PHYLUM PLATYHELMINTHES - (flatworms)

 

Characteristics:

 

Flatworms are unsegmented, bilaterally symmetrical worms that lack a coelom (body cavity).

 

Some forms are free living but many are parasitic.

 

Flatworms have a cephalized nervous system that consists of head ganglion (group of nerve cells), usually attached to longitudinal nerve cords that are interconnected across the body by transverse branches.

 

Excretion of liquid wastes by flatworms is controlled by "flame cells" located in protonephridia (simple Akidneys@).

 

Flatworms have no respiratory or circulatory systems; these functions take place by absorption through the body wall.

 

Nonparasitic forms have a simple, incomplete gut (mouth, but no anus); even this is lacking in many parasitic species.

 

Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle.

 

Others move along slime trails by the beating of epidermal cilia.

 

The development of directional movement is correlated with cephalization (one end is the head and the other the tail).

 

In some flatworms, cephalization includes light‑sensitive organs called ocelli (eyespots).

 

Other sense organs found in at least some members of this group (not necessarily on the head) include chemoreceptors, balance receptors (statocysts), and receptors that sense water movement (rheoreceptors).

 

Most flatworms can reproduce sexually or asexually.

 

Most are monoecious. Most of these have ways of avoiding self‑fertilization.

 

Development may be direct (eggs hatch into tiny worms that resemble the adults) or indirect (with a ciliated larval form).

 

Flatworms include a large number of parasitic forms, some of which are extremely damaging to human populations.

CLASS TURBELLARIA - PLANARIA

 

Turbellarians are free‑living and aquatic (found only in water).

 

They have an incomplete digestive tract, in which the mouth leads to a pharynx, then to temporary spaces containing cells that take in food particles by phagocytosis.

 

Digestion is intracellular.

 

These animals move by laying down slime from special skin glands, then gliding along these "slime trails" by the beating of epidermal cilia.

 

Turbellarians are mostly carnivorous, preying on tiny invertebrates that they locate by means of their chemoreceptors.

 

They are a diverse group, including over 4500 known species.

 

PLANARIA is the main example of this class

 

It is a free-living, flat bodied, freshwater creatures that exhibit the remarkable ability to regenerate lost body parts B can develop 2 heads or 2 tails if cut (describe).

 

It lives in lakes, streams, ponds, and other freshwater bodies

 

The planarian is non-parasitic, and eats decaying meat.

 

Has pharynx which is extended out when feeding

 

The body includes:

A simple nervous system that includes a Abrain@ or cerebral ganglion

 

Longitudinal nerve cords run length of the body & are connected by transverse commissures

 

Eyespots give Across-eyed@ look; light sensitive only, so no images.

 

Auricles for chemical response

 

An internal reproductive system for sexual reproduction

 

Reproduce sexually by copulation (describe).

 

Can reproduce asexually by transverse binary fission

 

A blind gut (mouth but no anus) branching through the body

An excretory system that includes specialized cells called 'flame cells'

 

They can be conditioned to respond to stimuli, display the ability to master a two-choice maze, and can transfer the memory of training from one individual to another (describe experiments).

 

CLASS TREMATODA - the flukes

 

All trematodes are parasitic, and most adult trematodes parasitize vertebrates.

 

Around 9000 species have been described.

 

Trematodes are characterized by one or two suckers (oral sucker & ventral sucker) for attachment to host tissues .

 

They are like turbellarians in having a relatively well developed alimentary canal, and their muscular, excretory, and reproductive systems are also relatively complete.

 

Most trematodes have complex life cycles, with larval stages parasitizing one or more species that are different from host of adults.

 

Larval stages of some medically important species include such forms as miracidium, redia, cercaria, and metacercaria, all of which do not resemble the adult worm.

 

They include several parasites that have an enormous impact on human populations, such as human liver flukes and the blood flukes that cause schistosomiasis.

 

 

FASCIOLA HEPATICA IS THE MAIN ANIMAL LIVER FLUKE:

 

The idea that a microscopic creature on a blade of grass can end up as a large parasite in the liver of cattle sounds like something out of a science fiction novel.

 

However, that is just what happens on a continual basis in some part of the U.S..

 

Very few beef cattle slaughtered in California, for example, are free of liver flukes.

