumin layer absorbs water, swells up and pushes the larvae out.

The hexacanth burrows through the gut wall into the haemocoel where it develops into a cysticercoid, which is a solid larva with an invaginated scolex (pulled in, not turned inside out) characteristic cestode larva in an arthropod. Note the hooks once again finish up at the opposite end of the larva to the scolex (suggest head = tail in adult). When a rat eats the beetle, the scolex part emerges and attaches to the intestinal wall. Most of the bulk of the cysticercoid is shed. The eversion of the scolex is triggered by the rise in temperature (37^oC) and the presence of specific bile salts.

Beetles infected with cysticercoids show behavioural changes, which may increase the chances of them being eaten by the final host. Infected beetles tend to show negative rather than positive geotropism and no longer respond to aggregation pheromone. Lots of examples are known where infected intermediate hosts show behavioural changes, but it is often hard to decide whether the changes are adaptive from the point of view of the parasite, or from the host (eliminating susceptible individuals) or purely fortuitous.

Although asexual reproduction in the intermediate host is not a feature of the life cycle of cestodes in the way that it is in Digeneans, it nevertheless does occur in a few species where cysticerci or in some cases cysticercoids undergo budding.