 

The common liver fluke of cattle (and sheep), Fasciola hepatica, has a bizarre life cycle.

 

The cattle ingest grass with an encysted stage (metacercarium) of the fluke present.

 

 

After the cattle eat this contaminated grass, the juvenile flukes "burrow" through the lining of the intestine, escape into the peritoneal cavity (the inside of the abdomen) and migrate to the liver.

 

The flukes bore their way into the liver and over the next 6 weeks or more make their way to the interior of the liver and finally arrive in the bile ducts where they begin to lay eggs (they are hermaphroditic).

 

The fluke eggs (over 100,000 per day) are shed into the manure of the cattle.

 

These eggs hatch into miracidia and make their way to fresh water snails, which they infect and where they undergo additional development.

 

They eventually emerge from the snail as young flukes (cercariae) and encyst (form a resistant coating) on blades of grass.

 

When cattle ingest them (metacercarium), the life cycle can be completed.

 

What damage does Fasciola hepatica cause?

 

The young flukes cause quite a lot of damage as they migrate through the liver.

 

If only a few flukes are migrating through the liver at one time, the damage to the cattle is minimal.

 

However, if many flukes are migrating at the same time, the damage to the liver can be extensive.

 

In these cases, diarrhea, weight loss, and jaundice (yellow mucous membranes) can be observed.

 

In addition to the direct damage to the liver, there is another problem liver flukes can precipitate and that is Redwater (named for the dark red, frothy urine).

 

Redwater can affect cattle at any time of the year; however, it is most common in the late spring, summer, and autumn.

 

Redwater is caused by a bacterium which colonizes in the liver of susceptible cattle and produces protein toxins that in turn destroy the body's red blood cells, damages other organ systems and rapidly causes death.

 

The migrating flukes damage local areas in the liver causing low oxygen concentration.  

 

The bacteria prefer these conditions and begin to grow rapidly in these damaged areas.

 

The disease has a short incubation period and the vast majority of affected cattle are usually found dead and bloated within 12 hours on start of the disease.

 

Another problem liver flukes seem to be associated with is decreased fertility.

 

Studies have been published that show decreased pregnancy rates in replacement heifers and increased age to puberty in heifers infected with liver flukes.

 

Thus, flukes can cause losses in a number of ways:

 

(1) direct damage to the liver, with weight loss and diarrhea,

 

(2) death loss due to Redwater secondary to liver damage of migrating flukes, and

 

(3) decreased reproductive performance.

 

Can we eliminate liver flukes?

 

In areas with  relatively mild winter conditions, the abundance of snails (the intermediate host), and wildlife reservoirs (animals that have the disease), it is doubtful we will be able to eliminate flukes.

 

How can we minimize the losses due to flukes?

 

If snails can be killed in the water, it breaks the life cycle.  Copper sulfate is used.

 

Another option is the use of drugs to kill the adult flukes during strategic times of the year.

 

Killing the adult flukes that are residing in the liver of cattle before turning them onto clean pastures seems to be the most cost-effective strategy.

 

This not only kills the flukes; but it prevents further shedding of eggs on the pastures.

 

Maximum transmission of flukes occurs in spring and summer in warmer regions and late summer to fall in cooler regions.

 

 

 

 

CLONORCHIS SINENSIS - THE HUMAN LIVER FLUKE:

 

Clonorchis sinensis is known as the oriental or Chinese liver fluke because it is distributed throughout Japan, Korea, China, Taiwan, and Vietnam .

 

It is estimated that this species infects more than 30,000,000 humans in these areas.

 

The parasite also infects a number of other animals, including dogs, cats, pigs, and rodents, and these animals serve as reservoirs of infection.

 

The adult worms measure between 10 and 25 mm in length.

 

They do not actually live in the host's liver, but are found in the bile ducts inside of the liver.

 

Eggs are passed in the host's feces and the first intermediate host is a snail.

 

Cercariae emerge from the first intermediate host and infect the second intermediate host.

 

The second intermediate host is a fish, and over 100 species of fish are susceptible to infection.

 

The definitive host (human or some other mammal) is infected when it eats raw or undercooked fish.  (Definitive host in the one in which the parasite reaches sexual maturity)

 

The metacercariae make their way to the host's small intestine, and the juvenile worm migrates up the common bile duct into the bile ducts inside of the liver.

 

The worms can live at least 10 years, and possibly as long as 20 years.

 

The parasite causes thickening of the lining of the bile duct and an inflammatory response in the surrounding liver tissue.

 

In heavy infections the bile duct epithelium can also be damaged, and the parasite's eggs will enter the liver tissue; in such cases the eggs are surrounded by a capsule.

 

Heavy infections can also result in stenosis (narrowing or blockage) of the bile ducts.

 

Since the parasites can live for years and the number of parasites tends to increase as a person ages, the damage to the liver and bile duct tends to accumulate over time and can result in death.

 

As with many trematodes, diagnosis of the infection depends on recovering and identifying the parasite's eggs in a stool sample.

SCHISTOSOMA, THE HUMAN BLOOD FLUKE:

 

The schistosomes are unusual trematodes in that the sexes are separate (they are dioecious).

 

The male and female look very different, but the female lies permanently in a groove of the         male.

 

They reside in the blood vessels of the definitive host (human).   

 

There are a number of species of schistosomes that can infect humans, but most human infections are caused by one of the three following species:

 

Schistosoma mansoni; (eggs in feces)

S. haematobium; (eggs in urine)

S. japonicum.  (eggs in feces)

 

Considering the distributions of all three species, schistosomiasis is distributed:  

 

throughout almost all of Africa,

 

parts of southeast Asia,

 

parts of northwest South America, and

 

some islands in the Caribbean Sea. 

 

It is estimated that approximately 200,000,000 million people are infected with schistosomes, resulting in 1,000,000 deaths each year. 

 

The life cycles of the three primary species of human schistosomes are similar. 

 

The male and female worms average about 10 mm in length and live in the veins of the abdominal cavity. 

 

Here they mate and the females produce eggs. 

 

The adult worms can live 20‑30 years and, depending on the species, and each female can produce several hundred eggs each day. 

 

The eggs escape from the body by penetrating the walls of the veins and small intestine or urinary bladder, and they are passed in the feces or urine. 

 

The eggs hatch in water, the intermediate host (a snail) is infected, and cercariae are liberated from the snails. 

When humans come in contact with water containing cercariae, the cercariae penetrate their skin and they become infected. 

 

This occurs when the humans swim, bath, wash clothes, etc., in rivers and streams. 

After the cercariae penetrate the skin the immature worms enter the circulatory system and migrate to the veins of the abdominal cavity, and in about six weeks they reach sexual maturity.

 

As the eggs of the schistosomes penetrate the walls of the veins and the small intestine or urinary bladder (depending on the species of the worm), they cause a significant amount of damage to the tissues. 

 

The tissues hemorrhage, so blood often appears in the urine or feces. 

 

As the infection progresses the tissues become inflamed and unable to function normally. 

 

Many of the eggs produced by the female worms do not escape from the veins, but are swept up in the circulatory system and deposited in the host's liver. 

 

The liver responds to the presence of the eggs by encapsulating them. 

 

The damage to the small intestine (or urinary bladder) and liver accumulate over time and result in a chronic, disabling disease that can be fatal.

 

As with most trematode infections, diagnosis most often depends on finding the parasite's eggs. 

 

In the case of S. haematobium, eggs are most often recovered in the urine; eggs of the other two species are most often recovered in the feces.

 

CLASS CESTODA  - TAPEWORMS

 

The cestodes, or tapeworms, differ in a number of ways from other flatworms.

 

Their bodies are long and flat, made up of many segments called proglottids.

 

Each proglottid is a reproductive unit, essentially a factory to produce gametes.

 

Their surface is covered with tiny projections, microvilli, which increase its surface area and thereby its ability to absorb nutrients from a host.

 

They have no digestive tract since they live in an environment (intestine) filled with digested food..

At the tapeworm's anterior end is a head called a scolex, which is usually covered with hooks or suckers and serves to anchor it to the host.

 

All of the 5000 or so known species of tapeworms are endoparasites.

 

Most require at least two hosts, with the host of the adult tapeworm a vertebrate.

 

Intermediate hosts are often invertebrates.

 

A number of tapeworm species inhabit humans.

 

BEEF/PORK TAPEWORM LIFE CYCLE: (Pork tapeworm is the most common in humans)

 

Taenia solium (pork tapeworm) are not often seen because most people cook pork thoroughly out of fear of other parasites, especially Trichina worms.

 

The adult Taenia .solium measures 8 to 10 ft. in length and include a scolex (head) armed with several hooks and a body composed of 1000 proglottids.

 

The proglottids vary from immature, to mature, to gravid as the move down the worm.

 

The life cycle resembles that of Taenia .saginata (beef tapeworm) except that hogs rather than cattle serve as the normal intermediate hosts.

 

Beef tapeworms are the most common type tapeworm in humans in the United States.

 

Human tapeworm infestations are most common in regions where there is fecal contamination of soil and water and where meat is eaten raw (beef tartare) or lightly cooked (rare).

 

Life cycle:

 

Each proglottid contains a complete set of male and female reproductive organs that produce the sex cells.

 

Fertilization is internal;

 

Cross fertilization between two adjacent worms is usually necessary, although sometimes a worm will loop around and fertilize itself. 

 

The eggs develop into embryos with a hard outer shell; these do not hatch until they are eaten by a suitable intermediate host.

 

In the case of the human tapeworm most common in the United States (the beef tapeworm, Taenia saginata). 

The usual intermediate host is a cow, which ingests the proglottid while drinking or grazing.

 

(In countries where cows are held to be sacred, the animals often follow                                    their worshipers into the wood in anticipation of a fecal meal.)

 

The round-bodied embryos hatch from the eggs and are equipped with sharp hooks, hatch and bore through the cow's intestinal wall into the bloodstream, where they are carried to the muscles (meatBperhaps steak).

 

Here each embryo encloses itself in a cyst, or bladder; at this stage it is called a bladder worm (cysticercus).

 

During the bladder worm stage the embryo develops into a miniature scolex turned inside-out;

 

It remains encysted until the muscle is eaten by a primary host, in this case a human.

 

If the scolex has not been killed by sufficient cooking of the meat, it sheds its covering and attaches to the intestinal wall, where it begins producing proglottids.

 

Humans may also act as intermediate hosts, either by ingesting eggs directly, or

 

By regurgitating gravid proglottids from the intestine to the stomach, where the embryos are released;

 

They penetrate the intestinal wall. 

 

Are carried to the subcutaneous tissue muscle, viscera and CNS.

 

Living cysticerci cause only a mild tissue reaction, but dead larvae provoke a vigorous reaction.

 

Symptoms and Signs

 

Infection with the adult worm is usually asymtomatic (no symptoms).

 

Heavy larval infection (cysticercosis) may cause muscle pains, weakness, fever, or if the CNS is involved, brain infection or epilepsy may occur.

 

Diagnosis

 

In adult worm infections, eggs may be found on skin around the anus or in feces.

Cysticercosis (infection with the bladderworm) should be suspected in any patient who lives or lived in an area where the parasite is found and develops nervous system problems.

 

Living bladderworms can be seen on CAT scan or MRI.

 

Calcified (dead) cysticerci may be seen on x-ray.

 

Prevention

 

Thoroughly cooked beef or pork prevents infection.

 

 

DIPHYLLOBOTHRIUM LATUM IS THE FISH TAPEWORM OF MAN.

 

It is particularly common in the Baltic region, Russia and the Great Lakes region of the U.S.A.

 

Structure of the adult tapeworm

 

The adult parasites are typically between 2 and 12 m in length by up to 2 cm in width, but may grow even longer in some cases.

 

The body is divided into proglottids, as is the case of all tapeworms.

 

These proglottids are more broad than they are long, except at the terminal end, where they are approximately square in shape.

 

Internally the proglottids are typical of  tapeworms, with numerous testes and a central bilobed ovary.

 

An important difference between this parasite and the other tapeworms of humans is that the uterus open to the exterior (most other tapeworms have closed uteruses).

 

Eggs are therefore actively deposited by the parasite, in contrast to the disintegration of the proglottids seen in the other human tapeworms.

 

Life cycle:

 

                        Diphylobothrium latum has the adult parasites in the definitive host, (i.e. humans).

 

There are two intermediate hosts containing larval stages.

 

Eggs are passed from humans in the feces and hatch in water to release a small swimming embryonic larvae, covered in cilia.

 

The larva is a free swimming stage, but cannot survive long.

 

For further development it must be ingested by the first intermediate host, a copepod (a copepod is a crustacean, looking a bit like a tiny shrimp).

 

On ingestion the embryonic larvae penetrates the copepod=s gut wall, entering the body cavity to develop into another larval stage. 

 

To continue the life cycle the copepod must be ingested by the next intermediate host, a fish.

 

The larva penetrates the gut wall of the fish, and develops into the next larval stage, measuring 4 ‑ 5 mm in length, in the viscera (internal organs) or musculature (meat) of the fish.

 

A number of different species of fish may act as intermediate hosts for the larvae of D. latum, but the highest densities are found in carnivorous fish such as the pike.

 

These high parasite loads result because, in addition to infection by ingestion of the copepod plus larvae, if another infected fish is eaten the larvae within the body tissues of this fish are released in the intestine of the carnivorous fish.

 

These then migrate through the intestinal wall, to invade the new host. 

 

The larvae are, in addition, very long lived, and very high parasite densities may be achieved.

 

Humans are infected by eating raw or undercooked fish, the larvae emerging in the intestine to grow into the adult parasite.

 

In addition to humans a number of other fish eating mammals may also be infected, including cats, dogs, pigs, bears.

 

In humans, multiple infections may occur. 

 

Sometimes of very high numbers (up to 143 worms have been reported from a single individual).

 

In these cases the individual parasites length is much less.

 

To avoid infection in humans, fish should be properly cooked, killing the infective larvae.

 

How it affects humans:

 

Infection, as is often the case with adult tapeworms, presents a wide range of symptoms, but is not commonly the cause of serious disease in humans.

 

Symptoms, when they occur, include:

 

A variety of non‑specific abdominal signs, including abdominal pain and loss of weight, and

 

Are often very similar to the symptoms displayed during infection with pork & beef tapeworms.

 

However D. latum differs from Taenia in absorbing much more vitamin B12, (between ten and fifty times more) than other tapeworms.

 

Infection may therefore result in anemia in some cases, vitamin B12 having an important role in formation of blood cells.

 

This feature of the disease is much more common in the Baltic region, particularly in Finland.

 

This tapeworm derived anemia may be due to genetic factors in the person.

 

It is also more commonly seen when the tapeworm is situated higher in the intestine.

 

ECHINOCOCCUS GRANULOSUS: (The dog tapeworm)

 

The life cycle of Echinococcus granulosus includes dogs (and other canines) as the definitive host. 

 

A variety of species of warm blooded vertebrates (sheep, cattle, goats, and humans) can serve as the intermediate host.

 

The adult worms are very small, usually consisting of only three proglottids (total length = 3-6 mm), and they live in the dog's small intestine.

 

Eggs are liberated in the dog's feces, and when these eggs are ingested by the intermediate host (maybe human) they hatch in the host's small intestine.

 

The larvae in the eggs penetrate the gut wall and enter the circulatory system.

 

The larvae can be distributed throughout the intermediate host=s body (although most end up in the liver) and grow into a stage called a hydatid cyst.  

Hydatid cysts have the ability to grow quite large; cysts the size of golf balls are not uncommon, and cysts the size of basketballs are reported on rare occasions.

 

The effects of hydatid disease in the intermediate host depends on the size of the cyst and its location.

 

One or two small cysts in the liver of a host might go unnoticed for years.

 

However, a single large cyst in the liver could prove fatal.

 

Hydatid disease is far more serious when the cysts are found in other locations, particularly the brain.

 

Under most circumstances humans are a "dead end" in the life cycle, but hydatid disease in humans remains a problem because the disease can cause such serious disease.

 

The infection is transmitted to the dog when the hydatid cyst from animals is eaten.

 

The interior of a hydatid cyst is filled with scolices, each of which has the ability to grow into an adult worm when ingested by a dog.

 

A small cyst might contain hundreds of scolices; a large cyst might contain tens of thousands!

 

This tremendous reproductive potential poses a problem in the intermediate host (particularly in humans).

 

If a hydatid cyst breaks open, each scolex could grow into a new hydatid cyst.

 

How might this happen?

 

A sharp blow to the abdomen might rupture a cyst in the liver.

 

A number of cases have been reported in which cysts have been damaged during routine surgery, allowing the cyst's contents to leak into the patient's abdominal cavity, resulting in death.